四级作文素材.docx

四级作文素材.docx

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四级作文素材

第五节写作内容储备

　　从上表可以看出，四级题目涉及到社会生活的各个方面，但可大体归纳为几个方面，下面就是对各个主要方面常见的内容的介绍。

　　一、环境（Environment）

　　Likeallotherlivingbeings,humanshaveclearlychangedtheirenvironment,buttheyhavedonesogenerallyonagranderscalethanhaveotherspecies.Someofthesechanges—suchasthedestructionoftheworld’stropicalrainforeststocreategrazinglandforcattleorthedryingupofalmostthree-quartersoftheAralSea,oncetheworld’sfourth-largestfreshwaterlake,forirrigationpurposes—haveledtoalteredclimatepatterns,whichinturnhavechangedthedistributionofspeciesofanimalsandplants.

Scientistsareworkingtounderstandthelong-termconsequencesthathumanactionshaveonecosystems,whileenvironmentalists—professionalsinvariousfields,aswellasconcernedcitizensintheUnitedStatesandothercountries—arestrugglingtolessentheimpactofhumanactivityonthenaturalworld.

PopulationGrowth（人口增长）

　　Humanpopulationgrowthmaybeseentobeattherootofvirtuallyalloftheworld’senvironmentalproblems.Increasinglylargenumbersofpeoplearebeingaddedtotheworldeveryyear.Asthenumberofpeopleincreases,morepollutionisgenerated,morehabitatsaredestroyed,andmorenaturalresourcesareusedup.Evenifnewtechnologicaladvanceswereabletocutinhalftheenvironmentalimpactthateachpersonhad,assoonastheworld’spopulationsizedoubled,theearthwouldbenobetteroffthanbefore.

　　ThePopulationDivisionoftheUnitedNationspredictsthatthe5.63billionhumansalivein1994willincreaseto6.23billionintheyear2000,8.47billionin2025,and10.02billionin2050.TheUN’sestimateassumesthatpopulationwillpeakandstabilizeat11.6billionin2200.

　　Althoughitistruethatratesofpopulationincreasearenowmuchslowerinthedevelopedworldthaninthedevelopingworld,itwouldbeamistaketoassumethatthepopulationgrowthproblemisprimarilyaproblemofdevelopingcountries.Infact,becauselargeramountsofresourcesperpersonareusedinthedevelopednations,eachcitizenfromthedevelopedworldhasamuchgreaterenvironmentalimpactthandoesacitizenfromadevelopingcountry.

　　Evidencenowexistssuggestingthatthemostimportantfactorsnecessarytolowerpopulationgrowthratesinthedevelopingworldaredemocracyandsocialjustice.Studiesshowthatpopulationgrowthrateshavefalleninareaswhereseveralsocialconditionshavebeenmet.Intheseareas,literacyrateshaveincreased,andwomenaregiveneconomicstatusequaltothatofmenandthusareabletoholdjobsandownproperty;also,birthcontrolinformationismorewidelyavailable,andwomenarefreetomaketheirownreproductivedecisions.

　　GlobalWarming（全球变暖）

　　Liketheglasspanesinagreenhouse,certaingasesintheearth’satmospherepermitthesun’sradiationtoheattheearthbutretardtheescapeintospaceoftheinfraredenergyradiatedbackoutbytheearth.Thisprocessisreferredtoasthegreenhouseeffect.Thesegases,primarilycarbondioxide,methane,nitrousoxide,andwatervapor,insulatetheearth’ssurface,helpingtomaintainwarmtemperatures.Iftheconcentrationofthesegaseswerehigher,moreheatwouldbetrappedwithintheatmosphere,andworldwidetemperatureswouldrise.

　　Withinthelastcentury,theamountofcarbondioxideintheatmospherehasincreaseddramatically,largelybecauseofthepracticeofburningfossilfuels—coalandpetroleumanditsderivatives.Atmosphericscientistshavenowconcludedthatatleasthalfofthatincreasecanbeattributedtohumanactivity,andtheyhavepredictedthatunlessdramaticactionistaken,temperaturewillcontinuetorisebybetween1°and3.5°Coverthenextcentury.Theconsequencesofsuchamodestincreaseintemperaturemaywellbedevastating.Sealevelswillrise,completelyinundatinganumberoflow-lyingislandnationsandfloodingmanycoastalcitiessuchasNewYorkandMiami.Manyplantandanimalspecieswillprobablybedrivenintoextinction,agriculturalregionswillbedisrupted,andthefrequencyofseverehurricanesanddroughtsislikelytoincrease.

　　Depletionoftheozonelayer（臭氧层变薄）

　　Theozonelayer,athinbandinthestratosphere（alayerintheupperatmosphere）,servestoshieldtheearthfromthesun’sharmfulultravioletrays.Inthe1970s,scientistsdiscoveredthatthelayerwasbeingattackedbychlorofluorocarbons（CFCs）,chemicalsusedinrefrigeration,air-conditioningsystems,cleaningsolvents,andaerosolsprays.

　　Theconsequencesofthedepletionoftheozonelayeraredramatic.Increasedultravioletradiationwillleadtoagrowingnumberofskincancersandcataractsandalsoreducetheabilityofpeople’simmunesystemstorespondtoinfection.Additionally,thegrowthratesoftheworld’soceanicplankton,thebaseofmostmarinefoodchains,willbenegativelyaffected,perhapsleadingtoincreasedatmosphericcarbondioxideandthustoglobalwarming.

