北极熊的生存环境因为全球气候变暖已经变得越来越恶劣了。但是近日有人证明北极熊可以在全球气候变暖的情况下生存，真的是这样吗，我们来看看下面的雅思阅读材料。
THISmonthhasseenqualifiedgoodnewsforpolarbears. OnDecember 16thresearchersfromtheUnitedStatesandCanadapredictedthateventhoughtheextentofthesummericeintheArcticOceanisexpectedtodeclineprecipitouslyoverthenextfewdecades, arefugeofsortswillremainforthem. Meanwhile, inapaperinNature, agroupAmericanclimatologistsarguedthatasuddenandirreversibledeclineoftheseaicemay, afterall, notbethemostlikelyoutcomeofglobalwarming.
TherefugewillbealongthenorthernshoresofCanadaandGreenland, StephaniePfirman, ofBarnardCollegeinNewYork, andhercolleaguestoldtheautumnmeetingoftheAmericanGeophysicalUnioninSanFrancisco. Eveninsummer, icewillpersisttherebecauseincreasedmeltingwillbecompensatedforbythearrivaloficefromthecentreoftheArcticOcean, andevenfromasfarawayasEuropeandAsia. Then, asnow, prevailingwindsandresultantoceancurrentscarryicetoNorthAmerica. ThatiswhytheiceontheEurasiansideistypicallyyoungandaboutonemetrethick, whereasthatnearCanadacanbeeightmetresthickandasmanyyearsold.
DrPfirmanarrivedatthisconclusionafterrunningacomputermodelcalledtheCommunityClimateSystemModelversion 3, orCCSM3, whichwasdevelopedbytheUniversityCorporationforAtmosphericResearchinBoulder, Colorado. Accordingtothismodel, theArcticOceanwillbemoreorlessice-freeby 2050 ifnospecialeffortismadetoreducegreenhouse-gasemissionsthebusinessasusualscenario. Thereasonisthatwhenlargestretchesofocean, forlackofreflectiveicecover, absorbmoreheatfromthesunthanisusuallythecasetheywillbeunlikelyevertofreezeoveragain. Evenafterthisdestructivefeedbackcyclehastakenitscourse, however, themodelscalculationsoficegrowthandtransportshowthattherefugeattheAmericansideoftheArctic, all 500,000 squarekilometresofit, willbethereforthelastpolarbears.
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