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. Theprimarypurposeofthepassageisto_____.
A.warn of the dangers of continued burning of fossil fuels B.discuss the significance of the amount of carbon dioxide in the atmosphere C.demonstrate the usefulness of mathematical models in predicting long-range climatic change D.describe the ways in which various atmospheric and climatic conditions contribute to the Earth’s weather正确答案B
