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CLIMATECHANGEIN THEANTHROPOCENE
KIERAND.O’HARA
UniversityofKentucky,Lexington,KY,UnitedStates
Elsevier
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Notices
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ISBN:978-0-12-820308-8
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Preface ix
PARTI1
1Ourgloballychangingclimate3
1.1 Introduction3
1.2 Globaltemperature4
1.3 Landsurfacetemperature5
1.4 Seasurfacetemperature6
1.5 Globalsurfacetemperature7
1.6 Trendsinglobaltemperatures7
1.7 Trendsinglobalprecipitation8
1.8 Extremeweatherevents9
1.9 Changesinthecryosphere10
1.10 Changesinsealevel14
1.11 Changesinlandprocesses14 References16
2Physicaldriversofclimatechange19
2.1 Theglobalradiationbudget19
2.2 Thegreenhouseeffect19
2.3 Radiationforcing22
2.4 Globalwarmingpotential24
2.5 Greenhousegases25
2.6 Aerosols29
2.7 Climateresponse30
2.8 Feedbacks32
2.9 Albedofeedbacks34
2.10 Oceanchemistry,ecosystems,andcirculation35
2.11 Permafrost38 References38
3Evaluationofclimatemodelperformance41
3.1 Introduction41
3.2 Modeltypes42
3.3 Modelimprovements43
3.4 Modelevaluation44
3.5 Ensembleapproachtoevaluation45
3.6 Modelintercomparisons45
3.7 Results46
3.8 Theocean48
3.9 Carboncycle52
3.10 TheParisAccords53
3.11 Representativeclimatepathways53
3.12 Near-termclimateprojections54
3.13 Long-termprojections57 References60
4Paleoclimates63
4.1 Introduction63
4.2 Preindustrialexternalradiativeforcings65
4.3 HighCO2 worlds67
4.4 Pleistoceneglacial-interglacialdynamics69
4.5 TheCLIMAPProject71
4.6 Holoceneclimate73 References76
PARTII79
5Climateimpacts:USsectorsandregions81
5.1 Introduction81
5.2 Keysectors81
5.3 Regionalclimateimpacts95 References101 6Adaptation105
6.1 Introduction105 References121
7Mitigation123
7.1 Introduction123
7.2 GHGemissiontrends125
7.3 Emissiondrivers127
7.4 Carbonintensityofenergy129
7.5 Sectors130
7.6 Buildings136
7.7 Sharedsocioeconomicpathways–quantifyingthepaths137
7.8 ComparisonofSSP1andSSP3138
7.9 SSP5.Fossilfueldevelopment139 References140
PARTIII143
81.5o Cversus2.0o Cwarming145
8.1 Introduction145
8.2 1.5o Cand2.0o Cwarming147
8.3 Naturalsystems149
8.4 Humansystems153 References155
9Gettingtonetzeroby2050157
9.1 Introduction157
9.2 Thecurrentsituation(2021)158
9.3 Roadtonet-zeroemissions2050160
9.4 PopulationandGDP161
9.5 EnergyandCO2 prices161
9.6 CO2 emissions162
9.7 Totalenergysupply162
9.8 Economicsectors163
9.9 Conclusions164 References164
10Climateengineering167
10.1 Introduction167
10.2 Solarradiationmanagement168
10.3 Aerosolinjectionintothestratosphere169
10.4 Albedoenhancementoflow-levelmarineclouds174
10.5 Surfacealbedoenhancement176
10.6 Carbondioxideremoval176
10.7 Discussion183 References184 Index 187
Preface
TheGreekwordforhumankindisanthropos.ThetermAnthropocene wasproposedovertwodecadesagobyPaulCrutzen(atmosphericscientist andNobellaureate)andEugeneStoermer(biologist)toindicateanew geologicalepochinwhichtheintensityofhumanactivitystronglyimpacted EarthSystems,therebymarkingtheendofthecurrentHoloceneepoch, andjustifyinganewepoch.TheAnthropocenehasnotbeenformalized asanewgeologicepochandeventheboundarybetweenitandtheearlier Holocenehasnotyetbeenagreedupon,butthetermneverthelesshasgained widespreadcurrencyinboththescientificandpopularliterature.
