Global warming

Globalwarming第PAGE页GlobalwarmingMeansurfacetemperatureanomaliesduringtheperiod2019to2019withrespecttotheaveragetemperaturesfrom1940to1980GlobalwarmingistheincreaseintheaveragetemperatureoftheEarth'snear-surfaceairandoceanssincethemid-20thcenturyanditsprojectedcontinuation.Globalsurfacetemperatureincreased0.74±0.18 °C(1.33±0.32 °F)duringthe100yearsendingin2019.[1][2]TheIntergovernmentalPanelonClimateChange(IPCC)concludesthatmostofthetemperatureincreasesincethemid-twentiethcenturyis"verylikely"duetotheincreaseinanthropogenicgreenhousegasconcentrations.[3][2]Naturalphenomenasuchassolarvariationandvolcanoesprobablyhadasmallwarmingeffectfrompre-industrialtimesto1950andasmallcoolingeffectfrom1950onward.[4][5]Thesebasicconclusionshavebeenendorsedbyatleast30scientificsocietiesandacademiesofscience,[6]includingallofthenationalacademiesofscienceofthemajorindustrializedcountries.[7][8][9]Whileindividualscientistshavevoiceddisagreementwiththesefindings,[10]theoverwhelmingmajorityofscientistsworkingonclimatechangeagreewiththeIPCC'smainconclusions.[11][12]Climatemodelprojectionsindicatethatglobalsurfacetemperaturewilllikelyriseafurther1.1to6.4 °C(2.0to11.5 °F)duringthetwenty-firstcentury.[3]Theuncertaintyinthisestimatecomesaboutfromdifferingestimatesoffuturegreenhousegasemissionsandfromtheuseofmodelswithdifferingclimatesensitivity.Anotheruncertaintyishowwarmingandrelatedchangeswillvaryfromregiontoregionaroundtheglobe.Althoughmoststudiesfocusontheperiodupto2100,warmingisexpectedtocontinueformorethanathousandyearsevenifgreenhousegaslevelsarestabilized.Thisresultsfromthelargeheatcapacityoftheoceans.[3]Increasingglobaltemperaturewillcausesealevelstoriseandwillchangetheamountandpatternofprecipitation,likelyincludinganexpanseofthesubtropicaldesertregions.[13]Otherlikelyeffectsincludeincreasesintheintensityofextremeweatherevents,changesinagriculturalyields,modificationsoftraderoutes,glacierretreat,speciesextinctionsandincreasesintherangesofdiseasevectors.MostnationalgovernmentshavesignedandratifiedtheKyotoProtocolaimedatreducinggreenhousegasemissions.Politicalandpublicdebatecontinuesregardingwhat,ifany,actionshouldbetakentoreduceorreversefuturewarmingortoadapttoitsexpectedconsequences.GreenhouseeffectThecausesoftherecentwarmingareanactivefieldofresearch.Thescientificconsensus[14][15]isthattheincreaseinatmosphericgreenhousegasesduetohumanactivitycausedmostofthewarmingobservedsincethestartoftheindustrialera,andtheobservedwarmingcannotbesatisfactorilyexplainedbynaturalcausesalone.[16]Thisattributionisclearestforthemostrecent50years,forwhichthemostdetaileddataareavailable.ThegreenhouseeffectwasdiscoveredbyJosephFourierin1824[17]andwasfirstinvestigatedquantitativelybySvanteArrheniusin1896.Itistheprocessbywhichabsorptionandemissionofinfraredradiationbyatmosphericgaseswarmaplanet'sloweratmosphereandsurface.Existenceofthegreenhouseeffectassuchisnotdisputed.Thequestionisinsteadhowthestrengthofthegreenhouseeffectchangeswhenhumanactivityincreasestheatmosphericconcentrationsofsomegreenhousegases.Recentincreasesinatmosphericcarbondioxide(CO2).ThemonthlyCO2measurementsdisplaysmallseasonaloscillationsinanoverallyearlyuptrend;eachyear'smaximumisreachedduringtheNorthernHemisphere'slatespring,anddeclinesduringtheNorthernHemispheregrowingseasonasplantsremovesomeCO2fromtheatmosphere.Naturallyoccurringgreenhousegaseshaveameanwarmingeffectofabout33 °C(59 °F),withoutwhichEarthwouldbeuninhabitable.