globalwarming,thegradualincreaseofthetemperatureoftheearth'sloweratmosphereasaresultoftheincreaseingreenhousegasessincetheIndustrialRevolution.

　　Thetemperatureoftheatmosphereneartheearth'ssurfaceiswarmedthroughanaturalprocesscalledthegreenhouseeffect.Visible,shortwavelightcomesfromthesuntotheearth,passingunimpededthroughablanketofthermal,orgreenhouse,gasescomposedlargelyofwatervapor,carbondioxide,methane,nitrousoxide,andozone.Infraredradiationreflectsofftheplanet'ssurfacetowardspacebutdoesnoteasilypassthroughthethermalblanket.Someofitistrappedandreflecteddownward,keepingtheplanetatanaveragetemperaturesuitabletolife,about60掳F(16掳C).

　　Growthinindustry,agriculture,andtransportationsincetheIndustrialRevolutionhasproducedadditionalquantitiesofthenaturalgreenhousegasespluschlorofluorocarbonsandothergases,augmentingthethermalblanket.Itisgenerallyacceptedthatthisincreaseinthequantityofgreenhousegasesistrappingmoreheatandincreasingglobaltemperatures,makingaprocessthathasbeenbeneficialtolifepotentiallydisruptiveandharmful.Duringthepastcentury,theatmospherictemperaturehasrisen1.1掳F(0.6掳C),andsealevelhasrisenseveralinches.Someprojected,longer-termresultsofglobalwarmingincludemeltingofpolarice,witharesultingriseinsealevelandcoastalflooding;disruptionofdrinkingwatersuppliesdependentonsnowmelts;profoundchangesinagricultureduetoclimatechange;extinctionofspeciesasecologicalnichesdisappear;morefrequenttropicalstorms;andanincreasedincidenceoftropicaldiseases.

　　Amongfactorsthatmaybecontributingtoglobalwarmingaretheburningofcoalandpetroleumproducts(sourcesofcarbondioxide,methane,nitrousoxide,ozone);deforestation,whichincreasestheamountofcarbondioxideintheatmosphere;methanegasreleasedinanimalwaste;andincreasedcattleproduction,whichcontributestodeforestation,methaneproduction,anduseoffossilfuels.

　　Muchofthedebatesurroundingglobalwarminghascenteredontheaccuracyofscientificpredictionsconcerningfuturewarming.Topredictglobalclimatictrends,climatologistsaccumulatelargehistoricaldatabasesandusethemtocreatecomputerizedmodelsthatsimulatetheearth'sclimate.Thevalidityofthesemodelshasbeenasubjectofcontroversy.Skepticssaythattheclimateistoocomplicatedtobeaccuratelymodeled,andthattherearetoomanyunknowns.Somealsoquestionwhethertheobservedclimatechangesmightsimplyrepresentnormalfluctuationsinglobaltemperature.Nonetheless,forsometimetherehasbeengeneralagreementthatatleastpartoftheobservedwarmingistheresultofhumanactivity,andthattheproblemneedstobeaddressed.In1992,attheUnitedNationsConferenceonEnvironmentandDevelopment,over150nationssignedabindingdeclarationontheneedtoreduceglobalwarming.

　　In1994,however,aUNscientificadvisorypanel,theIntergovernmentalPanelonClimateChange,concludedthatreductionsbeyondthoseenvisionedbythetreatywouldbeneededtoavoidglobalwarming.Thefollowingyear,theadvisorypanelforecastariseinglobaltemperatureoffrom1.44to6.3掳F(0.8鈥?.5掳C)by2100ifnoactionistakentocutdownontheproductionofgreenhousegases,andariseoffrom1to3.6掳F(0.5鈥?