Global Warming - NASA Earth Observatory

As the Earth moved out of ice ages over the past million years, the global temperature rose a total of 4 to 7 degrees Celsius over about 5,000 ... Atmosphere Heat HumanPresence Land Life Natural Events All Drought DustandHaze Earthquakes Fires Floods Landslides Man-made SeaandLakeIce SevereStorms TemperatureExtremes UniqueImagery Volcanoes WaterColor RemoteSensing Snow&Ice Water Collections ABCsfromSpace AmazonDeforestation AppliedSciences AstronautPhotography BlueMarble EarthatNight EarthfromAfar FallColors SeaLevelRise U.S.NationalParks WorldofChange EOExplorer Topics Search Images GlobalMaps Articles Blogs EOExplorer × Thispagecontainsarchivedcontentandisnolongerbeingupdated.Atthetimeofpublication,itrepresentedthebestavailablescience. GlobalWarming ByHolliRiebeek DesignbyRobertSimmon June3,2010 Throughoutitslonghistory,Earthhaswarmedandcooledtimeandagain.Climatehaschangedwhentheplanetreceivedmoreorlesssunlightduetosubtleshiftsinitsorbit,astheatmosphereorsurfacechanged,orwhentheSun’senergyvaried.Butinthepastcentury,anotherforcehasstartedtoinfluenceEarth’sclimate:humanity. (NASAastronautphotographISS022-E-6674.) WhatisGlobalWarming? GlobalwarmingistheunusuallyrapidincreaseinEarth’saveragesurfacetemperatureoverthepastcenturyprimarilyduetothegreenhousegasesreleasedbypeopleburningfossilfuels. HowDoesToday’sWarmingComparetoPastClimateChange? Earthhasexperiencedclimatechangeinthepastwithouthelpfromhumanity.Butthecurrentclimaticwarmingisoccurringmuchmorerapidlythanpastwarmingevents. WhyDoScientistsThinkCurrentWarmingIsn’tNatural? InEarth’shistorybeforetheIndustrialRevolution,Earth’sclimatechangedduetonaturalcausesunrelatedtohumanactivity.Thesenaturalcausesarestillinplaytoday,buttheirinfluenceistoosmallortheyoccurtooslowlytoexplaintherapidwarmingseeninrecentdecades. HowMuchMoreWillEarthWarm? Modelspredictthatastheworldconsumesevermorefossilfuel,greenhousegasconcentrationswillcontinuetorise,andEarth’saveragesurfacetemperaturewillrisewiththem.Basedonplausibleemissionscenarios,averagesurfacetemperaturescouldrisebetween2°Cand6°Cbytheendofthe21stcentury.Someofthiswarmingwilloccureveniffuturegreenhousegasemissionsarereduced,becausetheEarthsystemhasnotyetfullyadjustedtoenvironmentalchangeswehavealreadymade. HowWillEarthRespondtoWarmingTemperatures? Theimpactofglobalwarmingisfargreaterthanjustincreasingtemperatures.Warmingmodifiesrainfallpatterns,amplifiescoastalerosion,lengthensthegrowingseasoninsomeregions,meltsicecapsandglaciers,andalterstherangesofsomeinfectiousdiseases.Someofthesechangesarealreadyoccurring. ReferencesandRelatedResources GlobalWarming Throughoutitslonghistory,Earthhaswarmedandcooledtimeandagain.Climatehaschangedwhentheplanetreceivedmoreorlesssunlightduetosubtleshiftsinitsorbit,astheatmosphereorsurfacechanged,orwhentheSun’senergyvaried.Butinthepastcentury,anotherforcehasstartedtoinfluenceEarth’sclimate:humanity HowdoesthiswarmingcomparetopreviouschangesinEarth’sclimate?Howcanwebecertainthathuman-releasedgreenhousegasesarecausingthewarming?HowmuchmorewilltheEarthwarm?HowwillEarthrespond?Answeringthesequestionsisperhapsthemostsignificantscientificchallengeofourtime. WhatisGlobalWarming? GlobalwarmingistheunusuallyrapidincreaseinEarth’saveragesurfacetemperatureoverthepastcenturyprimarilyduetothegreenhousegasesreleasedaspeopleburnfossilfuels.Theglobalaveragesurfacetemperaturerose0.6to0.9degreesCelsius(1.1to1.6°F)between1906and2005,andtherateoftemperatureincreasehasnearlydoubledinthelast50years.Temperaturesarecertaintogoupfurther. Despiteupsanddownsfromyeartoyear,globalaveragesurfacetemperatureisrising.Bythebeginningofthe21stcentury,Earth’stemperaturewasroughly0.5degreesCelsiusabovethelong-term(1951–1980)average.