Bajrang Lal Kukana

ClimateChange Evidence&Causes Update2020 AnoverviewfromtheRoyalSocietyandthe USNationalAcademyofSciences Foreword CLIMATECHANGEISONEOFTHEDEFININGISSUESOFOURTIME.Itisnowmorecertain thanever,basedonmanylinesofevidence,thathumansarechangingEarth’sclimate.The atmosphere and oceans have warmed, which has been accompanied by sea level rise, a strong declineinArcticseaice,andotherclimate-relatedchanges.Theimpactsofclimatechangeon people and nature are increasingly apparent. Unprecedented flooding, heat waves, and wildfires have cost billions in damages. Habitats are undergoing rapid shifts in response to changing temperaturesandprecipitationpatterns. TheRoyalSocietyandtheUSNationalAcademyofSciences,withtheirsimilarmissionsto promote the use of science to benefit society and to inform critical policy debates, produced the originalClimateChange:EvidenceandCausesin2014.ItwaswrittenandreviewedbyaUK-US team of leading climate scientists. This new edition, prepared by the same author team, has been updated with the most recent climate data and scientific analyses, all of which reinforce our understandingofhuman-causedclimatechange. The evidence is clear. However, due to the nature of science, not every detail is ever totally settled orcertain.Norhaseverypertinentquestionyetbeenanswered.Scientificevidencecontinuesto be gathered around the world. Some things have become clearer and new insights have emerged. For example, the period of slower warming during the 2000s and early 2010s has ended with a dramaticjumptowarmertemperaturesbetween2014and2015.Antarcticseaiceextent,which hadbeenincreasing,begantodeclinein2014,reachingarecordlowin2017thathaspersisted. These and other recent observations have been woven into the discussions of the questions addressedinthisbooklet. Callsforactionaregettinglouder.The2020GlobalRisksPerceptionSurveyfromtheWorld EconomicForumrankedclimatechangeandrelatedenvironmentalissuesasthetopfiveglobal riskslikelytooccurwithinthenexttenyears.Yet,theinternationalcommunitystillhasfartogoin showing increased ambition on mitigation, adaptation, and other ways to tackle climate change. Scientificinformationisavitalcomponentforsocietytomakeinformeddecisionsabouthowto reduce the magnitude of climate change and how to adapt to its impacts. This booklet serves as a keyreferencedocumentfordecisionmakers,policymakers,educators,andothersseeking authoritative answers about the current state of climate-change science. We are grateful that six years ago, under the leadership of Dr. Ralph J. Cicerone, former President oftheNationalAcademyofSciences,andSirPaulNurse,formerPresidentoftheRoyalSociety, these two organizations partnered to produce a high-level overview of climate change science. As current Presidents of these organizations, we are pleased to offer an update to this key reference, supportedbythegenerosityoftheCiceroneFamily. MarciaMcNutt President,NationalAcademy ofSciences VenkiRamakrishnan President,RoyalSociety Forfurtherreading Formoredetaileddiscussionofthetopicsaddressedinthisdocument (includingreferencestotheunderlyingoriginalresearch),see: IntergovernmentalPanelonClimateChange(IPCC),2019:SpecialReportontheOceanandCryosphereinaChangingClimate [https://www.ipcc.ch/srocc] NationalAcademies of Sciences, Engineering, and Medicine (NASEM),2019:NegativeEmissionsTechnologiesandReliable Sequestration:AResearchAgenda [https://www.nap.edu/catalog/25259] RoyalSociety,2018:Greenhousegasremoval [https://raeng.org.uk/greenhousegasremoval] U.S.Global Change Research Program (USGCRP), 2018: Fourth NationalClimateAssessmentVolumeII:Impacts,Risks,and AdaptationintheUnitedStates[https://nca2018.globalchange.gov] IPCC,2018:GlobalWarmingof1.5°C[https://www.ipcc.ch/sr15] USGCRP,2017:FourthNationalClimateAssessmentVolumeI:Climate ScienceSpecialReports[https://science2017.globalchange.gov] NASEM,2016:AttributionofExtremeWeatherEventsinthe Context of Climate Change [https://www.nap.edu/catalog/21852] IPCC,2013:FifthAssessmentReport(AR5)WorkingGroup1. ClimateChange2013:ThePhysicalScienceBasis [https://www.ipcc.ch/report/ar5/wg1] NRC,2013:AbruptImpactsofClimateChange:Anticipating Surprises[https://www.nap.edu/catalog/18373] NRC, 2011: Climate Stabilization Targets: Emissions, Concentrations, andImpactsOverDecadestoMillennia [https://www.nap.edu/catalog/12877] RoyalSociety2010:ClimateChange:ASummaryoftheScience [https://royalsociety.org/topics-policy/publications/2010/ climate-change-summary-science] NRC, 2010: America’s Climate Choices:Advancing the Science ofClimateChange[https://www.nap.edu/catalog/12782] Much of the original data underlying the scientific findings discussedhereareavailableat: https://data.ucar.edu/ https://climatedataguide.ucar.edu https://iridl.ldeo.columbia.edu https://ess-dive.lbl.gov/ https://www.ncdc.noaa.gov/ https://www.esrl.noaa.gov/gmd/ccgg/trends/ http://scrippsco2.ucsd.edu http://hahana.soest.hawaii.edu/hot/ THE NATIONAL ACADEMY OF SCIENCES (NAS) wasestablishedtoadvisetheUnited States on scientific and technical issues when President Lincoln signed a Congressional charterin1863.TheNationalResearchCouncil,theoperatingarmoftheNationalAcademy ofSciencesandtheNationalAcademyofEngineering,hasissuednumerousreportsonthe causes of and potential responses to climate change. Climate change resources from the National Research Council are available at nationalacademies.org/climate. THEROYALSOCIETYisaself-governingFellowshipofmanyoftheworld’smost distinguishedscientists.Itsmembersaredrawnfromallareasofscience,engineering, and medicine. It is the national academy of science in the UK. The Society’s fundamental purpose,reflectedinitsfoundingChartersofthe1660s,istorecognise,promote,and support excellence in science, and to encourage the development and use of science for the benefit of humanity. More information on the Society’s climate change work is availableatroyalsociety.org/policy/climate-change Summary2 ClimateChangeQ&A 1 Istheclimatewarming?3 2 Howdoscientistsknowthatrecentclimatechangeislargelycausedbyhumanactivities?5 3 CO2isalreadyintheatmospherenaturally,sowhyareemissionsfrom humanactivitysignificant?6 4 WhatrolehastheSunplayedinclimatechangeinrecentdecades?7 5 Whatdochangesintheverticalstructureofatmospherictemperature—fromthe surfaceuptothestratosphere—tellusaboutthecausesofrecentclimatechange?8 6 Climateisalwayschanging.Whyisclimatechangeofconcernnow?9 7 IsthecurrentlevelofatmosphericCO2concentrationunprecedentedinEarth’shistory?9 8 IsthereapointatwhichaddingmoreCO2willnotcausefurtherwarming?10 9 Doestherateofwarmingvaryfromonedecadetoanother?11 10 Didtheslowdownofwarmingduringthe2000stoearly2010s meanthatclimatechangeisnolongerhappening?12 TheBasicsofClimateChangeB1–B8 ClimateChangeQ&A(continued) 11 Iftheworldiswarming,whyaresomewintersandsummersstillverycold?13 12 WhyisArcticseaicedecreasingwhileAntarcticseaicehaschangedlittle?14 13 Howdoesclimatechangeaffectthestrengthandfrequency offloods,droughts,hurricanes,andtornadoes?15 14 Howfastissealevelrising?16 15 Whatisoceanacidificationandwhydoesitmatter?17 16 HowconfidentarescientiststhatEarthwillwarmfurtheroverthecomingcentury?18 17 Areclimatechangesofafewdegreesacauseforconcern?19 18 Whatarescientistsdoingtoaddresskeyuncertainties inourunderstandingoftheclimatesystem?19 19 Aredisasterscenariosabouttippingpointslike“turningofftheGulfStream” andreleaseofmethanefromtheArcticacauseforconcern?21 20 Ifemissionsofgreenhousegaseswerestopped,wouldtheclimatereturn totheconditionsof200yearsago?22 Conclusion23 Acknowledgements24 GREENHOUSEGASES suchascarbondioxide(CO2)absorbheat(infraredradiation) emittedfromEarth’ssurface.Increasesintheatmosphericconcentrationsofthese gasescauseEarthtowarmbytrappingmoreofthisheat.Humanactivities—especially the burning of fossil fuels since the start of the Industrial Revolution—have increased atmosphericCO2concentrationsbymorethan40%,withoverhalftheincreaseoccurring since1970.Since1900,theglobalaveragesurfacetemperaturehasincreasedbyabout 1 °C (1.8 °F). This has been accompanied