EDITORIAL

www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004 917

If ever a phrase tripped lightly over the tongue, “the hydrogen economy” does. It appeals to thefuturist in all of us, and it sounds so simple: We currently have a carbon economy that pro-duces carbon dioxide (CO2), the most prominent of the greenhouse gases that are warming upthe world. Fortunately, however, we will eventually be able to power our cars and industrieswith climate-neutral hydrogen, which produces only water.

Well, can we? This issue of Science exposes some of the problems, and they’re serious. Toconvert the U.S. economy in this way will require a lot of hydrogen: about 150 million tons of it ineach year. That hydrogen will have to be made by extracting it from water or biomass, and that takesenergy. So, at least at first, we will have to burn fossil fuels to make the hydrogen,which means that we will have to sequester the CO2 that results lest it go into theatmosphere. That kind of dilemma is confronted in virtually all of the proposedroutes for hydrogen production: We find a way of supplying the energy tocreate the stuff, but then we have to develop other new technologies to dealwith the consequences of supplying that energy. In short, as the Viewpointby Turner in this issue (p. 972) makes clear, getting there will be a monumental challenge.

In a recent article (Science, 30 July, p. 616), Secretary of EnergySpencer Abraham calls attention to the Bush administration’s commit-ment to the hydrogen solution. The Hydrogen Fuel Initiative andFreedomCAR Partnership, announced in the 2003 State of the Union message, aims “to develop hydrogen fuel cell–powered vehicles.” The United States also led the formation of the International Partnership for theHydrogen Economy, a project in which Iceland, blessed with geothermalsources and an inventive spirit, appears to be ahead of everyone else (see p. 966).These and other initiatives are politically useful because they serve to focus public atten-tion on the long-range goal. They rely on the premise that when the research on these new tech-nologies is finished, we will have a better fix on the global warming problem; in the meantime, we’llput in place strictly voluntary measures to reduce CO2 emissions. That’s the case being made by theBush administration.

The trouble with the plan to focus on research and the future, of course, is that the exploding trajectory of greenhouse gas emissions won’t take time off while we are all waiting for the hydro-gen economy. The world is now adding 6.5 billion metric tons of carbon to the atmosphere in theform of CO2 annually. Some nations are cutting back on their share, but the United States, which isresponsible for about a quarter of the world’s total, is sticking firmly to business as usual. In eachyear, some of the added CO2 will be fixed (taken up by plants in the process of photosynthesis andthus converted to biomass) or absorbed by the oceans. But because the amount added exceeds theamount removed, the concentration of atmospheric CO2 continues to increase annually, and theadded carbon remains in the atmosphere for many decades.

In fact, even if the United States and all other nations reduced the growth rate of annual emis-sions to zero, the concentration of greenhouse gases would continue to rise for the rest of thecentury, and average global temperature would increase in response. How hot it will get dependson various feedback factors: clouds, changes in Earth’s reflectivity, and others. It is clear, how-ever, that steady and significant increases in average global temperature are certain to occur,along with increases in the frequency of extreme weather events, including, as shown in the paper by Meehl and Tebaldi in this issue (p. 994), droughts and heat waves.

Another kind of feedback factor, of course, would be a mix of social and economic changesthat might actually reduce current emissions, but current U.S. policy offers few incentives forthat. Instead, it is concentrating on research programs designed to bring us a hydrogen economythat will not be carbon-free and will not be with us any time soon. Meanwhile, our attention isdeflected from the hard, even painful measures that would be needed to slow our business-as-usual carbon trajectory. Postponing action on emissions reduction is like refusing medication fora developing infection: It guarantees that greater costs will have to be paid later.

Donald Kennedy

Editor-in-Chief

The Hydrogen Solution

CR

ED

IT:A

DA

PT

ED

FR

OM

AN

ILL

UST

RA

TIO

N B

Y J

EA

N-F

RA

NC

OIS

CO

LON

NA

(C

MA

P/E

CO

LE P

OLY

TEC

HN

IQU

E,F

T R

&D

)

13 AUGUST 2004 VOL 305 SCIENCE www.sciencemag.org926

NEWSSue’sterrible teens

Cancer andstem cells

Th is We e k

Deciding who will go down in history asAlvin’s last crew may be the biggest issuestill on the table now that the U.S. govern-ment has decided to retire its famous re-search submarine and build a faster, roomier,and deeper diving substitute. Last week, theNational Science Foundation (NSF) put anend to a decade of debate about the sub’s fu-ture by announcing that it will shelve the 40-

year-old Alvin in late 2007 and replace itwith a $21.6 million craft packed with fea-tures long coveted by deep-sea scientists.

“It’s a bittersweet moment. Alvin is abeloved symbol of ocean exploration,” says

Robert Gagosian, president of the WoodsHole Oceanographic Institution (WHOI) inMassachusetts, which operates Alvin andwill run the new craft. “But there’s a lot ofexcitement about the new things we’ll beable to do.”

The 6 August decision ended an oftenfeisty debate over how to replace Alvin,which entered service in 1967 and is one offive research subs in the world that can divebelow 4000 meters (Science, 19 July 2002, p.326). Its storied, nearly 4000-dive career has

witnessed many high-profile mo-ments, including the discovery ofsulfur-eating sea-floor ecosystemsand visits to the Titanic. Some re-searchers argued for replacing theaging Alvin with cheaper, in-

creasingly capable robotic vehi-cles. Others wanted a human-piloted craft able to reach the11,000-meter bottom of the

deepest ocean trench—far deep-er than Alvin’s 4500-meter rating, which en-ables it to reach just 63% of the sea floor.Last year, after examining the issues, a Na-tional Research Council panel endorsedbuilding a next-generation Alvin, but put ahigher priority on constructing a $5 million

robot that could dive to 7000 meters(Science, 14 November 2003, p. 1135).

That vehicle has yet to appear, althoughNSF officials say an automated sub currentlyunder construction at WHOI partly fills thebill. And NSF and WHOI have chosen whatthe panel judged the riskiest approach tobuilding a new Alvin: starting from scratchwith a new titanium hull able to reach 6500meters or 99% of the sea floor. The panelhad suggested using leftover Russian orU.S. hulls rated to at least 4500 meters,partly because few shipyards know how towork with titanium. WHOI engineers,

NSF Takes the Plunge on a Bigger, Faster Research Sub

M A R I N E E X P L O R AT I O N

NIH Declines to March In on Pricing AIDS Drug The National Institutes of Health (NIH) hasrejected a controversial plea to use its legalmuscle to rein in the spiraling cost of awidely used AIDS drug. NIH Director EliasZerhouni last week said his agency wouldnot “march in” and reclaim patents on adrug it helped develop because pricing is-sues are best “left to Congress.”

The decision disappointed AIDS ac-tivists, who said it opened the door to pricegouging by companies. But major researchuniversities were quietly pleased. “This wasthe only decision NIH could make [based]on the law,” says Andrew Neighbour, an as-sociate vice chancellor at the University ofCalifornia, Los Angeles.

The 4 August announcement was NIH’s

answer to a request filed in January by Essen-tial Inventions, a Washington, D.C.–based ad-vocacy group (Science, 4 June, p. 1427). Itasked NIH to invoke the 1980 Bayh-Dole Act,which allows the government to reclaimpatents on taxpayer-funded inventions if com-panies aren’t making the resulting productsavailable to the public. Specifically, the groupasked NIH to march in on four patents held byAbbott Laboratories of Chicago, Illinois. Allcover the anti-AIDS drug Norvir, which Ab-bott developed in the early 1990s with supportfrom a 5-year, $3.5 million NIH grant.

Last year, Abbott increased U.S. retailprices for some Norvir formulations by up to400%, prompting the call for NIH to interveneand allow other manufacturers to make the

drug. University groups and retired govern-ment officials who wrote the law, however, ar-gued that such a move would be a misreadingof Bayh-Dole and would undermine efforts tocommercialize government-funded inventions.

In a 29 July memo, Zerhouni concludedthat Abbott has made Norvir widely availableto the public and “that the extraordinary rem-edy of march-in is not an appropriate meansof controlling prices.” The price-gougingcharge, he added, should be investigated bythe Federal Trade Commission (which islooking into the matter). Essential Inventions,meanwhile, says it will appeal to NIH’s over-seer, Health and Human Services SecretaryTommy Thompson. Observers doubt Thomp-son will intervene. –DAVID MALAKOFF

PAT E N T S

CR

ED

ITS:W

OO

DS H

OLE

OC

EA

NO

GR

APH

IC I

NST

ITU

TIO

N

Coming out. Alvin’s last dive is scheduled for

late 2007.

▲

Going down.

New submersible will

be able to dive 6500 meters.

P A G E 9 2 9 9 3 0

however, are confident that hurdle can beovercome.

Overall, the new submarine will be aboutthe same size and shape as the current Alvin,so that it can operate from the existingmother ship, the Atlantis. But there will bemajor improvements.

One change is nearly 1 cubic meter moreelbowroom inside the sphere that holds the pi-lot and two passengers. It will also offer fiveportholes instead of the current three, and thescientists’ views will overlap with the pilot’s,eliminating a long-standing complaint. Asleeker design means researchers will sink tothe bottom faster and be able to stay longer.

Alvin currently lingers about 5 hours at 2500meters; the new craft will last up to 7 hours. Anew buoyancy system will allow the sub tohover in midwater, allowing researchers tostudy jellyfish and other creatures that spendmost of their lives suspended. And an abilityto carry more weight means researchers willbe able to bring more instruments—and haulmore samples from the depths.

At the same time, improved electronicswill allow colleagues left behind to partici-pate in real time. As the new vehicle sinks,it will spool out a 12-kilometer-long fiber-optic cable to relay data and images. “Itwill put scientists, children in classrooms,

and the public right in the sphere,” saysNSF’s Emma Dieter.

Officials predict a smooth transition be-tween the two craft. The biggest effect couldbe stiffer competition for time on board, be-cause the new submersible will be able toreach areas—such as deep-sea trenches withinteresting geology—once out of reach.

In the meantime, Alvin’s owner, the U.S.Navy (NSF will own the new craft), must de-cide its fate. NSF and WHOI officials willalso choose a name for the new vessel, al-though its current moniker, taken from a1960s cartoon chipmunk, appears to haveconsiderable support. –DAVID MALAKOFF

www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004 927

CR

ED

IT:I

MA

GE P

RO

DU

CED

BY

HA

L PIE

RC

E (

SSA

I/N

ASA

GSFC

)

A warmergreen

A river runsthrough him

Information--please

Foc u s

EVENT HORIZON +

+

–

1

2

3–

–

+

+

–BLACK HOLE

Facing pressure from Congress and theWhite House, NASA agreed last week torethink plans to retire a climate satellitethat weather forecasters have found usefulfor monitoring tropical storms. The spaceagency said it would extend the life of the$600 million Tropical Rainfall MeasuringMission (TRMM) until the end of the yearand ask the National Research Council(NRC) for advice on its future.

TRMM, launched on a Japanese rocketin 1997, measures rainfall and latent heat-ing in tropical oceans and land areas thattraditionally have been undersampled. Al-though designed for climate researchers,TRMM has also been used by meteorolo-gists eager to improve their predictions ofsevere storms. “TRMM has proven helpfulin complementing other satellite data,”says David Johnson, director of the Na-tional Oceanic and Atmospheric Adminis-tration’s (NOAA’s) weather service, whichrelies on a fleet of NOAA spacecraft.

Climate and weather scientists protestedlast month’s announcement by NASA that itintended to shut off TRMM on 1 August.NASA officials pleaded poverty and notedthat the mission had run 4 years longer thanplanned. The agency said it needed to putthe satellite immediately into a slow drift outof orbit before a controlled descent nextspring, a maneuver that would avoid a po-tential crash in populated areas.

The satellite’s users attracted the attentionof several legislators, who complained thatshutting down such a spacecraft at the startof the Atlantic hurricane season would put

their constituents in danger. “Your Adminis-tration should be able to find a few tens ofmillions of dollars over the next 4 years topreserve a key means of improving coastaland maritime safety,” chided RepresentativeNick Lampson (D–TX) in a 23 July letter tothe White House. “A viable funding arrange-ment can certainly be developed betweenNASA and the other agencies that use TRMM’s data if you desire it to happen.” Inan election year, that argument won the earof the Bush Administration, in particular,NOAA Chief Conrad C. Lautenbacher Jr.,

who urged NASA Administrator Sean O’Keefe to rethink his decision.

On 6 August, O’Keefe said he would keepTRMM going through December. He joinedwith Lautenbacher in asking NRC, the operat-ing arm of the National Academies, to hold aSeptember workshop to determine if and howTRMM’s operations should be continued.Whereas NOAA is responsible for weatherforecasting, NASA conducts research andwould prefer to divest itself of TRMM. “We’dbe happy to give it to NOAA or a university,”says one agency official. Keeping the satellite

going through December willcost an additional $4 million to$5 million—“and no one has de-cided who is going to pay,” theoff icial added. By extending TRMM’s life, NASA hopes “toaid NOAA in capturing anotherfull season of storm data,” saysGhassem Asrar, deputy associateadministrator of NASA’s new sci-ence directorate.

Technically, satellite operatorscould keep TRMM operating an-other 18 months, but this wouldcome with a hidden cost. NASAwould have to monitor the craftfor a further 3 years before put-ting it on a trajectory to burn up.That option would cost about$36 million. Now that TRMMhas so many highly placedfriends, its supporters hope thatone of them will also have deeppockets. –ANDREW LAWLER

NASA Climate Satellite Wins ReprieveS PA C E S C I E N C E

Eye opener. TRMM monitored the season’s first hurricane,

Alex, as it approached the North Carolina coast last week.

9 3 2 9 3 4 9 3 7

www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004

CR

ED

IT:C

DC

929

Federal Ethics Office FaultsNIH Consulting PracticesA government review of the ongoing ethicscontroversy at the National Institutes ofHealth (NIH) has found significant lapses inthe agency’s past procedures, according to apress report.

In a 20-page analysis, Office of Govern-ment Ethics (OGE) acting director MarilynGlynn charges NIH with a “permissive cul-ture on matters relating to outside compen-sation for more than a decade,” according toexcerpts in the 7 August Los Angeles Times.OGE reportedly found instances in whichNIH lagged in approving outside consultingdeals or did not approve them at all, and itconcluded that some deals raised “the ap-pearance of the use of public office for pri-vate gain.”The report, addressed to the De-partment of Health and Human Services(HHS), also questions whether NIH officialsshould oversee the agency’s ethics programgiven this spotty record. (As Science went topress, OGE and HHS had not released thereport.)

However, the report does not recom-mend a blanket ban on industry consulting,according to an official who has seen it. Andstrict new limits proposed by NIH DirectorElias Zerhouni—including no consulting byhigh-level employees—are consistent withthe report’s recommendations, says NIHspokesperson John Burklow.“We’re confi-dent that the strong policies we are devel-oping, in addition to the steps we have al-ready taken, will address the issues identi-fied.We look forward to working with OGEas we finalize these policies,” Burklow says.

–JOCELYN KAISER

Biopharming Fields Revealed? The U.S. Department of Agriculture (USDA)may have to disclose the locations ofbiotech field trials in Hawaii after losing around in court.The USDA issues permits forfield trials of biopharmaceuticals—drug andindustrial compounds produced in plants—and other genetically modified crops, but itconsiders the locations confidential busi-ness information.The agency is also worriedabout vandals.

The decision is part of a case that Earth-justice filed against USDA last year on be-half of environmental groups, arguing thatfield tests haven’t been adequately assessedfor environmental safety. Last week, a feder-al district court judge ruled that the field lo-cations must be revealed to the plantiffs toassess potential harm, but gave USDA 90days to make a stronger case against publicdisclosure. USDA says it is studying the deci-sion, and Earthjustice expects the agency toappeal. –ERIK STOKSTAD

ScienceScope

A prominent California stem cell lab says ithas hit on a cadre of cells that helps explainhow a form of leukemia transitions from rel-ative indolence to life-threatening aggres-sion. In an even more provocative claim,Irving Weissman of Stanford University andhis colleagues propose in this week’s NewEngland Journal of Medicine that thesecells, granulocyte-macrophage progenitors,metamorphose into stem cells as the cancerprogresses. Some cancer experts doubt thesolidity of the second claim, however.

The concept that stem cells launch andsustain a cancer has gained credence as sci-entists tied such cells to several blood can-

cers and, more recently, to breast cancer andother solid tumors (Science, 5 September2003, p. 1308). Weissman’s group exploreda facet of this hypothesis, asking: Can non-stem cells acquire such privileged status in acancer environment? The investigators fo-cused on chronic myelogenous leukemia(CML), which the drug Gleevec has earnedfame for treating.

The researchers gathered bone marrowsamples from 59 CML patients at differentstages of the disease. A hallmark of CML isits eventual shift, in patients who don’t re-spond to early treatment, from a chronicphase to the blast crisis, in which patientssuffer a massive proliferation of immatureblood cells. Weissman, his colleague Catri-ona Jamieson, and their team noticed thatamong blood cells, the proportion of granulocyte-macrophage progenitors, whichnormally differentiate into several types ofwhite blood cells, rose from 5% in chronic-phase patients to 40% in blast-crisis patients.

When grown in the lab, these cells ap-peared to self-renew—meaning that onegranulocyte-macrophage progenitor spawnedother functionally identical progenitor cells

rather than simply giving rise to more maturedaughter cells. This self-renewal, a definingfeature of a stem cell, seemed dependent onthe β-catenin pathway, which was previouslyimplicated in a number of cancers, includinga form of acute leukemia. Weissman and hisco-authors postulate that the pathway couldbe a new target for CML drugs aiming tostave off or control blast crisis.

Forcing expression of β-catenin proteinin granulocyte-macrophage progenitorsfrom healthy volunteers enabled the cells toself-renew in lab dishes, the researchers re-port. Whereas the first stage of CML is driv-en by a mutant gene called bcr-abl, whose

protein Gleevec targets, Weiss-man theorizes that a β-cateninsurge in granulocyte-macrophage progenitors leads tothe wild cell proliferation thatoccurs during the dangerousblast phase.

Some critics, however, saythat proof can’t come from thepetri dish. “To ultimately define astem cell” one needs to conducttests in animals, says John Dick,the University of Toronto biolo-gist who first proved the exis-tence of a cancer stem cell in the1990s. Studies of acute myeloge-nous leukemia uncovered numer-

ous progenitor cells that seemed to self-re-new, notes Dick. But when the cells were giv-en to mice, many turned out not to be stemcells after all.

Michael Clarke of the University ofMichigan, Ann Arbor, who first isolatedstem cells in breast cancer, is more im-pressed with Weissman’s results. The cells inquestion “clearly self-renew,” he says. “Theimplications of this are just incredible.” Thesuggestion that nonstem cells can acquirestemness could apply to other cancers andshed light on how they grow, he explains.

All agree that the next step is injectingmice with granulocyte-macrophage progenitors from CML patients to seewhether the cells create a blast crisis. Weiss-man’s lab is conducting those studies, andresults so far look “pretty good,” he says.

“What we really need to know is whatcells persist in those patients” who progressto blast crisis, concludes Brian Druker, aleukemia specialist at Oregon Health & Sci-ence University in Portland. That questionstill tops the CML agenda, although Weiss-man suspects that his team has found theculprits. –JENNIFER COUZIN

Proposed Leukemia Stem Cell Encounters a Blast of Scrutiny

C A N C E R R E S E A R C H

Outnumbered. Immature blood cells proliferate wildly as a

CML blast crisis takes hold.

13 AUGUST 2004 VOL 305 SCIENCE www.sciencemag.org930

Tyrannosaurus rex was a creature of super-latives. As big as a bull elephant, T. rexweighed 15 times as much as the largestcarnivores living on land today. Now, pale-ontologists have for the first time chartedthe colossal growth spurt that carried T. rexbeyond its tyrannosaurid relatives. “It wouldhave been the ultimate teenager in terms offood intake,” says Thomas Holtz of the Uni-versity of Maryland, College Park.

Growth rates have been studied in only

a half-dozen dinosaurs and no large carni-vores. That’s because the usual method oftelling ages—counting annual growth ringsin the leg bone—is a tricky task withtyrannosaurids. “I was told when I startedin this field that it was impossible to age T. rex,” recalls Gregory Erickson, a paleo-biologist at Florida State University in Tal-

lahassee, who led the study. The reason isthat the weight-bearing bones of largedinosaurs become hollow with age and theinternal tissue tends to get remodeled, thuserasing growth lines.

But leg bones aren’t the only placeto check age. While studying a tyran-nosaurid called Daspletosaurus at theField Museum of Natural History(FMNH) in Chicago, Illinois, Ericksonnoticed growth rings on the end of abroken rib. Looking around, he foundsimilar rings on hundreds of otherbone fragments in the museum draw-ers, including the fibula, gastralia, andthe pubis. These bones don’t bear sub-stantial loads, so they hadn’t been re-modeled or hollowed out.

Switching to modern alligators, croco-diles, and lizards, Erickson found that thegrowth rings accurately recorded the ani-mals’ ages. He and his colleagues then sam-pled more than 60 bones from 20 specimensof four closely related tyrannosaurids. Count-ing the growth rings with a microscope, theteam found that the tyrannosaurids had died

at ages ranging from 2 years to 28.By plotting the age of each animal

against its mass—conservatively estimatedfrom the circumference of its femur—theyconstructed growth curves for eachspecies. Gorgosaurus and Albertosaurus,both more primitive tyrannosaurids, beganto put on weight more rapidly at about age12. For 4 years or so, they added 310 to480 grams per day. By about age 15, theywere full-grown at about 1100 kilograms.The more advanced Daspletosaurus fol-lowed the same trend but grew faster andmaxed out at roughly 1800 kilograms.

T. rex, in comparison, was almost offthe chart. As the team describes this weekin Nature, it underwent a gigantic growthspurt starting at age 14 and packed on 2kilograms a day. By age 18.5 years, theheaviest of the lot, FMNH’s famous T. rexnamed Sue, weighed more than 5600 kilo-grams. Jack Horner of the Museum of theRockies in Bozeman, Montana, and KevinPadian of the University of California,Berkeley, have found the same growth pat-tern in other specimens of T. rex. Their pa-per is in press at the Proceedings of theRoyal Society of London, Series B.

It makes sense that T. rex would growthis way, experts say. Several lines of evi-dence suggest that dinosaurs had a highermetabolism and faster growth rates than liv-ing reptiles do (although not as fast asbirds’). Previous work by Erickson showedthat young dinosaurs stepped up the pace ofgrowth, then tapered off into adulthood; rep-tiles, in contrast, grow more slowly, but theykeep at it for longer. “Tyrannosaurus rexlived fast and died young,” Erickson says.“It’s the James Dean of dinosaurs.”

Being able to age the animals will helpshed light on the population structure oftyrannosaurids. For instance, the researchersdetermined the ages of more than half adozen Albertosaurs that apparently died

Bone Study Shows T. rex Bulked UpWith Massive Growth Spurt

PA L E O N T O L O G Y

Hungry. Growth rings (inset) in a rib show that Sue

grew fast during its teenage years.

Los Alamos’s Woes Spread to Pluto MissionThe impact of the shutdown of Los AlamosNational Laboratory in New Mexico couldripple out to the distant corners of the solarsystem. The lab’s closure last month due tosecurity concerns (Science, 23 July, p. 462)has jeopardized a NASA mission to Plutoand the Kuiper belt. “I am worried,” says S. Alan Stern, a planetary scientist with theSouthwest Research Institute in Boulder,Colorado, who is the principal investigator.

That spacecraft, slated for a 2006 launch,is the first in a series of outer planetaryflights. In those far reaches of space, solarpower is not an option. Instead, the missionwill be powered by plutonium-238, obtained

from Russia and converted by Los Alamosscientists into pellets. But the 16 July “standdown” at the lab has shut down that effort,which already was on a tight schedule due tothe lengthy review required for any space-craft containing nuclear material.

The 2006 launch date was chosen tomake use of a gravity assist from Jupiter torocket the probe to Pluto by 2015. A 1-yeardelay could cost an additional 3 to 4 years intransit time. “It won’t affect the science wewill be able to do in a serious way, but it willdelay it and introduce risks,” says Stern.Some researchers fear that Pluto’s thin atmos-phere could freeze and collapse later in the

next decade, although the likelihood andtiming of that possibility are in dispute.

Los Alamos officials are upbeat. “Labactivity is coming back on line,” saysspokesperson Nancy Ambrosiano. Even so,last week lab director George “Pete” Nanossuspended four more employees in connec-tion with the loss of several computer diskscontaining classif ied information, andNanos says that it could take as long as 2months before everyone is back at work.NASA off icials declined comment, butStern says “many people are working to findremedies.”

–ANDREW LAWLER

P L A N E TA RY S C I E N C E

CR

ED

ITS:T

HE F

IELD

MU

SEU

M

N E W S O F T H E W E E K▲

together. They ranged in age from 2 to 20in what might have been a pack. “You’vegot really young living with the really old,”Erickson says. “These things probablyweren’t loners.”

The technique could also help re-searchers interpret the medical history of in-dividuals. Sue, in particular, is riddled withpathologies, and the growth rings might re-veal at what age various kinds of injuries oc-

curred. “We could see if they had a reallyrotten childhood or lousy old age,” Holtzsays. And because a variety of scrap bonescan be analyzed for growth rings, more indi-viduals can be examined. “Not many muse-ums will let you cut a slice out of the femurof a mounted specimen,” notes co-authorPeter Makovicky of FMNH. “A great deal ofthe story about Sue was still locked in thedrawers,” Erickson adds. –ERIK STOKSTAD

www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004

CR

ED

IT:V

.SEM

EN

OV

ET

AL.

ScienceScope

931

Among the dark secrets that nestle in galac-tic cores, one of the most vexing is how thegargantuan energy fountains called radio-loud quasars propel tight beams of particlesand energy across hundreds of thousands oflight-years. Astrophysicists agree that thepower comes from supermassive blackholes, but they differ sharply about how themachinery works. According to a new mod-el, the answer might follow a familiar max-im: One good turn deserves another.

On page 978, three astrophysicists proposethat a whirling black hole at the centerof a galaxy can whip magneticfields into a coiled frenzy and ex-pel them along two narrow jets.The team’s simulations paintdramatic pictures of energyspiraling sharply into space.“It has a novelty to it—it’svery educational and illus-trative,” says astrophysi-cist Maurice van Putten ofthe Massachusetts Insti-tute of Technology inCambridge. But the mod-el’s simplified astrophysi-cal assumptions allow otherexplanations, he says.

The paper, by physicistVladimir Semenov of St. Pe-tersburg State University, Russia,and Russian and American col-leagues, is the latest word in an impassioneddebate about where quasars get their spark.Some astrophysicists think the energy comesfrom a small volume of space around theblack holes themselves, which are thought tospin like flywheels weighing a billion suns ormore. Others suspect the jets blast off fromblazingly hot “accretion disks” of gas thatswirl toward the holes. Astronomical obser-vations aren’t detailed enough to settle the ar-gument, and computer models require a com-plex mixture of general relativity, plasmaphysics, and magnetic fields. “We’re still afew years away from realistic time-dependentsimulations,” says astrophysicist Ken-IchiNishikawa of the National Space Science and

Technology Center in Huntsville, Alabama.Semenov and his colleagues depict the

churning matter near a black hole as individ-ual strands of charged gas, laced by strongmagnetic lines of force. Einstein’s equationsof relativity dictate the outcome, says co-author Brian Punsly of Boeing Space and In-telligence Systems in Torrance, California.The strands get sucked into the steep vortexof spacetime and tugged around the equatorjust outside the rapidly spinning hole, a rela-tivistic effect called frame dragging. Tension

within the magnetized ribbons keeps

them intact. Repeatedwindings at close to thespeed of light torque thestresses so high that themagnetic f ields springoutward in opposite direc-tions along the poles, ex-pelling matter as they go.

The violent spin neededto drive such outbursts aris-

es as a black hole consumesgas at the center of an active

galaxy, winding up like a mer-ry-go-round getting constant

shoves, Punsly says. In that environ-ment, he notes, “Frame dragging dominates

everything.”Van Putten agrees, although his research

suggests that parts of the black hole close tothe axis of rotation also play a key role informing jets by means of frame dragging.

Still, the basic picture—a fierce corkscrewof magnetized plasma unleashed by a fran-tically spinning black hole—is valuablefor quasar researchers, says astrophysicistRamesh Narayan of the Harvard-Smithsonian Center for Astrophysics inCambridge. “This gives me a physicalsense for how the black hole might domi-nate over the [accretion] disk in terms ofjet production,” he says. –ROBERT IRION

Do Black Hole Jets Do the Twist?A S T R O P H Y S I C S

Winding up. Coiled magnetic fields

launch jets from massive black

holes, a model claims.

Hubble Space TelescopeLoses Major InstrumentOne of the four main instruments on theaging Hubble Space Telescope has failed,due to an electrical fault in its power sys-tem. It will take several weeks to deter-mine whether the Space Telescope Imag-ing Spectrograph (STIS) is truly deceased,but officials have slim hopes of recovery,noting that even a shuttle repair missioncouldn’t revive it. “It doesn’t look good,”says Bruce Margon, the associate directorfor science at the Space Telescope ScienceInstitute in Baltimore, Maryland.

STIS, which splits incoming light intoits component colors, is particularly use-ful for studying galaxy dynamics, diffusegas, and black holes. Although STIS meas-urements account for nearly one-third ofthis year’s Hubble science portfolio, Mar-gon says that the telescope still has plen-ty of work it can do. “It will be no effortat all to keep Hubble busy,” says Margon,although it is a “sad and annoying loss ofcapability. … It’s a bit like being a gourmetchef and being told you can never cook achicken again.”

–CHARLES SEIFE

Britain to Consider RepatriatingHuman RemainsThe British government is requesting pub-lic comment on a proposal that could re-quire museums and academic collectionsto return human remains collectedaround the world. Department for Cultureofficials last month released a white pa-per (www.culture.gov.uk/global/consulta-tions) recommending that scientists iden-tify how bones or tissues became part oftheir collections and seek permissionfrom living descendants to keep identifi-able remains for study. It also calls for li-censing institutions that collect humanremains.

Indigenous groups have long cam-paigned for such measures, saying thatanthropologists and others have collectedremains without permission. But somescientists worry that the move couldharm research by putting materials out ofreach and lead to expensive legal wran-gles over ownership. Society needs to“balance likely harm against likely bene-fit,” says Sebastian Payne, chief scientistat English Heritage in London, adding that“older human remains without a clearand close family or cultural relationship”are probably best left in collections. Com-ments are due by 29 October.

–XAVIER BOSCH

PARIS—Decades of climate studies havemade some progress. Researchers have con-vinced themselves that the world has indeedwarmed by 0.6°C during the past century.And they have concluded that human activi-ties—mostly burning fossil fuels to producethe greenhouse gas carbon dioxide (CO2)—have caused most of that warming. But howwarm could it get? How bad is the green-house threat anyway?

For 25 years, official assessments of cli-mate science have been consistently vagueon future warming. In report afterreport, estimates of climatesensitivity, or how much agiven increase in atmos-pheric CO2 will warmthe world, fall into thesame subjective range.At the low end, dou-bling CO2—the tradi-tional benchmark—might eventually warmthe world by a modest1.5°C, or even less. Atthe other extreme, tem-peratures might soar bya scorching 4.5°, or morewarming might be possible,given all the uncertainties.

At an international work-shop* here late last month on cli-mate sensitivity, climatic wishy-washi-ness seemed to be on the wane. “We’ve gonefrom hand waving to real understanding,”said climate researcher Alan Robock of Rut-gers University in New Brunswick, New Jer-sey. Increasingly sophisticated climate mod-els seem to be converging on a most probablesensitivity. By running a model dozens oftimes under varying conditions, scientists arebeginning to pin down statistically the trueuncertainty of the models’ climate sensitivity.And studies of natural climate changes fromthe last century to the last ice age are alsoyielding climate sensitivities.

Although the next international assess-ment is not due out until 2007, workshop par-ticipants are already reaching a growing con-

sensus for a moderately strong climate sensi-tivity. “Almost all the evidence points to 3°C”as the most likely amount of warming for adoubling of CO2, said Robock. That kind ofsensitivity could make for a dangerous warm-ing by century’s end, when CO2 may havedoubled. At the same time, most attendeesdoubted that climate’s sensitivity to doubledCO2 could bem u c h

lesst h a n

1.5°C. Thatwould rule out

the feeble green-house warming

espoused by somegreenhouse contrarians.

But at the high and espe-cially dangerous end of climate

sensitivity, confidence faltered; an upperlimit to possible climate sensitivity remainshighly uncertain.

Hand-waving climate models

As climate modeler Syukuro Manabe ofPrinceton University tells it, formal assess-ment of climate sensitivity got off to a shakystart. In the summer of 1979, the late JuleCharney convened a committee of fellow me-teorological luminaries on Cape Cod to pre-pare a report for the National Academy of Sci-ences on the possible effects of increasedamounts of atmospheric CO2 on climate.None of the committee members actually didgreenhouse modeling themselves, so Charneycalled in the only two American researchersmodeling greenhouse warming, Manabe andJames Hansen of NASA’s Goddard Institute

for Climate Studies (GISS) in New York City.On the first day of deliberations, Manabe

told the committee that his model warmed2°C when CO2 was doubled. The next dayHansen said his model had recently gotten4°C for a doubling. According to Manabe,Charney chose 0.5°C as a not-unreasonablemargin of error, subtracted it from Manabe’snumber, and added it to Hansen’s. Thus wasborn the 1.5°C-to-4.5°C range of likely cli-mate sensitivity that has appeared in every

greenhouse assessment since, includingthe three by the Intergovernmental Panelon Climate Change (IPCC). More thanone researcher at the workshop calledCharney’s now-enshrined range and itsattached best estimate of 3°C so muchhand waving.

Model convergence, finally?

By the time of the IPCC’s second assess-ment report in 1995, the number of climatemodels available had increased to 13. After15 years of model development, their sensi-tivities still spread pretty much across Char-ney’s 1.5ºC-to-4.5ºC range. By IPCC’s thirdand most recent assessment report in 2001,the model-defined range still hadn’t budged.

Now model sensitivities may be begin-ning to converge. “The range of these mod-els, at least, appears to be narrowed,” saidclimate modeler Gerald Meehl of the Na-tional Center for Atmospheric Research(NCAR) in Boulder, Colorado, after pollingeight of the 14 models expected to be in-cluded in the IPCC’ s next assessment. Thesensitivities of the 14 models in the previousassessment ranged from 2.0ºC to 5.1ºC, butthe span of the eight currently availablemodels is only 2.6ºC to 4.0ºC, Meehl found.

If this limited sampling really has detect-ed a narrowing range, modelers believethere’s a good reason for it: More-powerfulcomputers and a better understanding of at-mospheric processes are making their mod-els more realistic. For example, researchersat the Geophysical Fluid Dynamics Labora-tory (GFDL) in Princeton, New Jersey, re-cently adopted a better way of calculatingthe thickness of the bottommost atmospher-ic layer—the boundary layer—where cloudsform that are crucial to the planet’s heat bal-

13 AUGUST 2004 VOL 305 SCIENCE www.sciencemag.org932

Climate researchers are finally homing in on just how bad greenhouse warming could get—and it seems increas-ingly unlikely that we will escape with a mild warming

Three Degrees of Consensus

News Focus

ILLU

ST

RA

TIO

N:T

IM S

MIT

H

* Workshop on Climate Sensitivity of the Inter-governmental Panel on Climate Change WorkingGroup I, 26–29 July 2004, Paris.

ance. When they made the change, the mod-el’s sensitivity dropped from a hefty 4.5ºC toa more mainstream 2.8ºC, said RonaldStouffer, who works at GFDL. Now thethree leading U.S. climate models—NCAR’s, GFDL’s, and GISS’s—have con-verged on a sensitivity of 2.5ºC to 3.0ºC.They once differed by a factor of 2.

Less-uncertain modeling

If computer models are increasingly brew-ing up similar numbers, however, theysometimes disagree sharply about the physi-cal processes that produce them. “Are wegetting [similar sensitivities] for the samereason? The answer is clearly no,” JeffreyKiehl of NCAR said of the NCAR andGFDL models. The problems come fromprocesses called feedbacks, which can am-plify or dampen the warming effect ofgreenhouse gases.

The biggest uncertainties have to do withclouds. The NCAR and GFDL modelsmight agree about clouds’ net effect on theplanet’s energy budget as CO2 doubles,Kiehl noted. But they get their similar num-bers by assuming different mixes of cloudproperties. As CO2 levels increase, clouds inboth models reflect more shorter-wave-length radiation, but the GFDL model’s in-crease is three times that of the NCAR mod-el. The NCAR model increases the amountof low-level clouds, whereas the GFDLmodel decreases it. And much of the UnitedStates gets wetter in the NCAR model whenit gets drier in the GFDL model.

In some cases, such widely varying as-sumptions about what is going on may havehuge effects on models’ estimates of sensitiv-ity; in others, none at all. To find out, re-searchers are borrowing a technique weatherforecasters use to quantify uncertainties intheir models. At the workshop and in thisweek’s issue of Nature, James Murphy of theHadley Center for Climate Prediction andResearch in Exeter, U.K., and colleagues de-scribed how they altered a total of 29 keymodel parameters one at a time—variablesthat control key physical properties of themodel, such as the behavior of clouds, theboundary layer, atmospheric convection, andwinds. Murphy and his team let each parame-

ter in the Hadley Center model vary over arange of values deemed reasonable by a teamof experts. Then the modelers ran simulationsof present-day and doubled-CO2 climates us-ing each altered version of the model.

Using this “perturbed physics” approachto generate a curve of the probability of awhole range of climate sensitivities (seefigure), the Hadley group found a sensitivi-

ty a bit higher thanthey would have gottenby simply polling the

eight independently built models. Their es-timates ranged from 2.4ºC to 5.4ºC (with5% to 95% confidence intervals), with amost probable climate sensitivity of 3.2ºC.In a nearly completed extension of themethod, many model parameters are beingvaried at once, Murphy reported at theworkshop. That is dropping the range andthe most probable value slightly, makingthem similar to the eight-model value aswell as Charney’s best guess.

Murphy isn’t claiming they have apanacea. “We don’t want to give a sense of ex-cessive precision,” he says. The perturbedphysics approach doesn’t account for manyuncertainties. For example, decisionssuch as the amount of geographic detailto build into the model introduce aplethora of uncertainties, as does themodel’s ocean. Like all model oceansused to estimate climate sensitivity, it hasbeen simplified to the point of having nocurrents in order to make the extensivesimulations computationally tractable.

Looking back

Faced with so many caveats, workshopattendees turned their attention to whatmay be the ultimate reality check for cli-mate models: the past of Earth itself. Al-though no previous change in Earth’s

climate is a perfect analog for the cominggreenhouse warming, researchers say model-ing paleoclimate can offer valuable clues tosensitivity. After all, all the relevant processeswere at work in the past, right down to theformation of the smallest cloud droplet.

One telling example from the recent pastwas the cataclysmic eruption of MountPinatubo in the Philippines in 1991. The de-

bris it blew into the strato-sphere, which stayed therefor more than 2 years, wasclosely monitored from or-bit and the ground, as wasthe global cooling that re-sulted from the debrisblocking the sun. Conve-niently, models show thatEarth’s climate systemgenerally does not distin-guish between a shift in itsenergy budget brought onby changing amounts ofgreenhouse gases and onecaused by a change in theamount of solar energy al-lowed to enter. From themagnitude and duration ofthe Pinatubo cooling, cli-mate researcher ThomasWigley of NCAR and his

colleagues have recently estimated Earth’ssensitivity to a CO2 doubling as 3.0ºC. Asimilar calculation for the eruption of Agungin 1963 yielded a sensitivity of 2.8ºC. Andestimates from the five largest eruptions ofthe 20th century would rule out a climatesensitivity of less than 1.5ºC.

Estimates from such a brief shock to theclimate system would not include moresluggish climate system feedbacks, such asthe expansion of ice cover that reflects radia-tion, thereby cooling the climate. But theglobally dominant feedbacks from water va-por and clouds would have had time towork. Water vapor is a powerful greenhousegas that’s more abundant at higher tempera-tures, whereas clouds can cool or warm byintercepting radiant energy.

www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004 933

Probably warm. Running a climate model over the full

range of parameter uncertainty suggests that climate sen-

sitivity is most likely a moderately high 3.2°C (red peak).

Volcanic chill. Debris from Pinatubo (above)

blocked the sun and chilled the world (left),

thanks in part to the amplifying effect of wa-

ter vapor.

1991 1992 1993 1994 1995 1996

0.2

0

–0.2

–0.4

–0.6

–0.8Tem

pera

ture

Cha

nge

(K)

ObservedObserved (El Niño removed)ModelModel (no water vapor feedback)E

rupt

ion

N E W S F O C U SC

RED

ITS:(

TO

P T

O B

OT

TO

M)

TIM

E L

IFE P

HO

TO

S/G

ET

TY

IM

AG

ES;A

DA

PT

ED

FR

OM

B.S

OD

EN

ET

AL.

,SC

IEN

CE

29

6,7

27

(20

02

);J.

MU

RPH

Y E

T A

L.,N

AT

UR

E4

30

(2

00

4)

More climate feedbacks come into playover centuries rather than years of climatechange. So climate researchers GabrieleHegerl and Thomas Crowley of Duke Uni-versity in Durham, North Carolina, consid-ered the climate effects from 1270 to 1850produced by three climate drivers: changesin solar brightness, calculated from sunspotnumbers; changing amounts of greenhousegases, recorded in ice cores; and volcanicshading, also recorded in ice cores. They putthese varying climate drivers in a simplemodel whose climate sensitivity could bevaried over a wide range. They then com-pared the simulated temperatures over theperiod with temperatures recorded in treerings and other proxy climate recordsaround the Northern Hemisphere.

The closest matches to observed tempera-tures came with sensitivities of 1.5ºC to3.0ºC, although a range of 1.0ºC to 5.5ºC waspossible. Other estimates of climate sensitiv-ity on a time scale of centuries to millenniahave generally fallen in the 2ºC-to-4ºC range,Hegerl noted, although all would benefit frombetter estimates of past climate drivers.

The biggest change in atmospheric CO2 inrecent times came in the depths of the last iceage, 20,000 years ago, which should providethe best chance to pick the greenhouse signalout of climatic noise. So Thomas Schneidervon Deimling and colleagues at the PotsdamInstitute for Climate Impact Research (PIK) inGermany have estimated climate sensitivityby modeling the temperature at the time usingthe perturbed-physics approach. As StefanRahmstorf of PIK explained at the workshop,they ran their intermediate complexity modelusing changing CO2 levels, as recorded in icecores. Then they compared model-simulatedtemperatures with temperatures recorded inmarine sediments. Their best estimate of sen-sitivity is 2.1ºC to 3.6ºC, with a range of 1.5ºCto 4.7ºC.

More confidence

In organizing the Paris workshop, the IPCCwas not yet asking for a formal conclusionon climate sensitivity. But participants clear-ly believed that they could strengthen thetraditional Charney range, at least at the lowend and for the best estimate. At the highend of climate sensitivity, however, mostparticipants threw up their hands. The calcu-lation of sensitivity probabilities goes highlynonlinear at the high end, producing a smallbut statistically real chance of an extremewarming. This led to calls for more tests ofmodels against real climate. They would in-clude not just present-day climate but a vari-ety of challenges, such as the details of ElNiño events and Pinatubo’s cooling.

Otherwise, the sense of the 75 or so scien-tists in attendance seemed to be that Char-ney’s range is holding up amazingly well,

possibly by luck. The lower bound of 1.5ºC isnow a much firmer one; it is very unlikelythat climate sensitivity is lower than that,most would say. Over the past decade, somecontrarians have used satellite observations toargue that the warming has been minimal,suggesting a relatively insensitive climatesystem. Contrarians have also proposed as-yet-unidentified feedbacks, usually involvingwater vapor, that could counteract most of thegreenhouse warming to produce a sensitivityof 0.5ºC or less. But the preferred lowerbound would rule out such claims.

Most meeting-goers polled by Science

generally agreed on a most probable sensi-tivity of around 3ºC, give or take a half-degree or so. With three complementary ap-proaches—a collection of expert-designedindependent models, a thoroughly variedsingle model, and paleoclimates over arange of time scales—all pointing to sensi-tivities in the same vicinity, the middle ofthe canonical range is looking like a goodbet. Support for such a strong sensitivity upsthe odds that the warming at the end of thiscentury will be dangerous for flora, fauna,and humankind. Charney, it seems, couldhave said he told us so. –RICHARD A. KERR

N E W S F O C U S

13 AUGUST 2004 VOL 305 SCIENCE www.sciencemag.org934

Take one set of the Encyclopedia Britan-nica. Dump it into an average-sized blackhole. Watch and wait. What happens? Andwho cares?

Physicists care, you might have thought,reading last month’s breathless headlinesfrom a conference in Dublin, Ireland. There,Stephen Hawking announced that, after pro-claiming for 30 years that black holes de-stroy information, he had decided theydon’t (Science, 30 July, p. 586). All ofwhich, you might well have concluded,seems a lot like debating how many angels

can dance on the head of a pin.Yet arguments about what a black hole

does with information hold physicists trans-fixed. “The question is incredibly interest-ing,” says Andrew Strominger, a string theo-rist at Harvard University. “It’s one of thethree or four most important puzzles inphysics.” That’s because it gives rise to aparadox that goes to the heart of the conflictbetween two pillars of physics: quantum the-ory and general relativity. Resolve the para-dox, and you might be on your way to re-solving the clash between those two theories.

A General Surrenders the Field,But Black Hole Battle Rages OnStephen Hawking may have changed his mind, but questions about the fate of informationcontinue to expose fault lines between relativity and quantum theories

Quantum Informat ion Theory

Eternal darkness? Spherical “event horizon” marks the region where a black hole’s gravity grows

so intense that even light can’t escape. But is the point of no return a one-way street? CR

ED

IT:G

SFC

/NA

SA

Yet, as Hawking and others convincethemselves that they have solved the para-dox, others are less sure—and everybody isdesperate to get real information about whatgoes on at the heart of a black hole.

The hairless hole

A black hole is a collapsed star—and a grav-itational monster. Like all massive bodies, itattracts and traps other objects through itsgravitational force. Earth’s gravity traps us,too, but you can break free if you strap on arocket that gets you moving beyond Earth’sescape velocity of about 11kilometers per second.

Black holes, on the otherhand, are so massive and com-pressed into so small a spacethat if you stray too close, yourescape velocity is faster thanthe speed of light. According tothe theory of relativity, no ob-ject can move that fast, so noth-ing, not even light, can escapethe black hole’s trap once itstrays too close. It’s as if theblack hole is surrounded by aninvisible sphere known as anevent horizon. This spheremarks the region of no return:Cross it, and you can nevercross back.

The event horizon shieldsthe star from prying eyes. Because nothingcan escape from beyond the horizon, an out-side observer will never be able to gatherany photons or other particles that would re-veal what’s going on inside. All you can everknow about a black hole are the characteris-tics that you can spot from a distance: itsmass, its charge, and how fast it’s spinning.Beyond that, black holes lack distinguishingfeatures. As Princeton physicist John Wheel-er put it in the 1960s, “A black hole has nohair.” The same principle applies to any mat-ter or energy a black hole swallows. Dumpin a ton of gas or a ton of books or a ton ofkittens, and the end product will be exactlythe same.

Not only is the information about the in-falling matter gone, but information uponthe infalling matter is as well. If you take anatom and put a message on it somehow (say,make it spin up for a “yes” or spin down fora “no”), that message is lost forever if theatom crosses a black hole’s event horizon.It’s as if the message were completely de-stroyed. So sayeth the theory of general rela-tivity. And therein lies a problem.

The laws of quantum theory say some-thing entirely different. The mathematics ofthe theory forbids information from dis-appearing. Particle physicists, string theo-rists, and quantum scientists agree that in-formation can be transferred from place to

place, that it can dissipate into the environ-ment or be misplaced, but it can never beobliterated. Just as someone with enoughenergy and patience (and glue) could, in the-ory, repair a shattered coffee cup, a diligentobserver could always reconstitute a chunkof information no matter how it’s abused—even if you dump it down a black hole.

“If the standard laws of quantum me-chanics are correct, for an observer outsidethe black hole, every little bit of informationhas to come back out,” says Stanford Uni-versity’s Leonard Susskind. Quantum me-

chanics and general relativity are telling sci-entists two contradictory things. It’s a para-dox. And there’s no obvious way out.

Can the black hole be storing the infor-mation forever rather than actually destroy-ing it? No. In the mid-1970s, Hawking real-ized that black holes don’t live forever; theyevaporate thanks to something now knownas Hawking radiation.

One of the stranger consequences ofquantum theory is that the universe isseething with activity, even in the deepestvacuum. Pairs of particles are constantlywinking in and out of existence (Science, 10January 1997, p. 158). But the vacuum near ablack hole isn’t ordinary spacetime. “Vacuaaren’t all created equal,” says Chris Adami, aphysicist at the Keck Graduate Institute inClaremont, California. Near the edge of theevent horizon, particles are flirting with theirdemise. Some pairs fall in; some pairs don’t.And they collide and disappear as abruptly asthey appeared. But occasionally, the pair isdivided by the event horizon. One falls in andis lost; the other flies away partnerless. With-out its twin, the particle doesn’t wink out ofexistence—it becomes a real particle and fliesaway (see diagram). An outside observerwould see these partnerless particles as asteady radiation emitted by the black hole.

Like the particles of any other radiation,the particles of Hawking radiation aren’t cre-

ated for free. When the black hole radiates, abit of its mass converts to energy. Accordingto Hawking’s equations, this slight shrinkageraises the “temperature” of the black hole bya tiny fraction of a degree; it radiates morestrongly than before. This makes it shrinkfaster, which makes it radiate more strongly,which makes it shrink faster. It gets smallerand brighter and smaller and brighter and—flash!—it disappears in a burst of radiation.This process takes zillions of years, manytimes longer than the present lifetime of theuniverse, but eventually the black hole disap-

pears. Thus it can’t store infor-mation forever.

If the black hole isn’t storinginformation eternally, can it beletting swallowed informationescape somehow? No, at leastnot according to general relativi-ty. Nothing can escape from be-yond the event horizon, so thatidea is a nonstarter. And physi-cists have shown that Hawkingradiation can’t carry informa-tion away either. What passesthe event horizon is gone, and itwon’t come out as the blackhole evaporates.

This seeming contradictionbetween relativity and quantummechanics is one of the burningunanswered questions in

physics. Solving the paradox, physicistshope, will give them a much deeper under-standing of the rules that govern nature—and that hold under all conditions. “We’retrying to develop a new set of physicallaws,” says Kip Thorne of the California In-stitute of Technology in Pasadena.

Paradox lost

Clearly, somebody’s old laws will have toyield—but whose? Relativity experts, in-cluding Stephen Hawking and Kip Thorne,long believed that quantum theory wasflawed and would have to discard the no-information-destruction dictum. Quantumtheorists such as Caltech’s John Preskill, onthe other hand, held that the relativisticview of the universe must be overlookingsomething that somehow salvages informa-tion from the jaws of destruction. Thathope was more than wishful thinking; in-deed, the quantum camp argued its caseconvincingly enough to sway most of thescientific community.

The clincher, many quantum and stringtheorists believed, lay in a mathematical cor-respondence rooted in a curious property ofblack holes. In the 1970s, Jacob Bekensteinof Hebrew University in Jerusalem andStephen Hawking came to realize that whena black hole swallows a volume of matter,that volume can be entirely described by the

N E W S F O C U S

www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004 935

ILLU

ST

RA

TIO

N:J

.MO

GLI

A/S

CIE

NC

E

EVENT HORIZON +

+

–

1

2

3–

–

+

+

–BLACK HOLE

CCoossmmiicc rreeffuuggeeeess.. Virtual particles that escape destruction near a black hole

(case 3) create detectable radiation but can’t carry information.

increase of surface area of the event horizon.In other words, if the dimension of time isignored, the essence of a three-dimensionalobject that falls into the black hole can beentirely described by its “shadow” on a two-dimensional object.

In the early 1990s, Susskind and the Uni-versity of Utrecht’s Gerard ’t Hooft general-ized this idea to what is now known as the“holographic principle.” Just as informationabout a three-dimensional object can be en-tirely encoded in a two-dimensional holo-gram, the holographic principle states thatobjects that move about and interact in ourthree-dimensional world can be entirelydescribed by the mathematics that resideson a two-dimensional surface that sur-rounds those objects. In a sense, our three-dimensionality is an illusion, and we are tru-ly two-dimensional creatures—at leastmathematically speaking.

Most physicists accept the holographicprinciple, although it hasn’t been proven. “Ihaven’t conducted any polls, but I think thata very large majoritybelieves in it,” saysBekenstein. Physicistsalso accept a relatedidea proposed in themid-1990s by stringtheorist Juan Malda-cena, currently at the In-stitute for AdvancedStudy in Princeton, NewJersey. Maldacena’s so-called AdS/CFT corre-spondence shows thatthe mathematics ofgravitational fields in avolume of space is es-sentially the same asthe nice clean gravity-free mathematics of theboundary of that space.

Although these ideasseem very abstract, theyare quite powerful. With the AdS/CFT corre-spondence in particular, the mathematics thatholds sway upon the boundary automaticallyconserves information; like that of quantumtheory, the boundary’s mathematical frame-work simply doesn’t allow information to belost. The mathematical equivalence betweenthe boundary and the volume of space meansthat even in a volume of space where gravityruns wild, information must be conserved. It’sas if you can ignore the troubling effects ofgravity altogether if you consider only themathematics on the boundary, even whenthere’s a black hole inside that volume. There-fore, black holes can’t destroy information;paradox solved—sort of.

“String theorists felt they completelynailed it,” says Susskind. “Relativity peopleknew something had happened; they knew

that perhaps they were fighting a losing bat-tle, but they didn’t understand it on theirown terms.” Or, at the very least, many gen-eral relativity experts didn’t think that thematter was settled—that information wouldstill have to be lost, AdS/CFT correspon-dence or no. Stephen Hawking was the mostprominent of the naysayers.

Paradox regained

Last month in Dublin, Hawking reversedhis 30-year-old stance. Convinced by hisown mathematical analysisthat was unrelated to theAdS/CFT correspondence, heconceded that black holes donot, in fact, destroy informa-tion—nor can a black holetransport information into an-other universe as Hawkingonce suggested. “The infor-mation remains firmly in ouruniverse,” he said. As a re-sult, he conceded a bet with

Preskill and handed over a baseball ency-clopedia (Science, 30 July, p. 586).

Despite the hoopla over the event, Hawk-ing’s concession changed few minds. Quan-tum and string theorists already believedthat information was indestructible, thanksto the AdS/CFT correspondence. “Every-body I know in the string theory communitywas completely convinced,” says Susskind.“What’s in [Hawking’s] own work is his wayof coming to terms with it, but it’s not likelyto paint a whole new picture.” Relativity ex-perts in the audience, meanwhile, wereskeptical about Hawking’s mathematicalmethod and considered the solution too un-realistic to be applied to actual, observableblack holes. “It doesn’t seem to me to beconvincing for the evolution of a black holewhere you actually see the black hole,” says

John Friedman of the University of Wiscon-sin, Milwaukee.

With battle lines much as they were,physicists hope some inspired theorist willbreak the stalemate. Susskind thinks the an-swer lies in a curious “complementarity” ofblack holes, analogous to the wave-particleduality of quantum mechanics. Just as a pho-ton can behave like either a wave or a particlebut not both, Susskind argues, you can lookat information from the point of view of anobserver behind the event horizon or in front

of the event horizon but not bothat the same time. “Paradoxes wereapparent because people tried tomix the two different experi-ments,” Susskind says.

Other scientists look else-where for the resolution of theparadox. Adami, for instance,sees an answer in the seethingvacuum outside a black hole.When a particle falls past theevent horizon, he says, it sparksthe vacuum to emit a duplicateparticle in a process similar to thestimulated emission that makesexcited atoms emit laser light. “Ifa black hole swallows up a parti-

cle, it spits one out that encodes preciselythe same information,” says Adami. “The in-formation is never lost.” When he analyzedthe process, Adami says, a key equation inquantum information theory—one that lim-its how much classical information quantumobjects can carry—made a surprise appear-ance. “It simply pops out. I didn’t expect itto be there,” says Adami. “At that moment, Iknew it was all over.”

Although it might be all over for Hawk-ing, Susskind, and Adami, it’s over for dif-ferent reasons—none of which has com-pletely convinced the physics community.For the moment, at least, the black hole is asdark and mysterious as ever, despite legionsof physicists trying to wring informationfrom it. Perhaps the answer lies just beyondthe horizon. –CHARLES SEIFE

N E W S F O C U S

13 AUGUST 2004 VOL 305 SCIENCE www.sciencemag.org936

CR

ED

ITS:(

LEFT

TO

RIG

HT

) C

ALT

EC

H;C

HA

RLE

S S

EIF

E

Gambling on nature. The 1997 wager among physicists Preskill, Thorne, and Hawk-

ing (above) became famous, but Hawking’s concession (right) left battle lines drawn.

www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004 937

STOLLSTEIMER CREEK, COLORADO—“Don’t be apin-headed snarf. … Read the river!” DaveRosgen booms as he sloshes through shin-deep water, a swaying surveying rodclutched in one hand and a toothpick in theother. Trailing in his wake are two dozen raptstudents—including natural resource man-agers from all over the world—who havegathered on the banks of this small RockyMountain stream to learn, in Rosgen’swords, “how to think like a river.” The lessonon this searing morning: how to measure andmap an abused waterway, the first step to-ward rescuing it from the snarfs—just one ofthe earthy epithets that Rosgen uses to de-scribe anyone, from narrow-minded engi-neers to loggers, who has harmed rivers.“Remember,” he says, tugging on the widebrim of his cowboy hat, “your job is to helpthe river be what it wants to be.”

It’s just another day at work for Rosgen, a62-year-old former forest ranger who is ar-guably the world’s most influential force inthe burgeoning field of river restoration.Over the past few decades, the folksy jack-of-all-trades—equally at home talkinghydrology, training horses, or driving a bull-dozer—has pioneered an approach to “natu-ral channel design” that is widely used bygovernment agencies and nonprofit groups.He has personally reconstructed nearly 160kilometers of small- and medium-sizedrivers, using bulldozers, uprooted trees, andmassive boulders to sculpt new channels thatmimic nature’s. And the 12,000-plus studentshe’s trained have reengineered many morewaterways. Rosgen is also the author of abest-selling textbook and one of the field’smost widely cited technical papers—and hejust recently earned a doctorate, some 40years after graduating from college.

“Dave’s indefatigable, and he’s had a re-markable influence on the practice of riverrestoration,” says Peggy Johnson, a civil en-gineer at Pennsylvania State University,University Park. “It’s almost impossible totalk about the subject without his namecoming up,” adds David Montgomery, ageomorphologist at the University of Wash-ington, Seattle.

But although many applaud Rosgen’swork, he’s also attracted a flood of criti-cism. Many academic researchers questionthe science underpinning his approach,saying it has led to oversimplified “cook-

book” restoration projects that do as muchharm as good. Rosgen-inspired projectshave suffered spectacular and expensivefailures, leaving behind eroded channelschoked with silt and debris. “There aretremendous doubts about what’s beingdone in Rosgen’s name,” says PeterWilcock, a geomorphologist who special-izes in river dynamics at Johns HopkinsUniversity in Baltimore, Maryland. “But

the people who hold the purse strings oftenrequire the use of his methods.”

All sides agree that the debate is far fromacademic. At stake: billions of dollars that areexpected to flow to tens of thousands of U.S.river restoration projects over the next fewdecades. Already, public and private groupshave spent more than $10 billion on morethan 30,000 U.S. projects, says MargaretPalmer, an ecologist at the University ofMaryland, College Park, who is involved in a

new effort to evaluate restoration efforts. “Be-fore we go further, it would be nice to knowwhat really works,” she says, noting that suchwork can cost $100,000 a kilometer or more.

Going with the flow

Rosgen is a lifelong river rat. Raised on anIdaho ranch, he says a love of forests and fish-ing led him to study “all of the ‘-ologies’ ” asan undergraduate in the early 1960s. He thenmoved on to a job with the U.S. Forest Serviceas a watershed forester—working in the sameIdaho mountains where he fished as a child.But things had changed. “The valleys I knewas a kid had been trashed by logging,” he re-called recently. “My trout streams were filledwith sand.” Angry, Rosgen confronted hisbosses: “But nothing I said changed anyone’s

mind; I didn’t have the data.”Rosgen set out to change

that, doggedly measuring wa-ter flows, soil types, and sedi-ments in a bid to predict howlogging and road buildingwould affect streams. As hewaded the icy waters, he be-gan to have the first inklingsof his current approach: “Irealized that the response [todisturbance] varied by streamtype: Some forms seemed re-silient, others didn’t.”

In the late 1960s, Rosgen’scuriosity led him to contactone of the giants of river sci-ence, Luna Leopold, a geo-morphologist at the Univer-sity of California, Berkeley,and a former head of the U.S.Geological Survey. Invited tovisit Leopold, the young cow-boy made the trek to what hestill calls “Berzerkley,” thenin its hippie heyday. “Talkabout culture shock,” Rosgensays. The two men ended upporing over stream data intothe wee hours.

By the early 1970s, thecollaboration had put Rosgenon the path to what has be-come his signature accom-

plishment: Drawing on more than a centuryof research by Leopold and many others, hedeveloped a system for lumping all rivers in-to a few categories based on eight funda-mental characteristics, including the channelwidth, depth, slope, and sediment load (seegraphic, p. 938). Land managers, he hoped,could use his system (there are many others)to easily classify a river and then predicthow it might respond to changes, such as in-creased sediment. But “what started out as a

The River DoctorDave Rosgen rides in rodeos, drives bulldozers, and has pioneered a widely usedapproach to restoring damaged rivers. But he’s gotten a flood of criticism too

Prof i le Dave Rosgen

Class act. Dave Rosgen’s system for classifying rivers is

widely used in stream restoration—and detractors say com-

monly misused.

CR

ED

IT:D

.MA

LAK

OFF

/SC

IEN

CE

description for management turned out to beso much more,” says Rosgen.

In particular, he wondered how a “fieldguide to rivers” might help the nascentrestoration movement. Frustrated by tradi-tional engineering approaches to flood anderosion control—which typically called forconverting biologically rich meandering

rivers to barren concrete channels or dump-ing tons of ugly rock “rip rap” on failingbanks—river advocates were searching foralternatives. Rosgen’s idea: Use the classifi-cation scheme to help identify naturally oc-curring, and often more aesthetically pleas-ing, channel shapes that could produce sta-ble rivers—that is, a waterway that couldcarry floods and sediment without signifi-cantly shifting its channel. Then, build it.

In 1985, after leaving the Forest Service ina dispute over a dam he opposed, Rosgen re-treated to his Colorado ranch to train horses,refine his ideas—and put them into action. Hefounded a company—Wildland Hydrology—and began offering training. (Courses cost upto $2700 per person.) And he embarked ontwo restoration projects, on overgrazed andchannelized reaches of the San Juan and Blan-co rivers in southern Col-orado, that became templatesfor what was to come.

After classifying the tar-get reaches, Rosgen de-signed new “natural” chan-nel geometries based on rel-atively undisturbed rivers,adding curves and boulder-strewn riffles to reduce ero-sion and improve fish habi-tat. He then carved the newbeds, sometimes driving theearthmovers himself. Al-though many people wereappalled by the idea of bull-dozing a river to rescue it,the projects—funded bypublic and private groups—ultimately won wide accept-ance, including a de factoendorsement in a 1992 Na-

tional Research Council report on restora-tion.

Two years later, with Leopold’s help, Ros-gen won greater visibility by publishing hisclassification scheme in Catena, a presti-gious peer-reviewed journal. Drawing on da-ta he and others had collected from 450rivers in the United States, Canada, and New

Zealand, Rosgen divided streams in-to seven major types and dozens ofsubtypes, each denoted by a letterand a number. (Rosgen’s current ver-sion has a total of 41 types.) Type“A” streams, for instance, are steep,narrow, rocky cascades; “E” chan-nels are gentler, wider, more mean-dering waterways.

Although the 30-page manifestocontains numerous caveats, Ros-gen’s system held a powerful prom-ise for restorationists. Using rela-tively straightforward field tech-niques—and avoiding what Rosgencalls “high puke-factor equations”—

users could classify a river. Then, using anincreasingly detailed four-step analysis, theycould decide whether its channel was cur-rently “stable” and forecast how it might al-ter its shape in response to changes, such asincreased sediment from overgrazed banks.For instance, they could predict that a nar-row, deep, meandering E stream with erod-ing banks would slowly degrade into a wide,shallow F river, then—if given enoughtime—restore itself back to an E. But moreimportant, Rosgen’s system held out hope ofpredictably speeding up the restorationprocess by reducing the sediment load andcarving a new E channel, for instance.

The Catena paper—which became thebasis for Rosgen’s 1996 textbook, AppliedRiver Morphology—distilled “decades offield observations into a practical tool,” says

Rosgen. At last, he had data. And peoplewere listening—and flocking to his talks andclasses. “It was an absolute revelation listen-ing to Dave back then,” recalls James Gracieof Brightwater Inc., a Maryland-basedrestoration firm, who met Rosgen in 1985.“He revolutionized river restoration.”

Rough waters

Not everyone has joined the revolution,however. Indeed, as Rosgen’s reputation hasgrown, so have doubts about his classifica-tion system—and complaints about how it isbeing used in practice.

Much of the criticism comes from aca-demic researchers. Rosgen’s classificationscheme provides a useful shorthand for de-scribing river segments, many concede. Butcivil engineers fault Rosgen for relying onnonquantitative “geomagic,” says RichardHey, a river engineer and Rosgen businessassociate at the University of East Anglia inthe United Kingdom. And geomorphologistsand hydrologists argue that his scheme over-simplif ies complex, watershed-wideprocesses that govern river behavior overlong time scales.

Last year, in one of the most recent cri-tiques, Kyle Juracek and Faith Fitzpatrick ofthe U.S. Geological Survey concluded thatRosgen’s Level II analysis—a commonlyused second step in his process—failed tocorrectly assess stream stability or channelresponse in a Wisconsin river that hadundergone extensive study. A competing an-alytical method did better, they reported inthe June 2003 issue of the Journal of theAmerican Water Resources Association. Theresult suggested that restorationists usingRosgen’s form-based approach would havegotten off on the wrong foot. “It’s a re-minder that classification has lots of limita-tions,” says Juracek, a hydrologist in

Lawrence, Kansas.Rosgen, however,

says the paper “is a pret-ty poor piece of work …that doesn’t correctlyclassify the streams. … Itseems like they didn’teven read my book.” Healso emphasizes that hisLevel III and IV analysesare designed to answerjust the kinds of ques-tions the researcherswere asking. Still, heconcedes that classifica-tion may be problematicon some kinds of rivers,particularly urban water-ways where massive dis-turbance has made itnearly impossible tomake key measurements.

13 AUGUST 2004 VOL 305 SCIENCE www.sciencemag.org938

CR

ED

ITS:(

TO

P T

O B

OT

TO

M)

D.M

ALA

KO

FF/S

CIE

NC

E;IL

LUST

RA

TIO

N:L

EE S

ILV

EY

,FR

OM

D.R

OSG

EN

,APPLI

ED F

LUV

IAL

GEO

MO

RPH

OLO

GY

A field guide to rivers. Drawing on data from more than 1000 waterways, Rosgen

grouped streams into nine major types.

N E W S F O C U S

One particularly problematic variable, allsides agree, is “bankfull discharge,” thepoint at which floodwaters begin to spill on-to the floodplain. Such flows are believed toplay a major role in determining channelform in many rivers.

Overall, Rosgen says he welcomes thecritiques, although hegripes that “my mostvocal critics are theones who know theleast about what I’mdoing.” And he recent-ly f ired back in a9000-word essay hewrote for his doctor-ate, which he earnedunder Hey.

Rosgen’s defend-ers, meanwhile, saythe attacks are mostlysour grapes. “The aca-demics were workingin this obscure littlef ield, f ighting overthree grants a year, andalong came this cow-boy who started get-ting millions of dollars for projects; therewas a lot of resentment,” says Gracie.

River revival?

The critics, however, say the real problem isthat many of the people who use Rosgen’smethods—and pay for them—aren’t awareof its limits. “It’s deceptively accessible;people come away from a week of trainingthinking they know more about rivers thanthey really do,” says Matthew Kondolf, ageomorphologist at the University of Cali-fornia, Berkeley. Compounding the problemis that Rosgen can be a little too inspira-tional, adds Scott Gillilin, a restoration con-sultant in Bozeman, Montana. “Studentscome out of Dave’s classes like they’ve beento a tent revival, their hands on the goodbook, proclaiming ‘I believe!’ ”

The result, critics say, is a growing list offailed projects designed by “Rosgenauts.” Inseveral cases in California, for instance, theyattempted to carve new meander bends re-inforced with boulders or root wads intohigh-energy rivers—only to see them buriedand abandoned by the next flood. In a muchcited example, restorationists in 1995 bull-dozed a healthy streamside forest alongDeep Run in Maryland in order to installseveral curves—then watched the several-hundred-thousand-dollar project blow out,twice, in successive years. “It’s the restora-tion that wrecked a river reach. … The curewas worse than the disease,” says geo-morphologist Sean Smith, a Johns Hopkinsdoctoral student who monitored the project.