[18][19]OnEarth,themajorgreenhousegasesarewatervapor,whichcausesabout36–70percentofthegreenhouseeffect(notincludingclouds);carbondioxide(CO2),whichcauses9–26percent;methane(CH4),whichcauses4–9percent;andozone,whichcauses3–7percent.[20][21]Humanactivitysincetheindustrialrevolutionhasincreasedtheconcentrationofvariousgreenhousegases,leadingtoincreasedradiativeforcingfromCO2,methane,troposphericozone,CFCsandnitrousoxide.TheatmosphericconcentrationsofCO2andmethanehaveincreasedby31%and149%respectivelysincethebeginningoftheindustrialrevolutioninthemid-1700s.Theselevelsareconsiderablyhigherthanatanytimeduringthelast650,000years,theperiodforwhichreliabledatahasbeenextractedfromicecores.[22]FromlessdirectgeologicalevidenceitisbelievedthatCO2valuesthishighwerelastattained20millionyearsago.[23]Fossilfuelburninghasproducedapproximatelythree-quartersoftheincreaseinCO2fromhumanactivityoverthepast20years.Mostoftherestisduetoland-usechange,inparticulardeforestation.[24]CO2concentrationsareexpectedtoriseduetoongoingburningoffossilfuelsandland-usechange.Therateofrisewilldependonuncertaineconomic,sociological,technological,andnaturaldevelopments.TheIPCCSpecialReportonEmissionsScenariosgivesawiderangeoffutureCO2scenarios,rangingfrom541to970ppmbytheyear2100.[25]Fossilfuelreservesaresufficienttoreachthislevelandcontinueemissionspast2100,ifcoal,tarsandsormethaneclathratesareextensivelyused.[26]SolarvariationSolarvariationoverthelastthirtyyears.Someotherhypothesesdepartingfromtheconsensusviewhavebeensuggestedtoexplainmostofthetemperatureincrease.Onesuchhypothesisproposesthatwarmingmaybetheresultofvariationsinsolaractivity.[27][28][29]ApaperbyPeterStottandotherresearcherssuggeststhatclimatemodelsoverestimatetherelativeeffectofgreenhousegasescomparedtosolarforcing;theyalsosuggestthatthecoolingeffectsofvolcanicdustandsulfateaerosolshavebeenunderestimated.[30]Theyneverthelessconcludethatevenwithanenhancedclimatesensitivitytosolarforcing,mostofthewarmingsincethemid-20thcenturyislikelyattributabletotheincreasesingreenhousegases.TworesearchersatDukeUniversity,BruceWestandNicolaScafetta,haveestimatedthattheSunmayhavecontributedabout45–50percentoftheincreaseintheaverageglobalsurfacetemperatureovertheperiod1900–2019,andabout25–35percentbetween1980and2019.[31]Adifferenthypothesisisthatvariationsinsolaroutput,possiblyamplifiedbycloudseedingviagalacticcosmicrays,mayhavecontributedtorecentwarming.[32]Itsuggestsmagneticactivityofthesunisacrucialfactorwhichdeflectscosmicraysthatmayinfluencethegenerationofcloudcondensationnucleiandtherebyaffecttheclimate.[33]Onepredictedeffectofanincreaseinsolaractivitywouldbeawarmingofmostofthestratosphere,whereasanincreaseingreenhousegasesshouldproducecoolingthere.[34]Theobservedtrendsinceatleast1960hasbeenacoolingofthelowerstratosphere.[35]Reductionofstratosphericozonealsohasacoolinginfluence,butsubstantialozonedepletiondidnotoccuruntilthelate1970s.[36]Solarvariationcombinedwithchangesinvolcanicactivityprobablydidhaveawarmingeffectfrompre-industrialtimesto1950,butacoolingeffectsince.[3]In2019,PeterFoukalandcolleaguesfoundnonetincreaseofsolarbrightnessoverthelast1,000years.Solarcyclesledtoasmallincreaseof0.07percentinbrightnessoverthelast30years.Thiseffectistoosmalltocontributesignificantlytoglobalwarming.