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Our Planet | NewScientist The Collection | 125



126 | NewScientist:The Collection| OurPlanet

during the thermalmaximum, geochemistJamesZachosof theUniversityof California,Santa Cruz, hasfoundthat thewarmingcoincided with a huge spike in atmospheric

CO2 . Between 5 and9 trilliontonnes ofcarbonentered theatmosphere in no more than20,000 years.Wherecouldsuch a hugeamount havecome from?

Volcanicactivity cannotaccount forthecarbonspike,Zachos says. Instead, heblamespeat decomposition,which wouldhavehappened not from meltingpermafrost –it was toowarm for permafrost – but throughclimaticdrying. Thefossilrecord ofplantsfromthistimetestifiesto justsuchadryingepisode.

Carbon spikeIfZachosand colleaguesare right, then55 million yearsagoEarth passed through acarboncrisisvery much likethe onefearedtoday: a sudden spikein CO 2 , followed bya runaway releaseof yetmoregreenhousegases.What happened next maygiveusa glimpseof whatto expect ifour currentcrisishitsfull force.

Geochemistshave longknownthat whenapulseofCO 2 entersthe air, much of it quicklydissolves in theupperlayerof theocean beforegraduallydispersing throughdeeper waters.Withina fewcenturies, an equilibriumisreached,withabout 85per centof theCO 2

dissolved inthe oceans and15 percent intheatmosphere.ThisCO 2 persistsfor tens orhundredsof thousandsofyears– what Archerbelieveswillbe the“longtail”of theAnthropocene. Until recently, though, climatemodellerswere a bitfuzzyon whatthistailwould look like.

“Until wehad some casestudies fromthepast, there wasalwayssome degreeof uncertaintyin themodels,”says Zachos.Hisstudiesare beginning to clear upthesedoubts.Carbonate rockslaiddown on thesea floor

during thecarbonspike,for example,revealthat theoceansquickly becamevery acidic.Butthis extremeacidification lasted just10,000 or20,000 years,barelya blinkof aneye by geologicalstandards,afterwhichtheoceansreturned to near-normal conditionsforthenext 150,000years.

Even thestoresof peat andmethanehydratesmust have regeneratedwithin2millionyears, Zachossays, becauseat that timethe planet underwentanother,smallercarboncrisis, whichmust alsohaveinvolved peator methane hydrates.Thissuggests that thelong tailof theAnthropocene

isunlikely to last longer than2 million years –still notlong at all by geologicalstandards.

However, today’s carbonspikediffersfromthat of thelate Palaeocene in oneimportant

way: our planet ismuchcoolerthan itwasback then, sowarming is likely tohave a moreprofound effect.During thelate Palaeocene,theworldwaswarmand largelyice-free.Nowwehavebright, shiny icecaps that reflectsunlight back into space.Thesewillmelt,givingway to dark, energy-absorbing rock andsoil. Andwith all that meltwater, sea levelswillriseand permafrostwillthawmore rapidly,boostingwarming still further.

This extra nudgecould conceivably tipEarthout ofits present cycleof glacialsandinterglacials andreturnit to an older,warmerstate.“TheEarth wasice-freeformanymillionsof years.The current iceages startedonlyabout 35millionyears ago, sowe mightkickourselvesoutof that,”saysPieterTans,an atmosphericscientistat theUS NationalOceanicand AtmosphericAdministrationinBoulder, Colorado.

Even so,the newly ice-freeworldwouldmerelybe revertingto a familiarstate.On thisreadingof theevidence,eventhe most drasticclimatecatastrophewould have little chanceofpushingEarth’s physicalsystemsintouncharted territory.

Notso, says JamesHansen, formerlydirectorof NASA’s GoddardInstitute forSpaceStudies, nowat Columbia Universityin NewYork. Hearguesthat pastepisodesarea poorguideto whatwill happen in the future,for thesimplereason that thesunis brighter nowthanitwasthen.Addthattothemixandthereleaseof methanehydrates could leadtocatastrophic,unstoppable global warming –a so-called“Venussyndrome”that causesthe

oceans to boil awayand doomsEarthto thefate of itsbroiling neighbour.

Somuch for Earth itself– whatof life?IfHansenis right,Earth isheadingforsterility.

Butif thelesserscenarioplaysoutinstead, it’sa very different story.Conservationbiologistssay wemay already

bein themidst ofan extinctioneventthatcould potentiallyturn into oneof thegreatestmass extinctionsever– onethat wouldalterthe trajectory of evolution.

Oddly enough, theclimatic turmoilof thethermal maximumled to very little lossof biodiversity.“Nobody hasever pickedthePalaeocene-Eoceneboundary asa majorextinction interval. It’s notevenin thesecondtier,”saysScottWing, a palaeobotanist at theSmithsonianInstitution in Washington DC.Instead, thefossilrecord showsthat speciessimply migrated, following theirpreferredclimateacrossthe globe.

Today, ofcourse, that isoften notpossiblebecause roads, citiesandfieldshavefragmented so many natural habitats.Polarandalpinespeciesmay find theirhabitatvanishesentirely, andthis isnot to mentionall theotherwayspeople imperil species.