Gracie, the Maryland consultant who

designed the Deep Run restoration, blamesthe disaster on inexperience and miscalcu-lating an important variable. “We under-sized the channel,” he says. But he says helearned from that mistake and hasn’t had asimilar failure in dozens of projects since.“This is an emerging profession; there is

going to be trial and er-ror,” he says. Rosgen,meanwhile, concedesthat overenthusiasticdisciples have misusedhis ideas and notes thathe’s added courses tobolster training. But hesays he’s had only one“major” failure himself—on Wolf Creek inCalifornia—out of nearly 50 projects. “Butthere [are] some things I sure as hell won’tdo again,” he adds.

What works?

Despite these black marks, critics note, agrowing number of state and federal agen-cies are requiring Rosgen training for any-one they fund. “It’s becoming a self-perpetuating machine; Dave is creating hisown legion of pin-headed snarfs who arelocked into a single approach,” saysGillilin, who believes the requirement isstifling innovation. “An expanding marketis being filled by folks with very limitedexperience in hydrology or geomorpholo-gy,” adds J. Steven Kite, a geomorphologistat West Virginia University in Morgantown.

Kite has seen the trend firsthand: One ofhis graduate students was recently rejectedfor a restoration-related job because helacked Rosgen training. “It seemed a bit oddthat years of academic training wasn’t con-sidered on par with a few weeks of work-shops,” he says. The experience helpedprompt Kite and other geomorphologists todraft a recent statement urging agencies to

increase their training requirements and uni-versities to get more involved (seewww.geo.wvu.edu/~kite). “The bulldozersare in the water,” says Kite. “We can’t justsit back and criticize.”

Improving training, however, is only oneneed, says the University of Maryland’s

Palmer. Another is improving theevaluation of new and existing proj-ects. “Monitoring is woefully inade-quate,” she says. In a bid to improvethe situation, a group led by Palmerand Emily Bernhardt of Duke Univer-sity in Durham, North Carolina, haswon funding from the National Sci-ence Foundation and others to under-take the first comprehensive nationalinventory and evaluation of restora-tion projects. Dubbed the National

River Restoration Science Synthesis, it hasalready collected data on more than 35,000projects. The next step: in-depth analysis of ahandful of projects in order to make prelimi-nary recommendations about what’s working,what’s not, and how success should be meas-ured. A smaller study evaluating certain typesof rock installations—including severalchampioned by Rosgen—is also under wayin North Carolina. “We’re already finding apretty horrendous failure rate,” says JerryMiller of Western Carolina University in Cul-lowhee, a co-author of one of the earliest cri-tiques of Rosgen’s Catena paper.

A National Research Council panel,meanwhile, is preparing to revisit the 1992study that helped boost Rosgen’s method.Many geomorphologists criticized that studyfor lacking any representatives from theirfield. But this time, they’ve been in on studytalks from day one.

Whatever these studies conclude, bothRosgen’s critics and supporters say his placein history is secure. “Dave’s legacy is that heput river restoration squarely on the table in avery tangible and doable way,” says Smith.“We wouldn’t be having this discussion if hehadn’t.” –DAVID MALAKOFF

www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004 939

CR

ED

ITS:M

AT

TH

EW

KO

ND

OLF

Errors on trial. Rosgen’s ideas have inspired ex-

pensive failures, critics say, such as engineered

meanders on California’s Uvas Creek (above) that

were soon destroyed by floods.

N E W S F O C U S

Virgin Rainforests and

Conservation

IN REVIEWING THE HISTORY OF RAINFOREST

clearance, K. J. Willis et al. (“How ‘virgin’is virgin rainforest?”, Perspectives, 16Apr., p. 402) conclude that rain-forests are “quite resilient,” andthat given time they “will almostcertainly regenerate” from modernslash-and-burn clearance. Out ofcontext, such statements maymislead policy-makers andweaken protection.

Although regrown rainforestmay appear floristically diverse orrestored (1), it may hold only asmall proportion of the prehuman(“natural”) richness and abun-dance of most taxa—includingvertebrates, invertebrates, lichens,mosses, and microbes. Such taxaare highly dependent on the struc-ture and microclimate of a forest(2, 3). How would we know theywere missing? Unfortunately, given thevery poor preservation opportunities formany taxa, paleoecological evidence of thenatural animal communities of rainforestsis even more sparse than that for plants:The rainforests as discovered by scientistswere possibly greatly impoverishedcompared with their prehuman state, yetwe could not detect this. The prehistoricloss of the majority of the Pleistocenemegafauna in some areas (e.g., giant slothsin the Amazon) means some forests cannever be restored. The loss of endemicspecies from isolated forests is also irre-versible. Few witnessing the loss of rain-forest in Madagascar, for example, couldbelieve it to be fully reversible.

We should not assume that modernslash-and-burn clearance is comparable inimpacts to that of early forest peoples—just as modern coppice management onforest reserves in Britain does not producethe same community as did “traditional”coppicing (3). Rainforests may be hypoth-esized to have been substantially impover-ished by traditional management and clear-ance, as were British forests. Contemporary

clearance—and hunting—may impoverishthem further and may also be hard tomonitor. A precautionary approach may be appropriate when advising forestmanagers.

CLIVE HAMBLER

Department of Zoology, University of Oxford,

South Parks Road, Oxford OX1 3PS, UK. E-mail:

clive.hambler@zoo.ox.ac.uk

References 1. T. C. Whitmore, An Introduction to Tropical Rain

Forests (Oxford Univ. Press, Oxford, 1998).2. T. R. E. Southwood et al., Biol. J. Linn. Soc. 12, 327

(1978).3. C. Hambler, Conservation (Cambridge Univ. Press,

Cambridge, 2004).

IN THEIR PERSPECTIVE “HOW ‘VIRGIN’ IS

virgin rainforest?” (16 Apr., p. 402), K. J.Willis et al. conclude that tropical humidforest regenerated quickly after the fall ofprehistoric tropical societies, and thatmuch of the “virgin” rainforest we seetoday is human-impacted and largelysecondary. We must note that most prac-ticing conservationists do not subscribe tothe concept of “virgin” rainforest (1), andwe disagree with the authors’ suggestionthat rapid rainforest regeneration may soonfollow the impacts of modern developmentin the humid tropical forest biome (2).

Most prehistoric societies in the humidtropics were unlike the mechanized andindustrialized societies that today dominatevirtually every developing country. Forexample, the modern counterparts exhibithigher population densities, higher resourceconsumption, widespread common language,and rapid movement of the labor force inresponse to economic opportunities (3).The authors cite New Georgia in theSolomon Islands as a place where matureand species-rich “modern” forests regener-ated quickly after the collapse and

dispersal of large prehistoric populationcenters. There we find today the majorimpacts produced by modern industrialactivities to be larger and certainly longer-lasting than the rural, traditional distur-bance regimes (swidden as well as site-stable agriculture, small-scale alluvialmining, gathering of forest products,small-scale cash-cropping) that we see inmodern and ancient forest societies. Today,New Georgia is beset by industrial-scaledevelopment that has seen large-scalelogging lead to forest clearance for oilpalm, bringing about wholesale destructionof watersheds and additional negative

impacts in adjacent lagoonal coralreef ecosystems. There is littlelikelihood that these high-impactdevelopment zones will revert tonative forest (4).

In Papua New Guinea, alsocited by the authors, the ruralcustomary communities inhab-iting the Lakekamu Basin contin-ually disturb the native forestthrough swidden agriculture,collection of a wide range offorest products, and artisanalgold-mining. However, that inte-rior forest basin today exhibits apredominance of “mature” nativerainforest, only intermittentlybroken by small human settle-ments and gardens (5). As with

typical rural prehistoric societies, the ruralsubsistence human demographics of theLakekamu produce a swidden gardeningcycle that leads to rapid reforestation andminimal loss of biodiversity. Contrast thiswith the massive-scale development of oilpalm in the fertile volcanic rainforestplains of Popondetta, about 100 km south-east of Lakekamu. There one finds large-scale monoculture that, because of itsemployment demands, has encouraged in-migration and a demographic shift thatwill, for the foreseeable future, spellintense pressure on any remaining naturalforested tracts in this area. As a result,instead of regenerating humid forest, onefinds continuing expansion of oil palm (asencouraged by the national government),intensive vegetable cash-cropping, andhabitat degradation, which over time leadsto a widespread proliferation of unproduc-tive rank grasslands (6, 7).

Overall, we see rural subsistence forestcommunities as forest stewards. Bycontrast, the large industrialized extractiveindustries are leading us inexorably to a world of degraded and low-biodiversity

Image not available for online use.

Rainforest near Tari, Southern Highlands, Papua New Guinea.

Letters to the EditorLetters (~300 words) discuss material published

in Science in the previous 6 months or issues

of general interest. They can be submitted

through the Web (www.submit2science.org)

or by regular mail (1200 New York Ave., NW,

Washington, DC 20005, USA). Letters are not

acknowledged upon receipt, nor are authors

generally consulted before publication.

Whether published in full or in part, letters are

subject to editing for clarity and space.

LETTERS

www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004 943

CR

ED

IT:W

OLF

GA

NG

KA

EH

LER

/CO

RB

IS

13 AUGUST 2004 VOL 305 SCIENCE www.sciencemag.org944

post-forest habitats where indigenous peopleshave a minimal role and no resources.

BRUCE M. BEEHLER, TODD C. STEVENSON,

MICHELLE BROWN

Melanesia Center for Biodiversity Conservation,

Conservation International, 1919 M Street, NW,

Washington, DC 20036, USA.

References1. J. B. Callicott, M. P. Nelson, Eds., The Great New

Wilderness Debate (Univ. of Georgia Press, Athens, GA,1998).

2. M. Williams, Deforesting the Earth: From Prehistory toGlobal Crisis (Univ. of Chicago Press, Chicago, IL, 2003).

3. B. Meggers, Science 302, 2067 (2003).4. E. Hviding, T. Bayliss-Smith, Islands of Rainforest:

Agroforestry, Logging and Eco-tourism in SolomonIslands (Ashgate Press, Aldershot, UK, 2000).

5. A. Mack, Ed., RAP Working Pap. 9, 1 (1998).6. L. Curran et al., Science 303, 1000 (2004).7. D. O. Fuller, T. C. Jessup, A. Salim, Conserv. Biol. 18, 249

(2004).

ResponseFORESTS ARE NOT MUSEUM PIECES BUT LIVING,dynamic ecosystems that have been affectedby various factors—climate change, humaninfluences, animal populations, and naturalcatastrophes—for millennia. The suggestionmade by Hambler that tropical forests areimpoverished because of prehistoric impact isnot only unfounded, but also seems to implythat evidence for forest regeneration afterclearance should be suppressed in case itdiminishes the case for preservation. The keypoint that we were making is that humanimpact has left a lasting legacy on some areasof tropical rainforests, and the biodiverselandscapes that we value today are not neces-sarily pristine. In both tropical and temperateforests, there are areas in which previoushuman activity has enhanced biodiversity (1, 2). For example, we now know thatmahogany-rich forests, and the diverse floraand fauna that they support, may have origi-nated following prehistoric catastrophicdisturbance (3, 4). Natural regeneration ofAfrican and Brazilian mahoganies is inhib-ited by the presence of more shade-tolerantrainforest tree species. In the face ofincreasing logging pressures, this discoveryallows us to understand the steps necessaryfor its conservation in areas of evergreenforest—an environment in which it cannotnormally regenerate (5).

We also argue that long-term data shouldbe central to reexamining deforestation issues,such as that described by Hambler forMadagascar. Although there is no doubt thatrapid deforestation is occurring in some areas,the process of deforestation is complex. Thehypothesis that, prior to human arrival, thewhole island had once been forested was over-turned in the 1980s by extensive palynologicalwork (6–8)—yet many estimates of deforesta-tion rates in Madagascar are based on theerroneous assumption of previous 100%forest cover [e.g., (9)].

In response to Beehler et al., we reiteratethat our Perspective referred to the process ofslash and burn and did not address the issue ofpermanent conversion of the forest followingindustrial-scale logging. Nor did we suggest“rapid” regeneration of forest. Indeed, thepaleo-record is important in this respectbecause in a number of instances, it has beendemonstrated that forest regenerationfollowing clearance can take hundreds if notthousands of years.

We agree with Beehler et al.’s assertionthat probably many conservationists workingon the ground are aware that prehistorichuman populations have affected currentlyundisturbed rainforest blocks. What they failto mention is that this information is rarelyacknowledged by the organizations for whichthey are working. For example, in their Websites, major conservation organizations suchas Conservation International, WildlifeConservation Society, and the World WildlifeFund rely on value-laden terms like “fragile,”“delicate,” “sensitive,” and “pristine” togenerate interest in rainforest projects.Although these terms certainly apply to manyof the macrofauna that face extinction fromcommercial trade, they may be unjustified inreference to the rainforest vegetation.

The Letters of Hambler and Beehler etal. highlight a growing dilemma in conser-vation: How can long-term data on ecolog-ical resilience and variability be reconciledwith a strong conservation message in theshort term? We suggest that information onthe long-term history of tropical rainforestscan aid conservation in several ways. First,as the mahogany example highlights,management of contemporary ecosystemscan be more effective if it utilizes all theecological knowledge available. Second,providing realistic estimates of the extentand rates of forest cover change enhancesthe long-term credibility of the conserva-tion movement. Such realistic estimates ofthe long time scales involved in therecovery of vegetation should aid thosearguing for careful planning in the utiliza-tion of forest resources. Third, inevitabledisturbance from rainforest exploitationshould not be justification for permanentconversion of land for plantations, agricul-ture, cattle ranching, and mining, becauselong-term data highlight the potential ofthis biodiverse ecosystem to recover.

K. J. WILLIS, L. GILLSON, T. M. BRNCIC

Oxford Long-term Ecology Laboratory, Biodiversity

Research Group, School of Geography and the

Environment, Oxford, OX2 7LE UK. E-mail:

kathy.willis@geog.ox.ac.uk

References

1. R. Tipping, J. Buchanan, A. Davies, E. Tisdall, J. Biogeogr.26, 33 (1999).

2. L. Kealhofer, Asian Perspect. 42, 72 (2003).3. L. J. T. White, African Rain Forest Ecology and

Conservation, B. Weber, L. J. T. White, A. Vedder, L.

Naughton-Treves, Eds. (Yale Univ. Press, New Haven,CT, 2001), p. 3.

4. L. K. Snook, Bot. J. Linn. Soc. 122, 35 (1996).5. N. D. Brown, S. Jennings, T. Clements, Perspect. Plant

Ecol. Evol. Syst. 6, 37 (2003).6. D. A. Burney, Quat. Res. 40, 98 (1993).7. D. A. Burney, Quat. Res. 28, 130 (1987).8. K. Matsumoto, D. A. Burney, Holocene 4, 14 (1994).9. G. M. Green, R. W. Sussman, Science 248, 212 (1990).

Stem Cell Research in

Korea

LAST FEBRUARY, A GROUP OF KOREAN

scientists led by W. S. Hwang and S. Y. Moonsurprised the world by deriving a humanembryonic stem cell line (SCNT hES-1) froma cloned blastocyst (“Evidence of apluripotent human embryonic stem cellline derived from a cloned blastocyst,”Reports, 12 Mar., p. 1669; published online12 Feb., 10.1126/science.1094515). This isthe first example of success in what mightbe considered a first step to human “thera-peutic cloning,” and it captured the atten-tion of the world media. In response to theannouncement, many have raised questionsabout the ethical and social environment ofKorea with regard to such biotechnologicalinvestigations.

In December 2003, the Korean NationalAssembly passed the “Bioethics andBiosafety Act,” which will go into effect inearly 2005. According to the Act, humanreproductive cloning and experiments suchas fusion of human and animal embryoswill be strictly banned [(1), Articles 11 and12]. However, therapeutic cloning will bepermitted in very limited cases for the cureof serious diseases. Such experiments willhave to undergo review by the NationalBioethics Committee (NBC) [(1), Article22]. According to the Act, every researcherand research institution attempting suchexperiments must be registered with theresponsible governmental agency [(1),Article 23]. Since the Act is not yet ineffect, the research done by Hwang et al.was done without any legal control orrestriction.

The Korean Bioethics Association(http://www.koreabioethics.net/), a leadingbioethics group in Korea, consisting ofbioethicists, philosophers, jurists, andscientists, announced “The SeoulDeclaration on Human Cloning” (2) in1999, demanding the ban of human repro-ductive cloning and the study of the socio-ethical implications of cloning research.Many nongovernment organizations andreligious groups in Korea agreed with andsupported the declaration.

We regret that Hwang and Moon did notwait until a social consensus about repro-ductive and therapeutic cloning was

L E T T E R S

L E T T E R S

www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004 945

achieved in Korea before performing theirresearch. Indeed, Hwang is Chairperson of theBioethics Committee of the Korean Societyfor Molecular Biology, and Moon is Presidentof the Stem Cell Research Center of Koreaand a member of its Ethics Committee. Theyargue that their research protocol wasapproved by an institutional review board(IRB). However, we are not convinced thatthis controversial research should be donewith the approval of only one IRB. We believethat it was premature to perform this researchbefore these issues had been resolved.

The Korean government is working toprepare regulations, guidelines, and reviewsystems for biotechnology research inkeeping with global standards (3). We hopethat there will be no more ethically dubiousresearch reports generated by Koreanscientists before these systems are in place.

SANG-YONG SONG*

Department of Philosophy, Hanyang University, 17

Haengdang-dong, Seoul 133 -791, Korea.

*President of the Korean Bioethics Association

2002–04

References

1. Biosafety and Bioethics Act, passed 2003.2. The Korean Bioethics Association, J. Kor. Bioethics

Assoc. 1 (no. 1), 195 (2000).3. Korean Association of Institutional Review Boards,

Guidelines for IRB Management, 10 Feb. 2003.

ResponseWE RECOGNIZE THAT OUR REPORT CHANGED

the ethical, legal, and social implications oftherapeutic cloning from a theoretical possi-bility to the first proof of principle that humanembryonic stem cells can be derived fromcloned blastocysts. Stem cell researchers andsociety at large must consider all the implica-tions associated with therapeutic cloning.Conversations on this important topic must beall-inclusive. However, it is important to reit-erate that the experiments included in ourmanuscript complied with all existing institu-tional and Korean regulations. In accordancewith both Korean government regulation, aswell as our own ethics, we neither have norwill conduct “human reproductive cloningand experiments such as fusion of human andanimal embryos.” We concur that all humanembryo experiments should be overseen byappropriate medical, scientific, and bioethicalexperts.

In Korea, as in other countries, there is agreat diversity of opinions regarding thenewest scientific discoveries and when or ifthey should be translated into clinicalresearch. The Korean Bioethics Association(KBA) is, in our opinion, not neutral andadvocates restricting the pace of biomedicaladvancements, viewing new techniques as

threats to society. For example, they havespoken publicly against the study of trans-genic mouse models for human disease andpreimplantation genetic diagnosis to helpparents have healthy children. Although werespect the opinions of the KBA, we, asmembers of a leading Korean stem cell andcloning laboratory, are committed to discov-ering the medical potential of stem cells andto participating in conversations with ethicaland religious groups regarding matters ofbioethical concern. Our research team hasalways and will continue to comply withethical regulations and any laws or guidelinespromulgated by the Korean government.

WOO-SUK HWANG1,2 AND SHIN YONG MOON3

1College of Veterinary Medicine, 2School of

Agricultural Biotechnology, Seoul National University,

Seoul 151-742, Korea. 3College of Medicine, Seoul

National University, Seoul, 110-744, Korea.

Changing Scientific

Publishing

WE SHARE THE CONCERNS OF Y.-L.WANG ET

al. that “[t]he direction of research isdictated more and more by publishabilityin high-profile journals, instead of strict

13 AUGUST 2004 VOL 305 SCIENCE www.sciencemag.org946

scientific considerations…” (“BiomedicalResearch Publication System,” Letters, 26Mar., p. 1974). We do not, however, sharetheir conclusions, as the major componentsof their proposed model to improve thepublication system already exist.

Wang et al. suggest that a post–Webpublication evaluation process to deter-mine which papers should appear in asmaller version of the printed journal thatis “influenced less by haggling and moreby quality” would be preferable to thecurrent practice. In fact, this servicealready exists in the form of Faculty of1000, to which we belong. The Facultyconsists of over 1600 highly respected biol-ogists, who choose and evaluate what theyconsider to be the best papers in their areasof biology, regardless of the journal inwhich the papers are published. Becausethis new online service evaluates eachpaper solely on its merits, it is beginning tomake the journal in which a paper appearsmuch less relevant.

Wang et al. also propose a “high-capacity Web site for posting peer-reviewed papers.” This too already exists inthe form of the open access site run byBioMed Central, where authors pay a flatfee to publish their research papers, which

are free to be read and downloaded byanyone with access to the Web.

As these two resources are alreadycatering to the needs delineated by Wang etal., we think it makes more sense tosupport them, rather than to reinvent thewheel.

MARTIN C. RAFF,1 CHARLES F. STEVENS,2

KEITH ROBERTS,3 CARLA J SHATZ,4

WILLIAM T. NEWSOME5

1MRC Laboratory for Molecular Cell Biology and

Cell Biology Unit, University College London,

London WC1E 6BT, UK. 2Molecular Neurobiology

Laboratory, The Salk Institute of Biological

Sciences, La Jolla, CA 92037, USA. 3Department of

Cell Biology, John Innes Centre, Norwich NR4 7UH,

UK. 4Department of Neurobiology, Harvard

Medical School, Boston, MA 02115, USA. 5HHMI,

Department of Neurobiology, Stanford University

School of Medicine, Palo Alto, CA 94305–2130,

USA.

CORRECTIONS AND CLARIFICATIONS

Reports: “Three-dimensional polarimetric imagingof coronal mass ejections” by T. G. Moran and J. M.Davila (2 July, p. 66). The e-mail address for T. G.Moran on p. 67 was incorrect; the correct e-mailaddress is moran@orpheus.nascom.nasa.gov. Alsoon p. 67, a date is incorrect in the last paragraph of

the second column. The correct sentence is “A haloCME was imaged three times on 29 June 1999 at2-h intervals, and another was imaged 17 times on4 November 1998 for 17 h at 1-h intervals.” In thefirst complete paragraph on p. 70, the secondsentence cites the wrong figure. The correctsentence is “In the topographical map (Fig. 3D),there are at least six of these linear structuresvisible that remain connected to the Sun, whichmay be legs or groups of legs of the arcade loops.”

Reports: “Sites of neocortical reorganization crit-ical for remote spatial memory” by T. Maviel et al.(2 July, p. 96). In the abstract, “cortex” and”cortices” were misplaced when author correctionswere made to the galley. The correct sentences areas follows: “By combining functional brain imagingand region-specific neuronal inactivation in mice,we identified prefrontal and anterior cingulatecortices as critical for storage and retrieval ofremote spatial memories… Long-term memorystorage within some of these neocortical regionswas accompanied by structural changes includingsynaptogenesis and laminar reorganization,concomitant with a functional disengagement ofthe hippocampus and posterior cingulate cortex.”

Reports: “Inhibition of netrin-mediated axonattraction by a receptor protein tyrosine phos-phatase” by C. Chang et al. (2 July, p. 103). The e-mail address given for the corresponding author,Marc Tessier-Lavigne, is incorrect. The correct e-mail address is marctl@gene.com.

L E T T E R S

947

Iam penning this review—one day pastdue—in a plane 35,000 feet above theAtlantic. Had I followed my original plans

and traveled earlier, I would have had therare pleasure of submitting a review on time.Unfortunately, a nod to our post-9/11 worldkept me out of the skies on America’sIndependence Day. It would somehow becomforting if we could ascribe this world tothe evil or greed of a few and believe that itwould be over when those few are capturedor removed from office. But Paul and AnneEhrlich’s One with Nineveh:Politics, Consumption, and theHuman Future suggests a dif-ferent reality. Although notclaiming to address the roots ofterrorism per se, the authorsmake a compelling case thatthe combination of populationgrowth, rampant consumption,and environmental degradationseriously threatens the liveli-hoods of the have-nots today and will in-creasingly threaten the haves in the none-too-distant future. Insecurity, hunger, and therecognition that one is entitled to a betterworld can breed a certain rage that will even-tually find a voice.

Of course the Ehrlichs are not so naïveas to think that choreographing a betterpopulation-consumption-environmentdance will rid the world of all hatred and in-tolerance. But surely ensuring an adequatesubsistence for the poorest of the planet,and securing a sustainable future for all,would go a long way toward diminishingthe power of those who preach fanaticism.

In many ways, our current environmentaland human dilemma is not a new problem,as the book’s title itself acknowledges. TheEhrlichs draw on a wealth of archaeologicalliterature to document the consequences ofpast collisions between human aspirationsand environmental limitations. We are onewith Nineveh in our predilection for weak-ening the natural resource base that shoresup the whole of human activity. However,we diverge from Nineveh in many other pro-found and unprecedented ways, including inour technological capacity, our global reach,and the rapidity with which we can inflict

change. These differences, the Ehrlichs as-sert, will mean that Nineveh’s fate cannot beours. Local collapses can no longer be con-tained. And global rescue will require a newevolutionary step—a “conscious culturalevolution” that allows us to overcome thelimitations of individual perception and for-mulate a more responsive societal whole.

A central thesis of the book, then, is thathumanity’s capacity to shape the planet hasbecome more profound than our ability torecognize the consequences of our collec-

tive activity. The authors thor-oughly document many ofthese consequences, such asland degradation, emergingdiseases, and the loss ofspecies. They offer some pro-vocative insights into the caus-es, including limitations of thehuman nervous system, fail-ures of education, and the non-linearities in Earth systems that

make effective management difficult. Andthey discuss potential sources for solutions:technology (which brings both promise andperil), better international institutions, andcivic and religious organizations that couldfoment the conscious cultural evolution.

One of the joys of reading One withNineveh is the sheer number of literaturesthe authors have reviewed. To any studentof the human predicament, the bibliogra-phy alone is worth the price of the book. Iparticularly enjoyed the sections on eco-nomics. The Ehrlichs distill the work ofmany thoughtful economists to revealsome limitations of current theory, includ-ing the imperfect “rationality” of actors inthe marketplace and the scaling issues thatmake group behavior difficult to predictfrom an understanding of individual pref-erences. More sobering, however, are thediscussions of how the current theories of afew economists have driven political dis-course in the wrong direction. Many con-temporary economists—particularly thosewho have come to understand the limita-tions on human activity imposed by thenatural environment—do not suggest thatunfettered growth is a sufficient key towealth, that markets alone can supply thenecessary ingredients for a sustainable so-ciety, or that unchecked corporate activitycan ensure the public good. Yet these senti-ments are increasingly represented in na-

tional and international policy dialogues.More of the environmentally aware work ineconomics, including the collaborativework between ecologists and economists(in which the Ehrlichs regularly engage),needs to find its way into the public arena.

Readers of Science should find at leasttwo important messages in the book. Thefirst addresses us as citizens. We are allcomplicit in the planet’s ills, and we can allcontribute to the solutions, at the very leastthrough civic engagement and ethical re-flection. The second speaks to us as scien-tists. There remain many unanswered ques-tions about the functioning of our planet. Asthe Ehrlichs point out, science has come along way in elucidating Earth’s biogeophys-ical components as a complex adaptive sys-tem. Science has also advanced significant-ly in its understanding of the complexity ofhuman perception and behavior acrossscales of social organization. We are only inthe early stages of successfully joining thesetwo perspectives to grasp how complex hu-man dynamics engender environmentalchange and vice versa. There have beensome steps, but more are urgently needed.Start the next leg of the journey by readingOne with Nineveh, and see where it takesyou as citizen and as scientist.

E N V I R O N M E N T

Our Once and Future FateAnn Kinzig

One with Nineveh

Politics, Consumption,

and the Human Future

by Paul R. Ehrlich

and Anne H. Ehrlich

Island Press, Washington,

DC, 2004. 459 pp. $27.

ISBN 1-55963-879-6.

The reviewer is in the School of Life Sciences, ArizonaState University, Tempe, AZ 85287, USA. E-mail:kinzig@asu.edu

et al.

CR

ED

IT:N

IK W

HEELE

R/C

OR

BIS

BOOKS

Ruins at the ancient Assyrian city of

Nineveh, Iraq.

www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004

948

M AT E R I A L S S C I E N C E

The Soft Sectorin PhysicsGerard C. L. Wong

Soft matter occupies a middle groundbetween the solid and fluid states.These materials have neither the crys-

talline symmetry of solids, nor the uniformdisorder of fluids. For instance, a smecticliquid crystal consists of a one-dimensional,solid-like, periodic stack of two-dimensionalfluid monolayers. Liquid crystals, poly-mers, and colloids are commonly cited ex-amples, but soft matter also encompassessurfactants, foams, granular matter, andnetworks (for example, glues, rubbers,gels, and cytoskeletons), to name a few.

The interactions that govern the behaviorof soft matter are often weak and compara-ble in strength to thermal fluctuations. Thusthese usually fragile forms of matter can re-spond much more strongly to stress, electric,or magnetic fields than can solid-state sys-tems. Common themes in the behavior ofsoft matter include the propensity for self-organized structures (usually at lengthscales larger than molecular sizes), self-organized dynamics, and complex adaptivebehavior (often in the form of large macro-scopic changes triggered by small micro-scopic stimuli). These themes can be seen ina wide range of examples from the recentliterature: shape-memory polymers for“smart,” self-knotting surgicalsutures (1), DNA-cationic mem-brane complexes in artificialgene delivery systems (2), col-loidal crystals for templatingphotonic-bandgap materials (3),cubic lipid matrices for crystal-lizing integral membrane pro-teins (4), and electronic liquidcrystalline phases in quantumHall systems (5). (In the lastcase, we have come full circle, towhere soft and hard condensed matterphysics meet.) To a traditional condensed-matter physicist, the above list may sound atbest like the animal classifications in JorgeLuis Borges’s imaginary Chinese encyclo-pedia (6), but the field’s broad conceptualreach is one of its strengths.

A young but already diverse field, softcondensed matter physics is expanding theprovince of physics in new and unexpecteddirections. For example, it has generated a

new branch of biophysics. Most largerphysics departments now have faculty whospecialize in soft matter, and such materialsare beginning to be covered in the under-graduate curricula in physics, chemistry, ma-terials science, and chemi-cal engineering. However,introducing students to thefield has been a challengebecause of the lack of suit-able textbooks. Thus the ap-pearance of StructuredFluids: Polymers, Colloids,Surfactants by Tom Wittenand Phil Pincus, two pio-neers in the field, is particu-larly welcome.

Witten and Pincus (fromthe physics departments atthe University of Chicagoand the University ofCalifornia, Santa Barbara, re-spectively) give us a tutorialfor thinking about polymers,colloids, and surfactants using a unified-scaling approach in the tradition of deGennes’s classic monograph in polymerphysics (7). They begin with a review of statis-tical mechanics, and then they proceed to de-velop the tools needed to make simple esti-mates by thinking in terms of important lengthscales and time scales in a given phenomenon.For example: How do we estimate viscosities?How do colloids aggregate? What does a poly-mer look like at different length scales in dif-ferent conditions, and how does that influencethe way it moves? What concentrations of sur-

factant do we need for entangledwormlike micelles to form?Witten and Pincus demonstratehow to come up with real num-bers for actual materials systems.

Another unusual strength ofthe book is the authors’ atten-tion to chemical and experi-mental details. Too few physicstextbooks explain how a poly-mer is made, much less men-tion recent synthetic strategies

for controlling sequence and length withrecombinant DNA technology. This bookalso offers an excellent, concise introduc-tion to scattering methods, in which dif-fraction is presented not so much as the in-terference of scattered waves from atomicplanes (as described in classic solid statephysics textbooks) but as a Fourier trans-form of a density-density correlation func-tion. This more powerful formulation facil-itates generalization to diffraction fromfractals and weakly ordered systems.

The authors describe a number of peda-gogical “home” experiments. These coverquestions including the elasticities of gelsand rubber, turbidity assays, and the elec-

trostatics of skim milk and employ suchreadily available household componentsas gelatin, rubber bands, and laser point-ers. Many interesting concepts are rele-gated to the appendices, which reward

careful reading. Theserange from a considera-tion of the dilational in-variance of random walksto a presentation of thecelebrated Gauss-Bonnettheorem (which seems asmuch a miracle as it is dif-ferential geometry).

The book’s fairly shortlength required the authorsto make hard choices. As aresult, the coverage is un-even and there are notableomissions. (For example,the rotational-isomeriza-tion-state model for poly-mer conformations is onlydiscussed qualitatively, as

are semiflexible chains.) In addition, read-ers would benefit from having moreworked problems. On the other hand, thebook is very readable, and it can be easilyadapted for a one-semester or a one-quartercourse. Instead of opting for an encyclope-dic treatment, Witten and Pincus cultivate aphysicist’s style of thought and intuition,which often renders knowledge weightless.Structured Fluids belongs on one’s shelfbeside recent works by Paul Chaikin andTom Lubensky (8), Jacob Israelachvili (9),and Ronald Larson (10). These books rec-tify and expand prevailing notions of whatcondensed matter physics can be.

References and Notes1. A. Lendlein, R. Langer, Science 296, 1673 (2002).2. Y. A. Vlasov, X. Z. Bo, J. Z. Sturn, D. J. Norris, Nature

393, 550 (1998).3. J. O. Rädler, I. Koltover, T. Salditt, C. R. Safinya, Science

275, 810 (1997).4. E. Pebay-Peyroula, G. Rummel, J. P. Rosenbusch, E. M.

Landau, Science 277, 1676 (1997).5. S. A. Kivelson, E. Fradkin, V. J. Emery, Nature 393, 550

(1998).6. Borges describes “a certain Chinese encyclopedia

called the Heavenly Emporium of BenevolentKnowledge. In its distant pages it is written that ani-mals are divided into (a) those that belong to theEmperor; (b) embalmed ones; (c) those that aretrained; (d) suckling pigs; (e) mermaids; (f) fabulousones; (g) stray dogs; (h) those that are included in thisclassification; (i) those that tremble as if they weremad; (j) innumerable ones; (k) those drawn with avery fine camel’s hair brush; (l) et cetera; (m) thosethat have just broken the flower vase; (n) those thatat a distance resemble flies.” J. L. Borges, SelectedNon-Fictions, E. Weinberger, Ed. (Penguin, New York,1999), pp. 229–232.

7. P.-G. de Gennes, Scaling Concepts in Polymer Physics(Cornell Univ. Press, Ithaca, NY, 1979).

8. P. M. Chaikin, T. C. Lubensky, Principles of CondensedMatter Physics (Cambridge Univ. Press, Cambridge,1995).

9. J. N. Israelachvili, Ed., Intermolecular and SurfaceForces (Academic Press, London, ed. 2, 1992).

10. R. G. Larson, The Structure and Rheology of ComplexFluids (Oxford Univ. Press, Oxford, 1999). C

RED

IT:A

DA

PT

ED

FR

OM

WIT

TEN

AN

D P

INC

US,2

00

4

Structured Fluids

Polymers, Colloids,

Surfactants

by Thomas A. Witten

with Philip A. Pincus

Oxford University Press,

Oxford, 2004. 230 pp.

$74.50, £39.95. ISBN

0-19-852688-1.

The reviewer is in the Department of MaterialsScience and Engineering, University of Illinois atUrbana-Champaign, 1304 West Green Street, Urbana,IL 61801, USA. E-mail: gclwong@uiuc.edu

13 AUGUST 2004 VOL 305 SCIENCE www.sciencemag.org

B O O K S E T A L .

949

The issue of ethics surrounding studiesfor regulatory decision–making hasbeen the subject of recent discussions

at the Environmental Protection Agency(EPA) that could have broad implications forhuman subject research. In 2000, a reportfrom a joint meeting of the Agency’s ScienceAdvisory Board (SAB) and the FederalInsecticide, Fungicide, and Rodenticide Act(FIFRA) Science Advisory Panel (SAP) rec-ommended that the Agency require “activeand aggressive” review of human studiesconducted by external groups (1). EPA an-nounced a moratorium indicating it wouldnot consider “third-party” generated data(i.e., from academia, industry, or public in-terest groups) in its regulatory process untilethical issues were resolved (2). This bancentered on several clinical studies submit-ted by pesticide manufacturers since 1998.However, EPA’s policy appeared to have im-plications for other toxicology and epidemi-ology studies. In 2001, EPA requested thatthe National Research Council (NRC) “fur-nish recommendations regarding the partic-ular factors and criteria EPA should considerto determine the potential acceptability ofthird-party studies.” EPA also asked theNRC to provide advice on a series of ques-tions, including “recommendations onwhether internationally accepted protocolsfor the protection of human subjects (the‘Common Rule’) could be used to developscientific and ethical criteria for EPA” (3).

In May 2003, EPA issued an AdvancedNotice of Proposed Rulemaking (ANPRM),the first formal step toward developing aregulatory standard and solicited publiccomment (4). The ANPRM noted thatthird-party research is not legally subject tothe Common Rule. The Common Rule,which is administered by the Department ofHealth and Human Services (DHHS), de-tails accepted ethical standards for the pro-tection of human subjects in research con-ducted or sponsored by all federal agencies

(5). In its ANPRM, EPA raised questionsregarding policy options being considered,including applicability of the CommonRule and whether the standard of accept-ability should vary depending on researchdesign, provenance, impact on regulatorystandard, or EPA’s assessment of the risksand benefits of the research. In addition,they requested input on a prospective andretroactive study review process.

We do not find a compelling reason forEPA to propose alternate and complex crite-ria. We believe that the best approach is theapplication of the Common Rule or equiva-lent international standards (6, 7). TheCommon Rule codifies existing ethicalguidance, is built on decades of experienceand practice, and thus is both necessary andsufficient to ensure protection of human re-search subjects. There should be no differ-ence in the standards based on the study de-sign, source of funding, or, most disturbing-ly, the impact of the study on a regulatorystandard. Otherwise, data that were obtainedin studies deemed ethically acceptable un-der the Common Rule could be excluded, or(perhaps worse) data from studies that donot meet these norms could be included.

We find troubling the notion that theethical standard for a human toxicity test ora clinical trial would be different whenconducted by a nonprofit organization oran industry. Whether or not studies withhuman subjects to test pesticides and in-dustrial chemicals will be judged ethicallyacceptable is not the point. We are alsoconcerned that different ethical normsmight be applied on the basis of whetherthe study’s conclusions strengthen or relaxan EPA regulatory position. Biasing theprocess in either direction is bad scienceand public policy.

In February 2004, the NRC recom-mended (8) that studies be conducted andused for regulatory purposes if they are ad-equately designed, societal benefits of thestudy outweigh any anticipated risks, andrecognized ethical standards and proce-dures are observed. It also stated that EPAshould ensure that all research it uses is re-viewed by an appropriately constitutedInstitutional Review Board (IRB) beforeinitiation, regardless of the source of fund-ing. These conclusions are consistent withother counsel that all research proposals in-

volving human subjects be submitted forscientific and ethical review (9).

Although we agree with these recom-mendations, we strongly disagree withNRC’s call for creation of an EPA reviewprocess and review board for human studiesproposed for use in formulating regulations.Private entities would submit research plansbefore beginning a study, and again beforesubmitting the study results. It is unclearhow post-study review can contribute toprotection of research subjects. Introductionof such a parallel review process will createconfusion regarding which set of rules ap-plies to a particular study. It is also likely tocreate resource and logistical problems. Wesuggest that EPA require that private entitiesobtain review under the Common Rule or itsforeign equivalent before undertaking astudy and provide documentation of this re-view in order to submit their data for regu-latory purposes. By requiring studies to fol-low the Common Rule or a foreign equiva-lent, EPA can strongly discourage the prac-tice of conducting human-subjects researchand clinical trials outside the United States,to avoid federal scrutiny.

By a strong endorsement and legallybinding adoption of the Common Rule andequivalent international standards, EPA canensure that ethical concerns are fully consid-ered. By joining the community of biomed-ical ethics, rather than establishing a separatepath, EPA will strengthen all of our efforts.

References and Notes1. Science Advisory Board and the FIFRA Scientific

Advisory Panel, EPA, “Comments on the use of data

from the testing of human subjects” (EPA-SAB-EC-

00-017, EPA, Washington, DC, 2000).

2. EPA, Agency requests National Academy of Sciences

input on consideration of certain human toxicity

studies; announces interim policy (press release, 14

December 2001).

3. National Research Council (NRC), Use of Third-PartyToxicity Research with Human Research Participants(National Academies Press, Washington, DC, 2002).

4. EPA, Human testing;Advance notice of proposed rule-

making, Docket no. OPP-2003-0132, Fed. Regist. 68,

24410 (2003).

5. DHHS, Protection of human subjects, Code of Federal

Regulations (CFR) 40, part 26 (2001).

6. World Medical Association, “Declaration of Helsinki:

Ethical principles for medical research involving hu-

man subjects” (World Medical Association, Edinburgh,

2000).

7. International Conference on Harmonisation of

Technical Requirements for Registration of Pharma-

ceuticals for Human Use (ICH Topic E6: Guideline for

Good Clinical Practice, Geneva, 1996).

8. NRC, Intentional Human Dosing Studies for EPARegulatory Purposes: Scientific and Ethical Issues(National Academies Press, Washington, DC, 2004).

9. The Council for International Organizations of

Medical Sciences (CIOMS), International EthicalGuidelines for Biomedical Research Involving HumanSubjects (National Academies Press, Washington, DC,

2002).

E T H I C S

Human Health Research EthicsE. Silbergeld, S. Lerman,* L. Hushka

E. K. Silbergeld is with the Johns Hopkins University,Bloomberg School of Public Health, Baltimore, MD21205, USA. S. E. Lerman is with ExxonMobilBiomedical Sciences Inc., Annandale, NJ 08801, USA.L. J. Hushka is with Exxon Mobil Corporation, HoustonTX 77079, USA.

*Author for correspondence. E-mail: steven.e.lerman@exxonmobil.com

POLICY FORUM

www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004

950

Only a few years after Bardeen,Cooper, and Schrieffer introducedtheir successful theory of supercon-

ductivity in metals (1, 2), the idea thatsomething similar might happen in semi-conductors was advanced (3). Electrons in asuperconductor, even though they repel oneanother, join to form pairs. Known asCooper pairs, these composite objects aremembers of a class of quantum particlescalled bosons. Unlike individual electronsand the other members of the particlescalled fermions, bosons are not bound bythe Pauli exclusion principle: Any numberof bosons can condense into the same quan-tum state. Bose condensation is at the rootof the bizarre properties of superfluid heli-um and is nowadays being intensely studiedin ultracold atomic vapors. The condensa-tion of Cooper pairs in a metal leads not on-ly to the well-known property of losslessconduction of electricity, but also to a vari-ety of other manifestations of quantum me-chanics on a macroscopic scale.

In a semiconductor, there are both elec-trons and holes. Holes are unfilled electronstates in the valence band of the material.Remarkably, holes behave in much thesame way as electrons, with one crucialdifference: Their electrical charge is posi-tive rather than negative. Electrons andholes naturally attract one another, andthus pairing seems very likely. LikeCooper pairs, these excitons, as they areknown, are bosons. If a suitably dense col-lection of excitons could be cooled to asufficiently low temperature, Bose conden-sation ought to occur and a new state ofmatter should emerge. Or so went thethinking in the early 1960s.

Alas, there is a problem: Excitons areunstable. They typically survive only abouta nanosecond before the electron simplyfalls into the hole, filling the empty va-lence band state and giving birth to a flashof light in the process. A nanosecond is notvery long, and this left the prospects forcreating a condensate of excitons in a bulksemiconductor pretty poor. Over the lastdecade the situation has improved consid-erably through the use of artificial semi-

conductor structures in which the electronsand holes are confined to thin slabs of ma-terial separated by a thin barrier layer. Thisphysical separation slows the recombi-nation substantially, and some very inter-esting, and provocative, results have beenobtained (4–6). Excitonic Bose condensa-tion has, however, remained elusive.

Last March, experimental results report-ed at the meeting of the American PhysicalSociety in Montreal by independentCalifornia Institute of Technology/Bell Labsand Princeton groups have revealed clearsigns of excitonic Bose condensation (7, 8).Remarkably, however, the findings weremade with samples consisting of two layersof electrons or two layers of holes. How canone have exciton condensation without elec-trons and holes in the same sample? Thetrick is to use a large magnetic field to levelthe playing field between electron-hole,electron-electron, and hole-hole double-lay-er systems (see the figure on this page).

Suppose that only electrons are presentin a thin layer of semiconductor. (This caneasily be achieved by doping with a suit-able impurity.) Applying a large magneticfield perpendicular to this system creates aladder of discrete energy levels for theseelectrons to reside in. If the field is large

enough, the electrons may only partiallyfill the lowest such level. Now, borrowingthe old viticultural metaphor, is the levelpartially filled or partially empty? Themagnetic field allows us to choose eitherpoint of view. If it is the latter, we maythink of the system as a collection ofholes, just as we always do with a partial-ly filled valence band in a semiconductor.Now bring in a second identical layer ofelectrons, and position it parallel to thefirst. We remain free to take either the par-tially full or partially empty point of viewwith this layer. Let us consider the firstlayer in terms of holes and the second interms of electrons. If the layers are closeenough together, the holes and electronswill bind to each other because of theirmutual attraction to form interlayer exci-tons. All we need to do is ensure that thereare no electrons or holes left over. A mo-ment’s thought shows that the way to dothis is to ensure that the total number ofelectrons in both of the original layers isjust enough to completely fill preciselyone of the energy levels created by themagnetic field. This is easily done by ad-justing the magnetic field strength to theright value (9–11).

An immense advantage of electron-elec-tron or hole-hole double-layer systems forcreating exciton condensates is that they arein equilibrium. In the electron-electroncase, only the conduction band of the semi-conductor is involved. In the hole-holecase, it is only the valence band. No opticalrecombination occurs in either system.Experimenters can proceed at their leisure.

The new results reported in Montrealclearly reveal that electrons and holes arebinding to each other to form electricallyneutral pairs. To demonstrate this, a varia-tion on a time-honored electrical measure-ment was performed. When an electricalcurrent flows at right-angles to a magneticfield, the Lorentz force on the carriers leadsto a voltage perpendicular to both the fieldand the current. This is the famous Hall ef-fect. One of the most important aspects ofthe Hall effect is that the sign of the Hallvoltage is determined by the sign of thecharge of the particles carrying the current.In the recent experiments, equal but oppo-sitely directed electrical currents were madeto flow through the two layers of electrons(or holes). This was done because a uniformflow of excitons in one direction, if present,would necessarily involve oppositely direct-ed electrical currents in the two layers.Meanwhile, the Hall voltage in one of thelayers was monitored. Normally one wouldexpect that the sign of this voltage would be

The author is at the California Institute ofTechnology, Pasadena, CA 91125, USA. E-mail:jpe@caltech.edu

Electrons

ExcitonsMagnetic field

Stabilizing excitons. With the application of a

strong magnetic field to a double layer (top) of

electrons in a semiconductor, electron-hole

pairs (excitons) can be stabilized against decay

and undergo Bose condensation (bottom).

P H Y S I C S

Half Full or Half Empty?J. P. Eisenstein

PERSPECTIVES

13 AUGUST 2004 VOL 305 SCIENCE www.sciencemag.org

951

determined by the sign of the charge carriersonly in the layer being measured. What theCalifornia Institute of Technology/Bell Labsteam and the researchers at Princeton foundwas that under the conditions in which exci-ton condensation was expected, the Hallvoltage simply vanished. The explanationfor this is simple: The oppositely directedcurrents in the two layers are being carriednot by individual particles, but by interlayerexcitons. Excitons have no net charge and sothere is no net Lorentz force on them, andhence no Hall voltage develops.

A vanishing Hall voltage is compellingevidence that excitons are present. By itself,

however, it does notprove that the excitongas possesses thekind of long-rangequantum coherenceexpected of a Bosecondensate. Althoughboth groups also

found that the conductivity of the excitongas appears to diverge as the temperatureapproaches absolute zero, an independentindicator of coherent behavior would makea much more compelling case. Inter-estingly, prior experiments by theCalifornia Institute of Technology/BellLabs group provided just such an indication(12). These earlier experiments revealed agigantic enhancement of the ability of elec-trons to quantum mechanically “tunnel”through the barrier separating the layers un-der the conditions in which exciton conden-sation was expected (see the figure on thispage). Taken together, the new Hall effect

measurements and the older tunneling stud-ies very strongly suggest that the vision ofexcitonic Bose condensation first advancedsome 40 years ago has finally beenachieved.

References1. J. Bardeen, L. N. Cooper, J. R. Schrieffer, Phys. Rev. 106,

162 (1957).

2. J. Bardeen, L. N. Cooper, J. R. Schrieffer, Phys. Rev. 108,

1175 (1957).

3. L. V. Keldysh, Y. V. Kopaev, Fiz. Tverd. Tela. (Leningrad)

6, 2791 (1964) [Sov. Phys. 6, 2219 (1965)].

4. D. B. Snoke, Science 298, 1368 (2002).

5. L. V. Butov, Solid State Commun. 127, 89 (2003).

6. C. W. Lai, J. Zoch, A. C. Gossard, D. S. Chemla, Science

303, 503 (2004).

7. M. Kellogg, J. P. Eisenstein, L. N. Pfeiffer, K. W. West,

Phys. Rev. Lett. 93, 036801 (2004).

8. E. Tutuc, M. Shayegan, D. Huse, Phys. Rev. Lett. 93,

036802 (2004).

9. H. Fertig, Phys. Rev. B 40, 1087 (1989).

10. E. H. Rezayi, A. H. MacDonald, Phys. Rev. B 42, 3224

(1990).

11. X. G. Wen, A. Zee, Phys. Rev. Lett. 69, 1811 (1992).

12. I. B. Spielman, J. P. Eisenstein, L. N. Pfeiffer, K. W. West,

Phys. Rev. Lett. 84, 5808 (2000).

How do you tell whether a rat that haslearned to self-administer a drug hasbecome an “addict”? Mere self-ad-

ministration is not evidence of addiction,because addiction refers to a specific pat-tern of compulsive drug-seeking and drug-taking behavior, one that predominates overmost other activities in life. Indeed, mostpeople have at some time self-administereda potentially addictive drug, but very fewbecome addicts. What accounts for thetransition from drug use to drug addiction,and why are some individuals more suscep-tible to this transition than others? Two pa-pers on pages 1014 (1) and 1017 (2) of thisissue represent a major advance in develop-ing realistic preclinical animal models toanswer these questions. Specifically, thetwo studies ask: How do you tell whether arat has made the transition to addiction?

Nonhuman animals learn to avidly per-form an action if it results immediately inthe intravenous delivery of a potentiallyaddictive drug, a phenomenon first report-ed in this journal by Weeks in 1962 (3).This self-administration animal model isstill the “gold standard” for assessing therewarding properties of drugs of abuse.From this model, we have learned a greatdeal about the conditions that support drugself-administration behavior. For example,nonhuman animals will self-administernearly every drug that is self-administeredby humans [with a few notable exceptions,such as hallucinogens (4)]. We also knowthat potentially addictive drugs usurp neu-ral systems that evolved to mediate behav-iors normally directed toward “natural re-wards” [such as food, water, shelter, andsex (5)].

However, despite enormous advances,drug self-administration studies have notprovided much insight into why some sus-ceptible individuals undergo a transition to

addiction, whereas others can maintaincontrolled drug use or forgo use altogether(6). This is in part because there have beenno good animal models to distinguish meredrug self-administration behavior fromthe compulsive drug self-administrationbehavior that characterizes addiction.Deroche-Gamonet et al. (1) and Vander-schuren and Everitt (2) approached thisproblem in a straightforward yet elegantway. They identified three key diagnosticcriteria for addiction and then simply askedwhether rats allowed to self-administer co-caine for an extended period developed anyof the symptoms of addiction described bythe criteria.

The first diagnostic criterion selected iscontinued drug-seeking behavior evenwhen the drug is known to be unavailable(1). This is reminiscent of the cocaine ad-dict, who has run out of drug, compulsive-ly searching the carpet for a few whitecrystals (“chasing ghosts”) that they knowwill most likely be sugar. Deroche-Gamonet et al. (1) measured this behaviorwith two signals: a “go” cue that drug isavailable and a “stop” cue that drug is notavailable (see the figure). Normal ratsquickly learn to work for drug only whenthe go cue is on, and refrain when the stop

N E U R O S C I E N C E

Addicted RatsTerry E. Robinson

The author is in the Department of Psychology andNeuroscience Program, University of Michigan, AnnArbor, MI 48109, USA. E-mail: ter@umich.edu

V

I

d

Exciton condensation. Onset of exciton condensation as detected in the

current, which quantum mechanically tunnels between the two layers in the

double layer two-dimensional electron system, as a function of the interlay-

er voltage V. A family of curves is shown, each one for a different effective

separation d between the layers. At large d, the tunneling current near V = 0

is strongly suppressed. As d is reduced, however, an abrupt jump in the cur-

rent (highlighted in red) develops around V = 0.This jump, reminiscent of the

Josephson effect in superconductivity, is a compelling indicator of the ex-

pected quantum coherence in the excitonic state.

www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004

P E R S P E C T I V E S

952

cue is on. Addicted rats keep working evenwhen signaled to stop.

The second criterion selected is unusu-ally high motivation (desire) for the drug(1). A defining characteristic of addictionis a pathological desire (“craving”) for thedrug, which drives a willingness to exertgreat effort in its procurement. This criteri-on was measured with a progressive ratioschedule in which the amount of work re-quired to obtain the drug progressively in-creased. At some point, the cost exceedsthe benefit and animals stop working; this“breaking point” is thought to provide ameasure of an animal’s motivation to ob-tain a reward (7). Addicted rats have an in-creased breaking point (see the figure).

The final criterion is continued drug useeven in the face of adverse consequences(1, 2). Addicts often continue drug use de-spite dire consequences. This feature of ad-diction was modeled by asking whetherrats would continue to work for cocaineeven when their actions produced an elec-tric shock along with the cocaine injection(1) or when the memory of past electricshocks was evoked (2). Addicted rats keptworking despite negative consequences.

Of particular importance are the condi-tions under which these symptoms of ad-

diction develop (which also explains whythis demonstration has been so long incoming). These symptoms of addictiononly appear after much more extensivedrug self-administration experience thanis the norm [see also (8)]. For the firstmonth that animals self-administered co-caine, they did not show any symptoms.Only after more than a month of exposureto cocaine (1), or after sessions with pro-longed drug access (2), did symptoms be-gin to emerge. Furthermore, Deroche-Gamonet et al. (1) report that after 3months, only a small subset of animalsbecame “addicts.” Although they all avid-ly self-administered cocaine, 41% of ratsfailed to meet any of the three diagnosticcriteria of addiction, 28% showed onlyone symptom, 14% two symptoms, and17% all three symptoms. In addition, theanimals that developed these symptomswere those that also showed a cardinalfeature of addiction: a high propensity torelapse [as indicated by reinstatement ofdrug-seeking behavior elicited by either adrug “prime” or a drug-associated cue(1)]. Also of keen interest are measuresnot associated with these symptoms of ad-diction, including measures of anxiety,“impulsivity,” and high versus low respon-

siveness to novelty (1). The researchersconclude that rats become “addicts” (i)only after extended experience with co-caine, and (ii) only if they are inherentlysusceptible.

Although extended access to cocaineled to continued drug-seeking in the face ofadverse consequences in both studies, onlyDeroche-Gamonet et al. (1) found in-creased motivation for the drug. Vander-schuren and Everitt (2), however, used avery different and less traditional proce-dure for assessing motivation for drug, andtheir measure may be less sensitive (7).Consistent with the Deroche-Gamonet etal. findings (1), long daily sessions withcontinuous access to cocaine, which leadsto escalation of intake (9), are associatedwith increased motivation for cocaine as-sessed using a progressive ratio schedule(10).

The demonstration that extended ac-cess to cocaine can lead to addiction-likebehavior in the rat raises many questions.Would daily access to even more drug accelerate this process (9)? Does thishappen with other addictive drugs? Whatdifferentiates susceptible from less sus-ceptible individuals? Do less susceptibleindividuals become susceptible if given C

RED

IT:T

AIN

A L

ITW

AK

A G

FDB

EC

H

PR

PR

Normal Rat

Addicted Rat

When more is not enough. An innovative rat model for the study of ad-

diction based on three diagnostic criteria (1, 2). Shown are rat cages, each

with a panel containing a hole through which a rat can poke its nose.

Above the hole, a green light signals that cocaine is available. If the rat

nose-pokes, it receives an intravenous injection of cocaine. (A and B)

Under usual limited-access conditions, normal rats (A) and addicted rats

(B) both self-administer cocaine at the same rate (1, 2). If given a longer

test session, however, addicted rats escalate their intake (1). (C and D) The

red light indicates that cocaine is not available. Normal rats (C) stop re-

sponding, but addicted rats (D) continue to nose-poke even though co-

caine is not delivered (1). (E and F) The green light signals that cocaine is

available, but the additional blue light either indicates that cocaine de-

livery will be accompanied by a footshock (the lightning bolt) (1) or rep-

resents a cue previously associated with a footshock (the memory of

shock) (2). Normal rats (E) decrease their responses in the presence of

the blue light, but addicted rats (F) keep responding (1, 2). (G and H) The

green light signals that cocaine is available, but it is now available on a

progressive ratio (PR) schedule where the number of responses required

for an injection is progressively increased (for example, from 10 to 20,

30, 45, 65, 85, 155). Under these conditions, addicted rats (H) work hard-

er than normal rats (G) for cocaine—that is, they show a higher “break-

ing point” (1).

13 AUGUST 2004 VOL 305 SCIENCE www.sciencemag.org

P E R S P E C T I V E S

953

more access to drug, or if exposed to, forexample, stress or different environments?How does extended access to cocainechange the brain (and only in susceptibleindividuals) to produce different symptomsof addiction? In providing more realisticpreclinical animal models of addiction thanpreviously available, the two new reportsset the stage for developing exciting new

approaches with which to unravel the psy-chology and neurobiology of addiction.

References1. V. Deroche-Gamonet, D. Belin, P. V. Piazza, Science

305, 1014 (2004).2. L. J. M. J. Vanderschuren, B. J. Everitt, Science 305,

1017 (2004).3. J. R. Weeks, Science 138, 143 (1962).4. R. A. Yokel, in Methods of Assessing the Reinforcing

Properties of Abused Drugs, M. A. Bozarth, Ed.

(Springer-Verlag, New York, 1987), pp. 1–33.5. A. E. Kelley, K. C. Berridge, J. Neurosci. 22, 3306

(2002).6. T. E. Robinson, K. C. Berridge, Annu. Rev. Psychol. 54,

25 (2003).7. J. M. Arnold, D. C. Roberts, Pharmacol. Biochem.

Behav. 57, 441 (1997).8. J. Wolffgramm, A. Heyne, Behav. Brain Res. 70, 77

(1995).9. S. H. Ahmed, G. F. Koob, Science 282, 298 (1998).

10. N. E. Paterson, A. Markou, Neuroreport 14, 2229(2003).

With even Hollywood aroused, thethermohaline circulation (THC)of the ocean has become a public

theme, and not without reason. The THChelps drive the ocean currents around theglobe and is important to the world’s

climate (see map onthis page). There is apossibility that theNorth Atlantic THCmay weaken sub-

stantially during this century, and thiswould have unpleasant effects on our cli-mate—not a disaster-movie ice age, butperhaps a cooling overparts of northern Europe.

The THC is a drivingmechanism for oceancurrents. Cooling and iceformation at high lati-tudes increase the densi-ty of surface waters suf-ficiently to cause them tosink. Several differentprocesses are involved,which collectively aretermed “ventilation.”When active, ventilationmaintains a persistentsupply of dense waters tothe deep high-latitudeoceans. At low latitudes,in contrast, vertical mix-ing heats the deep waterand reduces its density. Together, high-lati-tude ventilation and low-latitude mixingbuild up horizontal density differences inthe deep ocean, which generate forces. In

the North Atlantic, these forces help drivethe North Atlantic Deep Water (NADW)that supplies a large part of the deep watersof the world ocean.

Not everybody agrees that the THC isan important driving mechanism for theNADW flow. The north-south density dif-ferences observed at depth might be gener-ated by the flow rather than driving it (1).This argument is tempting, but it neglectssome salient features of the real ocean thatare at odds with many conceptual, analyti-cal, and even some numerical models.

The Greenland-Scotland Ridge splits the

North Atlantic into two basins (see the fig-ure on the next page). Most of the ventila-tion occurs in the northern basin, and thecold dense waters pass southward as deepoverflows across the Ridge. According tomeasurements (2–4), the total volume trans-port across the Ridge attributable to theseoverflows is only about one-third of the to-tal NADW production, but the volumetransported approximately doubles by en-trainment of ambient water within just a few

hundreds of kilometers after passing theRidge.

On their way toward the Ridge, the over-flow waters accelerate to current speeds ofmore than 1 m/s, which is clear evidence ofTHC forcing. After crossing the Ridge, theflows descend to great depths in bottomcurrents, which again are density-driven. Inthe present-day ocean, THC drives the over-flows, which together with the entrainedwater feed most of the NADW.

This is the reason why people worryabout a possible weakening of the THC. Inthe coming decades, global change via at-mospheric pathways is expected to increasethe freshwater supply to the Arctic. Thiswill reduce the salinity and hence the den-sity of surface waters, and thereby may re-duce ventilation. Even if the ventilationcomes to a total halt, this will not stop theoverflows immediately, because the reser-voir of dense water north of the Ridge sta-bilizes the overflow. Instead, the supply of

NADW would diminish ina matter of decades. Incontrast, large changes inlow-latitude mixing—even if conceivable—re-quire a much longer timebefore affecting the THC(5).

A potential weakeningof the North Atlantic THCwould affect the deep wa-ters of the world ocean inthe long run, but wouldhave more immediate ef-fects on the climate insome regions. The denseoverflow waters feedingthe deep Atlantic are re-plenished by a compen-sating northward flow in

the upper layers. These currents bringwarm saline water northward to the regionswhere ventilation and entrainment occur.This oceanic heat transport keeps largeArctic areas free of ice and parts of theNorth Atlantic several degrees warmerthan they would otherwise have been (6).

A substantially weakened THC reducesthis heat transport and regionally counter-balances global warming. In some areas, itmight even lead to cooling (7). This has in-

C L I M AT E S C I E N C E

Already the Day

After Tomorrow?Bogi Hansen, Svein Østerhus, Detlef Quadfasel, William Turrell

B. Hansen is at the Faroese Fisheries Laboratory,FO-110 Torshavn, Faroe Islands. S. Østerhus is atthe Bjerknes Center, NO-5007 Bergen, Norway.D. Quadfasel is at the Institut für Meereskunde,D-20146 Hamburg, Germany. W. Turrell is at theMarine Laboratory, Aberdeen AB11 9DB, Scotland.E-mail: bogihan@frs.fo

Enhanced online at

www.sciencemag.org/cgi/

content/full/305/5686/953

www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004

P E R S P E C T I V E S

Thermohaline circulation. Schematic map of the thermohaline circulation of the

world ocean. Purple ovals indicate ventilation areas, which feed the flow of deep dense

waters (blue lines with arrows). These waters flow into all of the oceans and slowly as-

cend throughout them. From there, they return to the ventilation areas as warm com-

pensating currents (red lines with arrows) in the upper layers.

954

spired a public debatefocused on a potentialcooling of northernEurope, which has thecompensating flowjust off the coast.Note that this part ofthe North AtlanticTHC is especiallydependent on ventila-tion north of theGreenland-ScotlandRidge, overflow, andentrainment (3).

The concept of aweakened THC issupported by somenumerical climatemodels (8), but not by all. Increased salini-ty of the compensating flow may balancethe salinity decrease from the increasedfreshwater supply and maintain ventilation(9). Climate models, so far, do not providea unique answer describing the future de-velopment of the THC, but what is thepresent observational evidence?

It is argued that early evidence forchanges should primarily be sought in theventilation and overflow rates. Indeed,some such changes have been reported.Since around 1960, large parts of theopen sea areas north of the Greenland-Scotland Ridge have freshened (10), andso have the overflows (11). At the sametime, low-latitude Atlantic waters becamemore saline in the upper layer (12), andthis is also reflected in the compensatingflow. Long-term observations in both ofthe main branches of compensating flowacross the Greenland-Scotland Ridge

have shown increasing salinity since themid-1970s, with a record high in 2003.

Even more convincing evidence for areduction of the North Atlantic THC hasbeen gained from monitoring both theoverflows and the compensating northwardflow by direct current measurements (13).For the Denmark Strait overflow, no per-sistent long-term trends in volume trans-port have been reported (2, 14), but theFaroe Bank Channel overflow was found tohave decreased by about 20% from 1950 to2000 (15).

We find evidence of freshening of theNordic Seas and a reduction of thestrength of the overflow, both of whichwill tend to weaken the North AtlanticTHC. On the other hand, the compensat-ing northward flow is getting moresaline, which may maintain ventilationand counterbalance the THC decrease.So the jury is still out. This emphasizes

the need for more refined climate mod-els and long-term observational systemsthat are capable of identifying potentialchanges in our climate system.

References1. C. Wunsch, Science 298, 1179 (2002).2. R. R. Dickson, J. Brown, J. Geophys. Res. 99, 12,319

(1994).3. B. Hansen, S. Østerhus, Prog. Oceanogr. 45,109

(2000).4. A. Ganachaud, C. Wunsch, Nature 408, 453 (2000).5. W. Munk, C.Wunsch, Deep-Sea Res. 45, 1976 (1998).6. R. Seager et al., Q. J. R. Meteorol. Soc. 128, 2563

(2002).7. M. Vellinga, R. A. Wood, Clim. Change 54, 251

(2002).8. S. Rahmstorf, Nature 399, 523 (1999).9. M. Latif et al., J. Clim. 13, 1809 (2000).

10. J. Blindheim et al., Deep-Sea Res. I 47, 655 (2000).11. R. R. Dickson et al., Nature 416, 832 (2002).12. R. Curry et al., Nature 426, 826 (2003).13. Arctic/Subarctic Ocean Fluxes (ASOF) (http://asof.

npolar.no).14. R. R. Dickson, personal communication.15. B. Hansen, W. R. Turrell, S. Østerhus, Nature 411, 927

(2001).

The cofactor nicotinamide adeninedinucleotide (NAD)—once con-signed to the oblivion of metabolic

pathway wall charts—has recently attainedcelebrity status as the link between meta-bolic activity, cellular resistance to stressor injury, and longevity. NAD influencesmany cell fate decisions—for example,NAD-dependent enzymes such as poly(ADP-ribose) polymerase (PARP) are im-portant for the DNA damage response, and

NAD-dependent protein deacetylases(Sirtuins) are involved in transcriptionalregulation, the stress response, and cellulardifferentiation. On page 1010 of this issue,Araki and colleagues (1) extend the influ-ence of NAD with their demonstration thatan increase in NAD biosynthesis or en-hanced activity of the NAD-dependentdeacetylase SIRT1 protects mouse neuronsfrom mechanical or chemical injury (2).

Axonal degeneration (termed Walleriandegeneration) often precedes the death ofneuronal cell bodies in neurodegenerativediseases such as Alzheimer’s (AD) andParkinson’s (PD). Mice carrying the spon-taneous dominant Wlds mutation show de-layed axonal degeneration following neu-

ronal injury. The Wlds mutation on mousechromosome 4 is a rare tandem triplicationof an 85-kb DNA fragment that harbors atranslocation. The translocation encodes afusion protein comprising the amino-ter-minal 70 amino acids of Ufd2a (ubiquitinfusion degradation protein 2a), an E4 ubiq-uitin ligase, and the entire coding region ofNmnat1 (nicotinamide mononucleotideadenylyltranferase 1), an NAD biosynthet-ic enzyme. Although the C57BL/Wlds

mouse was described 15 years ago (3) andexpression of the Wlds fusion protein isknown to delay Wallerian degeneration (4),the mechanism of neuroprotection has re-mained elusive. Given that proteasome in-hibitors block Wallerian degeneration bothin vitro and in vivo (5), the Ufd2a proteinfragment (a component of the ubiquitinproteasome system) has been the primecandidate for mediator of neuroprotectionin the Wlds mouse. Indeed, ubiquitin-medi-ated protein degradation by the proteasome

N E U R O S C I E N C E

NAD to the RescueAntonio Bedalov and Julian A. Simon

A. Bedalov is in the Clinical Research Division and J. A.Simon is in the Clinical Research and Human BiologyDivisions, Fred Hutchinson Cancer Research Center,Seattle, WA 98109, USA. E-mail: abedalov@fhcrc.org,jsimon@fhcrc.org

13 AUGUST 2004 VOL 305 SCIENCE www.sciencemag.org

Bottom depthCompensating flow

Upwelling

Overflowwater 0°C

Mixing

16–18 Sv

NADW 3°C

Labrador Sea

VentilationV

ent.: 4–6 Sv

6 Sv

12 Sv

Compensating flow

G r e e n l a n d

S c o t l a n d R i d g e

6 Sv

<500 m>500 m

GR

EE

NL

AN

D

Entrainm

ent6°C

EntrainmentCompensating

flow

DS-overflow

FBC-overflow

Arctic

North Atlantic flow. The exchange of water across the Greenland-Scotland Ridge is a fundamental component of the

North Atlantic THC. Arrows on the map indicate the main overflow (blue) and compensating inflow (red) branches. On

the schematic section to the right, temperatures in °C and volume transports in Sv (1 Sv = 106 m3/s) are approximate

values. DS, Denmark Strait; FBC, Faroe Bank Channel.