ThisbookfollowstheoriginalsuggestionthattheIndustrialRevolutionmarksthebeginningoftheAnthropocene,markedbythetransition fromapastorallifestyletoanindustrialonelargelybasedincities(circa 1800AD).Thistimeframecorrespondstoanincreaseinburningofcoal andincreasedemissionsofgreenhousegases,especiallycarbondioxide.Based onicecores,thepreindustrialatmosphericconcentrationofcarbondioxide wasabout280ppm(comparedto ∼420ppmin2020)andiscommonly usedasareferencepointwhendiscussingclimatechange.By2017,the globalmeansurfacetemperaturehadincreasedby1.0°C(± 0.2)(1.8°F)since preindustrialtimes,andbothofthesereferenceframesareusedthroughout thebook.
TheconceptoftheAnthropoceneprovidesalensthroughwhichinsight intoman’seffectsontheenvironmentcanbeviewedinastructuredhistoricalfashion.Itisworthnotingthatthegeologicalcommunityonaltering thegeologicaltimescalemovesataglacialpace:in1878,CharlesLapworth, proposedtheOrdovicianPeriodtobeplacedbetweentheyoungerSilurian PeriodandtheolderCambrianPeriod;theproposalwasformallyaccepted in1976.
ThisbookistoalargeextentbasedontheIntergovernmentalPanelon ClimateChange(IPCC)reports.TheWorldMeteorologicalOrganization (WMO)togetherwiththeUnitedNationsprovidesthebasisforthese reportswhicharepublishedapproximatelyeveryfiveorsixyears.The UnitedStatesGovernment’sFourthNationalClimateAssessment(NCA4, 2017),withinputfrom13governmentagencies,isalsoheavilyreliedupon anditsconclusionsagreecloselywiththoseoftheIPCCreports.Thefifth IPCCreport(IPCC-AR5)waspublishedin2013–2014andthelatestreport
(IPCC-AR6)waspublishedinAugustof2021,havingbeendelayedbythe globalpandemicof2020.Reportvolumesaredividedintothreeworking groups(WG1,II,III),andeachchaptercommonlyhastwentyormore internationalexpertauthorsandeachvolumeisweighty,oftenatathousand pagesormorepervolume.Thepeerreviewprocessofthesereportshas severalroundsandisextensiveandlengthy.Thisbookislargelyasummary ofthesereports.
FollowingCaesar’sGaul,thebookisdividedintothreeparts.PartI addressesthephysicalsciencebasisofclimatechangeandislargelybased onIPCC-AR5(2013).Chapter1addressesthebasicobservationsindicating climatechange,followedbythedriversofthischangeinchapter2.Chapter 3examinescomputerclimatemodelsandchapterfourlooksatpaleoclimate reconstructions.PartIIexaminesclimateimpactsinvariousregionsofthe USA(basedonNCA4,2017),followedbyadaptationandmitigationscenarios.PartIIIlooksatthedifferencebetween1.5and2.0°Cwarmingrisks (basedonIPCCSpecialReport,2018)followedbytheroadmaptonet-zero emissionsby2050(basedontheInternationalEnergyAgency2021report). Thefinalchapterexaminesclimateengineering(orgeoengineering),which iswidelyregardedasalastresortoption,andthischapterisbasedonthe currentscientificliterature.
AlthoughAnthroposappliestoallhumanity,itisclearthat,basedon geographyandsocioeconomicstatus,theimpactsofclimatechangeare relatedtosocialinequitiesandtheimpactsarenotandwillnotbedistributed evenly–thedevelopingcountriesandthepoorwillbemostaffected.The ParisAgreementof2015recognizedthisfactbutwhetherthedeveloped countrieswillfulfilltheirmonetarypromisestodevelopingnationsremains indoubt.TheUnitedStatesre-enteredtheParisagreementin2020.The UnitedNationsclimatesummitofNovember2021(COP26),heldin Glascow,agreedtoreducemethaneemissions(by30%)by2030andalso toeliminatedeforestationbythesamedate.Noagreementtoacoalbanwas reached,asChina,IndiaandRussiadidnotsignon.
