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1、Unit6carryingcapacity：earth’sbottomlineIttakesnostretchoftheimaginationtoseethatthehumanspeciesisnowanagentofchangeofgeologicproportions.Weliterallymovemountainstominetheearth’sminerals,redirectriverstobuildcitiesinthedesert,torchforeststomakewayforcropsandcattle,andalterthechemistryofthea
2、tmosphereindisposingofourwastes.Athumanity’shand,theearthisundergoingaprofoundtransformation-onewithconsequenceswecannotfullygrasp.没有一点点想象明白人类带来地理比例的变化,我们移山开采地球的矿物质,重定向的河流在沙漠中建造城市,为了农作物和畜牧烧森林来让路,并改变大气的化学处理的废物。对于人类,地球正在经历一个深远的改变-这一结果我们不能完全控制.Itmaybetheultimateironythat,inoureffortstomakethe
3、earthyieldmoreforourselves,wearediminishingitsabilitytosustainlifeofallkinds,humansincluded.Signsofenvironmentalconstraintsarenowpervasive.Croplandisscarcelyexpandinganymore,andagoodportionofexistingagriculturallandislosingfertility.Grasslandshavebeenovergrazedandfisheriesoverharvested,lim
4、itingtheamountofadditionalfoodfromthesesources.Waterbodieshavesufferedextensivedepletionandpollution,severelyrestrictingfuturefoodproductionandurbanexpansion.Andnaturalforests-whichhelpstabilizetheclimate,moderatewatersupplies,andharboramajorityoftheplanet’sterrestrialbiodiversity-continue
5、torecede.它可能是最终的讽刺,在我们努力使地球产量更多,我们正在消弱其维持生命的能力。包括人类,环境约束是现在普遍的迹象。农田几乎扩大了,和相当一部分现有的农业土地失去肥力。草地的过度放牧和渔业的过度捕捞，从这些来源限制额外的食物数量。水域已经遭受大量的损耗和污染，严重地限制未来的粮食生产和城市扩张。并且原始森林帮助稳定气候，减缓水的供应，并且保持大量地球的生物多样性，持续减弱。Thesetrendsarenotnew.Humansocietieshavebeenalteringtheearthsincetheybegan.Butthepaceandscaleofd
6、egradationthatstartedaboutmid-century-andcontinuestoday-ishistoricallynew.Thecentralconundrumofsustainabledevelopmentisnowalltooapparent:Populationandeconomiesgrowexponentially,butthenaturalresourcesthatsupportthemdonot.这些趋势并不新鲜。人类社会已经从他们开始改变地球。但退化的速度和规模,开始于中世纪一直持续到今天，是前所未有的。可持续发展的核心难题现在太明
7、显:人口和经济以指数形式增长，但自然资源不能维持他们。Biologistsoftenapplytheconceptof“carryingcapacity”toquestionsofpopulationpressuresonanenvironment.Carryingcapacityisthelargestnumberofanygivenspeciesthatahabitatcansupportindefinitely.Whenthatmaximumsustainablepopulationlevelissurpas
