chapter 20 global air pollution: ozone depletion,...

In June 1992, representatives from nearly 180 nations met in Rio deJaneiro to hammer out the final language of Agenda 21, a massiveglobal blueprint for sustainable development. They also negotiated

final language for various agreements on global climate change, biodi-versity, forest protection, and other subjects. (The outcome of the EarthSummit is described in Chapter 27.) This meeting was held in large partbecause of widespread recognition that human society is destroying thelife-support systems of the entire planet and that without global coop-

Global Air Pollution:Ozone Depletion, Acid Deposition, andGlobal Climate Change

Stratospheric OzoneDepletionAcid DepositionGlobal Climate ChangeSpotlight on SustainableDevelopment 20-1: IntelPledges Huge Purchase ofGreen Energy

20.3

20.2

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CHAPTER OUTLINE

CHAPTER 20

For 200 years we’ve been conquering nature. Now we’rebeating it to death.

—Tim McMillan

434

CRITICAL THINKING

ExerciseA business magazine article notes that “on theissue of global warming the scientific commu-nity is divided.” In support of this assertion, it quotes two prominent scientists. One saysthat he’s “convinced the world is in a human-induced warming phase,” and another claimsthat “there’s simply not enough evidence tosupport such a conclusion.” The author of thearticle goes on to say that because of the un-certainty among the scientific community, itmakes no sense to launch a global effort to re-duce carbon dioxide emissions. This position issupported by some segments of the businesscommunity (especially the oil and coal compa-nies) but not by others (such as the insuranceand natural gas industries). Can you detectany problem in this reportage? After you havefinished, list the critical thinking rules thatwere helpful to you in this exercise.

eration we could greatly decrease the habitability ofthe planet for humans and all other life-forms.

In the 1960s and 1970s, most environmentalissues were local or regional in scale. Today, how-ever, many problems have reached global propor-tions. Solving them will require global cooperationsuch as that witnessed during the Earth Summit.This chapter looks at three global issues—ozonedepletion, acid deposition, and global climatechange—and sustainable solutions.

Stratospheric OzoneDepletion

Encircling the Earth is a layer of ozone gas (O3),which screens out 99% of the sun’s harmful ultravi-olet (UV) radiation (FIGURE 20-1). Called the ozonelayer, this protective zone occupies the inner two-thirds of the stratosphere, which extends 15 to 45 kilometers (10 to 30 miles) above the Earth’ssurface. The ozone layer blocks 98% of the Sun’sultraviolet radiation and thus protects all terrestrialorganisms from UV radiation’s harmful effects. With-out the ozone layer, terrestrial life would all but van-ish. Some animals would suffer serious burns andwould develop cancer and lethal mutations. Hu-mans would be especially vulnerable to these

20.1

changes. Plants would be adversely affected. Unable to copewith the intense influx of ultraviolet radiation, many plantswould perish—and with them, millions of species that dependon them for nutrition. How does the ozone layer work?

When ultraviolet radiation strikes ozone molecules, itcauses them to split:

UV � O3 → O � O2

The products, however, quickly reunite, reforming ozoneand giving off heat:

O � O2 → O3 � heat

Thus, the ozone layer is a renewable form of protection thatconverts harmful ultraviolet radiation into heat.

KEY CONCEPTS

Activities That Deplete the Ozone LayerHuman civilization threatens the ozone layer through twoprincipal activities: (1) the use of a class of chemical com-pounds called chlorofluorocarbons and (2) jet travel throughthe stratosphere. Lets begin with chlorofluorocarbons.

The Use of Chlorofluorocarbons In 1951, manufacturersintroduced a promising new product on the market in theUnited States, spray cans that contained a chemical sub-stance known as Freon-11. It served as a propellant. Freon-11and a similar compound, Freon-12, belong to a group ofchemicals commonly referred to as chlorofluorocarbons(CFCs). Table 20-1 provides a few details on these CFCs. Asshown, Freon-11 is a propellant used in spray cans; Freon-12 is used in refrigerators, air conditioners, and freezers. Asyou can see from the chemical formulas, CFCs contain twoto three chlorine atoms (indicated by the symbol Cl) andone fluorine atom.

Chlorofluorocarbons have been used in other ways aswell. They were once widely used as foam blowing agents. Ablowing agent is a gas mixed with a liquid polymer such as

The ozone layer is a portion of the stratosphere with a slightlyhigher concentration of ozone molecules. It forms a protectiveshield that filters out harmful ultraviolet radiation.

435

Table 20-1Commonly Used Freons

Generic Chemical Name Use Chemical Name Formula

Freon-11 Spray can Trichloromonofluoromethanepropellant

Freon-12 Coolant in Dichlorodifluoromethanerefrigerators,freezers, andair conditioners

Cl�

Cl�C�Cl�F

Cl�

F�C�Cl�F

FIGURE 20-1 The ozone layer.(a) Ozone molecules absorb ultraviolet radiation striking theEarth’s atmosphere and convertit into heat. (b) Ozone concen-trations in the atmosphere andstratosphere.

436 PART V. Learning to Live with the Earth’s Carrying Capacity

polystyrene and blown or extruded into molds, to produceinsulated cups or foam insulation boards for building homes.CFCs form bubbles in the foam, making it a good insulator.One class of CFCs was also used to clean electronic equip-ment, precision ball bearings, and medical equipment.

CFCs were attractive substances because they werebelieved to be chemically unreactive—or inert. Until the early1970s, chemists believed that CFCs released from spray cansor escaping from refrigerators simply diffused into the upperlayers of the atmosphere. There, they were presumed to bepartially broken down by sunlight, a process that liberatedsome of the chlorine atoms. It was thought that this process,called photodissociation (breakdown in sunlight), wouldhave little if any effect on the upper atmosphere.

In the early 1970s, two U.S. scientists, Mario J. Molina andF. Sherwood Rowland, began to question this assumption.

They pointed out that chlorine free radicals (highly reactivechlorine atoms produced by the breakdown of CFCs) mightreact with stratospheric ozone. Shortly after their announce-ment, three research teams reported that a single chlorine freeradical could react with and destroy ozone molecules in thestratosphere. The reactions are shown in FIGURE 20-2. Becauseof the nature of these reactions, a single chlorine free radicalfrom a CFC molecule can destroy 100,000 molecules of ozone.

In addition to CFCs, scientists found that several chlo-rine and bromine compounds could deplete the ozone layer.Carbon tetrachloride, for example, is an ozone-depleting sol-vent that was widely used at the time in fire extinguishers andas a cleaning agent.

Like chlorine-containing compounds, bromine substancesalso diffuse into the stratosphere, where they photodissociate.Bromine then reacts with ozone, causing its concentration to

Troposphere (0–10 miles)

Stratosphere (10–30 miles)(containing the ozone layer)

40 km

15 km

Earth(not drawn to scale)

(a)

Incomingsolarradiation

~99% of the UV radiation is screened out by the ozone layer

35

30

25

20

15

10

5

05 10 15

Ozone amount (pressure, millipascals)

Atmospheric ozone

Stratospheric ozone(the ozone layer)

• Contains 90% ofatmospheric ozone

• Beneficial role:Acts as primaryUV radiation shield

–

• Current issues:Long-term globaldownward trends

–

Springtime Antarcticozone hole each year

–

• Contains 10% ofatmospheric ozone

• Harmful impact:Toxic effects onhumans andvegetation

–

• Current issues:Episodes of highsurface ozone inurban and rural areas

–

Troposphericozone

Alti

tude

(ki

lom

eter

s)

20

“Smog” ozone

25

(b)

FIGURE 20-3 Supersonic speed: at what price? Thissupersonic transport once flew through the strato-sphere, releasing nitric oxide, destroying ozone.

FIGURE 20-2 The chemistry of CFCs and ozonedepletion. (a) Chlorofluorocarbons are dissociatedby ultraviolet radiation in the stratosphere. Thisproduces a highly reactive chlorine free radical. (b) The free radical can react with ozone in theozone layer, forming chlorine oxide. This reactionreduces the ozone concentration. (c) A single mole-cule of Freon gas can eliminate many thousands ofmolecules of ozone because chlorine oxide breaksdown, reforming the chlorine free radical. Note too that chlorine oxide also reacts with ozone mole-cules, destroying them.

CHAPTER 20: Global Air Pollution: Ozone Depletion, Acid Deposition, and Global Climate Change 437

fall. However, bromine free radicals are far moredestructive of ozone than are chlorine free radi-cals and destroy hundreds of times more ozonemolecules than chlorine atoms.

KEY CONCEPTS

High-Altitude Jets High-flying aircraft, such as militaryjets that routinely travel through the stratosphere, also de-stroy ozone. How? Jet engines release a pollutant called ni-tric oxide (NO). Nitric oxide gas, in turn, reacts with ozoneto form nitrogen dioxide and oxygen:

NO (nitric oxide) � O3 (ozone) →NO2 (nitrogen dioxide) � O2 (oxygen)

The threat to the ozone layer from high-flying jets ismuch lower than that from CFCs. The Concorde, a super-sonic jet that once traveled between the UnitedStates and Europe in the stratosphere, was takenout of service in 2004 (FIGURE 20-3). In 1971, theU.S. Congress killed a proposal to subsidize theconstruction of 300 to 400 supersonic transports.Had they approved this proposal, jets would havebecome a major depleter of stratospheric ozone. Or-dinary commercial jets also produce nitric oxide,but their effect on the ozone layer is still in question.

Chlorofluorocarbon molecules were once usedas propellants, refrigerants, blowing agents, andcleaning agents. CFCs are stable molecules thatdiffuse into the stratosphere, where they breakdown, releasing chlorine atoms that react withozone molecules, destroying them. Other chlo-rine and bromine-containing compounds havealso been used widely and are known as ozonedepleters.

O

(a)

(b)

(c)

FluorineUV radiation

F

CCl

+

Carbon

Chlorofluorocarbon(CFC) molecule

Chlorine

Chlorinefree radical

Chlorinefree radical

Ozone moleculemade up of threeoxygen atoms (O3)

Chlorine oxide

Oxygen (O2)

+ +

Oxygenatom

Chlorine oxide

Oxygen (O2)

Cl

Cl

Cl

O

Cl

O

CL

O

CL

O

O

OO

OO

Ultraviolet radiation from the sun strikes the CFC moleculeand causes a chlorine atom to break away.

The chlorine atom reacts with an ozone molecule to form chlorine oxide and diatomic oxygen.

When a free atom of oxygen reacts with a chlorine oxidemolecule, diatomic oxygen is formed, and the chlorine atomis released to destroy more ozone.

+

KEY CONCEPTS

Ozone Depletion: The History of a Scientific DiscoveryConcern for the ozone layer originally began with projectionsof the effects of high-altitude jets. It increased dramaticallywhen, in 1974, the chemists Rowland and Molina announcedthat CFCs, previously considered safe, could react with and

All jets release nitric oxide gas, a pollutant that reacts withozone. Military jets that travel through the stratosphere have thegreatest impact. Jet travel poses a lower risk than the use of CFCs.


In 1988, researchers discovered that a similar hole wasforming over the Arctic. Ozone levels there, however, werenot as severely depleted because the vortex in the Arctic is notas strong as it is in the south. Invading winds can breakthrough, keeping the Arctic air much warmer and reducingozone destruction. Although the wintertime ozone deple-tion is only 10 to 30%, less than in Antarctica, the NorthernHemisphere is more heavily populated. When the Arcticozone hole breaks up, it also releases huge masses of ozone-depleted air that may linger over highly populated areas ofNorth America and Europe (FIGURE 20-4).

The decline in ozone throughout the atmosphere andespecially at the poles has resulted in a 3º decline at mid-latitudes in the Northern Hemisphere between 35º and65º North latitude. In the Southern Hemisphere, ozoneconcentrations have fallen by 6% between 35º and 65º Southlatitude. No significant changes have occured in the tropics.

In December 1990 (Antarctica’s summer), studies of ul-traviolet radiation reaching the ground at Palmer Station, aU.S. base on Antarctica, showed that levels had reached arecord high—twice their normal value. The researchers sug-gested that the high levels of ultraviolet radiation resultedfrom the longer-than-normal persistence of the springtimeozone hole. In October 1991, studies showed that ozone de-pletion over Antarctica was even greater, marking the thirdyear in a row that a severe ozone hole had formed overAntarctica.

30˚N30˚N

40˚N

53˚N

64˚N 64˚N

53˚N

40˚N

30˚N30˚N

40˚N

53˚N

64˚N 64˚N

53˚N

40˚NVancouver

Los Angeles

MexicoCity

New York

Toronto

Montreal

Chicago

–3.0%Average change

–2.3%

–1.7%

(a)

(b)FIGURE 20-4 Mapping ozone depletion. (a) This map of North America showsthe ozone depletion at different latitudes between 1969 and 1988. (b) Computer-generated images of the changing size of the ozone hole over the Arctic.

remove ozone molecules from the stratosphere. Their initialprojections indicated that CFCs could eventually destroy 20to 30% of the ozone layer. Their work on CFCs and ozone de-pletion eventually won them a Nobel prize.

Evidence in support of the claim that CFCs were deplet-ing the ozone layer began to be published in scientific journalsin the 1980s. In May 1985, for example, British scientists re-ported a large decrease in the ozone layer above Antarctica,which has since been dubbed an ozone hole because of theseverity of the decrease. In 1986 and 1987, intense study of theozone hole strongly suggested that it was caused in large partby CFCs. Studies showed that the Antarctic ozone hole was alsocaused by several natural climatic conditions. One such weatherphenomenon was a vortex of wind (a whirlpool in the atmo-sphere) that circles the South Pole during the winter months,blocking warmer air from penetrating the region. Studiesshowed that the vortex contributes to the formation of polarstratospheric clouds. CFCs and other ozone-depleting chem-icals adhere to the surface of the ice crystals in these clouds.After winter, when sunlight returns to this frigid region, CFCsare released and broken down by sunlight. Ozone levelsplummet as a result. During the 1990s, reductions of 40% to50% were common, but reductions of up to 70% have beenreported. After the vortex breaks down, huge masses ofozone-depleted air then move northward, where they hoverover nearby land masses such as Australia, New Zealand,and the southern tips of South America and Africa.


Ozone-depleting chemicals released on the Earth’s sur-face take many years to rise into the stratosphere; therefore,scientists predict that ozone depletion will grow worse—eventually reaching 10 to 30% over North America—eventhough, as you shall soon see, efforts have been made toeliminate the release of ozone-depleting substances. In April2011, for instance, Arctic ozone depletion reached a recordlow, causing a massive increase in ultraviolet radiation.

KEY CONCEPTS

The Many Effects of Ozone DepletionDo the declines in the concentration of ozone gas in theozone layer result in an increase in ultraviolet radiation strik-ing the Earth?

Extensive studies show that UVB radiation striking theEarth has increased significantly in the Northern and South-ern Hemispheres from about 40º N and 40º S. Denver, Coloradois near 40º North latitude. These increases in UV radiationaffect the northern U.S., Canada, most of Europe, Russia, andportions of Argentina, Chile, Australia, and New Zealand.