1.Ourgloballychangingclimate
Ourgloballychangingclimate
1.1Introduction
TheEarthsciencesstudyamultitudeofprocessesthatshapethespatialand temporalcharacterofourenvironment(Fig.1.1).Moderndayobservations, archivesofpastclimates,climatemodelprojections,andstatisticaltools, canallbeusedtoyieldsignificantinsightintoclimatechange,resulting inconclusionsthathavevariablelevelsofconfidencefromhightolow (see Cubaschetal.,2013).TheEarth’sclimatesystemispoweredbysolar radiationabouthalfofwhichisinthevisibleandultravioletrangeofthe electromagneticspectrum.Thesunprovidesitsenergyprimarilytothe tropics,whichisredistributedtohigherlatitudesbyatmosphereandocean transportprocesses.TherelativelycooltemperatureoftheEarth’ssurface meansitreradiatesenergyinthelongwavelengthpartofthespectrum (infrared)andmuchofthisradiationisabsorbedbygasesintheatmosphere suchaswatervapor,CO2 ,CH4 ,andN2 Oaswellashalocarbons–thisisthe greenhouseeffect.GiventheEarthhashadanearconstanttemperatureover thepastfewcenturiestheincomingsolarenergymustnearlybalancethe outgoingenergytospace,andcloudsplayanimportantroleinthisenergy balance.About30%oftheshortwaveradiationisreflectedbacktospace byclouds,causingcooling.Ontheotherhand,someclouds,depending onelevation,traplongwaveradiation,heatingthesurface,andthelower atmosphere.
Climateisaverageweatheroveraprolongedperiod,commonlytaken asthreedecadesorlonger,andclimatechangereferstoachangein thestateoftheclimate(basedonstatisticaltests),suchastemperature, precipitation,ordrought.Forexample,duringthelastglaciation,stadial, andinterstadialperiodswerecharacterizedbycold/dryclimates(stadials) alternatingwithwarm/wetclimates(interstadials),onamillennialtimescale (O’Hara,2014). Fig.1.1 summarizesseveralkeyelementsoftheclimate system;elementsinteractwithoneanotherincomplexwaysinvolvingboth positiveandnegativefeedbacks(seeChapter2).Thischaptersummarizes severalindicatorsthatourplanetiscurrentlywarming.Thewarmingdates backtothebeginningoftheanthropocene,wherethemeantemperature overtheperiod1850–1900istakenasthereferenceperiod.
ClimateChangeintheAnthropocene. Copyright©2022ElsevierInc. DOI: https://doi.org/10.1016/B978-0-12-820308-8.00005-2 Allrightsreserved. 3
1.2Globaltemperature
ThefourthIPCCassessmentreport(LeTreutetal.,2007)providesahistory ofearlyattemptsatconstructingaglobaltemperaturetimeseriesforthe nineteenthandtwentiethcenturies.Theglobalaveragetemperatureisone ofthemostimportantvariablesinthestudyofclimatechangeasitcorrelates withothervariablessuchasicemelting,sealevelrise,precipitation,and becauseithasthemostrobustrecordovertime.Theconceptofaglobal averagetemperatureissimpleinprinciplebutitscalculationisfarfromtrivial (Voseetal.,2012).Althoughthethermometerwasinventedasearlyasthe 1600sitwasnotuntilthe1900sthatdifferentglobalestimatesofaverageland temperaturebegantoagreewithoneother.
TheGermanclimatologistW.Köppen(1846–1940)wasoneofthefirst torecognizethemajorproblemsinvolvedintheglobalaveragetemperature estimatesnamely,accesstodatainusableform,qualitycontroltoremove erroneousdata,standardizationtoensurefidelityofdata,andareaaveraging inareasofsubstantialdatagaps.Köppenaveragedannualobservationsfrom 100stationsintolatitudebeltstoproduceanearglobaltimeseriesasearlyas thelatenineteenthcentury.TheInternationalMeteorologicalOrganization (IMO)formedin1873,anditssuccessortheWorldMeteorologicalOrganization(WMO),stillworktopromoteandstandardizeobservations.The
Figure1.1 Summaryofmajordriversofclimatechange. (Sourcewithpermission: Cubaschetal.,2007.)