[37][38]OnepaperbyMikeLockwoodandClausFröhlichfoundnorelationbetweenglobalwarmingandsolarradiationsince1985,whetherthroughvariationsinsolaroutputorvariationsincosmicrays.[39]HenrikSvensmarkandEigilFriis-Christensen,themainproponentsofcloudseedingbygalacticcosmicrays,disputedthiscriticismoftheirhypothesis.[40]A2019paperfoundthatinthelast20yearstherehasbeennosignificantlinkbetweenchangesincosmicrayscomingtoEarthandcloudinessandtemperature.[41][42][43]ForcingandfeedbackComponentsofthecurrentradiativeforcingasestimatedbytheIPCCFourthAssessmentReport.Noneoftheeffectsofforcingareinstantaneous.ThethermalinertiaoftheEarth'soceansandslowresponsesofotherindirecteffectsmeanthattheEarth'scurrentclimateisnotinequilibriumwiththeforcingimposed.Climatecommitmentstudiesindicatethatevenifgreenhousegaseswerestabilizedat2019levels,afurtherwarmingofabout0.5 °C(0.9 °F)wouldstilloccur.[44]ClimatevariabilityTheEarth'sclimatechangesinresponsetoexternalforcing,includinggreenhousegases,variationsinitsorbitaroundtheSun(orbitalforcing),[45][46][47]changesinsolarluminosity,andvolcaniceruptions;[48]allexamplesoftheearth'sownvariationintemperatures,forwhichtheUNFCCCusesthetermclimatevariability.FeedbackWhenawarmingtrendresultsineffectsthatinducefurtherwarming,theprocessisreferredtoasapositivefeedback;whentheeffectsinducecooling,theprocessisreferredtoasanegativefeedback.Theprimarypositivefeedbackinvolveswatervapor.Theprimarynegativefeedbackistheeffectoftemperatureonemissionofinfraredradiation:asthetemperatureofabodyincreases,theemittedradiationincreaseswiththefourthpowerofitsabsolutetemperature.[49]Thisprovidesapowerfulnegativefeedbackwhichstabilizestheclimatesystemovertime.Oneofthemostpronouncedpositivefeedbackeffectsrelatestotheevaporationofwater.Iftheatmosphereiswarmed,thesaturationvapourpressureincreases,andthequantityofwatervaporintheatmospherewilltendtoincrease.Sincewatervaporisagreenhousegas,theincreaseinwatervaporcontentmakestheatmospherewarmfurther;thiswarmingcausestheatmospheretoholdstillmorewatervapor(apositivefeedback),andsoonuntilotherprocessesstopthefeedbackloop.TheresultisamuchlargergreenhouseeffectthanthatduetoCO2alone.Althoughthisfeedbackprocesscausesanincreaseintheabsolutemoisturecontentoftheair,therelativehumiditystaysnearlyconstantorevendecreasesslightlybecausetheairiswarmer.[50]ThisfeedbackeffectcanonlybereversedslowlyasCO2hasalongaverageatmosphericlifetime.Feedbackeffectsduetocloudsareanareaofongoingresearch.Seenfrombelow,cloudsemitinfraredradiationbacktothesurface,andsoexertawarmingeffect;seenfromabove,cloudsreflectsunlightandemitinfraredradiationtospace,andsoexertacoolingeffect.Whethertheneteffectiswarmingorcoolingdependsondetailssuchasthetypeandaltitudeofthecloud.Thesedetailsaredifficulttorepresentinclimatemodels,inpartbecausecloudsaremuchsmallerthanthespacingbetweenpointsonthecomputationalgridsofclimatemodels.