P E R S P E C T I V E S

955

has been identified as a potential target fordeveloping drugs to treat neurodegenera-tive diseases such as AD, PD, and multiplesclerosis (6, 7).

Araki et al. (1) developed an in vitromodel of Wallerian degeneration compris-ing cultures of primary dorsal root gan-glion neurons derived from wild-typemice. The neurons overexpressed eitherthe Wlds fusion protein or one of the fu-sion protein fragments. Surprisingly, theauthors found that overexpression of theUfd2a protein fragment alone did not de-lay degeneration of axons injured by re-moval of the neuronal cell body (transec-

tion) or treatment with the neurotoxin vincristine. In contrast, overexpression ofNmnat1 or the addition of NAD to theneuronal cultures before injury delayedaxonal degeneration in response to me-chanical or chemical damage.

It is well established that increasedexpression of NAD salvage pathwaygenes in yeast, including the yeast ho-mologs of Nmnat1 (NMA1 and NMA2),lengthens life-span and boosts resistanceto stress, an effect that depends on theNAD-dependent deacetylase Sir2 (8).Based on this observation, Araki et al.tested whether the protective effect of increased Nmnat1 expression requiredNAD-dependent deacetylase activity.Expression of small interfering RNAsthat target each of the seven Sir2 mam-malian homologs (SIRT1 through SIRT7)decreased survival of the dorsal root gan-

glion cultures after injury only whenSIRT1 expression was reduced. The sameeffect was observed when SIRT1 activitywas blocked with a small-molecule inhibitor; a SIRT1 activator, on the otherhand, boosted neuronal survival follow-ing injury. These data suggest that pro-tection against Wallerian degeneration isthe result of increased expression ofNmnat1, a rise in nuclear NAD levels,and a consequent increase in SIRT1 ac-tivity. This conclusion does not negatethe involvement of the proteasome inWallerian degeneration, but it does indi-cate that the protective effect of the Wlds

fusion protein is independent of Ufd2aactivity. Indeed, the new findings throwopen the possibility that changes in NADlevels may indirectly regulate the ubiqui-tin-proteasome system.

The enzymes SIRT1 through SIRT7 be-long to a unique enzyme class that requiresa boost in NAD levels to maintain activity,because they consume this cofactor duringdeacetylation of target proteins. Anotherenzyme that depletes cellular NAD levelsis PARP. In the presence of NAD, inhibi-tion of PARP has little effect on Walleriandegeneration; however, in the absence ofexogenous NAD, inhibition of PARP in-creases the survival of dorsal root ganglioncultures after injury (1). This suggests thatneuronal survival requires the maintenanceof adequate NAD levels, but that a boost inNAD levels beyond this point confers noadditional benefit.

In intact neurons of C57BL/Wlds mice,the Wlds fusion protein is expressed almostexclusively in the nucleus (4). In fibro-blasts (9)—and, presumably, in neurons—SIRT1 also is expressed in the nucleus.SIRT1 and other NAD-dependent deacety-lases alter gene expression by targeting hi-stone proteins as well as key nuclear tran-scription factors such as p53 (9, 10), fork-head (11, 12), and NF-κB (13). In addition,Sirtuins also deacetylate cytoplasmic pro-teins, including α-tubulin. The protectiveeffect of the Wlds fusion protein appears tobe exerted in the nucleus, because additionof NAD after removal of cell bodies in theneuronal cultures is no longer protective.This suggests that an alternative programof gene expression is initiated by elevatedNAD levels in the nucleus, leading to theproduction of protective factors that active-ly block Wallerian degeneration. The ther-apeutic implication of this finding is that itmay be possible to design neuroprotectivedrugs that boost SIRT1 activity and preventfurther neurodegeneration in diseases likeAD and PD.

The Araki et al. study (1) addresses thelong-standing question of how the Wlds

fusion protein prevents Wallerian degener-ation. As with most groundbreaking stud-ies, new questions emerge. For example,what is the direct result of increasedNmnat1 expression? Overexpression ofNmnat1 leads to increased activity of thisenzyme but does not change total NADlevels or the ratio of NAD to NADH, rais-ing the possibility that increased Nmnat1activity may result in a decrease in nicoti-namide or other inhibitory molecules. It ispossible that the relevant target of SIRT1’sneuroprotective activity may be a tran-scription factor that responds to changes inthe cell’s metabolic state by switching onexpression of genes that encode neuropro-tective proteins. Identifying the targets ofSIRT1 that mediate the neuroprotective ef-fect may broaden the options for therapeu-tic intervention in AD, PD, and other neu-rodegenerative diseases.

References1. T. Araki, Y. Sasaki, J. Milbrandt, Science 305, 1010

(2004).2. A. Waller, Philos. Trans. R Soc. London 140, 423

(1850).3. E. R. Lunn et al., Eur. J. Neurosci. 1, 27 (1989).4. T. G. Mack et al., Nature Neurosci. 4, 1199 (2001).5. Q. Zhai et al., Neuron, 39, 217 (2003).6. M. P. Coleman, V. H. Perry, Trends Neurosci. 25, 532

(2002).7. A. Sadaji, B. L. Schneider, P. Aebischer, Curr. Biol. 14,

326 (2004).8. R. M. Anderson et al., J. Biol. Chem. 277, 18881

(2002).9. H. Vaziri et al., Cell 107, 149 (2001).

10. J. Luo et al., Cell 107, 137 (2001).11. A. Brunet et al., Science 303, 2011 (2004).12. M. C. Motta et al., Cell 116, 551 (2004).13. F. Yeung et al., EMBO J. 23, 2369 (2004).C

RED

IT:K

AT

HA

RIN

E S

UT

LIFF

/SC

IEN

CE

AWild-type

mouse

BWldS

mouse

CWldS

protein

Injury

Injury

Ufd2a Nmnat1

Fast

Slow

Cell body

Ubiquitinproteosome

system

?

? NADsalvagepathway

SIRT1 Walleriandegeneration

Axon

Energizing neuroprotection. (A) In wild-type mice, axons of injured neurons rapidly degenerate

(Wallerian degeneration) in a process that may be relevant to the neurodegeneration seen in dis-

eases like AD and PD. (B) In mice with the Wlds dominant mutation (a tandem triplication of a re-

gion on mouse chromosome 4), injured neurons show a delay in Wallerian degeneration due to ac-

tivity of the Wlds fusion protein. (C) The fusion protein consists of the amino terminus of Ufd2a

(an E4 ubiquitin-conjugating enzyme) and the entire sequence of Nmnat1 (an enzyme in the NAD

salvage pathway). Neuroprotection in the Wlds mouse may result from increased synthesis of

NAD, leading to a concomitant increase in the activity of the NAD-dependent deacetylase, SIRT1,

which may activate a transcription factor that induces expression of genes involved in neuropro-

tection (1).

www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004

P E R S P E C T I V E S

957www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004

T O W A R D A H Y D R O G E N E C O N O M Y SP

EC

IAL

SE

CT

ION

Seen up close, hydrogen looks like a recipe for success. Small and simple—one proton andone electron in its most common atomic form—hydrogen was the first element to assem-ble as the universe cooled off after the big bang, and it is still the most widespread. It ac-counts for 90% of the atoms in the universe, two-thirds of the atoms in water, and a fairproportion of the atoms in living organisms and their geologic legacy, fossil fuels.

To scientists and engineers, those atoms offer both promise and frustration. Highlyelectronegative, they are eager to bond, and they release energy generously when they do. That makesthem potentially useful, if you can find them. On Earth, however, unattached hydrogen is vanishingly

rare. It must be liberated by breaking chemical bonds, which requires energy. Once released, theatoms pair up into two-atom molecules, whose dumbbell-shaped electron clouds are so well bal-anced that fleeting charge differences can pull them into a liquid only at a frigid –252.89° Cel-sius, 20 kelvin above absolute zero. The result, at normal human-scale temperatures, is an invisi-ble gas: light, jittery, and slippery; hard to store, transport, liquefy, and handle safely; and capableof releasing only as much energy as human beings first pump into it. All of which indicates thatusing hydrogen as a common currency for an energy economy will be far from simple. The pa-pers and News stories in this special section explore some of its many facets.

Consider hydrogen’s green image. As a manufactured product, hydrogen is only as cleanor dirty as the processes that produce it in the first place. Turner (p.972) describes various options for large-scale hydrogen production inhis Viewpoint. Furthermore, as News writer Service points out (p.958), production is just one of many technologies that must matureand mesh for hydrogen power to become a reality, a fact that leadsmany experts to urge policymakers to cast as wide a net as possible.

In some places, the transition to hydrogen may berelatively straightforward. For her News story (p.966), Vogel visited Iceland, whose abundant naturalenergy resources have given it a clear head start.Elsewhere, though, various technological detoursand bridges may lie ahead. The Viewpoint byDemirdöven and Deutch (p. 974) and Cho’s Newsstory (p. 964) describe different intermediate tech-nologies that may shape the next generation of auto-mobiles. Meanwhile, the f ires of the fossilfuel–based “carbon economy” seem sure to burn in-tensely for at least another half-century or so [see theEditorial by Kennedy (p. 917)]. Service’s News storyon carbon sequestration (p. 962) and Pacala and So-colow’s Review (p. 968) explore strategies—includ-ing using hydrogen—for mitigating their effects.

Two generations down the line, the world may end up with a hydrogeneconomy completely different from the one it expected to develop. Perhapsthe intermediate steps on the road to hydrogen will turn out to be the destina-tion. The title we chose for this issue—Toward a Hydrogen Economy—reflects that basic uncertainty and the complexity of what is sure to be a long,scientifically engaging journey.

–ROBERT COONTZ AND BROOKS HANSON

Not So Simple

T I T L E O F S P E C I A L S E C T I O N

I N T R O D U C T I O N

C O N T E N T S

N E W S958 The Hydrogen Backlash

962 The Carbon ConundrumChoosing a CO2 Separation

Technology

964 Fire and ICE: Revving Up for H2

966 Will the Future Dawn in theNorth?

Can the Developing World Skip

Petroleum?

R E V I E W968 Stabilization Wedges: Solving

the Climate Problem for theNext 50 Years with CurrentTechnologies

S. Pacala and R. Socolow

V I E W P O I N T S972 Sustainable Hydrogen

Production J. A. Turner

974 Hybrid Cars Now, Fuel CellCars Later

N. Demirdöven and J. Deutch

See also related Editorial on p. 917.

H1

1.008

1IA1A

2IIA2A

3IIIB3B

4IVB4B

5VB5B

Li6.941

3

Be9.012

4

Na22.99

11

Mg24.31

12

K39.10

19

Ca40.08

20

Sc44.96

21

Rb85.47

37

Sr87.62

38

Y88.91

39

Ti47.88

22

Zr91.22

40

V50.94

23

Nb92.91

41

PE

RIO

DIC

TA

BL

EP

ER

IOD

IC T

AB

LE

13 AUGUST 2004 VOL 305 SCIENCE www.sciencemag.org958

In the glare of a July afternoon, the HydroGen3minivan threaded through the streets nearCapitol Hill. As a Science staffer put itthrough its stop-and-go paces, 200 fuel cellsunder the hood of the General Motors proto-type inhaled hydrogen molecules, strippedoff their electrons, and fed current to theelectric engine. The only emissions: a littleextra heat and humidity. The result was asmooth, eerily quiet ride—one that, withH3’s priced at $1 million each, workingjournalists won’t be repeating at their ownexpense anytime soon.

Hydrogen-powered vehicles may berareties on Pennsylvania Avenue, but inWashington, D.C., and other world capitalsthey and their technological kin are verymuch on people’s minds. Switching fromfossil fuels to hydrogen could dramaticallyreduce urban air pollution, lower depend-ence on foreign oil, and reduce the buildupof greenhouse gases that threaten to triggersevere climate change.

With those perceived benefits in view,the United States, the European Union,Japan, and other governments have sunk bil-lions of dollars into hydrogen initiativesaimed at revving up the technology and pro-pelling it to market. Car and energy compa-nies are pumping billions more into buildingdemonstration fleets and hydrogen fuelingstations. Many policymakers see the movefrom oil to hydrogen as manifest destiny,challenging but inevitable. In a recentspeech, Spencer Abraham, the U.S. secre-tary of energy, said such a transformationhas “the potential to change our country ona scale of the development of electricity andthe internal combustion engine.”

The only problem is that the bet on thehydrogen economy is at best a long shot.Recent reports from the U.S. NationalAcademy of Sciences (NAS) and theAmerican Physical Society (APS) concludethat researchers face daunting challenges infinding ways to produce and store hydrogen,

convert it to electricity,supply it to consumers,and overcome vexing safe-ty concerns. Any of thosehurdles could block abroad-based changeover.Solving them simultane-ously is “a very tall order,”says Mildred Dresselhaus,a physicist at the Massa-chusetts Institute of Tech-nology (MIT), who hasserved on recent hydrogenreview panels with theU.S. Department of Ener-gy (DOE) and APS as wellas serving as a reviewerfor the related NAS report.

As a result, the transition to a hydrogeneconomy, if it comes at all, won’t happensoon. “It’s very, very far away from substan-tial deployed impact,” says Ernest Moniz, aphysicist at MIT and a former undersecretaryof energy at DOE. “Let’s just say decades,and I don’t mean one or two.”

In the meantime, some energy researcherscomplain that, by skewing research towardcostly large-scale demonstrations of technol-ogy well before it’s ready for market, govern-ments risk repeating a pattern that has sunkprevious technologies such as synfuels in the1980s. By focusing research on technologiesthat aren’t likely to have a measurable impactuntil the second half of the century, the cur-rent hydrogen push fails to address the grow-ing threat from greenhouse gas emissionsfrom fossil fuels. “There is starting to besome backlash on the hydrogen economy,”says Howard Herzog, an MIT chemical engi-neer. “The hype has been way overblown. It’sjust not thought through.”

A perfect choice?Almost everyone agrees that producing a viable hydrogen economy is a worthy long-term goal. For starters, worldwide oil produc-tion is expected to peak within the next fewdecades, and although supplies will remainplentiful long afterward, oil prices are expect-ed to soar as international markets view thefuel as increasingly scarce. Natural gas pro-duction is likely to peak a couple of decadesafter oil. Coal, tar sands, and other fossil fuelsshould remain plentiful for at least anothercentury. But these dirtier fuels carry a steepenvironmental cost: Generating electricityfrom coal instead of natural gas, for example,releases twice as much carbon dioxide (CO2).And in order to power vehicles, they must be C

RED

IT:(

TO

P T

O B

OT

TO

M)

GEO

RG

E B

.DIE

BO

LD/C

OR

BIS

;PH

OTO

CO

UR

TESY

OF

GM

CO

RP.

T O W A R D A H Y D R O G E N E C O N O M Y

SP

EC

IAL

SE

CT

ION

N E W S

The Hydrogen BacklashAs policymakers around the world evoke grand visions of a hydrogen-fueled future, many experts say that a broader-based, nearer-term energypolicy would mark a surer route to the same goals

H

converted to a liquid or gas, which requiresenergy and therefore raises their cost.

Even with plenty of fossil fuels available,it’s doubtful we’ll want to use them all.Burning fossil fuels has already increasedthe concentration of CO2 in the atmospherefrom 280 to 370 parts per million (ppm)over the past 150 years. Unchecked, it’s ex-pected to pass 550 ppm this century, accord-ing to New York University physicist MartinHoffert and colleagues in a 2002 Sciencepaper (Science, 1 November 2002, p. 981).“If sustained, [it] could eventually produceglobal warming comparable in magnitudebut opposite in sign to the global cooling ofthe last Ice Age,” the authors write. Devel-opment and population growth can only aggravate the problems.

On the face of it, hydrogen seems likethe perfect alternative. When burned, or ox-idized in a fuel cell, it emits no pollution,including no greenhouse gases. Gram forgram, it releases more energy than any oth-er fuel. And as a constituent of water, hydrogen is all around us. No wonder it’sbeing touted as the clean fuel of the futureand the answer to modern society’s addic-tion to fossil fuels. In April 2003, Wiredmagazine laid out “How Hydrogen CanSave America.” Environmental gadfly Jeremy Rifkin has hailed the hydrogeneconomy as the next great economicrevolution. And General Motors has an-nounced plans to be the first companyto sell 1 million hydrogen fuel cell carsby the middle of the next decade.

Last year, the Bush Administra-tion plunged in, launching a 5-year,$1.7 billion initiative to commercializehydrogen-powered cars by 2020. InMarch, the European Commissionlaunched the first phase of an expected10-year, €2.8 billion public-private part-nership to develop hydrogen fuel cells.Last year, the Japanese government near-ly doubled its fuel cell R&D budget to$268 million. Canada, China, and othercountries have mounted efforts of theirown. Car companies have already spentbillions of dollars trying to reinvent theirwheels—or at least their engines—to run onhydrogen: They’ve turned out nearly 70 proto-type cars and trucks as well as dozens of bus-es. Energy and car companies have addedscores of hydrogen fueling stations worldwide,with many more on the drawing boards (see p.964). And the effort is still gaining steam.

The problem of priceStill, despite worthwhile goals and good intentions, many researchers and energy experts say current hydrogen programs fallpitifully short of what’s needed to bring ahydrogen economy to pass. The world’s

energy infrastructure is too vast, they say,and the challenges of making hydrogentechnology competitive with fossil fuels toodaunting unless substantially more funds areadded to the pot. The current initiatives arejust “a start,” Dresselhaus says. “None ofthe reports say it’s impossible,” she adds.However, Dresselhaus says, “the problem isvery difficult no matter how you slice it.”

Economic and political diff icultiesabound, but the most glaring barriers aretechnical. At the top of the list: finding asimple and cheap way to produce hydrogen.As is often pointed out, hydrogen is not afuel in itself, as oil and coal are. Rather, likeelectricity, it’s an energy carrier that must begenerated using another source of power.Hydrogen is the most common element inthe universe. But on Earth, nearly all of it isbound to other elements in molecules, suchas hydrocarbons and water. Hydrogen atomsmust be split off these molecules to generatedihydrogen gas (H2), the form it needs to bein to work in most fuel cells. These devicesthen combine hydrogen and oxygen to makewater and liberate electricity in the process.But every time a fuel is converted from one

source, such as oil, to another, such as elec-tricity or hydrogen, it costs energy andtherefore money.

Today, by far the cheapest way to pro-duce hydrogen is by using steam and cata-lysts to break down natural gas into H2 andCO2. But although the technology has beenaround for decades, current steam reform-ers are only 85% efficient, meaning that15% of the energy in natural gas is lost aswaste heat during the reforming process.The upshot, according to Peter Devlin, whoruns a hydrogen production program atDOE, is that it costs $5 to produce the

amount of hydrogen that releases as muchenergy as a gallon of gasoline. Currenttechniques for liberating hydrogen fromcoal, oil, or water are even less efficient.Renewable energy such as solar and windpower can also supply electricity to splitwater, without generating CO2. But thosetechnologies are even more expensive.Generating electricity with solar power, forexample, remains 10 times more expensivethan doing so with a coal plant. “The energy in hydrogen will always be moreexpensive than the sources used to makeit,” said Donald Huberts, chief executiveofficer of Shell Hydrogen, at a hearing before the U.S. House Science Committeein March. “It will be competitive only byits other benefits: cleaner air, lower green-house gases, et cetera.”

The good news, Devlin says, is that pro-duction costs have been coming down,dropping about $1 per gallon ($0.25/liter) ofgasoline equivalent over the past 3 years.The trouble is that DOE’s own road mapprojects that drivers will buy hydrogen-powered cars only if the cost of the fueldrops to $1.50 per gallon of gasoline equiv-

alent by 2010 and even lower in the yearsbeyond. “The easy stuff is over,” says Devlin. “There are going to have to be somefundamental breakthroughs to get to $1.50.”

There are ideas on the drawing board. Inaddition to stripping hydrogen from fossilfuels, DOE and other funding agencies arebacking innovative research ideas to producehydrogen with algae, use sunlight and cata-lysts to split water molecules directly, andsiphon hydrogen from agricultural waste andother types of “biomass.” Years of researchin all of these areas, however, have yet toyield decisive progress.

959www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004

T O W A R D A H Y D R O G E N E C O N O M Y SP

EC

IAL

SE

CT

ION

12,000

World Energy Production

OilO Gas uclearNu Comb. renew. & wasteHydro Geothermal/solar/wind

Over a barrel. The world is growing increasingly dependent on fossil fuels.

SO

UR

CE:I

NT

ER

NA

TIO

NA

L EN

ER

GY

AG

EN

CY

13 AUGUST 2004 VOL 305 SCIENCE www.sciencemag.org960

To have and to hold If producing hydrogen cheaply has researchers scratching their heads, storingenough of it on board a car has them posi-tively stymied. Because hydrogen is thelightest element, far less of it can fit into agiven volume than other fuels. At room temperature and pressure, hydrogen takes uproughly 3000 times as much space as gaso-line containing the same amount of energy.That means storing enough of it in a fueltank to drive 300 miles (483 kilometers)—DOE’s benchmark—requires either com-pressing it, liquefying it, or using some oth-er form of advanced storage system.

Unfortunately, none of these solutions isup to the task of carrying a vehicle 300miles on a tank. Nearly all of today’s proto-type hydrogen vehicles use compressed gas.But these are still bulky. Tanks pressurizedto 10,000 pounds per square inch (70 MPa)take up to eight times the volume of a cur-rent gas tank to store the equivalent amountof fuel. Because fuel cells are twice as effi-cient as gasoline internal combustion en-gines, they need fuel tanks four times aslarge to propel a car the same distance.

Liquid hydrogen takes up much less roombut poses other problems. The gas liquefies at

–253°C, just a few degrees above absolutezero. Chilling it to that temperature requiresabout 30% of the energy in the hydrogen.And the heavily insulated tanks needed tokeep liquid fuel from boiling away are stilllarger than ordinary gasoline tanks.

Other advanced materials are also beinginvestigated to store hydrogen, such as car-bon nanotubes, metal hydrides, and sub-stances such as sodium borohydride thatproduce hydrogen by means of a chemicalreaction. Each material has shown some

promise. But for now, each still has fataldrawbacks, such as requiring high tempera-ture or pressures, releasing the hydrogen tooslowly, or requiring complex and time-consuming materials recycling. As a result,many experts are pessimistic. A report lastyear from DOE’s Basic Energy SciencesAdvisory Committee concluded: “A newparadigm is required for the development ofhydrogen storage materials to facilitate a hydrogen economy.” Peter Eisenberger, viceprovost of Columbia University’s Earth Institute, who chaired the APS report, iseven more blunt. “Hydrogen storage is a potential showstopper,” he says.

Breakthroughs neededAnother area in need of serious progress isthe fuel cells that convert hydrogen to elec-tricity. Fuel cells have been around since the1800s and have been used successfully fordecades to power spacecraft. But their highcost and other drawbacks have kept themfrom being used for everyday applicationssuch as cars. Internal combustion enginestypically cost $30 for each kilowatt of powerthey produce. Fuel cells, which are loadedwith precious-metal catalysts, are 100 timesmore expensive than that.

If progress on renew-able technologies is anyindication, near-termprospects for cheap fuelcells aren’t bright, saysJoseph Romm, formeracting assistant secre-tary of energy for re-newable energy in theClinton Administrationand author of a recentbook, The Hype AboutHydrogen: Fact andFiction in the Race toSave the Climate. “Ithas taken wind powerand solar power eachabout twenty years tosee a tenfold decline inprices, after major gov-ernment and privatesector investments, andthey still each comprise

well under 1% of U.S. electricity genera-tion,” Romm said in written testimony inMarch before the House Science Commit-tee reviewing the Administration’s hydro-gen initiative. “A major technology break-through is needed in transportation fuelcells before they will be practical.” Varioustechnical challenges—such as making fuelcells rugged enough to withstand theshocks of driving and ensuring the safetyof cars loaded with flammable hydrogengas—are also likely to make hydrogen cars

costlier to engineer and slower to win pub-lic acceptance.

If they clear their internal technical hurdles, hydrogen fuel cell cars face an obstacle from outside: the infrastructurethey need to refuel. If hydrogen is generat-ed in centralized plants, it will have to betrucked or piped to its final destination. Butbecause of hydrogen’s low density, it wouldtake 21 tanker trucks to haul the amount ofenergy a single gasoline truck delivers today, according to a study by Switzerland-based energy researchers Baldur Eliassonand Ulf Bossel. A hydrogen tanker traveling500 kilometers would devour the equivalentof 40% of its cargo.

Ship the hydrogen as a liquid? Commer-cial-scale coolers are too energy-intensive forthe job, Eliasson and Bossel point out. Trans-porting hydrogen through long-distancepipelines wouldn’t improve matters much.Eliasson and Bossel calculate that 1.4% ofthe hydrogen flowing through a pipelinewould be required to power the compressorsneeded to pump it for every 150 kilometersthe gas must travel. The upshot, Eliasson andBossel report: “Only 60% to 70% of the hydrogen fed into a pipeline in NorthernAfrica would actually arrive in Europe.”

To lower those energy penalties, someanalysts favor making hydrogen at fuelingstations or in homes where it will be used,with equipment powered by the existingelectricity grid or natural gas. But onsiteproduction wouldn’t be cheap, either. Eliasson and Bossel calculate that to supplyhydrogen for 100 to 2000 cars per day, anelectrolysis-based fueling station would require between 5 and 81 megawatts of electricity. “The generation of hydrogen atf illing stations would make a threefold increase of electric power generating capaci-ty necessary,” they report. And at least forthe foreseeable future, that extra electricityis likely to come from fossil fuels.

Whichever approach wins out, it willneed a massive new hydrogen infrastructureto deliver the goods. The 9 million tons ofhydrogen (enough to power between 20 mil-lion and 30 million cars) that the UnitedStates produces yearly for use in gasolinerefining and chemical plants pale beside theneeds of a full-blown transportation sector.For a hydrogen economy to catch on, the fuel must be available in 30% to 50% of fill-ing stations when mass-market hydrogencars become available, says Bernard Bulkin,former chief scientist at BP. A recent studyby Marianne Mintz and colleagues at Argonne National Laboratory in Illinoisfound that creating the infrastructure neededto fuel 40% of America’s cars would cost astaggering $500 billion or more.

Energy and car companies are unlikely CR

ED

IT:A

DA

PT

ED

FR

OM

P.G

UA

Y,M

OD

ÉLIS

AT

ION

MO

NT

E-C

AR

LO D

E L'

AD

SO

RP

TIO

N D

E L'

HY

DR

OG

ÈNE

DA

NS

LES

NA

NO

ST

RU

CT

UR

ES D

E C

AR

BO

NE,

INR

S-E

MT,M

ON

TR

ÉA

L,2

00

3;(

IMA

GE)

RO

YA

LTY-F

REE/C

OR

BIS

T O W A R D A H Y D R O G E N E C O N O M Y

SP

EC

IAL

SE

CT

ION

Liquid hydrogen

High-temperature metal hydrides

Low-temperature metal hydrides

Compressed hydrogen

Diesel

GasolineDOE target

Chemical storage (NaBH4)

200

100

80

60

40

20

10

8

6

Volu

met

ric

den

sity

(kg

/m3 )

Gravimetric density (% weight H2)1.0 2 4 6 8 10 20

Showstopper? Current hydrogen storage technologies fall short of both

the U.S. Department of Energy target and the performance of petroleum.

to spend such sums unless they know mass-produced hydrogen vehicles are on the way.Carmakers, however, are unlikely to buildfleets of hydrogen vehicles without stationsto refuel them. “We face a ‘chicken andegg’ problem that will be difficult to over-come,” said Michael Ramage, a former executive vice president of ExxonMobil Research and Engineering, who chaired theNAS hydrogen report, when the report wasreleased in February.

Stress testEach of the problems facedby the hydrogen economy—production, storage, fuelcells, safety, and infrastruc-ture—would be thornyenough on its own. For a hy-drogen economy to succeed,however, all of these chal-lenges must be solved simul-taneously. One loose end andthe entire enterprise could un-ravel. Because many of thesolutions require fundamentalbreakthroughs, many U.S. re-searchers question their coun-try’s early heavy emphasis onexpensive demonstrationprojects of fuel cell cars, fuel-ing stations, and other technologies.

To illustrate the dangers of that approach,the APS report cites the fate of synfuels re-search in the 1970s and ’80s. President Ger-ald Ford proposed that effort in 1975 as a re-sponse to the oil crisis of the early 1970s.But declining oil prices in the 1980s and un-met expectations from demonstration proj-ects undermined industrial and congression-al support for the technology. For hydrogen,the report’s authors say, the “enormous per-formance gaps” between existing technologyand what is needed for a hydrogen economyto take root means that “the program needssubstantially greater emphasis on solving thefundamental science problems.”

Focusing the hydrogen program on basicresearch will naturally give it the appropriatelong-term focus it deserves, Romm and others believe. In the meantime, they say, thefocus should be on slowing the buildup ofgreenhouse gases. “If we fail to limit green-house gas emissions over the next decade—and especially if we fail to do so because wehave bought into the hype about hydrogen’snear-term prospects—we will be making anunforgivable national blunder that may lockin global warming for the U.S. of 1 degreeFahrenheit [0.56°C] per decade by mid-century,” Romm told the House ScienceCommittee in March in written testimony.

To combat the warming threat, fundingagencies should place a near-term priority on

promoting energy efficiency, research on re-newables, and development of hybrid cars,critics say. After all, many researchers pointout, as long as hydrogen for fuel cell cars isprovided from fossil fuels, much the sameenvironmental benefits can be gained byadopting hybrid gasoline-electric and advanced diesel engines. As MIT chemistand former DOE director of energy researchJohn Deutch and colleagues point out onpage 974, hybrid electric vehicles—a tech-nology already on the market—would im-

prove energy efficiencyand reduce greenhousegas emissions almost aswell as fuel cell vehiclesthat generate hydrogenfrom an onboard gasolinereformer, an approachthat obviates the need forbuilding a separate hydro-gen infrastructure.

Near-term help mayalso come from capturingCO2 emissions from pow-er and industrial plantsand storing them underground, a processknown as carbon sequestration (see p. 962).Research teams from around the world arecurrently testing a variety of schemes for do-ing that. But the process remains significant-ly more expensive than current energy. “Untilan economical solution to the sequestrationproblem is found, net reductions in overallCO2 emissions can only come through ad-vances in energy efficiency and renewableenergy,” the APS report concludes.

In response to the litany of concernsover making the transition to a hydrogeneconomy, JoAnn Milliken, who heads hy-drogen-storage research for DOE, pointsout that DOE and other funding agenciesaren’t promoting hydrogen to the exclusionof other energy research. Renewable ener-gy, carbon sequestration, and even fusion

energy all remain in the research mix. Criti-cism that too much is being spent ondemonstration projects is equally misguid-ed, she says, noting that such projects makeup only 13% of DOE’s hydrogen budget,compared with 85% for basic and appliedresearch. Both are necessary, she says:“We’ve been doing basic research onhydrogen for a long time. We can’t just doone or the other.” Finally, she points out,funding agencies have no illusions aboutthe challenge in launching the hydrogeneconomy. “We never said this is going to beeasy,” Milliken says. The inescapable truthis that “we need a substitute for gasoline.Gas hybrids are going to improve fuel econ-omy. But they can’t solve the problem.”

Yet, if that’s the case, many energy ex-perts argue, governments should be spendingfar more money to lower the technical andeconomic barriers to all types of alternativeenergy—hydrogen included—and bring it toreality sooner. “Energy is the single most im-portant problem facing humanity today,”says Richard Smalley of Rice University in

Houston, Texas, a1996 Nobel laureatein chemistry who hasbeen campaigning forincreased energy sci-ences funding for thelast 2 years. AmongSmalley’s proposals:a 5-cent-per-gallontax on gasoline in theUnited States to fund$10 billion annuallyin basic energy sci-ences research. Be-cause of the combi-n a t ion of c l imatechange and the soon-to-be-peaking pro-duction in fossil fu-els, Smalley says, “it

really ought to be the top project in world-wide science right now.”

Although not all researchers are willingto wade into the political minef ield ofbacking a gasoline tax, few disagree withhis stand. “I think he’s right,” Dresselhaussays of the need to boost the priority of ba-sic energy sciences research. With respectto the money needed to take a realistic stabat making an alternative energy economy areality, Dresselhaus says: “Most re-searchers think there isn’t enough moneybeing spent. I think the investment is pret-ty small compared to the job that has to bedone.” Even though it sounds like a no-brainer, the hydrogen economy will takeabundant gray matter and greenbacks tobring it to fruition.

–ROBERT F. SERVICE

961www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004

CR

ED

ITS:(

LEFT

TO

RIG

HT

) M

L SIN

IBA

LDI/

CO

RB

IS;P

HO

TO

S.C

OM

T O W A R D A H Y D R O G E N E C O N O M Y SP

EC

IAL

SE

CT

ION

CO2 free.. To be a clean energy technology,

hydrogen must be generated from wind,

solar, or other carbon-free sources.

Even if the hydrogen economy were techni-cally and economically feasible today, wean-ing the world off carbon-based fossil fuelswould still take decades. During that time,carbon combustion will continue to pourgreenhouse gases into the atmosphere—un-less scientists find a way to reroute them.Governments and energy companies aroundthe globe have launched numerous large-scale research and demonstration projects tocapture and store, or sequester, unwanted car-bon dioxide (see table). Although final resultsare years off, so far the testsappear heartening. “It seemsto look more and morepromising all the time,” saysSally Benson, a hydrogeolo-gist at Lawrence BerkeleyNational Laboratory in Cali-fornia. “For the first time, Ithink the technical feasibili-ty has been established.”

Last hope?Fossil fuels account formost of the 6.5 billion tons(gigatons) of carbon—theamount present in 25 giga-tons of CO2—that peoplearound the world vent intothe atmosphere every year.And as the amount of thegreenhouse gas increases,so does the likelihood oftriggering a debilitatingchange in Earth’s climate.Industrialization has already raised atmos-pheric CO2 levels from 280 to 370 parts permillion, which is likely responsible for alarge part of the 0.6°C rise in the averageglobal surface temperature over the past cen-tury. As populations explode and economiessurge, global energy use is expected to riseby 70% by 2020, according to a report lastyear from the European Commission, muchof it to be met by fossil fuels. If projectionsof future fossil fuel use are correct and noth-ing is done to change matters, CO2 emis-sions will increase by 50% by 2020.

To limit the amount of CO2 pumped intothe air, many scientists have argued for cap-turing a sizable fraction of that CO2 fromelectric plants, chemical factories, and thelike and piping it deep underground. InJune, Ronald Oxburgh, Shell’s chief in theUnited Kingdom, called sequestration es-

sentially the last best hope to combat cli-mate change. “If we don’t have sequestra-tion, then I see very little hope for theworld,” Oxburgh told the British newspaperThe Guardian.

Although no one has adopted the strate-gy on a large scale, oil companies have beenpiping CO2 underground for decades to ex-tract more oil from wells by reducing theviscosity of underground oil. Because theyweren’t trying to maximize CO2 storage,companies rarely tracked whether the CO2

remained underground or caused unwantedside effects.

That began to change in the early 1990s,when researchers began to consider seques-tering CO2 to keep it out of the atmosphere.The options for doing so are limited, saysRobert Kane, who heads carbon-sequestrationprograms at the U.S. Department of Energyin Washington, D.C. You can grow plantsthat consume CO2 to fuel their growth, orpipe the gas to the deep ocean or under-ground. But planted vegetation can burn orbe harvested, ultimately returning the CO2back into the atmosphere. And placing vastamounts of CO2 into the ocean creates anacidic plume, which can wreak havoc ondeep-water ecosystems (Science, 3 August2001, p. 790). As a result, Kane and otherssay, much recent research has focused onstoring the CO2 underground in depleted oil

and gas reservoirs, coal seams that are toodeep to mine, and underground pockets ofsaltwater called saline aquifers.

“Initially, it sounded like a wild idea,”Benson says, in part because the volume ofgas that would have to be stored is enor-mous. For example, storing just 1 gigaton ofCO2—about 4% of what we vent annuallyworldwide—would require moving 4.8 mil-lion cubic meters of gas a day, equivalent toabout one-third the volume of all the oilshipped daily around the globe. But earlystudies suggest that there is enough under-ground capacity to store hundreds of years’worth of CO2 injection, and that potential

underground storagesites exist worldwide.According to Benson,studies in the mid-1990spegged the undergroundstorage capacity between1000 and 10,000 giga-tons of CO2. More de-tailed recent analyses arebeginning to convergearound the middle ofthat range, Benson says.But even the low end iscomfortably higher thanthe 25 gigatons of CO2humans produce eachyear, she notes.

To test the technicalfeasibility, researchershave recently begunteaming up with oil andgas companies to studytheir CO2 piping proj-ects. One of the f irst,

and the biggest, is the Weyburn project inSaskatchewan, Canada. The site is home toan oil field discovered in 1954. Since then,about one-quarter of the reservoir’s oil hasbeen removed, producing 1.4 billion barrels.In 1999, the Calgary-based oil company EnCana launched a $1.5 billion, 30-year ef-fort to pipe 20 million metric tons of CO2into the reservoir after geologists estimatedthat it would increase the field’s yield by an-other third. For its CO2, EnCana teamed upwith the Dakota Gasification Co., which op-erates a plant in Beulah, North Dakota, thatconverts coal into a hydrogen-rich gas usedin industry and that emits CO2 as a byprod-uct. EnCana built a 320-km pipeline to car-ry pressurized CO2 to Weyburn, where it’sinjected underground.

In September 2000, EnCana began inject-ing an estimated 5000 metric tons of CO2 a

En route to hydrogen, the world will have to burn huge amounts of fossilfuels—and find ways to deal with their climate-changing byproducts

13 AUGUST 2004 VOL 305 SCIENCE www.sciencemag.org962

CR

ED

IT:S

TATO

IL

T O W A R D A H Y D R O G E N E C O N O M Y

SP

EC

IAL

SE

CT

ION

Burial at sea.. A million tons a year of CO2 from the Sleipner natural-gas field in the

North Sea are reinjected underground.

HN E W S

The Carbon Conundrum

963www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004

CR

ED

IT:(

BO

TTO

M)

RO

YA

LTY-F

REE/C

OR

BIS

SP

EC

IAL

SE

CT

ION

day 1500 meters beneath the sur-face. The technology essentiallyjust uses compressors to forcecompressed CO2 down a longpipe drilled into the undergroundreservoir. To date, nearly 3.5 mil-lion metric tons of CO2 havebeen locked away in the Wey-burn reservoir.

When the project began, theUnited States was still party tothe Kyoto Protocol, the inter-national treaty designed to re-duce greenhouse gas emissions. So the Unit-ed States, Canada, the European Union, andothers funded $28 million worth of model-ing, monitoring, and geologic studies to trackthe fate of Weyburn’s underground CO2.

For the first phase of that study, whichended in May, 80 researchers including geologists and soil scientists monitored thesite for 4 years. “The short answer is it’sworking,” says geologist and Weyburn teammember Ben Rostron of the University ofAlberta in Edmonton: “We’ve got no evi-dence of significant amounts of injectedCO2 coming out at the surface.” That waswhat they expected, Rostron says: Wells aresealed and capped, and four layers of rockthought to be impermeable to CO2 lie between the oil reservoir and the surface.

A similar early-stage success story is un-der way in the North Sea off the coast ofNorway. Statoil, Norway’s largest oil com-pany, launched a sequestration pilot projectfrom an oil rig there in 1996 to avoid a $55-a-ton CO2 tax that the Norwegian govern-ment levies on energy producers. The rigtaps a natural gas field known as Sleipner,which also contains large amounts of CO2.

Normally, gas producers separate the CO2from the natural gas before feeding the latterinto a pipeline or liquefying it for transport.The CO2 is typically vented into the air. Butfor the past 8 years, Statoil has been inject-ing about 1 million tons of CO2 a year backinto a layer of porous sandstone, which liesbetween 550 and 1500 meters beneath theocean floor. Sequestering the gas costsabout $15 per ton of CO2 but saves the com-pany $40 million a year in tax.

Researchers have monitored the fate ofthe CO2 with the help of seismic imagingand other tools. So far, says Stanford Univer-sity petroleum engineer Franklin Orr, every-thing suggests that the CO2 is staying put.Fueled by these early successes, other proj-ects are gearing up as well. “One can’t helpbut be struck by the dynamism in this com-munity right now,” says Princeton Universitysequestration expert Robert Socolow. “Thereis a great deal going on.”

Despite the upbeat early reviews, mostresearchers and observers are cautious aboutthe prospects for large-scale sequestration.“Like every environmental issue, there arecertain things that happen when the quantity

increases,” Socolowsays. “We haveenough history ofgetting this [type ofthing] wrong thateveryone is wary.”

Safety tops theconcerns. AlthoughCO2 is nontoxic (itconstitutes the bub-bles in mineral waterand beer), it can bedangerous. If it per-

colates into a freshwater aquifer, it can acidifythe water, potentially leaching lead, arsenic, or other dangerous trace elements in-to the mix. If the gas rises to the subsurface, itcan affect soil chemistry. And if it should es-cape above ground in a windless depression,the heavier-than-air gas could collect and suf-focate animals or people. Although such adisaster hasn’t happened yet with sequesteredCO2, the threat became tragically clear in1986, when an estimated 80 million cubicmeters of CO2 erupted from the Lake Nyoscrater in Cameroon, killing 1800 people.

Money is another issue. Howard Herzog, an economist at the MassachusettsInstitute of Technology in Cambridge andan expert on the cost of sequestration, esti-mates that large-scale carbon sequestrationwould add 2 to 3 cents per kilowatt-hour tothe cost of electricity delivered to the consumer—about one-third the averagecost of residential electricity in the UnitedStates. (A kilowatt-hour of electricity canpower 10 100-watt light bulbs for an hour.)Says Orr: “The costs are high enough thatthis won’t happen on a big scale without anincentive structure” such as Norway’s car-bon tax or an emissions-trading programakin to that used with sulfur dioxide, acomponent of acid rain.

But although sequestration may not becheap, Herzog says, “it’s affordable.” Gener-ating electricity with coal and storing thecarbon underground still costs only about14% as much as solar-powered electricity.And unlike most renewable energy, compa-nies can adopt it more easily on a large scaleand can retrofit existing power plants andchemical plants. That’s particularly impor-tant for dealing with the vast amounts ofcoal that are likely to be burned as countriessuch as China and India modernize theireconomies. “Coal is not going to go away,”Herzog says. “People need energy, and youcan’t make energy transitions easily.” Sequestration, he adds, “gives us time to develop 22nd century energy sources.” Thatcould give researchers a window in which todevelop and install the technologies neededto power the hydrogen economy.

–ROBERT F. SERVICE

Some CO2 Sequestration Projects

Project Location Tons of CO2 Source Statusto be injected

Sleipner North Sea 20 million Gas field Ongoing

Weyburn Canada 20 million Oil field Completed phase 1

In Salah Algeria 18 million Gas field Starts 2004

Gorgon Australia 125 million Saline aquifer In preparation

Frio U.S. 3000 Saline aquifer Pilot phase

RECOPOL Poland 3000 Coal seams Ongoing

Choosing a CO2 Separation Technology

If governments move to deep-six carbon dioxide, much

of the effort is likely to target emissions from coal-fired

power plants. Industrial companies have used detergent-

like chemicals and solvents for decades to “scrub” CO2

from flue gases, a technique that can be applied to exist-

ing power plants. The downside is that the technique is

energy intensive and reduces a coal plant’s efficiency by

as much as 14%. Another option is to burn coal with

pure oxygen, which produces only CO2 and water vapor

as exhaust gases. The water vapor can then be con-

densed, leaving just the CO2. But this technology too

consumes a great deal of energy to generate the pure

oxygen in the first place and reduces a coal plant’s over-

all efficiency by about 11%. A third approach extracts CO2 from coal before combustion.

This technique is expected to be cheaper and more efficient, but it requires building

plants based on a newer technology, known as Integrated Gasification Combined Cycle.

But it will take a carbon tax or some other incentive to drive utility companies away

from proven electricity-generating technology. –R.F.S.

Dark victory .. Coal can be

made cleaner, for a price.

T O W A R D A H Y D R O G E N E C O N O M Y

In the day we sweat it out in the streets ofa runaway American dream.At night we ride through mansions ofglory in suicide machines,Sprung from cages out on highway 9,Chrome wheeled, fuel injectedand steppin’out over the line …

Fear not, sports car aficionados and BruceSpringsteen fans: Even if the hydrogeneconomy takes off, it may be decades beforezero-emission fuel cells replace yourbeloved piston-pumping, fuel-burning,song-inspiring internal combustion engine.In the meantime, however, instead of fillingyour tank with gasoline, you may be pump-ing hydrogen.

A handful of automakers are developinginternal combustion engines that run on hydrogen, which burnsmore readily than gasolineand produces almost nopollutants. If manufactur-ers can get enough ofthem on the road in thenext few years, hydrogeninternal combustion en-gine (or H2 ICE) vehiclesmight spur the constructionof a larger infrastructure forproducing and distributinghydrogen—the very sameinfrastructure that fuel cell vehicles will require.

If all goes as hoped,H2 ICE vehicles couldsolve the chicken-or-the-egg problem of whichcomes first, the fuel cellcars or the hydrogen sta-tions to fuel them, says Robert Natkin, a me-chanical engineer at Ford Motor Co. in Dear-born, Michigan. “The prime reason for doingthis is to get the hydrogen economy underway as quickly as possible,” Natkin says. Infact, some experts say that in the race to eco-nomic and technological viability, the morecumbersome, less powerful fuel cell maynever catch up to the lighter, peppier, andcheaper H2 ICE. “If the hydrogen ICEs workthe way we think they can, you may never seefuel cells” powering cars, says Stephen Ciatti,a mechanical engineer at Argonne NationalLaboratory in Illinois.

BMW, Ford, and Mazda expect to start

producing H2 ICE vehicles for governmentand commercial fleets within a few years.But to create demand for hydrogen, thosecars and trucks will have to secure a nichein the broader consumer market, and thatwon’t be a drive in the countryside. The car-makers have taken different tacks to keepinghydrogen engines running smoothly andstoring enough hydrogen onboard a vehicleto allow it to wander far from a fueling sta-tion, and it remains to be seen which ap-proach will win out. And, of course, H2 ICEvehicles will require fueling stations, andmost experts agree that the public will haveto help pay for the first ones.

Most important, automakers will have toanswer a question that doesn’t lend itself tosimple, rational analysis: At a time whengasoline engines run far cleaner than they

once did and sales of gas-guzzling sportutility vehicles continue to grow in spite ofrising oil prices, what will it take to put theaverage driver behind the wheel of an exotichydrogen-burning car?

Running lean and greenAn internal combustion engine draws itspower from a symphony of tiny explosionsin four beats. Within an engine, pistons slideup and down within snug-fitting cylinders.First, a piston pushes up into its cylinder tocompress a mixture of air and fuel. Whenthe piston nears the top of its trajectory, thesparkplug ignites the vapors. Next, the ex-

plosion pushes the piston back down, turn-ing the engine’s crankshaft and, ultimately,the wheels of the car. Then, propelled by in-ertia and the other pistons, the piston pushesup again and forces the exhaust from the ex-plosion out valves in the top of the cylinder.Finally, the piston descends again, drawing afresh breath of the air-fuel mixture into thecylinder through a different set of valvesand beginning the four-stroke cycle anew.

A well-tuned gasoline engine mixes fueland air in just the right proportions to ensurethat the explosion consumes essentially everymolecule of fuel and every molecule of oxy-gen—a condition known as “running at stoichiometry.” Of course, burning gasolineproduces carbon monoxide, carbon dioxide,and hydrocarbons. And when running at stoichiometry, the combustion is hot enoughto burn some of the nitrogen in the air, cre-ating oxides of nitrogen (NOx), which seedthe brown clouds of smog that hang over

Los Angeles and otherurban areas.

In contrast, hydrogencoughs up almost no pol-lutants. Burning hydrogenproduces no carbon diox-ide, the most prevalentheat-trapping greenhousegas, or other carbon com-pounds. And unlike gaso-line, hydrogen burns evenwhen the air-fuel mixturecontains far less hydrogenthan is needed to con-sume all the oxygen—acondition known as “run-ning lean.” Reduce thehydrogen-air mixture toroughly half the stoichio-metric ratio, and the tem-perature of combustion

falls low enough to extinguish more than 90%of NOx production. Try that with a gasolineengine and it will run poorly, if at all.

But before they can take a victory lap,engineers working on H2 ICEs must solvesome problems with engine performance.Hydrogen packs more energy per kilogramthan gasoline, but it’s also the least densegas in nature, which means it takes up a lotof room in an engine’s cylinders, saysChristopher White, a mechanical engineer atSandia National Laboratories in Livermore,California. “That costs you power becausethere’s less oxygen to consume,” he says. Atthe same time, it takes so little energy to ig-

N E W S

Fire and ICE: Revving Up for H2The first hydrogen-powered cars will likely burn the stuff in good old internal combustion engines. But can they drive the construction of hydrogen infrastructure?

13 AUGUST 2004 VOL 305 SCIENCE www.sciencemag.org964

CR

ED

ITS:S

ON

G L

YR

ICS F

RO

M “

BO

RN

TO

RU

N,”

© 1

97

4 B

RU

CE S

PR

ING

ST

EEN

(A

SC

AP);

CO

PY

RIG

HT

20

04

,FO

RD

MO

TO

R C

O.A

ND

WIE

CK

PH

OTO

DA

TAB

ASE

T O W A R D A H Y D R O G E N E C O N O M Y

SP

EC

IAL

SE

CT

ION

Motoring. Hydrogen engines, such as the one that powers Ford’s Model U concept car,

may provide the technological steppingstone to fuel-cell vehicles.

H

965www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004

CR

ED

ITS:(

LEFT

TO

RIG

HT

) B

MW

AG

;MA

ZD

A M

OTO

R C

OR

P.T O W A R D A H Y D R O G E N E C O N O M Y S

PE

CIA

LS

EC

TIO

N

nite hydrogen that the hydrogen-air mixturetends to go off as soon as it gets close tosomething hot, like a sparkplug. Such“preignition” can make an engine “knock”or even backfire like an old Model T.

Power playTo surmount such problems, BMW, Ford,and Mazda are taking different tacks. Fordengineers use a mechanically driven pumpcalled a supercharger to force more airand fuel into the combustion chamber,increasing the energy of each detona-tion. “We basically stuff another one-and-a-half times more air, plus an ap-propriate amount of fuel, into thecylinders,” says Ford’s Natkin. Keepingthe hydrogen-air ratio very lean—lessthan 40% of the stoichiometric ratio—prevents preignition and backfire, hesays. A hydrogen-powered Focus com-pact car can travel about 240 kilome-ters before refueling its hydrogentanks, which are pressurized to 350times atmospheric pressure. And with anelectric hybrid system and tanks pressurizedto 700 atmospheres, or 70 MPa, Ford’s Mod-el U concept car can range twice as far.

Mazda’s H2 ICE prototype also carriesgaseous hydrogen, but it burns it in a rotaryengine driven by two triangular rotors. Toovercome hydrogen’s propensity to displaceair, Mazda engineers force the hydrogen in-to the combustion chamber only after thechamber has filled with air and the intakevalves have closed. As well as boostingpower, such “direct injection” eliminatesbackfiring by separating the hydrogen andoxygen until just before they’re supposed todetonate, explains Masanori Misumi, an en-gineer at Mazda Motor Corp. in Hiroshima,Japan. Mazda’s hydrogen engine will alsorun on gasoline.

When BMW’s H2 ICE needs maximumpower, it pours on the hydrogen to run atstoichiometry. Otherwise, it runs lean.A hydrogen-powered Beemer also carriesdenser liquid hydrogen, boiling away at–253ºC inside a heavily insulated tank,which greatly increases the distance a carcan travel between refueling stops. In futureengines, the chilly gas might cool the air-fuel mixture, making it denser and more potent than a warm mixture. The cold hydrogen gas might also cool engine parts,preventing backfire and preignition. BMW’sH2 ICE can run on gasoline as well.

Unlike Ford and Mazda, BMW has noimmediate plans to pursue fuel cell tech-nology alongside its H2 ICEs. A fuel cellcan wring more useful energy from a kilo-gram of hydrogen, but it cannot providethe wheel-spinning power that an internalcombustion engine can, says Andreas

Klugescheid, a spokesperson for the BMWGroup in Munich. “Our customers don’tbuy a car just to get from A to B, but to havefun in between,” he says. “At BMW we’repretty sure that the hydrogen internal com-bustion engine is the way to satisfy them.”

The first production H2 ICE vehicles willlikely roll off the assembly line within 5years, although the automakers won’t sayprecisely when. “We would anticipate a lot

more hydrogen internal combustion enginevehicles on the road sooner rather than lateras we continue to develop fuel cell vehicles,”says Michael Vaughn, a spokesperson forFord in Dearborn. Focusing on the marketfor luxury performance cars, BMW plans toproduce some hydrogen-powered vehicles inthe current several-year model cycle of itsflagship 7 Series cars. Automakers will introduce the cars into commercial and government fleets, taking advantage of thecentralized fueling facilities to carefullymonitor their performance.

Supplying demandIn the long run, most experts agree, the hydrogen fuel cell holds the most promisefor powering clean, ultraefficient cars. Ifthey improve as hoped, fuel cells might use-fully extract two-thirds of the chemical energy contained in a kilogram of hydrogen.In contrast, even with help from an electric hybrid system, an H2 ICE probably can ex-tract less than half. (A gasoline enginemakes use of about 25% of the energy in itsfuel, the rest going primarily to heat.) Andwhereas an internal combustion engine willalways produce some tiny amount of pollu-tion, a fuel cell promises true zero emis-sions. But H2 ICE vehicles enjoy one ad-vantage that could bring them to marketquickly and help increase the demand forhydrogen filling stations, says James Franc-fort, who manages the Department of Ener-gy’s Advanced Vehicle Testing Activity atthe Idaho National Engineering and Envi-ronmental Laboratory in Idaho Falls. “The

car guys know how to build engines,”Francfort says. “This looks like somethingthat could be done now.”

Developers are hoping that H2 ICE vehi-cles will attract enough attention in fleetsthat a few intrepid individuals will go out oftheir way to buy them, even if they have tofuel them at fleet depots and the odd pub-licly funded fueling station. If enough peo-ple follow the trendsetters, eventually the

demand for hydrogen refuelingstations will increase to the pointat which they become profitableto build and operate—or so thescenario goes.

All agree that if H2 ICEs are tomake it out of fleets and into cardealers’ showrooms, they’ll need apush from the public sector.

They’re already getting it in California,which gives manufacturers environmentalcredits for bringing H2 ICE vehicles to mar-ket. And in April, California GovernorArnold Schwarzenegger announced a plan toline California’s interstate highways with upto 200 hydrogen stations by 2010, just in timeto kick-start the market for H2 ICEs.

Ultimately, the fate of H2 ICE vehicleslies with consumers, who have previouslyturned a cold shoulder to alternative tech-nologies such as cars powered by electricity,methanol, and compressed natural gas. Withnear-zero emissions and an edge in powerover fuel cells, the H2 ICE might catch onwith car enthusiasts yearning to go green, ademographic that has few choices in today’smarket, says BMW’s Klugescheid. If theH2 ICE can help enough gearheads discov-er their inner tree-hugger, the technologymight just take off. “There are enough peo-ple who are deeply in love with perform-ance cars but also have an environmentalconscience,” Klugescheid says.

Developers hope that the H2 ICE vehi-cles possess just the right mixture of envi-ronmental friendliness, futuristic technology,and good old-fashioned horsepower to cap-ture the imagination of the car-buying pub-lic. A few short years should tell if they do.In the meantime, it wouldn’t hurt if BruceSpringsteen wrote a song about them, too.

–ADRIAN CHO

What a gas! H2 ICE vehicles, such as BMW’s

and Mazda’s prototypes, promise performance

as well as almost zero emissions.

REYKJAVIK—As commuters in this coastalcity board the route 2 bus, some get an un-expected chemistry lesson. The doors of thebus are emblazoned with a brightly painteddiagram of a water molecule, H2O. Whenthey swing open, oxygen goes right and hydrogen left, splitting the molecule in two.The bus’s sponsors hope that soon all of Iceland’s nearly 300,000 residents not onlywill know this chemical reaction but will berelying on it to get around: By 2050, theysay, Iceland should run on a completely hydrogen-based energy economy.

The hydrogen-powered fuel cell buses—three ply the route regularly—are the firststep toward weaning Iceland off importedfossil fuels, says Thorsteinn Sigfusson, aphysicist at the Universityof Iceland in Reykjavik andfounding chair of IcelandicNew Energy, a companylaunched to test and devel-op hydrogen-based trans-port and energy systems.

Although other Europeancountries are fielding similarpilot projects, this volcanicisland nation is uniquelypoised to tap hydrogen’s po-tential. Iceland already useswater—either hot waterfrom geothermal sources orfalling water from hydro-electric dams—to providehot showers, heat its homes,and light its streets andbuildings through the long,dark winter just south of theArctic Circle. “This is a sustainable Texas,”says Sigfusson, referring to the plentiful en-ergy welling from the ground. But althoughthe country has the capacity to producemuch more energy than it currently needs, itstill imports 30% of its energy as oil to pow-er cars and ships. Converting those vehiclesto hydrogen fuel could make the countryself-sufficient in energy.

Iceland started taking the idea of a hydro-gen economy seriously more than a decadeago, after the 1990 Kyoto Protocol requiredthat it cut its nonindustrial CO2 emissions.Having already converted more than 95% ofits heat and electricity generation to hydro-electric and geothermal energy, which emit

almost no CO2, the country focused on thetransportation sector. Spurred by an expertcommission that recommended hydrogenfuels as the most promising way to convertits renewable energy into a fuel to powercars, the government formed Icelandic NewEnergy in 1997. Today 51% of the sharesare owned by Icelandic sources, includingpower companies, the University of Iceland,and the government itself. The rest are heldby international corporations Norsk Hydro,DaimlerChrysler, and Shell Hydrogen. With$4 million from the European Union and $5million from its investors, the companybought three hydrogen-powered buses forthe city bus fleet and built a fueling station tokeep them running.

The fueling station is part of the coun-try’s busiest Shell gasoline station, clearlyvisible from the main road out of Reykjavik.It boldly proclaims to all passersby in Eng-lish that hydrogen is “the ultimate fuel” and“We’re making the fuel of the future.” Thehydrogen is produced overnight using elec-tricity from the city’s power grid to split wa-ter into hydrogen and oxygen. The hydrogenis then stored as a compressed gas. It takesjust over 6 minutes to fill a bus with enoughfuel to run a full day’s journey. The busesare built from standard bodies outfitted withrooftop hydrogen tanks that make themslightly taller than their diesel cousins. “Thefirst cars were horse carriages retrofitted

with engines. We’re seeing the same processhere,” Sigfusson says. Piped to the fuelcells, the hydrogen combines with oxygenfrom the air and produces electricity, whichdrives a motor behind the rear wheels.

As the buses tour the city, their exhaustpipes emit trails of steam strikingly reminis-cent of those that waft from the hot springsin the mountains outside the city. The ex-haust is more visible than that from othercars, Sigfusson says, because the fuel cellruns at about 70°C and so the steam is closeto the saturation point. But it is almost purewater, he says, so clean “you can drink it.”And because the buses are electric, they aresignificantly quieter than diesel buses.

The pilot project has not been trouble-free. On a recent Friday, for example, two ofthe three buses were out of service for re-pairs. The buses are serviced in a garageequipped with special vents that remove anyhighly explosive hydrogen that might escape

while a bus is being repaired. On coldwinter nights the vehicles must be keptwarm in specially designed bays, lest thewater vapor left in the system freeze anddamage the fuel cells. “They need to bekept like stallions in their stalls,” says Sig-fusson, who notes that newer generationsof fuel cells are drier and may not needsuch coddling.

But despite the hiccups, Sigfusson saysthe project so far has been encouraging. In9 months, the buses have driven a total of40,000 kilometers, while surveys showthat public support for a hydrogen econo-my has remained at a surprising 93%. Thenext step is a test fleet of passenger cars,Sigfusson says. Icelandic New Energy isnegotiating to buy more than a dozenhydrogen-powered cars for corporate orgovernment employees, he says.

Economic leaders are also optimistic.“I am not a believer that we will have a hy-drogen economy tomorrow,” says Fri∂rikSophusson, a former finance minister andnow managing director of the government-owned National Power Co., a shareholder inIcelandic New Energy. But he believes theinvestment will pay long-term dividends—not least to his company, which will supplyelectricity needed to produce the gas. “In20 years, I believe we will have vehiclesrunning on hydrogen efficiently generatedfrom renewable sources,” Sophusson says.“We are going to produce hydrogen in aclean way, and if the project takes off, wewill be in business.”

Although Iceland may harbor the most

N E W S

Will the Future Dawn in the North?With geothermal and hydroelectric sources supplying almost all of itsheat and electricity, Iceland is well on the way to energy self-suffiency.Now it is betting that hydrogen-fueled transportation will supply the lastbig piece of the puzzle

13 AUGUST 2004 VOL 305 SCIENCE www.sciencemag.org966

CR

ED

IT:C

OU

RT

ESY

OF

ICELA

ND

IC N

EW

EN

ER

GY

T O W A R D A H Y D R O G E N E C O N O M Y

SP

EC

IAL

SE

CT

ION

All aboard. Fuel cell buses and a planned car fleet are the latest en-

tries in Iceland’s marathon push for total energy independence.

H

967www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004

CR

ED

ITS:(

TO

P T

O B

OT

TO

M)

G.V

OG

EL;

SA

UR

AB

H D

AS/A

P P

HO

TO

T O W A R D A H Y D R O G E N E C O N O M Y

ambitious vision of a fossil fuel–free future,other countries without its natural advantagesin renewable energy are also experimentingwith hydrogen-based technologies. Sigfussonthinks the gas’s biggest potential could lie indeveloping countries that have not yet com-mitted themselves to fossil fuels (see side-bar), but industrialized nations are also push-ing hard. In a project partly funded by theEuropean Union (E.U.), nine other Europeancities now have small fleets of buses—simi-lar to the ones in Reykjavik—plying regularroutes. The E.U. imports 50% of its oil, andthat figure is expected to rise to 70% over thenext 20 to 30 years. In January, EuropeanCommission President Romano Prodipledged to create “a fully integrated hydrogeneconomy, based on renewable energysources, by the middle of the century.”

Realizing that bold ambition is now thejob of the Hydrogen and Fuel Cell Technolo-gy Platform, an E.U. body. Its advisory panel

of 35 prominent industry, research, and civicleaders will coordinate efforts in academiaand industry at both the national and Euro-pean levels and will draw up a research planand deployment strategy. Planned demon-stration projects include a fossil fuel powerplant that will produce electricity and hydro-

gen on an industrial scale while separatingand storing the CO2 it produces and a “hy-drogen village” where new technologies andhydrogen infrastructure can be tested. In all,the E.U.’s Framework research program in-tends to spend $300 million on hydrogen andfuel cell research during the 5-year periodfrom 2002 to 2006. Political and public in-terest in a hydrogen economy “is like asnowball growing and growing,” saysJoaquín Martín Bernejo, the E.U. official re-sponsible for research into hydrogen produc-tion and distribution.

As Iceland (not an E.U. member) treadsthat same path on a more modest scale, itsbiggest hurdle remains the conversion of itseconomically vital shipping fleet, which useshalf of the country’s imported oil. Boats posetougher technical problems than city busesdo. Whereas a bus can run its daily route ononly 40 kilograms of hydrogen, Sigfussonsays, a small trawler with a 500-kilowatt en-gine must carry a ton of the gas to spend 4 to5 days at sea. One way to store enough fuel,he says, might be to sequester the gas inhydrogen-absorbing compounds called metalhydrides. The compound could even serve asballast for the boat instead of the standardconcrete keel ballast.

Although Iceland’s leaders are eager forthe hydrogen economy to take off, Sigfussonacknowledges that it will have to appeal di-rectly to Iceland’s drivers and fishers. Gener-ous tax breaks to importers of hydrogen ve-hicles will help, he says, if hydrogen canmatch the price of heavily taxed fossil fuels:“People will be willing to pay a little more[for a hydrogen vehicle], but they’re not will-ing to pay a lot more. The market has to forcedown the price of an installed kilowatt.” Ac-cording to Sigfusson, that is already happen-ing, especially as research investments areramped up: “There has been a paradigmshift. We had had decades of coffee-roomdiscussions that never led anywhere.” Nowthe buses with their chemistry lesson, hesays, are pointing the way to the future.

–GRETCHEN VOGEL

With reporting by Daniel Clery.

Fill ’er up.. Reykjavik’s lone hydrogen station, which manufactures the fuel on site by splitting

water molecules, can fill a bus with pressurized gas in 6 minutes.

Can the Developing World Skip Petroleum?

If technologies for hydrogen fuel take off, one of the biggest winners could be the developing

world. Just as cell phones in poor countries have made land lines obsolete before they were

installed, hydrogen from renewable sources—in an ideal world—could enable developing

countries to leap over the developed world to energy independence. “The opportunity is

there for them to become leaders in this area,” says Thorsteinn Sigfusson of the University of

Iceland, one of the leaders of the International Partnership for a Hydrogen Economy (IPHE), a

cooperative effort of 15 countries, including the United States, Iceland, India, China, and

Brazil, founded last year to advance hydrogen research and technology development.

With their growing influence in global manufacturing, their technical expertise, and

their low labor costs, Sigfusson says, countries such as China and India could play ex-

tremely important roles in developing more efficient solar or biotech sources of hydro-

gen—as well as vehicles and power systems that use the fuel. “They have the opportu-

nity to take a leap into the hydrogen economy without all the troubles of going through

combustion and liquid fuel,” he says. The impact would be huge. The IPHE members al-

ready encompass 85% of the world’s population, he notes.

The current steps are small. For example, a joint U.S.-Indian project is working to build

a hydrogen-powered three-wheel test vehicle. The minicar, designed for crowded urban

streets, needs only one-tenth as much storage space as a standard passenger car. India’s

largest auto manufacturer, Mahindra and Mahindra, has shipped two of its popular gaso-

line-powered three-wheelers (currently a huge source of urban air pollution), to the

Michigan-based company Energy Conversion Devices (ECD). Engineers at ECD are work-

ing to convert the engine to run on hydrogen stored in a metal hydride. One model will

return to India for testing, and one will remain in the United States. The small project “is

just the beginning,” says a U.S. Department of Energy official. “But the point of bringing

in these countries is that they are huge

energy consumers. They simply have to be

part of the partnership, especially as we

start to use the technologies.”

Ideally, the developing world will be

able to harness the solar energy plentiful

in the tropics to power hydrogen systems,

Sigfusson says. “The most important re-

newable will be the sun,” he says.

“Mankind started as a solar civilization.

We spent 200 years flirting with fossil fu-

els, but I believe we’ll soon go back to be-

ing a solar civilization.” –G.V.Different future. Countries not yet committed

to fossil fuels might go straight to hydrogen.

SP

EC

IAL

SE

CT

ION

R E V I E W

Stabilization Wedges: Solving the Climate Problemfor the Next 50 Years with Current Technologies

S. Pacala1* and R. Socolow2*

Humanity already possesses the fundamental scientific, technical, and industrialknow-how to solve the carbon and climate problem for the next half-century. Aportfolio of technologies now exists to meet the world’s energy needs over the next50 years and limit atmospheric CO2 to a trajectory that avoids a doubling of thepreindustrial concentration. Every element in this portfolio has passed beyond thelaboratory bench and demonstration project; many are already implemented some-where at full industrial scale. Although no element is a credible candidate for doingthe entire job (or even half the job) by itself, the portfolio as a whole is large enoughthat not every element has to be used.

The debate in the current literature about stabi-lizing atmospheric CO2 at less than a doublingof the preindustrial concentration has led toneedless confusion about current options formitigation. On one side, the IntergovernmentalPanel on Climate Change (IPCC) has claimedthat “technologies that exist in operation or pilotstage today” are sufficient to follow a less-than-doubling trajectory “over the next hundredyears or more” [(1), p. 8]. On the other side, arecent review in Science asserts that the IPCCclaim demonstrates “misperceptions of techno-logical readiness” and calls for “revolutionarychanges” in mitigation technology, such as fu-sion, space-based solar electricity, and artificialphotosynthesis (2). We agree that fundamentalresearch is vital to develop the revolutionarymitigation strategies needed in the second halfof this century and beyond. But it is importantnot to become beguiled by the possibility ofrevolutionary technology. Humanity can solvethe carbon and climate problem in the first halfof this century simply by scaling up what wealready know how to do.

