TheGreenhouseeffectandGlobalwarming
Theheart-trappingcapacityoftheatmosphereispopularlyknownasthe“greenhouseeffect”.Despitepubliccontroversysurroundingglobalwarming,thenaturalgreenhouseeffecthasbeenlongestablishedasfactinthescientificcommunity.
Indeed,carbondioxide,andothergases,theearth’snaturalclimatewouldbeabout33℃coolerthanitis.Lifewouldhaveevolvedquitedifferentlyinsuchaclimate.
Mostscientistsbelievethattherapidexpansionofagricultureandindustrialactivitiesoverthelastseveralhundredyearshavegeneratedsignificantincreasesincarbondioxideandmethaneintheatmosphere.However,expertsdisagreeaboutwhethersuchchangershavecausedtheincreaseofapproximately0.5℃intheearth’ssurfacetemperaturethathasbeendocumentedoverthepastcentury.Manyscientistscitethefactthat1997wasthewarmestyearonrecord,followingadecadeinwhich9ofthe11hottestyearsofthiscenturywerereported,asstrong,butcircumstantial,evidencethathumanactivitieshavealteredtheearth’sclimate.Otherexperts,however,believethistemperaturetrendisanaturalvariation.
Alsodisputediswhetherprojectedworldpopulationgrowthtomorethan10billionpeoplebytheyear2100willresultinadoublingortriplingofatmosphericconcentrationsofcarbondioxide.Ifsuchabuildupoccurs,asmanyscientistspredict,globalsurfacetemperaturescouldincreasebyanywherefromaboutoneCelsiusdegreetoabout4Celsiusdegreesduringthenextcentury.Thehigherhalfofrangeinvolvestemperaturechangesoutsideofthoseexperiencedbyhumancivilizationssincetheendofthelasticeagesome10,000yearsago.Duringthelasticeage,averageglobaltemperatureswereonlyabout5Celsiusdegreescoolerthanthepresentperiod.
AtemperatureincreaseofseveraldegreesCelsiuscouldresultinasealevelriserangingfromabout10cmtoabout1m.Asealevelriseofabout0.5mwouldbenoticeableprimarilyinthemostvulnerable,low-lyingislandsandcoastalareas.Largersealevelriseswouldresultinextensivefloodingoflowlandbeaches,wetlands,andcoastalsettlements.Moreover,ahighersealevelbasewouldincreasestheriskofcatastrophicstormsurgesincoastalareas,sincefloodingwouldlikelyextendislandwellbeyondhistoriclevels.
Warmertemperaturecouldalsointensifydroughtsinsomeregions,destabilizeecosystems,andcausethedeclineorextinctionofsomespecies.Sincecarbondioxideenhancesphotosynthesis,somescientistshavecalculatedthathigherconcentrationsintheatmostpherewouldincreasetheproductivityofcropsandforests.Butothershavepointoutthatcarbondioxideincreaseandawarmingclimatecouldencouragethespreadofdestructivepests,includingweedsanddisease-carryinginsects.
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