What impacts could ozone depletion have? Like many nat-ural components of our environment, ultraviolet light is ben-eficial. In small amounts, ultraviolet radiation tans light skinand stimulates vitamin D production in the skin. However, ex-cess ultraviolet exposure can cause problems. In humans, itcan cause serious skin burns, cataracts (clouding of the eye’slens), skin cancer, and premature aging. Increased exposureto ultraviolet light may also suppress the human immune sys-tem, making us more susceptible to infectious diseases.

EPA researchers estimate that a 1% depletion of theozone layer would lead to a 0.7 to 2% increase in ultra-violet light striking the Earth. This, in turn, would lead to anincrease in skin cancer of about 4%. Over the next 5 decades,the EPA estimates that ozone depletion will result in ap-proximately 200,000 cases of skin cancer in the United States. Worldwide, the number will be much higher. Especiallyhard hit will be countries such as New Zealand and Australia.In November 1991, a panel of scientists convened by theUnited Nations released a report on the predicted effects ofglobal ozone depletion. They estimated that a 10% decreasein stratospheric ozone concentrations will annually cause300,000 additional cases of skin cancer and an additional1.6 million cases of cataracts (loss of opacity) worldwide.

Studies of skin cancer show that light-skinned people aremuch more sensitive to ultraviolet radiation than more heav-ily pigmented individuals. In addition, studies show thatsome chemicals that are commonly found in drugs, soaps,cosmetics, and detergents may sensitize the skin to ultravi-olet radiation. Thus, exposure to sunlight may increase theincidence of skin cancers among light-skinned people andusers of many commercial products.

Land- and water-dwelling plants could also suffer fromincreasing ultraviolet radiation. Intense ultraviolet radia-

Studies of the ozone layer show substantial declines over the globe, with the highest level of depletion in the SouthernHemisphere and Antarctica.

tion is usually lethal to plants; smaller, nonfatal doses dam-age leaves, inhibit photosynthesis, cause mutations, or stuntgrowth. Declining ozone and increasing ultraviolet radia-tion could, therefore, cause dramatic declines in commercialcrops such as corn, rice, and wheat, costing billions of dol-lars a year. It may also affect certain commercially valuabletree species.

In a hearing before the U.S. Senate in November 1991,Susan Weiler, head of the American Society of Limnology andOceanography, testified that studies in Antarctica by otherscientists have shown that phytoplankton (algae and otherfree-floating photosynthetic organisms) populations decreaseabout 6 to 12% when stratospheric ozone concentrationsover the region drop by 40%. Because phytoplankton form thebase of the aquatic food chain, damage to them could causewidespread ecological problems. One scientist thinks thatozone depletion and subsequent effects on the food chainmay be the reason why two species of penguin are decliningin Antarctica.

Finally, ultraviolet light is harmful to many products.Paints, plastics, and other materials deteriorate when ex-posed to ultraviolet light. Losses from further decreases inthe ozone layer could cost society enormous amounts ofmoney.

KEY CONCEPTS

Banning CFCs and Other Ozone-DepletingChemicals: A Global Success StoryIn the 1970s, fears caused by early projections of ozone de-pletion moved several nations, including the United States,Sweden, Finland, Norway, and Canada, to cut back on CFCemissions. In 1978, for example, the United States bannedCFC use in spray cans. Freon-12—the refrigerant, coolant,and blowing agent—was not affected by the ban.

Additional scientific evidence on the decline of theozone layer made it evident a decade later that worldwidecooperation was needed. As a result, in 1987, the UnitedNations sponsored negotiations aimed at reducing globalCFC production. In September of that year, 24 nationssigned a treaty called the Montreal Protocol, which wouldcut production of five CFCs in half by 1999 and freeze pro-duction of halons (used in fire prevention systems) at 1986levels. (In halons, bromine atoms replace some or all ofthe chlorine atoms.) Although halons are used in muchsmaller quantities, as noted earlier, bromine is far more ef-fective in destroying ozone than chlorine from CFCs andother sources.

This agreement paved the way for a gradual decline inCFC production in the industrial nations, but like so manyother pollution control strategies, it would only slow the

Ozone depletion is resulting in an increase in ultraviolet radia-tion striking the Earth, especially in unpolluted areas. Ultravi-olet radiation causes skin cancer, cataracts, and prematureaging. It could also seriously damage ecosystems, crops, mate-rials, and finishes.


rate of destruction, not stop it. EPA computer projectionsshowed that an 85% reduction in CFC emissions was neededto stabilize CFC levels in the atmosphere.

Before the Montreal Protocol went into effect, some-thing unusual happened. In March of 1988, an internationalpanel (described earlier) announced that ozone levels hadfallen throughout the world. Two weeks later, DuPont, amajor producer of CFCs, called for a total worldwide ban onCFC production—when only 2 weeks earlier it had said thatit would not support a ban.

Continuing bad news about ozone depletion broughtnegotiators to the table once again, this time in London,where in June 1990 they reached a new agreement. Thistreaty was signed by 93 nations and called for the completeelimination of CFCs and halons by the year 2000, if sub-stitutes were available by then. The signatories also agreedto phase out other ozone-depleting chemicals includinghydrochlorofluorocarbons (HCFCs), a class of chemicals (de-scribed shortly) once thought to be an excellent substitutefor CFCs.

The news about the ozone layer continued to worsen.In 1992, a team of 40 scientists announced record-high con-centrations of chlorine monoxide (ClO is produced whenchlorine free radicals from the breakdown of CFCs reactwith ozone molecules) in the air above New England andCanada. Concentrations such as these had never been seenbefore, even in the Antarctic ozone hole. If chlorine levels con-tinued to climb, chances were good that the Arctic ozonehole would begin to appear with great regularity, exposingCanada and parts of the United States, Europe, and Asia todangerous levels of UV radiation.

Aircraft measurements in 1992 also produced ratherdisturbing findings about global ozone outside the Arctic. Inflights as far south as the Caribbean, scientists detected ClOconcentrations of up to five times the amount they had anticipated.

In 1992, the nations of the world met in Copenhagen tosign another agreement calling for an acceleration of thephaseout of CFCs, carbon tetrachloride, and other ozone-depleting chemicals within 4 to 9 years. The success of theglobal efforts to phase out CFCs is shown in FIGURE 20-5, agraph of projected concentrations of ozone-depleting com-pounds. Although progress has been impressive, police arefinding that CFCs are being illegally imported into the UnitedStates (and presumably other countries as well) in massivequantities. Stopping this flow will be required to reach thegoals of the various international treaties.

KEY CONCEPTSAs scientific evidence on ozone depletion accumulated, the na-tions of the world tightened restrictions on the production ofozone-depleting chemicals. Three international treaties havealready been signed to eliminate the production of ozone-depleting compounds, and progress toward meeting these goalshas been very impressive.

Substitutes for Ozone-Destroying CFCsAs the previous section showed, scientific reports led to thesigning of the three treaties to ban CFCs and other ozone-depleting chemicals. The development of replacement chem-icals has also played a big role in the dramatic change. Itgave industry options and, in some cases, opportunities toprofit from the shift to less harmful means of refrigerating andof propelling liquids from spray cans. Many companies alsoled the way. AT&T, for example, was the first U.S. companyto set a goal of phasing out the use of CFCs, which they suc-ceeded in achieving in 1993.

CFCs are extremely important to modern society. Everyrefrigerator, freezer, and air conditioner once used them. Tocontinue to provide these desirable services and protect theozone layer, manufacturers pursued two routes: the use ofless stable CFC compounds, which break down before theyreach the stratosphere, and the production and use of non-CFC chemicals as substitutes.

Consider the first option, the production of less stableCFCs. This is achieved by substituting a hydrogen atom forone of the chlorine atoms of a CFC molecule. These mole-cules are known as hydrochlorofluorocarbons or HCFCs.HCFCs can be used in place of CFCs as refrigerants, clean-ing agents, blowing agents, and so on. They work well andare affordable. HCFCs are also released into the atmospherewhere they break down. Because they are less stable, how-ever, HCFCs break down in the lower atmosphere beforethey can ascend into the stratosphere, home to the ozonelayer. Nonetheless, some molecules of HCFC reach the strato-sphere where they photodissociate and destroy ozone. All inall, though, they cause much less damage to the ozone layerthan CFCs—about one-fifth as much as CFCs.

Because HCFCs cause the destruction of ozone and aregreenhouse gases, scientists and policymakers quickly cameto view them as an interim solution. The HCFCs are now alsoscheduled for phase out. A recent agreement among theworld’s nations calls for a 90% phase out of HCFCs by 2015.By 2020, the production and consumption of HCFCs will bevirtually eliminated.

15,000

1950

Abu

ndan

ce o

f str

atos

pher

ic c

hem

ical

s(p

arts

per

trill

ion)

Year

12,000

9000

6000

3000

0210020752050202520001975

No protocol

MontrealProtocol

CopenhagenAmendments

FIGURE 20-5 Graphing success. This graph shows the predictedconcentrations of ozone-depleting chemicals in the atmosphereunder various scenarios—no action, the Montreal Protocol, andthe Copenhagen Amendments.


The second option for replacing CFCs is the produc-tion of non-ozone-depleting substances—chemicals thatperform the same functions as CFCs and HCFCs but do notdeplete the ozone layer. Over the years, chemists have foundnumerous ozone-safe substitutes. Many of these chemicalsare now being used.

The Good News and Bad News about OzoneThe ozone story is one of humankind’s greatest success sto-ries. It not only illustrates how scientific knowledge can beused by society for the common good of humankind and allother species; it also shows how quickly changes can bemade to bring about an end to the destruction of the globalenvironment. It further illustrates the power of corporateresponsibility. Many of the changes were brought about vol-untarily without the need for governmental actions.

Already, studies are showing a decline in the concen-tration of ozone-depleting chemicals in the stratosphere.Unfortunately, many millions of tons of CFCs have alreadybeen released into the atmosphere and some CFCs last100 years in the atmosphere. Because of this, scientists pre-dict that the ozone layer will recover very slowly. Estimateson full recovery vary. National Oceanic and AtmosphericAdministration (NOAA) scientists predict that if interna-tional agreements are adhered to, recovery to pre-1980 lev-els will be achieved by 2050. Others’ predictions are lessoptimistic. They believe that at least 100 years will be re-quired to return the ozone layer to 1985 levels. Another 100to 200 years may be needed for full recovery.

Many people will contract skin cancer in the interim. TheEPA notes that skin cancer in the United States has alreadyreached epidemic proportions. One in five Americans will de-velop skin cancer in their lifetime and one American currentlydies from skin cancer every hour. Melanoma, the most seri-ous form of skin cancer, which has been linked to early ex-posure to excess ultraviolet light, is one of the fastest growingforms of cancer in the United States.

Nonetheless, many will be spared from skin cancer bythe phaseout. The EPA estimates that the successful phase-out of CFCs will result in 295 million fewer cases of skin can-cer over the next 100 years than would have occurred withoutthese important changes.

Finally, many ozone-depleting chemicals (and their re-placements) are greenhouse gases and thus contribute toglobal warming and climate change. They will acceleratethe warming of the Earth’s atmosphere, but their eventual de-cline may, when combined with other efforts, help us com-bat global warming.

KEY CONCEPTSThe concentration of CFCs and other ozone-depleting compoundsin the atmosphere has begun to decline. Despite this progress,the ozone layer will take many years to recover. Many people willcontract and die from skin cancer.

Acid DepositionIn the 1960s, forest ranger Bill Marleau built the cabin ofhis boyhood dreams on Woods Lake in the western part ofNew York’s Adirondack Mountains. Isolated in a dense for-est of birch, hemlock, and maple, the lake offered Marleauexcellent fishing. Ten years after Marleau finished his cabin,however, something bizarre happened: Woods Lake, once amurky green suspension of microscopic algae and zoo-plankton, teeming with trout, began to turn clear. As thelake went through this mysterious transformation, the troutstopped biting and soon disappeared altogether. Then the lilypads began to turn brown and die; soon afterward, the bull-frogs, otters, and loons disappeared.

What had happened to Woods Lake? What destroyed theweb of life at this small, isolated lake, far from any sourcesof pollution? Scientists from the New York Department of En-vironmental Conservation say that Woods Lake is “criticallyacidified.” As a result, virtually all forms of life in and aroundit have perished or moved elsewhere. The lake became acid-ified from acids and acid precursors deposited from the skiesin several forms.

Acid deposition—the deposition of acidic substancesgenerated from pollution largely human in origin—is com-monplace today, as are lakes like Marleau’s. In fact, WoodsLake is only one of about 375 lakes and ponds in the west-ern Adirondacks turned acidic and hazardous to virtuallyall forms of life by acid deposition (FIGURE 20-6). In easternCanada, 100 lakes have met a similar fate. In Scandinavia, thedeath count is 10,000. Across the globe thousands of lakeswill die unless something is done, and quickly.

Acid deposition is a phenomenon not just of environ-mental interest, but of grave social and economic impor-tance. Studies show that acid deposition turns lakes acidic,kills fish and other aquatic organisms, damages crops, de-stroys forests, alters soil fertility, and destroys statues and

20.2

FIGURE 20-6 This crystal clear lake in the Adirondacks is tooacidified to support aquatic life.

0 2 41 3

Acid rain

5 7 9 11 136 8 10 12 14Acidic

Lemon juice

Neutral Basic

Vinegar

Baking soda

Distilled water

“Pure” rain (5.7)

Mean pH of Adirondack Lakes-1975

Mean pH of Adirondack Lakes-1930s


buildings. Moreover, scientists are finding that acid precip-itation is more widespread than once thought and is takinga large toll on our environment and pocketbooks. Recentreports indicate that it poses a universal threat, affecting themore developed countries (MDCs) as well as many less de-veloped countries (LDCs). Earthscan, an international envi-ronmental group, reports that acid deposition is alreadydamaging soils, crops, and buildings in much of the devel-oping world. Rapidly growing urban centers, with theirpoorly regulated industry and traffic congestion, are largelythe culprits. Ironically, tough pollution laws in MDCs havegiven multinational corporations incentives to set up oper-ations in LDCs, whose pollution laws (if they have any) arecertainly much weaker.

KEY CONCEPTS

What Is an Acid?Understanding acid deposition requires a brief explanation ofacids. An acid is a chemical substance that adds hydrogen ionsto a solution. Hydrochloric acid (HCl), for example, dissoci-ates into hydrogen and chloride ions and is therefore an acid.

Acids come in varying strengths. Strong acids add manyhydrogen ions; weak acids add fewer. The degree of acidityof a solution, then, is related to the concentration of hydro-gen ions in it. Chemists and others measure acidity on thepH (potential hydrogen) scale, which ranges from 0 to 14(FIGURE 20-7). Substances that are acidic, such as vinegarand lemon juice, have low pH values—that is, less than 7. Ba-sic (alkaline) substances, such as baking soda and lime, havehigh pH values—greater than 7. Basic substances have verylow hydrogen ion concentrations. Neutral substances, suchas pure water, have a pH of 7.

The pH scale is logarithmic, like the decibel scale dis-cussed in Chapter 19 (which measures loudness). This meansthat a change of one unit on the scale—in this instance, 1 pHunit—represents a 10-fold change. Therefore, rain with a

Acid deposition from pollutants is a global problem with seri-ous social, economic, and environmental impacts.

pH of 4 is 10 times more acidic than rain with a pH of 5, 100times more acidic than rain with a pH of 6, and 1,000 timesmore acidic than rain with a pH of 7.