WorldWeatherRecords(WWR),formedbytheIMOin1923,provided monthlydatafortemperature(andalsopressureandprecipitation)estimates fromhundredsofstationsintheearlytwentiethcenturywithdatabeginning intheearly1800s. Callendar(1938) usedthesedatatoprovideoneofthefirst modernland-basedglobalaveragetemperaturetimeseries.Asmentionedin thePreface,theWorldMeteorologicalOrganization(WMO)togetherwith theUnitedNationstodayprovidesthebasisfortheIPCCscientificreports onclimatechangeandonwhichthisbookislargelybased.
Today,threeresearchgroupsstudyglobalseaandland-basedtemperatures puttogetherfrompiecemealrecords(Voseetal.,2012):theNationalOceanic andAtmosphericAdministration’sNationalClimaticDataCenter(NOAANCDC),theNationalAeronauticandSpaceAdministration’sGoddard InstituteforSpaceStudies(NASA-GISS)andtheMetOfficeHadley CenterandClimaticResearchUnit(HadCRUT).Eachgroupusessomewhatdifferentinputdatasetsandtheyalsoanalyzethedatawithdifferent methodologies.Forexample,GISSmakesextensiveuseofsatellitedata, whereasNCDCusesitinalimitedcapacityandHasCRUTmakesnouseof satellitedata.Similarly,GISSandNCDCprovidetemperatureestimatesin unsampledareas(usinginterpolation),whereasHasCRUTdoesnot.Despite thesedifferencesallthreegroupsreachasimilarconclusion:since1900the globalaveragesurfacetemperatureincreasehasbeenabout0.8 ± 0.2°C.The fifthIntergovernmentalPanelonClimateChange(IPCC-AR5,2013)and theUSgovernment’sFourthNationalClimateAssessment(NCA4,2017) reportsbothagreewiththisconclusionwithahighlevelofconfidence. Thesereportsalsoprojectthatbytheendofthiscentury(2100)theglobal averagetemperatureincreasewillbebetween2.0°Cand5.0°C,depending ongreenhousegasemissionsandpopulationandeconomicgrowthamong othervariables(seeChapter6).
1.3Landsurfacetemperature
ThedatasetusedbyNCDCconsistsofhistoricalmonthlydatagoingback acenturyfromover7000surfaceweatherstations.Thedatasetisreviewed forqualityassuranceandspatialinconsistencies.Landsurfacetemperatures requireadjustmentsduetoavarietyofcausessuchasstationrelocation, changeininstrumentation(e.g.,automation),urbanization(thecityheat effect)andlanduse,andmicroclimatechanges.Suchchangestypically produceanabruptjumprelativetoitsneighborstations.Theseartifacts areindentifiedautomaticallybycomparingsurroundingstationspairwise. RenoNevada,forexample,requiredanadjustmentof2°Cafterthestation
wasmovedfromdowntowntotheairport(Thorne,2016).Thetransition toelectronicsensorsintheUSinthelatetwentiethcenturyrequiredan adjustmentofabout0.25°Cnationwide(Voseetal.,2012).Averagedover theglobe,however,theseadjustmentshaveonlyaminorimpactonthelongtermLSTrecord.
Thetemperatureseriesisalsostandardizedtoaccountforelevation, latitude,coastalproximity,andseason.Ameantemperatureiscalculatedfor eachstationrelativetothereferenceperiod(1961–1990)andthenthismean issubtractedfromeachtemperaturevalueatthatstation.Theresultingvalues arereferredtoasanomaliesandthisisthemostcommonwaytheresultsare presentedingraphicform.Thisstandardizationprocedurereducesmuchof thevariabilityintheoriginaldataset.
Theunevenspatialdistributionofstationsistakenintoaccountby averagingmeasurementsin5-degreelongitudeandlatitudegridboxes.A singleaveragetemperatureiscalculatedforeachboxonamonthlyand annualbasisandthishelpspreventhigh-densitymeasurementboxestohave undueinfluence.Todaylandcoverageisabout90%andareasoflowcoverage includeforests,desertsandthepoles.Satellitedataaffordsglobalcoveragebut thedatamustbecalibratedwithgroundmeasurements;inaddition,because aninfraredspectrometerisusedfortemperaturemeasurements,theskies mustbecloud-free.