(NASAfigureadaptedfromGoddardInstituteforSpaceStudiesSurfaceTemperatureAnalysis.) Earth’snaturalgreenhouseeffect Earth’stemperaturebeginswiththeSun.Roughly30percentofincomingsunlightisreflectedbackintospacebybrightsurfaceslikecloudsandice.Oftheremaining70percent,mostisabsorbedbythelandandocean,andtherestisabsorbedbytheatmosphere.Theabsorbedsolarenergyheatsourplanet. Astherocks,theair,andtheseaswarm,theyradiate“heat”energy(thermalinfraredradiation).Fromthesurface,thisenergytravelsintotheatmospherewheremuchofitisabsorbedbywatervaporandlong-livedgreenhousegasessuchascarbondioxideandmethane. WhentheyabsorbtheenergyradiatingfromEarth’ssurface,microscopicwaterorgreenhousegasmoleculesturnintotinyheaters—likethebricksinafireplace,theyradiateheatevenafterthefiregoesout.Theyradiateinalldirections.TheenergythatradiatesbacktowardEarthheatsboththeloweratmosphereandthesurface,enhancingtheheatingtheygetfromdirectsunlight. Thisabsorptionandradiationofheatbytheatmosphere—thenaturalgreenhouseeffect—isbeneficialforlifeonEarth.Iftherewerenogreenhouseeffect,theEarth’saveragesurfacetemperaturewouldbeaverychilly-18°C(0°F)insteadofthecomfortable15°C(59°F)thatitistoday. SeeClimateandEarth’sEnergyBudgettoreadmoreabouthowsunlightfuelsEarth’sclimate. Theenhancedgreenhouseeffect Whathasscientistsconcernednowisthatoverthepast250years,humanshavebeenartificiallyraisingtheconcentrationofgreenhousegasesintheatmosphereatanever-increasingrate,mostlybyburningfossilfuels,butalsofromcuttingdowncarbon-absorbingforests.SincetheIndustrialRevolutionbeganinabout1750,carbondioxidelevelshaveincreasednearly38percentasof2009andmethanelevelshaveincreased148percent. Increasesinconcentrationsofcarbondioxide(top)andmethane(bottom)coincidedwiththestartoftheIndustrialRevolutioninabout1750.MeasurementsfromAntarcticicecores(greenlines)combinedwithdirectatmosphericmeasurements(bluelines)showtheincreaseofbothgasesovertime.(NASAgraphsbyRobertSimmon,basedondatafromtheNOAAPaleoclimatologyandEarthSystemResearchLaboratory.) Theatmospheretodaycontainsmoregreenhousegasmolecules,somoreoftheinfraredenergyemittedbythesurfaceendsupbeingabsorbedbytheatmosphere.Sincesomeoftheextraenergyfromawarmeratmosphereradiatesbackdowntothesurface,Earth’ssurfacetemperaturerises.Byincreasingtheconcentrationofgreenhousegases,wearemakingEarth’satmosphereamoreefficientgreenhouse. HowisToday’sWarmingDifferentfromthePast? Earthhasexperiencedclimatechangeinthepastwithouthelpfromhumanity.Weknowaboutpastclimatesbecauseofevidenceleftintreerings,layersoficeinglaciers,oceansediments,coralreefs,andlayersofsedimentaryrocks.Forexample,bubblesofairinglacialicetraptinysamplesofEarth’satmosphere,givingscientistsahistoryofgreenhousegasesthatstretchesbackmorethan800,000years.Thechemicalmake-upoftheiceprovidescluestotheaverageglobaltemperature. SeetheEarthObservatory’sseriesPaleoclimatologyfordetailsabouthowscientistsstudypastclimates. Glacialiceandairbubblestrappedinit(top)preservean800,000-yearrecordoftemperature&carbondioxide.Earthhascycledbetweeniceages(lowpoints,largenegativeanomalies)andwarminterglacials(peaks).(PhotographcourtesyNationalSnow&IceDataCenter.NASAgraphbyRobertSimmon,basedondatafromJouzeletal.,2007.) Usingthisancientevidence,scientistshavebuiltarecordofEarth’spastclimates,or“paleoclimates.”Thepaleoclimaterecordcombinedwithglobalmodelsshowspasticeagesaswellasperiodsevenwarmerthantoday.Butthepaleoclimaterecordalsorevealsthatthecurrentclimaticwarmingisoccurringmuchmorerapidlythanpastwarmingevents. AstheEarthmovedoutoficeagesoverthepastmillionyears,theglobaltemperatureroseatotalof4to7degreesCelsiusoverabout5,000years.Inthepastcenturyalone,thetemperaturehasclimbed0.7degreesCelsius,roughlytentimesfasterthantheaveragerateofice-age-recoverywarming. Temperaturehistoriesfrompaleoclimatedata(greenline)comparedtothehistorybasedonmoderninstruments(blueline)suggestthatglobaltemperatureiswarmernowthanithasbeeninthepast1,000years,andpossiblylonger.