KEY CONCEPTS

What Is Acid Deposition?In an unpolluted environment, rainwater is slightly acidic, hav-ing a pH of about 5.7. The normal acidity of rainwater is cre-ated when atmospheric CO2 is dissolved in water in clouds,mist, or fog and is converted into a mild acid (carbonic acid).Acid precipitation is rain and snow with a pH below 5.7.

Acid deposition includes two broad categories: wet dep-osition and dry deposition. Wet deposition refers to acids de-posited in rain and snow. These acids are formed when twoair pollutants, the sulfur and nitrogen oxides (which areboth gases), combine with water in the atmosphere. Sulfuroxides form sulfuric acid; nitrogen oxide gases react with wa-ter to form nitric acid. Sulfuric and nitric acid are two of thethree strongest acids known to science.

Produced in the atmosphere, these acids may accumu-late in clouds and fall from the sky in rain and snow. Evencoastal fogs may contain droplets of acid that, when de-posited on buildings or plants, can cause noticeable damage.One study shows that moisture droplets in low-lying clouds(fog) tend to contain higher concentrations of acid than rainor snow that falls from them. Therefore, fog and clouds maybathe trees in highly acidic water. Making matters worse,recent studies suggest that the evaporation of recently de-posited cloud water from forest canopies may result in acidconcentrations on leaf surfaces much higher than thosefound in the cloud droplets themselves.

Sulfate and nitrate particulates are also present in the at-mosphere. These pollutants may settle out of the atmospheremuch like fine dust particles. This process is one form ofdry deposition. Settling onto surfaces, these particulatescan combine with water to form acids. Sulfur and nitrogenoxide gases may also be adsorbed onto the surfaces of plantsor solid surfaces, where they, too, combine with water toform acids. This is another type of dry deposition.

KEY CONCEPTS

Where Do Acids Come From?Acid precursors, the primary pollutants that give rise toacids, come from natural and anthropogenic sources. Be-fore we look at them, let us review how these pollutants aregenerated. As noted in Chapter 19, fossil fuels such as coal

Rainfall in unpolluted areas has a pH of about 5.7 and is justslightly acidic. Acid deposition refers to rain and snow with a pHof less than 5.7 and the deposition of acid particles and gases.Acids reach the surface of the Earth either as wet deposition (rainor snow) or dry deposition (particulates and gases).

Acids are chemical substances that add hydrogen ions to a so-lution. Acids are measured on the pH scale, which ranges from0 to 14, with 7 being neutral—neither acidic nor basic.

FIGURE 20-7 The pH scale. A pH scale is used to denote levels ofacidity.


and oil contain sulfur impu-rities. When these fuels areburned, sulfur reacts withoxygen in the intense heatand forms sulfur oxide gases.These gases are released intothe atmosphere where theymay be converted into sulfu-ric acid. Chapter 19 also noted that the combustion of anyorganic matter in air, with its high concentration of nitrogengas, produces nitrogen oxide gases. These gases are convertedinto nitric acid in the atmosphere after reacting with water.

Several natural events can produce significant amountsof sulfur dioxide. The natural sources of sulfur oxides includevolcanoes and forest fires (FIGURE 20-8a). Bacterial decay of or-ganic material such as plants also produces hydrogen sulfidegas, which can be converted into sulfuric acid in the atmo-sphere. Anthropogenic sources of this pollutant are of majorconcern, however, because they are often concentrated in ur-ban and industrialized regions, causing local levels to be quitehigh. Two-thirds of all anthropogenic sulfur dioxide comes fromelectric power plants, most of which burn coal. Smelters, de-vices that melt mineral ores to extract pure minerals, also re-lease huge quantities of sulfur dioxide (FIGURE 20-8b).

Like the sulfur oxides, the nitrogen oxides arise from awide variety of sources. Forest fires, for example, can re-lease millions of tons. The two most important anthropogenicsources are electric power plants and motor vehicles. Amer-ican factories, cars, and power plants currently produce ap-proximately 10.4 million metric tons (11.5 million tons) ofsulfur dioxide and about 14.8 million metric tons (16.4 mil-lion tons) of nitrogen oxides a year.

KEY CONCEPTSAcid precursors come from natural and anthropogenic sources,the latter being the most important. Of the anthropogenicsources, the combustion of fossil fuels is the most significant.

The Transport of Acid PrecursorsAcid precursors and acids formed in the atmosphere can re-main airborne for 2 to 5 days and may travel hundreds, per-haps even thousands, of kilometers before being deposited.Studies have shown that acids falling from the sky in south-ern Norway and Sweden largely come from England andindustrialized Europe. In the United States, acids falling inthe Northeast come from the industrialized Midwest, pri-marily the Upper Mississippi and Ohio River valleys. Indianaand Ohio are the two major producers. Moving eastward,the mass of pollutants tends to converge on New York Stateand New England, where 50% of the lakes are endangeredbecause of low acid-neutralizing capacities.

As noted earlier, acid deposition is a global phenomenon.It is found in the United States, Canada, the Amazon Basin, Europe, and the Netherlands, among other places. All of theseplaces are downwind of heavily polluted areas. In Europe andScandinavia, rain and snow samples frequently have pHvalues between 3 and 5. In the White Mountains of New Hamp-shire, for example, the average annual pH of rainfall is about4 to 4.21, nearly 100 times more acidic than normal precipi-tation. Rain samples collected in Pasadena, California in the1970s had an average pH of 3.9. One of the lowest pHmeasurements was made in Kane, Pennsylvania, where a rain-fall sample with a pH of 2.7 was recorded—rain as acidicas vinegar. The grand prize for acidic rainfall, however, goesto Wheeling, West Virginia, where a rainfall sample had a pHof 2—stronger than lemon juice. More recent studies of the pHof fog downwind from the Los Angeles basin, however,revealed levels as low as 1.7.

In southern Norway and Sweden and in the northeast-ern United States, two ominous trends have been observed.First, acid precipitation is falling over a wider area than it was60 years ago; second, the areas over which the strongestacids are falling have been expanding until very recently(FIGURE 20-9).

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FIGURE 20-8 Sources of sulfur dioxide. (a) Volcanic eruptions, such as the Mount St. Helens blast shown here, is one natural source ofsulfur dioxide. (b) Coal-fired power plants like this one produce most of the anthropogenic sulfur dioxide.

(a) (b)


KEY CONCEPTS

The Social, Economic, and Environmental Impacts of Acid DepositionAcid deposition has many impacts on the environment, oureconomy, and society. This section describes some very realimpacts and some possible ones.

Acidification of Lakes Throughout the world, thousandsof lakes and rivers and their fish have fallen victim to aciddeposition. In the 1930s, for example, scientists surveyedlakes in the western part of the Adirondacks. Sampling thepH of 320 lakes, they found that most had pHs ranging from6 to 7.5. In a 1975 survey of 216 lakes in the same area, a largenumber of them had pH values below 5, a level at whichmost aquatic life perishes (FIGURE 20-10). Of the acidifiedlakes, 82% were devoid of fish life. A more recent study of1,500 lakes in New York’s Adirondack Park found that 25%of the lakes are so acidic that fish no longer live in them.Another 20% of lakes are acidic enough to be endangered.

In 1988, the National Wildlife Federation (NWF) pub-lished a list of U.S. lakes that had become acidified. Thestudy showed that eastern lakes had been particularly hardhit. One of every five lakes in Massachusetts, New Hampshire,New York, and Rhode Island was acidic enough to be harm-ful to aquatic life. As acid deposition continues to fall, theselakes could become lethal to virtually all forms of life.

Acid precursors can be transported hundreds of kilometers fromtheir site of production to their site of deposition. Acid depo-sition occurs downwind from virtually all major industrial and ur-ban centers. Acid deposition is increasing in strength (acidity)and expanding in geographic range.

Studies show that 4% of the lakes and 8% of the streamsin acid-sensitive areas in the United States are chronicallyacidic. This includes lakes in the mid-Appalachians, theAdirondack Mountains, New England, the Atlantic coast,Florida, and the upper Midwest. Although the percentagesmay seem small, they translate into thousands of lakes andstreams.

Studies have shown that acid deposition also occurswidely over the northern and central portions of Florida,with precipitation 10 times more acidic than normal. One-third of the lakes in Floridaare now acidic enough to beharmful to aquatic life.

In the mountains ofsouthern Scandinavia, acid-ification of surface waters hasoccurred at a rapid rate for

Lab pH

20061955

> 5.6

5.0–5.6

4.7–5.0

4.6–4.7

< 4.6

FIGURE 20-9 Acid pre-cipitation: a growingproblem. As shown here,acid precipitation in theeastern United States hasincreased between (a)1955 and (b) 2006. Notethe worsening of acid rainand the wider area experi-encing it. Thanks to regu-lations, emissions of acidprecursors have fallen andacid deposition has begunto decline. (NationalAtmospheric DepositionProgram [NRSP-3], 2007.)

(b)(a)

Per

cent

of l

akes

0

40

30

20

10

pH87654

1930–1938(320 lakes)

1969–1975(216 lakes)

FIGURE 20-10 Changing pH. These graphs show the pH level inAdirondack lakes. In the 1930s most lakes had a pH of 6.0 orhigher, but in the 1970s a large percentage had a pH of less than 5.

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more than 50 years. In Sweden, approximately 20,000 lakesare without or soon to be without fish. Swedish authoritiesestimated that about 17% of the nation’s lakes are acidifiedfrom acid deposition. Salmon runs in Norway have beeneliminated because of the impact of acid precipitation onegg development, putting an end to inland commercial fish-ing in some areas.

In Canada, nine of Nova Scotia’s famous salmon-fishingrivers have already lost their fish populations because ofacidity. Eleven more are teetering on the brink of destruction.In a survey of 8,500 lakes in southeastern Canada, 31% ofthem (2,635 lakes) had pHs lower than 6. Thirteen percent(1,100 lakes) had pH values below 5.5.

KEY CONCEPTS

Why Are Some Lakes More Susceptible Than Others? Alllakes are not created equal. Those that lie downwind frommajor industrial centers are most vulnerable, but location isnot a sufficient predictor of a lake’s future. Those found inwatersheds with thin soils with little buffering capacity fallvictim to this insidious form of pollution. Some lakes also con-tain buffers, chemical substances that allow aquatic systemsto resist changes in pH. When hydrogen ion (H�) levels in-crease, buffers combine with the free ions and eliminatethem. When levels fall, they release them, thus maintaininga constant pH. In general, the higher the buffering capacityof a lake and its surrounding watershed, the less vulnerableit is. FIGURE 20-11 shows acid-sensitive areas in the UnitedStates and Canada.

KEY CONCEPTS

How Does Acid Deposition Affect Aquatic Ecosystems?Many species of fish die when the pH drops below 4.5 to5.0 (FIGURE 20-12). Falling pH is only part of the reason fishdie. Scientists have found that acidic rainwater or snowmeltdissolves toxic elements such as aluminum from soil androcks. The acidic waters carry the metals to streams andlakes. Aluminum irritates the gills of brook trout, causing abuildup of mucus and, ultimately, death by asphyxiation(FIGURE 20-13).

Spring poses a special threat to fish and other aquatic or-ganisms. When the snow begins to melt, the surface meltsfirst. This water drains through the unmelted snowpack andleaches out most of the acids. In fact, the first 30% of the melt-water contains virtually all of the acid and typically has apH of 3 to 3.5, which is toxic to eggs, fry, and adult fish aswell. Thus, when snow begins to melt, the concentration ofacid in nearby lakes and streams rises rapidly. This surge ofacids coincides with the sensitive reproductive period formany species of fish.

The buffering capacity of the soil and surface waters—theirability to resist changes in pH—plays an important role in de-termining if a lake will be damaged by acid deposition.

Acid deposition has acidified lakes throughout the world. Hun-dreds of lakes no longer support aquatic life, and thousands areon the verge of ecological collapse.

KEY CONCEPTS

Widening the Circle of Destruction Many other speciesare affected by acid deposition. Songbirds living near acid-contaminated lakes in Scandinavia, for instance, lay eggswith softer shells than birds feeding near unaffected lakes.Scientists have found elevated levels of aluminum in thebones of affected birds and hypothesize that this comes fromeating aquatic insects living in acidified waters. The alu-minum interferes with normal calcium deposition, resultingin defective (soft) eggshells and fewer offspring.

Acidification of surface waters may also be partly re-sponsible for a nearly 50% decline in the population of blackducks on the East Coast from the 1950s to the early 1980swhen populations reached an all-time low. Today, populationsremain steady. Although other factors have accounted forpart of this decline, it appears that acids are killing the aquaticinsects needed by female ducks and their offspring.

Spotted salamanders are also adversely affected by acid-ity. In the laboratory, exposure to water with pH of 5 preventsnormal embryonic development and results in gross defor-mities that are usually fatal. In one study, it was found thatthe mortality of fertilized eggs was 60% at pH 6 but only 1%at pH 7.

In the wild, spotted salamanders breed in temporaryponds created by melted snow. These ponds are likely to be

Acidity kills aquatic organisms outright, but it also impairsgrowth and reproduction. Acidity also leaches heavy metals,which are toxic to fish, from the soil. The spring snowmelt re-sults in a sudden influx of acids and heavy metals that is highlydamaging to aquatic life.

30˚

45˚

60˚

30˚45˚60˚

75˚75˚

90˚105˚120˚135˚75˚150˚

165˚180˚

60˚

45˚

150˚

135˚

105˚ 90˚ 75˚

Sensitive areas

Major sources of sulfurdioxide, mostly coal-firedpower plants

Wind paths:Summer

Winter

FIGURE 20-11 Sensitive areas. This map of North America showsareas experiencing acid deposition and the most geologically vul-nerable regions.


highly acidic in regions where acid precipitation is prevalent;as a result, the fate of the spotted salamander is bleak.

The spotted salamander is as important as birds andsmall mammals in the food chain. A drastic change in itspopulation would very likely have serious repercussions inthe entire ecosystem. Many other amphibian species have dis-appeared or severely declined in their natural habitat, largelyas a result of acid deposition, as pointed out in Chapter 7.

KEY CONCEPTS

Forest and Crop Damage Researchers estimate that 120,000 hectares (300,000 acres) of forest has been destroyedin the former Czechoslovakia by pollution, mostly acid pre-cipitation. In Germany, 500,000 hectares (1.25 million acres)of forest is dead or dying (FIGURE 20-14). Even the famousBlack Forest is now severely damaged by acidic pollutantsfrom industry and automobiles. In Vermont’s Green Moun-tains, half the red spruce, a high-elevation tree, have died fromacid precipitation and acid fog. Lower-elevation sugar maplesare also on the decline. The death of trees exacts huge eco-logical and economic costs. Forest-dwelling species perishas their forests die. The loss of timber reduces revenues.Swiss scientists believe that damage to trees on mountainslopes may increase the likelihood of avalanches becausetrees help retain snow on steep mountainsides. Further lossof the “barrier forests” endangers the safety of mountain res-idents, skiers, and highway travelers.

