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,atmosphericcarbondioxideperformsallofthefollowingfunctionsEXCEPT:_____.
A.absorbing radiation at visible wavelengths
B.absorbing infrared radiation
C.absorbing outgoing radiation from the Earth
D.helping to retain heat near the Earth’s surface
正确答案是A