(GraphadaptedfromMannetal.,2008.) ModelspredictthatEarthwillwarmbetween2and6degreesCelsiusinthenextcentury.Whenglobalwarminghashappenedatvarioustimesinthepasttwomillionyears,ithastakentheplanetabout5,000yearstowarm5degrees.Thepredictedrateofwarmingforthenextcenturyisatleast20timesfaster.Thisrateofchangeisextremelyunusual. IsCurrentWarmingNatural? InEarth’shistorybeforetheIndustrialRevolution,Earth’sclimatechangedduetonaturalcausesnotrelatedtohumanactivity.Mostoften,globalclimatehaschangedbecauseofvariationsinsunlight.TinywobblesinEarth’sorbitalteredwhenandwheresunlightfallsonEarth’ssurface.VariationsintheSunitselfhavealternatelyincreasedanddecreasedtheamountofsolarenergyreachingEarth.Volcaniceruptionshavegeneratedparticlesthatreflectsunlight,brighteningtheplanetandcoolingtheclimate.Volcanicactivityhasalso,inthedeeppast,increasedgreenhousegasesovermillionsofyears,contributingtoepisodesofglobalwarming. AbiographicalsketchofMilutinMilankovitchdescribeshowchangesinEarth’sorbitaffectsitsclimate. Thesenaturalcausesarestillinplaytoday,buttheirinfluenceistoosmallortheyoccurtooslowlytoexplaintherapidwarmingseeninrecentdecades.Weknowthisbecausescientistscloselymonitorthenaturalandhumanactivitiesthatinfluenceclimatewithafleetofsatellitesandsurfaceinstruments. Remotemeteorologicalstations(left)andorbitingsatellites(right)helpscientistsmonitorthecausesandeffectsofglobalwarming.[ImagescourtesyNOAANetworkfortheDetectionofAtmosphericCompositionChange(left)andEnvironmentalVisualizationLaboratory(right).] NASAsatellitesrecordahostofvitalsignsincludingatmosphericaerosols(particlesfrombothnaturalsourcesandhumanactivities,suchasfactories,fires,deserts,anderuptingvolcanoes),atmosphericgases(includinggreenhousegases),energyradiatedfromEarth’ssurfaceandtheSun,oceansurfacetemperaturechanges,globalsealevel,theextentoficesheets,glaciersandseaice,plantgrowth,rainfall,cloudstructure,andmore. Ontheground,manyagenciesandnationssupportnetworksofweatherandclimate-monitoringstationsthatmaintaintemperature,rainfall,andsnowdepthrecords,andbuoysthatmeasuresurfacewateranddeepoceantemperatures.Takentogether,thesemeasurementsprovideanever-improvingrecordofbothnaturaleventsandhumanactivityforthepast150years. Scientistsintegratethesemeasurementsintoclimatemodelstorecreatetemperaturesrecordedoverthepast150years.Climatemodelsimulationsthatconsideronlynaturalsolarvariabilityandvolcanicaerosolssince1750—omittingobservedincreasesingreenhousegases—areabletofittheobservationsofglobaltemperaturesonlyupuntilabout1950.Afterthatpoint,thedecadaltrendinglobalsurfacewarmingcannotbeexplainedwithoutincludingthecontributionofthegreenhousegasesaddedbyhumans. Thoughpeoplehavehadthelargestimpactonourclimatesince1950,naturalchangestoEarth’sclimatehavealsooccurredinrecenttimes.Forexample,twomajorvolcaniceruptions,ElChichonin1982andPinatuboin1991,pumpedsulfurdioxidegashighintotheatmosphere.Thegaswasconvertedintotinyparticlesthatlingeredformorethanayear,reflectingsunlightandshadingEarth’ssurface.Temperaturesacrosstheglobedippedfortwotothreeyears. AlthoughEarth’stemperaturefluctuatesnaturally,humaninfluenceonclimatehaseclipsedthemagnitudeofnaturaltemperaturechangesoverthepast120years.Naturalinfluencesontemperature—ElNiño,solarvariability,andvolcanicaerosols—havevariedapproximatelyplusandminus0.2°C(0.4°F),(averagingtoaboutzero),whilehumaninfluenceshavecontributedroughly0.8°C(1°F)ofwarmingsince1889.(GraphsadaptedfromLeanetal.,2008.) Althoughvolcanoesareactivearoundtheworld,andcontinuetoemitcarbondioxideastheydidinthepast,theamountofcarbondioxidetheyreleaseisextremelysmallcomparedtohumanemissions.Onaverage,volcanoesemitbetween130and230milliontonnesofcarbondioxideperyear.Byburningfossilfuels,peoplereleaseinexcessof100timesmore,about26billiontonnesofcarbondioxide,intotheatmosphereeveryyear(asof2005).Asaresult,humanactivityovershadowsanycontributionvolcanoesmayhavemadetorecentglobalwarming. ChangesinthebrightnessoftheSuncaninfluencetheclimatefromdecadetodecade,butanincreaseinsolaroutputfallsshortasanexplanationforrecentwarming.NASAsatelliteshavebeenmeasuringtheSun’soutputsince1978.ThetotalenergytheSunradiatesvariesoveran11-yearcycle.Duringsolarmaxima,solarenergyisapproximately0.1percenthigheronaveragethanitisduringsolarminima. Thetransparenthaloknownasthesolarcoronachangesbetweensolarmaximum(left)andsolarminimum(right).