Numerous studies show that acid deposition damagesforests both directly and indirectly. Consider direct damagefirst. Acid deposition damages leaves of birch and needles ofpine trees. It also impairs germination of spruce seeds. Iterodes protective waxes from oak leaves and leaches nutri-ents from plant leaves. In 1988, Robert Brock, a forest epi-demiologist, reported findings from studies on Mt. Mitchellin North Carolina. In his study, he found that low-lyingclouds containing acids that often bathe spruce and fir trees

Acid deposition affects birds living near lakes and aquatic speciessuch as salamanders, which are a key element of the food chain.

Waterboatman

Whirligig

Yellow perch

Lake trout

Brown trout

Salamander(embryonic)

Mayfly

Smallmouthbass

Mussel

6.5 6.0 5.5 5.0 4.5 4.0 3.5pH

FIGURE 20-12 Differing sensitivity. The sensitiv-ity of fish and other aquatic organisms to acid levelsvaries. The figure indicates the lowest pH (highestacidity) at which the various organisms can survive.The yellow perch, for example, can withstand a pHof 4.5, but populations plummet if the pH falls anyfurther. Smallmouth bass and mussels are more acidsensitive, perishing at levels below 5.0 and 5.5, respectively.

FIGURE 20-13 Death by asphyxiation. These fish were confinedto a cage in a stream affected by acid rain. They died of asphyxia-tion caused by acid leaching aluminum from the soil. Aluminumirritates the gills and causes mucus buildup, which blocks oxygeninflux and kills the fish.


on the mountain are considerably more acidic than vinegar.Two days after a 2-day cloudy period, Brock found that nee-dle tips looked singed. The needles contained 7 to 11 timesmore sulfate than healthy ones. Needles and leaves dam-aged by acid develop brown spots. Photosynthesis, the processby which needles and leaves make foodstuffs, declines—causing the tree to suffer nutritionally. Badly damaged leavesand needles actually fall off.

Indirect effects can alsobe important. Acids, for ex-ample, dissolve nutrients andvaluable minerals from thesoil trees grow in. Theseimportant substances areflushed from the soil, thusdepriving plants of vital components essential for growthand reproduction. Acids also cause the release of toxic sub-stances in soil, such as naturally occurring aluminum, whichis normally chemically bound within the soil and thus nota problem. Recent evidence shows that aluminum damagescells in the water-transporting tubules of trees, closing off wa-ter transport. Trees die from thirst. The release of these sub-stances can damage trees and other plants.

In 1988, Professor Lee Klinger from the University of Col-orado proposed another hypothesis to help explain why theworld’s forests are dying. One of the chief culprits, he says,may be an acid-loving moss that grows on the forest floor.Klinger has studied 100 regions in 30 states where forests aredying. In each one he has found a thick layer of moss carpetingthe forest floor.

These mosses act as sponges, holding so much waterthat the surface soils become saturated. In affected areas,

the feeder roots of the trees and the trees themselves die, forthe same reason that a houseplant dies when it is overwatered:Water eliminates air from the soil. Plants literally suffocate.Mosses may also kill mycorrhizal fungi that help trees absorbnutrients. Mosses also acidify the water passing throughthem. Acidic water dissolves toxic trace metals such as alu-minum found in the soil, which can also kill the root system.It is likely that direct foliar damage and root damage may com-bine forces.

Acid deposition does not kill trees directly, according tothe U.S. EPA. It is more likely to weaken them, by damagingtheir leaves, limiting the nutrients available to them, or poi-soning them with toxic substances slowly released into thesoil. Weakened trees are more susceptible to diseases, in-sects, drought, and other pollutants such as ozone that ul-timately kill them. Combined, these forces may be responsiblefor the massive reduction in forest growth noticed by forestersin the eastern United States. It reminds us how important itis to consider the interaction of many changes we havewrought.

Just like trees, crops are affected directly and indirectlyby acids. The direct effects include damage to leaves andbuds. Acids falling on crop plants in the spring may impairgrowth at a very important time of year. In addition, acids mayinhibit photosynthesis, the process by which plants producecarbohydrates and other important chemicals.

Acids damage plants indirectly—by altering the soil.For example, they may leach important elements from thesoil, resulting in reduced growth. Acids impair soil bacte-ria and fungi that play an important role in nutrient cy-cling and nitrogen fixation, both essential to normal plantgrowth.

Concern for agriculture has also been raised by nu-merous researchers, but the results of many studies are in-conclusive. Some researchers have reported that simulatedacid precipitation decreases crop productivity, but othershave found increases. Still others have found no effect. Onbalance, the EPA concludes that food crops are not seriouslyaffected by acid deposition.

KEY CONCEPTS

Acids: Fertilizing Effect Aquatic and terrestrial plants re-quire sulfur and nitrogen to grow. In some instances, acid rainmay enhance soil fertility and improve crop growth.

On balance, acids probably do more damage than good.Direct damage to growing plants and damage to the soil off-set the fertilizing effect. Studies have also shown that nitro-gen in the form of nitrates or nitric acid deposited inChesapeake Bay actually stimulate the growth of algae andaquatic plants.

Plant overgrowth in Chesapeake Bay caused by nitric acidimpairs navigation. In addition, plants growing on the sur-face block sunlight needed for photosynthesis in plants and

Acid deposition damages forests in many parts of the world andmay affect crops as well. Trees and other plants are damaged di-rectly by acids but also indirectly, through changes in the soilchemistry and soil-dwelling organisms.

FIGURE 20-14 Forest die-off. This forest in Germany is nowlargely dead because of many years of acid deposition.

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algae in deeper layers. These deeper plants help maintainoxygen levels; without them, other forms of aquatic life mayperish. In addition, when aquatic vegetation dies in theautumn, it decays. The bacteria that break down this or-ganic matter rob the water of oxygen, killing many aquaticorganisms.

In a recent study of the effects of nitrogen fertilizationon the prairies of Minnesota (published in Science), re-searchers found that additional nitrogen benefited weedyspecies that overtook native prairie grasses. Native speciesrequire less nitrogen and were choked out by weedy species.This not only altered the species composition, it resulted ina greater emission of carbon dioxide, with implications forglobal warming. The excess carbon dioxide emissions re-sult from the fact that weedy species decompose much morerapidly than native prairie grass species.

KEY CONCEPTS

Damage to MaterialsAcid precipitation also corrodes humanmade structures. It hastaken its toll on some of special importance, such as the Statueof Liberty, the Canadian Parliament Building in Ottawa, Egypt’stemples of Karnak, and the caryatids of the Acropolis—notjust architectural works but works of art, priceless treasures (FIG-URE 20-15). Acid rain may also damage house paint and etchthe surfaces of automobiles and trucks. A U.S. report claims

The sulfur and nitrogen in sulfuric and nitric acid promote plantgrowth, but their negative effects (such as direct damage andchanges in the soil chemistry) typically outweigh any benefitsresulting from their fertilizing effect.

that acid precipitation causes an estimated $5 billion a year indamage to buildings in 17 northeastern and midwestern states.The price tag includes the cost of repairing mortar, galvanizedsteel, and stone structures as well as the cost of repainting. It doesnot include damage to automobile paint, roofing materials,and concrete, potentially adding billions of dollars to the cost.

KEY CONCEPTS

Solving a Growing Problem—Short-Term SolutionsIn 1984, the New York State legislature passed a bill that re-quired utilities to reduce sulfur emissions by 30% by 1991.Minnesota also passed legislation to curb the growing prob-lem. In 1984, nine European nations and Canada signed anagreement to make similar reductions, but over a 10-yearperiod. Canada’s program was finalized in 1985 with sevenprovinces agreeing to reduce their combined sulfur dioxideemissions by 2.3 million metric tons (2 million tons) peryear by 1994, a goal they met in 1993 primarily by switch-ing to low-sulfur coal.

The first significant U.S. governmental action came withthe passage of 1990 amendments to the Clean Air Act (Chap-ter 19), which contained provisions for substantial reductionsin sulfur oxide emissions but more modest cuts in nitrogen ox-ides by the year 2010. Although the goals were not met, therewere substantial reductions in the release of acid precursors.

In 1991, the United States and Canada signed an agree-ment to reduce emissions of sulfur dioxide and nitrogendioxide from power plants, smelters, and other stationarysources. Canada, highly motivated to protect its lakes fromfurther deterioration, met its goal by 1993. Huge reductionsoccurred at some facilities such as the world famous Inconickel smelter in Sudbury, Ontario, where 90% reductionswere achieved. The United States has not yet achieved either of its goals. Although sulfur dioxide emissions havebeen reduced, they’re still over 2 million metric tons (1.8 million tons) higher than the goal of the agreement.Nitrogen dioxide emissions have been even harder to bringunder control and have increased by over a million tonsper year in the past 5 years.

International efforts to control sulfur oxide emissionsgenerally rely on three strategies: (1) the installation of scrub-bers on new and existing coal-fired power plants, (2) thecombustion of low-sulfur coal or natural gas in utilities, and(3) the combustion of desulfurized coal—that is, coal that hashad most of its sulfur removed. Nitrogen dioxide levels aremore difficult to control because nitrogen oxide gases comefrom the nitrogen in air that reacts with oxygen in high-temperature furnaces. Changes in the temperature of com-bustion, however, can help reduce nitrogen oxide emissions.

Another approach to the problem involves treating thelakes themselves. In 1977, for example, the Swedish gov-ernment embarked on an expensive program to neutralize

Acids cause billions of dollars of damage to priceless statues,buildings, and materials.

FIGURE 20-15 Lady Liberty gets a face-lift. The Statue of Lib-erty was recently given a multimillion-dollar face-lift because ofdamage from air pollution, including acid deposition. The repairwork cost well over $35 million.


acidic lakes by applying lime to thousands of lakes and rivers(FIGURE 20-16). These actions improved the water quality inmany lakes, saving fish populations, but cost tens of mil-lions of dollars per year.

Critics argue that liming is a short-term, stopgap solu-tion, a little like CPR administered to a heart attack victim.In Canada liming costs $120 per hectare ($50 per acre).Treating a single lake can cost between $4,000 and $40,000.In 5 years, however, treated lakes turn acidic again.

Stopgap measures are essential, as noted earlier in thebook. They are the first line of defense against acid deposition,but many much less costly—indeed, profit-generating—options are available to us.

KEY CONCEPTS

Long-Term Sustainable StrategiesLiming lakes and other approaches outlined in the previoussection generally treat the symptoms of the problem whileignoring the root causes—in this case, our heavy dependenceon and inefficient use of fossil fuels. To solve this problemsustainably, we must find ways to address the root causes. In-creasing energy efficiency and reducing our dependence onfossil fuels by using solar and wind energy are key elementsof a sustainable strategy, which are discussed in Chapter 15.

Recognizing the importance of these alternative mea-sures, several countries—including Germany, Great Britain,Japan, Norway, and Sweden—are actively developing alter-native fuel sources of energy and increasing the efficiency withwhich they use fossil fuels. In Mexico, utilities, government,

Many stopgap measures have been initiated to help reduce thethreat of acid deposition, including the installation of smoke-stack scrubbers, combustion of low-sulfur or desulfurized coal,and liming lakes to neutralize acidity. Such measures are nec-essary in the short term but must eventually be replaced bylong-term, preventive actions.

and industry have teamed up in hopes of mak-ing energy efficiency a cornerstone of thecountry’s development. In India, private busi-nesses have launched a program to installthousands of energy-efficient compact fluo-rescent lightbulbs in homes and businesses.

Population stabilization, growth manage-ment, and recycling are also essential elementsof our prevention strategy. All of these reduceour demand for energy and, therefore, our pro-duction of pollutants. These strategies helpsolve more than acid deposition—they’ll helpreduce habitat destruction, the loss of species,urban air pollution, water pollution, and otherenvironmental ills.

KEY CONCEPTS

Are Controls on Acid Deposition Working?In 1980, the U.S. Congress passed the National Acid Pre-cipitation Act. Among other things, this law authorized theformation of the National Acid Precipitation Assessment Pro-gram (NAPAP)—a program staffed by people from a varietyof federal agencies including the EPA, Department of En-ergy, Department of Agriculture, and NOAA. Their goal wasto first assess the state of the environment—how much dam-age acid deposition has caused to determine how severe theproblem is. Their findings showed that although it is a prob-lem in some areas, it has not reached crisis stages in theUnited States. Some critics take issue with this statement, not-ing that huge numbers of lakes have been destroyed in thenortheastern United States with a huge impact on the tourismeconomy.

The Acid Deposition provisions of the 1990 Clean AirAct (discussed in Chapter 19) called for a dramatic reduc-tion of sulfur dioxide emissions to 9 million metric tons (10million tons) per year by 2010—over a 50% reduction. Italso required reduction, albeit a modest one (about 10%), of1.8 million metric tons (2 million tons) per year in nitrogenoxide emissions.

The 1990 Clean Air Act also directed the NAPAP to de-velop market-based controls on acid deposition—that is,strategies that encouraged electric utilities to cut emissionsthrough innovation and other methods that made more senseeconomically than old command and control regulations. Oneidea was the tradable permit, discussed in Chapter 19. Fur-thermore, the NAPAP was directed to monitor progress andeconomic costs and benefits of controls.

Their early findings were that market-based controlswere very effective in cutting sulfur emissions. In the first yearalone, emissions of sulfur dioxide from the worst polluters

Fuel efficiency, renewable fuels, recycling, population stabi-lization, and growth management are key elements of a sus-tainable design strategy to help prevent the production of acidprecursors—and hence reduce acid deposition.

FIGURE 20-16 Liming rivers. Millions of dollars are spent by theSwedish government to lime rivers each year to offset the influx ofacids from the sky.


fell by nearly 40%. Even more remarkably, costs of compli-ance have been far less than originally projected—only about15% of the estimates. Have sulfur dioxide and nitrogen ox-ide emissions fallen?

From 1980 to 2008, the most recent year for which dataare available, sulfur dioxide emissions declined by 50%. Ni-trogen oxide emissions decreased by 36%. Has the declinein acid precursors affected acid deposition and other prob-lems associated with it?

In response to the decreases in sulfur dioxide emissions,concentrations of sulfate and sulfuric acid in precipitationhave shown a significant decline in many parts of the UnitedStates. Decreases in wet sulfate deposition have averagedmore than 30% in the Eastern U.S. (FIGURE 20-17). Nitrateconcentrations in rain, as expected, have shown less change(FIGURE 20-18). Many lakes and streams also reflect changesin the chemical makeup of the rain. There is now evidenceof recovery of acidified lakes in New England, but the mostbadly damaged lakes of the Adirondacks have shown nochange. The NAPAP concluded that most forests in the na-tion are not adversely affected by acid deposition, but “ifdeposition levels are not reduced in areas where they arepresently high, adverse effects may develop due to chronicmultiple decade exposure.”