1.4Seasurfacetemperature
Theseasurfacetemperaturedatasetisprimarilyfrommarinemeteorological observationsfrombuoysandshipsintegratedfromnumeroushistorical sources.Buoyscanbeeitherdriftingormoored;buoyobservationsare givenaboutsixtimestheweightfromshipsonaccountofthenoisein thelatterobservations(e.g.,mistakesinnavigation,instrumentcalibration, datatranscription).Shiptemperaturemeasurementsshowachangein practiceovertime.Inpre-WorldWarIItimeswoodenorcanvasbuckets (someinsulated,somenot)werehauledondeckformeasurement.These measurementsrequireadjustmentsforseveralvariables:typeofbucket, heightofdeck,etc.Evaporativecooling,especiallyinhighwinds,requires adjustmentsofabout0.2°C(Thorne,2016).Lateron,themeasurements weremadeattheengine’scoolwaterintake,orsensorswereplacedonthe ship’shull.Globally,asmallergridbox(comparedtotheLSTs)of2 × 2 degreesisused.Eachboxvalueisanaverageofmeasurementsoveramonth andthemeanvalueforareferencetimeperiod(197–1990)issubtracted fromeachtemperaturemeasurement,asinthecaseforLSTs.
1.5Globalsurfacetemperature
BeforemergingtheLSTandSSTanomaliestheyareprocessedseparately becausetherearefundamentaldifferencesbetweenthetwodatasets(Vose etal.,2012).First,thespatialcoverageovertheoceansissubstantiallyless thanthatoverland(Thorne,2016)andsecondly,thedensityofocean measurementsissubstantiallylowerthanlandmeasurements.Inaddition, thetimeandspacescalesoftemperaturevariabilityoverlandareshorter comparedtotheocean,duetothehigherspecificheatofwateranditsslower speedofadvection.Beforemergingthedatasets,lowfrequencyvariations thatoccuroverlongerperiodsandhighfrequencyvariationsthatoccur overshorterperiodsareidentifiedandsmoothed,thenbothcomponentsare addedtogether.TheLSTandSSTdatasetsaremergedaftertheSSTgrid boxes(2o x2o )areaveragedinto5o x5o boxes.Thereferencetimeperiod overtheocean(1971–2000)isconvertedtothesametimeperiodastheland measurements(1961–1990).Otheradjustmentsaredescribedinmoredetail in Voseetal.(2012).Theglobalyearlyandmonthlyaveragesaresimplythe averageofallboxeshavingavalueinthatyearandmonth.Theannualglobal averagetemperatureissimplythearithmeticmeanof12monthlyaverages.
1.6Trendsinglobaltemperatures
Fig.1.2 showstheNCA4annual(top)anddecadal(bottom)averageglobal temperaturesoverlandandoceanfortheperiod1880–2016,relativeto thereferenceperiod1901–1960(Wuebblesetal.,2017).Theglobalannual averagetemperaturehasincreasedby0.7°C(1.2°F)fortheperiod1986–2016.YeartoyearfluctuationsareduetonaturalvariationssuchasElNiños andLaNiñasandvolcaniceruptions.Onadecadalscale(bottomdiagram) thesefluctuationsaresmoothedoutandeverydecadesince1966-1975 hasbeenwarmerthanthepreviousdecade.Recentdecadesshowgreater warmingduetoacceleratinggreenhousegasemissions.Sixteenofthe17 warmestyearssincethelate1800soccurredintheperiodfrom2001to2016. Ingeneral,winteriswarmingfasterthansummerandnightsarewarming fasterthandays.
Fig.1.3 showstheglobalsurfacetemperature(°F)increaseforthe period1986–2015referencedtothe1901–1960timeframe.Notethe oceansshowlesswarmingcomparedtothecontinentsonaccountoftheir higherheatcapacity.Onthecontinentsthelargestincreasesareseenin Eurasia,northwestNorthAmerica,centralSouthAmerica,andnorthwest Africa.
Figure1.2 Annual(top)anddecadal(bottom)combinedoceanandlandtemperatures fortheperiod1880–2010. (Sourcewithpermission: Wuebblesetal.,2017.)