(NASAExtremeUltravioletTelescopeimagesfromtheSOHODataArchive.) Eachcycleexhibitssubtledifferencesinintensityandduration.Asofearly2010,thesolarbrightnesssince2005hasbeenslightlylower,nothigher,thanitwasduringtheprevious11-yearminimuminsolaractivity,whichoccurredinthelate1990s.ThisimpliesthattheSun’simpactbetween2005and2010mighthavebeentoslightlydecreasethewarmingthatgreenhouseemissionsalonewouldhavecaused. Satellitemeasurementsofdaily(lightline)andmonthlyaverage(darkline)totalsolarirradiancesince1979havenotdetectedaclearlong-termtrend.(NASAgraphbyRobertSimmon,basedondatafromtheACRIMScienceTeam.) Scientiststheorizethattheremaybeamulti-decadaltrendinsolaroutput,thoughifoneexists,ithasnotbeenobservedasyet.EveniftheSunweregettingbrighter,however,thepatternofwarmingobservedonEarthsince1950doesnotmatchthetypeofwarmingtheSunalonewouldcause.WhentheSun’senergyisatitspeak(solarmaxima),temperaturesinboththeloweratmosphere(troposphere)andtheupperatmosphere(stratosphere)becomewarmer.Instead,observationsshowthepatternexpectedfromgreenhousegaseffects:Earth’ssurfaceandtropospherehavewarmed,butthestratospherehascooled. Satellitemeasurementsshowwarminginthetroposphere(loweratmosphere,greenline)butcoolinginthestratosphere(upperatmosphere,redline).Thisverticalpatternisconsistentwithglobalwarmingduetoincreasinggreenhousegases,butinconsistentwithwarmingfromnaturalcauses.(GraphbyRobertSimmon,basedondatafromRemoteSensingSystems,sponsoredbytheNOAAClimateandGlobalChangeProgram.) Thestratospheregetswarmerduringsolarmaximabecausetheozonelayerabsorbsultravioletlight;moreultravioletlightduringsolarmaximameanswarmertemperatures.Ozonedepletionexplainsthebiggestpartofthecoolingofthestratosphereoverrecentdecades,butitcan’taccountforallofit.Increasedconcentrationsofcarbondioxideinthetroposphereandstratospheretogethercontributetocoolinginthestratosphere. HowMuchMoreWillEarthWarm? Tofurtherexplorethecausesandeffectsofglobalwarmingandtopredictfuturewarming,scientistsbuildclimatemodels—computersimulationsoftheclimatesystem.Climatemodelsaredesignedtosimulatetheresponsesandinteractionsoftheoceansandatmosphere,andtoaccountforchangestothelandsurface,bothnaturalandhuman-induced.Theycomplywithfundamentallawsofphysics—conservationofenergy,mass,andmomentum—andaccountfordozensoffactorsthatinfluenceEarth’sclimate. Thoughthemodelsarecomplicated,rigoroustestswithreal-worlddatahonethemintopowerfultoolsthatallowscientiststoexploreourunderstandingofclimateinwaysnototherwisepossible.Byexperimentingwiththemodels—removinggreenhousegasesemittedbytheburningoffossilfuelsorchangingtheintensityoftheSuntoseehoweachinfluencestheclimate—scientistsusethemodelstobetterunderstandEarth’scurrentclimateandtopredictfutureclimate. Themodelspredictthatastheworldconsumesevermorefossilfuel,greenhousegasconcentrationswillcontinuetorise,andEarth’saveragesurfacetemperaturewillrisewiththem.Basedonarangeofplausibleemissionscenarios,averagesurfacetemperaturescouldrisebetween2°Cand6°Cbytheendofthe21stcentury. ModelsimulationsbytheIntergovernmentalPanelonClimateChangeestimatethatEarthwillwarmbetweentwoandsixdegreesCelsiusoverthenextcentury,dependingonhowfastcarbondioxideemissionsgrow.Scenariosthatassumethatpeoplewillburnmoreandmorefossilfuelprovidetheestimatesinthetopendofthetemperaturerange,whilescenariosthatassumethatgreenhousegasemissionswillgrowslowlygivelowertemperaturepredictions.Theorangelineprovidesanestimateofglobaltemperaturesifgreenhousegasesstayedatyear2000levels.