　　Predictingtherateofozonedepletionisdifficult.Optimistsclaimthatifinternationalagreementsforthephasingoutofozone-depletingchemicalsagreedtoinMontréalin1987arefollowed,ozonelosswillpeakintheyear2000.Withmanyoftheworld’sfastestgrowingcountriesintheprocessofindustrializingandmodernizing,thereisreasontobelievethatdestructionwillcontinuetoincreasewellbeyondthatyear.

　　Airpollution（空气污染）

　　Asignificantportionofindustryandtransportationisbasedontheburningoffossilfuels,suchasgasoline.Asthesefuelsareburned,chemicalsandparticulatematterarereleasedintotheatmosphere.Althoughavastnumberofsubstancescontributetoairpollution,themostcommoncontaincarbon,sulfur,andnitrogen.Thesechemicalsinteractwithoneanotherandwithultravioletradiationinsunlightinvariousdangerousways.Smog,usuallyfoundinurbanareaswithlargenumbersofautomobiles,cancauseserioushealthproblems.Acidrainisaseriousglobalproblembecausefewspeciesarecapableofsurvivinginthefaceofsuchacidicconditions.Acidrainhasmadenumerouslakessoacidicthattheynolongersupportfishpopulations.Acidrainisalsothoughttoberesponsibleforthedeclineofmanyforestecosystemsworldwide.

　　Waterpollution（水污染）

　　Estimatessuggestthatnearly1.5billionpeoplelacksafedrinkingwaterandthatatleast5milliondeathsperyearcanbeattributedtowaterbornediseases.Waterpollutionmaycomefrompointornonpointsources.Pointsourcesdischargepollutantsatspecificlocations—from,forexample,factories,sewagetreatmentplants,oroiltankers.Thetechnologyexistsforpointsourcesofpollutiontobemonitoredandregulated,althoughpoliticalfactorsmaycomplicatematters.Nonpointsources—runoffwatercontainingpesticidesandfertilizersfromacresofagriculturalland,forexample—aremuchmoredifficulttocontrol.Pollutionarisingfromnonpointsourcesaccountsforamajorityofthecontaminantsinstreamsandlakes.

　　Withalmost80percentoftheplanetcoveredbyoceans,peoplehavelongactedasifthosebodiesofwatercouldserveasalimitlessdumpinggroundforwastes.Rawsewage,garbage,andoilspillshavebeguntooverwhelmthedilutingcapabilitiesoftheoceans,andmostcoastalwatersarenowpolluted.Beachesaroundtheworldareclosedregularly,oftenbecauseofhighamountsofbacteriafromsewagedisposal,andmarinewildlifeisbeginningtosuffer.

　　Groundwaterpollution（地下水污染）

　　Waterthatseepsthroughporousrocksandisstoredbeneaththegroundiscalledgroundwater.Althoughgroundwaterisarenewableresource,reservesarereplenishedrelativelyslowly.Whengroundwaterisdepletedincoastalregions,oceanicsaltwatercommonlyintrudesintofreshwatersupplies.SaltwaterintrusionisthreateningthedrinkingwaterofmanyareasalongtheGulfandAtlanticcoasts.

　　TheEPAhasestimatedthat,onaverage,25percentofusablegroundwateriscontaminated,althoughinsomeareasasmuchas75percentiscontaminated.Contaminationarisesfromleakingundergroundstoragetanks,poorlydesignedindustrialwasteponds,andseepagefromthedeep-wellinjectionofhazardouswastesintoundergroundgeologicformations.Becausegroundwaterisrechargedandflowssoslowly,oncepolluteditwillremaincontaminatedforextendedperiods.

　　HabitatDestructionandSpeciesExtinction（居住环境破坏与物种灭绝）

　　Itisdifficulttoestimatetherateatwhichhumansaredrivingspeciesextinctbecausescientistsbelievethatonlyasmallpercentageoftheearth’sspecieshavebeendescribed.Whatisclearisthatspeciesaredyingoutatanunprecedentedrate;minimumestimatesareatleast4000speciesperyear,althoughsomescientistsbelievethenumbermaybeashighas50,000peryear.Theleadingcauseofextinctionishabitatdestruction,particularlyoftheworld’srichestecosystems—tropicalrainforestsandcoralreefs.Atthecurrentrateatwhichtheworld’srainforestsarebeingcutdown,theymaycompletelydisappearbytheyear2030.Ifgrowingpopulationsizeputsevenmorepressureonthesehabitats,theymightwellbedestroyedsooner.

　　Thislossiscriticalfromseveralperspectives.Theeconomicvalueofspecieslostandofnaturalproductsanddrugsthatneverwillbediscoveredorproducedisincalculable.Similarly,itisimpossibletoplaceeitheramoraloranaestheticvalueonourgrowinglistofextinctspecies.Ashabitatsaredestroyedandspecieslost,theworldisincreasinglylosingthreadsfromtheinterconnectedfabricoflife.

　　ChemicalRisks（化学成分污染）

　　Pesticideresiduesoncropsandmercuryinfishareexamplesoftoxicsubstancesthatmaybeencounteredindailylife.Manyindustriallyproducedchemicalsmaycausecancer,birthdefects,geneticmutations,ordeat