1.7Trendsinglobalprecipitation
TheClausius-Clapeyronrelationdescribesthewaterliquid-vaporequilibriumasafunctionofpressureandtemperature.Globalatmosphericwater vaporshouldincreasebyabout6%/°Cto7%/°Candsatellitedataoverthe oceansagreewiththisestimate(Santeretal.,2007);increasesinwatervapor shouldleadtoincreasedprecipitation.Globaltimeseriesofprecipitation
Figure1.3 Globalsurfacetemperaturefortheperiod1986–2015relativetothe1901–1960mean. (Source:NOAA.)
overthepastcenturyshowaslightrisebutarenotstatisticallysignificant becauseofthesparsityofdataintheearlyrecord(Wuebblesetal.,2017).The globaldistributionmapshowsincreasedprecipitationathigherlatitudesand lowerprecipitationatlowerlatitudesduetoHadleycellcirculation.Deficits inprecipitationarenotableinAfrica,theTibetanplateauandsouthern China,westernUSA,andeasternAustralia;asexpectedtheAmazonrain forestbasinshowshigherprecipitation.
1.8Extremeweatherevents
Thedistributionofextremeweathereventscanbeapproximatedbya Normaldistributionwhereextremeevents(hotorcold)arerareand correspondtothetailsofthedistribution(Fig.1.4).Inawarmingworld themeanofthedistributioncanbeexpectedtoshifttotherightgiving risetomoreextremehoteventsandalsofewercoldevents. Fig.1.5 shows decreasingnumberofcoldnightsanddays(toptwoinsets)andincreasing numberofwarmsnightsanddays(bottomtwoinsets)fortheperiod1950–2010relativetothereferencetimeperiod196–1990.Thepatternsaresimilar
Figure1.4 Climateextremesoccuronthetailsofanormaldistribution.Coldperiods becomelesscommonandhoteventsbecomemorecommonastheaverageclimate becomeswarmer. (Sourcewithpermission: Cubsachetal.,2013.)
tothosepredictedby Fig.1.4.Increasesinfrequencyofextremeprecipitation eventsareexpectedfromanincreaseinatmosphericwatervaporandannualmaximumdailyprecipitationeventshaveincreased8.5%overthepast 110yearsoverbothwetanddryregions(Wuebblesetal.,2017).Computer climatemodels(Chapter3)alsopredictincreasedextremeprecipitation events.
1.9Changesinthecryosphere
Thecryosphereincludescontinentalicesheets(GreenlandandAntarctica), seaice(e.g.,ArcticOcean),mountainglaciers,frozenground(permafrost), lakeandrivericeandsnow(Vaughanetal.,2013; Wuebblesetal.,2017). Thesedifferentcomponentsofthecryosphererespondtochanging conditionsondifferenttimescales:riverandlakeiceandsnowrespondona dailytimescale,glaciersonanannualbasis,mountainglaciersovercenturies, andlargeicesheetsonamillennialtimescale.Changesinland-basedpartsof thecryospherehaveamajorimpactonsealevelchangeandthearealextent ofice(see Box1.1).Bothonlandandsea-basedicehaveamajorinfluence onsunlightreflectivityoralbedo(e.g.,openwater ∼5%;ice ∼50–70%and snowcoveredice ∼90%),whichinturnaffectsclimatefeedbacks.Ona morelocalscale,meltingglaciersmayaffecttourismandimpactfreshwater resources.Forexample,earlierSpringmeltingofsnowintheSierraNevada mountainsresultsinincreasedrunoff,deprivingdownstreamaquifersof replenishmentandresultinginchanginghydroelectricgenerationschedules. Thecryosphereispartofacomplexclimatesystemandisoneofthebest barometersofcurrentclimatechange. Table1.1 showsthepercentareaof
Figure1.5 Globaldecreasingcoldnightsanddays(toptwoinsets)andincreasing warmsnightsanddays(bottomtwoinsets)overtheperiod1950–2010. (Sourcewith permission: Hartmannetal.,2013.)
variouscomponentsofthecryosphereonbothlandandsea.Thesealevel equivalentiftheseicecomponentsweretomeltisalsoshownanddoes notincludesealevelriseduetothermalexpansionofwarmingoceans. Thissealevelriseissimplyicevolumemeltdividedbytheareaoftheoceans
Table1.1 Landandoceancryospherecomponents.