(©2007IPCCWG1AR-4.) ClimateFeedbacks Greenhousegasesareonlypartofthestorywhenitcomestoglobalwarming.Changestoonepartoftheclimatesystemcancauseadditionalchangestothewaytheplanetabsorbsorreflectsenergy.Thesesecondarychangesarecalledclimatefeedbacks,andtheycouldmorethandoubletheamountofwarmingcausedbycarbondioxidealone.Theprimaryfeedbacksareduetosnowandice,watervapor,clouds,andthecarboncycle. Snowandice PerhapsthemostwellknownfeedbackcomesfrommeltingsnowandiceintheNorthernHemisphere.WarmingtemperaturesarealreadymeltingagrowingpercentageofArcticseaice,exposingdarkoceanwaterduringtheperpetualsunlightofsummer.Snowcoveronlandisalsodwindlinginmanyareas.Intheabsenceofsnowandice,theseareasgofromhavingbright,sunlight-reflectingsurfacesthatcooltheplanettohavingdark,sunlight-absorbingsurfacesthatbringmoreenergyintotheEarthsystemandcausemorewarming. Canada’sAthabascaGlacierhasbeenshrinkingbyabout15metersperyear.Inthepast125years,theglacierhaslosthalfitsvolumeandhasretreatedmorethan1.5kilometers.Asglaciersretreat,seaicedisappears,andsnowmeltsearlierinthespring,theEarthabsorbsmoresunlightthanitwouldifthereflectivesnowandiceremained.(Photograph©2005HughSaxby.) WaterVapor Thelargestfeedbackiswatervapor.Watervaporisastronggreenhousegas.Infact,becauseofitsabundanceintheatmosphere,watervaporcausesabouttwo-thirdsofgreenhousewarming,akeyfactorinkeepingtemperaturesinthehabitablerangeonEarth.Butastemperatureswarm,morewatervaporevaporatesfromthesurfaceintotheatmosphere,whereitcancausetemperaturestoclimbfurther. Thequestionthatscientistsaskis,howmuchwatervaporwillbeintheatmosphereinawarmingworld?Theatmospherecurrentlyhasanaverageequilibriumorbalancebetweenwatervaporconcentrationandtemperature.Astemperatureswarm,theatmospherebecomescapableofcontainingmorewatervapor,andsowatervaporconcentrationsgouptoregainequilibrium.Willthattrendholdastemperaturescontinuetowarm? Theamountofwatervaporthatenterstheatmosphereultimatelydetermineshowmuchadditionalwarmingwilloccurduetothewatervaporfeedback.Theatmosphererespondsquicklytothewatervaporfeedback.Sofar,mostoftheatmospherehasmaintainedanearconstantbalancebetweentemperatureandwatervaporconcentrationastemperatureshavegoneupinrecentdecades.Ifthistrendcontinues,andmanymodelssaythatitwill,watervaporhasthecapacitytodoublethewarmingcausedbycarbondioxidealone. Clouds Closelyrelatedtothewatervaporfeedbackisthecloudfeedback.Cloudscausecoolingbyreflectingsolarenergy,buttheyalsocausewarmingbyabsorbinginfraredenergy(likegreenhousegases)fromthesurfacewhentheyareoverareasthatarewarmerthantheyare.Inourcurrentclimate,cloudshaveacoolingeffectoverall,butthatcouldchangeinawarmerenvironment. Cloudscanbothcooltheplanet(byreflectingvisiblelightfromthesun)andwarmtheplanet(byabsorbingheatradiationemittedbythesurface).Onbalance,cloudsslightlycooltheEarth.(NASAAstronautPhotographSTS31-E-9552courtesyJohnsonspaceCenterEarthObservationsLab.) Ifcloudsbecomebrighter,orthegeographicalextentofbrightcloudsexpands,theywilltendtocoolEarth’ssurface.Cloudscanbecomebrighterifmoremoistureconvergesinaparticularregionorifmorefineparticles(aerosols)entertheair.Iffewerbrightcloudsform,itwillcontributetowarmingfromthecloudfeedback. SeeShip TracksSouthofAlaskatolearnhowaerosolscanmakecloudsbrighter. Clouds,likegreenhousegases,alsoabsorbandre-emitinfraredenergy.Low,warmcloudsemitmoreenergythanhigh,coldclouds.However,inmanypartsoftheworld,energyemittedbylowcloudscanbeabsorbedbytheabundantwatervaporabovethem.Further,lowcloudsoftenhavenearlythesametemperaturesastheEarth’ssurface,andsoemitsimilaramountsofinfraredenergy.Inaworldwithoutlowclouds,theamountofemittedinfraredenergyescapingtospacewouldnotbetoodifferentfromaworldwithlowclouds. Cloudsemitthermalinfrared(heat)radiationinproportiontotheirtemperature,whichisrelatedtoaltitude.ThisimageshowstheWesternHemisphereinthethermalinfrared.Warmoceanandlandsurfaceareasarewhiteandlightgray;cool,low-levelcloudsaremediumgray;andcold,high-altitudecloudsaredarkgrayandblack.