What Do We Mean by “Solving theCarbon and Climate Problem for theNext Half-Century”?Proposals to limit atmospheric CO2 to a con-centration that would prevent most damagingclimate change have focused on a goal of500 � 50 parts per million (ppm), or less thandouble the preindustrial concentration of 280ppm (3–7). The current concentration is �375ppm. The CO2 emissions reductions necessaryto achieve any such target depend on the emis-sions judged likely to occur in the absence of afocus on carbon [called a business-as-usual

(BAU) trajectory], the quantitative details of thestabilization target, and the future behavior ofnatural sinks for atmospheric CO2 (i.e., theoceans and terrestrial biosphere). We focus ex-clusively on CO2, because it is the dominantanthropogenic greenhouse gas; industrial-scalemitigation options also exist for subordinategases, such as methane and N2O.

Very roughly, stabilization at 500 ppmrequires that emissions be held near thepresent level of 7 billion tons of carbon peryear (GtC/year) for the next 50 years, eventhough they are currently on course to morethan double (Fig. 1A). The next 50 years isa sensible horizon from several perspec-tives. It is the length of a career, the life-time of a power plant, and an interval forwhich the technology is close enough toenvision. The calculations behind Fig. 1Aare explained in Section 1 of the supportingonline material (SOM) text. The BAU andstabilization emissions in Fig. 1A are nearthe center of the cloud of variation in thelarge published literature (8).

The Stabilization TriangleWe idealize the 50-year emissions reductionsas a perfect triangle in Fig. 1B. Stabilizationis represented by a “flat” trajectory of fossilfuel emissions at 7 GtC/year, and BAU isrepresented by a straight-line “ramp” trajec-tory rising to 14 GtC/year in 2054. The “sta-bilization triangle,” located between the flattrajectory and BAU, removes exactly one-third of BAU emissions.

To keep the focus on technologies that havethe potential to produce a material difference by2054, we divide the stabilization triangle intoseven equal “wedges.” A wedge represents anactivity that reduces emissions to the atmospherethat starts at zero today and increases linearlyuntil it accounts for 1 GtC/year of reduced car-bon emissions in 50 years. It thus represents acumulative total of 25 GtC of reduced emissionsover 50 years. In this paper, to “solve the carbon

and climate problem over the next half-century”means to deploy the technologies and/or lifestylechanges necessary to fill all seven wedges of thestabilization triangle.

Stabilization at any level requires that netemissions do not simply remain constant, buteventually drop to zero. For example, in onesimple model (9) that begins with the stabi-lization triangle but looks beyond 2054, 500-ppm stabilization is achieved by 50 years offlat emissions, followed by a linear decline ofabout two-thirds in the following 50 years,and a very slow decline thereafter that match-es the declining ocean sink. To develop therevolutionary technologies required for suchlarge emissions reductions in the second halfof the century, enhanced research and devel-opment would have to begin immediately.

Policies designed to stabilize at 500 ppmwould inevitably be renegotiated periodicallyto take into account the results of researchand development, experience with specificwedges, and revised estimates of the size ofthe stabilization triangle. But not filling thestabilization triangle will put 500-ppm stabi-lization out of reach. In that same simplemodel (9), 50 years of BAU emissions fol-lowed by 50 years of a flat trajectory at 14GtC/year leads to more than a tripling of thepreindustrial concentration.

It is important to understand that each ofthe seven wedges represents an effort beyondwhat would occur under BAU. Our BAUsimply continues the 1.5% annual carbonemissions growth of the past 30 years. Thishistoric trend in emissions has been accom-panied by 2% growth in primary energy con-sumption and 3% growth in gross worldproduct (GWP) (Section 1 of SOM text). Ifcarbon emissions were to grow 2% per year,then �10 wedges would be needed instead of7, and if carbon emissions were to grow at3% per year, then �18 wedges would berequired (Section 1 of SOM text). Thus, acontinuation of the historical rate of decar-bonization of the fuel mix prevents the needfor three additional wedges, and ongoing im-provements in energy efficiency prevent theneed for eight additional wedges. Most read-ers will reject at least one of the wedges listedhere, believing that the corresponding de-ployment is certain to occur in BAU, butreaders will disagree about which to reject onsuch grounds. On the other hand, our list ofmitigation options is not exhaustive.

1Department of Ecology and Evolutionary Biology,2Department of Mechanical and Aerospace Engineer-ing, Princeton University, Princeton, NJ 08544, USA.

*To whom correspondence should be addressed. E-mail: pacala@princeton.edu (S.P.); socolow@princeton.edu (R.S.)

T O W A R D A H Y D R O G E N E C O N O M Y

13 AUGUST 2004 VOL 305 SCIENCE www.sciencemag.org968

SPECIA

LSECTIO

N

What Current Options Could BeScaled Up to Produce at Least OneWedge?Wedges can be achieved from energy effi-ciency, from the decarbonization of the sup-ply of electricity and fuels (by means of fuelshifting, carbon capture and storage, nuclearenergy, and renewable energy), and from bi-ological storage in forests and agriculturalsoils. Below, we discuss 15 different exam-ples of options that are already deployed at anindustrial scale and that could be scaled upfurther to produce at least one wedge (sum-marized in Table 1). Although several op-tions could be scaled up to two or morewedges, we doubt that any could fill thestabilization triangle, or even half of it, alone.

Because the same BAU carbon emissionscannot be displaced twice, achieving onewedge often interacts with achieving another.The more the electricity system becomes decar-bonized, for example, the less the available sav-ings from greater efficiency of electricity use, andvice versa. Interactions among wedges are dis-cussed in the SOM text. Also, our focus is not oncosts. In general, the achievement of a wedge willrequire some price trajectory for carbon, the de-tails of which depend on many assumptions, in-cluding future fuels prices, public acceptance, andcost reductions by means of learning. Instead, ouranalysis is intended to complement the compre-hensive but complex “integrated assessments” (1)of carbon mitigation by letting the full-scale ex-amples that are already in the marketplace make asimple case for technological readiness.Category I: Efficiency and ConservationImprovements in efficiency and conservationprobably offer the greatest potential to pro-vide wedges. For example, in 2002, the Unit-ed States announced the goal of decreasing itscarbon intensity (carbon emissions per unitGDP) by 18% over the next decade, a de-crease of 1.96% per year. An entire wedgewould be created if the United States were toreset its carbon intensity goal to a decrease of2.11% per year and extend it to 50 years, and ifevery country were to follow suit by adding thesame 0.15% per year increment to its owncarbon intensity goal. However, efficiency andconservation options are less tangible thanthose from the other categories. Improvementsin energy efficiency will come from literallyhundreds of innovations that range from newcatalysts and chemical processes, to moreefficient lighting and insulation for buildings,to the growth of the service economy andtelecommuting. Here, we provide four ofmany possible comparisons of greater andless efficiency in 2054. (See references anddetails in Section 2 of the SOM text.)

Option 1: Improved fuel economy. Sup-pose that in 2054, 2 billion cars (roughly fourtimes as many as today) average 10,000 milesper year (as they do today). One wedge wouldbe achieved if, instead of averaging 30 miles

per gallon (mpg) on conventional fuel, cars in2054 averaged 60 mpg, with fuel type anddistance traveled unchanged.

Option 2: Reduced reliance on cars. Awedge would also be achieved if the averagefuel economy of the 2 billion 2054 cars were30 mpg, but the annual distance traveled were5000 miles instead of 10,000 miles.

Option 3: More efficient buildings. Accordingto a 1996 study by the IPCC, a wedge is thedifference between pursuing and not pursuing“known and established approaches” to energy-efficient space heating and cooling, water heating,lighting, and refriger-ation in residentialand commercialbuildings. These ap-proaches reduce mid-century emissionsfrom buildings byabout one-fourth.About half of poten-tial savings are in thebuildings in develop-ing countries (1).

Option 4: Im-proved power plantefficiency. In 2000,coal power plants,operating on averageat 32% efficiency,produced about one-fourth of all carbonemissions: 1.7 GtC/year out of 6.2 GtC/year. A wedge wouldbe created if twice to-day’s quantity ofcoal-based electricityin 2054 were pro-duced at 60% insteadof 40% efficiency.Category II: Decar-bonization of Elec-tricity and Fuels(See references anddetails in Section 3of the SOM text.)

Option 5: Substi-tuting natural gas forcoal. Carbon emis-sions per unit of elec-tricity are about halfas large from naturalgas power plants asfrom coal plants. As-sume that the capaci-ty factor of the aver-age baseload coalplant in 2054 has in-creased to 90% andthat its efficiency hasimproved to 50%.Because 700 GW ofsuch plants emit car-

bon at a rate of 1 GtC/year, a wedge would beachieved by displacing 1400 GW of baseload coalwith baseload gas by 2054. The power shifted togas for this wedge is four times as large as the totalcurrent gas-based power.

Option 6: Storage of carbon captured inpower plants. Carbon capture and storage(CCS) technology prevents about 90% of thefossil carbon from reaching the atmosphere,so a wedge would be provided by the instal-lation of CCS at 800 GW of baseload coalplants by 2054 or 1600 GW of baseloadnatural gas plants. The most likely approach

Fig. 1. (A) The top curve is a representative BAU emissions path for globalcarbon emissions as CO2 from fossil fuel combustion and cement manufac-ture: 1.5% per year growth starting from 7.0 GtC/year in 2004. The bottomcurve is a CO2 emissions path consistent with atmospheric CO2 stabilizationat 500 ppm by 2125 akin to the Wigley, Richels, and Edmonds (WRE) familyof stabilization curves described in (11), modified as described in Section 1 ofthe SOM text. The bottom curve assumes an ocean uptake calculated with theHigh-Latitude Exchange Interior Diffusion Advection (HILDA) ocean model(12) and a constant net land uptake of 0.5 GtC/year (Section 1 of the SOMtext). The area between the two curves represents the avoided carbonemissions required for stabilization. (B) Idealization of (A): A stabilizationtriangle of avoided emissions (green) and allowed emissions (blue). Theallowed emissions are fixed at 7 GtC/year beginning in 2004. The stabili-zation triangle is divided into seven wedges, each of which reaches 1GtC/year in 2054. With linear growth, the total avoided emissions perwedge is 25 GtC, and the total area of the stabilization triangle is 175 GtC.The arrow at the bottom right of the stabilization triangle points down-ward to emphasize that fossil fuel emissions must decline substantiallybelow 7 GtC/year after 2054 to achieve stabilization at 500 ppm.

T O W A R D A H Y D R O G E N E C O N O M Y

www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004 969

SPECIA

LSECTIO

N

has two steps: (i) precombustion capture ofCO2, in which hydrogen and CO2 are pro-duced and the hydrogen is then burned toproduce electricity, followed by (ii) geologicstorage, in which the waste CO2 is injectedinto subsurface geologic reservoirs. Hydro-gen production from fossil fuels is already avery large business. Globally, hydrogenplants consume about 2% of primary energyand emit 0.1 GtC/year of CO2. The capturepart of a wedge of CCS electricity would thusrequire only a tenfold expansion of plantsresembling today’s large hydrogen plantsover the next 50 years.

The scale of the storage part of this wedgecan be expressed as a multiple of the scale of

current enhanced oil recovery, or current season-al storage of natural gas, or the first geologicalstorage demonstration project. Today, about 0.01GtC/year of carbon as CO2 is injected into geo-logic reservoirs to spur enhanced oil recovery, soa wedge of geologic storage requires that CO2

injection be scaled up by a factor of 100 over thenext 50 years. To smooth out seasonal demandin the United States, the natural gas industryannually draws roughly 4000 billion standardcubic feet (Bscf) into and out of geologicstorage, and a carbon flow of 1 GtC/year(whether as methane or CO2) is a flow of69,000 Bscf/year (190 Bscf per day), so awedge would be a flow to storage 15 and 20times as large as the current flow. Norway’s

Sleipner project in the North Sea strips CO2

from natural gas offshore and reinjects 0.3million tons of carbon a year (MtC/year) intoa non–fossil-fuel–bearing formation, so a wedgewould be 3500 Sleipner-sized projects (or few-er, larger projects) over the next 50 years.

A worldwide effort is under way to assessthe capacity available for multicentury stor-age and to assess risks of leaks large enoughto endanger human or environmental health.

Option 7: Storage of carbon captured inhydrogen plants. The hydrogen resulting fromprecombustion capture of CO2 can be sent off-site to displace the consumption of convention-al fuels rather than being consumed onsite toproduce electricity. The capture part of a wedge

Table 1. Potential wedges: Strategies available to reduce the carbon emission rate in 2054 by 1 GtC/year or to reduce carbon emissions from2004 to 2054 by 25 GtC.

OptionEffort by 2054 for one wedge, relative to 14

GtC/year BAUComments, issues

Energy efficiency and conservationEconomy-wide carbon-intensity

reduction (emissions/$GDP)Increase reduction by additional 0.15% per year

(e.g., increase U.S. goal of 1.96% reduction peryear to 2.11% per year)

Can be tuned by carbon policy

1. Efficient vehicles Increase fuel economy for 2 billion cars from 30 to60 mpg

Car size, power

2. Reduced use of vehicles Decrease car travel for 2 billion 30-mpg cars from10,000 to 5000 miles per year

Urban design, mass transit, telecommuting

3. Efficient buildings Cut carbon emissions by one-fourth in buildingsand appliances projected for 2054

Weak incentives

4. Efficient baseload coal plants Produce twice today’s coal power output at 60%instead of 40% efficiency (compared with 32%today)

Advanced high-temperature materials

Fuel shift5. Gas baseload power for coal

baseload powerReplace 1400 GW 50%-efficient coal plants with

gas plants (four times the current production ofgas-based power)

Competing demands for natural gas

CO2 Capture and Storage (CCS)6. Capture CO2 at baseload power

plantIntroduce CCS at 800 GW coal or 1600 GW natural

gas (compared with 1060 GW coal in 1999)Technology already in use for H2 production

7. Capture CO2 at H2 plant Introduce CCS at plants producing 250 MtH2/yearfrom coal or 500 MtH2/year from natural gas(compared with 40 MtH2/year today from allsources)

H2 safety, infrastructure

8. Capture CO2 at coal-to-synfuelsplant

Introduce CCS at synfuels plants producing 30million barrels a day from coal (200 times Sasol),if half of feedstock carbon is available forcapture

Increased CO2 emissions, if synfuels areproduced without CCS

Geological storage Create 3500 Sleipners Durable storage, successful permitting

Nuclear fission9. Nuclear power for coal power Add 700 GW (twice the current capacity) Nuclear proliferation, terrorism, waste

Renewable electricity and fuels10. Wind power for coal power Add 2 million 1-MW-peak windmills (50 times the

current capacity) “occupying” 30 � 106 ha, onland or offshore

Multiple uses of land because windmills arewidely spaced

11. PV power for coal power Add 2000 GW-peak PV (700 times the currentcapacity) on 2 � 106 ha

PV production cost

12. Wind H2 in fuel-cell car forgasoline in hybrid car

Add 4 million 1-MW-peak windmills (100 times thecurrent capacity)

H2 safety, infrastructure

13. Biomass fuel for fossil fuel Add 100 times the current Brazil or U.S. ethanolproduction, with the use of 250 � 106 ha(one-sixth of world cropland)

Biodiversity, competing land use

Forests and agricultural soils14. Reduced deforestation, plus

reforestation, afforestation, andnew plantations.

Decrease tropical deforestation to zero instead of0.5 GtC/year, and establish 300 Mha of new treeplantations (twice the current rate)

Land demands of agriculture, benefits tobiodiversity from reduced deforestation

15. Conservation tillage Apply to all cropland (10 times the current usage) Reversibility, verification

T O W A R D A H Y D R O G E N E C O N O M Y

13 AUGUST 2004 VOL 305 SCIENCE www.sciencemag.org970

SPECIA

LSECTIO

N

would require the installation of CCS, by 2054,at coal plants producing 250 MtH2/year, or atnatural gas plants producing 500 MtH2/year.The former is six times the current rate ofhydrogen production. The storage part of thisoption is the same as in Option 6.

Option 8: Storage of carbon captured insynfuels plants. Looming over carbon manage-ment in 2054 is the possibility of large-scaleproduction of synthetic fuel (synfuel) from coal.Carbon emissions, however, need not exceedthose associated with fuel refined from crudeoil if synfuels production is accompanied byCCS. Assuming that half of the carbon enteringa 2054 synfuels plant leaves as fuel but theother half can be captured as CO2, the capturepart of a wedge in 2054 would be the differencebetween capturing and venting the CO2 fromcoal synfuels plants producing 30 million bar-rels of synfuels per day. (The flow of carbon in24 million barrels per day of crude oil is 1GtC/year; we assume the same value for theflow in synfuels and allow for imperfectcapture.) Currently, the Sasol plants inSouth Africa, the world’s largest synfuelsfacility, produce 165,000 barrels per dayfrom coal. Thus, a wedge requires 200Sasol-scale coal-to-synfuels facilities withCCS in 2054. The storage part of this op-tion is again the same as in Option 6.

Option 9: Nuclear fission. On the basis ofthe Option 5 estimates, a wedge of nuclearelectricity would displace 700 GW of effi-cient baseload coal capacity in 2054. Thiswould require 700 GW of nuclear power withthe same 90% capacity factor assumed for thecoal plants, or about twice the nuclear capac-ity currently deployed. The global pace ofnuclear power plant construction from 1975to 1990 would yield a wedge, if it contin-ued for 50 years (10). Substantial expan-sion in nuclear power requires restorationof public confidence in safety and wastedisposal, and international security agree-ments governing uranium enrichment andplutonium recycling.

Option 10: Wind electricity. We accountfor the intermittent output of windmills byequating 3 GW of nominal peak capacity (3GWp) with 1 GW of baseload capacity. Thus,a wedge of wind electricity would require thedeployment of 2000 GWp that displaces coalelectricity in 2054 (or 2 million 1-MWp windturbines). Installed wind capacity has beengrowing at about 30% per year for more than10 years and is currently about 40 GWp. Awedge of wind electricity would thus require50 times today’s deployment. The wind tur-bines would “occupy” about 30 million hect-ares (about 3% of the area of the UnitedStates), some on land and some offshore.Because windmills are widely spaced, landwith windmills can have multiple uses.

Option 11: Photovoltaic electricity. Sim-ilar to a wedge of wind electricity, a wedge

from photovoltaic (PV) electricity would re-quire 2000 GWp of installed capacity thatdisplaces coal electricity in 2054. Althoughonly 3 GWp of PV are currently installed, PVelectricity has been growing at a rate of 30%per year. A wedge of PV electricity wouldrequire 700 times today’s deployment, andabout 2 million hectares of land in 2054, or 2to 3 m2 per person.

Option 12: Renewable hydrogen. Re-newable electricity can produce carbon-free hydrogen for vehicle fuel by the elec-trolysis of water. The hydrogen producedby 4 million 1-MWp windmills in 2054, ifused in high-efficiency fuel-cell cars,would achieve a wedge of displaced gaso-line or diesel fuel. Compared with Option10, this is twice as many 1-MWp windmillsas would be required to produce the elec-tricity that achieves a wedge by displacinghigh-efficiency baseload coal. This inter-esting factor-of-two carbon-saving advan-tage of wind-electricity over wind-hydro-gen is still larger if the coal plant is lessefficient or the fuel-cell vehicle is lessspectacular.

Option 13: Biofuels. Fossil-carbon fuels canalso be replaced by biofuels such as ethanol. Awedge of biofuel would be achieved by theproduction of about 34 million barrels per dayof ethanol in 2054 that could displace gasoline,provided the ethanol itself were fossil-carbonfree. This ethanol production rate would beabout 50 times larger than today’s global pro-duction rate, almost all of which can be attrib-uted to Brazilian sugarcane and United Statescorn. An ethanol wedge would require 250million hectares committed to high-yield (15dry tons/hectare) plantations by 2054, an areaequal to about one-sixth of the world’s crop-land. An even larger area would be required tothe extent that the biofuels require fossil-carboninputs. Because land suitable for annually har-vested biofuels crops is also often suitable forconventional agriculture, biofuels productioncould compromise agricultural productivity.Category III: Natural SinksAlthough the literature on biological seques-tration includes a diverse array of options andsome very large estimates of the global po-tential, here we restrict our attention to thepair of options that are already implementedat large scale and that could be scaled up toa wedge or more without a lot of newresearch. (See Section 4 of the SOM textfor references and details.)

Option 14: Forest management. Conserva-tive assumptions lead to the conclusion that atleast one wedge would be available from re-duced tropical deforestation and the manage-ment of temperate and tropical forests. At leastone half-wedge would be created if the currentrate of clear-cutting of primary tropical forestwere reduced to zero over 50 years instead ofbeing halved. A second half-wedge would

be created by reforesting or afforesting ap-proximately 250 million hectares in thetropics or 400 million hectares in the tem-perate zone (current areas of tropical andtemperate forests are 1500 and 700 millionhectares, respectively). A third half-wedgewould be created by establishing approxi-mately 300 million hectares of plantationson nonforested land.

Option 15: Agricultural soils manage-ment. When forest or natural grassland is con-verted to cropland, up to one-half of the soilcarbon is lost, primarily because annual tillingincreases the rate of decomposition by aeratingundecomposed organic matter. About 55 GtC,or two wedges’ worth, has been lost historicallyin this way. Practices such as conservation till-age (e.g., seeds are drilled into the soil withoutplowing), the use of cover crops, and erosioncontrol can reverse the losses. By 1995, conser-vation tillage practices had been adopted on 110million hectares of the world’s 1600 millionhectares of cropland. If conservation tillagecould be extended to all cropland, accom-panied by a verification program that en-forces the adoption of soil conservationpractices that actually work as advertised, agood case could be made for the IPCC’sestimate that an additional half to onewedge could be stored in this way.

ConclusionsIn confronting the problem of greenhousewarming, the choice today is between actionand delay. Here, we presented a part of thecase for action by identifying a set of optionsthat have the capacity to provide the sevenstabilization wedges and solve the climateproblem for the next half-century. None ofthe options is a pipe dream or an unprovenidea. Today, one can buy electricity from awind turbine, PV array, gas turbine, or nucle-ar power plant. One can buy hydrogen pro-duced with the chemistry of carbon capture,biofuel to power one’s car, and hundreds ofdevices that improve energy efficiency. Onecan visit tropical forests where clear-cuttinghas ceased, farms practicing conservation till-age, and facilities that inject carbon into geo-logic reservoirs. Every one of these options isalready implemented at an industrial scaleand could be scaled up further over 50 yearsto provide at least one wedge.

References and Notes1. IPCC, Climate Change 2001: Mitigation, B. Metz et al.,

Eds. (IPCC Secretariat, Geneva, Switzerland, 2001);available at www.grida.no/climate/ipcc_tar/wg3/index.htm.

2. M. I. Hoffert et al., Science 298, 981 (2002).3. R. T. Watson et al., Climate Change 2001: Synthesis

Report. Contribution to the Third Assessment Reportof the Intergovernmental Panel on Climate Change(Cambridge Univ. Press, Cambridge, UK, 2001).

4. B. C. O’Neill, M. Oppenheimer, Science 296, 1971(2002).

5. Royal Commission on Environmental Pollution, En-

T O W A R D A H Y D R O G E N E C O N O M Y

www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004 971

SPECIA

LSECTIO

N

ergy: The Changing Climate (2000); available atwww.rcep.org.uk/energy.htm.

6. Environmental Defense, Adequacy of Commit-ments—Avoiding “Dangerous” Climate Change: ANarrow Time Window for Reductions and a SteepPriceforDelay (2002);availableatwww.environmentaldefense.org/documents/2422_COP_time.pdf.

7. “Climate OptiOns for the Long Term (COOL) synthe-sis report,” NRP Rep. 954281 (2002); available atwww.wau.nl/cool/reports/COOLVolumeAdef.pdf.

8. IPCC, Special Report on Emissions Scenarios (2001);available at www.grida.no/climate/ipcc/emission/index.htm.

9. R. Socolow, S. Pacala, J. Greenblatt, Proceedings ofthe Seventh International Conference on GreenhouseGas Control Technology, Vancouver, Canada, 5 to 9September, 2004, in press.

10. BP, Statistical Review of World Energy (2003); available atwww.bp.com/subsection.do?categoryId�95&contentId�2006480.

11. T. M. L. Wigley, in The Carbon Cycle, T. M. L. Wigley,D. S. Schimel, Eds. (Cambridge Univ. Press, Cam-bridge, 2000), pp. 258 –276.

12. G. Shaffer, J. L. Sarmiento, J. Geophys. Res. 100, 2659(1995).

13. The authors thank J. Greenblatt, R. Hotinski, and R.

Williams at Princeton; K. Keller at Penn State; and C.Mottershead at BP. This paper is a product of theCarbon Mitigation Initiative (CMI) of the PrincetonEnvironmental Institute at Princeton University.CMI (www.princeton.edu/�cmi) is sponsored by BPand Ford.

Supporting Online Materialwww.sciencemag.org/cgi/content/full/305/5686/968/DC1SOM TextFigs. S1 and S2Tables S1 to S5References

V I E W P O I N T

Sustainable Hydrogen ProductionJohn A. Turner

Identifying and building a sustainable energy system are perhaps two of the mostcritical issues that today’s society must address. Replacing our current energy carriermix with a sustainable fuel is one of the key pieces in that system. Hydrogen as anenergy carrier, primarily derived from water, can address issues of sustainability,environmental emissions, and energy security. Issues relating to hydrogen productionpathways are addressed here. Future energy systems require money and energy tobuild. Given that the United States has a finite supply of both, hard decisions mustbe made about the path forward, and this path must be followed with a sustainedand focused effort.

In his 2003 State of the Union Address, U.S.President Bush proposed “$1.2 billion in re-search funding so that America can lead theworld in developing clean, hydrogen-powered automobiles.” Since that time, arti-cles both pro and con have buffeted the wholeconcept. The hydrogen economy (1) is not anew idea. In 1874, Jules Verne, recognizingthe finite supply of coal and the possibilitiesof hydrogen derived from water electrolysis,made the comment that “water will be thecoal of the future” (2). Rudolf Erren in the1930s suggested using hydrogen producedfrom water electrolysis as a transportationfuel (3). His goal was to reduce automotiveemissions and oil imports into England. Sim-ilarly, Francis Bacon suggested using hydro-gen as an energy storage system (4). Thevision of using energy from electricity andelectrolysis to generate hydrogen from waterfor transportation and energy storage to re-duce environmental emissions and provideenergy security is compelling, but as yet re-mains unrealized.

If one assumes a full build-out of a hy-drogen economy, the amount of hydrogenneeded just for U.S. transportation needswould be about 150 million tons per year (5).One must question the efficacy of producing,storing, and distributing that much hydrogen.Because energy is required to extract hydro-gen from either water or biomass so that itcan be used as an energy carrier, if the United

States chooses a hydrogen-based future itneeds to think carefully about how muchenergy we need and where it is going tocome from. In addition, sustainability mustbe a hallmark of any proposed future infra-structure. What energy-producing technol-ogies can be envisioned that will last formillennia, and just how many people canthey support (6–8)?

Technologies for Hydrogen ProductionHydrogen can be generated from water, bio-mass, natural gas, or (after gasification) coal.Today, hydrogen is mainly produced fromnatural gas via steam methane reforming, andalthough this process can sustain an initialforay into the hydrogen economy, it repre-sents only a modest reduction in vehicleemissions as compared to emissions fromcurrent hybrid vehicles, and ultimately onlyexchanges oil imports for natural gas imports.It is clearly not sustainable.

Coal gasification could produce consider-able amounts of hydrogen and electricitymerely because of the large size of availablecoal deposits (9). Additionally, because of its rel-atively low cost, it is often cited as the best re-source for economically producing large quanti-ties of hydrogen. However, the energy requiredfor the necessary sequestration of CO2 wouldincrease the rate at which coal reserves are deplet-ed; converting the vehicle fleet to electric vehiclesand generating that electricity from “clean coal” ormaking hydrogen as a possible energy carrierwould accelerate that depletion. Couple that to amodest economic growth rate of �1%, and U.S.

250-year coal reserves drop to 75 years or so (6),which is not at all sustainable. That leaves solar-derived, wind, nuclear, and geothermal energy asmajor resources for sustainable hydrogen produc-tion. The hydrogen production pathways fromthese resources include electrolysis of water, ther-mal chemical cycles using heat, and biomass pro-cessing (using a variety of technologies rangingfrom reforming to fermentation).

Biomass processing techniques can bene-fit greatly from the wealth of research thathas been carried out over the years on refin-ing and converting liquid and gaseous fossilfuels. Some of these processes require con-siderable amounts of hydrogen, and many ofthese fossil-derived processes can be adaptedfor use with a large variety of biomass-derived feedstocks. Biomass can easily beconverted into a number of liquid fuels, in-cluding methanol, ethanol, biodiesel, and py-rolysis oil, which could be transported andused to generate hydrogen on site. For thehigh-biomass-yield processes, such as corn toethanol, hydrogen is required in the form ofammonia for fertilizer. Although biomass isclearly (and necessarily) sustainable, it can-not supply hydrogen in the amounts required.It remains to be seen, in a world that is bothfood-limited and carbon-constrained, wheth-er the best use of biomass is for food, as achemical feedstock, or as an energy source.

Because the direct thermal splitting ofwater requires temperatures of �2000°C andproduces a rapidly recombining mixture ofhydrogen and oxygen (10), a number of ther-mal chemical cycles have been identified thatcan use lower temperatures and produce hy-drogen and oxygen in separate steps. The onethat has received the greatest attention in-volves sulfuric acid (H2SO4) at 850°C andhydrogen iodide (HI) at 450°C (11). The nextgeneration of fission reactors includes de-signs that can provide the necessary heat;however, a number of critical material prop-erties must be satisfied to meet the requiredstability under the operating conditions of HI

National Renewable Energy Laboratory, Golden, CO80401–3393, USA. E-mail: jturner@nrel.gov

T O W A R D A H Y D R O G E N E C O N O M Y

13 AUGUST 2004 VOL 305 SCIENCE www.sciencemag.org972

SPECIALSECTION

and H2SO4. For safety reasons, a fairly longheat transfer line (�1 km) is necessary, sothat the hydrogen-producing chemical plantis located away from the reactor. If the issuesof nuclear proliferation and reprocessing canbe dealt with, then reactors based on thesedesigns could potentially supply many hun-dreds of years of energy, but even that is notultimately sustainable. Solar thermal systemscould also be used to drive such thermalchemical cycles, although more interestingcycles involve the use of metal/metal oxidesystems, in which solar heat is used to con-vert an oxide to the metal (releasing oxygen),and then the metal is reacted with water toproduce hydrogen and reform the oxide (12).

Any technology that produces electricitycan drive an electrolyzer to produce hydro-gen. Because of the enormous potential ofsolar and wind (13), it seems possible thatelectrolysis can supply future societies withwhatever hydrogen would be necessary. Fig-ure 1 shows the cost of hydrogenfrom electrolysis, based on thecost of the electricity and theefficiency of the electrolyzer(note that these are system effi-ciencies and include all losses)(14, 15). For example, some sys-tems provide high-pressure (70-MPa) hydrogen via electrochem-ical pressurization. Average U.S.electricity prices range from 4.8¢for large-scale industrial users to8.45¢ for commercial users (16).Based on thermodynamic consid-erations alone, improvements inthe efficiency of electrolysis arenot going to lead to major reduc-tions in the cost of produced hy-drogen. Additionally, as the costof electricity goes down [unsub-sidized wind is already below 4¢per kilowatt-hour (kWh)], efficiency has alower impact on the cost of the hydrogen.Rather, improvements and innovations inthe capital cost of the plant and the lifetimeof the cell and its maintenance require-ments are where the major cost savings willlikely be obtained.

System efficiencies of commercial elec-trolyzers range from 60 to 73%, so one argu-ment often used to discount electrolysis is itsperceived low efficiency. However, althoughefficiency is certainly important, it is neithera good proxy for deciding on new technolo-gy, nor should it be the determining factor. Ifcombined-cycle natural gas plants had thesame efficiency as coal plants, they wouldn’tbe economical at all; and even with theirhigher efficiency, they produce electricity at ahigher cost than coal.

The energy required to split water can beobtained from a combination of heat andelectricity. At 25°C, there is enough heat in

the environment that the electricity require-ment drops to 1.23 V. Increasing the electrol-ysis temperature can lower the electrolysisvoltage, but the total amount of energy re-quired to split water remains relatively con-stant (actually, the isothermal potential in-creases slightly). Thus, higher-temperatureelectrolysis only makes sense if the heat isfree and it only requires a small amount ofenergy to move it where you need it, or thereis an advantage in a new material set (lowercost, longer lifetime, etc.) or a significantdecrease in the electrolysis energy losses.Possible areas for heat plus electrolysis op-tions include nuclear, geothermal, and a num-ber of solar-based configurations.

The amount of water needed to producehydrogen for transportation is not great. Con-version of the current U.S. light-duty fleet(some 230 million vehicles) to fuel cell ve-hicles would require about 100 billion gallonsof water/year to supply the needed hydrogen

(17). Domestic personal water use in theUnited States is about 4800 billion gallons/year. The U.S. uses about 300 billion gallonsof water/year for the production of gasoline(18), and about 70 trillion gallons of water/year for thermoelectric power generation(19). Solar and wind power do not requirewater for their electricity generation. So notonly do these resources provide sustainablecarbon-free energy, they reduce the waterrequirements for power generation.

Impurities in the water can significantlyreduce the lifetime of the electrolysis cell.Water is usually purified on site, but watercleanup could add to the cost of the hydrogen.In a stationary system where hydrogen isused for energy storage, the water from thefuel cell could be cycled back to the electro-lyzer with minimal purification.

Sustainable hydrogen production technolo-gies that may affect hydrogen production in thefuture include photobiological (20) and photo-

electrochemical approaches (21–23). Thesesystems produce hydrogen directly from sun-light and water, and offer the possibility ofincreasing the efficiency of the solar-to-hydro-gen pathway (24) and lowering the capital costof the system, but they still require land area tocollect sunlight. These systems might allow theuse of seawater directly as the feedstock insteadof high-purity water.

General CommentsAn important consideration is the energy pay-back during a time of rapid growth of a newenergy or energy carrier technology. Therewill likely be an extended period of timewhen the new technologies consume moreenergy than they produce. The time frame forconversion to an alternative energy system istypically/historically 75 to 100 years. Withthis in mind, we need to think carefully abouthow many intermediate technology steps weintroduce and how long (and at what cost) we

must operate them in order tomake the energy payback posi-tive. The energy required to sus-tain a growth rate must also betaken into account.

Most hydrogen-producingsystems being proposed aresmaller than the current central-ized power plants. Instead ofbuilding a small number of largegenerating plants, a large numberof smaller plants such as windfarms and solar arrays are pro-posed that, when added together,can produce large amounts of en-ergy. To be considered then is thebenefit of a technology that isamenable to mass manufacturing.Much higher volumes can trans-late into cost savings. Electrolyz-ers, fuel cells, and battery tech-

nologies all fall into this area.Although a great deal of money, thought,

and energy are currently going into seques-tration technologies, the question still re-mains: Is this the best way to spend ourlimited supply of energy and financial capi-tal? As I said earlier, the best use of carbon-free sustainable electricity would be to re-place coal-burning power plants (13). Justbecause we have large coal reserves does notmean that we must use them. The question iswhether we have the will to leave that energyin the ground and move on to somethingmore advanced. Sustainable energy systemscan easily provide (albeit at some cost) suf-ficient amounts of both electricity and hydro-gen. Although current gasoline-powered hy-brid vehicles can reduce fossil fuel use, theycannot eliminate it. For transportation, theresearch, development, and demonstration ofthe hydrogen economy are well served byusing the existing natural gas–based infra-

Fig. 1. The cost of hydrogen based on the electricity prices alone; nocapital, operating, or maintenance costs are included in the calculation.HHV, higher heating value.

T O W A R D A H Y D R O G E N E C O N O M Y

www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004 973

SPECIALSECTION

structure. Integrating sustainable energy sys-tems into the infrastructure would allow rapidadoption of electrolysis-based hydrogen pro-duction, whenever these future transportationsystems become viable. Since the 1930s, therecognized vision of the hydrogen economyhas been to allow the storage of electricalenergy, reduce environmental emissions, andprovide a transportation fuel. This goal isclearly achievable, but only with a sustained,focused effort.

References and Notes1. For the purpose of this discussion, I use the following

definition of the hydrogen economy: the production,storage, distribution, and use of hydrogen as an en-ergy carrier.

2. Jules Verne, The Mysterious Island (available at http://www.literature-web.net/verne/mysteriousisland,1874).

3. P. Hoffmann, The Forever Fuel: The Story of Hydrogen(Westview Press, Boulder, CO, 1981).

4. D. Gregory, Sci. Am. 228, (no. 1), 13 (January 1973).5. Basic Research Needs for the Hydrogen Economy,

available at http://www.sc.doe.gov/bes/reports/files/

NHE_rpt.pdf (current U.S. production is about 9 mil-lion tons of hydrogen per year).

6. P. Weisz, Phys. Today 57 (no. 7), 47 (2004).7. A. Bartlett, Phys. Today 57 (no. 7), 53 (2004).8. G. Richard, ILEA Leaf, Winter 2002 (available at www.

ilea.org/leaf/richard2002.html).9. Energy Information Administration, unpublished file

data of the Coal Reserves Data Base (February 2004),available at http://www.eia.doe.gov/pub/international/iea2002/table82.xls.

10. A. Steinfeld, Solar Energy, in press (available online 3February 2004).

11. Nuclear Production of Hydrogen, Second InformationExchange Meeting–Argonne, Illinois, USA 2-3 October2003 (Organisation for Economic Cooperation and De-velopment, Paris) (available at http://oecdpublications.gfi-nb.com/cgi-bin/OECDBookShop.storefront/EN/product/662004021P1).

12. A. Steinfeld, Int. J. Hydrogen Energy 27, 611 (2002).13. J. A. Turner, Science 285, 5428 (1999).14. J. Ivy, Summary of Electrolytic Hydrogen Production:

Milestone Completion Report, available at www.osti.gov/servlets/purl/15007167-aF4rPu/native/.

15. In any discussion concerning the efficiency of elec-trolyzers, it is appropriate to use the higher heatingvalue to calculate the efficiency. This corresponds tothe isothermal potential (1.47 V � 39 kWh/kg) andrepresents the assumption that all the energy neededto split water comes from the electricity.

16. These figures are from the Energy Information Ad-

ministration, available at www.eia.doe.gov/cneaf/electricity/epm/tablees1a.html.

17. For an estimate of the amount of water needed forhydrogen-powered fuel cell vehicles, we will assumea vehicle fuel economy of 60 miles per kg of H2, thatvehicle miles traveled � 2.6 � 1012 miles/year (found at www.bts.gov/publications/national_transportation_statistics/2002/html/table_automo-bile_profile.html), and that 1 gallon of water contains0.42 kg of H2. Total water required for the U.S.fleet � (2.6 � 1012 miles/year)(1 kg of H2/60miles)(1 gal H2O/0.42 kg of H2) � 1.0 � 1011 gallonsof H2O/year. This represents the water used directlyfor fuel. If one considers all water uses along thechain; for example, from construction of wind farmsto the electrolysis systems (life cycle assessment),then the total water use would be in the range of3.3 � 1011gallons H2O/year.

18. This is a life cycle analysis (M. Mann and M. Whitaker,unpublished data). The United States used about 126billion gallons of gasoline in 2001 [see link in (17)].

19. See http://water.usgs.gov/pubs/circ/2004/circ1268/.20. A. Melis, Int. J. Hydrogen Energy 27, 1217 (2002).21. O. Khaselev, J. A. Turner, Science 280, 425 (1998).22. M. Graetzel, Nature 414, 338 (2001).23. N. Lewis, Nature 414, 589 (2001).24. O. Khaselev, A. Bansal, J. A. Turner, Int. J. Hydrogen

Energy 26, 127 (2001).25. Contributions by D. Sandor for careful manuscript

edits and by J. Ivy for Fig. 1 are gratefully acknowl-edged.

V I E W P O I N T

Hybrid Cars Now, Fuel Cell Cars LaterNurettin Demirdoven1 and John Deutch2*

We compare the energy efficiency of hybrid and fuel cell vehicles as well asconventional internal combustion engines. Our analysis indicates that fuel cellvehicles using hydrogen from fossil fuels offer no significant energy efficiencyadvantage over hybrid vehicles operating in an urban drive cycle. We conclude thatpriority should be placed on hybrid vehicles by industry and government.

Our interest in moving toward a hydrogeneconomy has its basis not in love of themolecule but in the prospect of meeting en-ergy needs at acceptable cost, with greaterefficiency and less environmental damagecompared to the use of conventional fuels.One goal is the replacement of today’s auto-mobile with a dramatically more energy-efficient vehicle. This will reduce carbon di-oxide emissions that cause adverse climatechange as well as dependence on importedoil. In 2001, the United States consumed 8.55million barrels of motor gasoline per day (1),of which an estimated 63.4% is refined fromimported crude oil (2). This consumption re-sulted in annual emissions of 308 million

metric tons (MMT) of carbon equivalent in2001, accounting for 16% of total U.S. car-bon emissions of 1892 MMT (3).

Two advanced vehicle technologies that arebeing considered to replace the current fleet, atleast partially, are hybrid vehicles and fuel cell(FC)–powered vehicles. Hybrid vehicles add aparallel direct electric drive train with motor andbatteries to the conventional internal combustionengine (ICE) drive train. This hybrid drive trainpermits significant reduction in idling losses andregeneration of braking losses that leads to great-er efficiency and improved fuel economy. Hy-brid technology is available now, although itrepresents less than 1% of new car sales. FCvehicles also operate by direct current electricdrive. They use the high efficiency of electro-chemical fuel cells to produce power from hy-drogen. For the foreseeable future, hydrogen willcome from fossil fuels by reforming natural gasor gasoline. FC vehicle technology is not heretoday, and commercialization will require a largeinvestment in research, development, andinfrastructure (4).

Here, we evaluate the potential of theseadvanced passenger vehicles to improve en-

ergy efficiency. We show that a tremendousincrease in energy efficiency can be realizedtoday by shifting to hybrid ICE vehicles,quite likely more than can be realized by ashift from hybrid ICE to hybrid FC vehicles.

Energy Efficiency ModelTo provide a basis for comparison of thesetwo technologies, we use a simple model (5)for obtaining the energy efficiency of thevarious power plant– drive train–fuel com-binations considered in more detailed stud-ies (6–11). In general, the energy efficiencyof ICEs with a hybrid drive train and fromFC-powered vehicles vary depending onthe vehicle configuration and the type ofengine, drive train, and fuel (natural gas,gasoline, or diesel).

For each configuration, we determinewell-to-wheel (WTW) energy efficiency for avehicle of a given weight operating on aspecified drive cycle. The overall WTW ef-ficiency is divided into a well-to-tank (WTT)and tank-to-wheel (TTW) efficiency so thatWTW � WTT � TTW.

We begin with the U.S. Department of En-ergy (DOE) specification of average passengerenergy use in a federal urban drive cycle, theso-called FUDS cycle (12). For example, fortoday’s ICE vehicle that uses a spark ignitionengine fueled by gasoline, the TTW efficiencyfor propulsion and braking is 12.6% (Fig. 1A).

1Technology and Policy Program, Engineering SystemsDivision, Massachusetts Institute of Technology,Cambridge, MA 02139, USA. 2Department of Chem-istry, Massachusetts Institute of Technology, Cam-bridge, MA 02139, USA. J.D. is a director of Cummins(a manufacturer of diesel engines) and an advisor toUnited Technologies Corporation, UTC Power, a fuelcell manufacturer.

*To whom correspondence should be addressed. E-mail: jmd@mit.edu

T O W A R D A H Y D R O G E N E C O N O M Y

13 AUGUST 2004 VOL 305 SCIENCE www.sciencemag.org974

SPECIALSECTION

The TTW efficiency of other configura-tions is estimated by making changes in thebaseline ICE parameters and calculating en-ergy requirements beginning with energy out-put. A hypothetical hybrid ICE (HICE),based on current hybrid technology, thatcompletely eliminates idling losses and cap-tures a portion (50%) of braking losses forproductive use (13) will have a TTW effi-ciency of 26.6% (Fig. 1B). Both the ICE andthe HICE use gasoline fuel directly, so nofuel processing is needed.

A likely hydrogen-based car might be aproton-exchange membrane (PEM) FC-powered vehicle with a hybrid power train.This advanced fuel cell (AFC) vehicle hasan on-board fuel processor that reformsgasoline to hydrogen fuel suitable for feedfor the PEM fuel cell. We assume a reform-er efficiency of 80% and 50% efficiency forthe FC stack operating over the urban drivecycle. We include a power train with thesame characteristics as the HICE vehicle.The TTW efficiency of this configuration is28.3% (Fig. 1C).

It is apparent that any alternative vehicleconfiguration of fuel–power plant–drive traincan be considered in a similar fashion. Forexample, if hydrogen were available withoutenergy cost, the overall efficiency would im-prove to 39.0%, over three times that for theconventional ICE (14). A diesel ICE with ahybrid power traincould achieve anefficiency of 31.9%,assuming that thishigher compressiondirect-injection en-gine has an efficiencyof 45.0% comparedto 37.6% for the gas-oline ICE.

Our results (Fig. 2)are in reasonably goodagreement with thoseof more detailed stud-ies but do not re-quire elaborate simula-tion models. Figure 2shows that, except for

the Argonne National Laboratory/General Motors(ANL/GM) (6) study, the relative gain in efficien-cy in moving from an ordinary ICE to a HICE ismore than twofold. The reason for this differenceis not clear, because the TTW analysis in thatstudy has its basis in a GM proprietarysimulation model.

Validating Our ModelTo test the validity of these comparisons and oursimple model, we have used an advanced vehiclesimulator called ADVISOR, developed by theNational Renewable Research Laboratory(NREL) of DOE (15). ADVISOR provides es-timates of energy efficiencies for different vehi-cle configurations. ADVISOR shows the broadrange of vehicle performance that is possiblewith a reasonable choice of system parameterssuch as maximum engine power, maximum mo-tor power, transmission type, and brake energyregeneration. The parameters we selected for thesimulation of the ICE, HICE, and AFC are givenin Table 1; for comparison, TTWs based on thissimulation are 28.8% for the Toyota Prius and26.2% for the Honda Insight. Except for theANL/GM results, all studies point to large po-tential energy efficiency gains from hybrid vehi-cles in urban drive cycles compared to cars withconventional ICEs (16).

Our analysis shows that hybrids offer thepotential for tremendous improvement in en-ergy use and significant reduction in carbonemissions compared to current ICE technol-ogy. But hybrid vehicles will only be adoptedin significant quantities if the cost to theconsumer is comparable to the conventionalICE alternative. Hybrid technology is heretoday, but, of course, hybrid vehicles costmore than equivalent ICE vehicles because ofthe parallel drive train. Estimates of the costdifferential vary, but a range of $1000 to$2000 is not unreasonable. Depending on themiles driven, the cost of ownership of a hy-brid vehicle may be lower than a convention-al ICE, because the discounted value of thefuel saving is greater than the incrementalcapital cost for the parallel drive train and

Fig. 1. Energy flow for various vehicle configurations. (A) ICE, the conventional internal combustion, sparkignition engine; (B) HICE, a hybrid vehicle that includes an electric motor and parallel drive train whicheliminates idling loss and captures some energy of braking; (C) AFC a fuel cell vehicle with parallel drivetrain. The configuration assumes on-board gasoline reforming to fuel suitable for PEM fuel cell operation.

Fig. 2. Comparison of WTW energy efficiencies of advanced vehicle systemsusing gasoline fuel. Color coding follows that in Fig. 1. 90% WTT efficiencyin all cases; thus WTW � 0.90 TTW. Data for ICE and HICE is from (7 ),table 5.3. Data for AFC is from (8), which does not give energy efficiencydirectly. We derive a range for energy efficiency by comparing data intables 8 and 9 for MJ/km for vehicle and fuel cycle for the 2020 ICE hybridto that of the gasoline FC hybrid given in (7), table 5.3. Data from (6 ),table 2.1. Data from NREL’s ADVISOR simulation; for details, see Table 1.

T O W A R D A H Y D R O G E N E C O N O M Y

www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004 975

SPECIALSECTION

electric motor. Thus, hybrid vehicles cancon tribute to lower emissions and lesspetroleum use at small or negative socialcost (17 ). Today only Toyota and Hondaoffer hybrids in the United States; Daimler-Chrysler, Ford, and General Motors areplanning to introduce hybrids in the periodfrom 2004 to 2006. At present there is afederal tax credit of $1500 for purchase ofa hybrid vehicle, but it is scheduled tophase out in 2006 (18).

Fuel cell technology is not here today.Both the Bush Administration’s Freedom-CAR program and the earlier Clinton Ad-ministration Partnership for a New Gener-ation of Vehicles (PNGV) launched majorDOE research and development initiativesfor FC-powered vehicles. The currentFreedomCAR program “focuses govern-ment support on fundamental, high-riskresearch that applies to multiple passenger-vehicle models and emphasizes the develop-ment of fuel cells and hydrogen infrastructuretechnologies” (19). A successful automotive FCprogram must develop high-durability FC stackswith lifetimes of 5 to 10 thousand hours, wellbeyond today’s experience. It is impossible toestimate today whether the manufacturing costrange that FC stacks must achieve for economicalpassenger cars can be reached even at the large-scale production runs that might be envisioned.

The government FC research and devel-opment initiative is welcome, but it is notclear whether the effort to develop economicFC power plants for passenger cars will besuccessful. In parallel, we should place pri-ority on deploying hybrid cars, beginningwith today’s automotive platforms and fuels.If the justification for federal support forresearch and development on fuel cells isreduction in imported oil and carbon dioxideemissions, then there is stronger justificationfor federal support for hybrid vehicles thatwill achieve similar results more quickly.Consideration should be given to expandinggovernment support for research and devel-opment on generic advanced hybrid technol-ogy and extending hybrid vehicle tax credits.

References and Notes1. Calculated from weekly data of supplied gasoline

products published by DOE, Energy InformationAgency; see www.eia.doe.gov/oil_gas/petroleum/info_glance/gasoline.html.

2. “National transportation statistics 2002,” U.S. Depart-ment of Transportation, Bureau of Transportation Sta-tistics (BTS02-08, Washington, DC, 2002), table 4-1.

3. “Inventory of U.S. greenhouse gas emissions andsinks: 1990-2001,” final version, U.S. EnvironmentalProtection Agency (EPA 430-R-03-004, Washington,DC, 2003), table A-1.

4. Only gasoline and natural gas are widely available asa transportation fuel today; a hydrogen or methanolfueled transportation system would take decades todeploy, at significant cost.

5. We define the average energy efficiency as the ratio

of the energy needed at the wheels, Eout, to drive andbrake a car of a given weight, M, on a specified testcycle to the total fuel energy, Ein, needed to drive thevehicle. Regenerative braking, if present, reduces thefuel needed to drive the car. Accessory power is notincluded in energy output. The TTW efficiency iscalculated as �TTW � Eout/Ein. For the vehicle con-figurations in Fig. 1, we keep Eout constant and cal-culate Ein by backward induction as

E in �1

�fp�e� Eout

�dt� B�rb � Eac � E idle�

where Eidle and Eac are the energies required in thespecified drive cycle for idling and for accessories,respectively. B is the recovered braking energy. Thevarious efficiencies of different stages are �fp, �e, �dt,and �rb for fuel processing, engine or fuel cell, drivetrain, and regenerative braking, respectively. We fo-cus on efficiency rather than the more common fueleconomy because the efficiency is less sensitive tovehicle weight than fuel economy.

6. In 2001, General Motors (GM) collaborated with Ar-gonne National Laboratories (ANL) to use ANL’sGreenhouse Gases, Regulated Emissions, and EnergyUse in Transportation (GREET) model. The report,“GM study: Well-to-wheel energy use and green-house gas emissions of advanced fuel/vehicle system,North American analysis,” is referred to as the ANL/GM study and is available online at http://greet.anl.gov/publications.html.

7. M. A. Weiss, J. B. Heywood, E. M. Drake, A. Schafer, F.AuYeung, “On the road in 2020: A life cycle analysis of newautomobile technologies,” (MIT Energy Laboratory ReportNo. MIT EL 00-003, Cambridge, MA, 2000). We thank M.Weiss for helpful discussions about this work.

8. M. A. Weiss, J. B. Heywood, A. Schafer, V. K. Natara-jan, “Comparative assessment of fuel cell cars” (MITLaboratory for Energy and Environment Report No.2003-001 RP, Cambridge, MA, 2003).

9. P. Ahlvi, A. Brandberg, Ecotraffic Research and Devel-opment, “Well to wheel efficiency for alternativefuels from natural gas to biomass,” Vagverket (Swed-ish National Road Administration), Publication 2001:85 (2001), appendix 1.8.

10. F. An, D. Santini, SAE Tech. Pap. 2003, no. 2003-01-0412 (2003).

11. B. Hohlein, G. Isenber, R. Edinger, T. Grube, Handbookof Fuel Cells, W. Vielstich, A. Gasteiger, A. Lamm, Ed.(Wiley, New York, 2003), vol. 3, chap. 21, p. 245.

12. More information is available at the DOE Web site:www.fueleconomy.gov/feg/atv.shtml.

13. We wish to keep the presentation of our modelsimple. The assumption of complete regenerativebraking and reduction in idling losses is not realis-tic. However, improvement in ICE engine efficiencyis also possible (7). The current performance ofhybrid ICE passenger vehicles such as the ToyotaPrius is impressive. Toyota reports TTW efficiencyof the Prius as 32%, compared to 16% for aconventional ICE: www.toyota.co.jp/en/tech/envi-ronment/fchv/fchv12.html. Prius regenerativebraking reportedly recaptures 30%; see www.ott.doe.gov/hev/regenerative.html.

14. For this case, there is no processor loss, and the FCstack efficiency improves to 55% because the FCfunctions better on pure hydrogen than reformate.

15. The NREL ADVISOR simulator is described online atwww.ctts.nrel.gov/analysis. Use of the model is de-scribed in several publications at www.ctts.nrel.gov/analysis/reading_room.html; see, for example, (20, 21).

16. GM quotes 15 to 20% fuel economy improvementsin 2007 for hybrid Tahoe and Yukon sport utilityvehicles. Not surprisingly, Toyota seems more opti-mistic about hybrids than GM.

17. In Europe, where fuel prices are much higher than inthe United States, the advantage of hybrids overconventional ICEs is significantly greater.

18. The 2003 Energy Act, currently under considerationby Congress, would extend the time period for thehybrid car tax credit.

19. Quote taken from www.eere.energy.gov/vehiclesandfuels/.20. T. Markel et al., J. Power Sources 110, 225 (2002).21. M. R. Cuddy, K. G. Wipke, SAE Tech. Pap. 1997, no.

970289 (1997).22. This work was supported by the Alfred P. Sloan Foundation.

Table 1. Input and output vehicle parameters obtained from NREL’s ADVISOR simulations. Weassumed 1500 kg for the total vehicle weight, including two passengers and fuel on board. Theactual weights of the Toyota Prius and Honda Insight with two passengers and fuel on board are1368 kg and 1000 kg, respectively. Auxiliary power is 700 W except for the Honda Insight, for whichit is 200 W. The simulations are over a FUDS cycle. Fuel use and TTW calculations follow thedefinition of efficiency given in (5), which is different than the “overall system efficiency” definedin the NREL’s ADVISOR. Of course, the underlying performance is the same.

ICE HICE AFC Prius Insight

VehicleMax power (kW) 102 83 70 74 60Power:weight ratio (W/kg) 68 55 47 54 60Frontal area (m2) 2 2 2 1.75 1.9Rolling resistance coefficient 0.009 0.009 0.009 0.009 0.0054

Engine–motor–fuel cell stackMax engine power (kW) 102 43 43 50Max engine efficiency (%) 38 38 39 40Max motor power (kW) 40 40 31 10Max motor efficiency (%) 92 92 91 96Max fuel cell power (kW) 30Max fuel cell stack efficiency (%) 56

AccelerationTime for 0 to 60 mph (s) 18 10 13 15 12

Fuel use1317 1189

Fuel energy use (kJ/km) 3282 1536 1553 (1274) (982)Fuel economy (mpg) 21 44 43 53 69

Average efficiencies (%)Engine efficiency 21 30 28 25Motor efficiency 79 84 81 90Reformer efficiency 80Fuel cell stack efficiency 51Round-trip battery efficiency 100 84 81 82Transmission efficiency 75 75 93 100 92Regenerative braking efficiency 35 39 41 38TTW efficiency 12.6 27.2 26.6 28.8 26.2

T O W A R D A H Y D R O G E N E C O N O M Y

13 AUGUST 2004 VOL 305 SCIENCE www.sciencemag.org976

SPECIALSECTION

A Compound from Smoke ThatPromotes Seed Germination

Gavin R. Flematti,1* Emilio L. Ghisalberti,1 Kingsley W. Dixon,2,3

Robert D. Trengove4

Smoke derived from burning plant materialhas been found to increase germination of awide range of plant species from Australia,North America, and South Africa (1). Wenow report the identity of a compound,present in plant- and cellulose-derivedsmoke, that promotes germination of a vari-ety of smoke-responsive taxa at a level sim-ilar to that of plant-derived smoke water.

The separation of the bioactive agent was fa-cilitated by bioassay-guided fractionation withLactuca sativa L. cv. Grand Rapids (2) and twosmoke-responsive Australian species, Conostylisaculeata R. Br. (Haemodoraceae) and Stylidiumaffine Sonder. (Stylidiaceae) (3). Extensive frac-tionation of the relatively less complex, cellulose-derived smoke (from combustion of filter paper)resulted in the isolation of a compound that pro-motes seed germination (4). The structure of thiscompound was elucidated from mass spectrome-try (MS) and spectroscopic data obtained by

1H, 13C, and two-dimensional (homonuclear cor-relation, heteronuclear single-quantum coherence,heteronuclear multi-bond correlation, and nuclearOverhauser effect spectroscopy) nuclear magneticresonance (NMR) techniques. Confirmation ofthe structure as the butenolide 3-methyl-2H-furo[2,3-c]pyran-2-one (1) (Scheme1) was achieved by synthesis. Thepresence of 1 in extracts of plant-derived smoke was confirmed by gaschromatography–MS analysis.

We compared the activity of thesynthetic form of the butenolide (1)with that of plant-derived smoke wa-ter by testing it at a range of concen-trations with the three bioassay species. The re-sults (Fig. 1) show that 1 stimulated the germina-tion of each test species to a level similar to thatachieved with plant-derived smoke water. Fur-thermore, activity is demonstrated at very lowconcentrations (�1 ppb, 10�9 M). Testing of

other smoke-responsive Australian species andsmoke-responsive South African (e.g., Syncarphavestita) and North American (e.g., Emmenanthependuliflora and Nicotiana attenuata) species hasfurther confirmed the activity of 1 (table S1).

The butenolide (1) conforms to the necessaryecological attributes of smoke that is producedfrom fires in natural environments. For example,the butenolide (1) is stable at high temperatures(its melting point is 118° to 119°C), water-soluble,active at a wide range of concentrations (1 ppm to100 ppt), and capable of germinating a widerange of fire-following species. The butenolideis derived from the combustion of cellulose,which, as a component of all plants, repre-

sents a universal combustion sub-strate that would be present innatural fires.

Given the broad and emerginguse of smoke as an ecological andrestoration tool (1), the identificationof 1 as a main contributor to thegermination-promoting activity ofsmoke could provide benefits for

horticulture, agriculture, mining, and disturbed-land restoration. In addition, the mode ofaction and mechanism by which 1 stimulates ger-mination can now be investigated. In this context,it is useful to note that the natural product (�)-strigol, which promotes the germination of theparasitic weed Striga (5), is active at similar con-centrations (10�9 M) and contains a butenolidemoiety and additional conjugated functionalitysimilar to those in 1.

References and Notes1. N. A. C. Brown, J. Van Staden, Plant Growth Regul. 22,

115 (1997).2. F. E. Drewes, M. T. Smith, J. Van Staden, Plant GrowthRegul. 16, 205 (1995).

3. S. Roche, K. W. Dixon, J. S. Pate,Aust. J. Bot.45, 783 (1997).4. Materials and methods are available as supporting

material on Science Online.5. S. C. M. Wigchert, B. Zwanenburg, J. Agric. FoodChem. 47, 1320 (1999).

6. We thank L. T. Byrne for assistance with the structuralelucidation of the active compound, D. Wege and S. K.Brayshaw for assistance with the synthetic approach, S. R.Turner and D. J. Merritt for assistance with germinationtrials, and Alcoa World Alumina and Iluka Resources forproviding seeds of native species for testing.

Supporting Online Materialwww.sciencemag.org/cgi/content/full/1099944/DC1Materials and MethodsTable S1

4 May 2004; accepted 25 June 2004Published online 8 July 2004;10.1126/science.1099944Include this information when citing this paper.

1School of Biomedical and Chemical Sciences, 2School ofPlant Biology, The University of Western Australia,Crawley, WA 6009, Australia. 3Kings Park and BotanicGarden, West Perth, WA 6005, Australia. 4School ofEngineering Science, Murdoch University, Rockingham,WA 6168, Australia.

*To whom correspondence should be addressed. E-mail: gflematt@chem.uwa.edu.au

Scheme 1.

Fig. 1. Comparison of the germination response of plant-derived smoke water and butenolide (1) atdifferent concentrations with three smoke-responsive species: Grand Rapids lettuce, C. aculeata, and S.affine. Water served as the control, and “neat” refers to undiluted smoke water. Values are means ofthree replicates � SE.

BREVIA

www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004 977

Simulations of Jets Driven byBlack Hole Rotation

Vladimir Semenov,1 Sergey Dyadechkin,1 Brian Punsly2,3*

The origin of jets emitted from black holes is not well understood; however,there are two possible energy sources: the accretion disk or the rotating blackhole. Magnetohydrodynamic simulations show a well-defined jet that extractsenergy from a black hole. If plasma near the black hole is threaded by large-scalemagnetic flux, it will rotate with respect to asymptotic infinity, creating largemagnetic stresses. These stresses are released as a relativistic jet at the expenseof black hole rotational energy. The physics of the jet initiation in the simu-lations is described by the theory of black hole gravitohydromagnetics.

Most quasars radiate a small fraction of theiremission in the radio band (radio quiet), yetabout 10% launch powerful radio jets (a high-ly collimated beam of energy and particles)with kinetic luminosities rivaling or some-times exceeding the luminosity of the quasarhost (radio loud) (1). There is no clear theo-retical understanding of the physics that oc-casionally switches on these powerful beamsof energy in quasars. Powerful extragalacticradio sources tend to be associated with largeelliptical galaxy hosts that harbor supermas-sive black holes [�109 solar masses (MJ) (2).The synchrotron-emitting jets are highlymagnetized and emanate from the environs ofthe central black hole, within the resolutionlimits of very long baseline interferometry(VLBI). Observations [see the supporting on-line material (SOM)] indicate that jets emit-ted from supermassive black hole magneto-spheres may be required for any theory to bein accord with the data (2–4). Previous per-fect magnetohydrodynamic (MHD) simula-tions of entire black hole magnetosphereshave shown some suggestive results. For ex-ample, the simulations of (5) were the first toshow energy extraction from the black hole,but there was no outflow of plasma. Numer-ical models of an entire magnetosphere in-volve a complex set of equations that reflectthe interaction of the background spacetimewith the plasma. In many instances, the onlyway to get any time evolution of the magne-tosphere is to assume unphysical initial con-ditions (5–8). Even so, previous simulationshave not shown a black hole radiating away

its potential energy into a pair of bipolar jets(5–10). Most important, in previous effortsthe underlying physics has been masked bythe complexity of the simulation. Thus, thisnumerical work has done little to clarify thefundamental physics that couples the jet tothe black hole.

We exploit the simplification that the fullset of MHD equations in curved spacetimeindicates that a magnetized plasma can beregarded as a fluid composed of nonlinearstrings in which the strings are mathematical-ly equivalent to thin magnetic flux tubes (11,12). In this treatment, a flux tube is thin bydefinition if the pressure variations across theflux tube are negligible compared to the totalexternal pressure P (gas plus magnetic pres-sure), which represents the effects of the en-veloping magnetized plasma (the magneto-sphere). By concentrating the calculation onindividual flux tubes in a magnetosphere, wecan focus the computational effort on thephysical mechanism of jet production (on allthe field lines). Thus, we are able to elucidatethe fundamental physics of black hole–drivenjets without burying the results in the effort tofind the P function. Our goal is to understandthe first-order physics of jet production, notall of the dissipative second-order effects thatmodify the efficiency, and for these purposesthe string depiction of MHD is suitable (seethe methods section of the SOM). The phys-ical mechanism of jet production is a theoret-ical process known as gravitohydromagnetics(GHM), in which the rotating spacetime ge-ometry near the black hole drags plasma rel-ative to the distant plasma in the same large-scale flux tube, thereby spring-loading thefield lines with strong torsional stresses (2).

The frame-dragging potential of the rotat-ing black hole geometry (described by Kerrspacetime) is responsible for driving the jets.The frame-dragging force is elucidated by theconcept of the minimum angular velocityabout the symmetry axis of the black hole as

viewed from asymptotic infinity, �p � �min,where �p is the angular velocity of the plas-ma [this frame is equivalent to Boyer-Lindquist (B-L) coordinates]. In flat space-time, �p � –c/rsin� � �min � 0, where c isthe speed of light and (r, �) are sphericalcoordinates. By contrast, within the ergo-sphere (located between the event horizon atr� and the stationary limit, where r, �, �, andt are B-L coordinates), �min � 0. The ulti-mate manifestation of frame dragging is thatall of the particle trajectories near the blackhole corotate with the event horizon, �min3� as r 3 r� (the horizon boundary condi-tion) (2). Now consider this �min condition ina local physical frame at a fixed poloidalcoordinate but rotating with d� /dt � �, so asto have zero angular momentum about thesymmetry axis of the black hole, m 0 (theZAMO frames). In this frame (and in allphysical frames; that is, those that move witha velocity less than c), a particle rotating at�min appears to be rotating backward, azi-muthally relative to black hole rotation at c,�p �minf c�� –c. From equation 3.49of (2), the mechanical energy of a particle inB-L coordinates (the astrophysical rest frameof the quasar), � � 0 if �� � c(r2 – 2Mr �a2)1/2 sin �/(� g��), in particular

lim��3 1

� �u0�min

c�g�� , �min

� � c(r2 2Mr � a2)1/2 sin�/g�� (1)

In Eq. 1, the four-velocity of a particle orplasma in the ZAMO frames is given by u� �u0(1, �), so u0 is the Lorentz contraction factorthat is familiar from special relativity; M is themass of the black hole in geometrized units; a isthe angular momentum per unit of mass of theblack hole in geometrized units; � is the speci-fic enthalpy; and the B-L metric coefficient,(g��)1/2, is just the curved-space analog of theazimuthal measure, rsin�, of flat spacetime up toa factor of a few. Because �min � 0 in theergosphere, Eq. 1 implies that if a particle rotatesso that �p � �min, then � �� 0. Similarly, thespecific angular momentum about the symmetryaxis of the hole, m u0 �� (g��)1/2, is negativefor these trajectories as well. Hence, the infall ofthese particles toward the black hole is tanta-mount to extracting its rotational energy.

Our simulation in Fig. 1 is of a perfectMHD (in terms of the field strength tensorF�� u� 0, � �) plasma, threading an ac-creting poloidal magnetic flux tube that be-gins aligned parallel to the black hole spinaxis. The perfect MHD condition means thatthe plasma can short out any electric fieldsthat are generated in its rest frame. There areno existing numerical methods that can incor-porate plasma dissipation into the black hole

1Institute of Physics, State University St. Petersburg,198504 Russia. 2Boeing Space and Intelligence Sys-tems, Boeing Electron Dynamic Devices, 3100 WestLomita Boulevard, Post Office Box 2999, Torrance, CA90509–1999, USA. 3International Center for Relativ-istic Astrophysics (ICRA), University of Rome La Sa-pienza, I-00185 Roma, Italy.

*To whom correspondence should be addressed. E-mail: brian.m.punsly@boeing.com

REPORTS

13 AUGUST 2004 VOL 305 SCIENCE www.sciencemag.org978

magnetosphere (that is, that can go beyondperfect MHD); only the analytical work of (2)captures certain non-MHD effects, and theirresults agree with our simulations. In order tochoose a realistic initial state, we note thatordered magnetic flux has been detected inthe central 100 pc of some galaxies (13).Because Lens-Thirring torques concentratethe accreting plasma toward the plane orthog-onal to the black hole spin axis, we expect thelarge-scale flux to become preferentiallyaligned with the spin axis (14). Long-termnumerical simulations of magnetized accre-tion show a concentration of vertical fluxnear the black hole (15). Even an inefficientaccretion of flux (reconnection of oppositelydirected field lines and resistive diffusion)has been shown to lead to an enhanced fluxdistribution near the hole (15, 16). Conse-quently, we have chosen our simulations tofollow the accretion of a poloidal magneticflux tube that begins aligned parallel to theblack hole spin axis. The consistent magnet-ospheric field configuration is a result of thelong-term accumulation of similar flux tubes.A rapidly spinning black hole is chosen in thesimulation (a/M 0.9998), as is often arguedto be likely in a quasar (17).