KEY CONCEPTS

Global Climate ChangeScientists have long known that air pollution can affect lo-cal weather. For example, smoke from factories can sub-stantially increase rainfall in areas downwind. Many scientistsalso believe that air pollution can affect weather globally.Evidence for this phenomenon has been growing steadilyfor many years. In 1995, a group of 2,500 atmospheric sci-entists concluded that the bulk of the evidence showed thathuman activities (pollution and deforestation) were havinga discernible effect on global climate that could have profoundeffects on people, the economy, and the environment. Theycontended that human activities were increasing pollutantsthat caused global warming, an increase in average globaltemperature. Global warming, in turn, was altering the Earth’s

20.3

Market-based strategies, in particular tradable permits, haveproven successful in reducing sulfur dioxide emissions in theUnited States, with corresponding changes being seen in the acid-ity of rainfall as well as many lakes and streams. Still, Americansare a long way from the goals of reducing sulfur and nitrogen diox-ide set out in the 1990 Clean Air Act.

1989–1991 2006–2008

FIGURE 20-17 Acid deposition on the decline. These maps show a significant decrease in wet sulfate emissions in the U.S. (Source: Environ-mental Protection Agency http://www.epa.gov/airmarkt/progress/ARP_4.html)

1989–1991 2006–2008

FIGURE 20-18 Acid deposition on the decline. These maps show the decrease in wet nitrate deposition resulting from reductions in atmo-spheric nitrogen oxide emissions. (Source: Environmental Protection Agency http://www.epa.gov/airmarkt/progress/ARP_4.html)

climate, that is, causing global climate change. Climate is theaverage weather conditions. Their announcement was metwith skepticism by a handful of scientists and by industry,especially the oil and coal industries.

In December 1999, the head of the U.S. National Oceanicand Atmospheric Administration and the head of the BritishMeteorological Office, agencies that normally do not ventureinto the political arena, agreed with this assessment. Theypublished a joint letter in a London newspaper, The Inde-pendent, in which they wrote, “Our climate is changing rap-idly and it’s important that we take action now. . . . webelieve the evidence is almost incontrovertible, that manhas an effect (on climate) and therefore we need to actaccordingly.”

Some oil companies, such as BP (formerly British Pe-troleum) accepted the scientific consensus and began to takesteps, for example, by investing in renewable energy anddecreasing their output of CO2. BP has become a major playerin commercial wind farms. Exxon Mobil, however, did notagree and has since spent several million dollars to convincethe American public that global warming is not a threat. Tounderstand this issue, we first look at the science of global en-ergy balance, the basis for the planet’s climate.

The Science Global Energy Balance and the Greenhouse EffectEach day the Earth is bathed in sunlight. Approximatelyone-third of the sunlight striking the Earth and its atmosphereis reflected back into space. The rest is absorbed by the air,water, land, and plants. As anyone who has sat in the sunknows, sunlight is absorbed by surfaces and converted intoheat or, more technically, infrared radiation. As shown inFIGURE 20-19a, this heat is slowly radiated back into the at-mosphere and eventually escapes into outer space. As a re-sult, energy input is balanced by energy output.

Scientists have long known that certain chemical sub-stances in the atmosphere alter this balance. These includewater vapor, carbon dioxide (CO2), nitrous oxide (N2O),methane (CH4), and chlorofluorocarbons (Table 20-2). Howdo these gases affect the Earth’s energy balance? Consider car-bon dioxide. Carbon dioxide molecules in the air absorb in-frared radiation escaping from the Earth’s surface and radiateit back to Earth, acting much like the glass in a greenhouse(FIGURE 20-19b). Other molecules act similarly.

This phenomenon, known as the greenhouse effect, isessential to life on planet Earth, for it helps maintain theEarth’s surface temperature. Without it, the Earth would beat least 30°C (55°F) cooler than it is today—and inhospitableto most life-forms.

The Earth’s surface temperature can be altered by changesin the concentrations of CO2 and other chemical substancespreviously mentioned. An increase in carbon dioxide lev-els, for instance, causes more heat to be radiated back to theEarth, which warms the planet. Decreases in carbon dioxideor other similar compounds have the opposite effect.

The chemical substances that increase the Earth’s sur-face temperature are called greenhouse gases because theyact somewhat like the glass in a greenhouse. You’ve observedthe greenhouse effect in a car or house heated by the sun. Theglass permits sunlight to enter the car’s interior, where it isabsorbed by interior surfaces and converted to heat. Theglass prevents heat from escaping. On Earth, sunlight is ab-sorbed by plants, dirt, buildings, parking lots, and roadwaysand is converted into heat. The heat or infrared radiationthen rises, but some of it is trapped in the atmosphere bygreenhouse gases.

Studies of the global energy balance show that the Earth’ssurface temperature is influenced by two key factors: thosethat affect the amount of sunlight striking the Earth andthose that alter the amount of heat lost or retained. In otherwords, the Earth’s temperature is affected by the influx of


(a) (b)

Absorbed by air, land, and water and converted to heat

6% Reflected fromwater, soil, air,vegetation

5% Reflected from dust

Total reflected 3%

1%–2% Absorbed by plants

Water vaporCH4

Heat Heat

Heat

HeatNO2

CO2

21% Reflected from clouds

67% Heat

FIGURE 20-19 Global energy balance. (a) This drawing shows the influx of solar radiation and its fate. (b) Carbon dioxide and othergreenhouse gases radiate heat back to Earth, causing the Earth’s surface temperature to increase.


solar energy and loss of heat. Both natural and anthro-pogenic factors influence these processes.

KEY CONCEPTS

Natural Factors That Influence GlobalTemperatureThe average temperature of the Earth and, as a consequence,the Earth’s climate, changes naturally over time. In fact, theEarth cycles between cold spells (ice ages) and interglacialwarm periods. We’re currently in an interglacial warmingperiod.

These natural changes in climate, while not completelyunderstood, are the result of at least five natural phenomena:(1) changes in the Earth’s orbit around the sun, (2) changesin the Earth’s tilt, (3) increases or decreases in solar activity(sunspots), (4) increases or decreases in volcanic activity, and(5) chaotic interactions of the climate system.

As you may recall from early science classes, the Earthorbits around the sun in an elliptical path. Scientists havefound that the Earth’s orbit changes over time, sometimesbringing the Earth slightly closer to the sun, causing warm-ing, sometimes moving the Earth slightly farther away fromthe sun, causing cooling.

You may also recall from earlier studies that the Earth istilted at approximately a 23.5-degree angle (see Figure 5-1 onpage 74). Surprisingly, the Earth’s tilt can also change veryslightly, altering the average temperature and either heatingor cooling the planet.

Much of the sunlight striking the Earth and its atmosphere is con-verted into heat and is eventually radiated back into space. Nat-ural and anthropogenic factors affect the amount of solarradiation striking the Earth and the rate at which heat escapesand, thus, influence the temperature of the Earth’s atmosphere.

Solar activity is also inconstant. In fact, scientists havefound that the sun’s output goes through an 11-year cycle.Increased output results in natural planetary warming.

Changes in volcanic activity also play a role in climate.Increased volcanism, for instance, results in the release of ashthat reduces solar gain, cooling the Earth. According to sci-entists, volcanic activity seems to play an important role inyear-to-year climate variability, and there is not much evidenceto suggest that prolonged periods of volcanic eruptions causelong-term changes.

The last cause of long-term natural climate change ispotentially chaotic interactions within the climate sys-tem. Complex and inherently unpredictable interactionstake place between major components of the Earth’s cli-mate, for example, the oceans and the atmosphere. Suchinteractions could, say some scientists, cause the climatesystem to change over time, resulting, by chance, in ei-ther a period of cooling or warming that could last fordecades to centuries. The continued action of these ef-fects could even restore conditions to something resemblingthe earlier state.

As shown in FIGURE 20-20, the Earth’s temperature and theconcentration of two naturally occurring greenhouse gases,methane and carbon dioxide, also change cyclically—approximately every 100,000 years or so. What causes thesechanges? Scientists believe that natural causes—for example,changes in the Earth’s tilt or solar activity—stimulate increasesin temperature. It in turn increases forest and grassland firesand the decay of organic material, which increase carbon diox-ide and methane levels. These greenhouse gases, in turn, in-crease global average temperature up to a point.

KEY CONCEPTSThe Earth’s climate shifts naturally as a result of many factors,including changes in solar activity and volcanic activity, as wellas changes in the Earth’s orbit and the tilt of the Earth.

Table 20-2Major Greenhouse Gases and Their Characteristics

Relative CurrentAtmospheric Annual Life Greenhouse GreenhouseConcentration Increase Span Efficiency Contribution Principal Sources

Gas (ppm) (%) (Years) (CO2 � 1) (%) of Gas

Carbon dioxide (CO2) (from fossil fuels)Carbon dioxide (from biological sources)Chlorofluorocarbons(CFCs)Methane (CH4)

Nitrous oxide (N2O)

Sources: Data from World Watch Institute, U.S. EPA, and Journal of Geophysical Research.1Carbon dioxide is a stable molecule with a 2- to 4-year average residence time in the atmosphere.

380

0.000225

1.774

0.319

0.4

5

1

0.2

x1

75–111

11

150

1

15,000

25

230

57 (44)

(13)

25

12

6

Coal, oil, natural gas, deforestation

Foams, aerosols, refrigerants, solventsWetlands, rice, fossil fuels, livestockFossil fuels, fertilizers,deforestation


CH4 = 1755 parts per billion in 2004

CO2 = 377 parts per million in 2004

Carbon dioxide (CO2) parts per million

Methane (CH4) parts per billion

Earth’s temperature (degrees C)

Thousands of years before 1850

400

400

350

300

250

200

700

600

500

400

20

–2–4–6–8

–10350 300 250 200 150 100 50

Date

0

1900

2000

FIGURE 20-20 Natural cycles. The Earth’s temperature (bottompanel) cycles approximately every 100,000 years. Methane andcarbon dioxide levels also undergo long-term cycles, as explainedin the text. Notice that today, carbon dioxide and methane levelsfar exceed those found in the previous 450,000 years. What effectwill this have on average global temperature? (Reproduced withkind permission from Springer Science & Business Media; Clim.Change, A Slippery Slope: How much global warming constitutes“dangerous anthropogenic interference?” vol. 68, 2005,pp. 269–279, J. E. Hansen.)

their life spans. It also lists their relative greenhouse effi-ciency—that is, how they compare as a greenhouse gas to car-bon dioxide. As shown, one molecule of CFCs is equivalentto 15,000 molecules of carbon dioxide, partly because CFCslast so long. Pay special attention to the contribution of eachto the greenhouse effect in column 6. This takes into ac-count the relative effect of each one and the amount in theatmosphere.

As you can see, carbon dioxide is the major greenhousegas and emissions of this greenhouse gas are on the rise (Fig-ure 20-21a). Not all carbon dioxide comes from the com-bustion of fossil fuels. Deforestation and torching of clearedforests, for example, also produce a significant amount of car-bon dioxide. How does deforestation result in an increase inatmospheric carbon dioxide? As you may recall from study-ing the ecology chapters, all plants, including trees, take upcarbon dioxide from the atmosphere, which they convertinto plant matter. As trees are stripped from the land, theEarth’s capacity to absorb carbon dioxide decreases. At-mospheric levels increase.

KEY CONCEPTS

Are Global Warming and Global ClimateChange Occurring?While many people believe that the Earth’s oceans and ouratmosphere are warming, others are not so sure. What ishappening?

As shown in FIGURE 20-21c, the Earth’s temperature is in-deed increasing and the climate is shifting as a result. The tem-perature increase is determined from data obtained fromthousands of temperature sensors distributed over land andsea throughout the Earth’s surface. Temperature readingsshow a steady increase in global average temperature sincethe 1880s with only a small dip in temperature from 1940 to1960. Scientists believe that this transient decline may havebeen caused by particulate pollution from factories duringthe post-war surge in economic activity. Particulates reducesunlight striking the Earth. Starting in the 1960s, air pollu-tion laws reduced particulate pollution, eliminating thiscooling effect.

FIGURE 20-22 looks at the magnitude of change by com-paring the average temperature during the period of 2001 to2005 to the average from 1951 to 1980. As you can see, tem-perature has increased nearly 2 degrees Celsius over thepoles, especially the North Pole.

While rising average global temperature shows that theEarth is warming, there’s an abundance of additional evi-dence that change is underway. Here is a summary of evidencefor global warming and global climate change:

Rising sea level. In the past 50 years, sea level hasrisen 10 to 12.5 centimeters (4 to 6 inches), as a re-sult of melting glaciers in mountainous areas and inAntarctica.

Greenhouse gases come from natural and anthropogenic sources,the latter of which have been increasing dramatically over thepast 60 years.

Anthropogenic Factors That IncreaseGlobal TemperatureHuman activities also affect global average temperature. Twoactivities are most important: the release of greenhouse gasesand deforestation. As noted earlier, greenhouse gases, suchas carbon dioxide and methane, come from natural and an-thropogenic sources. Although the release of greenhousegases from natural sources has remained fairly constant overthe past 100 years, emissions from human sources have in-creased dramatically. Annual carbon dioxide emissions fromthe combustion of fossil fuels, for instance, have climbedfrom a mere 534 million tons (of carbon) to about nearly8,500 million tons per year in 2010 (FIGURE 20-21a) in thepast 100 years. Chapter 19 described the sources of these andother pollutants.

Not all greenhouse gases have a natural source. Chlo-rofluorocarbons and their replacements, the hydrochloro-fluorocarbons, for example, have no natural source. Theirproduction and release have risen rapidly since 1950, al-though CFCs release has declined dramatically as a result ofa use ban and HCFCs release will also begin to decline as well.

Table 20-2 lists the four greenhouse gases and several im-portant facts about each one, including their sources, therates at which they are increasing in the atmosphere, and


Rising ocean temperatures. Studies show that the oceansare warming to a depth of 3,000 meters (10,000 feet). Theoceans currently absorb about 80% of the heat added tothe climate system by global warming. As the tempera-ture of water increases, water expands. This is thoughtto be part of the reason for rising sea level and chang-ing intensity of storms, especially hurricanes.

Melting glaciers. Global warming is evidenced by rap-idly melting glaciers throughout the world.

Breakup of Antarctic ice sheet. Huge blocks of ice, oftenthe size of small New England states, are breaking off theAntarctic ice sheet, signaling a melting and break-up ofthis massive expanse of ice.

Dramatic decrease in Arctic polar sea ice and land-basedice in Greenland (FIGURE 20-23). A research ship oncetook 2 years to traverse the North Pole; in 2000, theship made the trip in a few months because of the lackof ice. Over a period of 2 years, a huge ice shelf that hasextended into the Arctic Ocean for the past 3,000 yearshas broken up, threatening polar bear habitat.

Heat waves and drought on the rise. Heat and droughtare taking their toll on agriculture worldwide, causingsevere economic problems in some areas. In the early2000s, severe drought plagued many parts of the UnitedStates. Heat waves are also responsible for numerous

Mill

ion

tons

0

8,000

7,000

6,000

5,000

4,000

3,000

2,000

1,000

Year

Annual CO2 Emissions

1951 1976 2001 20082006192619011876185118261776 18011751

Atm

osph

eric

car

bon

diox

ide

(par

ts p

er m

illio

n)

0

400

375

350

325

300

Year201020001990198019701960

Atmospheric Concentration of CO2

Global Land–Ocean Temperature Index

Tem

pera

ture

Ano

mal

y (°

C) Annual Mean

5–year Running Mean

.6

.4

.2

.0

−.2

−.4

1880 1900 1920 1940 1960 1980 2000

FIGURE 20-21 Global carbon dioxide emissions and global temperature.(a) Annual carbon emissions from fossil fuel burning, 1751–2004. (Data fromUN, BP, DOE, and IEA.) (b) Annual mean carbon dioxide levels in the atmo-sphere have risen dramatically since 1960. (Data from Scripps Institute ofOceanography.) (c) Graph of average global temperature since 1950. (Datafrom GISS, BP, IEA, CDIAC, DOE, and Scripps Institute of Oceanography.)