(NASAimagecourtesyGOESProjectScience.) Highcoldclouds,however,forminapartoftheatmospherewhereenergy-absorbingwatervaporisscarce.Thesecloudstrap(absorb)energycomingfromtheloweratmosphere,andemitlittleenergytospacebecauseoftheirfrigidtemperatures.Inaworldwithhighclouds,asignificantamountofenergythatwouldotherwiseescapetospaceiscapturedintheatmosphere.Asaresult,globaltemperaturesarehigherthaninaworldwithouthighclouds. Ifwarmertemperaturesresultinagreateramountofhighclouds,thenlessinfraredenergywillbeemittedtospace.Inotherwords,morehighcloudswouldenhancethegreenhouseeffect,reducingtheEarth’scapabilitytocoolandcausingtemperaturestowarm. SeeCloudsandRadiationforamorecompletedescription. Scientistsaren’tentirelysurewhereandtowhatdegreecloudswillendupamplifyingormoderatingwarming,butmostclimatemodelspredictaslightoverallpositivefeedbackoramplificationofwarmingduetoareductioninlowcloudcover.Arecentobservationalstudyfoundthatfewerlow,densecloudsformedoveraregioninthePacificOceanwhentemperatureswarmed,suggestingapositivecloudfeedbackinthisregionasthemodelspredicted.Suchdirectobservationalevidenceislimited,however,andcloudsremainthebiggestsourceofuncertainty--apartfromhumanchoicestocontrolgreenhousegases—inpredictinghowmuchtheclimatewillchange. TheCarbonCycle IncreasedatmosphericcarbondioxideconcentrationsandwarmingtemperaturesarecausingchangesintheEarth’snaturalcarboncyclethatalsocanfeedbackonatmosphericcarbondioxideconcentration.Fornow,primarilyoceanwater,andtosomeextentecosystemsonland,aretakingupabouthalfofourfossilfuelandbiomassburningemissions.Thisbehaviorslowsglobalwarmingbydecreasingtherateofatmosphericcarbondioxideincrease,butthattrendmaynotcontinue.Warmeroceanwaterswillholdlessdissolvedcarbon,leavingmoreintheatmosphere. Abouthalfthecarbondioxideemittedintotheairfromburningfossilfuelsdissolvesintheocean.Thismapshowsthetotalamountofhuman-madecarbondioxideinoceanwaterfromthesurfacetotheseafloor.Blueareashavelowamounts,whileyellowregionsarerichinanthropogeniccarbondioxide.Highamountsoccurwherecurrentscarrythecarbon-dioxide-richsurfacewaterintotheoceandepths.(MapadaptedfromSabineetal.,2004.) SeeTheOcean’sCarbonBalanceontheEarthObservatory. Onland,changesinthecarboncyclearemorecomplicated.Underawarmerclimate,soils,especiallythawingArctictundra,couldreleasetrappedcarbondioxideormethanetotheatmosphere.Increasedfirefrequencyandinsectinfestationsalsoreleasemorecarbonastreesburnordieanddecay. Ontheotherhand,extracarbondioxidecanstimulateplantgrowthinsomeecosystems,allowingtheseplantstotakeadditionalcarbonoutoftheatmosphere.However,thiseffectmaybereducedwhenplantgrowthislimitedbywater,nitrogen,andtemperature.Thiseffectmayalsodiminishascarbondioxideincreasestolevelsthatbecomesaturatingforphotosynthesis.Becauseofthesecomplications,itisnotclearhowmuchadditionalcarbondioxideplantscantakeoutoftheatmosphereandhowlongtheycouldcontinuetodoso. Theimpactofclimatechangeonthelandcarboncycleisextremelycomplex,butonbalance,landcarbonsinkswillbecomelessefficientasplantsreachsaturation,wheretheycannolongertakeupadditionalcarbondioxide,andotherlimitationsongrowthoccur,andaslandstartstoaddmorecarbontotheatmospherefromwarmingsoil,fires,andinsectinfestations.Thiswillresultinafasterincreaseinatmosphericcarbondioxideandmorerapidglobalwarming.Insomeclimatemodels,carboncyclefeedbacksfrombothlandandoceanaddmorethanadegreeCelsiustoglobaltemperaturesby2100. EmissionScenarios