Initially the flux tube is rotating with anangular velocity, �F, equal to the local ZAMOangular velocity �0: a rapidly decreasing func-tion with radial coordinate (2). Thus, initially, �0

�� �H, because the flux tube is far from theevent horizon. The flux tube accretes toward theblack hole under the influence of the gravitation-al force. The natural state of plasma motion(geodesic motion) induced by frame dragging isto spiral inward faster and faster as the plasmaapproaches corotation with the event horizon(the horizon boundary condition). By contrast,the natural state of plasma motion in a magneticfield is a helical Larmor orbit that is threadedonto the field lines. Generally, these two naturalstates of motion are in conflict near a black hole.These two strong opposing forces create a glo-bally distributed torsion, creating the dynamicaleffect that drives the simulation (Fig. 1). Theplasma far from the hole is still rotating slowlynear �0 in frame A. As the plasma penetrates theergosphere, �min3� as r3 r�, and �p mustexceed �0 in short order because �0 �� �H

(within t � 0.1 GM/c3 after crossing the station-ary limit). Thus, the ergospheric plasma getsdragged forward, azimuthally, relative to the dis-tance portions of the flux tube, by the gravita-tional field. The back reaction of the field rees-tablishes the Larmor helical trajectories bytorquing the plasma back onto the field lineswith J � B forces (the cross-field current densityJ driven by this torsional stress is sunk within theenveloping magnetosphere). By Ampere’s law, Jcreates a negative azimuthal magnetic field, B�,upstream of the current flow. The J � B back-reaction forces eventually torque the plasma onto� � 0 trajectories as per Eq. 1. Frames B and C

illustrate the fact that the B� created in theergosphere propagates upstream in the formof an MHD plasma wave at later times (t �70 GM/c3) as more and more negative ener-gy (the red portion of the field line) iscreated in the ergospheric region of the fluxtube. In frame C, a bona fide jet (a collimat-ed relativistic outflow of mass) emergesfrom the ergosphere.

The dynamo region for B� in the ergosphereis expanded in frame D. Because B� � 0 up-stream of the dynamo, there is an energy (�–�FB�BP) and angular momentum (� –B�BP)flux along BP, away from the hole in the jet. Thered portion of the field line indicates that the totalplasma energy per particle is negative, E � 0(because the magnetic field is primarily azimuth-al near the black hole, E � � � S/k, where S isthe poloidal Poynting flux along B, and k is thepoloidal particle flux along B), downstream ofthe dynamo. Because B� � 0 downstream, thefield transports energy and angular momentumtoward the hole with the inflowing plasma. Thus,S/k � 0 in the downstream state, implying that ��� 0 in order for E � 0, downstream. The J �B back-reaction forces on the twisted field linestorque the plasma onto trajectories with �p ��min. The ingoing � �� 0 plasma extracts therotational energy of the hole by Eq. 1; thus,black hole rotational inertia is powering the jetin the simulation. This is the fundamentalphysics of GHM (2).

Three additional movies of simulationsare provided with different initial conditions[movie S3 has the same P as Fig. 1, but thefield line geometry is different (the flux tubeis highly inclined); movie S4 has a differentpressure function, P � (r – r�) 2.2; andmovie S5 has four flux tubes] to show thegenerality of the results of the simulation inmovies S1 and S2 (Fig. 1).

It is important to make a connection be-tween what is modeled here and what isobserved in quasar jets. The jet can be con-sidered as a bundle of thin flux tubes similarto those in our simulations (this is clearlyvisualized in movie S5). The jet is composedof Poynting flux and a relativistic outflow ofparticles. In the simulation of Fig. 1, jet plas-ma attains a bulk flow Lorentz factor of 2.5,at r � 50 GM/c2 (which is � 1016 cm from a109 MJ black hole, which is assumed in all ofthe following estimates) in frame C (at t �250 GM/c3). In quasars, VLBI observationsindicate that jet Lorentz factors are in therange of 2 to 30, with most �10 (18). Theresolution of VLBI is on the order of 100 to1000 times the length of the jet in the simu-lations. Physically, it is believed that Poynt-ing flux is required to accelerate the jet plas-ma to very high bulk flow Lorentz factorsof � 10 on parsec scales (19, 20). Further-more, the magnetic tower created by B� inFig. 1, frame C, in combination with the

Fig. 1. Black hole–drivenjet. A jet is produced onthe magnetic flux tubesthat experience thetorsional stress inducedby the opposition ofthe gravitational frame-dragging force with theJ � B electromagneticforce, in which J is thecurrent density and B isthe magnetic field. Thesimulation is of a singleflux tube in an envelop-ing magnetosphere ofsimilar flux tubes. Theentire flux tube rotatesin the same sense as theblack hole, where �H isthe angular velocity ofthe event horizon asviewed from asymptoticinfinity. The red portionsof the field line indicateplasma with negativetotal energy, as viewedfrom asymptotic infini-ty. The black hole has aradius r � GM/c2, which is � 1.5 � 10 cm (14) for a 109-MJ black hole. Frame A is a snapshot of theinitiation of negative energy generation in the ergosphere: the outer boundary of the ergosphere is given byr � GM/c3 (1 � sin�). This effect is seen in (5), but that simulation ends at this early stage. In frame B, anoutgoing Poynting flux emerges from the ergosphere, and frame C shows a well-formed jet. The time lapsesbetween frames, asmeasured by a distant stationary observer, are: A to B, t 78GM/c3 and A to C, t 241GM/c3 � 13 days. The simulation endswith a pair of jets, each over 50GM/c2 in length. FrameD is a close-upof the dynamo region, inwhich B� is created in the jet (that is, where B� changes sign). The pure Alfven speed[ameasure of the ratio ofmagnetic energy to plasma inertia,UA BP/(4�n�c2)1/2] in the simulation isUA �12c to 13c in the region in which Poynting flux is injected into the jet just above the dynamo.

R E P O R T S

www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004 979

poloidal field component BP, naturally pro-vides stable hoop stresses that are the onlyknown collimation mechanism for the large-scale jet morphology of quasars (21, 22). Thenonrelativistic outer layer observed in somejets can be supported by a coexisting, envel-oping, relatively low-power wind or jet fromthe accretion disk (23).

The jets composed of a bundle of stringslike those in the simulation are energeticenough to power quasar jets. The Poyntingflux transported by a pair of bipolar collimat-ed jets is approximately (2)

S �[�F�]2

2�2c(2)

The total magnetic flux in the jet is �. Thefield line angular velocity �F varies from nearzero at the outer boundary of the ergosphere tothe horizon angular velocity �H � 10�4 s�1 inthe inner ergosphere. Thus, S � (�H�)2 �(a�/2Mr�)2 � (a�/M)2 for rapidly rotatingblack holes. Consequently, there are three im-portant parameters that determine jet power: M,a, and B. For rapidly spinning black holes, thesurface area of the equatorial plane in the ergo-sphere becomes quite large; for a/M � 0.996, itis �30 M2 (2). Various accretion flow modelsyield a range of achievable ergospheric fieldstrengths B � 103 G to 2 104 G that equateto � � 1033 G-cm2 to 1034 G-cm2 (2, 14, 24).Inserting these results into Eq. 2 yields a jetluminosity of �1045 to 5 1047 ergs/s. This isconsistent with the estimates of the kinetic lu-minosity of powerful quasar jets (1). The max-imum value of the flux noted above occurswhen the persistent accretion of magnetic fluxhas a pressure that is capable of pushing theinner edge of the accretion disk out of theergosphere (known as magnetically arrestedaccretion) (15, 16). This maximum flux in-serted in Eq. 2 equates to a jet power thatis � 5 to 25 times the bolometric thermalluminosity of the accretion flow in the diskmodels considered in (2, 24).

If 10% of the central black holes in qua-sars were magnetized by the accretion ofvertical flux, this would explain the radioloud/radio quiet quasar dichotomy. To ele-vate this above a conjecture requires obser-vational corroboration of the putative magnet-osphere in radio loud quasars. A significantflux trapped between the black hole and theaccretion disk should modify the innermostregions of the accretion flow. Thus, one canlook for a distinction between radio loud andradio quiet quasar thermal emission at thehighest frequencies. There might already beevidence to support this. The accretion flowradiation has a high-frequency tail in theEUV (extreme ultraviolet). Hubble SpaceTelescope (HST) data indicate that radio qui-et quasars have an EUV excess as comparedto radio loud quasars (25). The EUV suppres-

sion has been explained by the interaction ofthe magnetic field with the inner edge of thedisk, displacing the EUV-emitting gas in ra-dio loud quasars (26).

The simulations presented here explainfive important observations of radio loudquasars: the production of a collimated jet(based on radio observations); a power source(the black hole) that is decoupled from theaccretion flow properties to first order[broadband radio-to-ultraviolet observationsindicate that a quasar can emit most of itsenergy in a jet without disrupting the radia-tive signatures of the accretion flow (see theSOM)]; the suppression of the EUV in radioloud quasars (from HST observations); therelativistic velocity of the jet (from VLBIdata); and the maximal kinetic luminosity ofthe quasar jets (from broadband radio andx-ray observations of radio lobes). The GHMprocess might also drive jets in other systemssuch as microquasars or gamma-ray bursts(27, 28). However, microquasars show corre-lations between accretion disk emission andjet properties, unlike quasars (29). Conse-quently, there is no strong observational rea-son to prefer the black hole over the accretiondisk as the primary power source for micro-quasars, as there is with quasar jets.

References1. K. Blundell, S. Rawlings, Astron. J. 119, 1111 (2000).2. B. Punsly, Black Hole Gravitohydromagnetics

(Springer-Verlag, New York, 2001).3. M. J. Rees, E. S. Phinney, M. C. Begelman, R. D.

Blandford, Nature 295, 17 (1982).4. M. C. Begelman, R. D. Blandford, M. J. Rees, Rev. Mod.

Phys. 56, 255 (1984).5. S. Koide, K. Shibata, T. Kudoh, D. L. Meier, Science295, 1688 (2002).

6. S. Koide, Phys. Rev. D 67, 104010 (2003).

7. S. Koide, Astrophys. J. Lett. 606, L45 (2004).8. S. Komissarov,Mon.Not. R. Astron. Soc.350, 1431 (2004).9. S. Hirose, J. Krolik, J. De Villiers, Astrophys. J. 606,

1083 (2004).10. J. McKinney, C. Gammie, Astrophys. J., in press; pre-

print available at http://arxiv.org/abs/astro-ph/0404512.

11. M. Christensson, M. Hindmarsh, Phys. Rev. D60,063001-1 (1999).

12. V. S. Semenov, S. A. Dyadechkin, I. B. Ivanov, H. K.Biernat, Phys. Scripta 65, 13 (2002).

13. T. J. Jones, Astron. J. 120, 2920-2927 (2000).14. D. Macdonald, K. Thorne, R. Price, X.-H. Zhang, in

Black Holes The Membrane Paradigm, K. Thorne, R.Price, D. Macdonald, Eds. (Yale Univ. Press, NewHaven, CT, 1986), pp. 121–145.

15. I. V. Igumenshchev, R. Narayan, M. A. Abramowicz,Astrophys. J. 592, 1042 (2003).

16. R. Narayan, I. V. Igumenshchev, M. A. Abramowicz,Publ. Astron. Soc. Jpn. 55, 69 (2003).

17. J. Bardeen, Nature 226, 64 (1970).18. K. Kellerman et al. Astrophys. J. 609, 539 (2004).19. R. V. E. Lovelace, M. M. Romanova, Astrophys. J. Lett.

596, 159 (2003).20. V. Nektarios, A. Konigl, Astrophys. J. 605, 656 (2004).21. G. Benford, Mon. Not. R. Astron. Soc. 183, 29 (1978).22. P. Hardee, in Cygnus A–Study of a Radio Galaxy, C. L.

Carilli, D. E. Harris, Eds. (Cambridge Univ. Press, NewYork, 1996), pp. 113–120.

23. G. Henri, G. Pelletier, Astrophys. J. 383, L7 (1991).24. F. Casse, R. Keppens, Astrophys. J. 601, 90 (2004).25. W. Zheng, G. A. Kriss, R. C. Telfer, J. P. Grimes, A. F.

Davidsen, Astrophys. J. 475, 469 (1997).26. B. Punsly, Astrophys. J. 527, 609 (1999).27. S. Eikenberry et al., Astrophys. J. 494, L61 (1998).28. J. Katz, Astrophys. J. 490, 633 (1997).29. R. Fender, in Compact Stellar X-Ray Sources, W. H. G.

Lewin, M. van der Klis, Eds. (Cambridge Univ. Press,Cambridge, in press) (preprint available at http://arxiv.org/abs/astro-ph/0303339).

Supporting Online Materialwww.sciencemag.org/cgi/content/full/305/5686/978/DC1MethodsSOM TextFigs. S1 and S2ReferencesMovies S1 to S5

24 May 2004; accepted 8 July 2004

Localization of FractionallyCharged Quasi-Particles

Jens Martin,1* Shahal Ilani,1† Basile Verdene,1 Jurgen Smet,2

Vladimir Umansky,1 Diana Mahalu,1 Dieter Schuh,3

Gerhard Abstreiter,3 Amir Yacoby1

An outstanding question pertaining to the microscopic properties of the fractionalquantum Hall effect is understanding the nature of the particles that participatein the localization but that do not contribute to electronic transport. By using ascanning single electron transistor, we imaged the individual localized states in thefractional quantum Hall regime and determined the charge of the localizing par-ticles. Highlighting the symmetry between filling factors 1/3 and 2/3, our mea-surements show that quasi-particles with fractional charge e* � e/3 localize inspace to submicrometer dimensions, where e is the electron charge.

The quantum Hall effect (QHE) arises whenelectrons confined to two dimensions are subjectto a strong perpendicular magnetic field. Themagnetic field quantizes the kinetic energy andleads to the formation of Landau levels (LL).Energy gaps appear in the spectrum whenever an

integer number of LLs is filled. Disorder broad-ens the LLs and gives rise to bands of extendedstates surrounded by bands of localized states.Localization plays a fundamental role in theuniversality and robustness of quantum Hallphenomena. In the localized regime, as the den-

R E P O R T S

13 AUGUST 2004 VOL 305 SCIENCE www.sciencemag.org980

sity of electrons increases, only localized statesare being populated and hence the transport co-efficients remain universally quantized (1). Con-versely, when the Fermi energy lies within thebands of extended states, the transport coeffi-cients vary indicating transitions between quan-tum Hall phases (2, 3).

When the filling of the lowest LL is less thanone, the fractional quantum Hall effect (FQHE)emerges (4) as a result of Coulomb interactionsbetween electrons. The Coulomb interactionsgive rise to a new set of energy gaps occurring at

fractional fillings �q

2q � 1with q being an

integer. Soon after the discovery of the FQHE, atheory was presented for the ground state prop-erties of these unique phases (5), in which thelow energy excitations above these ground states

are fractionally charged with e* �e

2q � 1. Only

recently, through use of resonant tunneling andshot noise measurements, were such fractionallycharged quasi-particles shown to exist experi-mentally (6–10). These measurements convinc-ingly demonstrate that the fractionally chargedquasi-particles participate in transport and arehence extended along the edge of the sample.However, what is the nature of the particles thatparticipate in the localization and do not contrib-ute to transport? We address this question withthe use of a scanning single electron transistor(SET), which enables us to detect directly theposition and charge of the localizing particlesacross the various integer and fractional quan-tum Hall phases.

Our experimental method is described indetail elsewhere (11, 12). A SET is used tomeasure the local electrostatic potential of atwo-dimensional electron gas (2DEG) (13).At equilibrium, changes in the local electro-static potential result from changes in thelocal chemical potential, which can be in-duced, for example, by varying the averageelectron density (14) with the use of a backgate. The local derivative of the chemicalpotential, �, with respect to the electronicdensity, d�/dn, is inversely proportional tothe local compressibility and depends strong-ly on the nature of the underlying electronicstates (15, 16). In the case of extended elec-trons, charge is spread over large areas;hence, the chemical potential will follow con-tinuously the variations in density, and themeasured inverse compressibility will besmall and smooth. However, in the case of

localized electrons the local charge densitycan increase only in steps of e/ 2, where isthe localization length. Hence, the systembecomes compressible only when a localizedstate is being populated, producing a jump inthe local chemical potential and a spike in itsderivative, the inverse compressibility. Thus,by using the scanning SET in the localizedregime, we were able to image the positionand the average density at which each local-ized state is populated. Three different GaAs-based 2DEGs with peak mobilities of 2 106

cm2/ V–1 s–1 (V6-94 and V6-131) and 8 106 cm2/ V–1 s–1 (S11-27-01.1) were studied.

We start by reviewing the properties of lo-calized states in the integer QHE. Figure 1Ashows a typical scan of d�/dn as function of theaverage electron density, n, and position, x, nearthe integer quantum Hall phase � 1. Eachblack arc corresponds to the charging line of anindividual localized state. At any particular loca-tion, as the density is varied through � 1,electrons occupying localized states give rise tonegative spikes. The tip detects only charging oflocalized states situated directly underneath it.The measured spatial extent of each localizedstate (black arc) is therefore determined by thesize of the localized state convoluted with thespatial resolution of the tip. A small tip bias isresponsible for the arcing shape of each chargingline. Figure 1A shows that electrons do notlocalize randomly in space but rather pile up atparticular locations. Moreover, the spacing with-in each charging spectrum is regular. Suchcharging spectra are reminiscent of Coulombblockade physics, where charge quantizationgoverns the addition spectrum of a quantum dot.In the microscopic description of localization inthe integer QHE (17), the formation of dots wasexplained with use of a simple model that incor-porates Coulomb interaction between electronsand thereby accounts for changes in the screen-ing properties of the 2DEG as the filling factor isvaried (18–21). For completeness, we brieflysketch the model for � 1 because it will proveto be essential for understanding the measuredspectra at fractional filling factors.

An intuitive picture of localization driven byCoulomb interaction is obtained by tracing theself-consistent density distribution as a functionof B and n. Far from integer filling, the largecompressibility within an LL provides nearlyperfect screening of the disorder potential. Thisis accomplished by creating a nonuniform den-sity profile with a typical length scale larger thanthe magnetic length, lm � �h/(eB) (Fig. 1B).The corresponding potential distribution in theplane of the 2DEG is equal and opposite in signto the disorder potential. Because of a largeenergy gap between the LLs, the density, nLL,cannot exceed one electron per flux quanta,nmax � B/�0, and is therefore constrained by 0 �nLL � nmax. At the center of the LL (far frominteger filling), this constraint is irrelevant be-cause the variations in density required for

screening are smaller than nmax, thus leading tonearly perfect screening (Fig. 1B). Each electronadded to the system experiences this flat poten-tial and thus, within this approximation, is com-pletely delocalized. With increasing filling fac-tor, the density distribution increases uniformly,maintaining its spatial structure. However, near � 1 the average density approaches nmax andthe required density distribution for perfectscreening exceeds nmax at certain locations. Be-cause of a large energy gap between the LLs, nLL

cannot exceed nmax, and the density at theselocations becomes pinned to nmax. Local incom-pressible regions are formed in which the baredisorder potential is no longer screened, coexist-ing with compressible regions where the LL isstill only partially full [0 � nLL(x) � nmax, where

1Weizmann Institute of Science, Condensed MatterPhysics, 76100 Rehovot, Israel. 2Max-Planck Institutfur Festkorperforschung, D-70569 Stuttgart, Germa-ny. 3Walter-Schottky Institut, Technische UniversitatMunchen, D-85748 Garching, Germany.

*To whom correspondence should be addressed. E-mail: jens.martin@weizmann.ac.il†Present address: Laboratory of Atomic and SolidState Physics, Cornell University, Ithaca, New York14853, USA.

9.9

7.3

2.50.0 x ( m)µ

schematic density profiles

D

A

2.50.0 x ( m)µ

n (a

.u)

incompr.dot

incompressibleanti-dot

dot

compressible

C

1

nmin

nmax

n (1

0

cm )

10-2

nmax

nmin

B

Fig. 1. (A) Density-position scan of d�/dn near � 1 [B � 3.5 T, temperature (T) � 300 mK,sample V6-94]. Each black line corresponds to alocalized state. The central white area corre-sponds to the incompressible region near com-plete filling of the first LL (the dashed line marks � 1). Localized states group into families, whichappear as QD spectra above (and antidot spectrabelow) the incompressible region. (B) Far frominteger filling, the large compressibility within anLL provides nearly perfect screening at the cost ofa nonuniform density profile. Here, a schematicdensity profile is shown. nmax and nmin indicatethe maximum and minimal densities, respective-ly, within an LL. a.u., arbitrary units. (C) In case theaverage density is slightly less than completefilling, the 2DEG becomes incompressible when-ever the maximum allowed density nmax isreached and remains compressible only in a smallarea. This area forms an antidot whose energylevels are determined by the charging energy. (D)Once the average density is slightly higher thancomplete filling, compressible areas appear in thenext LL and form QDs.

R E P O R T S

www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004 981

x is the position] and the local potential isscreened (Fig. 1C). Once an incompressible re-gion surrounds a compressible region, it behavesas an antidot whose charging is governed byCoulomb blockade physics. Antidots are there-fore formed in the regions of lowest localdensity, and as the filling increases further theantidot will be completely emptied. Spatiallyseparated from such local density minima are themaxima of the density profile, where the fillingof the next LL will first occur, now as quantumdots (QDs) (Fig. 1D).

An important consequence of the abovemodel is that the spectra of localized statesare defined only by the bare disorder poten-tial, the presence of an energy gap, causing anincompressible quantum Hall liquid sur-rounding the QD, and the charge of the lo-calizing particles. It is the character of theboundary that determines the addition spectraof the quantum dot, and it is because ofCoulomb blockade that the charge directlydetermines the number of charging lines andtheir separation and their strength. Therefore,the number of localized states does not de-pend on the magnetic field. A different mag-netic field will simply shift nmax and hencethe average density at which localizationcommences. Therefore, the local chargingspectra will evolve as a function of densityand magnetic field exactly parallel to thequantized slope of the quantum Hall phases,dn/dB � e/h (Fig. 2, A, I and III).

The presence of an energy gap at fractionalfilling factors implies that the model for local-ization described above may also apply to thefractional quantum Hall regime. As in the integercase, the local charging spectra contain a fixednumber of localized states (independent of B)that evolve in density and magnetic field accord-ing to dn/dB � e/h. The difference between theinteger and fractional case is that now the slopesare quantized to fractional values (Fig. 2, A, IIand IV, and B). The invariance of the localcharging spectra along fractional filling factorscan also be inferred from Fig. 2, C and D, wherewe show that the spatially dependent chargingspectra at � 1/3 for two different magneticfields are identical. Our observations confirmthat the microscopic mechanism for localizationin the fractional quantum Hall regime is identicalto that of the integer case; i.e., localization isdriven by Coulomb interaction. The QDs thatappear near integer or fractional filling factorsare, therefore, identical. Their position, shape,and size are solely determined by the underlyingbare disorder potential. This conclusion allowsus to determine the charge of localizing particlesin the fractional quantum Hall regime by com-paring the charging spectra of integer and frac-tional filling factors. Charging spectra corre-sponding to the localization of electrons shouldlook identical to those seen at integer filling. Onthe other hand, the charging spectra of fraction-ally charged quasi-particles, with e* � e/3 for

example, would have a denser spectrum withthree times more charging lines, i.e, localizedquasi-particle states. We emphasize that theslope of the localized states in the n-B planemerely indicates the filling factor they belong torather than their charge.

The spatially resolved charging spectra forinteger � 1 and � 3 and fractional � 1/3and � 2/3 are shown in Fig. 3. All the scans aretaken along the same line in space and cover thesame density interval. The scans differ only inthe starting density and the applied magneticfield. The spatially resolved charging spectra forinteger fillings in Fig. 3, A and B, look identical.As expected, despite the different filling fac-tors the measured spectra of localized states ap-pear at the same position in space and withidentical spacing. Figure 3, C and D, showsspatially resolved charging spectra at � 1/3and � 2/3. Here also, the two spectra lookidentical. Moreover, the charging spectra areseen at the same locations as in the integer case.The only difference between the spectra taken atinteger and fractional filling is the number of

charging lines within a given range of den-sities. At � 1/3 and � 2/3, there arethree times more charging lines and theseparation between charging lines is threetimes smaller. This can be also seen in Fig.3, E to H.

Our model assumes large energy gaps rela-tive to the bare disorder potential. In practice,however, the gap for fractional filling factors isconsiderably smaller than in the integer case. Inorder to have a comparable gap in the integer andfractional regime, we first measured the integerquantum Hall phases at low magnetic field andthen measured the fractions at high field. Such acomparison is meaningful because the number oflocalized states is independent of magnetic field.In order to eliminate any doubt that the highernumber of localized states in the fractional re-gime result from the measurement at higherfields, we also compare in Fig. 3, G and H,spectra of � 1 and � 1/3 measured at thesame magnetic field, B � 7.25 T. Regardless offield or density, the spectrum of � 1/3 is threetimes denser than that of � 1.

6.5

5.4

13.07.3 B (T)

1.00.0 x (µm)

10.4

6.4

D

1/3, B = 11.4T

1.00.0 x (µm)

9.2

8.1

C

∆B = 0.3T ∆B = 0.1T

III

A,I BII

IV

1 1/3

2 2/3

2/5

1/3, B = 8.0T

1/3

∆n1

10

cm10

-2

n(1

0cm

)10

-2n

(10

cm)

10-2

n(1

0cm

)10

-2 ∆n

110

cm10

-2

Fig. 2. (A) Density-magnetic field scans for integer � 2 (I) and � 1 (III) and for fractional �2/3 (II) and � 1/3 (IV) (sample V6-131). All scans cover the same density range �n. The rangein magnetic field is three times larger for the fractional fillings. Each black line corresponds to alocalized state. (B) This density-magnetic field scan covers a much larger range in field and densitythan (A) and shows localized states for � 1/3 and � 2/5 (sample S11-27-01.1, T � 100 mK).Even for a change of the magnetic field by a factor of 2, the number of localized states does notchange. (C) Density-position scan for � 1/3 at B � 8 T (sample V6-131, T � 300 mK). Just asfor the integer QHE, the localized states appear as QD spectra. (D) Scan at the same position asin (C) but at B � 11.4 T. The same QD spectra are recovered at higher density. They are betterresolved as a result of the enhanced gap size at higher fields.

R E P O R T S

13 AUGUST 2004 VOL 305 SCIENCE www.sciencemag.org982

Our results constitute direct evidence thatquasi-particles with charge e/3 localize at �1/3 and � 2/3. Moreover, our results highlightthe symmetry between filling factors 1/3 and 2/3,indicating directly that at � 2/3 the quasi-

particle charge is e/3. In contrast to the experi-ments on resonant tunneling across an artificialantidot (6, 7), our measurements probe genericlocalized states in the bulk of the 2DEG. Thelocalization area, 2, can be inferred from the

charging spectra by using the condition forcharge neutrality: e* � e�n 2, where is themeasured spacing between charging lines inunits of density. For the left spectrum in Fig. 1D,e* � e/3 and �n � 1 1011 cm–2, whichcorresponds to � 200 nm, indicating quasi-particle localization to submicrometer dimen-sions. The extracted localization length providesfurther support for the validity of our model,which assumes long-range disorder relative tothe magnetic length.

So far we have concentrated on the levelspacing of the charging spectra. We now turn toaddress the amplitude of a single charging event.The integrated signal across a single chargingevent (spike) is the change in the local chemicalpotential associated with the addition of a singlecharge quantum, given by �� � e*/Ctot, whereCtot is the total capacitance of the QD. Becausethe capacitance between the QD and its sur-rounding is unknown experimentally, one cannotuse this jump in chemical potential to determinethe absolute charge of the localized particle.However, knowing that the QDs formed at inte-ger and fractional filling are identical, one ex-pects Ctot to be unchanged. Therefore, the ratioof �� at different filling factors is a measure ofthe ration of quasi-particle charge. Figure 4shows a cross section of the measured d�/dnthrough the center of a QD and the integratedsignal ��. One can clearly see that the stepheight in the fractional regime is only about 1/3of the step height in the integer regime, confirm-ing the localization of quasi-particles with e* �e/3 for both � 1/3 and � 2/3.

References and Notes1. R. E. Prange, S. M. Girvin, Eds., The Quantum Hall

Effect (Springer-Verlag, New York, ed. 2, 1990), chap.1.

2. S. Kivelson, D. H. Lee, S. C. Zhang, Phys. Rev. B 46,2223 (1992).

3. B. Huckestein, Rev. Mod. Phys. 67, 357 (1995).4. D. C. Tsui, H. L. Stormer, A. C. Godard, Phys. Rev. Lett.48, 1559 (1982).

5. R. B. Laughlin, Phys. Rev. Lett. 50, 1359 (1983).6. V. J. Goldman, B. Su, Science 267, 1010 (1995).7. J. D. F. Franklin et al., Surf. Sci. 361-362, 17 (1996).8. L. Saminadayar, R. V. Glattli, Y. Jin, B. Etienne, Phys.

Rev. Lett. 79, 2526 (1997).9. M. Reznikov, R. de Picciotto, T. G. Griffiths, M.

Heiblum, V. Umansky, Nature 399, 238 (1999).10. R. de Picciotto et al., Nature 389, 162 (1997).11. M. J. Yoo et al., Science 276, 579 (1997).12. A. Yacoby, H. F. Hess, T. A. Fulton, L. N. Pfeiffer, K. W.

West, Solid State Commun. 111, 1 (1999).13. N. B. Zhitenev et al., Nature 404, 473 (2000).14. J. P. Eisenstein, L. N. Pfeiffer, K. W. West, Phys. Rev. B

50, 1760 (1994).15. S. Ilani, A. Yacoby, D. Mahalu, H. Shtrikman, Phys.

Rev. Lett. 84, 3133 (2000).16. S. Ilani, A. Yacoby, D. Mahalu, H. Shtrikman, Science

292, 1354 (2001).17. S. Ilani et al., Nature 427, 328 (2004).18. A. L. Efros, A. F. Ioffe, Solid State Commun. 67, 1019 (1988).19. D. B. Chklovskii, P. A. Lee, Phys. Rev. B 48, 18060 (1993).20. N. R. Cooper, J. T. Chalker, Phys. Rev. B 48, 4530 (1993).21. I. Ruzin, N. Cooper, B. Halperin, Phys. Rev. B 53, 1558

(1996).22. This work is supported by the Israel Science Founda-

tion, the Minerva Foundation, and the Fritz ThyssenStiftung.

5 May 2004; accepted 9 July 2004

B = 11.4TB = 3.8T B = 7.25T B = 7.25T

1.00.0 x (µm)µm)

13.8

12.73

8.8

7.1

A

D16.1

15.02/3

C

1.00.0 x ( same fieldsame density

9.2

8.1

11 1/3

1/3

1/3

4.6

3.5 1

B

HE F G

n(1

0cm

)10

-2n

(10

cm)

10-2

n(1

0cm

)10

-2n

(10

cm)

10-2

n(1

0cm

)10

-2

∆n

= 0

.710

cm10

-2

Fig. 3. (A) Density-position scan for � 3(B � 1.9 T, T � 300 mK,sample V6-131). The levelspacing decreases slightlywith increasing density,indicating the spectra ofQDs. (B) Density-positionscan at the same positionas (A) for � 1 (B � 1.9T). (C) Density-positionscan for � 2/3 (B � 10T) at the same position asthe integer scans. (D)Density-position scan for � 1/3 (B � 11.4 T,same as in Fig. 2D). Com-parison between integerand fractional filling fac-tors reveals three timesdenser LS spectra for thefractional regime. (E) Leftdot at � 1 measured atthe same density (one-third the magnetic field)as scan for � 1/3 in (F).(G) Left dot at � 1measured at the samemagnetic field (3 timesthe density) as the scanfor � 1/3 in (H). Thedashed lines serve as aguide to the eye to em-phasize the difference inthe level spacing between � 1 and � 1/3. Thelevel spacing is indepen-dent of magnetic fieldand density.

filling factor3 1 2/3 1/3

ratio

of

(a.u

)∆µ

1

2/3

1/3

E

C

∆µµ(

V)

0.0

710

3.45 n (10 cm )10 -24.35n (10 cm )10 -2 8.15 8.95

180 Vµ

0.0

300

A

1

-0.1

0.0

1/(a

.u)

κ

1/(a

.u)

κ

-0.5

0.0

∆µµ(

V)

80 Vµ

D 1/3

B 1/31Fig. 4. (A) Cross section of themeasured compressibility, �,through the center of the leftdot for � 1 (Fig. 3B). For inte-gration, the background com-pressibility (dotted line) is sub-tracted. (B) Cross sectionthrough the center of the leftdot for � 1/3 (Fig. 3D). (C)Integrated signal (chemical po-tential �) with background cor-rection for � 1. Each step cor-responds to one electron or qua-si-particle entering the QD. (D)Integrated signal with back-ground correction for � 1/3.(E) Step height for the left dotfor the various integer and frac-tional filling factors.

R E P O R T S

www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004 983

DNA-Functionalized NanotubeMembranes with Single-Base

Mismatch SelectivityPunit Kohli, C. Chad Harrell, Zehui Cao, Rahela Gasparac,

Weihong Tan, Charles R. Martin*

We describe synthetic membranes in which the molecular recognition chem-istry used to accomplish selective permeation is DNA hybridization. Thesemembranes contain template-synthesized gold nanotubes with inside diame-ters of 12 nanometers, and a “transporter” DNA-hairpin molecule is attachedto the inside walls of these nanotubes. These DNA-functionalized nanotubemembranes selectively recognize and transport the DNA strand that is com-plementary to the transporter strand, relative to DNA strands that are notcomplementary to the transporter. Under optimal conditions, single-base mis-match transport selectivity can be obtained.

In both biology and technology, molecularrecognition (MR) chemistry is used to se-lectively transport chemical species acrossmembranes. For example, the transmem-brane proteins that transport molecules andions selectively across cell membranes con-tain MR sites that are responsible for thisselective-permeation function (1–3). In asimilar manner, MR agents such as antibod-ies have been incorporated into syntheticmembranes so that the membranes will selec-tively transport the species that binds to theMR agent (4, 5). However, there appear to beno previous examples of either biological orsynthetic membranes where nucleic acid hy-bridization is used as the MR event to facil-itate DNA or RNA transport through themembrane (6, 7). If such membranes couldbe developed, they might prove useful forDNA separations and for the sensors needed,for example, in genomic research.

We describe synthetic MR membranes forselective DNA transport. We prepared thesemembranes by incorporating a “transporter”DNA, in this case a DNA-hairpin (8, 9) mol-ecule (Table 1), within the nanotubes of agold nanotube membrane (10, 11). We foundthat these membranes selectively recognizedand transported the DNA molecule that wascomplementary to the transporter DNA. Therate of transport (flux) of the complementarystrand was higher than the fluxes of perme-ating DNA molecules (Table 1) that con-tained as few as a single-base mismatch withthe transporter DNA.

We prepared gold nanotube membraneswith the template synthesis method (12), byelectrolessly depositing gold along the pore

walls of a polycarbonate template membrane(10, 11). The template was a commerciallyavailable filter (Osmonics), 6 �m thick, withcylindrical, 30-nm-diameter pores and 6 �108 pores per cm2 of membrane surface area.The inside diameters of the gold nanotubesdeposited within the pores of the template canbe controlled by varying the deposition time.The membranes used here contained goldnanotubes with inside diameters of 12 � 2nm, as determined by a gas-flux measure-ment on three identical samples (10).

We chose a DNA hairpin (8, 9) as ourtransporter strand. DNA hairpins contain acomplementary base sequence at each end ofthe molecule (Table 1), and in an appropriateelectrolyte solution, intramolecular hybridiza-tion causes a closed stem/loop structure to form.In order to form the duplex, the complementarystrand must open this structure, and this is acompetitive process in that the intramolecularhybridization that closes the stem must be dis-placed by hybridization of the complementarystrand to the loop. As a result, hybridization canbe very selective, and in optimal cases, single-base mismatch selectivity is observed (8, 9).That is, the perfect complement hybridizes tothe hairpin, but a strand containing even a sin-gle mismatch does not.

Our hairpin-DNA transporter (Table 1)was 30 bases long and contained a thiolsubstituent at the 5� end that allowed it to becovalently attached to the inside walls of thegold nanotubes (13). The first six bases ateach end of this molecule are complimentaryto each other and form the stem of the hair-pin, and the middle 18 bases form the loop(Table 1). The permeating DNA moleculeswere 18 bases long and were either perfectlycomplementary to the bases in the loop orcontained one or more mismatches with theloop (Table 1). A second thiol-terminatedDNA transporter was also investigated (Table1). This DNA transporter was also 30 bases

long, and the 18 bases in the middle of thestrand were identical to the 18 bases in theloop of the hairpin-DNA transporter. Howev-er, this second DNA transporter did not havethe complementary stem-forming bases oneither end and thus could not form a hairpin.We used this linear-DNA transporter to testthe hypothesis that the hairpin-DNA providesbetter transport selectivity because of its en-hanced ability to discriminate the perfect-complement permeating DNA from the per-meating DNAs that contained mismatches.

The transport experiments were done ina U-tube permeation cell (10) in which thegold nanotube membrane separated thefeed half-cell containing one of the perme-ating DNA molecules (Table 1), dissolvedin pH 7.2 phosphate buffer (ionic strength�0.2 M), from the permeate half-cell thatinitially contained only buffer. We deter-mined the rate of transport (flux) of thepermeating DNA molecule from the feedhalf-cell through the membrane into thepermeate half-cell by periodically measur-

Department of Chemistry and Center for Research atthe Bio/Nano Interface, University of Florida, Gaines-ville, FL 32611–7200, USA.

*To whom correspondence should be addressed. E-mail: crmartin@chem.ufl.edu

DN

A (

nm

ol t

ran

spo

rted

)0

0.2

0.4

0.6

0.8

0 100 200 300Time (min)

0 30 60 90 120

0.0

0.6

1.2

1.8

Flu

x (n

mo

l cm

-2 h

r-1 )

Feed Concentration (µM)

A

B

Fig. 1. (A) Transport plots for PC-DNA throughgold nanotube membranes with (blue triangles)and without (red circles) the immobilizedhairpin-DNA transporter. The feed solutionconcentration was 9 �M. (B) Flux versus feedconcentration for PC-DNA. The data in red andblue were obtained for a gold nanotube mem-brane containing the hairpin-DNA transporter.At feed concentrations of 9 �M and above, thetransport plot shows two linear regions. Thedata in blue (squares) were obtained from thehigh slope region at longer times. The data inred (circles) were obtained from the low sloperegion at shorter times. The data in pink (tri-angles) were obtained for an analogous nano-tube membrane with no DNA transporter.

R E P O R T S

13 AUGUST 2004 VOL 305 SCIENCE www.sciencemag.org984

ing the ultraviolet absorbance of the per-meate half-cell solution, at 260 nm, thatarose from the permeating DNA molecule.

Transport plots (Figs. 1A and 2) show thenumber of nanomoles of the permeatingDNA transported through the nanotube mem-brane versus permeation time. When the hair-pin DNA was not attached, a straight-linetransport plot was obtained for the perfect-complement DNA (PC-DNA) (Fig. 1A), andthe slope of this line provides the flux ofPC-DNA across the membrane (Table 2).The analogous transport plot for the mem-brane containing the hairpin-DNA transporteris not linear, but instead can be approximatedby two straight-line segments: a lower slopesegment at short times followed by a higherslope segment at times longer than a criticaltransition time, �. This transition is very re-producible; for example, for a feed concen-tration of 9 �M, � was 110 � 15 min (aver-age of three membranes).

Figure 1A shows that the flux of the perme-ating PC-DNA in the membrane containing thehairpin-DNA transporter was at all times higherthan the flux for an otherwise identical mem-brane without the transporter (Table 2). Hence,the hairpin DNA acted as an MR agent tofacilitate the transport (4, 5, 14, 15) of thePC-DNA. Additional evidence for this conclu-sion was obtained from studies of the effect ofconcentration of the PC-DNA in the feed solu-tion on the PC-DNA flux. If the hairpin DNAfacilitated the transport of the PC-DNA, thisplot should show a characteristic “Lang-muirian” shape (4, 5). Figure 1B shows that thisis indeed the case, for transport data both beforeand after �. The analogous plot for the identicalmembrane without the hairpin-DNA transporteris linear (Fig. 1B), showing that transport is notfacilitated but rather described simply by Fick’sfirst law of diffusion. The transition to thehigher slope segment was not observed, duringpermeation experiments with a total duration of300 min, for feed concentrations below 9 �M(Fig. 1B).

Analogous permeation data were ob-tained for the various mismatch-containingpermeating DNA molecules (Table 1). Thetransport plots for these mismatch DNAsshow only one straight-line segment (Fig.2), and their fluxes were always lower thanthe flux for the PC-DNA obtained from thehigher slope region of the PC-DNA trans-port plot (Table 2). In particular, the mem-brane containing the hairpin-DNA trans-porter showed higher flux for PC-DNAthan for the two permeating DNAs thatcontained only a single-base mismatch.

To illustrate this point more clearly, wedefined a selectivity coefficient �HP,PC/1MM,which is the flux for the PC-DNA divided bythe flux for a single-base mismatch DNA inthe membrane with the hairpin (HP)–DNAtransporter. A selectivity coefficient of�HP,PC/1MM � 3 can be derived from the data inTable 1. The analogous selectivity coefficient forthe PC-DNA versus the DNA with seven mis-matches is �HP,PC/7MM � 7. These selectivitycoefficients show that nanotube membranescontaining the hairpin-DNA transporter selec-tively transport PC-DNA and that single-basemismatch transport selectivity can be obtained.

The importance of the hairpin structure tomembrane selectivity is illustrated by analo-gous transport data for membranes containingthe linear-DNA transporter (Table 1). Withthis transporter, all of the transport plotsshow only a single straight-line segment, andthe fluxes for the single-mismatch DNAswere identical to the flux for the PC-DNA(Table 2); i.e., the single-base mismatch se-

lectivity coefficient for this linear (LN) DNAtransporter is �LN,PC/1MM � 1. The linear-DNA transporter does, however, show sometransport selectivity for the PC-DNA versusthe seven-mismatch DNA, �LN,PC/7MM � 5.

We also investigated the mechanism oftransport in these membranes. In such MR-based, facilitated-transport membranes, thepermeating species is transported by se-quential binding and unbinding events withthe MR agent (4, 5, 14 ). For these DNA-based membranes, the binding and unbind-ing events are sequential hybridization anddehybridization reactions between the per-meating DNA molecule and the DNA trans-porter attached to the nanotubes. To showthat hybridization occurred in the mem-brane with the hairpin-DNA transporter,the membrane was exposed to PC-DNAand then to a restriction enzyme (Sfc I,New England Biolabs) (13). If hybridiza-tion between the PC-DNA and the hairpintransporter occurs, this enzyme would cutthe resulting double-stranded DNA suchthat the last five bases of the binding loop,and all of the stem-forming region, at the 3�end of the hairpin would be removed. Thisreaction would substantially damage thebinding site, and on the basis of our priorwork (4 ), we predicted that if this mem-brane were subsequently used in a perme-ation experiment, a lower PC-DNA fluxwould be obtained (16 ).

After exposure to the restriction enzyme, themembrane was extensively rinsed to removethe enzyme and DNA fragments and was then

DN

A (

nm

ol t

ran

spo

rted

)

Time (min)

0

0.18

0.36

0.54

0.72

0 100 200 300

Fig. 2. Transport plots for a gold nanotube mem-brane containing the hairpin-DNA transporter.The permeating DNA was either PC-DNA (redcircles), single-mismatch (end) (brown circles),seven-mismatch (blue triangles), or single-mismatch (middle) (orange squares). The feedsolution concentration was 9 �M.

Table 1. DNA molecules used. For transporter DNAs, the 18 bases that bind to the permeating DNAs arein bold. For permeating DNAs, the mismatched bases are underlined. FAM is a fluorescein derivative(Applied Biosystems), and Cy5 is a cyanine dye (Amersham Biosciences).

Type Sequence

Transporter DNAsHairpin 5�HS-(CH2)6-CGCGAGAAGTTACATGACCTGTAGCTCGCG3�Linear 5�HS-(CH2)6-CGCGAGAAGTTACATGACCTGTAGACGATC3�

Permeating DNAsPerfect complement (PC-DNA) 3�TTCAATGTACTGGACATC5�Single-base mismatch (3� end) 3�CTCAATGTACTGGACATC5�Single-base mismatch (middle) 3�TTCAATGTAGTGGACATC5�Seven-mismatch 3�AAGTTACATGACCTGTAG5�FAM-labeled perfect complement 3�TTCAATGTACTGGACATC-(CH2)6-FAM 5�Cy5-labeled single-base mismatch 3�CTCAATGTACTGGACATC-(CH2)6 Cy5 5�

Table 2. Fluxes for a feed concentration of 9 �M.

Transporter DNA Permeating DNA Flux (nmol cm2 h1)

Hairpin Perfect complement 0.57, 1.14*Linear Perfect complement 0.94None Perfect complement 0.20Hairpin Single mismatch (middle) 0.37Hairpin Single mismatch (end) 0.44Linear Single mismatch (middle) 0.94Hairpin Seven-mismatch 0.17Linear Seven-mismatch 0.20

*Two fluxes were obtained because the transport plot showed two slopes (Fig. 1A).

R E P O R T S

www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004 985

used for a transport experiment with PC-DNAas the permeating species. Unlike the data inFig. 1A, the transport plot for this damaged-transporter membrane showed only onestraight-line segment (13), corresponding to aflux of 0.2 nmol cm�2 h�1. This value is wellbelow those we observed from membranes withan undamaged DNA-hairpin transporter (Table2). The damaged DNA-transporter was thenremoved from the nanotubes, and fresh DNA-hairpin transporter was applied. A subsequenttransport experiment with PC-DNA showed atransport plot identical to that obtained beforeexposure to the restriction enzyme (13). Thesedata suggest that hybridization is, indeed, in-volved in the transport mechanism for theDNA-hairpin–containing membranes.

To show that dehybridization occurs ona reasonable time scale in these mem-branes, we exposed a hairpin-DNA mem-brane to a fluorescently labeled version ofthe PC-DNA (Table 1). The membrane wasthen rinsed with buffer solution and im-mersed into a solution of either pure bufferor buffer containing unlabeled PC-DNA. Ifthe dehybridization reaction is facile, thefluorescently labeled PC-DNA should bereleased into the solution. We found thatdehybridization did occur, but it wasstrongly accelerated when unlabeled PC-DNA was present in the solution (Fig. 3).Hence, dehybridization is much fasterwhen it occurs through a cooperative pro-cess whereby one PC-DNA molecule dis-places another from an extant duplex (17 ).

We also investigated transport selectiv-ity for a feed solution containing fluores-cently labeled versions (Table 1) of boththe PC-DNA and the single-mismatchDNA. The fluorescent labels allowed forquantification of both of these permeatingDNAs simultaneously in the permeate so-lution. In analogy to the single-moleculepermeation experiment, the flux of the PC-DNA was five times higher than the flux ofthe single-mismatch DNA (13). To assess

the practical utility of these membranes,transport studies with more realistic sam-ples (such as cell lysates) will be needed.

Finally, we have not observed a sponta-neous transition from a low-flux to a high-flux state (Fig. 1A) with our previous MR-based membranes (4, 5). The fact thatwhether this transition is observed dependson the feed concentration suggests that thetransition is a transport-related phenome-non. It is possible that this transition relatesto the concept of cooperative (high-flux)versus noncooperative (low-flux) dehybrid-ization (Fig. 3), but further studies, bothexperimental and modeling, will be re-quired before a definitive mechanism forthis transition can be proposed.

References and Notes1. D. A. Doyle et al., Science 280, 69 (1998).2. J. Abramson et al., Science 301, 610 (2003).3. B. Hille, Ion Channels of Excitable Membranes

(Sinauer, Sunderland, MA, ed. 3, 2001), pp. 441–470.4. S. B. Lee et al., Science 296, 2198 (2002).5. B. B. Lakshmi, C. R. Martin, Nature 388, 758 (1997).6. An interesting example of attaching a single-stranded

DNA molecule to a protein channel to make a newtype of DNA sensor has been reported (18).

7. H. Fried, U. Kutay, Cell. Mol. Life Sci. 60, 1659 (2003).8. G. Bonnet, S. Tyagi, A. Libchaber, F. R. Kramer, Proc.

Natl. Acad. Sci. U.S.A. 96, 6171 (1999).9. B. Dubertret, M. Calame, A. J. Libchaber, Nature Bio-

technol. 19, 365 (2001).10. C. R. Martin, M. Nishizawa, K. Jirage, M. Kang, J. Phys.

Chem. B 105, 1925 (2001).11. K. B. Jirage, J. C. Hulteen, C. R. Martin, Science 278,

655 (1997).12. C. R. Martin, Science 266, 1961 (1994).13. Materials and methods are available as supporting

material on Science Online.14. M. Mulder, Basic Principles of Membrane Technology

(Kluwer, Dordrecht, Netherlands, 1996), pp. 342–351.15. Y. Osada, T. Nakagawa, in Membrane Science and

Technology, Y. Osada, T. Nakagawa, Eds. (MarcelDekker, New York, 1992), pp. 377–391.

16. A fluorescence-based method was used to providedirect evidence for clipping of the double-strandedDNA by the restriction enzyme (13).

17. M. C. Hall, H. Wang, D. A. Erie, T. A. Kunkel, J. Mol.Biol. 312, 637 (2001).

18. S. Howorka, S. Cheley, H. Bayley, Nature Biotech. 19,636 (2001).

19. Supported by the National Science Foundation andby the Defense Advanced Research Projects Agency.

Supporting Online Materialwww.sciencemag.org/cgi/content/full/305/5686/984/DC1Materials and MethodsFigs. S1 to S3References and Notes

6 May 2004; accepted 9 July 2004

Sample Dimensions InfluenceStrength and Crystal Plasticity

Michael D. Uchic,1* Dennis M. Dimiduk,1 Jeffrey N. Florando,2

William D. Nix3

When a crystal deforms plastically, phenomena such as dislocation storage,multiplication, motion, pinning, and nucleation occur over the submicron-to-nanometer scale. Here we report measurements of plastic yielding for singlecrystals of micrometer-sized dimensions for three different types of metals. Wefind that within the tests, the overall sample dimensions artificially limit thelength scales available for plastic processes. The results show dramatic sizeeffects at surprisingly large sample dimensions. These results emphasize thatat the micrometer scale, one must define both the external geometry andinternal structure to characterize the strength of a material.

A size-scale effect can be defined as achange in material properties—mechanical,electrical, optical, or magnetic—that is dueto a change in either the dimensions of aninternal feature or structure or in the overallphysical dimensions of a sample. For met-als, size-scale effects related to changes ininternal length scales are readily observedand are often exploited for industrial use.For example, it is well known that the yieldstrengths of metallic alloys can be im-

proved through refinement of the grain size(1–3), where the yield strength is propor-tional to the inverse square root of theaverage grain diameter, and this relation isgenerally valid for grains that range in sizefrom millimeters to tens of nanometers. Bycomparison, changes in the mechanical re-sponse of materials due solely to the phys-ical geometry of a sample have been largelyoverlooked. Large increases in yieldstrength (approaching the theoretical limit)were observed over 40 years ago in tensiontesting of single-crystal metallic whiskershaving micrometer-scale diameters (4–6 ).However, whisker testing is restricted tomaterials that can be grown in that form.Conversely, no changes in strength andonly mild decreases in work hardeningwere observed during the deformation of

1Air Force Research Laboratory, Materials & Manufac-turing Directorate, Wright-Patterson Air Force Base,OH 45433–7817, USA. 2Lawrence Livermore NationalLaboratory, Livermore, CA 94550, USA. 3Departmentof Materials Science and Engineering, Stanford Uni-versity, Stanford, CA 94305–2205, USA.

*To whom correspondence should be addressed. E-mail: michael.uchic@wpafb.af.mil

0

1

2

3

0 300 600Rel

ativ

e fl

uo

resc

ence

inte

nsi

ty

Time (min)

Fig. 3. Release of fluorescently labeled PC-DNA from a membrane containing the hairpin-DNA transporter. The fluorescently labeledPC-DNA was released into a buffer solutioncontaining no unlabeled PC-DNA (lower curve)or into a buffer containing 9 �M unlabeledPC-DNA (upper curve).

R E P O R T S

13 AUGUST 2004 VOL 305 SCIENCE www.sciencemag.org986

simple metals at submillimeter sample di-ameters (7–10), but those studies only start-ed to explore the gap between millimeterand whisker dimensions.

There remains a fundamental challengeto systematically investigate externallength-scale effects in the submillimeter-to-nanometer size regime. Such small dimen-sions are pervasive in modern devices andalso encompass the size range in which dis-location-based plasticity mechanisms occur.External length-scale effects may be ob-served at multiple stages over this wide rangeof sizes, because the mechanisms associatedwith dislocation storage, multiplication, mo-tion, pinning, and nucleation are generallyactive over different length scales. Withoutsuch an understanding, it is impossible toknow the appropriate material properties touse in the design of small devices. At present,one can question whether features havingmicrometer-sized dimensions should be de-signed using the extraordinary strengths ofdefect-free “whiskers” or using behaviormore akin to that of bulk metal crystals.

Recently, size-scale effects in materialsmechanics received renewed attention underconditions where deformation gradients areimposed at the micrometer scale (11–14).These studies explore the evolution of geo-metrically necessary dislocations (GNDs)(15, 16) that are required to accommodate theplastic strain gradients that may be inducedby the test condition or by the internal struc-ture of the material. For example, the perma-nent change in the profile of a surface duringindentation testing, which is due to the defor-mation gradient imposed by the indentationtip, may be wholly accommodated by thegeneration and motion of GNDs. These stud-ies find that gradient-induced increases indefect evolution result in concomitant localchanges in the strength and hardening rates ofmaterials. However, the studies do not con-sider other changes in the fundamental defor-mation mechanisms associated with limitingthe physical dimensions of the deformingvolume; that is, one might speculate that thedeformation micromechanisms themselvesare affected by the size of the deformingvolume. We suggest that a more completeunderstanding of size effects for a given ma-terial can only be realized after testing forgeometric effects and under test conditionsthat minimize imposed deformation gradi-ents, thus limiting the GND density.

We have developed a test methodology (17)that allows the exploration of size-scale effects invirtually any bulk inorganic material, using afocused ion beam (FIB) microscope for samplepreparation, together with mechanical testingthat is a simple extension of nanoindentationtechnology. The FIB is used to machine cylin-drical compression samples into the surface of abulk crystal, leaving the samples attached to the

bulk at one end. Samples were prepared in thesize range from 0.5 to 40 �m in diameter andwith an aspect ratio ranging from 2:1 to 4:1.Once prepared, the samples were tested using aconventional nanoindentation device outfittedwith a flat-punch indentation tip. Nanoindenta-tion systems are normally used for depth-sensingindentation experiments using a sharp tip, buthere the same platform is used to perform con-ventional uniaxial compression tests at pre-scribed displacement rates ranging from 1 to 5nm/s. This technique can be used to study exter-nal size effects in single crystals in the absenceof grain boundaries, which are strong internalbarriers to dislocation glide. Although there havebeen notable advances in mechanical test meth-ods that operate on micrometer-sized samples(18–20), these test techniques use samples thathave been fabricated with wafer processingmethods that are specific to the microelectronicindustry. The microstructures of those samples aretypically polycrystalline, having a submicrometergrain size, which can complicate the interpretationof observed external size effects (21).

The mechanical behavior of bulk singlecrystals of pure Ni is well known, so thismaterial was used as a model system for thetest method. A single-slip orientation wasselected to simplify the defect evolution

and hardening conditions. The stress-straincurves for Ni microcompression sampleshaving diameters in the 20- to 40-�m rangeare similar to those for bulk samples (Fig.1A), because the yield strength and overallwork-hardening rates are within 30% of themeasured properties of millimeter-sized spec-imens. After testing, fine discrete slip bandscan be observed along the gauge length of thesamples, which are also found in the bulkspecimen tests (Fig. 1B).

For samples 5 �m in diameter, there aredistinct changes in the stress-strain curves thatare indicative of physical size limitations. Thesesamples display large strain bursts: very rapidflow to values up to 19% strain upon yielding, incontrast to bulk samples that show a smoothtransition from elastic to plastic flow and asteady rate of work hardening. The yield stressfor each of the four 5-�m-diameter samples ishigher than that for microsamples that are equalto or larger than 20 �m in diameter and variesover a range of 70 MPa. Differences may also beobserved in the appearance of the microsamplesafter testing. There are fewer slip bands, butthose that exist appear to be much more active,as indicated by large single-slip plane displace-ments (Fig. 1C). Strain bursts are also observedfor samples 10 �m in diameter, although the

Fig. 1. Mechanical be-havior at room temper-ature for pure Ni mi-crosamples having a�134� orientation.(A) Stress-strain curvesfor microsamples rang-ing in size from 40 to 5�m in diameter, as wellas the stress-straincurve for a bulk singlecrystal having approxi-mate dimensions 2.6 �2.6 � 7.4 mm. (B) Ascanning electron mi-crograph (SEM) imageof a 20-�m-diameter microsample tested to �4% strain. The circle milled into the top surface of themicrosample is a fiducial mark used during sample machining. (C) A SEM image of 5-�m-diametermicrosample after testing, where the sample achieved �19% strain during a rapid burst of deformationthat occurred in less than 0.2 s.

Fig. 2. Mechanical be-havior at room tem-perature for Ni3Al-Tamicrosamples having a�123� orientation. (A)Representative stress-strain curves for micro-samples ranging in sizefrom 20 to 0.5 �m indiameter, as comparedto the behavior of a bulksingle crystal having ap-proximate dimensions2.5� 2.5� 7.5mm. (B)A SEM image of 20-�m-diameter microsampleafter testing, where the sample achieved �10% strain during the rapid burst of deformation. (C) A SEMimage of 1-�m-diameter microsample after testing, where the top of the sample has completely sheared offduring the rapid strain burst. This behavior is observed for both the 1- and 0.5-�m-diameter samples.

R E P O R T S

www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004 987

extent of these is typically less than 1% strain.For samples 10 �m in diameter and larger, mostof the plastic deformation consists of short peri-ods of stable flow with low work-hardeningrates, separated by increments of nearly elasticloading. There is a gradual progression betweenbulk and size-limited behavior as the sample sizedecreases from 40 to 5 �m in diameter. Theseattributes are distinct from the common behaviorof both bulk materials and whiskers. Whiskers ofpure metals typically display much higher yieldstresses than bulk materials. In one study, thestrength of Cu whiskers 16 �m in diameter andsmaller exhibited yield stresses in the range from0.3 to 6 GPa (6), whereas the yield stress forbulk Cu is on the order of 10 to 50 MPa (de-pending on purity levels and heat treatment con-ditions). In addition, after yielding, whiskers donot maintain this high flow stress; rather, theflow stress drops to the level observed in bulk Cuor the whisker simply fractures. The reasons forthis are understood to be related to the fact that,unlike most common metals, the whiskers startout defect-free before loading.

One interpretation of these results is thatdecreasing sample diameter affects the mecha-nisms for defect multiplication and storage thatare associated with plastic flow, before the dis-location-source–limited regime attributed towhiskers is achieved. The increases in flowstress and extremely low hardening rates falloutside the regimes known for bulk tests but donot enter the regime of high stresses known formetal whiskers. The increase in the spread andthe rise of the values of the yield stress forsmaller samples suggest aspects of self-organi-zation and criticality events at the elastic-plastictransition. That is, the transition appears to bestochastic, showing a progression toward a sin-gle catastrophic event as the ability to multiplydislocations or the number of dislocation sourcesis truncated. This occurs either through increas-ing levels of deformation or through shrinkingthe total volume of the sample.

The same method was used to examine anintermetallic alloy, Ni3Al-Ta, which is widelyknown to exhibit fundamentally different flowmechanisms. One physical manifestation of thisbehavior is an anomalous increase in strengthwith increasing temperature. There is consider-able evidence that at temperatures in the anom-alous flow regime, the mobility of screw-char-acter dislocations is greatly influenced by thelateral motion of large jogs and kinks along thelength of the dislocations (22–24), and it islikely that dislocation kinetics are strongly in-fluenced by the characteristic active line lengthof dislocations known to be on the order of afew micrometers (23, 25). The characteristicscales for multiplication are unknown. In thepresent study, the sample sizes are equivalent tothe length scales for the physical processesgoverning flow.

We observed a dramatic size effect onstrength for a Ni3Al-1% Ta alloy deformingunder nominally single-slip conditions (Fig.2A). The flow stress increased from 250 MPafor a 20-�m-diameter sample to 2 GPa for a0.5-�m-diameter sample. These flow stressesare much higher than those found for bulkcrystals, which themselves exhibit a flowstress of only 81 MPa. Although these stress-es exceed those for the bulk material, theinfluence of sample size occurs at dimensionsthat are large by comparison to whisker-typetests. After testing, slip traces are very fineand are homogeneously distributed along thegage section (Fig. 2B), except for the 0.5- and1-�m-diameter samples, because they havecompletely sheared apart during large strainbursts. Closer inspection of the loadingcurves for all of the tests before the largestrain bursts show small events of plasticactivity that occur sporadically during theloading of the sample, separated by nearlyelastic loading, again akin to self-organizedprocesses. These aspects of work-hardeningbehavior are similar to what we have ob-served in the smaller Ni samples but have notbeen reported for bulk samples.

Examination of the flow stress in Ni3Al-Taas a function of sample diameter (Fig. 3) showstwo regimes of size-dependent strengtheningthat scale with the inverse of the square root ofthe sample diameter—coincidently similar tograin-size hardening. However, although suchstrength scaling in metals usually arises fromthe presence of internal kinematic barriers toflow, these samples have no known internalbarriers. One may speculate that this re-markable behavior is associated withchanges in the self-exhaustion or annihila-tion of dislocations, specifically those ofscrew character. That said, it is surprisingthat significant length-scale effects are ob-served for such large sample sizes; notethat the transition to bulk behavior is pre-dicted from the scaling relation in Fig. 3to occur for samples greater than 42�m in diameter.

Finally, we examined a Ni superalloy sin-gle crystal that consisted of a Ni solid-solution matrix having a high volume fractionof Ni3Al-based precipitates that are �250 nmin diameter and are uniformly distributed.Both solid-solution alloying and the precipi-tates provide additional strengthening mech-anisms and help to determine internal de-formation length scales. A 10-�m-diametermicrocompression sample, which had about30 precipitates spanning the width of thesample, displayed a mechanical responsethat matched the behavior of a bulk tensiontest (Fig. 4). The agreement is not surpris-ing, because the strong internal hardeningmechanisms that control plastic deforma-tion operate at the dimensional scale of theprecipitates and are still effective at thissample size, thus preempting influencesfrom limited sample dimensions.

We have demonstrated a method to char-acterize aspects of length-scale effects ondeformation and strength by shrinking thetraditional uniaxial compression test to themicrometer scale. From these tests it is clearthat when the external dimensions of the

Fig. 3. Dependence of the yield strength onthe inverse of the square root of the samplediameter for Ni3Al-Ta. The linear fit to thedata predicts a transition from bulk to size-limited behavior at �42 �m. �ys, the stressfor breakaway flow.

Fig. 4. Mechanical behavior at room temperature of a Ni superalloy microsample having anear-�001� orientation. (A) A stress-strain curve for a 10-�m-diameter microsample tested incompression as compared to the behavior of a bulk single tested in tension. The microsample wasmachined from an undeformed region of the grip region of the bulk sample after testing. (B) A SEMimage of the microsample after testing.

R E P O R T S

13 AUGUST 2004 VOL 305 SCIENCE www.sciencemag.org988

sample become smaller than a few tens ofmicrometers, the basic processes of plasticdeformation are affected; thus, it may not bepossible to define the strength of a givenmaterial in the absence of physical conditionsthat are completely specified. The resultsshow that such influences occur at muchlarger dimensions than are classically under-stood for metal whisker-like behavior (6).Emerging strain-gradient–based continuumtheories of deformation (that is, models thatincorporate a physical length scale into theconstitutive relations for the mechanical re-sponse of materials) must carefully accountfor these fundamental changes of deforma-tion mechanisms that extend beyond thegradient-induced storage of defects.

References and Notes1. E. O. Hall, Proc. Phys. Soc. London B 64, 747 (1951).2. N. J. Petch, J. Iron Steel Inst. 174, 25 (1953).3. S. Yip, Nature 391, 532 (1998).4. S. S. Brenner, J. Appl. Phys. 27, 1484 (1956).5. S. S. Brenner, J. Appl. Phys. 28, 1023 (1957).6. S. S. Brenner, in Growth and Perfection of Crystals,

R. H. Doremus, B. W. Roberts, D. Turnbull, Eds. (Wiley,New York, 1959), pp. 157–190.

7. H. Suzuki, S. Ikeda, S. Takeuchi, J. Phys. Soc. Jpn. 11,382 (1956).

8. J. T. Fourie, Philos. Mag. 17, 735 (1968).9. S. J. Bazinski, Z. S. Bazinski, in Dislocations in Solids,

F. R. N. Nabarro, Ed. (North Holland, Amsterdam,1979), vol. 4, pp. 261–362.

10. G. Sevillano, in Materials Science and Technology,Vol. 6 Plastic Deformation and Fracture of Materials,H. Mughrabi, Ed. (VCH, Weinheim, Germany, 1993),vol. 6, pp. 19–88.

11. N. A. Fleck, G. M. Muller, M. F. Ashby, J. W. Hutchin-son, Acta Metall. Mater. 42, 475 (1994).

12. Q. Ma, D. R. Clarke, J. Mater. Res. 10, 853 (1995).13. W. D. Nix, H. Gao, J. Mech. Phys. Solids 46, 411 (1998).14. J. S. Stolken, A. G. Evans, Acta Mater. 46, 5109 (1998).15. J. F. Nye, Acta Metall. 1, 153 (1953).16. M. F. Ashby, Philos. Mag. 21, 399 (1970).17. M. D. Uchic, D. M. Dimiduk, J. N. Florando, W. D. Nix, in

Materials Research Society Symposium Proceedings, E. P.George et al., Eds. (Materials Research Society, Pittsburgh,PA, 2003), vol. 753, pp. BB1.4.1–BB1.4.6.

18. W. N. Sharpe, K. M. Jackson, K. J. Hemker, Z. Xie, J.MEMS Syst. 10, 317 (2001).

19. M. A. Haque, M. T. A. Saif, Sensors Actuators A 97-98,239 (2002).

20. H. D. Espinosa, B. C. Prorok, M. Fischer, J. Mech. Phys.Solids 51, 47 (2003).

21. H. D. Espinosa, B. C. Prorok, B. Peng, J. Mater. Res. 52,667 (2004).

22. M. Mills, N. Baluc, H. P. Karnthaler, in Materials Re-search Society Symposium Proceedings, C. T. Liu etal., Eds. (Materials Research Society, Pittsburgh, PA,1989), vol. 133, pp. 203–208.

23. P. Veyssiere, G. Saada, in Dislocations in Solids,F. R. N. Nabarro, M. S. Duesbery, Eds. (North Holland,Amsterdam, 1996), vol. 10, pp. 253–440.

24. P. B. Hirsch, Philos. Mag. A 65, 569 (1992).25. X. Shi, G. Saada, P. Veyssiere, Philos.Mag. Lett.71, 1 (1995).26. Supported by the Air Force Office of Scientific Re-

search and the Accelerated Insertion of Materialsprogram of the Defense Advanced Research ProjectsAgency (M.D.U. and D.M.D.) under the direction of C.Hartley and L. Christodoulou, respectively, and by theU.S. Department of Energy and NSF (J.N.F. andW.D.N.). The Ni superalloy single crystal and corre-sponding bulk mechanical test data were provided byT. Pollock of the University of Michigan. We grate-fully acknowledge useful discussions with T. A.Parthasarathy, K. Hemker, and R. LeSar. We alsoacknowledge H. Fraser, whose efforts enabled manyaspects of the instruments used in this work.

9 April 2004; accepted 21 July 2004

Discovery of Mass Anomalieson Ganymede

John D. Anderson,1* Gerald Schubert,2,3 Robert A. Jacobson,1

Eunice L. Lau,1 William B. Moore,2 Jennifer L. Palguta2

We present the discovery of mass anomalies on Ganymede, Jupiter’s third andlargest Galilean satellite. This discovery is surprising for such a large icy satellite.We used the radio Doppler data generated with the Galileo spacecraft duringits second encounter with Ganymede on 6 September 1996 to model the massanomalies. Two surface mass anomalies, one a positive mass at high latitudeand the other a negative mass at low latitude, can explain the data. There areno obvious geological features that can be identified with the anomalies.

Jupiter’s four Galilean satellites can be ap-proximated by fluid bodies that are distortedby rotational flattening and by a static tideraised by Jupiter. All four satellites are insynchronous rotation with their orbital peri-ods, and all four are in nearly circular orbitsin Jupiter’s equatorial plane (1). Previouslywe reported on the interior structure of thefour Galilean satellites as inferred from theirmean densities and second-degree (quadru-pole) gravity moments (2). The inner threesatellites, Io, Europa, and Ganymede, havedifferentiated into an inner metallic core andan outer rocky mantle. In addition, Europaand Ganymede have deep icy shells on top oftheir rocky mantles. The outermost satellite,Callisto, is an exception. It has no metalliccore, and rock (plus metal) and ice are mixedthroughout most if not all of its deep interior.