[50]NorthernHemisphereicetrendsSouthernHemisphereicetrends.Asubtlerfeedbackprocessrelatestochangesinthelapserateastheatmospherewarms.Theatmosphere'stemperaturedecreaseswithheightinthetroposphere.Sinceemissionofinfraredradiationvarieswiththefourthpoweroftemperature,longwaveradiationemittedfromtheupperatmosphereislessthanthatemittedfromtheloweratmosphere.Mostoftheradiationemittedfromtheupperatmosphereescapestospace,whilemostoftheradiationemittedfromtheloweratmosphereisre-absorbedbythesurfaceortheatmosphere.Thus,thestrengthofthegreenhouseeffectdependsontheatmosphere'srateoftemperaturedecreasewithheight:iftherateoftemperaturedecreaseisgreaterthegreenhouseeffectwillbestronger,andiftherateoftemperaturedecreaseissmallerthenthegreenhouseeffectwillbeweaker.Boththeoryandclimatemodelsindicatethatwithincreasedgreenhousegascontenttherateoftemperaturedecreasewithheightwillbereduced,producinganegativelapseratefeedbackthatweakensthegreenhouseeffect.Measurementsoftherateoftemperaturechangewithheightareverysensitivetosmallerrorsinobservations,makingitdifficulttoestablishwhetherthemodelsagreewithobservations.[51]Anotherimportantfeedbackprocessisice-albedofeedback.[52]Whenglobaltemperaturesincrease,icenearthepolesmeltsatanincreasingrate.Astheicemelts,landoropenwatertakesitsplace.Bothlandandopenwaterareonaveragelessreflectivethanice,andthusabsorbmoresolarradiation.Thiscausesmorewarming,whichinturncausesmoremelting,andthiscyclecontinues.Warmingisalsothetriggeringvariableforthereleaseofmethanefromsourcesbothonlandandonthedeepoceanfloor,makingbothofthesepossiblefeedbackeffects.Thawingpermafrost,suchasthefrozenpeatbogsinSiberia,createsapositivefeedbackduetoreleaseofCO2andCH4.[53]Methanedischargefrompermafrostispresentlyunderintensivestudy.Warmerdeepoceantemperatures,likewise,couldreleasethegreenhousegasmethanefromthe'frozen'stateofthevastdeepoceandepositsofmethaneclathrate/methanehydrate,accordingtotheClathrateGunHypothesis,Oceanecosystems'abilitytosequestercarbonareexpectedtodeclineasitwarms.Thisisbecausetheresultinglownutrientlevelsofthemesopelagiczone(about200to1000mdepth)limitsthegrowthofdiatomsinfavorofsmallerphytoplanktonthatarepoorerbiologicalpumpsofcarbon.[54]TemperaturechangesRecentTwomillenniaofmeansurfacetemperaturesaccordingtodifferentreconstructions,eachsmoothedonadecadalscale.Theunsmoothed,annualvaluefor2019isalsoplottedforreference.Globaltemperatureshaveincreasedby0.75 °C(1.35 °F)relativetotheperiod1860–1900,accordingtotheinstrumentaltemperaturerecord.Thismeasuredtemperatureincreaseisnotsignificantlyaffectedbytheurbanheatislandeffect.[55]Since1979,landtemperatureshaveincreasedabouttwiceasfastasoceantemperatures(0.25 °Cperdecadeagainst0.13 °Cperdecade).[56]Temperaturesinthelowertropospherehaveincreasedbetween0.12and0.22 °C(0.22and0.4 °F)perdecadesince1979,accordingtosatellitetemperaturemeasurements.Temperatureisbelievedtohavebeenrelativelystableovertheoneortwothousandyearsbefore1850,withpossiblyregionalfluctuationssuchastheMedievalWarmPeriodortheLittleIceAge.[citationneeded]Seatemperaturesincreasemoreslowlythanthoseonlandbothbecauseofthelargereffectiveheatcapacityoftheoceansandbecausetheoceancanloseheatbyevaporationmorereadilythantheland.[57]TheNorthernHemispherehasmorelandthantheSouthernHemisphere,soitwarmsfaster.TheNorthernHemispherealsohasextensiveareasofseasonalsnowandsea-icecoversubjecttotheice-albedofeedback.MoregreenhousegasesareemittedintheNorthernthanSouthernHemisphere,butthisdoesnotcontributetothedifferenceinwarmingbecausethemajorgreenhousegasespersistlongenoughtomixbetweenhemispheres.[58]BasedonestimatesbyNASA'sGoddardInstituteforSpaceStudies,2019wasthewarmestyearsincereliable,widespreadinstrumentalmeasurementsbecameavailableinthelate1800s,exceedingthepreviousrecordsetin2019byafewhundredthsofadegree.