by warming of the ocean, a rise in sea level, a strongdeclineinArcticseaice,widespreadincreasesinthefrequencyandintensity of heatwaves, and many other associated climate effects. Much of this warming has occurredinthelastfivedecades.Detailedanalyseshaveshownthatthewarming during this period is mainly a result of the increased concentrations of CO2 and other greenhousegases.Continuedemissionsofthesegaseswillcausefurtherclimatechange, includingsubstantialincreasesinglobalaveragesurfacetemperatureandimportant changesinregionalclimate.Themagnitudeandtimingofthesechangeswilldependon manyfactors,andslowdownsandaccelerationsinwarminglastingadecadeormorewill continuetooccur.However,long-termclimatechangeovermanydecadeswilldepend mainlyonthetotalamountofCO2andothergreenhousegasesemittedasaresultof humanactivities. Istheclimatewarming? Theclearestevidenceforsurfacewarmingcomesfromwidespreadthermometerrecordsthat,insome places,extendbacktothelate19thcentury.Today,temperaturesaremonitoredatmanythousandsof locations,overboththelandandoceansurface.Indirectestimatesoftemperaturechangefromsuch sources as tree rings and ice cores help to place recent temperature changes in the context of the past. In terms of the average surface temperature of Earth, these indirect estimates show that 1989 to 2019 was very likely the warmest 30-year period in more than 800 years; the most recent decade,-, is the warmestdecadeintheinstrumentalrecordsofar(since1850). A wide range of other observations provides a more comprehensive picture of warming throughout the climatesystem.Forexample,theloweratmosphereandtheupperlayersoftheoceanhavealsowarmed, snowandicecoveraredecreasingintheNorthernHemisphere,theGreenlandicesheetisshrinking,and sealevelisrising[Figure1b].Thesemeasurementsaremadewithavarietyofland-,ocean-,andspace-based monitoringsystems,whichgivesaddedconfidenceintherealityofglobal-scalewarmingofEarth’sclimate. Figure1a.Earth’sglobalaverage surfacetemperaturehasrisenas showninthisplotofcombined landandoceanmeasurements from1850to2019,derivedfrom threeindependentanalysesofthe available data sets. The temperaturechangesarerelativetotheglobal averagesurfacetemperatureof 1961−1990.Source:NOAAClimate. gov;datafromUKMetOfffceHadley Centre (maroon), US National AeronauticsandSpaceAdministration GoddardInstituteforSpaceStudies Annualglobalsurfacetemperature(1850−- -0.2 -0.4 -0.6 -0.8 HadleyCentre(UKMet) (red), and US National Oceanic andAtmosphericAdministration NationalCentersforEnvironmental Information(orange)- Year Figure1b.A large amount of observational evidence besides surface temperature records shows thatEarth’sclimateischanging. Forexample,additionalevidence ofawarmingtrendcanbefound inthedramaticdecreaseinthe extentofArcticseaiceatits summerminimum(whichoccurs inSeptember),thedecreasein JunesnowcoverintheNorthern Hemisphere,theincreasesinthe Arcticseaiceextentinwinterandsummer(1979−2019) 20 0 -20 -40 - global average upper ocean (upper700mor2300feet)heatcontent (shownrelativetothe- average), and the rise in global sea level. Source: NOAA Climate.gov Year NorthernHemisphereJunesnowcover(1967−2019) 6 4 2 Data:NSIDC 0 -2 -4 -6 1955 - Upperoceanheatcontent(1955−2019) 20 Year Data:RutgersSnow Lab 15 10 5 0 -5 - Globalsealevel- -20 -40 -60 -80 -100 Year Data:NOAANODC- YearData:Church&White2011,UHSLC Howdoscientistsknowthatrecentclimatechangeislargelycausedbyhumanactivities? Sincethemid-1800s,scientistshaveknownthatCO2isoneofthemaingreenhousegasesofimportance toEarth’senergybalance.DirectmeasurementsofCO2intheatmosphereandinairtrappediniceshow thatatmosphericCO2increasedbymorethan40%from1800to2019.Measurementsofdifferentforms ofcarbon(isotopes,seeQuestion3)revealthatthisincreaseisduetohumanactivities.Othergreenhouse gases (notably methane and nitrous oxide) are also increasing as a consequence of human activities. The observed global surface temperature rise since 1900 is consistent with detailed calculations of the impacts oftheobservedincreaseinatmosphericgreenhousegases(andotherhuman-inducedchanges)onEarth’s