These interior models are consistent withthe satellites’ external gravitational fields, asinferred from radio Doppler data from closespacecraft flybys, with one exception. It isimpossible to obtain a satisfactory fit to theDoppler data from the second Ganymedeflyby (G2) without including all gravity mo-ments to the fourth degree and order in thefitting model. The required truncated spheri-cal harmonic expansion for Ganymede’sgravitational potential function V takes theform (3)

V�r,,� �

GM

r �1��n 2

4 �m 0

n �R

r� n

�Cnm cos m �

Snm sin m� Pnm(sin)� (1)

The spherical coordinates (r, , ) are re-ferred to the center of mass, with r the radialdistance, the latitude, and the longitudeon the equator. Pnm is the associated Legendrepolynomial of degree n and order m, and Cnm

and Snm are the corresponding harmonic co-efficients, determined from the Doppler databy least squares analysis.

By taking the satellite’s center of massat the origin, its first degree coefficients arezero by definition, although all harmonicsof degree greater than one can be nonzero(3). The gravity parameters needed to fitthe G2 Doppler data to the noise level areGM, where M is the mass of the satelliteand G is the gravitational constant; fivesecond-degree coefficients; seven third-degree coefficients; and nine fourth-degreecoefficients, for a total of 22 gravity param-eters. With only two flybys and no globalcoverage of Ganymede’s gravitational po-tential, the truncated harmonic expansion isnot unique. Consequently, the 22 gravityparameters have little physical meaning.

The reference radius R for the potentialfunction is set equal to the best determinationof Ganymede’s physical radius from space-craft images, 2631.2 � 1.7 km (4). With thisradius, Ganymede is the largest satellite in thesolar system, larger than Saturn’s satelliteTitan and even larger than the planet Mercu-ry. Its GM value, as determined by four fly-bys (G1, G2, G7, G29), is 9887.83 � 0.03km3 s�2 (4), which yields a mean density of1941.6 � 3.8 kg m�3, consistent with adifferentiated metal-rock interior and an icyshell about 800 km deep (2). Ganymede’stotal mass is (1.48150 � 0.00022) � 1023 kg,where the error is dominated by the uncer-tainty in G (5), not the uncertainty in GM.

The higher degree coefficients required tofit the Ganymede G2 data must be a reflectionof some other, and more localized, distortion ofthe gravitational field. In order to describe thismore localized field, we first obtained a best fitto Ganymede’s global field with just the param-eters GM and the five second-degree gravitycoefficients. Using this field, we calculatedDoppler residuals about the best fit. Before any

1Jet Propulsion Laboratory, California Institute ofTechnology, Pasadena, CA 91109–8099, USA. 2De-partment of Earth and Space Sciences, University ofCalifornia, Los Angeles, CA 90095–1567, USA. 3Insti-tute of Geophysics and Planetary Physics, Los Ange-les, CA 90095–1567, USA.

*To whom correspondence should be addressed; E-mail: john.d.anderson@jpl.nasa.gov

R E P O R T S

www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004 989

fitting model was applied, the structure of theDoppler data was dominated by the GM term(Fig. 1). After removal of the best-fit model forGM and the second-degree field, the residualswere dominated by the localized gravity anom-aly or anomalies (Fig. 2). These residuals werenumerically differentiated by the cubic-splinetechnique developed for lunar mascons (6),thereby yielding acceleration data along the lineof sight (Fig. 3).

Mass points were moved around on thesurface until the acceleration data were fit withan inverse square Newtonian acceleration onthe spacecraft in free fall (Gm/d2), with m themass of a mass point fixed in the body ofGanymede and d its distance from the space-craft as a function of time. This Newtonianacceleration was then projected on the line ofsight in order to produce a model for the ob-served acceleration. Nonlinear least squaresanalysis was used to find the best fit for themasses and for the locations of the mass points.

A reasonably good fit to the accelerationdata can be achieved with just two masses,although a better fit is achieved with threemasses (Table 1 and Fig. 3). This suggeststhat there are at least two distinct gravityanomalies on Ganymede. The first can berepresented by a positive mass of about 2.6 10�6 the mass of Ganymede on the surface athigh latitude near the closest approach point,and the second by a negative mass of about5.1 10�6 times the mass of Ganymede atlow latitude. The first mass is needed to fitthe positive peak in the acceleration data atclosest approach. The second mass fits thepeak after closest approach and fills in thelarge depression in the acceleration data,thereby providing a better overall fit. A third,smaller positive mass of about 8.2 10�7 themass of Ganymede improves the overall fitand produces a better fit to the accelerationdata just before closest approach (Table 1 andFig. 3). Because these results and standarderrors are obtained by formal nonlinear leastsquares analysis, the results are model depen-dent with three independent variables (mass,latitude, longitude) for each anomaly. The re-sults do not necessarily imply that the physicalanomalies are known to a similar accuracy.

The results of Table 1 indicate that thefirst two larger masses are stable againstchanges to the fitting procedure, but that theirlocations can change appreciably. This can bedemonstrated by changing the starting condi-tions for the fit from mass values near the twosolutions of Table 1 to mass values of zero.The least-squares procedure converges to thesame solutions regardless of the starting val-ues for the masses. This suggests that thesolutions represent the best global fit to thedata, at least for small masses placed on thesurface. The values of the masses are stable towithin a standard deviation, although the lo-cations can change by tens of a standard

deviation. There are no obvious geologicalstructures at the locations of the mass anom-alies on Ganymede’s surface that could beidentified as the sources of the anomalies.

There may be additional gravity anomalieson Ganymede, but they are undetectable with

only the two close flybys available. There mayalso be gravity anomalies on other Galilean sat-ellites, especially on Europa, which has a differ-entiated structure similar to that of Ganymede.The only Europa flyby suitable for anomalydetection is the one on the 12th orbital revolution

400

200

0

-200

-400

-600

-800

Do

pp

ler

velo

city

(m

s-1)

Time from Ganymede closest approach (s)

-4000 -3000 -2000 -1000 0 1000 2000 3000 4000

Fig. 1. Radio Dopplerresiduals before the ap-plication of any fittingmodel. The time tags forthe rawDoppler data arein seconds from J2000(JD 2451545.0 UTC) asmeasured by the stationclock. The time tags forthe plot are referencedto the G2 closest ap-proach time of 6 Sep-tember 1996, 19:38:34UTC, ground receivetime. The gap in the plotbefore closest approachis a result of a failure ofthe spacecraft receiverto phase lock to the up-link radio carrier wave. Areference frequency of 2.296268568 GHz has been subtracted from the raw data, and the result has beenconverted to Doppler velocity by the unit conversion factor Hz � 0.065278 m s�1. The data are generatedby sending a radio wave to the spacecraft, which returns it to the station by means of a radio transponder(two-way Doppler). Therefore the frequency reference for the Doppler shift is a hydrogen maser at thestation, not the spacecraft’s crystal oscillator. The sample interval for the data is 10 s.

Do

pp

ler

velo

city

(m

m s

-1)

Time from Ganymede closest approach (s)3000200010000-1000-2000

-2

-1

0

1

2 Fig. 2. Doppler resid-uals of Fig. 1, afterthe application of afitting model that in-cludes Ganymede’smass (GM) and itssecond degree andorder gravity field.The remaining resid-uals are evidence ofone or more gravityanomalies near the Ga-lileo trajectory track.

Fig. 3. Acceleration datain units ofmgal (10�5ms�2) as derived from theDoppler residuals of Fig.2. The best-fit accelera-tion model for two sur-face masses is shown inred. The best-fit modelfor three surface massesis in green.

R E P O R T S

13 AUGUST 2004 VOL 305 SCIENCE www.sciencemag.org990

(E12) at an altitude of 201 km. The E12 closestapproach point is near the equator at a latitude of�8.7° and a west longitude of 225.7°. However,unlike G2 at an altitude of 264 km, Doppler datafrom E12, as well as three other more distantflybys (E4 at 692 km, E6 at 586 km, and E11 at2043 km), can be fit to the noise level withsecond-degree harmonics. The two Callisto fly-bys that yield gravity information are more dis-tant (C10 at 535 km and C21 at 1048 km). Noanomalies are required to fit data from four Ioflybys (I24 at 611 km, I25 at 300 km, I27 at 198km, and I33 at 102 km). A satisfactory fit can beachieved with a second degree and order har-monic expansion for all the satellite flybys ex-cept G2, and for that one flyby even a thirddegree and order expansion leaves systematicDoppler residuals. The G2 flyby is unique.

The surface mass-point model provides asimple approach to fitting the data. Furtheranalysis will be required to determine if othermass anomalies at different locations anddepths below the surface might also yieldacceptable fits to the Doppler residuals. Ourfitting model of point masses does not allowspecification of the horizontal dimensionsover which the density heterogeneities ex-tend, although these are likely to be hundredsof kilometers, comparable to the distancesfrom the anomalies to the spacecraft. Withadditional study of the point-mass model andincorporation of more realistic anomalyshapes (disks, spheres) into the analysis, itmay be possible to identify the physicalsources of the anomalies. If the anomalies areat the surface, or near to it, then they could besupported for a lengthy period of geologicaltime by the cold and stiff outer layers ofGanymede’s ice shell.

References and Notes1. A compilation of satellite data can be found in D. J.

Tholen, V. G. Tejfel, A. N. Cox, Allen’s AstrophysicalQuantities, A. N. Cox, Ed. (Springer-Verlag, New York,ed. 4, 2000), pp. 302–310.

2. The interior composition, structure, and dynamics ofthe four Galilean satellites have been summarized,along with a bibliography, by G. Schubert, J. D. Ander-son, T. Spohn, W. B. McKinnon, in Jupiter, F. Bagenal,

T. E. Dowling, W. B. McKinnon, Eds. (Cambridge Univ.Press, New York, 2004), chap. 13.

3. W. M. Kaula, Theory of Satellite Geodesy (Blaisdell,Waltham, MA, 1966).

4. J. D. Anderson et al., Galileo Gravity Science Team,

Bull. Am. Astron. Soc. 33, 1101 (2001). This referenceincludes the best determination of Ganymede’s radi-us currently available.

5. P. J. Mohr, B. N. Taylor, Phys. Today 55, BG6 (2002).Because of recent determinations, the adopted valueof G has fluctuated over the past few years. We usethe current (2002) value recommended by the Com-mittee on Data for Science and Technology(CODATA), G � 6.6742 10�11 m3 kg�1 s�2, witha relative standard uncertainty of 1.5 10�4.

6. P. M. Muller, W. L. Sjogren, Science 161, 680(1968).

7. We acknowledge the work of O. Olsen for finding fitsto the fourth-degree gravitational field. We thank W.L. Sjogren, A. S. Konopliv, and D.-N. Yuan for theirassistance, especially for providing us with a recom-piled version of their gravity-anomaly software Grav-ity Tools. We also thank D. Sandwell for helpfuldiscussions about the nature of Ganymede’s gravityanomalies, and S. Asmar, G. Giampieri, and D.Johnston for helpful discussions. This work was per-formed at the Jet Propulsion Laboratory, CaliforniaInstitute of Technology, under contract withNASA. G.S., W.B.M., and J.L.P. acknowledge supportby grants from NASA through the Planetary Geologyand Geophysics program.

12 April 2004; accepted 8 July 2004

Probing the AccumulationHistory of the Voluminous

Toba MagmaJorge A. Vazquez1*† and Mary R. Reid1,2

The age and compositional zonation in crystals from the Youngest Toba Tuff recordtheprelude to Earth’s largestQuaternary eruption.Weused allanite crystals to dateand decipher this zoning and found that the crystals retain a record of at least150,000 years of magma storage and evolution. The dominant subvolcanicmagma was relatively homogeneous and thermally stagnant for 110,000years. In the 35,000 years before eruption, the diversity of melts increasedsubstantially as the system grew in size before erupting 75,000 years ago.

Toba caldera, a continental arc volcano inSumatra, Indonesia, produced Earth’s largestQuaternary eruption, ejecting �3000 km3 ofmagma 73,000 � 4000 years ago (1). Atmo-spheric loading by aerosols and ash from theToba eruption may have accelerated coolingof Earth’s climate (2) and resulted in near-extinction of humans (3). How quickly thisand other huge volumes of magma can amassis unclear, especially because large volumesof eruptible magma have not been detectedbeneath areas of active and/or long-livedmagmatism (4, 5). The rate of magma accu-mulation can dictate whether reservoirs ofmagma simply cool and solidify or persist at

magmatic conditions (6, 7), and may influ-ence the probability of volcanic eruption andthe characteristics of associated plutonic in-trusions (8, 9). A detailed record of magmaticevolution is that retained by the composition-al zoning of major minerals (10, 11), and thismight reveal how magma chambers accumu-late and change (12, 13). However, currentanalytical techniques are not sufficiently sen-sitive to put the chemical zoning in majorminerals into an absolute time frame. Hence,it is impossible to relate the zoning stratigra-phy of one crystal to another or evaluate theage of magma associated with crystallization.Here we use a combination of in situ compo-sitional and isotopic analyses on single crys-tals of a less abundant mineral, the epidote-group mineral allanite, to date and quantifycompositional zoning within and betweencrystals in the Youngest Toba Tuff (YTT)and to establish how this voluminous magmaevolved before eruption.

Allanite is a common accessory mineral inrhyodacitic and rhyolitic magmas and may haveconsiderable compositional zoning in major and

1Department of Earth and Space Sciences, Univer-sity of California, Los Angeles, CA 90095–1567,USA. 2Department of Geology, Northern ArizonaUniversity, Flagstaff, AZ 86011, USA.

*To whom correspondence should be addressed. E-mail: jvazquez@ess.ucla.edu†Present address: Department of Geological Sciences,California State University, Northridge, CA 91330–8266,USA.

Table 1. Least-squares fits to the acceleration data for two masses on Ganymede’s surface and also forthree masses on the surface. The three independent variables in the fitting model for each mass are Gm,and the geographic coordinates latitude and west longitude. For reference, the closest approach locationis at latitude 79.3° and west longitude 123.7° at an altitude of 264 km. The measure of goodness of fitis given by the variance �2 for the acceleration residuals. A qualitative measure of the goodness of fit isgiven by Fig. 3.

Six-parameter fit for two masses (�2 � 0.0244 mgal2)First mass Second mass Third mass

Gm (km3 s�2) 0.0237 � 0.0056 �0.0558 � 0.0084 �Latitude (°) 58.9 � 1.5 24.2 � 5.5 �Longitude W (°) 65.2 � 1.6 61.8 � 5.4 �

Nine-parameter fit for three masses (�2 � 0.0192 mgal2)First mass Second mass Third mass

Gm (km3 s�2) 0.0256 � 0.0038 �0.0500 � 0.0058 0.0081 � 0.0021Latitude (°) 77.7 � 1.0 39.9 � 2.6 53.6 � 2.3Longitude W (°) 337.3 � 5.1 355.6 � 4.6 140.1 � 4.8

R E P O R T S

www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004 991

minor elements. High Th concentrations (1to 2 weight %) and a large degree of U-Thfractionation between allanite and meltmake it ideal for in situ dating by 238U-230Thdisequilibrium methods, with an age resolu-tion of tens of thousands of years. Whereasindividual zircons are also amenable to insitu dating (14 ), the composition of allaniteis particularly sensitive to the differentiationof magma. The YTT is compositionallyzoned from 68 to 77 weight % SiO2, with themajority (�70%) of erupted magma being�73 weight % SiO2 (15). Chesner (15) con-cluded that this diversity of compositionswas largely produced by crystal fraction-ation. We analyzed allanites from a repre-sentative 75 weight % SiO2 rhyolite with theUCLA high-resolution ion microprobe andan electron microprobe (16 ).

When the 238U-230Th isotope characteris-tics of the host rhyolite are used to estimateinitial 230Th/232Th activity ratios, the cores ofthe YTT allanites are found to have crystal-lization ages ranging from 100 to 225 thou-sand years ago (ka), and most rims have agesidentical to or within analytical error of the�75-ka eruption age (17). Allanite composi-tions oscillate on scales of 10 to 30 �m (Fig.1). The greatest compositional variations(factor of 2 to 3) are in elements that can bedivided into two groups that covary inverse-ly: One group contains Mg, La, Ce, Ca, Ti,and Al, and the other Mn, Y, Sm, Nd, Th, Pr,and Fe (table S2). The zoning cannot reflectgrowth in a boundary layer that was depletedor enriched in allanite-compatible elementsbecause melt trapped in the growing allaniteslack such depletions or enrichments (18).

Ratios between the concentrations ofchemically similar elements that substituteinto the same crystallographic site in allanite,such as between the light and middle rare

earth elements, can mirror compositionalchanges in melt from which the allanite grewin the same way as, for example, the Fe/Mgratio in olivine mirrors that of the melt fromwhich it grew (19). Two ratios that vary by afactor of �2 in the allanites are MnO/MgOand La/Nd (Fig. 1). Each traces a distinctcomponent of fractionation in rhyolitic mag-mas (20) and is essentially not fractionated bykinetic effects or coupled substitution in al-lanite because elements within each pair aresimilarly sized and charged and fit in thesame crystallographic site. The effect of in-creasing fractionation on the composition ofallanite is to progressively lower La/Nd andincrease MnO/MgO in response to concomi-tant changes in the host melt (Fig. 2).

The variations in La/Nd and MnO/MgO inYTT allanites correlate smoothly (Fig. 2). Insome crystals, zoning is normal (trending tolower La/Nd and higher MnO/MgO) or re-verse, but in more than two-thirds of them, itis oscillatory, with a near-rim trend to a sim-ilar, more evolved composition (Fig. 1). Eventhough these allanites are present in one ofthe most evolved YTT pumices, some com-positions match those for representative al-lanites from the least evolved YTT rhyolite(Fig. 2). Reversals to less evolved compo-sitions (lower MnO/MgO and higher La/Nd) are typically abrupt and correspond toirregular boundaries marking zones withcontrasting tone in backscattered electron(BSE) images.

La/Nd and MnO/MgO exchange coeffi-cients enable us to predict how the YTT allanitecompositions are related to fractionation of theirparental melts (Fig. 2). Estimated La/Nd andMnO/MgO ratios for the rhyolitic melts overlapthose for erupted glasses reported by Chesner(15), and the variation can be related by �45%fractional crystallization (Fig. 2). This is com-

parable to an estimate of 40 to 50% fraction-ation based on the major element variability ofYTT pumices (15). Affinity between allanitecompositions and YTT melts is further suggest-ed by the agreement between measured andpredicted melt concentrations for elements suchas Mn and Mg (fig. S1).

The young age of the allanites shows thatthe YTT eruption tapped a rhyolitic magmaproduced after the demise of the preceding(Middle Toba Tuff, 500 ka) caldera magmachamber. In addition, the continuity of thegrowth record shows that the antiquity of theallanites is not due solely to preferential pres-ervation and/or recycling of crystals from olderintrusions. The �35,000 to 150,000 years ofmagmatic evolution recorded by individual al-lanites is comparable to crystallization intervalsestimated largely by zircon dating (21) of theother voluminous (�1000 km3) silicic magmas.Within this time frame, voluminous crystal-richmagmas can undergo fractionation differentia-tion by compaction and/or hindered crystal set-tling or can be thermally rejuvenated by aninflux of mafic magma into the base of thesubvolcanic reservoir (22, 23).

The oscillatory zoning and disparate historiesof the allanite (Fig. 1) require heterogeneousconditions of crystallization, whether in mush(�40% crystals) or liquid-rich domains. Theirregular boundaries and compositional reversalsin the allanites are suggestive of episodic disso-lution due to mixing with hotter, less evolvedmagmas. Crystals may have been cycled be-tween distinct batches of magma in Toba’s res-ervoir by differential movement along bound-aries between convectional zones (24, 25) ormingling between recharge and resident magmasand/or during intrareservoir self-mixing (26, 27).Although the range in melt variation required bythe allanite zoning does not require some of thecrystallization to have occurred in magma mush,

0.4

0.6

0.8

1.8

2.2

2.6

0 100 200 300

Grain 1

T3 Grain 3

La/

Nd

Mn

O/M

gO

0.4

0.6

0.81.0

La/

Nd

Mn

O/M

gO

1.8

2.2

2.6

20 40 60 80 1000

89±10

176±13

94±7

85±8 66±7

84±7

103±8

Rim

Rim

193±16

190±17 181±15

151±12 194±31

165±13

148±10

Core

Core

0.81.31.82.3

0 50 100 150 200

0.61.01.41.8

La/

Nd

Mn

O/M

gO

Rim

Core

Grain 2

93±9 102±10

93±6

81±7 79±10 Grain 3

Rim

1.8

2.2

2.63.0

0 40 80 120 160

0.4

0.6

0.8

La/

Nd

Mn

O/M

gO

111±989±7

222±35224±15

236±16181±9141±8

124±11

84±8

Distance from rim (µm)Distance from rim (µm)

Core

Fig. 1. Compositionaland age results for YTTallanites illustrated by(i) 238U-230Th agedistributions super-imposed on BSE im-ages of representa-tive YT T allanitesand (ii) core-to-rimcompositional varia-tions in MnO/MgOand La/Nd (note thedifferent composi-tional scale for grain2). Locations of insitu analyses areshown by open cir-cles (ion probe) andwhite circles (elec-tron probe). The con-trast in BSE imagesrepresents differenc-es in the abundancesof high-mass elements, such as U, Th, and rare earth elements, and correlates with compositional changes measured by electron probe.Uncertainties are 1�. The scale bar in BSE images is 100 �m.

R E P O R T S

13 AUGUST 2004 VOL 305 SCIENCE www.sciencemag.org992

it does not preclude it either, in which case themush must have been periodically invaded by newsilicic magma [compare (28)]. Evidently, mag-matic conditions were frequently disrupted as thecrystals grew, and the ages imply that greater dis-ruption was closer to the time of eruption.

From �225 to 110 ka, the allanite com-positions are relatively restricted, excluding asingle grain with evolved compositions (Fig.3). Between �110 and 75 ka, the composi-tional variability of the allanites is high. Onlyclose to their rims do the allanite composi-tions converge (Figs. 1 and 3). On the basis ofthese patterns, we suggest that initially, andfor a protracted interval of time, much of theYoungest Toba Tuff reservoir was relativelyhomogeneous, with melt compositions vary-ing by 15% fractionation (or �74 to 77weight % SiO2). The reservoir was nearlythermally stagnant, reflecting a heat balanceperched between magmatic influxes andcooling of the system. Nearer to eruption, thediversity of melts sampled by the crystalsincreased substantially (melts related by up to45% fractionation or �70 to 77 weight %

SiO2). Mixing was probably less efficient asthe system grew in size and diverse condi-tions of magma storage developed, resultingin domains of variably fractionated magmasand compositional zoning of the reservoir.Zoning of the magma that finally erupts (68to 77 weight % SiO2) could have developedeven closer to eruption if the crystal-chemicalvariations arose in a mush rather than aliquid-dominated reservoir. Different magmabatches may have intermittently coalesced inresponse to mass and heat input from theinflux of new magma, as documented for therecent eruptions of Soufriere Hills and Rua-

pehu volcanoes (8, 29), or when melts wereexpelled by gravitational collapse of criticallythickened batches of magma mush (22). Thelikely voluminous domains of not-yet rigidmagma mush probably enhanced the likeli-hood of cumulate crystals being reentrained(12), but those rare crystals with distinct com-positions were probably harvested from rela-tively isolated batches of magma that wereeven closer to solidification. Final merging ofmagma in the Toba reservoir, rather than theperiodic recharge that sustained the magmaticsystem for �100,000 years, could have cat-alyzed the cataclysmic eruption.

Our results demonstrate that the compo-nents of a huge subvolcanic magma reservoirmay unite crystals that probe magmatic evo-lution in space and time, and that intrusionsof silicic magma may undergo a transitionbetween homogeneous and heterogeneousstates during their storage in Earth’s crust. Acorollary is that in chemically and/or isotopi-cally zoned bodies of magma (10, 27), dif-ferent crystal-zoning profiles may reflect spa-tial as well as temporal variations in themagma reservoir. Our results predict that thecrystal-rich residue remaining after eruptionof the YTT magma would form a composi-tionally zoned pluton that is locally monoto-nous but complexly zoned at a mineral scale,features that are increasingly observed in theplutonic record (12, 30). Because the YTTmagma reservoir grew by piecemeal accumu-lation [compare (6, 7)] with mingling be-tween successive additions of magma, crys-tals from domains of the reservoir that did noterupt, such as any cumulate pile underpin-ning the more liquid portions of the reservoir(31, 32), might record this evolution as well.Generation of the YTT magma by meltingand remobilization of a young granitic pluton(33) is unlikely because the amount of crys-tallization recorded by the allanites is somuch less than expected for solidification ofan intrusion. Instead, the YTT magma accu-mulated and evolved over a period of�100,000 years.

References and Notes1. C. A. Chesner, W. I. Rose, A. Deino, R. Drake, J. A.

Westgate, Geology 19, 200 (1991).2. M. R. Rampino, S. Self, Nature 359, 50 (1992).3. M. R. Rampino, S. Self, Science 262, 1955 (1993).4. H. M. Iyer in, Volcanic Seismology, P. Gasparini, R.

Scarpa, K. Aki, Eds. (Springer-Verlag, Berlin, 1992), pp.299–338.

5. A. F. Glazner, J. M. Bartley, D. S. Coleman, W. Gray,R. Z. Taylor, GSA Today 14, 4 (2004).

6. R. B. Hanson, A. F. Glazner, Geology 23, 213 (1995).7. A. S. Yoshinobu, D. A. Okaya, S. R. Paterson, J. Struct.Geol. 20, 1205 (1998).

8. M. D. Murphy, R. S. J. Sparks, J. Barclay, M. R. Carroll,T. S. Brewer, J. Petrol. 41, 21 (2000).

9. S. Blake, Nature 289, 783 (1981).10. J. P. Davidson, F. J. T. Tepley, Science 275, 826 (1997).11. G. S. Wallace, G. W. Bergantz, Earth Planet. Sci. Lett.

202, 133 (2002).12. J. Blundy, N. Shimizu, Earth Planet. Sci. Lett. 102, 178

(1991).

1.0

1.4

1.8

2.2

2.6

3.0

0 0.5 1.0 1.5MnO/MgO

La/

Nd

10

20

30

40

50D

ifferentiation

10

20

30

40

50

Fig. 2. Plot of La/Nd versus MnO/MgO for allan-ites from evolved YTT rhyolite. Curve shows al-lanite compositions expected for crystallizationfrom melts related by fractionating the quartz-plagioclase-sanidine-biotite phenocryst assem-blage in modal proportions observed by (15),beginning from the least evolved melt composi-tion. A La/Nd exchange coefficient, D La/Nd, relat-ing allanite composition to melt composition is1.7 � 0.1 [1 SD; computed from data of (38, 39)]and agrees well with values of 1.5 to 1.6 calcu-lated by applying the model of Blundy and Wood(40) to the structural data of Dollase (41). D La/Nd

is essentially constant over much of the range oflow-to-high silica rhyolites, even though absolutepartition coefficients increase. A D MnO/MgO valueof 1.4 � 0.3 (1 SD) based on data for high-silicarhyolites [data of (39, 42)] is less constrainedbecause of a smaller number of partitioning data,but it agrees with the value of 1.4 based on theBlundy and Wood (40) model. Compositions insingle grains (e.g., grain 3, black circles) may over-lap nearly the entire range of observed allanitecompositions, including allanites from the leastevolved (68 weight % SiO2) rhyolite (gray field)reported by (43).

Age (ka)100 200 3000

Mn

O/M

gO

1.6

1.2

0.8

0.4

10%/40 ky

10%/25 ky

~225-110 ka~110-75 ka

10%/75 ky

780°760°

740°

720°Rims

710°

Eru

ptio

n

Temp

erature (C

)

700°

800°

10 k

mToba calderaToba caldera

dF/dt =

Fig. 3. Temporal variation in MnO/MgO forYTT allanites and cartoons depicting the mag-machronology of rhyolite beneath Tobacaldera. The depth of the system is from (15).Temperatures of magma during allanite growthare based on covariation of temperature andMnO/MgO reported by (44) for experimentalcrystallization of YTT magma compositions at100 to 200 MPa. Reference to dashed curves fordifferent rates of MnO/MgO fractionation (dF/dt � rate of crystal-liquid fractionation, start-ing at two different compositions recorded inthe oldest allanite cores) based on the miner-alogy the YTT emphasizes the divergence ofthe data from simple evolutionary trends. Mostallanite compositions (gray circles) between�225 and 110 ka are restricted and reflect abody of rhyolitic magma that is relatively in-variant compositionally. Rare grains (e.g., grain2) reflect isolated batches of highly evolvedmagmas (white circles with black dots). Theincrease in diversity of allanite compositionbetween 110 and 75 ka is produced by theinteraction and mingling of differentially frac-tionated batches of rhyolitic magma with tem-peratures between �760° and 715°C. Finalmixing (not shown) gathers crystals into themost evolved batch of rhyolite in the reservoirthat then erupts at 75 ka.

R E P O R T S

www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004 993

13. A. T. Anderson, A. M. Davis, F. Lu, J. Petrol. 41, 449 (2000).14. M. R. Reid, C. D. Coath, T. M. Harrison, K. D. McKee-

gan, Earth Planet. Sci. Lett. 150, 27 (1997).15. C. A. Chesner, J. Petrol. 39, 397 (1998).16. Materials and analytical methods are available as

supporting material on Science Online.17. Model 238U-230Th ages are derived as described in

(14). See (34) for a review of 230Th dating in mag-matic systems. The reported ages are taken to bethose of crystallization because of the relatively tightpacking of ions within allanite and the tetravalentcharge of Th [compare (35)]. Reequilibration of Thduring magmatic residence will be insignificant:Based on the predictive model of Fortier and Giletti(36), Th diffusion would only affect �2 �m in allaniteover a period of 100,000 years at the highest report-ed temperature of the YTT magma (�780°C). Un-certainty in the calculated ages due to possiblevariation of initial 230Th/232Th during magmatic evo-lution can be evaluated by assuming that observederuption-age 230Th/232Th activity ratio variations ofthe YTT (0.358 to 0.433) are representative of theinitial range of Th-isotope composition. For reportedages �120 ka, the uncertainty in age associated withthe initial ratio is within the analytical uncertainty onthe ages, except for the youngest allanite domainswhich could not have grown from melts that had Thisotope compositions significantly different from thatof their host. Reported ages that are 120 to 200 kacould be at most a few percent to, for the older ofthese, as much as 27% older than allowed by theanalytical uncertainty. Those few allanites with ages�200 ka could be substantially older. Thus, the ageranges reported here are conservative.

18. J. B. Thomas, R. J. Bodnar, N. Shimizu, C. Chesner, inZircon, J. M. Hanchar, P. W. O. Hoskin, Eds. (Mineral-ogical Society of America, Washington, DC, 2004),vol. 53, chap. 3.

19. P. L. Roeder, R. F. Emslie, Contrib. Mineral. Petrol. 29,275 (1970).

20. MnO/MgO in residual melts of silicic magmas typi-cally increases with fractionation of major maficsilicates. La/Nd also increases except when the frac-tionating assemblage includes sufficient quantities ofallanite, chevkinite, and/or monazite that are rich inlight rare earth elements (37).

21. M. R Reid, in Treatise on Geochemistry, H. D. Holland,K. K Turekian, Eds. (Elsevier, Amsterdam, 2003), vol.3, chap. 3.05.

22. O. Bachmann, G. W. Bergantz, J. Petrol., 45, 1565 (2004).23. O. Bachmann, G. W. Bergantz, Geology 27, 447 (2003).24. B. D. Marsh, M. R. Maxey, J. Volcanol. Geotherm. Res.

24, 95 (1985).25. V. R. Troll, H. U. Schmincke, J. Petrol. 43, 243 (2002).26. G. W. Bergantz, J. Struct. Geol. 22, 1297 (2000).27. S. Couch, R. S. J. Sparks, M. R. Carroll, Nature 411,

1037 (2001).28. S. Turner, R. George, D. A. Jerram,N. Carpenter, C. Hawkes-

worth, Earth Planet. Sci. Lett. 214, 279 (2003).29. M. Nakagawa, K. Wada, T. Thordarson, C. P. Wood,

J. A. Gamble, Bull. Volcanol. 61, 15 (1999).30. D. M. Robinson, C. F. Miller, Am. Mineral. 84, 1346 (1999).31. C. A. Bachl, C. F. Miller, J. S. Miller, J. E. Faulds, GSA

Bull. 113, 1213 (2001).32. T. H. Druitt, C. R. Bacon, Trans. R. Soc. Edinburgh

Earth Sci. 79, 289 (1988).33. I. N. Bindeman, J. W. Valley, Geology 28, 719 (2000).34. M. Condomines, P.-J. Gauthier, O. Sigmarsson, in

Uranium-Series Geochemistry, B. Bourdon, G. M. Hen-derson, C. C. Lundstrom, S. P. Turner, Eds. (Mineral-ogical Society of America, Washington, DC, 2003),vol. 52, chap. 4.

35. E. Dowty, Am. Mineral. 65, 174 (1980).36. S. M. Fortier, B. J. Giletti, Science 245, 1481 (1989).37. C. F. Miller, D. W. Mittlefehldt, Geology 10, (1982).38. C. K. Brooks, P. Henderson, J. G. Ronsbo, Mineral.

Mag. 44, 157 (1981).39. G. A. Mahood, W. Hildreth, Geochim. Cosmochim.

Acta 47, 11 (1983).40. J. Blundy, B. Wood, Nature 372, 452 (1994).41. W. A. Dollase, Am. Mineral. 56, 447 (1971).42. A. Ewart, W. L. Griffin, Chem. Geol. 117, 251 (1994).43. C. A. Chesner, A. D. Ettlinger, Am. Mineral. 74, 750 (1989).44. J. E. Gardner, P. W. Layer, M. J. Rutherford, Geology

30, 347 (2002).

45. We are grateful to C. Chesner for samples; C. Coath, F.Ramos, and F. Kyte for analytical help; and especially J.Simon and G. Bergantz for insightful discussions. Anony-mous referees provided very helpful reviews. Funded byNSF grants EAR-9706519 and EAR-0003601. The Univer-sity of California, Los Angeles (UCLA), ion microprobe ispartially subsidized by a grant from the NSF Instrumenta-tions and Facilities Program.

Supporting Online Materialwww.sciencemag.org/cgi/content/full/305/5686/991/DC1Materials and MethodsTables S1 and S2Fig. S1References

19 February 2004; accepted 9 July 2004

More Intense, More Frequent, andLonger Lasting Heat Waves in

the 21st CenturyGerald A. Meehl* and Claudia Tebaldi

A global coupled climate model shows that there is a distinct geographic patternto future changes in heat waves. Model results for areas of Europe and NorthAmerica, associated with the severe heat waves in Chicago in 1995 and Paris in2003, show that future heat waves in these areas will become more intense, morefrequent, and longer lasting in the second half of the 21st century. Observationsand the model show that present-day heat waves over Europe and North Americacoincide with a specific atmospheric circulation pattern that is intensified byongoing increases in greenhouse gases, indicating that it will produce more severeheat waves in those regions in the future.

There is no universal definition of a heatwave, but such extreme events associatedwith particularly hot sustained tempera-tures have been known to produce notableimpacts on human mortality, regional econ-omies, and ecosystems (1–3). Two well-documented examples are the 1995 Chica-go heat wave (4) and the Paris heat wave of2003 (5). In each case, severe hot temper-atures contributed to human mortality andcaused widespread economic impacts, in-convenience, and discomfort.

In a future warmer climate with increasedmean temperatures, it seems that heat waveswould become more intense, longer lasting, and/or more frequent (6, 7). However, analyses offuture changes in other types of extreme events,such as frost days, show that changes are notevenly distributed in space but are characterizedinstead by particular patterns related to largerscale climate changes (8). Here, we examinefuture behavior of heat waves in a global coupledclimate model, the Parallel Climate Model(PCM). This model has a latitude-longitude res-olution of about 2.8° in the atmosphere and alatitude-longitude resolution of less than 1° inthe ocean, and it contains interacting compo-nents of atmosphere, ocean, land surface, and seaice. The PCM has been used extensively tosimulate climate variability and climate changein a variety of applications for 20th- and 21st-century climate (6, 8–13). We analyzed a four-member ensemble (i.e., the model was run four

times from different initial states and the fourmembers were averaged together to reducenoise) for 20th-century climate and a five-mem-ber ensemble for 21st-century climate. Theformer includes the major observed forcings forthe 20th century encompassing greenhouse gas-es, sulfate aerosols, ozone, volcanic aerosols,and solar variability (13). The latter uses a “busi-ness-as-usual” scenario, which assumes little inthe way of policy intervention to mitigate green-house gas emissions in the 21st century (14). Wedefine the present-day reference period as 1961to 1990 for model and observations and thefuture as the time period from 2080 to 2099.

First, we sought to define a heat wave.Many definitions could apply to heat wavesthat quantify the duration and/or intensity ofeither nighttime minima or daytime maxima(4, 5, 15, 16). Here, we used two definitionsof heat waves; each has been shown to beassociated with substantial societal impactson human health and economies. The first (4)evolved from a study of the 1995 Chicagoheat wave; it concentrates on the severity ofan annual “worst heat event” and suggeststhat several consecutive nights with no relieffrom very warm nighttime minimum temper-atures may be most important for health im-pacts. For present-day climate for NorthAmerica and Europe (Fig. 1), the means ofthree consecutive warmest nights for obser-vations and the model show good agreement.Heat waves presently are more severe in thesoutheast United States (large areas greaterthan 24°C) and less severe in the northwestUnited States (equally large areas less than16°C; Fig. 1, A and C). For Europe, there ismore of a north-south gradient in both obser-

National Center for Atmospheric Research (NCAR),Post Office Box 3000, Boulder, CO 80307, USA.

*To whom correspondence should be addressed. E-mail: meehl@ncar.ucar.edu

R E P O R T S

13 AUGUST 2004 VOL 305 SCIENCE www.sciencemag.org994

vations and the model (Fig. 1, B and D), withmore severe heat waves in the Mediterraneanregion (most countries bordering the Medi-terranean have values greater than 20°C) andless severe heat in northern Europe (manyareas less than 16°C).

Future changes of worst 3-day heat wavesdefined in this way in the model are not uniform-ly distributed in space but instead show a distinctgeographical pattern (Fig. 1, E and F). Thoughdifferences are positive in all areas, indicative ofthe general increase of nighttime minima, heatwave severity increases more in the western andsouthern United States and in the Mediterraneanregion, with heat wave severity showing positiveanomalies greater than 3°C in those regions.Thus, many of the areas most susceptible to heatwaves in the present climate (greatest heat waveseverity in Fig. 1, A to D) experience the greatestincrease in heat wave severity in the future. Butother areas not currently as susceptible, such asnorthwest North America, France, Germany, andthe Balkans, also experience increased heat waveseverity in the 21st century in the model.

The second way we chose to define a heatwave is based on the concept of exceeding spe-cific thresholds, thus allowing analyses of heatwave duration and frequency. Three criteriawere used to define heat waves in this way,which relied on two location-specific thresholdsfor maximum temperatures. Threshold 1 (T1)was defined as the 97.5th percentile of the dis-tribution of maximum temperatures in the obser-vations and in the simulated present-day climate(seasonal climatology at the given location), andT2 was defined as the 81st percentile. A heatwave was then defined as the longest period ofconsecutive days satisfying the following threeconditions: (i) The daily maximum temperaturemust be above T1 for at least 3 days, (ii) theaverage daily maximum temperature must beabove T1 for the entire period, and (iii) the dailymaximum temperature must be above T2 forevery day of the entire period (16).

Because the Chicago heat wave of 1995and the Paris heat wave of 2003 had partic-ularly severe impacts, we chose grid pointsfrom the model that were close to those twolocations to illustrate heat wave characteris-tics. This choice was subjective and illustra-tive given that there are, of course, otherwell-known heat waves from other locations.Also, we are not suggesting that a model gridpoint is similar to a particular weather station;we picked these grid points because theyrepresent heat wave conditions for regionsrepresentative of Illinois and France in themodel, and therefore they can help identifyprocesses that contribute to changes in heatwaves in the future climate in those regions.We chose comparable grid points from theNational Centers for Environmental Predic-tion (NCEP)/NCAR reanalyses that used as-similated observational data (17, 18) for com-parison to the model results.

For both the Paris- and Chicago-area gridpoints for the five-ensemble members, a fu-ture increase in heat wave occurrence is pre-dicted (Fig. 2, A and B). For Chicago, thenumber of present-day heat waves (1961 to1990) ranges from 1.09 to 2.14 heat wavesper year for the four-ensemble members,whereas for future climate, the range shifts tobetween 1.65 and 2.44. Thus, the ensemblemean heat wave occurrence increases 25%from 1.66 to 2.08 heat waves per year. Thecurrent observed value from NCEP for 1961to 1990 lies within the present-day rangefrom the model with a value of 1.40 eventsper year. For Paris, the model ranges from1.18 to 2.17 heat waves per year at present(the value from the NCEP reanalysis liesbarely outside this range at 1.10 days), withthe future range shifting to between 1.70 and2.38. Thus, the ensemble mean heat waveoccurrence for the Paris grid point increases31% from 1.64 to 2.15 heat waves per year.Both observed values from NCEP fall well

short of the future range from the model,indicative of the shift to more heat waves peryear in the future climate.

There is a corresponding increase induration at both locations (Fig. 2, C and D).For Chicago, present-day average durationof heat waves from the four–model ensem-ble members ranges from 5.39 to 8.85 days,encompassing the observed value fromNCEP at 6.29 days. For Paris, the present-day model range is 8.33 to 12.69 days, withthe NCEP observation lying within thatrange at 8.40 days. For future climate at theParis grid point, there is a shift to longerlived heat waves with average duration in-creasing from 11.39 days to 17.04 days. Forboth of these regions, similar to what wasfound for the number of heat waves, thecorresponding grid point values from theNCEP reanalyses show the duration to bewithin or very near the range of the present-day model ensemble members but not thefuture ensemble members, indicative of the

Fig. 1. Heat wave severity as the mean annual 3-day worst (warmest) nighttime minima event (4)from NCEP/NCAR reanalyses, 1961 to 1990, for North America (°C) (A) and Europe (B), and fromthe model for North America (C) and Europe (D). The changes of 3-day worst (warmest) nighttimeminima event from the model, future (2080 to 2099) minus present (1961 to 1990) for NorthAmerica (°C) (E) and Europe (F) are also shown.

R E P O R T S

www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004 995

shift in the model to more and longer livedheat waves in future climate.

Heat waves are generally associated withspecific atmospheric circulation patterns repre-sented by semistationary 500-hPa positiveheight anomalies that dynamically produce sub-sidence, clear skies, light winds, warm-air ad-vection, and prolonged hot conditions at thesurface (15, 19). This was the case for the 1995Chicago heat wave and 2003 Paris heat wave(Fig. 3, A and B), for which 500-hPa heightanomalies of over �120 geopotential meters(gpm) over Lake Michigan for 13 to 14 July1995 and �180 gpm over northern France for 1to 13 August 2003 are significant at greater thanthe 5% level according to a Student’s t test. Astratification based on composite present-dayheat waves from the model for these twolocations over the period of 1961 to 1990(Fig. 3, C and D) shows comparable ampli-tudes and patterns, with positive 500-hPaheight anomalies in both regions greater than�120 gpm and significance exceeding the5% level for anomalies of that magnitude.

There is an amplification of the positive500-hPa height anomalies associated with agiven heat wave for Chicago and Paris forfuture minus present climate (Fig. 4, A andB). Statistically significant (at greater thanthe 5% level) ensemble mean heat wave 500-hPa differences for Chicago and Paris in thefuture climate compared with present-day arelarger by about 20 gpm in the model (com-paring Fig. 4, A and B, with Fig. 3, C and D).

The future modification of heat wave char-acteristics with a distinct geographical pattern(Fig. 1, E and F) suggests that a change inclimate base state from increasing greenhousegases could influence the pattern of those chang-es. The mean base state change for future climateshows 500-hPa height anomalies of nearly �55gpm over the upper Midwest, and about �50gpm over France for the end of the 21st century(Fig. 4, C and D, all significant above the 5%level). The 500-hPa height increases over theMediterranean and western and southern UnitedStates for future climate are directly associatedwith more intense heat waves in those regions(Fig. 1, E and F), thus confirming the link be-tween the pattern of increased 500-hPa heightsfor future minus present-day climate and in-creased heat wave intensity in the future climate.A comparable pattern is present in an ensembleof seven additional models for North Americafor future minus present-day climate, with some-what less agreement over Europe (fig. S1). Inthat region, there is still the general character oflargest positive anomalies over the Mediterra-nean and southern Europe regions, and smallerpositive anomalies to the north (fig. S1), butlargest positive values occur near Spain, as op-posed to the region near Greece as in our model(Fig. 4D). This also corresponds to a similarpattern for increased standard deviations of bothsummertime nighttime minimum and daytime

Fig. 2. Based on the threshold definition of heat wave (16), mean number of heat waves per yearnear Chicago (A) and Paris (B) and mean heat wave duration near Chicago (C) and Paris (D) areshown. In each panel, the blue diamond marked NCEP indicates the value computed fromNCEP/NCAR reanalysis data. The black segment indicates the range of values obtained from thefour ensemble members of the present-day (1961 to 1990) model simulation. The red segmentindicates the range of values obtained from the five ensemble members of the future (2080 to2099) model simulation. The single members are marked by individual symbols along the segments.Dotted vertical lines facilitate comparisons of the simulated ranges/observed value.

-50

0

50

100

150

Observed Heat Wave 500hPa Height Anomalies July 13-14, 1995, minus July 1948-2003

0

50

100

150

200

Observed Heat Wave 500hPa Height Anomalies August 1-13, 2003, minus August 1948-2003

-50

0

50

100

150

Simulated Composite Heat Wave 500 hPa Height Anomalies (JJA, 1961-1990)

0

50

100

150

200

Simulated Composite Heat Wave 500 hPa Height Anomalies (JJA, 1961-1990)

48.8°N

48.8°N

23.7°N

23.7°N

65.5°N

65.5°N

26.5°N

26.5°N

123.7°W

123.7°W

70.3°W

70.3°W

16.8°W

16.8°W

30.9°E

30.9°E

A B

C D

Fig. 3. Height anomalies at 500 hPa (gpm) for the 1995 Chicago heat wave (anomalies for 13 to 14 July1995 from July 1948 to 2003 as base period), from NCEP/NCAR reanalysis data (A) and the 2003 Parisheat wave (anomalies for 1 to 13 August 2003 from August 1948 to 2003 as base period), fromNCEP/NCAR reanalysis data (B). Also shown are anomalies for events that satisfy the heat wave criteriain the model in present-day climate (1961 to 1990), computed at grid points near Chicago (C) and Paris(D). In both cases, the base period is summer [ June, July, August ( JJA)], 1961 to 1990.

R E P O R T S

13 AUGUST 2004 VOL 305 SCIENCE www.sciencemag.org996

maximum temperatures (fig. S2). This is consis-tent with a widening of the distribution of tem-peratures in addition to a shift in the mean (5),and suggests that there is an increase in heatwave occurrence beyond that driven by changesin the mean circulation.

The 500-hPa height anomalies are moststrongly related to positive warm season precipi-tation anomalies over the Indian monsoon regionand associated positive convective heating anom-alies that drive mid-latitude teleconnection pat-terns (such as those in Fig. 4, C and D) in responseto anomalous tropical convective heating in futureclimate (figs. S3 to S5). Thus, areas alreadyexperiencing strong heat waves (e.g., southwest,midwest, and southeast United States and theMediterranean region) could experience evenmore intense heat waves in the future. But otherareas (e.g., northwest United States, France, Ger-many, and the Balkans) could see increases ofheat wave intensity that could have more seriousimpacts because these areas are not currently aswell adapted to heat waves.

References and Notes1. C. Parmesan et al., Bull. Am. Meteorol. Soc. 81, 443

(2000).2. D. R. Easterling et al., Science 289, 2068 (2000).3. World Health Organization (WHO), “The health im-

pacts of 2003 summer heat waves,” WHO BriefingNote for the Delegations of the 53rd session of theWHO Regional Committee for Europe, Vienna, Aus-tria, 8 to 11 September 2003; available at www.euro.who.int/document/Gch/HEAT-WAVES%20RC3.pdf.

4. T. R. Karl et al., Bull. Am. Meteorol. Soc. 78, 1107(1997).

5. C. Schar et al., Nature 427, 332 (2004).6. U. Cubasch et al., in Climate Change 2001: The Scientific

Basis. Contribution of Working Group I to the Third

Assessment Report of the Intergovernmental Panel onClimate Change, J. T. Houghton et al., Eds. (CambridgeUniv. Press, Cambridge, 2001), pp. 525–582.

7. T. R. Karl, K. E. Trenberth, Science 302, 1719 (2003).8. G. A. Meehl et al., Clim. Dyn., in press.9. W. M. Washington et al., Clim. Dyn. 16, 755 (2000).10. A. Dai et al., Geophys. Res. Lett. 28, 4511 (2001).11. G. A. Meehl et al., J. Clim. 16, 426 (2003).12. B. D. Santer et al., Science 301, 479 (2003).13. G. A. Meehl et al., J. Clim., in press.14. A. Dai et al., J. Climate 14, 485 (2001) describes the

business-as-usual scenario as similar to the A1Bemissions scenario of the Intergovernmental Panel onClimate Change (IPCC) Special Report on EmissionScenarios (SRES) (20), with CO2 rising to about 710parts per million by volume by 2100, SO2 emissionsdeclining to less than half the present value by 2100,CH4 stabilized at 2500 parts per billion by volume in2100, N2O as in the IPCC IS92 emissions scenario(21), and halocarbons following a preliminary versionof the SRES A1B scenario.

15. M. A. Palecki et al., Bull. Am. Meteorol. Soc. 82, 1353(2001).

16. R. Huth et al., Clim. Change 46, 29 (2000).17. E. Kalnay et al., Bull. Am. Meteorol. Soc. 77, 437 (1996).18. Archived observations from surface weather stations,

weather balloons, satellites, and other sources areinterpolated to a regular grid in a weather forecastmodel, and the model is run at regular intervals overpast time periods to produce a dynamically consis-tent time-evolving representation of the observedhistorical climate state.

19. K. E. Kunkel et al., Bull. Am. Meteorol. Soc. 77, 1507(1996).

20. N. Nakicenovic et al., IPCC Special Report on EmissionScenarios (Cambridge Univ. Press, Cambridge, 2000).

21. J. Legget et al., Climate Change 1992: The Supple-mentary Report to the IPCC Scientific Assessment(Cambridge Univ. Press, New York, 1992), pp. 69–75.

22. We thank D. Nychka for discussions; G. Branstator forthe convective heating anomaly results; and L. Buja, J.Arblaster, and G. Strand for assistance on theCMIP2� results from the Coupled Model Intercom-parison Project, phase 2 plus (www-pcmdi.llrl.gov/cmip). This work was supported in part by theWeather and Climate Impact Assessment Initiative atthe National Center for Atmospheric Research. Aportion of this study was also supported by theOffice of Biological and Environmental Research, U.S.Department of Energy, as part of its Climate ChangePrediction Program, and the National Center for At-mospheric Research. The National Center for Atmo-spheric Research is sponsored by NSF.

Supporting Online Materialwww.sciencemag.org/cgi/content/full/305/5686/994/DC1Figs. S1 to S5References

2 April 2004; accepted 9 July 2004

Discovery of SymbioticNitrogen-Fixing Cyanobacteria

in CoralsMichael P. Lesser,1* Charles H. Mazel,2 Maxim Y. Gorbunov,3

Paul G. Falkowski3,4

Colonies of the Caribbean coral Montastraea cavernosa exhibit a solar-stimulated orange-red fluorescence that is spectrally similar to a variety offluorescent proteins expressed by corals. The source of this fluorescence isphycoerythrin in unicellular, nonheterocystis, symbiotic cyanobacteriawithin the host cells of the coral. The cyanobacteria coexist with thesymbiotic dinoflagellates (zooxanthellae) of the coral and express the ni-trogen-fixing enzyme nitrogenase. The presence of this prokaryotic sym-biont in a nitrogen-limited zooxanthellate coral suggests that nitrogenfixation may be an important source of this limiting element for the sym-biotic association.

The success of scleractinian corals sincethe Triassic (1) has been attributed to theestablishment of a mutualistic symbiosisbetween the cnidarian host and a diverse

group of endosymbiotic dinoflagellates(zooxanthellae). Zooxanthellae, which arelocalized within gastrodermal cells of thecnidarian host, can provide more than

-50

0

50

100

150

Simulated Future Heat Wave 500 hPa Height Anomalies

0

50

100

150

200

Simulated Future Heat Wave 500 hPa Height Anomalies

46

48

50

52

54

56

Simulated Difference in 500hPa Height Anomalies Future minus Present

40

45

50

55

60

Simulated Difference in 500hPa Height Anomalies Future minus Present

65.5°N

65.5°N

26.5°N

26.5°N

16.8°W

16.8°W

30.9°E

30.9°E

48.8°N

48.8°N

23.7°N

23.7°N

123.7°W

123.7°W

70.3°W

70.3°W

A B

C D

Fig. 4. Height anomalies at 500 hPa (gpm) for events that satisfy the heat wave criteria in themodel in future climate (2080 to 2099) for grid points near Chicago (A) and Paris (B), using thesame base period as in Fig. 3, C and D. Also shown are changes (future minus present) in themodel’s 500-hPa height mean base state, for North America (C) and Europe (D).

R E P O R T S

www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004 997

100% of the carbon requirements of theanimal partner, primarily in the form ofcarbohydrates and low–molecular-weightlipids. Experimental manipulations of zooxan-thellate corals suggest that inorganic nitrogenlimits the growth and abundance of zooxanthel-lae in the coral; indeed, this limitation has beensuggested to be essential for the stability of thesymbiotic association (2, 3).

In addition to zooxanthellae, a variety ofbacteria appear to be associated with scler-actinian corals (4–6 ), and although theseassociations also appear to be widely dis-tributed, stable, and nonpathogenic, thefunction of these bacteria remains largelyunknown. However, the presence of cya-nobacteria is associated with photosynthe-

sis-dependent nitrogen fixation on coralreefs (7 ) and is suggested to be responsiblefor nitrogen fixation in living coral tissue(8). In this paper, we show that large num-bers of endosymbiotic cyanobacteria capa-ble of fixing nitrogen occur in a commonscleractinian coral, Montastraea caver-nosa.

Scleractinian corals, including M. cav-ernosa (9–11), express a variety of fluores-cent proteins, but colonies of M. cavernosahave also been observed to fluoresce or-ange during the daytime (Fig. 1A). Thisfluorescence is not due to a fluorescentprotein but to phycoerythrin. In vivo exci-tation/emission spectra of these coralsshowed an emission peak at 580 nm and ashoulder at 630 nm, with excitation bandsat 505 and 571 nm (Fig. 1B). Although thisspectral signature is similar to those report-ed for red fluorescent proteins of corals(12, 13), the two excitation peaks also cor-responded to absorption by phycoerythrinin marine cyanobacteria that contain boththe phycourobilin and phycoerythrobilinchromphores (14, 15 ). Immunoblots (16 )of coral homogenates challenged with apolyclonal antibody against phycoerythrin

revealed a positive cross-reaction with the18- to 20-kD �-polypeptide of phyco-erythrin (Fig. 1C). These results clearlysuggest that intracellular cyanobacteria areassociated with the coral.

Fluorescence lifetime analyses (16 ) in-dicate that the 580-nm excited state is dom-inated by a single component with a 3.93-ns time constant (Fig. 1D). The slow sin-gle-exponential decay of the orangepigment is longer than described for fluo-rescent proteins (2.6 to 3.7 ns) (11, 17, 18)and suggests energetic isolation and theabsence of excitation energy transfer out ofthe chromophore. In contrast, phyco-erythrin fluorescence normally observed incyanobacteria exhibits faster kinetics invivo owing to efficient energy transferwithin the phycobilisomes. The lifetimedata and the daytime fluorescence indicatethat the energy coupling of these pigmentsto primary photochemistry in the symbioticcyanobacteria is weak, leading to the relative-ly high quantum yield of fluorescence forthis pigment.

Epifluorescence microscopy (16) of hosttissue homogenates revealed zooxanthellaeexhibiting red chlorophyll fluorescence, as

1Department of Zoology and Center for Marine Biol-ogy, University of New Hampshire, Durham, NH03824, USA. 2Physical Sciences, 20 New EnglandBusiness Center, Andover, MA 01810, USA. 3Environ-mental Biophysics and Molecular Ecology Program,Institute of Marine and Coastal Sciences, RutgersUniversity, 71 Dudley Road, New Brunswick, NJ08901, USA. 4Department of Geological Sciences,Rutgers University, Piscataway, NJ, USA.

*To whom correspondence should be addressed. E-mail: mpl@cisunix.unh.edu

Fig. 1. (A) Underwater photograph of M. cavernosa exhibiting orange daytime fluorescencethroughout the colony. The colony is approximately 0.6 m high. (B) Measured fluorescenceexcitation/emission spectrum of an orange-fluorescing M. cavernosa. Orange-fluorescingcorals have an emission peak at 580 nm and excitation peaks at 505, 533, and 571 nm. (C)Immunoblot of zooxanthellae-free tissues of M. cavernosa, showing the presence of positivestaining for the 18- to 20-kD �-polypeptide of phycoerythrin. (D) Fluorescence lifetimeanalysis for the orange fluorescent chromophore from M. cavernosa single decay at 580 nm.The results produce a single lifetime component at 3.93 ns.

R E P O R T S

13 AUGUST 2004 VOL 305 SCIENCE www.sciencemag.org998

well as many smaller orange-fluorescentcells resembling cyanobacteria (Fig. 2A).An analysis of tissue homogenates usingflow cytometry (16) showed a distinct phy-coerythrin signature and a size range forthese cells of 1.0 to 3.0 �m in diameter.The number of phycoerythrin-positive cellsfrom fluorescent samples of M. cavernosa,normalized to surface area, ranged from1.14 � 107 to 2.55 � 107 cells cm�2,

whereas nonfluorescent colonies have ��102 phycoerythrin-positive cells per squarecentimeter. Transmission electron micro-graphs (16) of the coral tissue revealed thatthe cyanobacteria-like cells are located inthe epithelial cells of the animal host andare surrounded by host membrane (Fig.2B). The cyanobacteria-like cells exhibit anunusual arrangement of their thylakoidmembranes that cross randomly throughout

the cell and occasionally appear both ex-panded and appressed. These cells also ap-pear to have fewer electron-dense phyco-bilisomes associated with the thylakoidmembranes than has been reported for othercyanobacteria. Immunogold probing of thinsections (Fig. 2C), using the antibody tophycoerythrin, showed that the antibodybinds significantly [analysis of variance(ANOVA), P � 0.001] to the cyanobacte-rial cells [134.9 particles per cell � 19.4(SE)] when compared to controls [6.5 par-ticles per cell � 2.1 (SE)] without theprimary antibody. These cells also bindpositively to a cyanobacteria-specific(CYA762) 16S ribosomal RNA–targetedoligonucleotide probe (Fig. 2D) whichcross-reacts with a large number of cyanobac-teria (16, 19). Additionally, 16S ribosomalDNA sequencing using cyanobacteria-specificprimers (16, 20) yielded a 556-bp sequence(GenBank accession number AY580333) fromgenomic DNA preparations that match cya-nobacterial sequences (n � 100) related toSynechococcus sp., Prochlorococcus sp., or un-cultured cyanobacteria in the Order Chroococ-cales, a paraphyletic group (21), with 93 to 97%sequence homology.

To assess the potential for nitrogen fix-ation in this coral, we challenged proteinextracts with a polyclonal antibody to the32-kD Fe protein subunit of nitrogenase(Fig. 3A). The results yielded a single pos-itive cross-reaction, strongly indicating ex-pression of the gene in the coral. Again,using the same antibody to nitrogenase, weused immunogold probing of thin sections(Fig. 3B) and found that the antibody bindssignificantly (ANOVA, P � 0.006) to thecyanobacterial cells [8.8 particles percell � 1.3 (SE)] when compared to controls[4.5 particles per cell � 0.5 (SE)] withoutthe primary antibody. Many members ofthe Order Chroococcales are capable offixing nitrogen. Our results clearly suggestthat endosymbiotic cyanobacteria capableof fixing nitrogen are present in M. caver-nosa and form a stable long-term associa-tion within host cells. This symbiont couldpotentially be a source of the limiting ele-ment nitrogen for the symbiosis through therelease of fixed nitrogen products to thecoral host.

Like the symbiotic cyanobacteria of M.cavernosa, free-living cyanobacteria andprochlorophytes that contain phycoerythrinoften exhibit strong fluorescence under cer-tain conditions, with a maximum emissionbetween 570 to 580 nm (22, 23). Uncou-pled phycoerythrin has been proposed toserve as a storage pool of nitrogen in phy-cobilin-containing cyanobacteria (22) butnot in prochlorophytes (23). Phycoerythrindetachment from the photosynthetic appa-ratus in cyanobacteria and prochlorophytes

Fig. 2. (A) Epifluorescent micrograph (magni-fication,�1000) of coral homogenate showingred-fluorescing (685-nm fluorescence fromchlorophyll) zooxanthellae and 1- to 3-�morange-fluorescing cyanobacteria (arrows). (B)Electron micrograph of epithelial tissues of M.cavernosa. Scale bar, 1.0 �m. C, cyanobacte-rium; N, nematocyst; T, thylakoid; HM, hostmembrane; SV, secretory vesicle. (C) Immuno-

gold labeling (20-nm gold particles, arrows) of thin sections for phycoerythrin (magnification,�50,000). (D) Fluorescent in situ hybridization micrograph (magnification,�1000) ofM. cavernosaepithelial tissues showing positive binding of cyanobacterial-specific probe (arrows).

Fig. 3. (A) Positive im-munoblot for the 32-kD Fe protein of nitro-genase in zooxanthel-lae-free tissues of M.cavernosa. (B) Immu-nogold labeling (20-nmgold particles, arrow) ofthin sections for nitro-genase (magnification,�50,000).

R E P O R T S

www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004 999

can be caused by exposure to glycerol andresults in strong fluorescence by eliminat-ing the quenching associated with energytransfer from phycoerythrin to the reactioncenters (22, 23). The cyanobacterial sym-bionts of M. cavernosa are exposed to highconcentrations of glycerol in the coral, be-cause it is the major carbon compoundtranslocated from the symbiotic zooxan-thellae to the host tissues (24 ), and this mayexplain both the unusual ultrastructure andthe characteristic orange fluorescenceemission of the symbiotic cyanobacteria.Because little or no energy is transferredfrom phycoerythrin to primary photochem-istry, glycerol supplied from the zooxan-thellae may serve as an energy source forthe cyanobacteria operating heterotrophi-cally and provide a steady supply of reduc-tant and adenosine triphosphate for nitro-gen fixation in the symbionts. Nitrogenaseis also sensitive to molecular and reactivespecies of oxygen that accumulate duringphotosynthesis (25, 26 ), and the symbioticcyanobacteria, operating heterotrophically,could quench molecular oxygen via respi-ration and/or by the Mehler reaction (27 ).Additionally, the coral environment is wellsuited for the temporal separation of pho-tosynthesis and nitrogen fixation, becausecoral tissues experience extreme hypoxia atnight (25 ). The presence of an additionalsymbiont in a zooxanthellate coral that isnitrogen-limited (2) suggests that nitrogenfixation may be an important supplementalsource of the limiting element for the sym-biotic association, and it highlights the po-tential significance of microbial consortiacomposed of photosynthetic eukaryotes andprokaryotes (28). Cyanobacteria are in-volved in many diverse mutualistic symbi-oses in both terrestrial and marine environ-ments, and they provide critical ecologicalservices, including important contributionsto the global nitrogen cycle (29).

References and Notes1. J. E. N. Veron, Corals in Space and Time (Cornell Univ.

Press, Ithaca, NY, 1995).2. P. G. Falkowski, Z. Dubinsky, L. Muscatine, L. R. Mc-

Closkey, Bioscience 43, 606 (1993).3. Z. Dubinsky, P. L. Jokiel, Pacific Sci. 48, 313 (1994).4. F. Rohwer, M. Breitbart, J. Jara, F. Azam, N. Knowlton,

Coral Reefs 20, 85 (2001).5. F. Rohwer, V. Seguritan, F. Azam, N. Knowlton, Mar.

Ecol. Prog. Ser. 243, 1 (2002).6. H. W. Ducklow, in Coral Reefs, Ecosystems of the

World, Z. Dubinsky, Ed. (Elsevier, Amsterdam, 1990),pp. 265–290.

7. W. J. Wiebe, R. E. Johannes, K. L. Webb, Science 188,257 (1975).

8. N. Shashar, Y. Cohen,Y. Loya, N. Star, Mar. Ecol. Prog.Ser. 111, 259 (1994).

9. M. V. Matz et al., Nature Biotechnol. 17, 969 (1999).10. S. G. Dove, O. Hoegh-Guldberg, S. Ranganathan, Cor-

al Reefs 19, 197 (2001).11. C. H. Mazel et al., Limnol. Oceanogr. 48, 402 (2003).12. I. V. Kelmanson, M. V. Matz, Mol. Biol. Evol. 20, 1125

(2003).

13. Y. A. Labas et al., Proc. Natl. Acad. Sci. U.S.A. 99,4256 (2002).

14. C. H. Mazel, Mar. Ecol. Prog. Ser. 120, 185 (1995).15. A. N. Glazer, J. A. West, C. Chan, Biochem. Syst. Ecol.

10, 203 (1982).16. Materials and methods are available as supporting

material on Science Online.17. A. M. Gilmore et al., Photochem. Photobiol. 77, 515

(2003).18. A. A. Heikal, S. T. Hess, G. S. Baird, R. Y. Tsein, W. W.

Webb, Proc. Natl. Acad. Sci. U.S.A. 97, 11996 (2000).19. W. Schonhuber et al., Appl. Environ. Microbiol. 65,

1259 (1999).20. U. Nubel, F. Garcia-Pichel, G. Muyzer, Appl. Environ.

Microbiol. 63, 3327 (1997).21. M. K. Litvaitis, Hydrobiologia 468, 135 (2002).22. M. Wyman, R. P. F. Gregory, N. G. Carr, Science 230,

818 (1985).23. H. Lokstein, C. Steglich, W. R. Hess, Biochim. Biophys.

Acta 1410, 97 (1999).24. L. Muscatine, in Coral Reefs, Ecosystems of the World, Z.

Dubinsky, Ed. (Elsevier, Amsterdam, 1990), pp. 75–87.25. M. Kuhl, Y. Cohen, T. Dalsgaard, B. B. Jørgensen, N. P.

Revsbech, Mar. Ecol. Prog. Ser. 117, 159 (1995).26. M. P. Lesser, Coral Reefs 16, 187 (1997).

27. I. Berman-Frank et al., Science 294, 1534 (2001).28. N. Knowlton, F. Rowher, Am. Nat. 162, S51 (2003).29. D. G. Adams, in The Ecology of Cyanobacteria, B. A.

Whitton, M. Potts, Eds. (Kluwer, Dordrecht, Nether-lands, 2000), pp. 523–561.

30. The authors thank A. Blakeslee, J. H. Farrell, V. A.Kruse, A. Mumford, and E. Sullivan for technical as-sistance; J. Zehr for the nitrogenase antibody; E.Gantt for the phycoerythrin antibody; and M. Litvaitisfor cyanobacterial primers. This work was funded bygrants from the Office of Naval Research–Environ-mental Optics Program, and logistical support wasprovided by the Caribbean Marine Research Center,Lee Stocking Island, Bahamas. The experiments con-ducted for this study comply with the current laws ofthe Bahamas and the United States.

Supporting Online Materialwww.sciencemag.org/cgi/content/full/305/5686/997/DC1Materials and MethodsFig. S1

14 April 2004; accepted 16 July 2004

Modulation of HematopoieticStem Cell Homing andEngraftment by CD26

Kent W. Christopherson II,1,2* Giao Hangoc,1,2 Charlie R.

Mantel,1,2 Hal E. Broxmeyer1,2†

Hematopoietic stem cell homing and engraftment are crucial to transplan-tation efficiency, and clinical engraftment is severely compromised whendonor-cell numbers are limiting. The peptidase CD26 (DPPIV/dipeptidylpep-tidase IV) removes dipeptides from the amino terminus of proteins. Wepresent evidence that endogenous CD26 expression on donor cells nega-tively regulates homing and engraftment. By inhibition or deletion of CD26,it was possible to increase greatly the efficiency of transplantation. Theseresults suggest that hematopoietic stem cell engraftment is not absolute,as previously suggested, and indicate that improvement of bone marrowtransplant efficiency may be possible in the clinic.

