托福阅读真题分类――词汇题(二)

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1、智课网托福备考资料托福阅读真题分类――词汇题（二）DesertFormationThedeserts,whichalreadyoccupyapproximatelyafourthoftheEarth'slandsurface,haveinrecentdecadesbeenincreasingatanalarmingpace.Theexpansionofdesert-likeconditionsintoareaswheretheydidnotpreviouslyexistiscalleddesertification.Ithasbeenestimatedthatanadditionalo

2、ne-fourthoftheEarth'slandsurfaceisthreatenedbythisprocess.Desertificationisaccomplishedprimarilythroughthelossofstabilizingnaturalvegetationandthesubsequentacceleratederosionofthesoilbywindandwater.Insomecasestheloosesoilisblowncompletelyaway,leavingastonysurface.Inothercases,thefinerparticlesma

3、yberemoved,whilethesand-sizedparticlesareaccumulatedtoformmobilehillsorridgesofsand.Evenintheareasthatretainasoilcover,thereductionofvegetationtypicallyresultsinthelossofthesoil'sabilitytoabsorbsubstantialquantitiesofwater.Theimpactofraindropsontheloosesoiltendstotransferfineclayparticlesintothe

4、tiniestsoilspaces,sealingthemandproducingasurfacethatallowsverylittlewaterpenetration.Waterabsorptionisgreatlyreduced;consequentlyrunoffisincreased,resultinginacceleratederosionrates.Thegradualdryingofthesoilcausedbyitsdiminishedabilitytoabsorbwaterresultsinthefurtherlossofvegetation,sothatacycl

5、eofprogressivesurfacedeteriorationisestablished.Insomeregions,theincreaseindesertareasisoccurringlargelyastheresultofatrendtowarddrierclimaticconditions.Continuedgradualglobalwarminghasproducedanincreaseinaridityforsomeareasoverthepastfewthousandyears.Theprocessmaybeacceleratedinsubsequentdecade

6、sifglobalwarmingresultingfromairpollutionseriouslyincreases.Thereislittledoubt,however,thatdesertificationinmostareasresultsprimarilyfromhumanactivitiesratherthannaturalprocesses.Thesemiaridlandsborderingthedesertsexistinadelicateecologicalbalanceandarelimitedintheirpotentialtoadjusttoincreasede

7、nvironmentalpressures.Expandingpopulationsaresubjectingthelandtoincreasingpressurestoprovidethemwithfoodandfuel.Inwetperiods,thelandmaybeabletorespondtothesestresses.Duringthedryperiodsthatarecommonphenomenaalongthedesertmar