Iceinocean Oceanarea(%) Volume(103 km3 )
Antarcticseaiceaustralsummer(spring) 0.8(5.2) 3.4(11.1) Arcticseaiceautumn(winter) 1.7(3.9) 13.0(16.5) Total 5.3–7.3
Sourcewithpermission: adapted IPCC-AR5(2013),table4.1.
BOX1.1Polaramplification
Polaramplificationoccurswhenthemagnitudeofzonalaveragedtemperature changeathighlatitudesexceedstheglobalaveragetemperature,inresponseto climateforcings(e.g.,orbitalforcingorgreenhousegases)ontimescalesgreater thantheannualcycle.Amplificationhasimportantimplicationsforpolaricesheet stabilityandhencesealevelandalsoforthecarboncycleinvolvedinpermafrost thawing.IthasbeenknownsinceMilankovitch’stimethatorbitalforcingwas moreimportantathighlatitudes(ImbrieandImbrie,1986).Today,orbitalforcing iscommonlycalculatedat50o to60o latitudeinthenorthernhemisphere(NH). IntheArctic,theseaice/oceanalbedofeedbackisalsoimportant.Retreatingseaicedecreasesthesurfacealbedocausingoceanwarmingandfurthermelting.In continentalArcticregionssnowcoverchangealsochangesthesurfacealbedo. Surfacevegetationchangesoccuronalongertimescale(decadestocenturies) andalsoaffectthealbedo.Onglacial-interglacialtimescales(thousandsofyears), theslowretreatoftheicealsoleadstoalbedochangeandisimportanttopolar amplificationintheNH.
IntheSouthernOcean,seasurfacetemperatureisamplifiedinresponseto changesinradiativeforcings,alsoduetotheseaice/oceanalbedofeedback.But astheSouthernOceanislessstratifiedcomparedtotheArctic,itcanabsorbmuch moreheat(astratifiedoceandoesnotallowasmuchdownwardheattransport). ThismeanstheSouthernOceantemperatureresponseisdampedincomparison totheArctic.
(363 × 106 km2 ).Note:anicemeltvolumeof363Gtwouldcausea1mm riseinglobalsealevel;1Gt(109 metrictons)oficeisequivalenttoavolume of1km3
SatellitedatafromGravityRecoveryandClimateExperiment (GRACE)haveprovidedgravimetriclandicemeasurementsindicatingmass lossfromtheglobalcryosphere(VelicognaandWahr,2006a, 2006b).These measurementsindicatemasslossesfromtheAntarcticandGreenlandice sheetsandmountainglaciersaroundtheworld.Theannuallyaveragedmass lossfrom37referenceglaciershasincreasedeveryyearsince1984.Over theperiod2003–2009,19referencemountainglaciersallshowmassloss withAlaska,Greenland,SouthernAndes,CanadianArctic,andtheAsian mountainsaccountingfor80%ofglobaliceloss(Vaughanetal.,2013). Arcticseaiceextentandthicknessandvolumehavealldecreasedsince 1979whensatellitedatafirstbegan(http://nsidc.org/arcticseaicenews/). Typically,ArcticseaiceisatmaximuminMarchandaminimumin Septemberaftersummermelting.Septemberseaiceextentdecreased by13.3percentperdecadebetween1979and2016.Climatemodels (Chapter3)projectanearlyice-freeArcticOceaninsummertimebymidcentury(2050).Theloweralbedoofanice-freeArcticwouldbesubstantial andisequivalenttoaradiativeforcingof0.3Wm 2 (Chapter2).Overthe pastdecademasslossfromtheGreenlandicesheethasaccelerated,losing 244 ± 6Gtperyearbetween2003and2013.Satellitedatafor2012–2013 showedalossof562Gt,twicetheannualaverage.Greenlandicesheetmass lossisbysurfacemeltingaswellasdischargefromthebaseofthesheet.