[59]EstimatespreparedbytheWorldMeteorologicalOrganizationandtheClimaticResearchUnitconcludedthat2019wasthesecondwarmestyear,behind2019.[60][61]Temperaturesin2019wereunusuallywarmbecausethestrongestElNiño-SouthernOscillationinthepastcenturyoccurredduringthatyear.[62]Anthropogenicemissionsofotherpollutants—notablysulfateaerosols—canexertacoolingeffectbyincreasingthereflectionofincomingsunlight.Thispartiallyaccountsforthecoolingseeninthetemperaturerecordinthemiddleofthetwentiethcentury,[63]thoughthecoolingmayalsobedueinparttonaturalvariability.JamesHansenandcolleagueshaveproposedthattheeffectsoftheproductsoffossilfuelcombustion—CO2andaerosols—havelargelyoffsetoneanother,sothatwarminginrecentdecadeshasbeendrivenmainlybynon-CO2greenhousegases.[64]PaleoclimatologistWilliamRuddimanhasarguedthathumaninfluenceontheglobalclimatebeganaround8,000yearsagowiththestartofforestclearingtoprovidelandforagricultureand5,000yearsagowiththestartofAsianriceirrigation.[65][66]Ruddiman'sinterpretationofthehistoricalrecord,withrespecttothemethanedata,hasbeendisputed.[67]Pre-humanclimatevariationsCurvesofreconstructedtemperatureattwolocationsinAntarcticaandaglobalrecordofvariationsinglacialicevolume.Today'sdateisontheleftsideofthegraph.Earthhasexperiencedwarmingandcoolingmanytimesinthepast.TherecentAntarcticEPICAicecorespans800,000years,includingeightglacialcyclestimedbyorbitalvariationswithinterglacialwarmperiodscomparabletopresenttemperatures.[68]ArapidbuildupofgreenhousegasesamplifiedwarmingintheearlyJurassicperiod(about180millionyearsago),withaveragetemperaturesrisingby5 °C(9 °F).ResearchbytheOpenUniversityindicatesthatthewarmingcausedtherateofrockweatheringtoincreaseby400%.Assuchweatheringlocksawaycarbonincalciteanddolomite,CO2levelsdroppedbacktonormaloverroughlythenext150,000years.[69]Suddenreleasesofmethanefromclathratecompounds(theclathrategunhypothesis)havebeenhypothesizedasbothacauseforandaneffectofotherwarmingeventsinthedistantpast,includingthePermian–Triassicextinctionevent(about251millionyearsago)andthePaleocene–EoceneThermalMaximum(about55millionyearsago).ClimatemodelsCalculationsofglobalwarmingpreparedinorbefore2019fromarangeofclimatemodelsundertheSRESA2emissionsscenario,whichassumesnoactionistakentoreduceemissions.Thegeographicdistributionofsurfacewarmingduringthe21stcenturycalculatedbytheHadCM3climatemodelifabusinessasusualscenarioisassumedforeconomicgrowthandgreenhousegasemissions.Inthisfigure,thegloballyaveragedwarmingcorrespondsto3.0 °C(5.4 °F).Scientistshavestudiedglobalwarmingwithcomputermodelsoftheclimate.Thesemodelsarebasedonphysicalprinciplesoffluiddynamics,radiativetransfer,andotherprocesses,withsimplificationsbeingnecessarybecauseoflimitationsincomputerpowerandthecomplexityoftheclimatesystem.Allmodernclimatemodelsincludeanatmosphericmodelthatiscoupledtoanoceanmodelandmodelsforicecoveronlandandsea.Somemodelsalsoincludetreatmentsofchemicalandbiologicalprocesses.[70]Thesemodelsprojectawarmerclimateduetoincreasinglevelsofgreenhousegases.[71]However,evenwhenthesameassumptionsoffuturegreenhousegaslevelsareused,therestillremainsaconsiderablerangeofclimatesensitivity.Includinguncertaintiesinfuturegreenhousegasconcentrationsandclimatemodeling,theIPCCanticipatesawarmingof1.1 °Cto6.4 °C(2.0 °Fto11.5 °F)bytheendofthe21stcentury,relativeto1980–2019.