ThemoleculesofcarbondioxideintheEarth’satmosphereaffecttheheatbalanceoftheEarthbyactingasaone-wayscreen.Althoughthesemoleculesallowradiationatvisiblewavelengths,wheremostoftheenergyofsunlightisconcentrated,topassthrough,theyabsorbsomeofthelonger-wavelength,infraredemissionsradiatedfromtheEarth’ssurface,radiationthatwouldotherwisebetransmittedbackintospace.FortheEarthtomaintainaconstantaveragetemperature,suchemissionsfromtheplanetmustbalanceincomingsolarradiation.Iftherewerenocarbondioxideintheatmosphere,heatwouldescapefromtheEarthmuchmoreeasily.Thesurfacetemperaturewouldbesomuchlowerthattheoceansmightbeasolidmassofice. Today,however,thepotentialproblemistoomuchcarbondioxide.Theburningoffossilfuelsandtheclearingofforestshaveincreasedatmosphericcarbondioxidebyabout15percentinthelasthundredyears,andwecontinuetoaddcarbondioxidetotheatmosphere.Couldtheincreaseincarbondioxidecauseaglobalriseinaveragetemperature,andcouldsucharisehaveseriousconsequencesforhumansociety?Mathematicalmodelsthatallowustocalculatetheriseintemperatureasafunctionoftheincreaseindicatethattheanswerisprobablyyes. Underpresentconditionsatemperatureof-18℃canbeobservedatanaltitudeof5to6kilometersabovetheEarth.Belowthisaltitude(calledtheradiatinglevel),thetemperatureincreasesbyabout6℃perkilometerapproachingtheEarth’ssurface,wheretheaveragetemperatureisabout15℃.Anincreaseintheamountofcarbondioxidemeansthattherearemoremoleculesofcarbondioxidetoabsorbinfraredradiation.Asthecapacityoftheatmospheretoabsorbinfraredradiationincreases,theradiatinglevelandthetemperatureofthesurfacemustrise.Onemathematicalmodelpredictsthatdoublingtheatmosphericcarbondioxidewouldraisetheglobalmeansurfacetemperatureby2.5℃:Thismodelassumesthattheatmosphere’srelativehumidityremainsconstantandthetemperaturedecreaseswithaltitudeatarateof6.5℃perkilometer.Theassumptionofconstantrelativehumidityisimportant,becausewatervaporintheatmosphereisanotherefficientabsorberofradiationatinfraredwavelengths.Becausewarmaircanholdmoremoisturethancoolair,therelativehumiditywillbeconstantonlyiftheamountofwatervaporintheatmosphereincreasesasthetemperaturerises.Therefore,moreinfraredradiationwouldbeabsorbedandreradiatedbacktotheEarth’ssurface.Theresultantwarmingatthesurfacecouldbeexpectedtomeltsnowandice,reducingtheEarth’sreflectivity.Moresolarradiationwouldthenbeabsorbed,leadingtoafurtherincreaseintemperature. Accordingtothepassage,whichofthefollowingistrueofthelasthundredyears?
A.Fossil fuels were burned for the first time.
B.Greater amounts of land were cleared than at any time before.
C.The average temperature at the Earth’s surface has become 2℃ cooler.
D.The amount of carbon dioxide in the atmosphere has increased measurably.
正确答案是D