“We’re a perfect stormas faras biodiversityisconcerned,”saysDavidJablonski,apalaeontologistat theUniversity ofChicago.“We’renot just overhuntingand overfishing.We’re not just changing thechemistryof theatmosphereand acidifying theoceans. We’renotjust taking thelarge-bodied animals.We’redoingall this stuffsimultaneously.”Evenso,Jablonski thinkshumansare unlikely to becapableofcausing an extinction comparableto the one atthe end ofthe Permian,251millionyearsago, when an estimated96per centof allmarinespeciesand70 per

”Recoveries from mass extinctions are geologicallyrapid, but from a human point of view grindinglylong. We’re talking about millions of years”

Some have warned ofa “Venus syndrome”creating hell on Earth R

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OurPlanet | NewScientist:The Collection| 127

cent of terrestrial ones bit thedust.Whether theAnthropocenemassextinction

eventually rankswiththe Permianor withlesserones, it wouldstill reshuffle theevolutionarydeck.Once again, thepast givesussomeideaof whatwe couldexpect.

Thefossilrecord tells us that everymassextinction playsout differently, becauseeachhas itsown uniquecauses. However,there is onecommonfactor: thespeciesat greatest risk are thosewiththe narrowestgeographic ranges. Jablonski’s studiesof fossil marinesnailsshowthat specieswithplanktoniclarvae– whichdispersewidely – fare betterthan species withamorerestricted distribution.

Cockroach world

Addto that massive habitatdisturbances,saysJablonski,anda pictureemergesof lifeaftertheAnthropoceneextinction.Small bodysizes, fastreproductive ratesandan abilityto exploitdisturbed habitats willall prove

advantageous.“It’s a rats,weedsandcockroacheskind of world,”says Jablonski.Thewaveof extinctionsis likely to sweep

throughspecies in a fairlypredictableway.“First wewouldprobably losethe speciesthatarealready endangered, thenit wouldwork itswaydown,”saysBarnosky.“Eventually itwouldhit someof the speciesthatwedon’tconsider at risk today– for example, many ofthe African herbivores that today seem to havehealthy populations.”

However, predictions about the fate of anyparticular species are almost impossible, asluck will also play a part. The survivors will

probably be a more-or-less random selectionof weedy plants and opportunistic animals,notes Doug Erwin, a palaeobiologist at theSmithsonian Institution.

If the Anthropocene does end with a massextinction, the fossil record tells us a lot aboutwhat the recovery might look like. Whetherthe news is good or bad depends on yourperspective. “Recoveries from massextinctions are geologically rapid, but from ahuman point of view grindingly long. We’retalking millions of years,” says Jablonski.

Immediately after a mass extinction, thefossil evidence suggests that ecosystemsgo into a state of shock for several millionyears. For many millions of years after thePermian extinction, for example, marineenvironments the world over were dominatedby the same 25 or 30 species. “It’s prettyboring,” says Erwin.

Something similar happened on land afterthe Cretaceous extinction. Pre-extinctionplant fossils from western North Americatestify to flourishing ecosystems, with a

variety of insects feeding on a wideassortment of plants. After the extinction,though, both plant and insect diversity dropsdramatically, with some insect feedingmethods vanishing almost completely.

After that, confusion reigns for 10 millionyears. There are fossil assemblages with only afew insects and plants, ones with many insectsbut few plants, others with many plants butfew insects – just about everything exceptwhat ecologists would call “normal”. “At notime did we have what I would call a healthyecosystem, with diverse insects feeding ondiverse plants,” says Wilf. All the while

biodiversity remainslow,with fewnew

speciesevolving.“You’re just trying to hangon,”says Erwin.A study ofmarinefossildiversitybears this

out.In 2000, JamesKirchnerof theUniversityof California,Berkeley, andAnne WeilofDukeUniversityin Durham, North Carolina, tookadatabase ofall known marinefossilsand usedit to work outhow closelypeaksofspeciationfollowpeaks ofextinction.“We went into thisthinking, likeeverybodyelse, that whenyouhave an extinction,you begin repopulatingalmost immediately,”says Kirchner. Instead,theyfoundthat speciation peaks laggedabout 10 million yearsbehindextinctionpeaks.“Weprettymuch fellout ofourchairs,”hesays.

In fact, forthe first fewmillion yearsafter an extinction the speciation rate actuallyfalls. “That suggests to us a sort of woundedbiosphere. Extinction events don’t justremove organisms from an ecosystem,leaving lots of opportunity for new species todiversify. Instead, what we think happens isthat the niches themselves collapse, so youwon’t have new organisms emerging tooccupy them. The niches themselves don’texist any more,” says Kirchner.

Eventually, though, evolution wins theday, and after a few tens of millions of yearsbiodiversity rebounds. Sometimes, as afterthe Ordovician mass extinction 440 millionyears ago, the new regime looks a lot likethe old one. But more often a new worldemerges. “You’re not re-establishing the oldchessboard, you’re designing a whole newgame,” says Erwin.

In the Permian, the oceans were dominatedby filter-feeding animals such as brachiopodsand sea lilies, which lived their whole livesattached to the bottom. Predators were rare.All that changed after the extinction, leaving

a more dynamic and richer ecosystem. “Frommy point of view, the end-Permian massextinction was the best thing that everhappened to life,” says Erwin.

In a perverse way, then, the bottom lineis an encouraging one. Even if we manage tooverpopulate and overconsume ourselvesback to the Stone Age, the Earth will probablysurvive. Life will go on. By the time the longtail of the Anthropocene is over, what littlewas left of humanity will probably be gone.A new geological age will dawn. Shamethere won’t be anybody around to give ita name. ■

Lake Natron in Tanzania is the kind of low-diversityenvironment that could become the norm

KAZUYO

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