tpo1-34解析包中含审核后题目可作为权威真题-阅读及译文passage

Thedeserts,whichalreadyoccupyapproximayafourthoftheEarth'slandsurface,haveinrecentdecadesbeenincreasingatanalarmingpace.Theexpansionofdesertlikeconditionsintoareaswheretheydidnotpreviouslyexistiscalleddesertification.Ithasbeenestimatedthatanadditionalone-fourthoftheEarth'slandsurfaceisthreatenedbythisprocess.四分之一的地方正沙漠化。Desertificationis plishedprimarilythroughthelossofstabilizingnaturalvegetationandthesubsequentacceleratederosionofthesoilbywindandwater.Insomecasestheloosesoilisblowncompleyaway,leavingastonysurface.Inothercases,thefinerparticlesmayberemoved,whilethesand-sizedparticlesareaccumulatedtoformmobilehillsorridgesofsand.Evenintheareasthatretainasoilcover,thereductionofvegetationtypicallyresultsinthelossofthesoil'sabilitytoabsorbsubstantialtiesofwater.Theimpactofraindropsontheloosesoiltendstotransferfineclayparticlesintothetiniestsoilspaces,sealingthemandproducingasurfacethatallowsverylittlewaterpenetration.Waterabsorptionisgreatlyreduced;consequentlyrunoffisincreased,resultinginacceleratederosionrates.Thegradualdryingofthesoilcausedbyitsdiminishedabilitytoabsorbwaterresultsinthefurtherlossofvegetation,sothatacycleofprogressivesurfacedeteriorationisestablished.即便是在保留了土壤表层的区域植被减少也已成为土壤大量吸取水的能力Insomeregions,theincreaseindesertareasisoccurringlargelyastheresultofatrendtowarddrierclimaticconditions.Continuedgradualglobalwarminghasproducedanincreaseinaridityforsomeareasoverthepastfewthousandyears.fromairpollutionseriouslyincreases.的温室效应继续，沙漠化进程会在未来数十年内加速实现。Thereislittledoubt,however,thatdesertificationinmostareasresultsprimarilyfromhumanactivitiesratherthannaturalprocesses.Thesemiaridlandsborderingthedesertsexistinadelicateecologicalbalanceandarelimitedintheirpotentialtoadjusttoincreasedenvironmentalpressures.Expandingpopulationsaresubjectingthelandtoincreasingpressurestoprovidethemwithfoodandfuel.Inwetperiods,thelandmaybeabletorespondtothesestresses.Duringthedryperiodsthatarecommonphenomenaalongthedesertmargins,though,thepressureonthelandisoftenfarinexcessofitsdiminishedcapacity,anddesertificationresults.Fourspecificactivitieshavebeenidentifiedasmajorcontributorstothedesertificationprocesses:overcultivation,overgrazing,firewoodgathering,andoverirrigation.Thecultivationofcropshasexpandedintoprogressivelydrierregionsaspopulationdensitieshavegrown.Theseregionsareespeciallylikelytohaveperiodsofseveredryness,sothatcropfailuresarecommon.Sincetheofmostcropsnecessitatesthepriorremovalofthenaturalvegetation,cropfailuresleaveextensivetractsoflanddevoidofaplantcoverandsusceptibletowindandwatererosion.Theraisingoflivestockisamajoreconomicactivityinsemiaridlands,wheregrassesaregenerallythedominanttypeofnaturalvegetation.■Theconsequencesofanexcessivenumberoflivestockgrazinginanareaarethereductionofthevegetationcoverandthetramplingandpulverizationofthesoil.■ThisisusuallyfollowedbythedryingofthesoilandacceleratedFirewoodisthechieffuelusedforcookingandheatinginmanycountries.Theincreasedpressuresofexpandingpopulationshaveledtotheremovalofwoodyplantssothatmanycitiesandtownsaresurroundedbylargeareascompleylackingintreesandshrubs.Theincreasinguseofdriedanimalwasteasasubstitutefuelhasalsohurtthesoilbecausethisvaluablesoilconditionerandsourceofplantnutrientsisnolongerbeingreturnedtotheland.在很多国家木材是用来做饭和加热的最主要人口增加带来的压力促使人们使用烘干的动物排泄物作为替代同样对土壤不利因为这些珍贵的土壤成分Thefinalmajorhumancauseofdesertificationissoilsalinizationresultingfromoverirrigation.Excesswaterfromirrigationsinksdownintothewatertable.IfnoThewaterevaporatesandthesaltsareleftbehind,creatingawhitecrustallayerthatpreventsairandwaterfromreachingtheunderlying灌溉多余的水渗透到水位。假如没有排水系统的存在，那么水位上升，这一地壳层了空气和水接触地的土壤。Theextremeseriousnessofdesertificationresultsfromthevastareasoflandandthetremendousnumbersofpeopleaffected,aswellasfromthegreatdifficultyofreversingorevenslowingtheprocess.Oncethesoilhasbeenremovedbyerosion,onlythepassageofcenturiesormillenniawillenablenewsoiltoform.Inar