Scientistspredicttherangeoflikelytemperatureincreasebyrunningmanypossiblefuturescenariosthroughclimatemodels.Althoughsomeoftheuncertaintyinclimateforecastscomesfromimperfectknowledgeofclimatefeedbacks,themostsignificantsourceofuncertaintyinthesepredictionsisthatscientistsdon’tknowwhatchoicespeoplewillmaketocontrolgreenhousegasemissions. Thehigherestimatesaremadeontheassumptionthattheentireworldwillcontinueusingmoreandmorefossilfuelpercapita,ascenarioscientistscall“business-as-usual.”Moremodestestimatescomefromscenariosinwhichenvironmentallyfriendlytechnologiessuchasfuelcells,solarpanels,andwindenergyreplacemuchoftoday’sfossilfuelcombustion. IttakesdecadestocenturiesforEarthtofullyreacttoincreasesingreenhousegases.Carbondioxide,amongothergreenhousegases,willremainintheatmospherelongafteremissionsarereduced,contributingtocontinuingwarming.Inaddition,asEarthhaswarmed,muchoftheexcessenergyhasgoneintoheatingtheupperlayersoftheocean.Likeahotwaterbottleonacoldnight,theheatedoceanwillcontinuewarmingtheloweratmospherewellaftergreenhousegaseshavestoppedincreasing. Theseconsiderationsmeanthatpeoplewon’timmediatelyseetheimpactofreducedgreenhousegasemissions.Evenifgreenhousegasconcentrationsstabilizedtoday,theplanetwouldcontinuetowarmbyabout0.6°Coverthenextcenturybecauseofgreenhousesgasesalreadyintheatmosphere. SeeEarth’sBigHeatBucket,CorrectingOceanCooling,andClimateQ&A:Ifweimmediatelystoppedemittinggreenhousegases,wouldglobalwarmingstop?tolearnmoreabouttheoceanheatandglobalwarming. HowWillGlobalWarmingChangeEarth? Theimpactofincreasedsurfacetemperaturesissignificantinitself.Butglobalwarmingwillhaveadditional,far-reachingeffectsontheplanet.Warmingmodifiesrainfallpatterns,amplifiescoastalerosion,lengthensthegrowingseasoninsomeregions,meltsicecapsandglaciers,andalterstherangesofsomeinfectiousdiseases.Someofthesechangesarealreadyoccurring. Globalwarmingwillshiftmajorclimatepatterns,possiblyprolongingandintensifyingthecurrentdroughtintheU.S.Southwest.Thewhiteringofbleachedrockontheonce-redcliffsthatholdLakePowellindicatethedropinwaterleveloverthepastdecade—theresultofrepeatedwinterswithlowsnowfall.(Photograph©2006Tigresblanco.) ChangingWeather Formostplaces,globalwarmingwillresultinmorefrequenthotdaysandfewercooldays,withthegreatestwarmingoccurringoverland.Longer,moreintenseheatwaveswillbecomemorecommon.Storms,floods,anddroughtswillgenerallybemoresevereasprecipitationpatternschange.Hurricanesmayincreaseinintensityduetowarmeroceansurfacetemperatures. Apartfromdrivingtemperaturesup,globalwarmingislikelytocausebigger,moredestructivestorms,leadingtoanoverallincreaseinprecipitation.Withsomeexceptions,thetropicswilllikelyreceivelessrain(orange)astheplanetwarms,whilethepolarregionswillreceivemoreprecipitation(green).Whiteareasindicatethatfewerthantwo-thirdsoftheclimatemodelsagreedonhowprecipitationwillchange.Stippledareasrevealwheremorethan90percentofthemodelsagreed.(©2007IPCCWG1AR-4.) Itisimpossibletopinanysingleunusualweathereventonglobalwarming,butemergingevidencesuggeststhatglobalwarmingisalreadyinfluencingtheweather.Heatwaves,droughts,andintenseraineventshaveincreasedinfrequencyduringthelast50years,andhuman-inducedglobalwarmingmorelikelythannotcontributedtothetrend. RisingSeaLevels Theweatherisn’ttheonlythingglobalwarmingwillimpact:risingsealevelswillerodecoastsandcausemorefrequentcoastalflooding.Someislandnationswilldisappear.Theproblemisseriousbecauseupto10percentoftheworld’spopulationlivesinvulnerableareaslessthan10meters(about30feet)abovesealevel. Between1870and2000,thesealevelincreasedby1.7millimetersperyearonaverage,foratotalsealevelriseof221millimeters(0.7feetor8.7inches).Andtherateofsealevelriseisaccelerating.Since1993,NASAsatelliteshaveshownthatsealevelsarerisingmorequickly,about3millimetersperyear,foratotalsealevelriseof48millimeters(0.16feetor1.89inches)between1993and2009. Sealevelscreptupabout20centimeters(7.9inches)duringthetwentiethcentury.Sealevelsarepredictedtogoupbetween18and59cm(7.1and23inches)overthenextcentury,thoughtheincreasecouldbegreaterificesheetsinGreenlandandAntarcticameltmorequicklythanpredicted.Highersealevelswillerodecoastlinesandcausemorefrequentflooding.