[3]Modelshavealsobeenusedtohelpinvestigatethecausesofrecentclimatechangebycomparingtheobservedchangestothosethatthemodelsprojectfromvariousnaturalandhuman-derivedcauses.Currentclimatemodelsproduceagoodmatchtoobservationsofglobaltemperaturechangesoverthelastcentury,butdonotsimulateallaspectsofclimate.[72]Thesemodelsdonotunambiguouslyattributethewarmingthatoccurredfromapproximately1910to1945toeithernaturalvariationorhumaneffects;however,theysuggestthatthewarmingsince1975isdominatedbyman-madegreenhousegasemissions.Globalclimatemodelprojectionsoffutureclimateareforcedbyimposedgreenhousegasemissionscenarios,mostoftenfromtheIPCCSpecialReportonEmissionsScenarios(SRES).Lesscommonly,modelsmayalsoincludeasimulationofthecarboncycle;thisgenerallyshowsapositivefeedback,thoughthisresponseisuncertain(undertheA2SRESscenario,responsesvarybetweenanextra20and200 ppmofCO2).Someobservationalstudiesalsoshowapositivefeedback.[73][74][75]InMay2019,itwaspredictedthat"globalsurfacetemperaturemaynotincreaseoverthenextdecade,asnaturalclimatevariationsintheNorthAtlanticandtropicalPacifictemporarilyoffsettheprojectedanthropogenicwarming",basedontheinclusionofoceantemperatureobservations.[76]Therepresentationofcloudsisoneofthemainsourcesofuncertaintyinpresent-generationmodels,thoughprogressisbeingmadeonthisproblem.[77]Aminorissueinclimatemodelingistheperceivedmismatchbetweenactualconditionsandthoseprojectedbythemodels.A2019studybyDavidDouglassandcolleaguescomparedthecompositeoutputof22leadingglobalclimatemodelswithactualclimatedataandfoundthatthemodelsdidnotaccuratelyprojectobservedchangestothetemperatureprofileinthetropicaltroposphere.Theauthorsnotethattheirconclusionscontraststronglywiththoseofrecentpublicationsbasedonessentiallythesamedata.[78]A2019paperpublishedbya17-memberteamledbyBenSanterofLawrenceLivermoreNationalLaboratorynotedseriousmathematicalflawsintheDouglassstudy,andfoundinsteadthatdeviationsbetweenthemodelsandobservationswerestatisticallyinsignificant.[79]AttributedandexpectedeffectsEnvironmentalSparserecordsindicatethatglaciershavebeenretreatingsincetheearly1800s.Inthe1950smeasurementsbeganthatallowthemonitoringofglacialmassbalance,reportedtotheWGMSandtheNSIDC.Althoughitisdifficulttoconnectspecificweathereventstoglobalwarming,anincreaseinglobaltemperaturesmayinturncausebroaderchanges,includingglacialretreat,Arcticshrinkage,andworldwidesealevelrise.Changesintheamountandpatternofprecipitationmayresultinfloodinganddrought.Theremayalsobechangesinthefrequencyandintensityofextremeweatherevents.Othereffectsmayincludechangesinagriculturalyields,additionofnewtraderoutes,[80]reducedsummerstreamflows,speciesextinctions,andincreasesintherangeofdiseasevectors.Someeffectsonboththenaturalenvironmentandhumanlifeare,atleastinpart,alreadybeingattributedtoglobalwarming.A2019reportbytheIPCCsuggeststhatglacierretreat,iceshelfdisruptionsuchasthatoftheLarsenIceShelf,sealevelrise,changesinrainfallpatterns,andincreasedintensityandfrequencyofextremeweathereventsareattributableinparttoglobalwarming.[81]Otherexpectedeffectsincludewaterscarcityinsomeregionsandincreasedprecipitationinothers,changesinmountainsnowpack,andadversehealtheffectsfromwarmertemperatures.[82]Socialandeconomiceffectsofglobalwarmingmaybeexacerbatedbygrowingpopulationdensitiesinaffectedareas.Temperateregionsareprojectedtoexperiencesomebenefits,suchasfewerdeathsduetocoldexposure.[83]Asummaryofprobableeffectsandrec