2018年6月大学英语六级听力常考时文：Obtaining Fresh Water from Icebergs思维导图

Theconceptofobtainingfreshwaterfromicebergsthataretowedtopopulatedareasandaridregionsoftheworldwasoncetreatedasajokemoreappropriatetocartoonsthanreallife.Butnowitisbeingconsideredquiteseriouslybymanynations,especiallysincescientistshavewarnedthatthehumanracewilloutgrowitsfreshwatersupplyfasterthanitrunsoutoffood.Glaciersareapossiblesourceoffreshwaterthathasbeenoverlookeduntilrecently.Three-quartersoftheEarth’sfreshwatersupplyisstilltiedupinglacialice,areservoirofuntappedfreshwatersoimmensethatitcouldsustainalltheriversoftheworldfor1,000years.Hugeglaciersthatstretchovertheshallowcontinentalshelfgivebirthtoicebergsthroughouttheyear.Icebergsarenotlikeseaice,whichisformedwhentheseaitselffreezes,rather,theyareformedentirelyonland,breakingoffwhenglaciersspreadoverthesea.Astheydriftawayfromthepolarregion,icebergssometimesmovemysteriouslyinadirectionoppositetothewind,pulledbysubsurfacecurrents.Becausetheymeltmoreslowlythansmallerpiecesofice,icebergshavebeenknowntodriftasfarnorthas35degreessouthoftheequatorintheAtlanticOcean.Tocorralthemandsteerthemtopartsoftheworldwheretheyareneededwouldnotbetoodifficult.Thedifficultyarisesinothertechnicalmatters,suchasthepreventionofrapidmeltinginwarmerclimatesandthefunnelingoffreshwatertoshoreingreatvolume.Buteveniftheicebergslosthalfoftheirvolumeintowing,thewatertheycouldprovidewouldbefarcheaperthanthatproducedbydesalinization,orremovingsaltfromwater.