(Graph©2007RobertRohde.) TheIntergovernmentalPanelonClimateChange(IPCC)estimatesthatsealevelswillrisebetween0.18and0.59meters(0.59to1.9feet)by2099aswarmingseawaterexpands,andmountainandpolarglaciersmelt.Thesesealevelchangepredictionsmaybeunderestimates,however,becausetheydonotaccountforanyincreasesintherateatwhichtheworld’smajoricesheetsaremelting.Astemperaturesrise,icewillmeltmorequickly.SatellitemeasurementsrevealthattheGreenlandandWestAntarcticicesheetsaresheddingabout125billiontonsoficeperyear—enoughtoraisesealevelsby0.35millimeters(0.01inches)peryear.Ifthemeltingaccelerates,theincreaseinsealevelcouldbesignificantlyhigher. ImpactingEcosystems Moreimportantly,perhaps,globalwarmingisalreadyputtingpressureonecosystems,theplantsandanimalsthatco-existinaparticularclimatezone,bothonlandandintheocean.Warmertemperatureshavealreadyshiftedthegrowingseasoninmanypartsoftheglobe.ThegrowingseasoninpartsoftheNorthernHemispherebecametwoweekslongerinthesecondhalfofthe20thcentury.Springiscomingearlierinbothhemispheres. Thischangeinthegrowingseasonaffectsthebroaderecosystem.Migratinganimalshavetostartseekingfoodsourcesearlier.Theshiftinseasonsmayalreadybecausingthelifecyclesofpollinators,likebees,tobeoutofsynchwithfloweringplantsandtrees.Thismismatchcanlimittheabilityofbothpollinatorsandplantstosurviveandreproduce,whichwouldreducefoodavailabilitythroughoutthefoodchain. SeeBuzzingAboutClimateChangetoreadmoreabouthowthelifecycleofbeesissynchedwithfloweringplants. Warmertemperaturesalsoextendthegrowingseason.Thismeansthatplantsneedmorewatertokeepgrowingthroughouttheseasonortheywilldryout,increasingtheriskoffailedcropsandwildfires.Oncethegrowingseasonends,shorter,milderwintersfailtokilldormantinsects,increasingtheriskoflarge,damaginginfestationsinsubsequentseasons. Insomeecosystems,maximumdailytemperaturesmightclimbbeyondthetoleranceofindigenousplantoranimal.Tosurvivetheextremetemperatures,bothmarineandland-basedplantsandanimalshavestartedtomigratetowardsthepoles.Thosespecies,andinsomecases,entireecosystems,thatcannotquicklymigrateoradapt,faceextinction.TheIPCCestimatesthat20-30percentofplantandanimalspecieswillbeatriskofextinctioniftemperaturesclimbmorethan1.5°to2.5°C. ImpactingPeople Thechangestoweatherandecosystemswillalsoaffectpeoplemoredirectly.Hardesthitwillbethoselivinginlow-lyingcoastalareas,andresidentsofpoorercountrieswhodonothavetheresourcestoadapttochangesintemperatureextremesandwaterresources.Astropicaltemperaturezonesexpand,thereachofsomeinfectiousdiseases,suchasmalaria,willchange.Moreintenserainsandhurricanesandrisingsealevelswillleadtomoreseverefloodingandpotentiallossofpropertyandlife. Oneinevitableconsequenceofglobalwarmingissea-levelrise.Inthefaceofhighersealevelsandmoreintensestorms,coastalcommunitiesfacegreaterriskofrapidbeacherosionfromdestructivestormsliketheintensenor’easterofApril2007thatcausedthisdamage.(Photograph©2007metimbers2000.) Hottersummersandmorefrequentfireswillleadtomorecasesofheatstrokeanddeaths,andtohigherlevelsofnear-surfaceozoneandsmoke,whichwouldcausemore‘codered’airqualitydays.Intensedroughtscanleadtoanincreaseinmalnutrition.Onalongertimescale,freshwaterwillbecomescarcer,especiallyduringthesummer,asmountainglaciersdisappear,particularlyinAsiaandpartsofNorthAmerica. Ontheflipside,therecouldbe“winners”inafewplaces.Forexample,aslongastheriseinglobalaveragetemperaturestaysbelow3degreesCelsius,somemodelspredictthatglobalfoodproductioncouldincreasebecauseofthelongergrowingseasonatmid-tohigh-latitudes,providedadequatewaterresourcesareavailable.Thesamesmallchangeintemperature,however,wouldreducefoodproductionatlowerlatitudes,wheremanycountriesalreadyfacefoodshortages.Onbalance,mostresearchsuggeststhatthenegativeimpactsofachangingclimatefaroutweighthepositiveimpacts.Currentcivilization—agricultureandpopulationdistribution—hasdevelopedbasedonthecurrentclimate.Themoretheclimatechanges,andthemorerapidlyitchanges,thegreaterthecostofadaptation. 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