energybalance. Differentinfluencesonclimatehavedifferentsignaturesinclimaterecords.Theseuniquefingerprintsare easiertoseebyprobingbeyondasinglenumber(suchastheaveragetemperatureofEarth’ssurface),and bylookinginsteadatthegeographicalandseasonalpatternsofclimatechange.Theobservedpatternsof surfacewarming,temperaturechangesthroughtheatmosphere,increasesinoceanheatcontent,increases inatmosphericmoisture,sealevelrise,andincreasedmeltingoflandandseaicealsomatchthepatterns scientistsexpecttoseeduetohumanactivities(seeQuestion5). Theexpectedchangesinclimatearebasedonourunderstandingofhowgreenhousegasestrapheat.Both thisfundamentalunderstandingofthephysicsofgreenhousegasesandpattern-basedfingerprintstudies show that natural causes alone are inadequate to explain the recent observed changes in climate. Natural causesincludevariationsintheSun’soutputandinEarth’sorbitaroundtheSun,volcaniceruptions,and internal fluctuations in the climate system (such as El Niño and La Niña). Calculations using climate models (seeinfobox,p.20)havebeenusedtosimulatewhatwouldhavehappenedtoglobaltemperaturesifonly naturalfactorswereinfluencingtheclimatesystem.Thesesimulationsyieldlittlesurfacewarming,orevena slightcooling,overthe20thcenturyandintothe21st.Onlywhenmodelsincludehumaninfluencesonthe compositionoftheatmospherearetheresultingtemperaturechangesconsistentwithobservedchanges. CO2isalreadyintheatmospherenaturally,sowhyareemissionsfromhumanactivitysignificant? Innature,CO2isexchangedcontinuallybetweentheatmosphere,plants,andanimalsthrough photosynthesis,respiration,anddecomposition,andbetweentheatmosphereandoceanthroughgas exchange.AverysmallamountofCO2(roughly1%oftheemissionratefromfossilfuelcombustion)is alsoemittedinvolcaniceruptions.Thisisbalancedbyanequivalentamountthatisremovedbychemical weatheringofrocks. The CO2 level in 2019 was more than 40% higher than it was in the 19th century. Most of this CO2 increase has taken place since 1970, about the time when global energy consumption accelerated. Measured decreases in the fraction of other forms of carbon (the isotopes 14C and 13C) and a small decrease in atmospheric oxygen concentration (observations of which have been available since 1990) show that theriseinCO2islargelyfromcombustionoffossilfuels(whichhavelow13Cfractionsandno14C). Deforestationandotherlandusechangeshavealsoreleasedcarbonfromthebiosphere(livingworld) whereitnormallyresidesfordecadestocenturies.TheadditionalCO2fromfossilfuelburningand deforestation has disturbed the balance of the carbon cycle, because the natural processes that could restorethebalancearetooslowcomparedtotheratesatwhichhumanactivitiesareaddingCO2tothe atmosphere.Asaresult,asubstantialfractionoftheCO2emittedfromhumanactivitiesaccumulates in the atmosphere, where some of it will remain not just for decades or centuries, but for thousands of years. Comparison with the CO2 levels measured in air extracted from ice cores indicates that the current concentrations are substantially higher than they have been in at least 800,000 years (see Question 6). For periods before the onset of satellite measurements, knowledge about solar changes is less certain becausethechangesareinferredfromindirectsources—includingthenumberofsunspotsandthe abundanceofcertainforms(isotopes)ofcarbonorberylliumatoms,whoseproductionratesinEarth’s atmosphereareinfluencedbyvariationsintheSun.Thereisevidencethatthe11-yearsolarcycle,during which the Sun’s energy output varies by roughly 0.1%, can influence ozone concentrations, temperatures, andwindsinthestratosphere(thelayerintheatmosphereabovethetroposphere,typicallyfrom12to 50kmaboveearth’ssurface,dependingonlatitudeandseason).Thesestratosphericchangesmayhave a small effect on surface climate over the 11-year cycle. However, the available evidence does not indicate pronouncedlong-termchangesintheSun’soutputoverthepastcentury,duringwhichtimehuman- inducedincreasesinCO2concentrationshavebeenthedominantinfluenceonthelong-termglobal