(a)(c)

(b)

SPOTLIGHT ON SUSTAINABLE DEVELOPMENT

20-1 Intel Pledges Huge Purchase of Green Energy

In 2008, computer chip giant Intel announced that thecompany will purchase more than 1.3 billion kilowatt hoursa year of renewable energy certificates (RECs) from SterlingPlanet to offset carbon dioxide emissions from their oper-ations. The electricity will be generated by wind, solar,small hydroelectric facilities, and biomass. According tothe U.S. EPA, this purchase makes Intel the single-largestcorporate buyer of green power—electricity generated fromclean, renewable resources—in the United States.

Besides offsetting their emissions, Intel officials hopedthat their purchase would help stimulate other companiesto follow suit and, hence, boost the market for green power.

Their efforts, combined with those of other companies,could help lower the generating cost of electricity from re-newable resources such as wind.

To learn of other companies, check out EPA’s GreenPower Partners list online. At this writing, Intel takes thenumber one spot, and PepsiCo is number two. Companiesare not the only entities that have made major commit-ments to purchase green power. The Commonwealth ofPennsylvania, the city of Houston, and the city of Dallas areall major purchasers, as are Sprint Nextel, IBM, and theU.S. Department of Veterans Affairs.

deaths. In 2003, an estimated 40,000 people died in Eu-rope as a result of a record-breaking heat wave.

Tornadoes and violent storms are on the rise. Heat en-ergy stored in oceans and in the atmosphere spawnsmore violent storms. A recent study of the severity ofstorms in the United States showed that violent down-pours are on the rise. Studies also show that the occur-rence of tornadoes is on the rise in the United States(FIGURE 20-24). In the 1980s, 600 to 800 tornadoes oc-curred in the United States. In the 2000s, the number hadincreased to 1,000 to 1,700 per year. In 2008, the UnitedStates experienced nearly 1,700 tornadoes that killedover 1,700 people; at this writing (May 2011) there werealready over 1,000 reported tornadoes. Worldwide, theincidence of major weather events has also increased. In1999, there were four times as many major storms andfloods as there were in the 1960s, and damage was seven

times higher. Plagued by devastating floods in Europe inrecent years that many experts think were caused byglobal climate change, the European insurance industryhas become a major supporter of international efforts tocurb the emission of greenhouse gases.

Severity of hurricanes is on the rise. Studies show thatalthough the number of hurricanes has not increased,their severity has (FIGURE 20-25). Hurricane Katrina isa good example. It killed 1,836 people, and displaced1 million residents in Louisiana and Mississippi. Katrinadestroyed 200,000 homes, costing the insurance in-dustry $25.3 billion. It destroyed 500,000 hectares,(1.3 million acres) of forest, costing $5 billion. The to-tal economic impact of this event is estimated to bearound $150 billion.

Forest fires on the rise. During the 16-year period from1987 to 2003, fires have burned 6.5 times as much area


FIGURE 20-23 Loss of arctic sea ice. The yellow line is the aver-age boundary of the ice pack from 1979 to 2004. This photo wastaken in 2005, showing a dramatic decrease in the extent of theice.

Num

ber

of r

epor

ted

torn

adoe

s

600

1600

1700

1800

1900

1400

1500

1300

1000

1100

1200

900

800

700

Year2000 2004 2008199619921988

FIGURE 20-24 Tornadoes. The number of tornadoes each year ison the rise.

FIGURE 20-22 This figure showsthe change in average temperaturethroughout the world. Note that thegreatest increases are occurring inthe poles.


per year as they did between 1970 and 1986. While theincrease in forest and grass fires typically has been blamedon fire suppression and grazing policies, it is more likelythe cause of rising temperature.

KEY CONCEPTS

Are Human Activities Causing GlobalWarming?To most objective observers the evidence is pretty straight-forward: the Earth’s temperature—and its climate—are chang-ing dramatically. Even though these potentially devastatingchanges are occurring, critics say that this does not mean thathumans are the cause? Natural factors could be playing arole.

In a recent report of the International Panel on ClimateChange, researchers presented sound evidence to suggestthat while natural factors are indeed warming the planet,they pale in comparison to human causes. As shown in FIG-URE 20-26, scientists determined the rate of heating (or cool-ing). They expressed these values, known as radiative forcing,in watts per square meter, that is, the amount of heat per

Evidence clearly shows that global warming is occurring andthat the climate is changing as a result.

RF Terms

Long-livedgreenhouse gases

Ozone

Ant

hrop

ogen

icN

atur

al

Land use

–2 –1 0 1 2

CO2

CH4

N2O

Surface albedo

Direct effect

Linear controls

Solar irradiance

Total netanthropogenic

Totalaerosol Cloud albedo

effect

Stratospheric watervapor from CH4

Spatial scale

Global High

Global

Continentalto global


Local tocontinental


Continental

Global

Global

High

Med

Low

Med –Low

Med –Low

Low

Low

Low

LOSURF values (Wm–2)

Radiative forcing (Wm–2)

1.66 [1.49 to 1.83]

0.07 [0.02 to 0.12]

–0.5 [–0.9 to –0.1]

–0.7 [–1.8 to –0.3]

0.01 [0.003 to 0.03]

0.12 [0.08 to 0.30]

1.6 [0.6 to 2.4]

–0.05 [–0.15 to 0.05]0.35 [0.25 to 0.65]

–0.2 [–0.4 to 0.0]0.1 [0.0 to 0.2]

0.48 [0.43 to 0.53}0.16 [0.14 to 0.18]0.34 [0.31 to 0.37]

Stratospheric Tropospheric

Black carbonon snow

Halocarbons

FIGURE 20-26 Radiative forcing (RF) components. This graph shows the warming and cooling effects of humanpollutants and natural forces, measured as radiative forcing. The natural effects are dwarfed by human factors.(LOSU = level of scientific understanding.) Reproduced from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change.Cambridge University Press.

Per

cent

of t

otal

hur

rican

esCategories 2 & 3

Categories 4 & 5

Category 1

50

40

30

20

10

01970–1974

1975–1979

1980–1984

1985–1989

1990–1994

1994–1999

2000–2004

FIGURE 20-25 Category 1, 2, and 3 hurricanes are onthe decline while more violent and destructive cate-gory 4 and 5 hurricanes are on the rise. (Adapted fromP. J. Webster et al., Science 309 [2005]: 1844–1846.)


square meter of the Earth’s surface. As illustrated, some hu-man activities increase temperature; others decrease globaltemperature. On balance, though, human activities are in-creasing the Earth’s temperature. But how do they compareto natural factors?

As shown in the figure, natural radiative forcing is onlya small fraction of human-induced warming. Computerstudies bear out this contention. Using complex and in-creasingly more reliable computer models of global climate,scientists plugged in data on natural and anthropogenic fac-tors that affect global climate to estimate the average tem-perature of various continents as well as future temperature.The results are shown in FIGURE 20-27. In this graph, theblue-shaded area indicates the average temperature thatwould be expected if only natural factors were involved inglobal temperature. The pink shaded areas indicate the pre-dicted temperature if natural and human factors were in-volved. The black lines represent the actual average temperaturebased on observations. Asyou can see, there’s a verygood correlation between ac-tual and predicted tempera-ture if both natural andhuman factors are taken intoconsideration.

KEY CONCEPTS

Predicting Future Effects of GlobalWarming and Global Climate ChangeWhile scientific evidence strongly suggests that global warm-ing and global climate change are occurring and that hu-man activities are the main cause, what will the future bring?How much change will occur? How will future releases ofgreenhouse gases and deforestation affect our climate, oureconomy, our society, and ecosystems?

Predicting future impacts is fraught with difficulty. Manyfactors will determine the level of greenhouse gases in the fu-ture. Population growth and the rate of economic growth andindustrialization will be primary determinants, however, in-dustrialization alone is not a sufficient predictor. What ismost important is the amount of fossil fuel that will be burnedto spur economic growth and industrial development.

Public policy will also have a profound effect on futureconcentrations of greenhouse gases. Measures to promote re-newable energy and efficient energy use could reduce thegrowth in the emissions of carbon dioxide. International

While natural forces affect global warming and global climatechange, human activities appear to be the main driving forces.

FIGURE 20-27 Actual and predicted temperature. The blue-shaded areas show the predictedtemperature of various regions based on natural factors. The pink-shaded areas show predictedtemperature if both natural and anthropogenic factors were included. The black line represents theactual temperature. Notice that there’s a very good correlation between the pink-shaded area andthe black line. Modified and based on Figure SPM.4 of the Summary for Policymakers of the WorkingGroup I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on ClimateChange, Climate Change 2007: The Physical Science Basis. Cambridge University Press. Used withpermission of IPCC.

GO GREEN

Recycle all waste, and use re-cycled products to reduce ournation’s energy demand and en-vironmental pollution.


treaties are also important. The treaties dis-cussed earlier in this chapter that call for thephaseout of CFCs, for example, are reducingone greenhouse gas. Corporate actions and in-dividual actions will also influence how muchcarbon dioxide is released into the atmosphereeach year.

To predict the effects of different levelsof atmospheric greenhouse gases, scientistsuse global climate models, computer pro-grams that contain mathematical equationsthat attempt to simulate the various aspects ofour climate and how they change. Scientistsfeed data into the computer regarding pro-jected greenhouse gas levels, and the com-puters use this information to predict possibleclimatic effects.

Global climate models have been criticized by some asbeing inaccurate. Although early climate models were crude,considerable efforts have gone into fine-tuning models tomake them much more valuable tools for predicting futureclimate. With this in mind, let’s take a look at some of the po-tential impacts of rising greenhouse gas emissions anddeforestation.

Rising Temperatures, Rising Sea Levels In 2007, the UN-sponsored Intergovernmental Panel on Climate Change(IPCC) predicted a 1.8° to 4.0°C increase in average globaltemperature by year 2100. Although a 2° to 4°C increasemay seem insignificant, it could drastically alter global cli-mate and sea level. In 2007, IPCC scientists predicted thatsea level could increase 0.2 to 0.6 meters (8 to 24 inches) by2100 as a result of the melting of glaciers and the land-basedAntarctic ice pack and an expansion of the seas resultingfrom warmer temperatures.

Rising sea levels would threaten coastal cities through-out the world. Today, over half the world’s population livesin coastal cities and towns. In fact, over 40 of the world’slargest cities are in coastal regions. Among cities most at riskare Miami, New Orleans, Bangkok, Hamburg, London, St. Pe-tersburg, Shanghai, Sydney, Alexandria, and Dhaka. In theUnited States, more than half of the population lives within83 kilometers (50 miles) of the ocean. Even a modest in-crease in sea level would flood coastal wetlands, low-lyingfields, and cities. The rise in sea level would also worsen thedamage from storms. Waves produced during hurricanesand other storms would sweep further inland, damagingmore homes and cities than they do today. Many peoplewould have to relocate. Cities may be forced to build leveesto hold back the seas or gradually move to higher ground asbuildings are retired.

The inland creep of the ocean would usurp farmlandand wildlife habitat and create more crowding as peoplecompete for a limited land base. The changes would notcome cheaply, and they would not be easy.

Less-developed nations would also suffer enormously ifthe oceans rise. In Asia, for instance, rice is produced inmany low-lying regions that might be reclaimed by the sea.

Storm surges could carry saltwater onto some of the re-maining fields, killing crops and poisoning the soil.

Low-lying island nations, many of them tropical resortplaces, would also be dramatically affected by rising sealevel. Many island nations in the South Pacific are already be-ginning to feel the effect as ocean waters rise and rob themof valuable beaches and shoreline (FIGURE 20-28). In fact, acoalition of island nations is one of the leading proponentsin global climate change negotiations.

KEY CONCEPTS

Rising Temperatures, Changing Rainfall Patterns Globaltemperature is expected to continue to increase after 2100even if the emission of greenhouse gases ceases. Accordingto some computer models, not only will temperature andsea level increase, rainfall patterns will change (FIGURE 20-29).Much of Europe, Africa, and South America, for example, arealready experiencing decreased rainfall. Such decreases re-duce agricultural production, causing food shortages. Manyareas, however, are experiencing increased rainfall. Althoughthat may seem beneficial, it too can disrupt farming and leadto decreased food production. In 2000, heavy spring rains inwestern New York, for example, flooded fields and preventedfarmers from planting their crops on time. By the Fourth ofJuly, corn was only ankle high.

KEY CONCEPTSGlobal temperature increases could shift rainfall patterns, in-creasing precipitation in some areas and decreasing it in oth-ers. Too little rain in some areas and too much in others couldhave a profound effect on food production and the economy.

Scientists predict a climatically significant increase in temper-ature over the next 100 years that could result in a rise in sealevel with potentially devastating effects on coastal populationsand ecosystems.

FIGURE 20-28 A victim of global warming. This tiny island, Funafuti, in Tuvalu, is losing ground to rising sea levels caused by global warming. It won’t be long before the island disappears. Residents are already fleeing to make new homes.


Rising Temperatures, Violent Storms Computer modelspredict that global warming could spawn bizarre and in-creasingly more violent weather, notably tornadoes and hur-ricanes, such as those witnessed in growing numbers inrecent years. Warming seas impart more energy to the atmo-sphere, which can greatly intensify a hurricane as it did inthe case of Hurricane Katrina, which struck Louisiana andMississippi in 2005. An increase in devastating storms couldhave potentially serious economic impacts as well, by de-stroying crops and making food more expensive. Property in-surance would rise.

KEY CONCEPTS

The Ecological and Health Impacts of Global Climate ChangeIn 2000, a report by the Na-tional Research Council pre-dicted a temperature increasein North America of 3° to 6°C(5° to 10°F) over the next100 years. In this report, sci-entists predicted a major shiftin climate zones in North America. The authors suggest thatthe tropical climate of equatorial regions will shift north-ward into the lower tier states. Their climate, in turn, will shiftto the mid-tier states and the climate of the top-tier stateswould move into Canada. The effect of such a rapid shift onvegetation and wildlife could be devastating, with manytrees simply dying as temperatures climb outside of theirrange of tolerance.

Computer models and current evidence suggests that rising tem-peratures could result in more violent and costly storms, in-cluding tornadoes and hurricanes.

A great many plants and animals could face difficulttimes as the planet warms and its climate shifts. In fact, if thechange in temperature continues at the predicted rate, manyspecies could become extinct. Others could suffer enormousdeclines in their populations. A limited number will be ableto adapt or migrate to suitable habitat.

