Environmental Problems, Their Causes, and Sustainability 1

global warming during the past 50 years. If so, sootwould be the second biggest human contribution toglobal warming, after the greenhouse gas CO 2 .One possible effect of increased aerosols in the troposphereis global or solar dimming. In 2004, scientistsreported that measurements showed a drop in theamount of sunshine reaching the earth’s surface by asmuch as 10% between 1960 and 1990, with a 37% dropin Hong Kong. Satellite measurements showed that adecrease in the amount of energy from the sun—solarradiation—could not account for this effect. Scientistshypothesize that pollution can dim sunlight in twoways. One is that soot particles in the atmosphere reflectsome of the sunlight back into space. Anotherpossibility is that the airborne particles cause morewater droplets to condense out of the air, leading tothicker and darker clouds, which can reduce incomingsunlight. However, preliminary measurementsshowed that the amount of sunlight reaching theearth’s surface increased slightly between 2001 and2003. Scientists are trying to sort out the complexitiesand causes of these phenomena.Climate scientists do not expect aerosol pollutantsto counteract or enhance projected global warmingvery much in the next 50 years for two reasons. One isthat aerosols and soot fall back to the earth or arewashed out of the lower atmosphere within weeks ormonths, whereas CO 2 and other greenhouse gases remainin the troposphere for decades to several hundredyears. The other is that aerosol inputs into thetroposphere are being reduced—especially in developedcountries.Can Increased CO 2 Levels Stimulate Photosynthesisand Remove More CO 2 from the Air?A Temporary and Limited EffectIncreased CO 2 in the troposphere could increaseplant photosynthesis, but several factors can limit oroffset this effect.Some studies suggest that more CO 2 in the tropospherecould increase the rate of plant photosynthesisin areas with adequate water and soil nutrients. Thiswould remove more CO 2 from the troposphere andhelp slow atmospheric warming.However, recent studies indicate that this CO 2 removalwould be temporary for two reasons. One isthat it would slow as the plants reach maturity andtake up less CO 2 from the troposphere. The other isthat carbon stored by the plants as organic compoundswould be returned to the troposphere as CO 2 when theplants die and decompose or burn.A 2004 study by a team of U.S. and Brazilian scientistsshowed that undisturbed old-growth Amazonrainforests are experiencing rapid changes in speciescomposition apparently because of rising atmosphericlevels of carbon dioxide. Higher CO 2 levels are fertilizingmany species of trees and fast growing larger treesare out competing smaller younger trees. This ischanging the mix of tree and wildlife species or biodiversitymakeup of theses rainforests. Initially, this canincrease the uptake of CO 2 from the atmosphere. Butas these larger trees mature and die out sooner the reductionin denser wood and foliage could eventuallylead to a drop in the amount of carbon dioxide theserainforests remove from the atmosphere. In addition,plant-eating insects that breed more rapidly and yearroundin warmer temperatures could offset much ofthe increased plant growth.How Might a Warmer TroposphereAffect Methane Emissions? AcceleratedWarmingWarmer air can release methane gas stored in bogs,wetlands, and tundra soils, causing a feedback loopthat makes the air warmer.Global warming could be accelerated by an increasedrelease of methane (a potent greenhouse gas) from twomajor sources. One is bogs and other wetlands and theother is ice-like compounds called methane hydratestrapped beneath the arctic permafrost. Significantamounts of methane would be released into the troposphereif the permafrost in tundra and boreal forestsoils partially or completely melts, as is occurring inparts of Canada, Alaska, China, and Mongolia. The resultingtropospheric warming could lead to moremethane release and still more warming.21-6 POSSIBLE EFFECTSOF A WARMER WORLDWhat Are Some Possible Effects of a WarmerTroposphere? Winners and LosersAwarmer troposphere would have beneficial andharmful effects, but poor nations in the tropics willsuffer the most.A warmer troposphere could have a number of beneficialand harmful effects, listed in Figures 21-13 and21-14, for humans, other species, and ecosystems, dependingmostly on their locations and on how rapidlythe temperature changes. Study these figures carefully.However, betting on living in an area with favorableclimate change in the future is like playing a game ofRussian roulette. Global climate models are improving,but so far we cannot make reliable projectionsabout how the climates of particular regions are likelyto change.474 CHAPTER 21 Climate Change and Ozone Loss