WestAntarcticaischaracterizedbylandicethattransitionstocoastal iceandseaicesheets.Airtemperaturesaretoocoldforsurfacemeltingin Antarcticaeveninthesummer,andrecenticelossfromWestAntarcticasea iceisattributedtowarmingofthesurroundingocean.Evidencesuggeststhat theAmundsenSeasectorisexpectedtoentirelydisintegratebythiscentury, correspondingtoasealevelriseof1.2meters(Wuebblesetal.,2017).Areas ofEastAntarcticseaiceshowgainsof1.2%to1.8%since1979;thesegains aremuchsmallerthanlossesseeninArcticice.
Terrestrialpermafrostshowsatemperatureincreaseof1°Cto2°Cfrom avarietyofnorthernregionsovertheperiodfromthe1970sto2010in boreholesatadepthof5mto20m;thesedepthsarebelowseasonal temperaturevariations.Thesouthernboundaryofpermafrosthasmovedup to80kmnorthinsomehighlatitudelocations(e.g.,Siberia).Thethawingof permafrostraisesconcernsofthereleaseofbothCO2 andCH4 bybacterial activityresultinginapositivefeedbackforwarming.Otherconcernsarethe
impactoninfrastructure(instabilityofroadsandbuildings).Forexample,a fuelstoragedepotinnorthernSiberia(Norilsk)collapsedinMayof2020 spilling17,500metrictonsofdieselfuelintothelocalriverthatflowstothe ArcticOcean.Thecollapsewasattributedtothawingpermafrost.
1.10Changesinsealevel
Statisticalanalysesoftidegaugedataindicatethatglobalmeansealevel (GMS)hasincreasedby20–23cmsince1880witharateofabout1.2cm to1.5cmperdecadefrom1901to1990.However,sincetheearly1990sboth tidegaugeandsatellitealtimetershaverecordedafasterrateofabout3cm perdecade,resultinginaboutan8cmriseinGMSsincetheearly1990s. Thisriseisattributedtotwocomponents,twothirdsofwhichisdueto meltingofland-basediceandonethirdtothermalexpansionoftheoceans (Gehrels,2016).
Futureprojectionsofsealevelrisedependonwhichrepresentative climatepathway(RCP)wefollow(Cubaschetal.,2013;seealsoTable3.1), whichinturndependsoffuturegreenhouseemissions,populationgrowth, andsocietaleconomicprogressamongothervariables(Chapter6).Model projectionsforthehighscenario(RCP8.5;similartowhatissometimes termed“businessasusual”)indicateasealevelriseof0.5–1.3mby2100. Thisscenarioinvolvesaglobaltemperatureincreaseofabout4°C.Thelow scenario(RCP2.6)indicatesasealevelriseof0.24–0.8mandthisscenario involvesatemperatureincreaseofabout2°Cbyendofthecentury.
1.11Changesinlandprocesses
Changesinlandcovercanhaveimportanteffectsonclimateandconversely changesinclimatecausechangesinlandcoverleadingtofeedbackmechanismsbothpositiveandnegative.Northernhemispheresnowcoverhas decreasedbyabout0.5millionkm2 intheSpringduetoearlierSpring melting,largelyduetoglobalwarmingsincethe1970s,resultinginreduced albedoofthelandsurface.Globally,land-usechangesincethe1750shasbeen typifiedbydeforestationreplacedbyintensivefarmingandurbanization, therebyincreasingalbedo,resultinginasmallcoolingeffect(seeTable2.3 foralbedovalues).Thisdeforestationhasreleasedabout190 ± 65GtCover thistimeperiod.Overthesametimeperiodanthropogenicemissionswere 600 ± 70GtC,sothatcumulativelandusechangeamountstoabout32%of totalemissions.Tropicaldeforestationbybiomassburningemitsabout0.1–1.7GtCperyear.Globaldeforestationemitsabout3GtCperyear,butthisis
Figure1.6 Summarydiagramofglobalclimatechangeproperties.Fromtoptobottom: landsurfacetemperaturerise(4datasets),seasurfacetemperaturerise(3datasets),sea levelrise(4datasets);Insets,lefttoright:Tropospheretemperaturerise(5datasets), oceanheatcontentrise(5datasets),humidityrise(4datasets),northernhemisphere snowcoverfall(1dataset),Arcticseaicefall(annualandSeptember),glaciermassloss (1dataset). (Sourcewithpermission: Wuebblesetal.,2017.)