The efficiency of hematopoietic stem cell(HSC) transplantation is important whendonor-cell numbers are limiting. For exam-ple, since the first cord blood transplants(1–3), the use of cord blood has been main-ly restricted to children, not adults, as aresult of apprehension about limited cellnumbers. Attempts at ex vivo expansion ofstem cells for clinical transplantation havenot been encouraging (4, 5). An alternativemeans to enhance engraftment is to in-crease HSC homing efficiency to bone mar-row (BM) niches. Recently it was suggest-

ed that HSCs engrafted mice with absoluteefficiency (6–8). However, if all HSCshomed with absolute efficiency and en-graftment, problems of limiting donor cellswould not be a concern for clinical trans-plantation (3). Thus, enhancement of hom-ing and engraftment of HSC is needed ifadvances in transplantation with limitingnumbers of HSC are to be realized. On thebasis of our work implicating CD26 ingranulocyte colony-stimulating factor (G-CSF)–induced mobilization of HSCs andhematopoietic progenitor cells (HPCs) (9–11), we investigated the involvement ofCD26 in homing and engraftment. Inhibi-tion or deletion of CD26 on donor cellsenhanced short-term homing, long-term en-graftment, competitive repopulation, sec-ondary transplantation, and mouse survival,which suggests that CD26 is a novel targetfor increasing transplantation efficiency.

Mouse bone marrow HSCs were definedas cells within the Sca-1�lin– population

1Department of Microbiology and Immunologyand the Walther Oncology Center, Indiana Univer-sity School of Medicine, Indianapolis, IN 46202,USA. 2Walther Cancer Institute, Indianapolis, IN46208, USA.

*Present address: Institute of Molecular Medicine, TheUniversity of Texas Health Science Center, Houston,TX 77030, USA.†To whom correspondence should be addressed:hbroxmey@iupui.edu

R E P O R T S

13 AUGUST 2004 VOL 305 SCIENCE www.sciencemag.org1000

(12). Using chemotaxis assays, we previouslyestablished that Diprotin A (Ile-Pro-Ile) is aspecific inhibitor of CD26 (10). We showhere that Diprotin A–treated C57BL/6 Sca-1�lin– BM cells exhibited twofold increasesin CXCL12-induced migration (Fig. 1A).CD26-deficient (CD26�/�) Sca-1�lin– BMcells had up to threefold greater migratoryresponse, compared with control Sca-1�lin–

BM cells (Fig. 1A). Diprotin A treatment ofCD26�/� cells did not further enhance che-motaxis (Fig. 1A). Thus, in vitro migration ofSca-1�lin– HSC cells to CXCL12 was en-hanced by specific inhibition and even moreby the absence of CD26 peptidase activity.

Short-term homing experiments usedcongenic C57Bl/6 (CD45.2�) and BoyJ(CD45.1�) cells to assess recruitment oftransplanted HSCs to BM (13). Treatmentof 1 � 104 to 2 � 104 sorted Sca-1�lin–

BM C57Bl/6 donor cells with CD26 inhib-itor (Diprotin A) for 15 min before trans-plant resulted in ninefold increases in hom-ing efficiency in BoyJ recipients comparedwith untreated cells (Fig. 1B). Transplanta-tion of sorted CD26�/� Sca-1�lin– BMcells (14) resulted in 11-fold increases inhoming efficiency (Fig. 1B). This suggeststhat inhibition, or loss of CD26 activity,significantly increases homing of sortedSca-1�lin– HSCs in vivo. Pretreatment of20 � 106 low-density (LD) BM donor cellswith CD26 inhibitors resulted in 1.5-foldincreases in homing efficiency of C57BL/6Sca-1�lin– cells (within the LDBM donorpopulation) into BoyJ recipient BM 24hours after transplant (Fig. 1C). Transplan-tation of CD26�/� cells provided a 2.6-foldincrease in homing efficiency (Fig. 1C).Thus, inhibition or loss of CD26 activity inthe total LDBM donor unit (containing dif-ferentiated cells and progenitors) increasesin vivo homing of Sca-1�lin– HSCs withinthe LDBM fraction. The use of LDBM cellsmore accurately represents clinical proto-cols than does the use of sorted Sca-1�lin–

HSCs. Differences in homing efficiencybetween sorted Sca-1�lin– cells and LDBMcells may be partially explained by largernumbers of Sca-1�lin– donor cells (3 �104) contained within the 20 � 106 cellLDBM donor unit or by accessory cellscontained within the LDBM, but not sorted,cell population.

Treatment of 10 � 106 LDBM donorcells with CXCR4 antagonist AMD3100(15) for 15 min before transplantation re-versed increases in homing efficiency ofCD26-inhibited or deleted Sca-1�lin– cells(Fig. 1D). AMD3100 treatment itself re-duced homing efficiency compared withcontrol cells (Fig. 1D). Migration data fromtreatment with AMD3100 and in vitroCD26�/� HSC/HPC, combined with ourprevious studies (10), suggest that CXCL12

is a logical downstream target of enhancedtransplant efficiency. This is consistentwith an important role for CXCL12 in mi-gration (16 ), mobilization (17–19), hom-ing, and engraftment of HSCs (3, 20–22),holding HSCs and HPCs in the bone mar-row (23), and enhancing cell survival, anadditional component of HSC engraftment(24, 25).

Although CD26 peptidase activity israpidly lost after treatment with CD26 in-hibitors (Diprotin A or Val-Pyr), recoverybegins within 4 hours after treatment (Fig.1E), which might explain homing and en-hancement differences of inhibitor-treated,compared with CD26�/�, donor cells (Fig.1, B and C). As reported for CD34� cells,cytokine treatment (with interleukin-6 andstem cell factor) of Sca-1�lin– cells result-ed in increased CXCR4 expression (fig. S1)(26 ). Cytokine treatment did not affect

CD26 expression or activity (fig. S1),which suggests that these cytokines do notregulate CD26.

Transplant efficiency requires consider-ation of long-term donor HSC engraftment.Transplants were performed with 5 � 105

LDBM C57Bl/6 (CD45.2�) or CD26�/�

(CD45.2�) cells into lethally irradiatedcongenic BoyJ (CD45.1�) recipients.CD26�/� donor cells made a significantlygreater contribution to peripheral blood(PB) leukocytes 6 months after transplant(Fig. 2A and fig. S2). This was especiallyapparent at limiting cell dilutions (Fig. 2A)and correlated with changes in mouse sur-vival (Fig. 2, B and C). At day 60, 0%survival was observed with 2.5 � 104 con-trol cells (Fig. 2B); 80% survival was ob-served with an equivalent number ofCD26�/� cells (Fig. 2C). Transplantationof 2.5 � 104 normal cells was below that

Fig. 1. Inhibition/loss of CD26 increasesCXCL12 chemotaxis and CXCR4-dependentshort-term homing of Sca-1�lin– mouse BMcells. (A) Diprotin A–treated C57BL/6 Sca-1�lin– cells (squares) and CD26�/� BM cells(triangles) had enhanced CXCL12-induced mi-gratory response compared with untreatedC57BL/6 cells (circles) (P � 0.01). Diprotin Atreatment of CD26�/� cells (diamonds) hadno further effect. n � 8 samples. (B) SortedSca-1�lin– C57Bl/6 cells pretreated with 5mM

Diprotin A for 15 min and CD26�/� cells have increased short-term homing into BoyJ recipientmice (P � 0.05; n � 10 mice; total from two experiments). (C) Sca-1�lin– cells within donorLDBM pretreated with CD26 inhibitors (Diprotin A or Val-Pyr) or transplantation of CD26�/�

cells significantly increases short-term homing of donor cells (P � 0.01; n � 6 mice; totalfrom two experiments). (D) Increased homing efficiency of Sca-1�lin– C57BL/6 HSC withinLDBM cells noted with Diprotin A treatment or with CD26�/� cells is reversible by theirtreatment with CXCR4 antagonist AMD3100 for 15 min before transplant. AMD3100 alsoreduces homing efficiency of C57BL/6 donor cells in the absence of CD26 inhibition (P � 0.05;n � 5 mice). (E) CD26 peptidase activity (U/1000 cells; 1U � 1 pmol p-nitroanilide per min)of C57BL/6 BM cells is rapidly lost with 15 min inhibitor treatment (P � 0.01); recovery beginswithin 4 hours.

R E P O R T S

www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004 1001

required for mouse survival. Recipient sur-vival is dependent on short- and long-termreconstitution of marrow. The absence ofsurviving mice in this group by day 21suggests that loss of short-term reconstitu-tion may be responsible for lethality at thisdonor-cell dose. At limiting transplanteddonor cells, long-term engraftment andmouse survival increased with CD26�/�

donor cells. At nonlimiting donor-cell num-bers (2 � 105), improvement is also ob-served in engraftment and survival withCD26�/� cells at day 60, which sug-gests that long-term reconstitution isalso targeted.

At nonlimiting cell doses and in a non-competitive assay, treatment of C57BL/6

donor cells with either CD26 inhibitor re-sulted in a one-third increase in donor-cellcontribution to leukocyte formation in le-thally irradiated BoyJ recipients relative tountreated cells (Fig. 2D). In secondarytransplanted recipient mice, a threefold in-crease in donor-cell contribution to PB leu-kocytes was seen with CD26 inhibition(Fig. 2E and fig. S3). Increases insecondary repopulating HSCs comparedwith repopulating HSCs in primary recipi-ents indicates an increased homing/engraftment of self-renewing stem cellswith CD26 inhibition.

Competitive repopulating HSC assaysprovide the most functional assessment ofHSC by direct comparison of engraftment

from experimental donor cells (CD45.2�)relative to constant numbers of competitorcells (CD45.1�) (13, 27 ). Six months aftertransplant, increased donor contribution tochimerism was observed with Diprotin A orVal-Pyr treatment relative to cotrans-planted cells (Fig. 3A and fig. S4A). Atlimiting donor-cell numbers (1.25 � 105

and 0.625 � 105), no significant increasesin donor contribution were observed withCD26 inhibitor treatment (Fig. 3A). How-ever, CD26�/� donor cells significantly en-hanced chimerism at all donor-cell numbersmeasured (Fig. 3A). Even greater increasesin donor-cell contribution were observedwith CD26 inhibitor–treated and CD26�/�

donor cells in secondary transplanted BoyJrecipients 4 months after transplant (Fig.3B); this result was more striking whenCD26�/� donor cells were used (Fig. 3B andfig. S4B). It is unlikely that some increases inCD26�/� donor-cell engraftment are the resultof increased HSC cell numbers in this popula-tion. Numbers of Sca-1�lin– cells (2.69 �0.49 � 104 per femur pair in C57BL/6 and2.75 � 0.15 � 104 per femur pair in CD26�/�)and CFU-GM, BFU-E, and CFU-GEMM inPB, BM, and spleen (11) are comparable inCD26�/� and control C57BL/6 mice. Cyclingstatus of CD26�/� cells was examined becauseHSCs/HPCs not in G0/G1 are reported to man-ifest decreased homing and engraftment (28).No significant differences were seen in cyclingstatus between CD26�/� and control C57BL/6Sca-1�lin– BM cells, either when freshly iso-lated or after 24 hours of preincubation withgrowth factors (fig. S5).

A

0

20

40

60

80

100

2x10^5 1x10^5 5x10^4 2.5x10^4

Donor Cell Number

% D

on

or

Co

ntr

ibu

tio

n

C57BL/6 CD26-/-

* * *

B

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60Days Post Transplant

% S

urv

ival No Cells

2.5x10^4

5x10^4

1x10^5

2x10^5

C

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60Days Post Transplant

% S

urv

ival No Cells

2.5x10^4

5x10^4

1x10^5

2x10^5

D

0

10

20

30

40

50

60

70

80

Control Diprotin A Val-Pyr

% D

on

or

Co

ntr

ibu

tio

n

**

E

02468

101214161820

Control Diprotin A Val-Pyr

% D

on

or

Co

ntr

ibu

tio

n

**Fig. 2. CD26�/� donor cells have enhanced

long-term engraftment at limiting cell dilutionsand increase recipient survival. (A) CD26�/�

donor cells into congenic BoyJ recipient micesignificantly increases noncompetitive long-term engraftment 6 months after transplant(P � 0.01; n � 3 to 5 mice). (B and C) Mousesurvival following limiting cell dilutions. In-creased survival is observed 60 days after trans-plant in recipients receiving low numbers ofCD26�/� cells (C) versus control C57BL/6 cells(B) (P � 0.01; n � 3 to 5 mice). (D) Increaseddonor-cell contribution to formation of PB leukocytes during noncom-petitive long-term engraftment assays was also observed with CD26inhibitor treatment (Diprotin A or Val-Pyr) 6 months after transplant

(P � 0.05; n � 5 mice). (E) Even greater donor contribution to chimerismwas observed in 6-month secondary transplants receiving cells fromprimary mice engrafted with CD26-inhibited cells (P � 0.01; n � 5 mice).

A

0102030405060708090

100

% D

on

or

Co

ntr

ibu

tio

n

Control Diprotin A Val-Pyr CD26-/-

**

*

*

*

*

*

*

B

01020304050607080

% D

on

or

Co

ntr

ibu

tio

n

Control Diprotin A Val-Pyr CD26-/-

**

*

* *

*

* *

*

**

*

Donor Cell Number Donor Cell Number500000 250000 125000 62500 500000 250000 125000 62500

Fig. 3. CD26-inhibited and CD26�/� cells have increased competitive repopulation and engraft-ment of secondary repopulating HSCs compared with control cells. (A) Increased donor-cellchimerism in competitive repopulation assays was observed with Diprotin A or Val-Pyr treatmentof 5 � 105 or 2.5 � 105 donor cells in direct competition with 5 � 105 competitor recipient BoyJcells (P � 0.01). CD26�/� cells had a greatly enhanced contribution to chimerism at all donor-cellnumbers (P � 0.01; n � 5 mice). (B) An even greater increase in donor-cell contribution wasobserved with CD26 inhibitor–treated donors and CD26�/� donors in secondary transplantedrecipient BoyJ mice (P � 0.01; n � 5).

R E P O R T S

13 AUGUST 2004 VOL 305 SCIENCE www.sciencemag.org1002

Enhancing transplant efficiency has clinicalimplication but is being debated. Recent reportssuggest that HSCs engraft mice with absoluteefficiency (7, 8). One report (7) was heavilyinfluenced by mathematical correction factors,and the other (8) addressed single-cell trans-plants by a subset of HSCs among competitorcells that themselves could save the lethallyirradiated recipient. Contrary to this is the real-ity of the clinical situation (3) and studies inwhich injection of HSCs directly into BMshowed enhanced engraftment compared withintravenous administering of cells (29–31). Re-moval of endogenous CD26 activity on donorHSCs increased homing and engraftment.Thus, improvement in transplant efficiency ispossible. Further advancement may requiremore effective use of CD26 inhibitors, whichmay translate into the use of HSCs for clinicaltransplantation from sources containing limit-ing cell numbers, such as cord blood.

References and Notes1. H. E. Broxmeyer et al., Proc. Natl. Acad. Sci. U.S.A.

86, 3828 (1989).

2. E. Gluckman et al., N. Engl. J. Med. 321, 1174 (1989).3. H. E. Broxmeyer, F. Smith, in Thomas’ Hematopoietic

Cell Transplantation, K. G. Blume, S. J. Forman, F. R.Appelbaum, Eds. (Blackwell Science, Oxford; Malden,MA, 2004), chap. 43, pp. 550–564.

4. E. J. Shpall et al., Biol. Blood Marrow Transplant. 8,368 (2002).

5. J. Jaroscak et al., Blood 101, 5061 (2003).6. H. Ema, H. Nakauchi, Immunity 20, 1 (2004).7. P. Benveniste, C. Cantin, D. Hyam, N. N. Iscove,

Nature Immunol. 4, 708 (2003).8. Y. Matsuzaki, K. Kinjo, R. C. Mulligan, H. Okano,

Immunity 20, 87 (2004).9. K. W. Christopherson 2nd, G. Hangoc, H. E. Brox-

meyer, J. Immunol. 169, 7000 (2002).10. K. W. Christopherson 2nd, S. Cooper, H. E. Broxmeyer,

Blood 101, 4680 (2003).11. K. W. Christopherson 2nd, S. Cooper, G. Hangoc, H. E.

Broxmeyer, Exp. Hematol. 31, 1126 (2003).12. G. J. Spangrude, S. Heimfeld, I. L. Weissman, Science

241, 58 (1988).13. D. E. Harrison, Blood 55, 77 (1980).14. Materials and methods are available as supporting

material on Science Online.15. M. M. Rosenkilde et al., J. Biol. Chem. 279, 3033

(2004).16. D. E. Wright, E. P. Bowman, A. J. Wagers, E. C. Butcher,

I. L. Weissman, J. Exp. Med. 195, 1145 (2002).17. K. Hattori et al., Blood 97, 3354 (2001).18. H. E. Broxmeyer et al., Blood 100, 609a (abstr. no.

2397) (2002).

19. C. W. Liles et al., Blood 102, 2728 (2003).20. A. Peled et al., Science 283, 845 (1999).21. H. E. Broxmeyer, Int. J. Hematol. 74, 9 (2001).22. T. Ara et al., Immunity 19, 257 (2003).23. C. H. Kim, H. E. Broxmeyer, Blood 91, 100 (1998).24. H. E. Broxmeyer et al., J. Immunol. 170, 421 (2003).25. H. E. Broxmeyer et al., J. Leukoc. Biol. 73, 630 (2003).26. O. Kollet et al., Blood 100, 2778 (2002).27. D. E. Harrison, C. T. Jordan, R. K. Zhong, C. M. Astle,

Exp. Hematol. 21, 206 (1993).28. A. Gothot, J. C. van der Loo, D. W. Clapp, E. F. Srour,

Blood 92, 2641 (1998).29. T. Yahata et al., Blood 101, 2905 (2003).30. J. Wang et al., Blood 101, 2924 (2003).31. F. Mazurier, M. Doedens, O. I. Gan, J. E. Dick, Nature

Med. 9, 959 (2003).32. These studies were supported by U.S. Public Health

Science Grants R01 DK53674, R01 HL67384, and R01HL56416 to H.E.B. K.W.C. was supported sequentiallyduring these studies by NIH training grant T32DK07519 to H.E.B. and by a Fellow Award from theLeukemia and Lymphoma Society to K.W.C.

Supporting Online Materialwww.sciencemag.org/cgi/content/full/305/5686/1000/DC1Materials and MethodsFigs. S1 to S5References

23 February 2004; accepted 15 July 2004

Natural Antibiotic Function of aHuman Gastric Mucin AgainstHelicobacter pylori InfectionMasatomo Kawakubo,1,2 Yuki Ito,1 Yukie Okimura,2

Motohiro Kobayashi,1,4 Kyoko Sakura,2 Susumu Kasama,1

Michiko N. Fukuda,4 Minoru Fukuda,4 Tsutomu Katsuyama,2

Jun Nakayama1,3*

Helicobacter pylori infects the stomachs of nearly a half the human population,yet most infected individuals remain asymptomatic, which suggests that thereis a host defense against this bacterium. Because H. pylori is rarely found indeeper portions of the gastric mucosa, whereO-glycans are expressed that haveterminal �1,4-linked N-acetylglucosamine, we tested whether these O-glycansmight affect H. pylori growth. Here, we report that these O-glycans haveantimicrobial activity against H. pylori, inhibiting its biosynthesis of cho-lesteryl-�-D-glucopyranoside, a major cell wall component. Thus, the uniqueO-glycans in gastric mucin appeared to function as a natural antibiotic, pro-tecting the host from H. pylori infection.

Helicobacter pylori colonizes the gastricmucosa of about half the world’s popula-tion and is considered a leading cause ofgastric malignancies (1–3). However, most

infected individuals remain asymptomaticor are affected merely by chronic activegastritis (2). Only a fraction of infectedpatients develop peptic ulcer, gastric can-cer, and malignant lymphoma. This sug-gests the presence of host defense mecha-nisms against H. pylori pathogenesis.

Gastric mucins are classified into twotypes based on their histochemical properties(4). The first is a surface mucous cell–typemucin, secreted from the surface mucouscells. The second is found in deeper portionsof the mucosa and is secreted by gland mu-cous cells, including mucous neck cells,

cardiac gland cells, and pyloric gland cells.In H. pylori infection, the bacteria are

associated solely with surface mucous cell–type mucin (5), and two carbohydrate struc-tures, Lewis b and sialyl dimeric Lewis X insurface mucous cells, serve as specific li-gands for H. pylori adhesins, BabA andSabA, respectively (6, 7). H. pylori rarelycolonizes the deeper portions of gastric mu-cosa, where the gland mucous cells pro-duce mucins having terminal �1,4-linked N-acetylglucosamine (�1,4-GlcNAc) residuesattached to core 2–branched O-glycans[GlcNAc�134Gal�134GlcNAc�136(GlcNAc�13 4Gal�133)GalNAc�3Ser/Thr] (8). Development of pyloric glandatrophy enhances the risk of peptic ulcer orgastric cancer two- to three-fold comparedwith chronic gastritis without pyloric glandatrophy (3). These findings raise the possi-bility that �1,4-GlcNAc–capped O-glycans have protective properties againstH. pylori infection.

To test this hypothesis, we generated mu-cin-type glycoproteins containing terminal�1,4-GlcNAc and determined its effect on H.pylori in vitro. Because CD43 serves as apreferential core protein of these O-glycans(8), we generated recombinant soluble CD43having �1,4-GlcNAc–capped O-glycans intransfected Chinese hamster ovary cells (9).Soluble CD43 without �1,4-GlcNAc wasused as a control.

H. pylori (ATCC43504), incubated withthe medium containing varying amounts ofrecombinant soluble CD43, showed littlegrowth during the first 2.5 days, irrespec-tive of the presence or absence of �1,4-

1Department of Pathology and 2Department of Labora-tory Medicine, Shinshu University School of Medicine,and 3Institute of Organ Transplants, ReconstructiveMedicine and Tissue Engineering, Shinshu UniversityGraduate School of Medicine, Asahi 3-1-1, Matsumoto390-8621, Japan. 4Glycobiology Program, Cancer Re-search Center, The Burnham Institute, 10901 NorthTorrey Pines Road, La Jolla, CA 92037, USA.

*To whom correspondence should be addressed. E-mail: jun@hsp.md.shinshu-u.ac.jp

R E P O R T S

www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004 1003

GlcNAc–capped O-glycans, characteristicof the lag phase of H. pylori growth (Fig.1A). After 3 days, microbes cultured in thepresence of control soluble CD43 grew rap-idly, corresponding to the log phase ofbacterial growth. In contrast, soluble CD43containing more than 62.5 mU/ml of ter-minal �1,4-GlcNAc impaired log-phasegrowth. Although growth inhibition wasnot obvious at a lower concentration (31.2mU/ml), time-lapse images of the microbesrevealed significant reduction of motilityunder this condition (Fig. 1B). Morpholog-ic examination at the lower concentrationrevealed abnormalities of the microbe, suchas elongation, segmental narrowing, andfolding (Fig. 1C). These morphologicchanges are distinct from conversion tococcoid form, because reduction of growth,associated with conversion from the bacil-lary to the coccoid form (10), was notapparent under these conditions. These in-hibitory effects of soluble CD43 containingterminal �1,4-GlcNAc were also detectedagainst various H. pylori strains, includinganother authentic strain, ATCC43526, andthree clinical isolates with a minimum in-

hibitory concentration between 15.6 mU/mland 125.0 mU/ml. By contrast, neither in-hibitory growth nor abnormal morphologyof H. pylori was observed at any concen-trations of soluble CD43 lacking �1,4-GlcNAc (Fig. 1, A to C). These resultsindicate that �1,4-GlcNAc–capped O-gly-cans specifically suppress the growth ofH. pylori in a manner similar to otherantimicrobial agents. Similar inhibitoryeffects on H. pylori were also found inanother mucin-like glycoprotein, CD34(11) having terminal �1,4-GlcNAc (12).In addition, p-nitrophenyl-�-N-acetylglu-cosamine (GlcNAc�-PNP) suppressed thegrowth of H. pylori in a dose-dependentmanner (Fig. 1D), although the effects werenot as strong with soluble CD43 havingterminal �1,4-GlcNAc (Fig. 1A). These re-sults provide evidence that the terminal�1,4-GlcNAc residues, rather than scaffoldproteins, are critical for growth inhibitoryactivity against H. pylori, and that thepresentation of multiple terminal �1,4-GlcNAc residues as a cluster on mucin-typeglycoprotein may be important for achiev-ing the optimal activity.

To determine whether natural gastricmucins containing terminal �1,4-GlcNAccan also inhibit growth of H. pylori, subsetsof human gastric mucins were preparedfrom the surface mucous cells and pyloricgland cells (9). The growth of H. pylori wassignificantly suppressed with mucin de-rived from pyloric gland cells at 125.0mU/ml during the log phase (Fig. 1E). Asimilar inhibitory effect was also observedwhen the glandular mucin prepared fromhuman gastric juice was tested (13). Bycontrast, mucin derived from surface mu-cous cells, MUC5AC, stimulated growth.These results support the hypothesis thatnatural gastric mucins containing ter-minal �1,4-GlcNAc, secreted from glandmucous cells, have antimicrobial activityagainst H. pylori.

The morphologic abnormalities of H.pylori induced by �1,4-GlcNAc–capped O-glycans are similar to those induced byantibiotics such as �-lactamase inhibitors,which disrupt biosynthesis of peptidogly-can in the cell wall (14, 15). Therefore,these O-glycans may inhibit cell wall bio-synthesis in H. pylori. The cell wall of

Fig. 1. �1,4-GlcNAc–capped O-glycans in-hibit the growth andmotility of H. pylori.(A) Growth curves ofH. pylori cultured inthe presence of solu-ble CD43 with ter-minal �1,4-GlcNAc[�GlcNAc (�)] or sol-uble CD43 withoutterminal �1,4-GlcNAc[�GlcNAc (–)]; theprotein concentrationof �GlcNAc (–) wasthe same as that of125.0 mU/ml of�GlcNAc (�). Onemiliunit of �GlcNAc(�) corresponds to 1�g (2.9 nmol) ofGlcNAc�-PNP. A600,absorbance at 600nm. (B) Motility of H.pylori cultured with31.2 mU/ml of�GlcNAc (�) or thesame protein concen-tration of �GlcNAc(–) for 3 days bytime-lapse recordingwith 1-s intervals.Representative H. py-lori is indicated by arrowheads. The mean velocity of seven H. pyloricultured in the presence of �GlcNAc (�) and �GlcNAc (–) is 3.1 � 3.5�m/s (mean � SD) and 21.2 � 2.6 �m/s (P � 0.001). Scale bar, 50�m. (C) Scanning electron micrographs of H. pylori incubated with31.2 mU/ml of �GlcNAc (�) or the same protein concentration of�GlcNAc (–) for 3 days. Note abnormal morphologies such as elon-gation, segmental narrowing, and folding in the culture with �GlcNAc(�). All photographs were taken at the same magnification. Scale bar,1 �m. (D) Growth curves of H. pylori cultured in the medium supple-

mented with various amounts of GlcNAc�-PNP. Growth of the bac-teria is suppressed by GlcNAc�-PNP in a dose-dependent manner. (E)Growth curves of H. pylori cultured in the medium supplemented withpyloric gland cell-derived mucin containing 125 mU/ml of �1,4-GlcNAc or the same protein concentration of surface mucous cell-derived mucin isolated from the human gastric mucosa. The deathphase started from 3.5 days, and saline instead of each mucin wassupplemented as a control experiment. In (A), (D), and (E), each valuerepresents the average of duplicate measurements.

R E P O R T S

13 AUGUST 2004 VOL 305 SCIENCE www.sciencemag.org1004

Helicobacter species characteristically con-tains �-cholesteryl glucosides (�-CGs), ofwhich the major components are cho-lesteryl-�-D-glucopyranoside (CGL), cho-lesteryl-6-O-tetradecanoyl-�-D-glucopyr-anoside (CAG), and cholesteryl-6-O-phosphatidyl-�-D-glucopyranoside (CPG)(16). Mass spectrometric analysis of thecell wall components from H. pylori cul-tured with �1,4-GlcNAc–capped O-glycansdisplayed reduced lipid-extractable cellwall constituents (Fig. 2B). In particular,the levels of CGL, relative to phosphatidicacid (17), were significantly reduced ascompared with controls (Fig. 2, A and B).These results suggest that �1,4-GlcNAc–

capped O-glycans directly inhibit biosyn-thesis of CGL in vivo by H. pylori.

CGL is likely formed by a UDP-Glc:sterol �-glucosyltransferase, which trans-fers glucose (Glc) from UDP-Glc to the C3position of cholesterol with �-linkage. In-cubation of cholesterol and UDP-Glc withH. pylori lysates revealed substantialamounts of CGL by mass spectrometry(Fig. 2C), demonstrating the activity ofUDP-Glc:sterol �-glucosyltransferase in H.pylori. When soluble CD43 containing ter-minal �1,4-GlcNAc was added to this as-say, production of CGL was suppressed(Fig. 2D), whereas no effect was seen withcontrol soluble CD43 (Fig. 2E). Consider-ing structural similarity between �-linkedGlcNAc found in the gland mucous cell–type mucin and the �-linked Glc found inCGL, these findings suggest that the termi-nal �1,4-GlcNAc residues could direct-ly inhibit the �-glucosyltransferase acti-vity through an end-product inhibitionmechanism (18), resulting in decreasedCGL biosynthesis.

Genes involved in the biosynthesis ofcholesterol are not found in the genomedatabase of H. pylori (19). Thus, H. pylorimay not be able to synthesize CGL in the

absence of exogenous cholesterol. When H.pylori was cultured for 5 days without cho-lesterol, bacterial growth was significantlyreduced (table S1). In such cultures, H.pylori was elongated and no motile mi-crobes were found. When H. pylori wasfurther cultured without cholesterol for upto 21 days, the microbes died off complete-ly. By contrast, when H. pylori was cul-tured with cholesterol, bacteria grew well,and no signs of abnormality were detected(table S1). H. pylori cultured with choles-terol (9) revealed a typical triplet of �-CGsincluding CGL (Fig. 3, lane 2), while�-CGs were not detected in H. pylori cul-tured without cholesterol (Fig. 3, lane 1).Moreover, no antibacterial effect of solubleCD43 containing terminal �1,4-GlcNAcwas observed on bacterial strains lackingCGL such as Escherichia coli, Pseudomo-nas aeruginosa, Klebsiella pneumoniae,Staphylococcus aureus, �-Streptococcus, andStreptococcus pneumoniae (9). These resultscollectively indicate that synthesis of CGL byusing exogenously supplied cholesterol is re-quired for the survival of H. pylori and thatantimicrobial activity of �1,4-GlcNAc–capped O-glycans may be restricted to bacte-rial strains expressing CGL.

Fig. 2. Soluble CD43 with terminal �1,4-GlcNAc suppresses CGL biosynthesis in H.pylori as determined by matrix-assisted laser desorption/ionization–time-of-flight(MALDI-TOF) mass spectrometry. (A) Sodium-adducted CGL, [CGL � Na]� at m/z571.6, is detected in the lipid fraction of H. pylori incubated with control solubleCD43 (arrow). (B) CGL in H. pylori incubated with 4.0 mU/ml of �GlcNAc-cappedsoluble CD43 is reduced to 29.5% of the control experiment (arrow). In both (A)and (B), amounts of an endogenous standard, phosphatidic acid (17), are normalizedas 100%, and a representative result of duplicate experiments is shown. (C)MALDI-TOF mass spectrum of products synthesized from UDP-Glc and cholesterolby sonicated H. pylori. [CGL � Na]� at m/z 571.6 is shown. (D and E) Massspectrum of products synthesized from UDP-Glc and cholesterol by sonicated H. pylori in the presence of 50.0 mU/ml of �1,4-GlcNAc–capped solubleCD43 (D) or control soluble CD43 (E). Note that CGL is not synthesized in the presence of �1,4-GlcNAc–capped soluble CD43 in (D).

Fig. 3. Absence of �-CGs includ-ing CAG, CGL, and CPG in H.pylori cultured without exoge-nous cholesterol. Total glycolip-ids extracted from H. pylori in-cubated with Brucella brothlacking cholesterol (lane 1) orcontaining 0.005% cholesterol(lane 2) were analyzed by thin-layer chromatography.

R E P O R T S

www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004 1005

To test whether mucous cells expressing�1,4-GlcNAc–capped O-glycans protectthemselves against H. pylori infection, gas-tric adenocarcinoma AGS-�4GnT cells sta-bly transfected with �4GnT cDNA werecocultured with H. pylori (9). With a short-term incubation (8 hours), the microbesattached equally well to AGS-�4GnT cellsand mock-transfected AGS cells. No signif-icant damage was observed in either groupof cells (Fig. 4A). Upon prolonged incuba-tion (24 hours), mock-transfected AGScells exhibited remarkable deterioration,such as flatness or shrinkage, with in-creased number of associated H. pylori(Fig. 4B), and the number of viable AGScells was dramatically reduced after thethird day (Fig. 4C). This cellular damagemay be attributed to the perturbed signaltransduction in AGS cells, where a tyrosinphosphatase, SHP-2, is constitutively acti-vated by H. pylori CagA protein (20). Bycontrast, growth of H. pylori in cultureswith AGS-�4GnT cells was markedlysuppressed, and cellular damage found inmock-transfected AGS cells was barelydetected in these cells (Fig. 4B). Thus,the viability of AGS-�4GnT cells wasfully maintained for up to 4 days (Fig.4C). These results indicate that �1,4-GlcNAc–capped O-glycans have no effecton the adhesion of H. pylori to AGS-

�4GnT cells, but protect the host cells fromH. pylori infection.

Glycan chains play diverse roles as li-gands for cell surface receptors (11, 21–23)and as modulators of receptors and adhe-sive proteins (24–26). The present studyreveals a new aspect of mammalian glycanfunction as a natural antibiotic. Because�1,4-GlcNAc–capped O-glycans are pro-duced by human gastric gland mucouscells, the present study provides a basisfor development of novel and potentiallysafe therapeutic agents to prevent and treatH. pylori infection in humans without ad-verse reactions.

References and Notes1. R. M. Peek Jr., M. J. Blaser, Nature Rev. Cancer 2, 28

(2002).2. D. R. Cave, Semin. Gastrointest. Dis. 12, 196 (2001).3. P. Sipponen, H. Hyvarinen, Scand. J. Gastroenterol.

Suppl.196, 3 (1993).4. H. Ota et al., Histochem. J. 23, 22 (1991).5. E. Hidaka et al., Gut 49, 474 (2001).6. D. Ilver et al., Science 279, 373 (1998).7. J. Mahdavi et al., Science 297, 573 (2002).8. J. Nakayama et al., Proc. Natl. Acad. Sci. U.S.A. 96,

8991 (1999).9. Materials and methods are available as supplemental

material on Science Online.10. H. Enroth et al., Helicobacter 4, 7 (1999).11. J.-C. Yeh et al., Cell 105, 957 (2001).12. M. Kawakubo, J. Nakayama, unpublished observa-

tions.13. Y. Ito, M. Kawakubo, J. Nakayama, unpublished ob-

servations.

14. T. Horii et al., Helicobacter 7, 39 (2002).15. J. Finlay, L. Miller, J. A. Poupard, J. Antimicrob. Che-

mother. 52, 18 (2003).16. Y. Hirai et al., J. Bacteriol. 177, 5327 (1995).17. Y. Inamoto et al., J. Clin. Gastroenterol. 17, S136

(1993).18. J. Nakayama et al., J. Biol. Chem. 271, 3684 (1996).19. J. F. Tomb et al., Nature 388, 539 (1997).20. H. Higashi et al., Science 295, 683 (2002).21. J. B. Lowe, Cell 104, 809 (2001).22. T. O. Akama et al., Science 295, 124 (2002).23. N. L. Perillo, K. E. Pace, J. J. Seilhamer, L. G. Baum,

Nature 378, 736 (1995).24. D. J. Moloney et al., Nature 406, 369 (2000).25. M. Demetriou, M. Granovsky, S. Quaggin, J. W. Den-

nis, Nature 409, 733 (2001).26. J. Nakayama, M. N. Fukuda, B. Fredette, B. Ranscht,

M. Fukuda, Proc. Natl. Acad. Sci. U.S.A. 92, 7031(1995).

27. K. Ishihara et al., Biochem. J. 318, 409 (1996).28. This work was supported by a Grant-in-Aid for

Scientific Research on Priority Area 14082201 fromthe Ministry of Education, Culture, Sports, Scienceand Technology of Japan (J.N.) and by grants CA71932 (M.N.F.) and CA 33000 (M.F.) from theNational Cancer Institute. The authors thank H.Ota, Y. Kawakami, T. Taketomi, and O. Harada fordiscussions; E. Ruoslahti, R. C. Liddington, and E.Lamar for critical reading of the manuscript; and E.Hidaka, Y. Takahashi, S. Kubota, and A. Ishida fortechnical assistance. This report is dedicated to thememory of Hideki Matsumoto.

Supporting Online Materialwww.sciencemag.org/cgi/content/full/305/5686/1003/DC1Materials and MethodsTable S1References and Notes

16 April 2004; accepted 21 June 2004

Fig. 4. �1,4-GlcNAc–capped O-glycans pro-tect the host cells. AGScells were incubatedwithH. pylori for 8 hours(A) or 24 hours (B), anddoubly stained with an-ti-H. pylori antibody(red) and HIK1083 anti-body specific for termi-nal �1,4-GlcNAc (27)(green). (A) Note thatcomparable number ofH. pylori adhered toboth mock-transfectedAGS cells and AGS-�4GnT cells. (B) After 24hours, marked damagesuch as cell flatness orshrinkage are noted (ar-rows) in mock-trans-fected AGS cells; no cel-lular damage and fewattached bacteria arefound in AGS-�4GnTcells. ( Top) Nomarskiphotographs of thesame field. Scale bar, 50�m. (C) Viabilities ofAGS cells coculturedwith H. pylori for 4 daysdetermined by MTS as-say. Note that viabili-ty of mock-transfectedAGS cells was signifi-cantly reduced after thethird day, whereas AGS-�4GnT cells were fully viable for up to 4 days. The assay was done with triplicate measurements, and error bars indicate SD.

R E P O R T S

13 AUGUST 2004 VOL 305 SCIENCE www.sciencemag.org1006

Optical Sectioning Deep InsideLive Embryos by Selective PlaneIllumination Microscopy

Jan Huisken,* Jim Swoger, Filippo Del Bene, Joachim Wittbrodt,

Ernst H. K. Stelzer*

Large, living biological specimens present challenges to existing opticalimaging techniques because of their absorptive and scattering properties.We developed selective plane illumination microscopy (SPIM) to generatemultidimensional images of samples up to a few millimeters in size. Thesystem combines two-dimensional illumination with orthogonal camera-based detection to achieve high-resolution, optically sectioned imagingthroughout the sample, with minimal photodamage and at speeds capableof capturing transient biological phenomena. We used SPIM to visualize allmuscles in vivo in the transgenic Medaka line Arnie, which expresses greenfluorescent protein in muscle tissue. We also demonstrate that SPIM can beapplied to visualize the embryogenesis of the relatively opaque Drosophilamelanogaster in vivo.

Modern life science research often requiresmultidimensional imaging of a completespatiotemporal pattern of gene and proteinexpression or tracking of tissues during thedevelopment of an intact embryo (1). Inorder to visualize the precise distribution ofdevelopmental events such as activation ofspecific genes, a wide range of processes,from small-scale (subcellular) to large-scale (millimeters), needs to be followed.Ideally, such events, which can last fromseconds to days, will be observed in liveand fully intact embryos.

Several techniques have been developedthat allow mapping of the three-dimensional(3D) structure of large samples (2). Geneexpression has been monitored by in situhybridization and block-face imaging (3).Techniques that provide noninvasive (opti-cal) sectioning, as opposed to those thatdestroy the sample, are indispensable forlive studies. Optical projection tomographycan image fixed embryos at high resolution(4 ). Magnetic resonance imaging (5) andoptical coherence tomography (6 ) featurenoninvasive imaging, but do not providespecific contrasts easily.

In optical microscopy, green fluorescentprotein (GFP) and its spectral variants are usedfor high-resolution visualization of protein lo-calization patterns in living organisms (7).When GFP-labeled samples are viewed, op-tical sectioning (which is essential for itselimination of out-of-focus light) is obtain-able by laser scanning microscopy (LSM),

either by detection through a pinhole (con-focal LSM) (8) or by exploitation of thenonlinear properties of a fluorophore (mul-tiphoton microscopy) (9). Despite the im-

proved resolution, LSM suffers from two ma-jor limitations: a limited penetration depth inheterogeneous samples and a marked differ-ence between the lateral and axial resolution.

We developed selective plane illumina-tion microscopy (SPIM), in which opticalsectioning is achieved by illuminating thesample along a separate optical path or-thogonal to the detection axis (Fig. 1 andfig. S1). A similar approach in confocaltheta microscopy has been demonstrated toimprove axial resolution (10 –12). In SPIM,the excitation light is focused by a cylin-drical lens to a sheet of light that illumi-nates only the focal plane of the detectionoptics, so that no out-of-focus fluorescenceis generated (optical sectioning). The neteffect is similar to that achieved by con-focal LSM. However, in SPIM, only theplane currently observed is illuminated andtherefore affected by bleaching. Therefore,the total number of fluorophore excitationsrequired to image a 3D sample is greatlyreduced compared to the number in con-focal LSM (supporting online text).

GFP-labeled transgenic embryos of theteleost fish Medaka (Oryzias latipes) (13)were imaged with SPIM. In order to visu-

European Molecular Biology Laboratory (EMBL),Meyerhofstra�e 1, D-69117 Heidelberg, Germany.

*To whom correspondence should be addressed. E-mail: huisken@embl.de (J.H.) and stelzer@embl.de(E.H.K.S.)

Fig. 1. (A) Schematic of the samplechamber. The sample is embedded in acylinder of agarose gel. The solidifiedagarose is extruded from a syringe(not shown) that is held in a mechan-ical translation and rotation stage.The agarose cylinder is immersed inan aqueous medium that fills thechamber. The excitation light entersthe chamber through a thin glass win-dow. The microscope objective lens,which collects the fluorescence light,dips into the medium with its opticalaxis orthogonal to the plane of theexcitation light. The objective lens issealed with an O-ring and can bemoved axially to focus on the plane offluorescence excited by the lightsheet. In a modified setup, for low-magnification lenses not corrected forwater immersion, a chamber with fourwindows and no O-ring can be used.In this case, the objective lens imagesthe sample from outside the cham-ber. det., detection; ill., illumination;proj., projection. (B to E) A Medakaembryo imaged with SPIM by two dif-ferent modes of illumination. Lateral[(B) and (C)] and dorsal-ventral [(D)and (E)] maximum projections areshown. In (B) and (D), the sample wasilluminated uniformly, i.e., withoutthe cylindrical lens, as with a conven-tional widefield microscope. There isno optical sectioning. The elongationof fluorescent features along the de-tection axis is clearly visible in (D). Incontrast, selective (select.) plane illu-mination [(C) and (E)] provided optical sectioning because the cylindrical lens focused theexcitation light to a light sheet. Both image stacks were taken with a Zeiss Fluar 5�, 0.25objective lens.

R E P O R T S

www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004 1007

alize the internal structure, we imaged thetransgenic line Arnie, which expresses GFPin somatic and smooth muscles as well as inthe heart (14 ). A 4-day-old fixed Arnieembryo [stage 32 (15)] is shown in Fig. 1.SPIM was capable of resolving the internalstructures of the entire organism with highresolution (better than 6 �m) as deep as 500�m inside the fish, a penetration depth thatcannot be reached using confocal LSM (fig.S6). The axial resolution in SPIM is deter-mined by the lateral width of the lightsheet; for the configuration shown in Fig. 1,the axial extent of the point spread function(PSF) was about 6 �m, whereas without thelight sheet it was more than 20 �m (sup-porting online text).

Any fluorescence imaging system suf-fers from scattering and absorption in thetissue; in large and highly scattering sam-ples, the image quality decreases as theoptical path length in the sample increases.This problem can be reduced by a multi-view reconstruction, in which multiple 3Ddata sets of the same object are collectedfrom different directions and combined in apostprocessing step (16–18). The high-quality information is extracted from eachdata set and merged into a single, superior3D image (supporting online text). Oneway to do this is by parallel image acqui-sition, using more than one lens for thedetection of fluorescence (18).

We collected SPIM data for a multiviewreconstruction sequentially by generatingmultiple image stacks between which thesample was rotated. Sample deformationswere avoided with a rotation axis parallel togravity (Fig. 1). In contrast to tomographicreconstruction techniques [such as those in(4)], which require extensive processing ofthe data to yield any meaningful 3D informa-tion, rotation and subsequent data processingare optional in SPIM. They allow a furtherincrease in image quality and axial resolutioncompared to a single stack, but in many casesa single, unprocessed 3D SPIM stack aloneprovides sufficient information.

We performed a multiview reconstructionwith four stacks taken with four orientationsof the same sample (figs. S2 and S3). Com-bination of these stacks (supporting onlinetext) yielded a complete view of the sample,�1.5 mm long and �0.9 mm wide. In Fig. 2,the complete fused data set is shown and themost pronounced tissues are labeled. The de-crease in image quality with penetrationdepth is corrected by the fusion process. Ityielded an increased information content inregions that were obscured (by absorption orscattering in the sample) in some of the un-processed single views.

The method of embedding the sample ina low-concentration agarose cylinder isnondisruptive and easily applied to live

embryos. We routinely image live Medakaand Drosophila embryos over periods of upto 3 days without detrimental effects onembryogenesis and development. To dem-onstrate the potential of SPIM technology,we investigated the Medaka heart, a struc-ture barely accessible by conventional con-focal LSM because of its ventral position in

the yolk cell. We imaged transgenicMedaka Arnie embryos and show a recon-struction of the inner heart surface (Fig.3A) derived from the data set shown in Fig.2. This reveals the internal structure of theheart ventricle and atrium. In a slightlyearlier stage, internal organs such as theheart and other mesoendodermal deriva-

Fig. 2. A Medaka embryo (the same asin Fig. 1) imaged with SPIM and pro-cessed by multiview reconstruction(figs. S2, S3, and S6 and movies S1 andS2). (A) Overlay of a single stack (green)and the fusion of four data sets (red andgreen). (B) Dorsal-ventral and (C) lateralmaximum intensity projections of thefused data. The high resolutionthroughout the entire fish allows iden-tification of different tissues: rgc, retinalganglion cells; so, superior oblique; io,inferior oblique; ir, inferior rectus; sr,superior rectus; im, intermandibualaris;hh, hyohyal; rc, rectus communis; dpw,dorsal pharyngeal wall; fad, fin adduc-tor; fab, fin abductor; sm, somitic me-soderm; tv, transverse ventrals. Thestack has a size of 1201 by 659 by 688pixels (1549 �m by 850 �m by 888�m).

Fig. 3. AMedaka heart imaged withSPIM (movies S3 and S4). (A) Sur-face rendering of the heart takenfrom the data shown in Fig. 2. Theheart has been cut open computa-tionally to make the internal struc-ture visible. hv, heart ventricle; ha,heart atrium. (B) Schematic repre-sentation of a Medaka embryo atstage 26 of development (13), 2days post-fertilization. Three opti-cally sectioned planes are indicated.At this stage, ventral structures suchas the heart are deeply buried in theyolk sphere. d, dorsal; v, ventral; a,anterior; p, posterior; y, yolk; ey, eye.(C) Optical section of an Arnie em-bryo showing the eye and the opticnerve labeling and the dorsal part ofthe heart ventricle. on, optic nerve.(D) Optical section showing theheart ventricle chamber and thedorsal wall of the heart atrium. (E)Optical section showing the atrium chamber.

R E P O R T S

13 AUGUST 2004 VOL 305 SCIENCE www.sciencemag.org1008

tives are deeply buried in the yolk sphere,under the body of the embryo (Fig. 3B). InFig. 3, C to E, three optical sections atdifferent depths illustrate GFP expressionin the muscles of the living heart. Fastframe recording (10 frames per s) allowsimaging of the heartbeat (movies S3 andS4); similar imaging has previously onlybeen demonstrated at stages when the heartis exposed and by cooling the embryo toreduce the heart rate (19).

To demonstrate that SPIM can also beused to image the internal structures of rela-tively opaque embryos, we recorded a timeseries (movie S5) of the embryogenesis of thefruit fly Drosophila melanogaster (Fig. 4).GFP-moesin labeled the plasma membranethroughout the embryo (20). Even withoutmultiview reconstruction, structures insidethe embryo are clearly identifiable and trace-able. Stacks (56 planes each) were taken au-tomatically every 5 min over a period of 17hours, without refocusing or realignment.Even after being irradiated for 11,480 imag-

es, the embryo was unaffected and completedembryogenesis normally.

In summary, we present an optical wide-field microscope capable of imaging pro-tein expression patterns deep inside bothfixed and live embryos. By selective illu-mination of a single plane, the excitationlight is used efficiently to achieve opticalsectioning and reduced photodamage inlarge samples, key features in the study ofembryonic development. The method ofsample mounting allows positioning androtation to orient the sample for op-timal imaging conditions. The optionalmultiview reconstruction combines inde-pendently acquired data sets into an opti-mal representation of the sample. Theimplementation of other contrasts such asscattered light will be straightforward. Thesystem is compact, fast, optically stable,and easy to use.

SPIM is well suited for the visualizationof high-resolution gene and protein ex-pression patterns in three dimensions in the

context of morphogenesis. Heart functionand development can be precisely followedin vivo using SPIM in Arnie transgenicembryos. Because of its speed and its au-tomatable operation, SPIM can serve as atool for large-scale studies of developingorganisms and the systematic and compre-hensive acquisition and collection of ex-pression data. Even screens for moleculesthat interfere with development andregeneration on a medium-throughput scaleseem feasible. SPIM technology can bereadily applied to a wide range of organ-isms, from whole embryos to single cells.Subcellular resolution can be obtained inlive samples kept in a biologically relevantenvironment within the organism or in cul-ture. Therefore, SPIM also has the po-tential to be of use in the promising fieldsof 3D cultured cells (21) and 3D cellmigration (22).

References and Notes1. S. G. Megason, S. E. Fraser, Mech. Dev. 120, 1407

(2003).2. S. W. Ruffins, R. E. Jacobs, S. E. Fraser, Curr. Opin.

Neurobiol. 12, 580 (2002).3. W. J. Weninger, T. Mohun, Nature Genet. 30, 59

(2002).4. J. Sharpe et al., Science 296, 541 (2002).5. A. Y. Louie et al., Nature Biotechnol. 18, 321 (2000).6. D. Huang et al., Science 254, 1178 (1991).7. M. Chalfie, Y. Tu, G. Euskirchen, W. W. Ward, D. C.

Prasher, Science 263, 802 (1994).8. J. B. Pawley, Handbook of Biological Confocal Micros-

copy (Plenum, New York, 1995).9. W. Denk, J. H. Strickler, W. W. Webb, Science 248, 73

(1990).10. E. H. K. Stelzer, S. Lindek, Opt. Commun. 111, 536

(1994).11. E. H. K. Stelzer et al., J. Microsc. 179, 1 (1995).12. S. Lindek, J. Swoger, E. H. K. Stelzer, J. Mod. Opt. 46,

843 (1999).13. J. Wittbrodt, A. Shima, M. Schartl, Nature Rev. Genet.

3, 53 (2002).14. Materials and methods are available as supporting

material on Science Online.15. T. Iwamatsu, Zool. Sci. 11, 825 (1994).16. P. J. Shaw, J. Microsc. 158, 165 (1990).17. S. Kikuchi, K. Sonobe, S. Mashiko, Y. Hiraoka, N.

Ohyama, Opt. Commun. 138, 21 (1997).18. J. Swoger, J. Huisken, E. H. K. Stelzer, Opt. Lett. 28,

1654 (2003).19. J. R. Hove et al., Nature 421, 172 (2003).20. K. A. Edwards, M. Demsky, R. A. Montague, N.

Weymouth, D. P. Kiehart, Dev. Biol. 191, 103(1997).

21. A. Abbott, Nature 424, 870 (2003).22. D. J. Webb, A. F. Horowitz, Nature Cell Biol. 5, 690

(2003).23. We thank S. Enders and K. Greger for contributions to

the instrumentation and F. Jankovics and D. Brunnerfor providing the Drosophila samples. The beating-heart data was recorded by K. Greger.

Supporting Online Materialwww.sciencemag.org/cgi/content/full/305/5686/1007/DC1Materials and MethodsSOM TextFigs. S1 to S6References and NotesMovies S1 to S5

6 May 2004; accepted 15 July 2004

Fig. 4. Time-lapse imaging of Drosophila melanogaster embryogenesis. Six out of 205 time pointsacquired are shown (movie S5). At each time point, 56 planes were recorded, from which two (atdepths of 49 �m and 85 �m below the cortex) are shown. No multiview reconstruction wasnecessary. The optical sectioning capability and the good lateral resolution are apparent. Despitethe optically dense structure of the Drosophila embryo, features are well resolved at these depthsin the sample. For this figure, the images were oriented so that the illumination occurs from below.This results in a slight drop in intensity and clarity from the bottom to the top of each slice.Nevertheless, the information content across the embryo is nearly uniform, and the overallmorphogenetic movements during embryonic development can be followed. The images werenormalized to exhibit the same overall intensity, thus compensating the continuous production ofGFP-moesin. We took 205 stacks at 5-min intervals with a Zeiss Achroplan 10�, 0.30W objectivelens (56 planes per stack at 4-�m spacing) for 11,480 images in total.

R E P O R T S

www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004 1009

Increased Nuclear NAD Biosynthesisand SIRT1 Activation PreventAxonal Degeneration

Toshiyuki Araki, Yo Sasaki, Jeffrey Milbrandt*

Axonal degeneration is an active program of self-destruction that is ob-served in many physiological and pathological settings. In Wallerian de-generation slow (wlds) mice, Wallerian degeneration in response to axonalinjury is delayed because of a mutation that results in overexpression of achimeric protein (Wlds) composed of the ubiquitin assembly protein Ufd2aand the nicotinamide adenine dinucleotide (NAD) biosynthetic enzymeNmnat1. We demonstrate that increased Nmnat activity is responsible forthe axon-sparing activity of the Wlds protein. Furthermore, we demonstratethat SIRT1, a mammalian ortholog of Sir2, is the downstream effector ofincreased Nmnat activity that leads to axonal protection. These findingssuggest that novel therapeutic strategies directed at increasing the supplyof NAD and/or Sir2 activation may be effective for treatment of diseasescharacterized by axonopathy and neurodegeneration.

Axonopathy is a critical feature of many pe-ripheral neuropathies, and axonal degenera-tion often precedes the death of neuronal cellbodies in neurodegenerative diseases such asParkinson’s and Alzheimer’s disease (1).These axonal deficits are an important com-ponent of the patient’s disability and poten-tially represent a therapeutic target for com-bating these diseases (2).

The discovery of a spontaneous domi-nant mutation in mice that results in de-layed axonal degeneration, the Walleriandegeneration slow (wlds) mice, suggeststhat axonal degeneration is an active pro-cess of self-destruction (3). Genetic analy-sis has shown that the wlds mutation com-prises an 85-kb tandem triplication, whichresults in overexpression of a chimeric nu-clear molecule (Wlds protein). This proteinis composed of the N-terminal 70 aminoacids of Ufd2a (ubiquitin fusion degrada-tion protein 2a), a ubiquitin-chain assemblyfactor, fused to the complete sequence ofnicotinamide mononucleotide adenylyl-transferase1 (Nmnat1), an enzyme in theNAD biosynthetic pathway that generatesNAD within the nucleus (4, 5). The Wlds

protein has Nmnat activity but lacks ubiq-uitin ligase function, suggesting that axonalprotection is derived from either increasedNmnat1 activity or a “dominant negative”inhibition of Ufd2a function.

To determine the mechanism of delayedaxonal degeneration mediated by the Wlds pro-tein, we used an in vitro Wallerian degenerationmodel. Primary dorsal root ganglion (DRG)

explant neurons were infected with lentivirusexpressing recombinant proteins, and axonswere injured by either removal of the neuronalcell body (transection) or growth in vincristine(toxic). We first demonstrated that transectedaxons from neurons expressing the Wlds pro-tein degenerated with the delayed kinetics char-acteristic of neurons derived from wlds mice(Fig. 1A) (6). Next, we compared axonal de-generation after transection in wild-type neu-rons that express the chimeric Wlds proteinwith those that express only the Ufd2a orNmnat1 portions of the Wlds protein, linked toenhanced green fluorescent protein (EGFP)(Fig. 1B). We found that expression of EGFP-Nmnat1 delayed axonal degeneration compara-ble to Wlds protein itself, whereas the N-termi-nal 70 amino acids of Ufd2a (fused to EGFP),targeted to either the nucleus or cytoplasm, didnot affect axonal degeneration. Quantificationof these effects was performed by counting thepercentage of remaining neurites at varioustimes after removal of neuronal cell bodies.This analysis showed that EGFP-Nmnat1, likeWlds protein itself, resulted in a �10-fold in-crease in intact neurites 72 hours after injury.To further exclude direct involvement of theubiquitin-proteasome system in Wlds protein-mediated axonal protection, we examined theeffect of Ufd2a inhibition using either a domi-nant-negative Ufd2a mutant or a Ufd2a smallinterfering RNA (siRNA) construct. However,neither method of Ufd2a inhibition resulted indelayed axonal degradation in response to axo-tomy. Together, these experiments demonstrat-ed that the Nmnat1 portion of the Wlds proteinis responsible for the delayed axonal degenera-tion observed in wlds mice.

Nmnat1 is an enzyme in the nuclearNAD biosynthetic pathway that catalyzesthe conversion of nicotinamide mononucle-

otide (NMN) and nicotinate mononucleo-tide (NaMN) to NAD and nicotinate ade-nine dinucleotide (NaAD), respectively (7 ).The axonal protection observed in Nmnat1overexpressing neurons could be mediatedby its ability to synthesize NAD (i.e., itsenzymatic activity), or perhaps by otherunknown functions of this protein. To ad-dress this question, we used the Nmnat1crystal structure to identify several residuespredicted to participate in substrate binding(8). A mutation in one of these residues wasengineered into full length Nmnat1(W170A) and Wlds (W258A) protein. Invitro enzymatic assays confirmed that bothof these mutant proteins were severely lim-ited in their ability to synthesize NAD (fig.S2). Each of these mutants and their respec-tive wild-type counterparts was singly in-troduced into neurons to assess their abilityto protect axons from degradation. Wefound that neurons expressing these enzy-matically inactive mutants had no axonalprotective effects (Fig. 1C), which indi-cates that NAD/NaAD production is re-sponsible for the ability of Nmnat1 to pre-vent axonal degradation.

In addition to mechanical transection,axonal protection in wlds mice is also ob-served against other damaging agents suchas ischemia and toxins (2, 9). We sought todetermine whether increased Nmnat activ-ity would also delay axonal degradation inresponse to other types of axonal injury,such as vincristine, a cancer chemothera-peutic reagent with well-characterized ax-onal toxicity. Neurons expressing eitherNmnat1 or EGFP (control) were grown in0.5 �M vincristine for up to 9 days. Wefound that axons of neurons expressingNmnat1 maintained their original lengthand refractility, whereas axons emanatingfrom uninfected neurons or those express-ing EGFP gradually retracted and hadmostly degenerated by day 9 (Fig. 2). Theseresults indicate that increased Nmnat activ-ity itself can protect axons from both me-chanical and toxic insults.

Previous experiments have shown thatneuronal cells express membrane proteinsthat can bind and transport extracellularNAD into the cell (10). This encouraged usto investigate whether exogenously admin-istered NAD could prevent axonal degen-eration. We added various concentrationsof NAD to neuronal cultures before axonaltransection and examined the extent of ax-onal degradation. We found that 0.1 to 1mM NAD added 24 hours before axotomysignificantly delayed axonal degenera-tion, although exogenously applied NAD(1 mM) was slightly less effective in pro-tecting axons than lentivirus-mediatedNmnat1 expression (Fig. 3A). These re-sults provide direct support for the idea

Department of Pathology, Washington UniversitySchool of Medicine, St. Louis, Missouri 63110, USA.

*To whom correspondence should be addressed. E-mail: jmilbrandt@wustl.edu

R E P O R T S

13 AUGUST 2004 VOL 305 SCIENCE www.sciencemag.org1010

that increased NAD supply can preventaxonal degradation.

NAD plays a variety of roles in the cell.In the mitochondria, it is involved in elec-tron-transport processes important in ener-gy metabolism, whereas in the nucleusNAD regulates aspects of DNA repair andtranscription. In yeast, the Nmnat ho-mologs are nuclear proteins that participatein the nuclear NAD salvage pathway (11,12), which suggests that NAD could bemediating its axonal protective effects by anuclear mechanism. Indeed, both Wlds andNmnat1 were found in the nucleus withimmunohistochemistry and EGFP fluores-cence (fig. S3). Interestingly, the activationof the NAD salvage pathway in yeast doesnot alter total cellular NAD levels (11).Similarly, tissue NAD levels in wild-typeand wlds brain are similar, despite the in-creased NAD synthetic activity in wlds tis-sues (5). We measured NAD levels in wild-type and Nmnat1-expressing cells using

sensitive microscale enzymatic assays (13)and found that increased Nmnat activity didnot result in changes in overall cellularNAD levels (14 ). Together, these datasuggest that an NAD-dependent enzyma-tic activity in the nucleus, as opposed tocytoplasmic NAD-dependent processes,is likely to mediate the axonal protec-tion observed in response to increasedNmnat activity.

To gain further insight into the mecha-nism of NAD-dependent axonal protection(NDAP), we examined whether NAD wasrequired prior to the removal of the neuro-nal cell bodies or whether direct exposureof the severed axons to high levels of NADwas sufficient to provide protection (Fig.3B). Neuronal cultures were prepared, and1 mM NAD was added to the culture me-dium at the time of axonal transection or atvarious times (4 to 48 hours) before injury.We found that administering NAD at thetime of axonal transection or for up to 8

hours before injury had no protective ef-fects on axons. However, significant axonsparing was observed when neurons wereincubated with NAD for longer periods oftime before injury, with the greatest effectsoccurring after 24 hours of NAD pretreat-ment. These results indicate that NDAP isnot mediated by a rapid posttranslationalmodification within the axons themselves.Instead, they suggest that the protectiveprocess requires de novo transcriptionaland/or translational events. The activenature of axonal self-destruction wasfurther emphasized by our observationsthat treatment of neurons for 24 hours be-fore axotomy with inhibitors of eitherRNA (actinomycin D) or protein (cyclo-heximide) synthesis resulted in axonalprotection (15 ).

The Sir2 family of protein deacetylasesand poly(ADP-ribose) polymerase (PARP)are involved in major NAD-dependent nu-clear enzymatic activities. Sir2 is an NAD-

Fig. 1. Nmnat1 activity ofthe Wlds fusion protein isresponsible for the de-layed axonal degenera-tion in wlds mutant mice.(A) In vitro Wallerian de-generation model usinglentivirus-infected DRGneuronal explant culturesexpressing Wlds proteinor EGFP alone. Tubulin�III-immunoreactive neu-rites are shown beforetransection and at 12, 24,48, and 72 hours aftertransection. The * de-notes the location of thecell bodies prior to re-moval. Insets demon-strate EGFP signal to con-firm transgene expres-sion. Scale bar, 1 mm. (B)In vitro Wallerian degen-eration model with lentivirus-infected DRG neurons expressing EGFPonly, Wlds protein, Ufd2a portion (70 residues) of Wlds protein fusedto EGFP [Ufd2a(1-70)-EGFP], Ufd2a(1-70)-EGFP with C-terminal nuclearlocalization signal, Nmnat1 portion of Wlds protein fused to EGFP, domi-nant-negative Ufd2a [Ufd2a(P1140A)], or Ufd2a siRNA construct. Represen-tative images of neurites and quantitative analysis of remaining neuritenumbers (percentage of remaining neurites relative to pretransection � SD)at the indicated time point with each construct are shown. The * indicatessignificant difference (P � 0.0001) with EGFP-infected neurons. The EGFP

signal before transection confirms transgene expression (bottom row). Scalebar, 50 �m. (C) In vitro Wallerian degeneration model of lentivirus-infectedDRG neurons expressing Nmnat1 or Wlds protein, mutants of these proteinsthat lack NAD-synthesis activity Nmnat1(W170A) and Wlds(W258A), orEGFP (see color code). Quantitative analysis of the number of re-maining neurites at the indicated time points for each construct(percentage of remaining neurites relative to pretransection �SD). The * indicates significant difference (P � 0.0001) with EGFP-infected neurons.

R E P O R T S

www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004 1011

dependent deacetylase of histones (15) andother proteins, and its activation is centralto promoting increased longevity in yeastand Caenorhabditis elegans (17, 18).PARP is activated by DNA damage and isinvolved in DNA repair (19). The impor-tance of these NAD-dependent enzymes inregulating gene activity prompted us to in-vestigate their potential role in the self-destructive process of axonal degradation.We tested whether inhibitors of Sir2 (Sirti-nol) (20) and PARP [3-aminobenzamide(3AB)] (21) could affect NDAP (Fig. 4A).Neurons were cultured in the presence of 1mM NAD and either Sirtinol (100 �M) or3AB (20 mM). Axonal transection was per-formed by removal of the neuronal cellbodies, and the extent of axonal degrada-

tion was assessed 12 to 72 hours later. Wefound that, although Sirtinol had no axonaltoxicity on uninjured axons (fig. S5), iteffectively blocked NDAP after transec-tion, indicating that Sir2 proteins are likelyeffectors of this process. In contrast, 3ABhad no effect on NDAP, indicating thatPARP does not play a role in axonal pro-tection. Interestingly, 3AB alone did stim-ulate limited axonal protection (Fig. 4A),presumably as a consequence of PARP in-hibition, which decreases NAD consump-tion and raises nuclear NAD levels. Toconfirm the involvement of Sir2 proteins inNDAP, we tested the effects of resveratrol(10 to 100 �M), a polyphenol compoundfound in grapes that enhances Sir2 activity(22). We found that neurons treated with

resveratrol prior to axotomy showed a de-crease in axonal degradation that was com-parable to that obtained with NAD (Fig.4B), providing further support for the idea thatSir2 proteins are effectors of the axonal protec-tion mediated by increased Nmnat activity.

In humans and rodents, seven moleculesthat share the Sir2 conserved domain [sir-tuin (SIRT) 1 to 7] have been identified(23). SIRT1 is located in the nucleus and isinvolved in chromatin remodeling and theregulation of transcription factors such asp53 (24 ), whereas other SIRT proteins arelocated within the cytoplasm and mitochon-dria (25, 26 ). To determine which SIRTprotein(s) is involved in NDAP, we per-formed knockdown experiments usingsiRNA constructs to specifically targeteach member of the SIRT family. Neuronswere infected with lentiviruses expressingspecific SIRT siRNA constructs that effec-tively suppressed expression of their in-tended target (table S1). The infected neu-rons were cultured in 1 mM NAD, andaxonal transection was performed by re-moving the cell bodies. Inhibiting the ex-pression of most SIRT proteins did notsignificantly affect NDAP; however, theknockdown of SIRT1 blocked NDAP aseffectively as Sirtinol (Fig. 4C). Like Sirti-nol treatment, SIRT1 inhibition by siRNAdid not affect the rate of degeneration inuntreated neurons or the axonal integrity inuninjured neurons (fig. S5). These resultsindicate that SIRT1 is the major effector ofthe increased NAD supply that effectivelyprevents axonal self-destruction. Although,SIRT1 may deacetylate proteins directlyinvolved in axonal stability, its predomi-

Fig. 2. Increased Nmnat1 activity protects axons from degeneration caused by vincristine toxicity.(A) DRG neuronal explants expressing either Nmnat1 or EGFP (control) were cultured with 0.5 �Mvincristine. Representative images of neurites (phase-contrast) at the indicated times after vincris-tine addition are shown. Scale bar, 1 mm. (B) Quantification of the protective effect at theindicated time points is plotted as the area covered by neurites relative to that covered by neuritesbefore treatment.

Fig. 3. Axonal protection requires pretreatment of neurons with NAD beforeinjury. (A) In vitroWallerian degeneration using DRG explants cultured in thepresence of various concentrations of NAD added 24 hours before axonaltransection. (B) DRG explants preincubated with 1mM NAD for 4, 8, 12, 24,

or 48 hours prior to transection. In each experiment, the number ofremaining neurites (percentage of remaining neurites relative to pretransec-tion � SD) is shown at each of the indicated time points. The * indicatessignificant axonal protection compared with control (P � 0.0001).

R E P O R T S

13 AUGUST 2004 VOL 305 SCIENCE www.sciencemag.org1012

nantly nuclear location, along with the re-quirement for NAD �24 hours prior toinjury for effective protection, suggest thatSIRT1 regulates a genetic program thatleads to axonal protection.

In wlds mice, axonal protection throughWlds protein overexpression has been dem-onstrated in models of motor neuron andParkinson’s disease and in peripheral sen-sory neurons affected by chemotherapeuticagents (2, 27 ). Our results indicate that themolecular mechanism of axonal protectionin the wlds mice is due to the increasednuclear NAD biosynthesis that results fromincreased Nmnat1 activity and consequentactivation of the protein deacetylaseSIRT1. Other intracellular events that af-fect NAD levels or NAD/NADH ratios,such as energy production through respira-tion, may also affect physiological andpathological processes in the nervous sys-tem through SIRT1-dependent pathways(28). It is possible that the alteration ofNAD levels by manipulation of the NADbiosynthetic pathway, Sir2 protein activity,or other downstream effectors will providenew therapeutic opportunities for the treat-

ment of diseases involving axonopathy andneurodegeneration.