Interestingly, most species and habitats have dealt withchanging conditions for eons, so warming itself is less a con-cern than the rate at which it is likely to occur. ProfessorMargaret Davis of the University of Minnesota shows why,in a computer simulation she performed to predict the effectsof a global temperature increase on several eastern treespecies. She found that if global atmospheric carbon diox-ide concentrations double by 2050 and temperatures rise aspredicted, hardwood trees east of the Mississippi would facetremendous losses. Her studies predict that beech trees wouldprobably disappear from the southeastern United States, ex-cept in a few mountainous regions. Suitable beech tree habi-tat would shift north to New England and southeasternCanada, which is now the extreme northern limit of thetree’s range.

Although species can shift their range, most would notbe able to move anywhere as fast as required. At the end ofthe last ice age, for instance, beech trees “migrated” north-ward by dispersing seeds at a rate of about 10 kilometers (6 miles) per 50 years—far slower than the 500-kilometer(300 miles) migration needed to avoid destruction fromglobal warming taking place within the next 50 years. Manyother trees in the United States would face a similar fate.Over time, these trees will die out, possibly being replacedby less desirable heat-resistant species.

Practically every ecosystem on Earth would be affectedby global warming. Some of the most important and mostthreatened are coastal ecosystems, notably mangrove swampsand coastal marshes. The future of these areas and the services

FIGURE 20-29 Changing rainfallpatterns. This map shows measuredchanges in global rainfall over thepast 100 years. Source: Reproducedfrom Climate Change 2001: Synthe-sis Report. A contribution of Work-ing Groups I, II and III to the ThirdAssessment Report of the Intergov-ernmental Panel on Climate Change.Cambridge University Press. Usedwith permission of IPCC.

GO GREEN

Combine trips to save energyand reduce pollution—or walkor ride your bike to do errandsif you can.


they provide—among them protecting coastal regions fromerosion and providing habitat for commercially importantfood species—would not be bright. As sea level rises, mostmangrove swamps would be lost.

Some studies suggest that plants might thrive in a CO2-rich world. After all, carbon dioxide is essential to plantgrowth. Higher levels of carbon dioxide are used to increasegrowth of plants such as tomatoes in commercial green-houses. Some studies show that plants use water more effi-ciently when exposed to higher levels of carbon dioxide.Some individuals have interpreted this to suggest that a car-bon dioxide-rich world would be a greener world.

A closer examination of the issue, however, suggeststhat the benefits of increasing levels of CO2 are overstated.In fact, studies have shown that elevated CO2 levels benefitcertain plants but harm others—among the latter corn, sugarcane, and many grasses. In addition, studies show trees intropical regions grow more slowly when exposed to highertemperatures. Moreover, photosynthesis grinds to a haltwhen temperatures exceed 38°C (100°F). Although someplants may grow better in higher levels of CO2, at highertemperatures that will accompany higher CO2, the benefitwould be lost. Although plants may use water more effi-ciently, this feature won’t help them in severe droughtsspawned by global warming.

Making matters worse, elevated levels of CO2 result ina reduction in nitrogen levels in virtually all plants, for rea-sons not understood. Changes in the nutritional quality ofplants could affect the entire food web. Studies show that tocompensate for the lower nutritional value of plants grownin elevated CO2, insects eat more. In other words, they’dcause more crop damage.

If insects cannot get enough to eat of natural plants, de-clines in insect populations might occur. This, of course,would have devastating effects on birds and other insect-eating species.

Animals, like plants, respond to warming trends by shift-ing to new habitats, but not all animals are capable of mov-ing as far as necessary. Stanford University researcher DennisMurphy studied the Great Basin Mountains, lying betweenthe Cascades and Sierra Nevadas on the west and the Rock-ies on the east. His studies indicated that 44% of the mam-mals, 23% of the butterflies, and a smaller percentage ofbirds would be destroyed by a 3°C (5.4°F) increase in globaltemperature. A study published in the prestigious journalNature says that global climate change could drive as manyas 1 million species to extinction by 2050.

Some species may actually thrive amid global warming—for example, organisms that are responsible for infectious dis-eases. Robert Shope of the Yale University School of Medicinepredicts that some diseases now restricted to tropical areascould invade new territory as the planet warms. For exam-ple, one form of rabies, now transmitted by vampire bats,could spread northward from Mexico and could result indamage of about $1 billion a year to the Texas cattle indus-try. Some scientists believe that the recent outbreaks of thedeadly Hanta virus, which is transmitted by rodents in thedesert Southwest, may be a result of rising global temperature.

Warmer climates are already causing the spread of insectsthat carry malaria and dengue fever to higher altitudes andhigher latitudes. In Africa, studies show that malaria isspreading to higher altitude regions in five countries, whichscientists attribute to the warming climate. In Mexico andCosta Rica, dengue fever is spreading into the highlands aswell. “The control issue looms largest in the developingworld, where resources for prevention and treatment can bescarce,” writes Dr. Paul Epstein, Associate Director of theCenter for Health and the Global Environment at HarvardMedical School in an article in Scientific American. “But thetechnologically advanced nations, too, can fall victim to sur-prise attacks—as happened last year (1999) when the WestNile virus broke out for the first time in North America,killing seven New Yorkers. In these days of internationalcommerce and travel, an infectious disorder that appears inone part of the world can quickly become a problem conti-nents away if the disease-causing agent, or pathogen, findsitself in a hospitable environment.”

KEY CONCEPTS

Cooling and Changing Ocean CurrentsWhile most of the discussion of global climate change focuseson the effects of higher temperatures, many scientists be-lieve that global warming may plunge some areas of theplanet into a period of intense and catastrophic cooling.Global warming could bring on ice-age conditions in GreatBritain, Europe, and Russia. How?

As noted in Chapter 5, seawater circulates throughoutthe oceans. For example, warm water from tropical regionscirculates northward in the Atlantic Ocean via the GulfStream (see Figure 5-4 on page 76). This makes Great Britainand Europe (located on the same latitude as northern Canada)much warmer than it would otherwise be. Warm saltwatercirculating northward cools as it moves toward the NorthPole. The cooler, denser water then sinks and flows back to-ward the equator, only to rise again. This gigantic currentforms a huge conveyor shown in FIGURE 20-30 that distrib-utes heat from the equator to the poles. It makes the poleswarmer and the equator cooler than they would be otherwise,but global climate change could alter all of this, and abruptly.How?

Scientists believe that melting glaciers and increasedrainfall in the northern latitudes could infuse the ocean withless dense, cool fresh water that could shut down the con-veyor belt. The cool less-dense fresh water would essentiallyput a halt to an essential driving force of the global conveyorbelt—the sinking of dense, cool, salt water. Studies of oceancurrents and climate suggest that such changes could occurabruptly, and soon, and may take decades, even centuries, toreverse. The result? Areas like Europe could become muchcolder, while the equator becomes hotter. As you shall soonsee, scientists have already detected significant changes in this

Organisms and ecosystems could be profoundly influenced byglobal climate change, especially if the rate of change occursfaster than their ability to adapt, which seems inevitable.


system, which has caused considerable concern in Europeand stimulated major efforts to curb greenhouse gases.

National and International SecurityAs the previous material illustrates, climate change couldhave extremely serious economic and environmental con-sequences throughout the world. The U.S. Department of De-fense noted in a classified report that global climate changecould result in a global catastrophe that would cost millionsof lives through warfare and recurrent natural disasters. Thereport warns that major European cities could sink beneathrising seas as Great Britain is plunged into a Siberian cli-mate as early as 2020. It also warns of nuclear conflict, seri-ous droughts, famine, and widespread civil unrest that coulderupt throughout the world.

The document predicts that abrupt climate change couldcause many nations to develop nuclear weapons to defendand secure dwindling food, water, and energy supplies. Someexperts who have viewed the classified document argue thatthe threat of terrorism is currently eclipsed by the potentialglobal instability that could result from global climate change.“Disruption and conflict will be endemic features of life,” con-cludes the Pentagon. “Once again, warfare would define hu-man life.” Climate change “should be elevated beyond ascientific debate to a U.S. national security concern,” saythe authors, Peter Schwartz, CIA consultant and former headof planning at Royal Dutch/Shell Group, and Doug Randallof the California-based Global Business Network.

KEY CONCEPTS

Scientific Uncertainties: What We Don’t KnowAlthough there is strong evidence that the Earth is warmingup, that climate is changing, and that human activities arethe primary cause of these trends, there are some scientificuncertainties worth considering.

One significant uncertainty is the potential for changesthat could cause the Earth’s temperature to rise more quickly.These factors are not taken into account in the projectionsof global warming by the IPCC, leading some scientists toview the IPCC projections as conservative. Because they donot take into account factors that could result in a rapid de-terioration of climatic conditions, climate could deterioratemuch more quickly than anticipated.

Evidence of this is already being seen. FIGURE 20-31, forinstance, shows the predicted melting of arctic sea ice, us-ing computer models. The red line shows that observedmelting, indicating a much faster rate of disappearance thanpredicted.

Changes now in motion that could stimulate dangerouspositive feedbacks are shown in FIGURE 20-32. Many of thesechanges could lead to dangerous positive feedbacks. A positive

Global climate change is not just an economic and environ-mental issue, it has potentially serious implications for globalpeace.

90° W 60° W 30° W 0° 30° E 60° E 90° E 120° E

90° W 60° W 30° W 0° 30° E 60° E 90° E 120° E

ArcticCircle

60° N

30° NTropic

of Cancer

0° Equator

Tropic of Capricorn

60° S

Antarctic Circle

Arctic Circle

60° N

30° NTropic of Cancer

Tropic of Capricorn30° S

60° S

Antarctic Circle

0° Equator

150° E 180° W 150° W 120° W

150° E 180° W 150° W 120° W

30° SS H A L L O W

D E E P F L O W

F L O W

FIGURE 20-30 Surface and deep water exchange.


feedback occurs when one factor leads to an increase in another.In nature, positive feedback mechanisms are rare. Homeo-static mechanisms, discussed in Chapter 6, tend to operatethrough negative feedback mechanisms that maintain moreor less constant conditions. In natural systems altered byhumans, positive feedback mechanisms are dangerouslycommon. To understand this phenomenon, consider someexamples of positive feedbacks.

The oceans currently serve as a major reservoir for CO2.In fact, they store 60% more CO2 than the atmosphere does.Without the oceans, CO2 levels in the atmosphere would beconsiderably higher than they are today. As the Earth’s tem-perature rises, however, the ocean’s ability to dissolve and hold

CO2 will very likely decline (Figure 20-32). (Warm liquids,like a warm soda, hold less CO2 gas.)

If the amount of CO2 dissolved in the oceans declined,the oceans would release CO2 they have absorbed in the past100 years. This would cause atmospheric CO2 levels to in-crease much more rapidly, accelerating global warming andglobal climate change. Rising temperatures would further re-duce the amount of CO2 held in ocean waters, creating adangerous positive feedback cycle. Research suggests that ris-ing temperatures are already reducing the ability of oceansto hold CO2.

Several additional positive feedback mechanisms mayalso be initiated. Melting glaciers, for instance, decrease thereflective surface of the Earth, resulting in an increase in theabsorption of sunlight. This would result in a further warm-ing of the atmosphere, for reasons explained at the beginningof this section. Rising temperatures, in turn, would acceler-ate the melting of glaciers. Rising temperatures would alsoincrease the amount of energy people use to cool theirhouses—which adds more CO2 to the atmosphere, onceagain accelerating the rise in temperature.

As noted earlier, the rise in atmospheric CO2 concen-trations is partly the result of deforestation—the rapid lossof trees, especially in the tropics and in parts of the UnitedStates, such as the Pacific Northwest. Trees absorb CO2,which they use to produce food molecules and structuraltissues. Worldwide, forests are being cut much faster than theyregrow. As a result, deforestation “contributes” approxi-mately one-fourth of the annual global increase in CO2 and,as noted in Table 20-2, about 13% of the annual rise in globaltemperature. Trees may also be destroyed by climatic change(rising temperature and drought) and by fires. This losscould also become part of a wildly accelerating positive feed-back mechanism that causes a rapid increase in CO2 in the

FIGURE 20-31 This graph shows the computer projections of seaice (blue) and actual measurements (red), indicating that thecomputer models underestimate melting. © 2008 University Corpo-ration for Atmospheric Research; illustration by Steve Beyo.

Melting glaciers and polar icecaps

Increased global temperature(greenhouse effect)

Increasing atmosphericcarbon dioxide

Flooding ofcoastal regions

Deforestation

Decreasedreflectivesurface

Fossil fuel combustion

Rising sea level

Warm oceans

Decreased watercarbon dioxidesolubility

FIGURE 20-32 Dangerous feedback. If the planetcontinues to warm, many changes could occur. Somecould accelerate the increase in atmospheric carbondioxide levels and result in an acceleration in therate of global warming.


atmosphere and a more rapid shift in global temperature.An increase in forest fires, already witnessed, could also con-tribute increasing CO2 levels, resulting in further warming.

Although most potential feedbacks are positive, there area few factors that could counteract these forces, either negat-ing the increase in temperature or reducing it. The oceans,for instance, tend to act as a temperature buffer, keepingtemperatures from rising as rapidly. In addition, rising tem-peratures may result in an increase in evaporation of waterfrom lakes and oceans, increasing cloudiness. This couldreduce sunlight penetration, reducing the climb in globaltemperature. However, some forms of clouds trap heat. In-creased particulates in dusty air from denuded, parchedlands could also reflect incident solar energy.

Current evidence suggests that the depletion of theozone layer will have a cooling effect on global climate.Other information suggests an adjustment in the other di-rection. A study of 17 computer models published in Sci-ence in 1991 also suggests that global warming may affectsnowfall and average temperature in unanticipated ways.As a rule, climate modelers think that global warming willresult in a net decrease in planetary snow cover. This, theypredict, would increase absorption and increase global warm-ing. However, five of the world’s best climate models showthat a reduction in snow cover would cool the atmosphereand counteract a small part of the greenhouse warming. Thecooling effect may result from an increase in cloud cover oran increase in heat radiation into space. In sum, many climatemodelers acknowledge that current models don’t do a verygood job of simulating cloud behavior.

How these factors play out is anyone’s guess at this time,but many scientists believe that, overall, the positive feed-back loops—factors that increase temperature—maydominate.

KEY CONCEPTS

Solving a Problem in a Climate of Uncertainty: Weighing Risks and BenefitsGlobal climate is a very complex system, and our under-standing, while growing, is still not complete. Many uncer-tainties still exist with regard to the behavior of the CO2

sinks such as the oceans and forests. Future levels of green-house gases and global warming activities such as defor-estation are unknown.

With this much uncertainty, some people prefer to takea wait-and-see approach. Some oil companies, coal compa-nies, and the former President Bush have opposed measuresto cut greenhouse gas emissions, arguing that the threat isnot real and that such changes will hurt the nation’s econ-omy. Critics of this viewpoint argue that the stakes are much

Many factors could complicate predictions of future climate.Some factors could result in a runaway increase in atmosphericCO2 levels, which would accelerate warming. Others could reducewarming.

too high to take no action. They note the economic and en-vironmental benefits of cutting fossil fuel combustion areso significant that we should make the conversion despite theuncertainty. In short, they argue, the cost of accepting thegreenhouse hypothesis and acting accordingly is far lowerthan the potential cost of rejecting it.

Today, many nations and companies like Microsoft,Google, Toyota, BP, and Wal-Mart, to name a few, are tak-ing steps to reduce CO2 emissions. Germany plans to de-velop a 100% renewable energy economy. Great Britain haspledged to reduce its CO2 emissions by 60%. Meanwhile,the U.S. government has steadfastly opposed action. Infact, the Bush Administration ruled that CO2 cannot beregulated. The Obama administration supports efforts to re-duce carbon dioxide emission, and has taken many steps tocurb emissions.

Fortunately, many state and local governments havetaken the lead in reducing greenhouse gas emissions by tak-ing steps to reduce their carbon footprint. Many of their ef-forts, like energy efficiency, could have profound economicbenefits, too, and could help spawn a renewable energy in-dustry vital to the world’s long-term future.

KEY CONCEPTS

Solving the Problem SustainablyHumans produce over 8.0 billion tons of carbon (in the formof CO2) a year—40% from the less developed nations and 60%from the more developed industrial nations. To create a sus-tainable society, free from the impacts of global warming,the Intergovernmental Panel on Climate Change believesthat CO2 emissions need to be cut by about 75%, down toabout 2 billion tons per year. Unfortunately, the world ispoised to increase emissions. Emissions from the less de-veloped nations are projected to quadruple in the next halfcentury. China’s emissions could soon exceed those of theUnited States, the leader in greenhouse gas emissions. By2020, the less developed nations could be producing as muchCO2 as the industrial nations. Emissions from the more de-veloped nations are projected to increase by 30%.

Further adding to the dilemma, tropical deforestationand burning of forests could greatly accelerate CO2 emis-sions and global climate change. The fires that occurred inAsia in 1997 and 1998 released more CO2 than westernEurope produces in a year. These fires were intentionallyignited in many cases. In others, however, extraordinarilydry rain forests caught fire in lightning storms. The dryconditions were caused by deforestation in upwind areas,which tends to reduce rainfall, a phenomenon discussed inChapter 12.

Global warming is ultimately a symptom of overpopu-lation and unsustainable human systems and technologies

Some uncertainty exists on global climate change, which hasslowed progress. Even though there are some uncertainties,many people believe that the costs of reducing or even elimi-nating greenhouse gases are outweighed by the potential social,economic, and environmental costs of global climate change.


designed to meet the needs of people. Especially notewor-thy in the group of human systems are energy, transportation,housing, industry, and timber production. The energy sys-tem, for example, relies heavily on fossil fuel, the combus-tion of which produces huge quantities of CO2.

The good news is that numerous solutions are availableto reduce, even eliminate, greenhouse gas emissions. Banson ozone-depleting and greenhouse-enhancing CFCs, dis-cussed earlier in the chapter, are one example. Other sus-tainable solutions, those that strike at the roots of theproblems, include measures to reduce population growthand eliminate the harmful emissions of CO2, methane, CFCs,and nitrous oxide. This section discusses some of the mosteffective measures. Not surprisingly, the most effective ap-proaches turn out to be applications of the operating prin-ciples of sustainability discussed in earlier chapters:conservation, recycling, renewable resource use, restora-tion, and population stabilization.

KEY CONCEPTS

Population Stabilization and Restoration To reduce at-mospheric CO2 levels and stave off or stop the inevitableincrease in global temperature will require a massive refor-estation of the Earth. Australia recently announced that it wasembarking on an ambitious program to plant 1 billion trees,partly to offset global warming. A few other countries are fol-lowing suit, among them China, although efforts are belowwhat’s required to have any appreciable effect.

Norman Myers, an international expert on tropicalforests, argues that replanting 2.6 million square kilometers(1 million square miles) of tropical rain forest would reduceannual emissions of CO2 by 2.25 billion metric tons (2.5 bil-lion tons), or about 33%. Although this ambitious projectwould cost approximately $100 billion, Myers argues that itis a small price to pay, especially when one takes into ac-count the potential economic damage caused by climatechange. A bad hurricane can easily cause several billion dol-lars worth of damage if it hits a metropolitan area. Accord-ing to the EPA, protecting the U.S. east coast from rising seawaters could cost the nation $75 to $110 billion. Globalwarming would necessitate costly modifications of irriga-tion systems and hydroelectric dams estimated at $100 bil-lion. Replanting could also bring direct economic benefits toless developed nations through reduced soil erosion andsustainable harvest of forests.

Individuals can help plant trees in clear-cuts, roadsides,abandoned fields, and backyards. If you really want to off-set the carbon dioxide produced by your lifestyle, you wouldhave to plant 400 trees. A family of four would need to plant2.5 hectares (6 acres) of fast-growing trees to offset its life-time carbon dioxide production. Obviously, not everyonecan replant trees, but you can help by supporting public andprivate reforestation projects.

Redesigning human systems according to the principles of sus-tainability could help alleviate global warming and help createa prosperous, safe future.

KEY CONCEPTS

Recycling, Energy Efficiency, and Renewable Energy In-dividuals and businesses throughout the world can also helpreduce global CO2 levels by recycling and reusing all mate-rials to the maximum extent possible. Table 20-3 shows acomparison of energy use in manufacturing cans and bottles.It shows that recycled glass and aluminum require far less en-ergy to produce—and, thus, produce less CO2 air pollution—than if they were made from raw ore and used only once.Refillable bottles use much less energy even if they are onlyused 10 times. (For more on recycling and reuse, see Chap-ter 23.)

Energy efficiency is another means of reducing CO2 andmethane emissions. According to energy experts AmoryLovins of the Rocky Mountain Institute and ChristopherFlavin of the Worldwatch Institute, global emissions could becut by 2.7 billion metric tons (3 billion tons) per year within2 decades by cost-effective and profitable technologies. Notechnical breakthroughs are needed, either. Combined withreforestation efforts outlined above, these efforts could nearlyeliminate anthropogenic global CO2 emissions.

Individuals can cut energy demand by walking, bicy-cling, or riding a bus to school or work; by building smaller,more energy-efficient homes; by insulating existing homes;by recycling; and by using efficient appliances or doing somethings by hand (for example, mixing by hand rather than us-ing an electric mixer, or drying clothes on a line rather thanusing a dryer). When buying appliances, choose the mostenergy-efficient ones available. New energy-efficient refriger-ators, the leading user of electricity in homes, can cut elec-trical demand from 1,200 kilowatt-hours per year to 240. (SeeChapter 15 for more ideas on energy efficiency.)

Stabilizing population growth can help reduce humanity’s needfor fossil fuels and other greenhouse-enhancing activities suchas deforestation. Restoring forests, especially in the tropics,and restoring other carbon sinks like wetlands, could have aprofound effect on global carbon dioxide levels.

Table 20-3Energy Consumption per Use for 12-Ounce Beverage Containers

Container Energy Use (BTUs)

Aluminum can, used once 7050Steel can, used once 5950Recycled steel can 3880Glass beer bottle, used once 3730Recycled aluminum can 2550Recycled glass beer bottle 2530Refillable glass bottle, used 10 times 610

Source: From L.R. Brown, C. Flavin, and S. Postel (1991). Saving thePlanet: How to Shape an Environmentally Sustainable Economy. NewYork: Norton.


2005

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Path for 80% reduction

Year

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. car

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emis

sion

s (M

tC/y

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EEWindBiofuelsBiomassPVCSPGeothermal

Renewable energy technologies can dramatically reduceCO2 and methane emissions (see Chapter 15). Many op-tions are available, and several are quite cost competitive.

How effective are these and other measures outlined inthis section? According to a study by the American SolarEnergy Society (FIGURE 20-33), widespread implementationof energy efficiency, wind energy, biofuels, biomass, solarelectricity, and other renewable energy technologies could re-duce the United States CO2 emissions by 60 to 80%, wellwithin the range of what is needed to reach climatic stabil-ity. Their studies show that these measures would actuallyimprove the economy. Efficiency measures especially wouldsave individuals and companies huge sums of money.

KEY CONCEPTS

International Cooperation to Halt Global Warming Thischapter opened with a discussion of the Earth Summit, thehighly acclaimed global conference that addressed suchpressing problems as forest protection and global warming,among others. One outcome of the meeting was an agreementcalling on nations to reduce CO2 emissions. The nations

Recycling and energy efficiency greatly reduce energy demandand cut greenhouse gas emissions. Renewable energy tech-nologies can provide us with much-needed power with little orno impact on the global climate.

FIGURE 20-33 Energy efficiency and renewable energy could easily help the United States meet its energy needswhile reducing carbon dioxide emissions by 60% to 80%. (EE = energy efficiency, PV = solar electricity, CSP = con-centrated solar power.) (Reproduced from American Solar Energy Society, Tackling Climate Change in the U.S.:Potential Carbon Emissions Reductions from Energy Efficiency and Renewable Energy by 2030. January, 2007.)

also signed an agreement to protect forests. Details of theseagreements are outlined in Chapter 27.

Although both agreements are important, they fall seri-ously short of the job at hand. Further, the global climateagreement is largely voluntary. Making matters worse, manyU.S. industries that initially supported the global climatetreaty reneged on their support and stonewalled efforts tocurb greenhouse gas emissions. By 1997, little progress hadbeen made in reducing CO2 levels. During that year, theUnited Nations sponsored a meeting in Kyoto, Japan, to ne-gotiate an international agreement on global climate change.Known as the 1997 Kyoto Protocol and signed by 174 nations(as of November 2007), this agreement calls for a slow, steadydecline in greenhouse gas emissions. It went into effect in2005 when Russia signed on (the United States did not). Thisagreement commits the industrial nations and the formerEastern Bloc nations to cut CO2 and other greenhouse gasemissions 5.2% below 1990 levels between 2008 and 2012.The signatories have met twice since then, but so far successin achieving these goals has been poor. Many countries haveexperienced increases in greenhouse gas emissions of 9 to12%, among them Japan, the United States, and Australia.The fastest growth in greenhouse emissions has occurred inthe less developed nations, with growth rates of about 40%,in large part because of industrialization. There is some goodnews, however, notably in the United Kingdom, France, and


CRITICAL THINKING AND CONCEPT REVIEW1. What is the ozone layer? Why is it important to life on

Earth?2. Describe the major activities and pollutants that de-

plete the ozone layer.3. What measures have been taken to reduce the destruc-

tion of the ozone layer? Will they reverse the declinequickly? Why or why not?

4. HCFCs, replacements for CFCs, also deplete the ozonelayer, but to a lesser degree. Consequently, they areconsidered interim solutions. What other reason ac-counts for their interim-solution status?

5. Some critics of the environmental movement suggestthat stratospheric ozone depletion is a hoax. Look upsome of their writings and, if possible, the work onwhich they base their claims. What do you find? Arethe conclusions based on good scientific evidence?With your knowledge of the issue, debate the mainpoints they make.

6. Critically analyze the following statement: “We mustbalance the harm done by ozone depletion and main-

taining CFCs for refrigeration with the potential loss ofrefrigeration in developing countries, which could re-sult in many lost lives due to foodborne disease.”

7. Define the terms acid deposition, acid precursor, wetdeposition, and dry deposition.

8. How are acid precursors formed?9. Using your critical thinking skills, analyze the follow-

ing statement: “Natural events such as volcanoes pro-duce far more acid precursors than human activities, sowe shouldn’t worry about controlling anthropogenicsources. Such controls won’t do anything to affect acidlevels.”

10. Using your critical thinking skills and your knowledgeof biology, how would you assess current efforts aimedat reducing acid deposition or counteracting its impact(for example, smokestack scrubbers, liming lakes, andbreeding acid-resistant fish)?

11. Outline key elements of a sustainable strategy for re-ducing acid deposition.

CRITICAL THINKING

Exercise AnalysisCritical thinking rules encourage us to question sources of information and their conclusions. In this exam-ple, we find that the author of this article is grossly in error when he claims that the scientific communityis divided on the issue. The key word is divided, which makes it sound as if there’s a 50–50 split of opinion.

In truth, about 99% of the United States’ 700 atmospheric scientists believe that global warming is a reality; only a handful embrace the opposite view. The evidence the author introduces to support his assertion—a quote from each side of the issue—is terribly misleading.

This type of reporting is quite common in newspapers, television, and magazines. In journalism, it iscustomary to get both sides of the issue, but one must not mistake the notion of two sides for an equalsplit of opinion. Statements such as the one we’re analyzing here, although intended to give a balancedview of issues, in reality provide an extremely unbalanced view.

What would have been a more accurate statement? The author might have noted opposing views butalso noted that, although not all scientists agree that global warming is occurring, the vast majority do.

That said, it is still important to note that even though the majority of the world’s atmospheric scien-tists agree that global warming is happening, it doesn’t mean they’re right. For many centuries, scientistsheld that the Earth was at the center of the solar system.

This exercise shows that digging a little deeper and finding out more often throws conclusions intoquestion.

Germany, where greenhouse gas emissions are falling as a re-sult of energy-efficiency policies. Russia and many membersof the former Soviet Union have also recorded outstanding de-clines in CO2 emission, as a result of the collapse of theireconomies following the dissolution of the USSR.

Many observers believe that, as was the case with the firsttreaty on ozone protection (the Montreal Protocol), nationswill see the need to take more drastic steps to curb or even

stop the rise in greenhouse gas, which the vast majority ofthe world’s atmospheric scientists believe is responsible forrecord-hot temperatures in the past 2 decades and a host ofother changes, including a rise in sea level. Continuing badnews—hot years, devastating floods, and violent storms—could spur further agreements that eventually lead us to amore sustainable system of energy production and a climatewe can all live with.

CHAPTER 20: Global Air Pollution: Ozone Depletion, Acid Deposition, and Climate Change 467

Connect to this book's website:http://environment.jbpub.com/9e/The site features eLearning, an online reviewarea that provides quizzes, chapter outlines,and other tools to help you study for yourclass. You can also follow useful links for in-depth information, research the differingviews in the Point/Counterpoints, or keep up on the latest environmental news.

REFERENCES AND FURTHER READINGTo save on paper and allow for updates, additional readingrecommendations and the list of sources for the informationdiscussed in this chapter are available at http://environment.jbpub.com/9e/.

KEY TERMS acid depositionacid precursorsbuffering capacitybufferschlorine free radicalschlorofluorocarbons (CFCs)dry depositionFreon-11

Freon-12global climate changeglobal climate modelsglobal warminggreenhouse effectgreenhouse gaseshydrochlorofluorocarbons (HCFCs)Kyoto Protocol

Montreal ProtocolNational Acid Precipitation ActNational Acid Precipitation

Assessment Program (NAPAP)ozone layerpH (potential hydrogen)positive feedbackwet deposition

12. What is the greenhouse effect? What gases are respon-sible for it? Which ones are natural? Which ones are an-thropogenic?

13. Describe the potential social, economic, and environ-mental impacts of the continued rise of greenhousegases. Describe how environmental impacts could leadto social and economic impacts.

14. Describe the factors that could accelerate greenhousewarming and those that may lessen it.

15. Outline the short-term and long-term solutions thathave been proposed for addressing global climatechange and critically analyze each one. Which oneswould be the most effective? Which ones would be themost politically acceptable?

16. Compare the difference between heeding warningsabout global warming and ignoring them. What actiondo you recommend? Why?

chapter 20 global air pollution: ozone depletion,...

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