References and Notes1. M. C. Raff, A. V. Whitmore, J. T. Finn, Science 296,

868 (2002).2. M. P. Coleman, V. H. Perry, Trends Neurosci. 25, 532.3. E. R. Lunn et al., Eur. J. Neurosci. 1, 27 (1989).4. L. Conforti et al., Proc. Natl. Acad. Sci. U.S.A. 97,

11377 (2000).5. T. G. Mack et al., Nat. Neurosci. 4, 1199 (2001).6. E. A. Buckmaster, V. H. Perry, M. C. Brown, Eur.J. Neurosci. 7, 1596 (1995).

7. G. Magni et al., Cell. Mol. Life Sci. 61, 19 (2004).8. T. Zhou et al., J. Biol. Chem. 277, 13148 (2002).9. T. H. Gillingwater et al., J. Cereb. Blood Flow Metab.

24, 62 (2004).10. S. Bruzzone et al., FASEB J. 15, 10 (2001).11. R. M. Anderson et al., J. Biol. Chem. 277, 18881

(2002).12. W. K. Huh et al., Nature 425, 686 (2003).13. C. Szabo et al., Proc. Natl. Acad. Sci. U.S.A. 93, 1753

(1996).14. T. Araki et al., unpublished observations.15. T. Araki et al., unpublished observations.16. S. Imai et al., Nature 403, 795 (2000).17. M. Kaeberlein, M. McVey, L. Guarente, Genes Dev. 13,

2570 (1999).18. H. A. Tissenbaum, L. Guarente, Nature 410, 227

(2001).19. S. D. Skaper, Ann. N.Y. Acad. Sci. 993, 217, discussion

287 (2003).20. C. M. Grozinger et al., J. Biol. Chem. 276, 38837

(2001).

21. G. J. Southan, C. Szabo, Curr. Med. Chem. 10, 321(2003).

22. K. T. Howitz et al., Nature 425, 191 (2003).23. S. W. Buck, C. M. Gallo, J. S. Smith, J. Leukoc. Biol. 75,

939, 2004.24. J. Luo et al., Cell 107, 137 (2001).25. P. Onyango et al., Proc. Natl. Acad. Sci. U.S.A. 99,

13653 (2002).26. B. J. North et al., Mol. Cell 11, 437 (2003).27. A. Sajadi, B. L. Schneider, P. Aebischer, Curr. Biol. 14,

326 (2004).28. S. J. Lin et al., Genes Dev. 18, 12 (2004).29. We thank D. Baltimore for the lentiviral expression

system, Kazusa DNA Research Institute for the mu-rine Ufd2a cDNA, and J. Manchester and J. Gordon forassistance with NAD measurements. We thank mem-bers of the laboratory and our colleagues E. Johnson,J. Gordon, S. Imai, R. Van Gelder, and R. Heuckerothfor helpful discussion and comments on the manu-script. The work was supported by grants from theNational Institute of Neurological Disorders andStroke NS40745 and National Institute on AgingAG13730, and a pilot grant from the Alzheimer’sDisease Research Center at Washington University(National Institute on Aging AG05681).

Supporting Online Materialwww.sciencemag.org/cgi/content/full/305/5686/1010/DC1Materials and MethodsSOM TextFigs. S1 to S5Table S1References

17 March 2004; accepted 29 June 2004

Fig. 4. NDAP is mediated by SIRT1 activation. (A) In vitroWallerian degeneration using DRG explant cultures prein-cubated with 1 mM NAD alone (control) or in the presenceof either 100 �M Sirtinol (a Sir2 inhibitor) or 20 mM3-aminobenzimide (3AB), a PARP inhibitor. The * indicatessignificant inhibition of axonal protection (P � 0.0001)compared with the no-inhibitor control. (B) In vitro Wal-lerian degeneration using DRG explant cultures incubatedwith resveratrol (10, 50, or 100 �M), a Sir2 protein acti-vator. The * indicates significant axonal protection (P �0.0001) compared with the no-resveratrol control. (C) Invitro Wallerian degeneration using DRG explant culturesinfected with lentivirus expressing siRNA specific for eachmember of the SIRT family (SIRT1 to 7) and preincubatedwith 1 mM NAD. Quantitative analysis of the number ofremaining neurites (percentage of remaining neurites rel-ative to pretransection � SD) at indicated time point foreach condition is shown. The * indicates points significant-ly different from the no-siRNA control (P � 0.0001).

R E P O R T S

www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004 1013

Evidence for Addiction-likeBehavior in the Rat

Veronique Deroche-Gamonet, David Belin, Pier Vincenzo Piazza*

Although the voluntary intake of drugs of abuse is a behavior largelypreserved throughout phylogeny, it is currently unclear whether patholog-ical drug use (“addiction”) can be observed in species other than humans.Here, we report that behaviors that resemble three of the essential diag-nostic criteria for addiction appear over time in rats trained to self-administer cocaine. As in humans, this addiction-like behavior is presentonly in a small proportion of subjects using cocaine and is highly predictiveof relapse after withdrawal. These findings provide a new basis for devel-oping a true understanding and treatment of addiction.

The voluntary intake of drugs of abuse is abehavior largely conserved throughout phy-logeny. Preferences for drug-associated en-vironments or drug-reinforced learning oftasks have been found in several species(1–6 ). The possibility of studying thesebehaviors in animals has helped us to un-derstand the neurobiological basis of drugtaking (7–10) and, more generally, thebrain systems for reward (11).

As important as the comprehension ofdrug taking and reward is, however, themajor goal of drug abuse research is touncover the mechanisms of addiction. Ad-diction is not just the taking of drugs butcompulsive drug use maintained despite ad-verse consequences for the user (12). Thispathological behavior appears only in asmall proportion (15 to 17%) of those usingdrugs (13) and has the characteristics of achronic disease (12). Indeed, even after aprolonged period of withdrawal, 90% ofaddicted individuals relapse to drug taking(14 ). Unfortunately, our knowledge of thebiological basis of addiction lags behindour knowledge of the mechanisms of drugtaking, probably because convincing evi-dence of addiction in animals is lacking.

We thus investigated whether addiction-like behaviors can be observed in rodents.Our experiments used intravenous self-administration (SA), the most common pro-cedure for the study of voluntary drug intakein laboratory animals. Freely moving ratslearned to obtain intravenous infusions ofcocaine by poking their noses into a hole. Toallow for addiction-like behavior to appear,we studied SA over a time frame of about 3months, much longer than is typical in SA

experiments (i.e., between 10 and 30 days).During this prolonged SA period, we repeat-edly evaluated the intensity of three behav-iors resembling those currently consideredthe hallmarks of substance dependence in theDSM-IV (12):

(i) The subject has difficulty stoppingdrug use or limiting drug intake. We mea-sured the persistence of cocaine seeking dur-ing a period of signaled nonavailability ofcocaine. The daily SA session included three40-min “drug periods” that were separated bytwo 15-min “no-drug periods.” During thedrug periods, a standard FR5 reinforcementschedule was in effect: Five nose-pokes re-sulted in an infusion of 0.8 mg of cocaine perkilogram of body weight (mg/kg). During theno-drug periods, nose-pokes had no effect.The two different periods of drug availabilitywere signaled by a change in the illuminationof the SA chamber (15).

(ii) The subject has an extremely highmotivation to take the drug, with activitiesfocused on its procurement and consumption.We used a progressive-ratio schedule: Thenumber of responses required to receive oneinfusion of cocaine (i.e., the ratio of respond-ing to reward) was increased progressivelywithin the SA session. The maximal amountof work that the animal will perform beforecessation of responding, referred to as thebreaking point, is considered a reliable indexof the motivation for the drug (16).

(iii) Substance use is continued despite itsharmful consequences. We measured the per-sistence of the animals’ responding for thedrug when drug delivery was associated witha punishment. During these sessions, nose-pokes on the standard FR5 schedule resultedin the delivery of both the drug and an elec-tric shock. This shock punishment was sig-naled by a new cue light that was turned on atthe time of the first nose-poke and off afterthe delivery of the shock (15).

To provide further validity to the addiction-like behaviors studied here, we analyzed theirdevelopment as a function of the propensity

of an individual to relapse to drug seeking.This approach was chosen because, as men-tioned above, in humans the most predictableoutcome of a first diagnosis of addiction is a90% chance of relapse to drug use even afterlong periods of withdrawal (14). To study thepropensity to relapse, we used the “reinstate-ment” procedure (17). After a 5- or 30-dayperiod of withdrawal that followed the 3months of SA, rats were exposed to stimuliknown to induce relapse in humans, such assmall amounts of the abused drug or a con-ditioned stimulus associated with drug tak-ing. These challenges induce high levels ofresponding (reinstatement) on the device pre-viously associated with drug delivery. Therate of responding during the test for rein-statement is considered a measure of the pro-pensity to relapse.

In a first experiment, rats (n � 17) wereassigned to two groups on the basis of theirbehavior on the test for reinstatement, in-duced here by the infusion of small quantitiesof cocaine given after 5 days of withdrawalthat followed 76 days of testing for SA (15).The two groups (n � 7 each) contained therats with the 40% highest (HRein) and 40%lowest (LRein) cocaine-induced reinstate-ment of responding (Fig. 1D) (15). HReinand LRein differed profoundly on the occur-rence of addiction-like behaviors (Fig. 1, A toC) (15). HRein rats progressively increasedtheir drug-seeking behavior during the no-drug periods (F2,12 � 3.54, P � 0.05) andafter punishment (F1, 6 � 8.14, P � 0.05) andalso had higher breaking points on theprogressive-ratio schedule (F1,12 � 22.07,P � 0.0005). In contrast, none of thesebehaviors increased over time in LRein rats,and in fact they tended to decrease. Finally,correlation analyses revealed that eachaddiction-like behavior strongly predicts thepropensity to reinstatement (persistence indrug seeking, r � 0.96; resistance to punish-ment, r � 0.67; motivation for the drug, r �0.79; P � 0.001 in all cases). A regressionanalysis including the three addiction-likebehaviors as independent variables showed amultiple R equal to 0.82 (P � 0.001).

In a second experiment (n � 15), weassessed whether addiction-like behaviorscould also be related to the propensity toreinstatement after a longer period of with-drawal (30 days). This time, reinstatementof responding induced by both cocaine anda cocaine-associated conditioned stimulus(CS) was studied (15). HRein rats (n � 6)showed higher levels of reinstatement re-sponding induced by cocaine (F3,30 � 4.07,P � 0.01) and by the CS (F1,10 � 4.62, P �0.05) than did LRein rats (n � 6) (Fig. 2, Dand E) (15). Again (Fig. 2, A to C) (15),HRein rats displayed higher levels ofaddiction-like behaviors than did LReinrats (group effect for each behavior,

INSERM U588, Laboratoire de Physiopathologie desComportements, Bordeaux Institute for Neuro-sciences, University Victor Segalen–Bordeaux 2, Do-maine de Carreire, Rue Camille Saint-Saens, 33077Bordeaux Cedex, France.

*To whom correspondence should be addressed. E-mail: piazza@bordeaux.inserm.fr

R E P O R T S

13 AUGUST 2004 VOL 305 SCIENCE www.sciencemag.org1014

F1,10 � 7.09 to 13.73, P � 0.05 to 0.005).In humans, the diagnosis of addiction is

performed by counting the number of diag-nostic criteria that are met by an individualsubject; a positive diagnosis is made when apreestablished number of criteria are found(12). We used a similar approach in rats byscoring them for each of the three addiction-like behaviors. For this analysis we addedrats from a third experiment (n � 26) toincrease the total number of subjects (n � 58)that completed the SA procedure. An individ-ual was considered positive for an addiction-like criterion when its score for one of thethree addiction-like behaviors was in the 66thto 99th percentile of the distribution (15).This allowed us to separate our sample of ratsinto four groups according to the number ofpositive criteria met (zero to three). The in-tensity of the three addiction-like behaviorswas proportional to the number of criteriamet by the subject (criteria effect for eachbehavior, F3, 54 � 16.99 to 30.7, P � 0.0001)(Fig. 3, A to C) (15). Strikingly, the groupthat met all three positive criteria represented17% of the entire sample (Fig. 3D), a per-centage similar to that of human cocaineusers diagnosed as addicts (13). Finally, de-spite this profound difference in addiction-like behavior scores, rats showing zero or

three addiction-like behaviors did not differon intake of cocaine during the entire SAperiod (Fig. 3E) or on sensitivity to the un-conditioned effects of the drug, as measuredby locomotion during SA (Fig. 3F) (15).

A factor analysis was then performed todetermine whether the three addiction-likebehaviors and the level of responding duringextinction (15) were indices of two differentunderlying constructs. Extinction conditionsallow for the measurement of persistence ofresponding for the drug when it is no longeravailable. Continued responding under theseconditions is considered a measure of impul-sivity/disinhibition (18), a general factor thatcould influence addiction-like behaviors (19,20). Remarkably, the factor analysisshowed that the three addiction-like behav-iors loaded equally on one factor (r � 0.70to 0.88) and extinction loaded on a secondindependent factor (r � 0.94), with mini-mal cross-loading (Fig. 4A) (tables S1 andS2) (15). These findings indicate that thethree addiction-like behaviors are measuresof a single factor that may reflect compul-sive drug use.

Finally, complementary behavioral testswere performed in rats from a fourth ex-periment (n � 44). These studies (15) con-firmed that other dimensions previously re-

lated to vulnerability to drugs (19–24) donot explain individual differences in addic-tion-like behaviors. For example, rats withzero or three positive criteria did not differ(Fig. 4, B and C) (15) with respect tospontaneous motor activity (19–22) andanxiety-like behaviors (23, 24 ). Similarly,a higher sensitivity to the unconditionedeffects of the drug did not seem to beinvolved, because drug seeking persisted ina drug-free state (F1, 23 � 8.74, P � 0.005)(Fig. 4D) (15). In contrast, as predicted byDSM-IV criteria, addiction-like behaviorswere associated with difficulty in limitingdrug intake when access to the drug wasprolonged (F1, 23 � 4.4, P � 0.05) (Fig. 4E).

These experiments show that after aprolonged period of SA, addiction-like be-haviors can be found in rats. Although it isalways difficult to translate findings fromrats to humans, our data show strikingsimilarities between the two species. Somerats develop behaviors similar to the diag-nostic criteria for addiction described in theDSM-IV. Addiction-like behaviors are notpresent after a short period of SA butdevelop, as does addiction in humans, onlyafter a prolonged exposure to the drug.Furthermore, as do human addicts, ratsshowing an addiction-like behavior have a

13 38 32 35 52 0.2 0.4 0.8 1.67454

Act

ive

no

se-p

oke

s

% o

f b

asel

ine

infu

sio

ns

Bre

akin

g p

oin

t

Act

ive

no

se-p

oke

s

400

350

300

250

200

150

100

50

0

80

60

40

20

0

800

700

600

500

400

300

200

100

0

A B C250

225

200

175

150

125

100

75

50

25

0

D

Time Time Cocaine doses (mg/kg)Time(Days of self-administration)

Fig. 1. Development of addiction-likebehaviors over subsequent cocaineSA sessions in rats showing high (F,HRein) or low (E, LRein) cocaine-induced reinstatement after 5 days ofwithdrawal. (A) Persistence in drugseeking, as measured by number ofnose-pokes in the cocaine-associateddevice during the no-drug period. (B)Resistance to punishment, as mea-sured by change in the number ofcocaine self-infusions (expressed aspercentage of baseline SA) when co-caine delivery was associated with anelectric shock. (C) Motivation for the drug, as measured by the breakingpoint during a progressive-ratio schedule. (D) Drug-induced reinstatement,as measured by number of nose-pokes in the drug-associated device as a

function of the priming dose of cocaine. LRein and HRein contained the rats(n � 7 per group) with the lowest and highest reinstatement, respectively,induced by cocaine infusion at 1.6 mg/kg.

LRein HRein

Act

ive

no

se-p

oke

s

Act

ive

no

se-p

oke

s

Bre

akin

g p

oin

t

% o

f b

asel

ine

infu

sio

ns

300

250

200

150

100

50

0

Act

ive

no

se-p

oke

s

300

250

200

150

100

50

0

Cocaine doses (mg/kg)0.2 0.4 0.8 1.6

150

125

100

75

50

25

0LRein HRein LRein HRein

100

80

60

40

20

0LRein HRein

600

500

400

300

200

100

0

A B C D EFig. 2. Development of addiction-like behaviors over subsequent co-caine SA sessions in rats showinghigh (HRein) or low (LRein) cocaine-induced reinstatement after a 30-day withdrawal period. (A) Persis-tence in drug seeking, as measuredby number of nose-pokes in thecocaine-associated device duringthe no-drug period of the 54th SAsession. (B) Resistance to punish-ment, as measured by change in thenumber of cocaine self-infusions (expressed as percentage of baseline SA)when cocaine delivery was associated with an electric shock during the72nd SA session. (C) Motivation for the drug, as measured by the breakingpoint during the progressive-ratio schedule conducted during the 60th SAsession. (D) Drug-induced reinstatement, as measured by number ofnose-pokes in the drug-associated device as a function of the primingdose of cocaine. (E) Reinstatement induced by a conditioned stimulus

(CS), as measured by the number of nose-pokes in the drug-associateddevice when responding was associated with the contingent presentationof the CS. LRein and HRein contained the rats (n � 6 per group) with thelowest and highest reinstatement, respectively, induced by cocaine in-fusion at 1.6 mg/kg. Tests for cocaine- and CS-induced reinstatementswere performed after 30 and 32 days of withdrawal, respectively, usinga latin square design.

R E P O R T S

www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004 1015

high propensity to relapse even after a longperiod of withdrawal. Finally, the percent-age of rats (17%) that show a high score forall three addiction-like criteria is similar tothe percentage (15%) of human cocaineusers diagnosed as addicts (13).

It could seem surprising that the capacityof drugs of abuse to induce addiction-likebehavior exists across species. As alreadymentioned, however, voluntary intake ofdrugs abused by humans is present in sev-eral species (1–6 ). Drugs of abuse havereinforcing effects by activating endoge-nous reward systems that are similar indifferent species. Thus, the mechanismsmediating the neuroadaptations induced bychronic drug exposure and their behavioralconsequences (addiction) may be similar indifferent species. Indeed, preliminary re-sults show similar changes in brain activitybetween rats showing addiction-like behav-iors and human addicts (15).

Our results allow us to propose a unifiedvision of the origin of addiction that inte-grates the experimental and clinical perspec-tives. The major hypotheses driving experi-mental research consider the degree of drugexposure as the key factor leading to addic-tion (7–10, 25). By contrast, clinical visionsof drug abuse have been progressively shift-ing weight from the role of drug exposure tothe role of the higher vulnerability to drugs incertain individuals (26–30). Our data indicatethat addiction results from the interaction ofthese two variables: (i) the degree of expo-sure to drugs (because addiction-like behav-ior appears only after extended access tococaine), and (ii) the degree of vulnerabilityin the exposed individual (because, despite asimilar drug intake in all subjects, addiction-

0 1 2 3

Act

ive

no

se-p

oke

s

0

100

200

300

400

500

600

700

800

Number of positive criteria0 1 2 3

% o

f b

asel

ine

infu

sio

ns

0102030405060708090

100

0 1 2 3

Bre

akin

g p

oin

t

0

100

200

300

400

500

600

700

800

900A B C

41.4%

27.6%

13.8%

17.2%

D

0 criteria

1 criterion3 criteria

2 criteria

0 3

Ph

oto

cell

cou

nts

0

200

400

600

800

1000

1200

1400

1600

Sessions1 5 9 13 17 21 25 29 33 37 41 45 49 53 57 60

Co

cain

e (m

g/k

g)

0

5

10

15

20

25

30

35

40

45 E F

Number of positive criteria

0 criteria3 criteria

Fig. 3. (A to D) Addiction-like behaviors in ratspositive for the presence of zero, one, two, orthree addiction-like criteria. An individual wasconsidered positive for an addiction-like crite-rion when its score for one of the three addic-tion-like behaviors was in the 66th to 99thpercentile of the distribution. (A) Persistence indrug seeking, as measured by number of nose-pokes in the cocaine-associated device duringthe no-drug period of the 54th SA session. (B)Resistance to punishment, as measured bychange in the number of cocaine self-infusions(expressed as percentage of baseline SA) whencocaine delivery was associated with an electricshock between the 72nd and 74th SA sessions.(C) Motivation for the drug, as measured by thebreaking point during a progressive-ratioschedule performed between the 52nd and60th SA sessions. (D) Percentage of the totalpopulation (n � 58) of rats positive for zero,one, two, or three addiction-like criteria. (E andF) Drug intake and motor activity during base-line SA in rats positive for the presence of zeroor three addiction-like criteria. (E) Cocaine in-take per session during baseline SA sessions (every other session is represented). (F) Horizontal motor activity during SA, as measured by number ofphotocell beam breaks. Results are expressed as the mean over three baseline SA sessions (between sessions 49 and 59).

0 3

Ph

oto

cell

cou

nts

0

100

200

300

400

500

600

700

800

900 B

0 3

Act

ive

no

se-p

oke

s

0

50

100

150

200

250

300

350

400

450

D

Number of positive criteria

Factor 1 (52.1%)

Factor 2 (25.8%)

Extinction

Punishment

Motivation

Drug seeking

1.0

1.0-1.0

-1.0

A

Time (min)30 60 90 120 150 180 210 240 270 300

Cu

mu

late

d in

fusi

on

s

0

20

40

60

80

100 E

Number of positive criteria

0 criteria3 criteria

Number of positive criteria0 3

Tim

e (s

ec)

0

50

100

150

200

250 C Open armsClosed arms

Fig. 4. (A) Factor analysis of SAvariables. Two factors were ex-tracted; factor 1 is representedby the horizontal axis and fac-tor 2 by the vertical axis. Fac-tor 1 (compulsive drug intake)accounts for 52.1% of the totalvariance, factor 2 (extinction)for 25.8%. The locations of thevariables (F) correspond to thefollowing parameters: Persis-tence in drug seeking wasmeasured by the number ofnose-pokes in the cocaine-associated device during theno-drug period of the 54th SAsession. Resistance to punish-ment was measured by changein the number of cocaine self-infusions (expressed as per-centage of baseline SA) whencocaine delivery was associat-ed with an electric shock be-tween the 72nd and 74th SA ses-sions. Motivation for the drugwas measured by the breakingpoint during a progressive-ratioschedule performed betweenthe 52nd and 60th SA sessions.Extinction was measured by thenumber of active nose-pokesduring a 1-hour extinction ses-sion conducted between the60th and 63rd sessions. (B andC) Measures of other potentiallydrug-related behaviors in rats positive for the presence of zero or three addiction-like criteria.(B) Spontaneous horizontal motor activity, as measured by number of photocell beam breaksexhibited during a 2-hour exposure to a novel environment. (C) Anxiety-related behavior, asmeasured by the comparison of the time spent in the open versus the closed arms during a5-min exposure to an elevated plus-maze. (D and E) Measure of drug seeking in a drug-freestate and of drug taking during extended access to cocaine in rats positive for the presence ofzero or three addiction-like criteria. (D) Persistence in drug seeking in a drug-free state, asmeasured by the number of nose-pokes in the cocaine-associated device when a no-drug periodprecedes the SA session (mean of five consecutive tests performed between the 47th and 58th SAsessions). (E) SA during extended access to the drug. Cocaine was continuously accessible for 5hours, and SA was estimated by the cumulated number of self-infusions over time.

R E P O R T S

13 AUGUST 2004 VOL 305 SCIENCE www.sciencemag.org1016

like behavior appears only in a few). It is thusthe interaction between a long exposure todrug and a vulnerable phenotype, not one orthe other factor in itself, that seems to deter-mine the development of addiction.

References and Notes1. T. Kusayama, S. Watanabe, Neuroreport 11, 2511

(2000).2. C. I. Abramson et al., Alcohol. Clin. Exp. Res. 24, 1153

(2000).3. C. R. Schuster, T. Thompson, Annu. Rev. Pharmacol. 9,

483 (1969).4. R. Pickens, W. C. Harris, Psychopharmacologia 12,

158 (1968).5. J. R. Weeks, Science 138, 143 (1962).6. S. R. Goldberg, J. H. Woods, C. R. Schuster, Science

166, 1306 (1969).7. E. J. Nestler, G. K. Aghajanian, Science 278, 58 (1997).8. S. E. Hyman, R. C. Malenka, Nature Rev. Neurosci. 2,

695 (2001).9. G. F. Koob, M. Le Moal, Science 278, 52 (1997).

10. B. J. Everitt, M. E. Wolf, J. Neurosci. 22, 3312 (2002).11. R. A. Wise, Neuron 36, 229 (2002).

12. Diagnostic and Statistical Manual of Mental Disorders(American Psychiatric Association, Washington, DC,ed. 4, revised version, 2000).

13. J. C. Anthony et al., Exp. Clin. Psychopharmacol. 2,244 (1994).

14. W. DeJong, Int. J. Addict. 29, 681 (1994).15. See supporting data at Science Online.16. N. R. Richardson, D. C. Roberts, J. Neurosci. Methods

66, 1 (1996).17. Y. Shaham, U. Shalev, L. Lu, H. De Wit, J. Stewart,

Psychopharmacology 168, 3 (2003).18. Y. Shaham, S. Erb, J. Stewart, Brain Res. Rev. 33, 13

(2000).19. R. N. Cardinal, D. R. Pennicott, C. L. Sugathapala, T. W.

Robbins, B. J. Everitt, Science 292, 2499 (2001).20. R. Ito, T. W. Robbins, B. J. Everitt, Nature Neurosci. 7,

389 (2004).21. P. V. Piazza, J.-M. Deminiere, M. Le Moal, H. Simon,

Science 245, 1511 (1989).22. P. V. Piazza, V. Deroche-Gamonet, F. Rouge-Pont, M.

Le Moal, J. Neurosci. 20, 4226 (2000).23. J. R. Homberg et al., Eur. J. Neurosci. 15, 1542 (2002).24. R. Spanagel et al., Psychopharmacology 122, 369

(1995).25. T. E. Robinson, K. C. Berridge, Brain Res. Rev. 18, 247

(1993).

26. C. P. O’Brien, R. N. Ehrman, J. N. Terns, in BehavioralAnalysis of Drug Dependence, S. R. Goldberg, I. P.Stolerman, Eds. (Academic Press, New York, 1986),pp. 329–356.

27. H. de Wit, E. H. Uhlenhuth, C. E. Johanson, DrugAlcohol Depend. 16, 341 (1986).

28. M. A. Enoch, Am. J. Pharmacogenomics 3, 217 (2003).29. T. J. Crowley et al., Drug Alcohol Depend. 49, 225

(1998).30. D. M. Ferguson, L. J. Horwood, M. T. Lynskey, P. A.

Madden, Arch. Gen. Psychiatry 60, 1033 (2003).31. We thank E. Balado for precious technical help and

D. H. Epstein for insightful comments of this manu-script. Supported by INSERM, Bordeaux Institute forNeurosciences (IFR8), University Victor Segalen–Bordeaux 2, and Region Aquitaine.

Supporting Online Materialwww.sciencemag.org/cgi/content/full/305/5686/1014/DC1Materials and MethodsSOM TextTables S1 and S2

12 April 2004; accepted 7 July 2004

Drug Seeking BecomesCompulsive After ProlongedCocaine Self-AdministrationLouk J. M. J. Vanderschuren*† and Barry J. Everitt

Compulsive drug use in the face of adverse consequences is a hallmark featureof addiction, yet there is little preclinical evidence demonstrating the actualprogression from casual to compulsive drug use. Presentation of an aversiveconditioned stimulus suppressed drug seeking in rats with limited cocaineself-administration experience, but no longer did so after an extended cocaine-taking history. In contrast, after equivalent extended sucrose experience, su-crose seeking was still suppressed by an aversive conditioned stimulus. Per-sistent cocaine seeking in the presence of signals of environmental adversityafter a prolonged cocaine-taking history was not due to impaired fear condi-tioning, nor to an increase in the incentive value of cocaine, and may reflectthe establishment of compulsive behavior.

Compulsive drug seeking and drug takingdistinguishes drug addicts from casual drugusers. Addicts display drug-dominated, in-flexible behavior and are unable to shift theirthoughts and behavior away from drugs anddrug-related activities. Even with awarenessof the deleterious consequences of this drug-centered behavior, addicts have enormousdifficulty in abstaining from drug seeking anduse (1, 2). Several hypotheses try to explainthe occurrence of compulsive drug use; itmay reflect the establishment of an automaticstimulus-response habit (3, 4), drug-inducedloss of impulse control (5), sensitization of an

incentive (“wanting”) system (6), or disrup-tion of hedonic homeostasis (7). Remarkably,there is little evidence from animal studiesdemonstrating the actual progression fromcasual to compulsive drug use, although drugintake in rats escalates after weeks of pro-longed drug self-administration (8). Model-ing compulsive drug seeking in animalswould clarify our understanding of the neu-ropsychology of drug addiction and may alsolead to the development of novel treatments.

Here, we tested the hypothesis that anextended drug-taking history renders drugseeking impervious to environmental adver-sity (such as signals of punishment), captur-ing one element of its compulsive nature (1).Appetitive behavior for natural and drug re-wards is readily suppressed by aversive en-vironmental stimuli or outcomes, a phenom-enon termed conditioned suppression (9–11).We investigated whether the ability of afootshock-paired conditioned stimulus (CS)

to suppress cocaine-seeking behavior dimin-ishes after a prolonged cocaine-taking historyand whether this reduced susceptibility toconditioned suppression also followed a sim-ilarly prolonged history of seeking sucrose, ahigh-incentive natural reinforcer.

In experiment 1, 21 rats were trained toself-administer cocaine under a heterogeneousseeking-taking chain schedule (12), in whichdrug seeking and taking are separate acts (13).Thus, meeting a response requirement on onelever (the seeking lever) in an operant chambernever resulted in drug, but instead gave accessto a second lever (the taking lever), respondingon which resulted in an intravenous infusion ofcocaine. Immediately after the rats reachedtraining criterion on this schedule—that is, aftera limited cocaine-taking history—12 rats re-ceived tone-footshock pairings (the CS-shockgroup), whereas the other nine rats receivedpresentations of the same tone not paired withfootshock (the control group). Several days lat-er, conditioned suppression of drug seeking wasassessed in a session in which the rats hadaccess to the seeking lever only and the foot-shock CS was presented for three 2-min periodsinterspersed with three 2-min periods when noCS was presented (13). The CS-shock groupshowed a profound conditioned suppression ofdrug seeking during presentation of the CS[F(CS) � 5.32, P � 0.05; F(CS � group) �25.65, P � 0.001] (Fig. 1A). There was also amarked increase in the time taken to make thefirst seeking response (seeking latency) in theCS-shock group [F(group) � 7.43, P � 0.01](Fig. 1B). Thus, in rats with limited cocaineself-administration experience, drug seekingwas greatly suppressed by presentation of anaversive CS, showing that it was sensitive to anadverse outcome.

Next, we tested whether cocaine seekingwould become less susceptible to an aversiveCS after prolonged cocaine self-administration

Department of Experimental Psychology, Universityof Cambridge, Cambridge CB2 3EB, UK.

*Present address: Rudolf Magnus Institute of Neuro-science, Department of Pharmacology and Anatomy,University Medical Center Utrecht, 3584 CG Utrecht,Netherlands.†To whom correspondence should be addressed. E-mail: l.j.m.j.vanderschuren@med.uu.nl

R E P O R T S

www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004 1017

experience. The same 21 rats were thereforeallowed a further 20 cocaine self-administrationsessions. These included eight “extended-ac-cess” sessions in which the rats could respondfor cocaine under a simple continuous rein-forcement (FR1) schedule for a maximum of 80

infusions, thereby greatly increasing the extentof cocaine-taking experience and cocaine expo-sure. Subsequently, the rats were recondi-tioned (CS-shock pairings) and tested un-der circumstances identical to those in theprevious phase of the experiment. Rats with

an extended cocaine-taking history showedvirtually no conditioned suppression ofdrug seeking [F(CS) � 0.47, not significant(n.s.); F(CS � group) � 1.55, n.s.] (Fig.1C), although the response latency in theCS-shock group was still somewhat in-creased [F(group) � 8.39, P � 0.01](Fig. 1D).

An advantageous characteristic of the seek-ing-taking chain schedule used is that the rate ofresponding on the seeking lever is a function ofreinforcer magnitude. Thus, rats responding forhigher unit doses of cocaine or higher concen-trations of sucrose show increased rates of re-sponding on the seeking lever (12); seeking ratecan therefore be used as a measure of theincentive value of the reinforcer. A plausibleexplanation for the reduced susceptibility ofcocaine seeking to presentation of the footshockCS is that the incentive value of cocaine in-creases after prolonged cocaine exposure.Therefore, rats may have been less prepared toreduce their cocaine-seeking rates when facedwith signals of an adverse environmental eventbecause cocaine had become a more valuablecommodity. To test this possibility, we com-pared cocaine-seeking rates after limited andextended cocaine exposure but before CS-shock conditioning. Remarkably, the seekingrates were not different under these conditions;that is, they were not affected by the amountand duration of cocaine exposure [F(experi-ment phase) � 0.01, n.s.] (Fig. 1E), suggestingthat the incentive value of cocaine had notchanged over the course of the extended self-administration history.

In experiment 2, we aimed to exclude thepossibility that different degrees of between-session extinction of the footshock CS ac-

Fig. 1. Presentation of an aversive CS suppresses cocaine seeking after limited (A and B) but notprolonged (C and D) cocaine self-administration, independent of changes in the incentive value ofcocaine (E). (A) Mean (� SEM) cocaine-seeking responses per 2-min interval in the CS-shock andcontrol groups after limited cocaine exposure, with the aversive CS on or off during alternating2-min periods. **P � 0.01 (Student-Newman-Keuls). (B) Latency to make the first seeking responseduring the test for conditioned suppression of cocaine seeking in the CS-shock and control groupsafter limited cocaine exposure. **P � 0.01 [analysis of variance (ANOVA)]. (C) Mean (� SEM)cocaine-seeking responses per 2-min interval in the CS-shock and control groups after extendedcocaine exposure, with the aversive CS on or off during alternating 2-min periods. (D) Seekinglatency during the test for conditioned suppression of cocaine seeking in the CS-shock and controlgroups after extended cocaine exposure. **P � 0.01 (ANOVA). (E) Mean (� SEM) number ofcocaine-seeking responses per seeking-taking chain cycle after limited and extended cocaineself-administration.

0 on off on off on off

Time block (2 min)

05

101520253035

No.

of r

espo

nses

control CS-shock

A

CS 0

30

60

90

120

150

Late

ncy

(sec

)

control CS-shock

B

0

5

10

15

20

25

30

No.

of

resp

onse

s pe

r cy

cle

C

Limited cocaine

Extended cocaine

0 on off on off on off

Time block (2 min)

0

10

20

30

40

No.

of r

espo

nses

control CS-shock

D

CS 0

30

60

90

120

150

Late

ncy

(sec

)

control CS-shock

E

***

**

pre-CS CS0

20

40

60

80

% F

reez

ing

limitedcocaine

extendedcocaine

extendedsucrose

F

Fig. 2. Presentation of an aversive CS doesnot suppress cocaine seeking after prolongedcocaine self-administration (A and B), inde-pendent of changes in the incentive value ofcocaine (C). Presentation of an aversive CSsuppresses sucrose seeking after prolongedsucrose self-administration (D and E). Thedifferences in conditioned suppression werenot the result of differences in conditionedfear (F). (A) Mean (� SEM) cocaine-seekingresponses per 2-min interval in the CS-shockand control groups after extended cocaineexposure, with the aversive CS on or offduring alternating 2-min periods. (B) Seekinglatency during the test for conditioned sup-pression of cocaine seeking in the CS-shockand control groups after extended cocaineexposure. (C) Mean (� SEM) number of co-caine-seeking responses per seeking-takingchain cycle after limited and extended co-caine self-administration. (D) Mean (� SEM)sucrose-seeking responses per 2-min intervalin the CS-shock and control groups afterextended sucrose exposure, with the aver-sive CS on or off during alternating 2-min periods. **P � 0.01 (Student-Newman-Keuls). (E) Seeking latency during the test for conditioned suppres-sion of sucrose seeking in the CS-shock and control groups after extendedsucrose exposure. ***P � 0.001 (ANOVA). (F) Conditioned freezing to a

footshock CS in rats with limited cocaine, extended cocaine, and extendedsucrose self-administration experience. Percentage of time spent freezingwas scored for 2 min before (pre-CS, left panel) and during 2 min ofpresentation of the CS (right panel).

R E P O R T S

13 AUGUST 2004 VOL 305 SCIENCE www.sciencemag.org1018

counted for the diminished conditioned sup-pression seen after extended, as compared tolimited, cocaine exposure—that is, that ratshad learned during the suppression tests,when the CS was repeatedly presented, that itno longer predicted footshock when subse-quently presented in the self-administrationenvironment. Therefore, rats (CS-shockgroup, n � 11; control group, n � 12) weretrained to self-administer cocaine under con-ditions identical to those in experiment 1.However, the first suppression test was omit-ted, so that rats were conditioned and testedonly after extended cocaine exposure, includ-ing eight extended-access sessions (13). Un-like the limited-exposure rats, and identical tothe extended-exposure rats in experiment 1,cocaine seeking in the CS-shock group wasnot suppressed at all during presentation ofthe footshock CS [F(CS) � 2.53, n.s.;F(CS � group) � 4.40, P � 0.05] (Fig. 2A).Moreover, the seeking latency in the CS-shock group was no different from that in thecontrol group [F(group) � 2.15, n.s.] (Fig.2B). Consistent with experiment 1, there wasalso no change in the incentive value ofcocaine, as assessed by the rates of seekingbefore and after the eight extended-accesssessions [F(experiment phase) � 0.08, n.s.](Fig. 2C). Thus, cocaine seeking is greatlysuppressed by a footshock CS, but only inrats with a limited cocaine-taking history.This indicates that the flexibility of drugseeking strongly depends on the extent ofdrug self-administration experience.

Conditioned suppression of appetitive be-havior has been readily observed in a variety ofsettings and is commonly used as an index ofconditioned fear in animals responding for nat-ural reinforcers, such as food (9, 10). However,it is not known whether food seeking can alsobecome insensitive to the suppressive effects ofan aversive CS after prolonged experience. Inexperiment 3, we therefore replicated experi-ment 2 with rats trained to seek and ingestsucrose (13). Rats (CS-shock group, n � 11;control group, n � 11) were trained to respondfor a sucrose solution under the same seeking-taking chain schedule. To make an optimalcomparison between cocaine and sucrose self-administration, we chose a unit amount andconcentration of sucrose that led to rates ofresponding on the seeking lever comparable tothose in experiment 2 (experiment 3 versusexperiment 2: sucrose seeking, 13.9 � 1.8responses/min; cocaine, 12.6 � 1.7 responses/min). In addition, the sucrose-trained rats weretrained for a comparable number of sessionsand received a comparable number of totalreinforcer presentations before assessing condi-tioned suppression (total number of sucrosereinforcers in experiment 3: 1148 � 29; totalnumber of cocaine reinforcers in experiment 2:1110 � 14). After extended experience ofsucrose seeking, profound conditioned suppres-

sion during presentation of the footshock CSwas still observed [F(CS) � 5.31, P � 0.05;F(CS � group) � 8.35, P � 0.01] (Fig. 2D),together with a marked increase in the seekinglatency in the CS-shock group [F(group) �17.27, P � 0.001] (Fig. 2E). Thus, lengthytraining under this seeking-taking chainschedule does not itself result in diminishedsensitivity of appetitive behavior to presen-tation of an aversive CS. Rather, these datasuggest that the nature of the reinforcer(drug versus natural) determines whethercompulsive behavior resistant to adverseenvironmental events will develop aftersimilarly prolonged periods of self-administration (14, 15).

It is important to exclude the possibilitythat the failure to observe conditioned sup-pression in the extended cocaine exposuregroup reflected weaker fear conditioning.Therefore, the long-term sucrose-trained ratsfrom experiment 3, the long-term cocaine-trained rats from experiment 2, and a newgroup of rats with limited cocaine exposure(as in experiment 1) all underwent fear con-ditioning and were tested for conditionedfreezing to a discrete auditory (clicker) CS (9,13, 16). Twenty-four hours after condition-ing, the rats were placed in the training con-text and after 2 min, the clicker CS wasplayed for 2 min (13). Rats in all three groupsexhibited profound freezing during the CS[F(CS) � 552.2, P � 0.01], and there wereno differences in fear behavior among thethree groups during the pre-CS or CS periods[F(CS � group) � 0.77, n.s.; pre-CS freez-ing: F(group) � 0.003, n.s.; CS freezing:F(group) � 0.82, n.s.] (Fig. 2F). Thus, thedifferences in conditioned suppression cannotbe attributed to altered pain sensitivity or aninability to encode or express a CS-footshockassociation after extended cocaine exposure.Conditioned suppression and conditionedfreezing are well known to be highly corre-lated (9), but this correlation between freez-ing and the suppression of cocaine-seekingbehavior was lost in rats with a prolongedcocaine-taking history because they were stillfearful, yet their appetitive behavior was notaffected by an aversive CS during the unflag-ging pursuit of drug.

Dysfunction of prefrontal cortical-striatalsystems is likely to underlie loss of controlover drug use. These systems subserve thecoordination of goal-directed and habitual ap-petitive behavior (17, 18) and have been im-plicated in both obsessive-compulsive disor-der (19) and drug addiction (20). Indeed,animal studies suggest a critical role for theprefrontal cortex in drug seeking (21). More-over, functional neuroimaging studies in hu-man drug addicts have consistently shownactivation of the orbitofrontal and dorsolater-al prefrontal cortex during cocaine craving(20), and cocaine addicts are impaired in

cognitive and decision-making abilities thatdepend on the orbital and other prefrontalcortical areas (22).

Our results show that cocaine seekingcan be suppressed by presentation of anaversive CS, but after extended exposure toself-administered cocaine, drug seekingbecomes impervious to adversity. Interest-ingly, inflexible drug seeking appears todevelop with prolonged drug-taking expe-rience independently of alterations in theincentive value of cocaine. The attenuatedconditioned suppression of seeking doesnot occur after identically prolonged expo-sure to sucrose, which suggests that appet-itive behavior may more readily becomeresistant to aversive environmental eventswhen directed toward obtaining drugs rath-er than natural reinforcers. We thereforeconclude that a prolonged cocaine self-ad-ministration history endows drug seekingwith an inflexible, compulsive dimension.

References and Notes1. Diagnostic and Statistical Manual of Mental Disorders

(American Psychiatric Association, Washington, DC,ed. 4, 1994).

2. C. P. O’Brien, A. T. McLellan, Lancet 347, 237 (1996).3. S. T. Tiffany, Psychol. Rev. 97, 147 (1990).4. B. J. Everitt, A. Dickinson, T. W. Robbins, Brain Res.

Rev. 36, 129 (2001).5. J. D. Jentsch, J. R. Taylor, Psychopharmacology 146,

373 (1999).6. T. E. Robinson, K. C. Berridge, Brain Res. Rev. 18, 247

(1993).7. G. F. Koob, M. Le Moal, Science 278, 52 (1997).8. S. H. Ahmed, G. F. Koob, Science 282, 298 (1998).9. M. E. Bouton, R. C. Bolles, Anim. Learn. Behav. 8, 429

(1980).10. S. Killcross, T. W. Robbins, B. J. Everitt, Nature 388,

377 (1997).11. D. N. Kearns, S. J. Weiss, L. V. Panlilio, Drug Alcohol

Depend. 65, 253 (2002).12. M. C. Olmstead et al., Psychopharmacology 152, 123

(2000).13. See supporting data at Science Online.14. J. D. Berke, S. E. Hyman, Neuron 25, 515 (2000).15. E. J. Nestler, Nature Rev. Neurosci. 2, 119 (2001).16. J. E. LeDoux, A. Sakaguchi, D. J. Reis, J. Neurosci. 4,

683 (1984).17. S. Killcross, E. Coutureau, Cereb. Cortex 13, 400 (2003).18. H. H. Yin, B. J. Knowlton, B. W. Balleine, Eur. J. Neu-

rosci. 19, 181 (2004).19. A. M. Graybiel, S. L. Rauch, Neuron 28, 343 (2000).20. R. Z. Goldstein, N. D. Volkow, Am. J. Psychiatry 159,

1642 (2002).21. P. W. Kalivas, K. McFarland, Psychopharmacology

168, 44 (2003).22. R. D. Rogers, T. W. Robbins, Curr. Opin. Neurobiol.

11, 250 (2001).23. This research was funded by an MRC Programme

Grant and was conducted within the Cambridge MRCCentre for Clinical and Behavioral Neuroscience.L.J.M.J.V. was a visiting scientist from the ResearchInstitute Neurosciences VU, Department of MedicalPharmacology, VU Medical Center, Amsterdam, sup-ported by a Wellcome Trust Travelling Research Fel-lowship. We thank P. Di Ciano and J. L. C. Lee forpractical assistance and help with the design of theexperiments and R. N. Cardinal for additional soft-ware programming.

Supporting Online Materialwww.sciencemag.org/cgi/content/full/305/5686/1017/DC1Materials and MethodsReferences

9 April 2004; accepted 2 July 2004

R E P O R T S

www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004 1019

Visual Pattern Recognition inDrosophila Is Invariant for

Retinal PositionShiming Tang,1*† Reinhard Wolf,2* Shuping Xu,1

Martin Heisenberg2†

Vision relies on constancy mechanisms. Yet, these are little understood, becausethey are difficult to investigate in freely moving organisms. One such mech-anism, translation invariance, enables organisms to recognize visual patternsindependent of the region of their visual field where they had originally seenthem. Tethered flies (Drosophila melanogaster) in a flight simulator can rec-ognize visual patterns. Because their eyes are fixed in space and patterns canbe displayed in defined parts of their visual field, they can be tested fortranslation invariance. Here, we show that flies recognize patterns at retinalpositions where the patterns had not been presented before.

In the flight simulator (Fig. 1A), the fly’s(Drosophila melanogaster) head and thoraxand, hence, its eyes are fixed in space whileits yaw torque can still control the angularvelocity of a panorama surrounding it (1). Ifthe panorama displays different patterns thefly can be trained to discriminate them (Fig.1B) (2). The flight simulator lends itself toan investigation of translation invariance aspatterns rotate around the fly at a fixedheight and can be vertically displaced be-tween training and test. To our surprise,flies failed in such tests to recognize pat-terns shifted up or down by 9° or more aftertraining. Pattern recognition seemed to re-quire the same retinal coordinates for ac-quisition and retrieval (3–5). This findingwas in line with earlier experiments in ants,which had failed to show interocular trans-fer for landmark recognition (6 ).

Subsequent studies (7–9) identified someof the pattern parameters (features) the fliesused for discrimination. These were size,color, vertical compactness, and vertical po-sition of the centers of gravity (COGs) of thepatterns in the panorama. For many patternpairs carrying none of these features, noconditioned discrimination could be detected,although flies often discriminated themspontaneously (7). In the earlier quest fortranslation invariance (3–5), flies had beenconditioned to discriminate patterns solely bythe vertical position of their COGs. For in-stance, if in Fig. 1A the vertical positions ofthe COGs of upright and inverted Ts werealigned, flies were unable to discriminate

them after conditioning [shown for trianglesin ref. (7) and discussed in Supplement]. Thisraised the possibility that perhaps verticaldisplacement specifically interfered with thefeature “vertical position” (10, 11).

Only two of the four parameters, sizeand color, are independent of the verticalposition of the pattern elements in the are-na. These, therefore, were chosen in thepresent study. To test for conditioned dis-crimination of (horizontal) size (Fig. 2A,left dotted bars), we presented two blackrectangles of the same height but differingby about a factor of two in width in neigh-boring quadrants. They were all shown atthe same vertical position in the arenaslightly above the fly’s horizon. Flies readi-ly learned to avoid the larger or smallerfigure after the training (P � 0.001). Next,these patterns were vertically displaced be-tween training and test (12). In contrast tothe earlier experiments with patterns differ-ing in the vertical position of their COGs,no decrement of the memory score wasobserved after a vertical displacement of�H � 20° (Fig. 2A, right cross-hatchedbars). In the same way, we tested color.Flies remembered blue and green rectan-gles of the same size and presented at thesame vertical position (8, 9). They had nodifficulty recognizing them after verticaldisplacement at the new position (Fig. 2B).

Edge orientation is a feature that hasbeen extensively documented in the honey-bee (13–17 ) to serve in conditioned patterndiscrimination. We found a robust condi-tioned preference for bars tilted �45° and– 45° to the vertical [Fig. 2C, left dottedbars; (18); but see (7 )]. Vertical displace-ment of the bars after training had no sig-nificant effect on the memory score [Fig.2C, right cross-hatched bars; (19)].

Next, more complex patterns were test-ed. The rectangles in the four quadrants in

Fig. 2D were each composed of a blue anda green horizontal bar. They differed onlyin whether green was above blue or blueabove green. In principle, flies had twooptions to discriminate the two figures.They could combine the two features ver-tical position and color to give a new fea-ture with relational cues (e.g., “green aboveblue”). Alternatively, they could evaluatethe two colors separately and remember foreach whether the high or low rectangleswere safe or dangerous. This would havedifferent consequences in the transfer ex-periment. If the colors were processed sep-arately, flies would have to rely on verticalpositions and could recognize neither thegreen nor the blue patterns at the newretinal positions. For composite figures,however, the vertical positions would betransformed into relational cues, whichmight still be recognized after vertical dis-placement. The latter was observed (Fig.2D, right cross-hatched bars). Apparently,within each rectangle, flies evaluated thepositions of the colored pattern elements

1Institute of Biophysics Academia Sinica, 15 Datun Road,Chaoyang, Beijing 100101, P.R. China. 2Lehrstuhl furGenetik und Neurobiologie, Universitat Wurzburg,Biozentrum (Am Hubland), 97074 Wurzburg, Germany.

*These authors contributed equally to this work.†To whom correspondence should be addressed. E-mail:tang-shm@sohu.com, heisenberg@biozentrum.uni-wuerzburg.de

Fig. 1. Visual pattern discrimination learning inthe flight simulator. Angular velocity of an ar-tificial visual panorama is made negatively pro-portional to the fly’s yaw torque [gray arrowsin (A)], allowing the fly to change its flightdirection (yaw torque � 0), or to maintainstable orientation (yaw torque � 0) with re-spect to visual landmarks in the panorama (up-right and upside-down T-shaped black pat-terns). During training, a heat beam (not shownin figure), directed to the fly’s thorax and headfrom behind, is switched on or off at theboundaries between quadrants containing theone or the other pattern type in their center.(B) Standard learning experiment. Performanceindex (PI) is calculated as PI � (tc – th)/(tc � th),where tc is the fraction of time with heat offand th the remaining time with heat on in a2-min interval. The arena is rotated to a ran-dom angular position at the beginning of each2-min interval. Empty bars indicate test inter-vals without any heat; hatch bars denote train-ing intervals. PI 8 and PI 9 (dotted bars) quan-tify the flies’ conclusive pattern memory. Errorbars are SEMs.

R E P O R T S

13 AUGUST 2004 VOL 305 SCIENCE www.sciencemag.org1020

relative to each other and directed theirflight with respect to these cues.

Similar relational cues were effective inFig. 2E. Each of the alternative figures con-sisted of two orthogonally oriented obliquebars, one above the other. In the two figuresthe two orientations were exchanged. In one,the top bar was tilted by �45°, in the other by–45°. If the flies relied on the integral of theorientations of all edges in each compositefigure they would not be able to discriminatethe two. Flies were able to evaluate the spatialrelations in the composite figures. Even forthese complex patterns, translation invariancefor vertical displacement was found. Whenthe two patterns were rotated by 90°, whichplaced the two bars side-by-side at the samevertical position, no conditioned discrimina-tion was obtained (Fig. 2F).

The only patterns Drosophila failed torecognize after vertical displacement werethose that it can discriminate only by theirvertical position. This suggests a special in-terference between the feature vertical posi-tion and the vertical displacement. Horizontaldisplacement of these patterns cannot be test-ed in the flight simulator, because horizontalmotion is controlled by the fly, which has tochoose a certain azimuth relative to the land-marks for its direction of flight. We thereforedeveloped an alternative paradigm to investi-gate visual pattern recognition and, in partic-ular, horizontal translation invariance. Flieswere conditioned at the torque meter by heatto restrict their yaw torque range to only left

or only right turns (2). Two patterns weredisplayed at stable retinal positions duringtraining (Fig. 3), for instance at �45° and–45° from the frontal direction. Betweentraining and memory test the patterns wereexchanged. Flies shifted their restricted yawtorque range to the other side (i.e. from leftturns to right turns or vice versa) (Fig. 3A).

When patterns were shifted to a new po-sition on the same side (from �30° to �80°or vice versa), the yaw torque bias stayed onthe side of the yaw torque range to which ithad been confined during training (Fig. 3B).Obviously, flies recognized the patterns at thenew retinal positions after horizontal dis-placement. The T-shaped patterns used in thisexperiment could be discriminated only bythe vertical positions of their COGs, and theywere the very patterns for which no transla-tion invariance had been found in the flightsimulator after vertical displacement.

All five pattern parameters tested (size,color, edge orientation, relational cues, verti-cal position) showed visual pattern recogni-tion in Drosophila to be translation invariant.Vertical position was the only parameter thatthe flies could not recognize after verticaldisplacement, but they did recognize this pa-rameter after a horizontal shift.

Little is known so far about translationinvariance in flies. In the present study, it hasbeen demonstrated for horizontal displace-ments between �45° and –45° from the fron-tal direction. These positions are well outsidethe region of binocular overlap (20). Hence,

the pattern information generalized for posi-tion must be made available to both brainhemispheres (interocular transfer).

Our yaw torque learning paradigm revealsintriguing properties of visual processing.First, it shows that visual motion is not aprerequisite for pattern recognition. Flieswith their eyes fixed in space can recognizestationary visual objects (21). No motion isrequired for the perceptual process. Althoughthe fly can still move the optical axes of itsphotoreceptors by a few degrees (22), this istoo little to generate directional motion.Moreover, flies can recognize visual patternsin the flight simulator if during acquisitionthese are kept stationary (23). In the presentexperiment, the patterns were stationary even

Fig. 2. Pattern discrimination learning and retinal transfer with vertical displacements for variousparameters. Flies are trained with different pairs of patterns following the protocol of the standardlearning experiment (see Fig.1). Bar graphs: (dotted) pattern memory tested without verticaldisplacement; (cross-hatched) the complete panorama was shifted upward by 20° after the lasttraining block (at t � 14 min). Bars are averaged means of PI 8 and PI 9. Error bars are SEMs.***P � 0.001; **P � 0.01 (From a one-sample t test, 2-tailed P value).

Fig. 3. Yaw torque conditioning with fixed vi-sual patterns. Two stationary patterns are pre-sented to the fly at �45° and –45° from thefrontal direction. The fly’s yaw torque is record-ed and its range is divided in two domainsroughly corresponding to intended left andright turns. If torque is to the right, heat isswitched on, if torque is to the left, heat isswitched off. Flies can learn to keep theirtorque persistently in the safe domain (for thisexperiment without visual patterns see ref. (2).The first test between the two training blocks iscarried out with patterns in the training posi-tions. The positive PI indicates that the fliescontinue to direct their yaw torque to the“safe” side. (A) The two patterns are exchangedafter a second training period with the arenalight switched off during the shift. Negative PIafter the second training indicates that fliesrecognize the patterns in their new positions.Higher time resolution of PIs shows gradualtransition from positive to negative PIs (inset incircle), which indicates different dynamics ofvisual and motor memory. (B) As a control,patterns were shifted within the same visualhalf-field (from � � �30° to � � �80° or viceversa). ***P � 0.001; **P � 0.01 (From aone-sample t test, two-tailed P value).

R E P O R T S

www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004 1021

during retrieval. The fly’s turning tendencyindicated that it recognized the patterns.

Second, during intended turns to one sideflies selectively followed the directional mo-tion cues of landmarks on that side and ne-glected the symmetrical motion cues of acorresponding landmark on the other side(24). From the present experiment, we candeduce that the fly associated the heat withthe pattern to which it tried to turn whilebeing heated, and the no-heat condition withthe pattern to which it tried to turn whileflying in the cold. Because the flies wereexposed to the two patterns in equivalentretinal positions, they must be able to activatea gating process for a part of the visual arrayin the optic lobes corresponding to one or theother of the visual half-fields. Studies ofwalking flies have provided similar phenom-ena (25). The ability to confine visual pro-cessing to a visual field region of choice iscalled selective visual attention (26).

Selective attention may be relevant alsofor the flight simulator experiment. For trans-lation invariance in the flight simulator, thefly has to store not only a feature of a patternfor recognition (“what”) but also an azimuthvalue for orientation (“where”). We proposethat, while being heated, the fly associatesthe pattern that happens to be in the win-dow of attention with the heat. In the flightsimulator, the fly most of the time keeps thewindow of attention in a frontal position(27 ). In this way, the pattern would belabeled “dangerous if approached.”

Third, the flies in the yaw torque learn-ing paradigm not only associated heat withthe turning tendency to one side but alsowith the pattern on the side to which theytried to turn. In the memory test after thepatterns had been exchanged, the fly invert-ed its turning tendency. The preference forthe previously “safe” torque domain quick-ly fades, whereas the pattern preference,expressed by the fly’s yaw torque to theside of the attractive pattern, persists. If, forinstance, the fly expects yaw torque to theleft to entail heat but suddenly finds on thatside the previously safe pattern, it overridesits negative predisposition for left turns andtries to turn into that direction. The fadingof the behavioral memory component hasalso been reported for 3-way associationstesting colors instead of patterns (23).

Feature detectors for edge orientationare a hallmark of mammalian visual sys-

tems (28) and have also extensively beenstudied in the honeybee (13–17 ). Liketranslation invariance, edge orientation is afurther basic property that is shared be-tween the visual systems of Drosophila andlarger animals. Finally, flies also evaluaterelational cues such as {A above B} versus{B above A}. So far this fascinating abilityhas been demonstrated only for two colors(blue and green) and two edge orientations(�45° and – 45°). The negative outcome ofthe experiment with two horizontally ar-ranged oblique bars in the flight simulatorcannot be generalized. As mentionedabove, the exclusively horizontal motion inthe flight simulator may specifically inter-fere with this arrangement. Also horizontalcompactness is not a discriminating param-eter (7 ), although a grouping effect forvertical bars can be observed in fixation(27 ). In any case, the discovery of relation-al cues seems to vastly increase the poten-tial number of pattern parameters the flymight be able to discriminate.

Our data suggest a basic scheme (a mini-mal circuit) for translation invariance. Asmentioned above (10, 11), the experimentalparadigms conceptually demand a distinctionbetween orientation and recognition, i.e., awhere and what network (28, 29). Both net-works must have a centripetal (afferent) and acentrifugal (efferent) branch. The model isoutlined in the supplement (fig. S1).

References and Notes1. M. Heisenberg, R. Wolf, J. Comp. Physiol. A Sens.

Neural Behav. Physiol. 130, 113 (1979).2. R. Wolf, M. Heisenberg, J. Comp. Physiol. A Sens.

Neural Behav. Physiol. 169, 699 (1991).3. M. Dill, R. Wolf, M. Heisenberg, Nature 365, 751

(1993).4. M. Dill, M. Heisenberg, Philos. Trans. R. Soc. London B

Biol. Sci. 349, 143 (1995).5. M. Dill, R. Wolf, M. Heisenberg, Learn. Mem. 2, 152

(1995).6. R. Wehner, M. Muller, Nature 315, 228 (1985).7. R. Ernst, M. Heisenberg, Vision Res. 39, 3920 (1999).8. S. M. Tang, A. K. Guo, Science 294, 1543 (2001).9. Note that flies discriminated the rectangles by hue

rather than brightness, because varying the relativeintensities of the two colors by a factor of 10 be-tween training and test has no significant effect onmemory performance.

10. M. Heisenberg, Curr. Opin. Neurobiol. 5, 475 (1995).11. In the flight simulator, an orientation task is used to

measure pattern recognition and translation invariance.In order to retrieve a particular flight direction relativeto the panorama in the memory test, the fly has tostore the respective pattern (feature) during the trainingnot only for recognition but also by an azimuth valuefor orientation (e.g., direction of flight).

12. In previous experiments (3, 6), the transparency inthe arena carrying the figures had been exchanged

between training and test. This procedure required 30to 60 s. Also, for half of the flies, the pattern changedfrom a lower to a higher position; for the other half,the sequence was the opposite. In the present exper-iments, the whole arena was shifted after the finaltraining and the shift was always 20° upward. Thenew procedure implied that, not only the figures, butalso the upper and lower margins of the arena weredisplaced, but the shift took only an instant and didnot entail any visual disturbances from handling.Control experiments showed the same basic resultswith the old and new procedures. For instance, fliestrained with horizontal bars at different heights(�H � 20°) to avoid certain flight directions wereunable to retrieve this information if, after the train-ing, the whole arena was shifted upward by 20° (30),this inability confirmed that the flies’ pattern recog-nition system does not tolerate the vertical displace-ment if vertical position is the discriminating patternparameter.

13. R. Wehner, Nature 215, 1244 (1967).14. R. Wehner, M. Lindauer, Z. Vgl. Physiol. 52, 290

(1966).15. J. H. van Hateren, M. V. Srinivasan, P. B. Wait,

J. Comp. Physiol. A Sens. Neural Behav. Physiol. 167,649 (1990).

16. M. V. Srinivasan, S.W. Zhang, K. Witney, Philos. Trans.R. Soc. London B Biol. Sci.343, 199 (1994).

17. A. Horridge, J. Insect. Physiol. 44, 343 (1998).18. M. Dill, thesis, University of Wurzburg (1992).19. As pattern recognition in honey bees is studied with

freely moving animals, a formal test for translationinvariance has not been possible. Nevertheless, reti-notopic template-matching can be excluded as thesole mechanism (31). In particular, the data clearlyindicate that the orientation of edges can be recog-nized independent of their precise location on thevisual array (32).

20. E. Buchner, thesis, University of Tubingen (1971)21. B. Bausenwein, R. Wolf, M. Heisenberg, J. Neuro-

genet. 3, 87 (1985).22. R. Hengstenberg, Kybernetik 9, 56 (1971).23. M. Heisenberg, R. Wolf, B. Brembs, Learn. Mem. 8, 1

(2001).24. R. Wolf, M. Heisenberg, J. Comp. Physiol. A Sens.

Neural Behav. Physiol. 140, 69 (1980).25. S. Schuster, thesis, University of Tubingen (1996).26. M. I. Posner, C. R. R. Snyder, B. J. Davidson, J. Exp.

Psychol. Genet. 109, 160 (1980).27. M. Heisenberg, R. Wolf, Vision in Drosophila: Genetics

of Microbehavior, V. Braitenberg, Ed. (Studies of BrainFunction, vol. 12, Springer-Verlag, Berlin, 1984).

28. D. H. Hubel, Eye, Brain, and Vision (Scientific Ameri-can Library, New York, 1988).

29. M. Livingstone, D. Hubel, Science 240, 740 (1988).30. S. Tang, R. Wolf, S. Xu, M. Heisenberg, unpublished

results.31. D. Efler, B. Ronacher, Vision Res. 40, 3391 (2000).32. M. V. Srinivasan, S. W. Zhang, B. Rolfe, Nature 362,

539 (1993).33. We thank B. Ronacher and L. Wiskott for valuable

comments on the manuscript. This work was sup-ported by the German Science Foundation (SFB 554)and the Multidisciplinary Research program of theChinese Academy of Science (S.T.).

Supporting Online Materialwww.sciencemag.org/cgi/content/full/305/5686/1020/DC1Material and MethodsSOM TextFig. S1References

3 May 2004; accepted 8 July 2004

R E P O R T S

13 AUGUST 2004 VOL 305 SCIENCE www.sciencemag.org1022

www.sciencemag.org SCIENCE VOL 305 13 AUGUST 2004 1023

PEPTIDE HYDROGELBD PuraMatrix Peptide Hydrogel isa novel synthetic matrix used tocreate defined three-dimensional(3D) microenvironments for a vari-ety of cell culture experiments. This

material consists of standard amino acids and more than 99%water. Under physiological conditions, the peptide componentself-assembles into a 3D hydrogel that exhibits a nanometer scalefibrous structure. In the presence of key bioactive molecules, thishydrogel promotes the attachment, growth, and differentiation ofmultiple cell types. It is biocompatible, resorbable, and injectable,and devoid of animal-derived material and pathogens.

ULTRA-SENSITIVE LUMINOMETERClarity is an ultra-sensitive microplate luminometer that com-bines sensitivity, ergonomic design,and data analysis. Applications include reporter gene assays (luci-

ferase, dual luciferase), ATP assays (cell proliferation, cytotoxicity),DNA assays, reactive oxygen species assays, and chemilumines-cence assays. Fast photon counting and high-quality optics ensure the best sensitivity available. The use of disposable tipsfor injection, a contamination-free stainless steel work surface,and a dead volume below 500 µl are just a few of the features.

ANTIOXIDANT ASSAYThis Antioxidant Assay is designedto measure the overall antioxidantcapacity within a given sample. Theassay relies on the ability of anti-oxidants in the sample to inhibit

the oxidation of ABTS in comparison to Trolox, a water-solubletocopherol analog. By quantifying the cumulative effect of allantioxidants present, more relevant biological information is ac-quired compared with the measurement of individual compo-nents alone. The 96-well plate format can be used for the rapidmeasurement of antioxidant capacity in a variety of sampletypes, including plasma, serum, urine, saliva, and cell lysates.

GENOMIC DNAOmniPrep is for the extraction ofhigh-quality genomic DNA fromany species or tissue. The uniquefeature of OmniPrep is that the ge-nomic DNA hydrates in minutes,

not hours. OmniPrep generates pure genomic DNA through atwo-step protocol that removes proteins and other impurities.The procedure takes as little as 30 min. The DNA extracted is onaverage 100 kb in size and has an A260/280 ratio between1.8–2.0. The genomic DNA can be easily digested by many re-striction endonucleases.

REAL-TIME NUCLEIC ACIDDETECTION SYSTEMQuantica is a real-time nucleic aciddetection system that offers openchemistry versatility and multiplex

capability in which up to four dyes can be detected in any onewell. Quantica makes use of a cold solid-state white light sourcethat has an impressive range of excitation wavelengths that doesnot limit the user to the choice of methods possible, comparedwith those offered by single wavelength systems. It features aphoton-counting photo multiplier tube detection system thathas a wide detection range suitable for use with fluorochromescurrently used in real-time detection techniques. It also featureseasily interchangeable filter sets and two methods of controllinglight levels. Its optical heated lid can be set at different tempera-tures for program stages requiring special incubations.

MICRO-RULED COVERSLIPThe Cellattice micro-ruled coverslipconsists of a cell culture surfacewith microscopic identification andmeasurement markers. It is manu-factured with high optical-quality

plastic suitable for phase contrast and other standard cell-basedassays measurement techniques. The coverslip thickness is 0.13mm to 0.16 mm. The large micro-ruled area is 10 mm by 10 mm.Cells cultured on the Cellattice coverslip are identifiable within25 µm. The built-in markerson the coverslip allow directmeasurement of cell growthand movement without im-age acquisition. The samecell or cell cluster can bemeasured throughout thecourse of the experiment,even after each incubation period. The Cellattice coverslip can beused to monitor morphology, cell movement, and differentiationat the individual cell level. Cell proliferation is measured directlywith multiple readings.

REPORTER VECTORSThe Promega Rapid Response Re-porter Vectors are designed to allowscientists to measure what theymight have missed in their cell-basedassays. The reporter vectors reduce

the risk of secondary effects and allow measurements of smallerand more transient changes in transcription compared with theirpredecessors. The reporter vectors contain degradation sequencesthat yield a dramatic improvement in the temporal coupling be-tween transcription and reporter signal. The relative magnitude ofreporter response is also greater. The combined effect producesgreater changes in less time, enabling researchers to reduce assaytimes by up to 75%.They are available in firefly or Renilla luciferaseconfigurations, and are compatible with a wide range of Promegaluminescent detection reagents. Primary uses include quantitativecellular analysis and functional genomics applications.

Newly offered instrumentation, apparatus, and laboratory materials of interest to re-searchers in all disciplines in academic, industrial, and government organizations arefeatured in this space. Emphasis is given to purpose, chief characteristics, and availabil-ity of products and materials. Endorsement by Science or AAAS of any products ormaterials mentioned is not implied. Additional information may be obtained from themanufacturer or supplier by visiting www.scienceproductlink.org on the Web, whereyou can request that the information be sent to you by e-mail, fax, mail, or telephone.

N E W P R O D U C T SBD Biosciences

For more information

800-343-2035

www.bdbiosciences.com

www.scienceproductlink.org

Promega

For more information

608-277-2545

www.promega.com

www.scienceproductlink.org

Nexcelom Bioscience

For more information

978-397-1125

www.nexcelom.com

www.scienceproductlink.org

Bio-Tek

For more information

888-451-5171

www.biotek.com

www.scienceproductlink.org

Cayman Chemical

For more information

800-364-9897

www.caymanchem.com

www.scienceproductlink.org

Genotech

For more information

314-991-6034

www.genotech.com

www.scienceproductlink.org

Techne

For more information

+44 (0) 1223 832401

www.techne.com

www.scienceproductlink.org