surfacetemperatureincrease.Furtherevidencethatcurrentwarmingisnotaresultofsolarchangescan befoundinthetemperaturetrendsatdifferentaltitudesintheatmosphere(seeQuestion5). Figure2.Measurementsofthe Sun’senergyincidentonEarth shownonetincreaseinsolar forcingduringthepast40years, andthereforethiscannotbe responsible for warming during thatperiod.Thedatashowonly smallperiodicamplitudevariations associatedwiththeSun’s11-year cycle.Source:TSIdatafrom Physikalisch-Meteorologisches Observatorium Davos, Switzerland, onthenewVIRGOscalefrom1978to mid-2018; temperature data for same timeperiodfromtheHadCRUT4 dataset, UK Met Offfce, Hadley Centre. 1.2 1.0 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4 1362.5 1362.0 1361.5 1361.0 1360.5 1360.0 1359.5 Whatdochangesintheverticalstructureofatmospherictemperature —fromthesurfaceuptothestratosphere—tellusaboutthe causesofrecentclimatechange? In the early 1960s, results from mathematical/physical models of the climate system first showed that human-inducedincreasesinCO2 wouldbeexpectedtoleadtogradualwarmingoftheloweratmosphere (the troposphere) and cooling of higher levels of the atmosphere (the stratosphere). In contrast, increasesintheSun’soutputwouldwarmboththetroposphereandthefullverticalextentofthestratosphere.At thattime,therewasinsufficientobservationaldatatotestthisprediction,buttemperaturemeasurements fromweatherballoonsandsatelliteshavesinceconfirmedtheseearlyforecasts.Itisnowknownthatthe observed pattern of tropospheric warming and stratospheric cooling over the past 40 years is broadly consistent with computer model simulations that include increases in CO2 and decreases in stratospheric ozone, each caused by human activities. The observed pattern is not consistent with purely natural changes in the Sun’s energy output, volcanic activity, or natural climate variations such as El Niño and La Niña. Despitethisagreementbetweentheglobal-scalepatternsofmodelledandobservedatmospheric temperature change, there are still some differences. The most noticeable differences are in the tropics, wheremodelscurrentlyshowmorewarminginthetropospherethanhasbeenobserved,andintheArctic, wheretheobservedwarmingofthetroposphereisgreaterthaninmostmodels. Climateisalwayschanging.Whyisclimatechangeofconcernnow? Thelargestglobal-scaleclimatevariationsinEarth’srecentgeologicalpastaretheiceagecycles(seeinfobox, p.B4),whicharecoldglacialperiodsfollowedbyshorterwarmperiods[Figure3].Thelastfewofthesenatural cycles have recurred roughly every 100,000 years. They are mainly paced by slow changes in Earth’s orbit,whichalterthewaytheSun’senergyisdistributedwithlatitudeandbyseasononEarth.Theseorbital changesareverysmalloverthelastseveralhundredyears,andalonearenotsufficienttocausetheobserved magnitudeofchangeintemperaturesincetheIndustrialRevolution,nortoactonthewholeEarth.On ice-age timescales, these gradual orbital variations have led to changes in the extent of ice sheets and in the abundanceofCO2andothergreenhousegases,whichinturnhaveamplifiedtheinitialtemperaturechange. Recentestimatesoftheincreaseinglobalaveragetemperaturesincetheendofthelasticeageare4to5°C(7to9°F).Thatchangeoccurredoveraperiodofabout7,000years,starting18,000yearsago.CO2 hasrisenmorethan40%injustthepast200years,muchofthissincethe1970s,contributingtohuman alteration of the planet’s energy budget that has so far warmed Earth by about 1 °C (1.8 °F). If the rise in CO2continues unchecked, warming of the same magnitude as the increase out of the ice age can be expectedbytheendofthiscenturyorsoonafter.Thisspeedofwarmingismorethantentimesthatattheendofan iceage,thefastestknownnaturalsustainedchangeonaglobalscale. Isthecurrentlevelofatmospheric CO2concentrationunprecedentedin Earth’shistory? Measurementsofairinicecoresshowthatforthepast800,000yearsupuntilthe20thcentury,the atmosphericCO2concentrationstayedwithintherange170to300partspermillion(ppm),makingtherecent rapidrisetomorethan400ppmover200yearsparticularlyremarkable[figure3].Duringtheglacialcycles ofthepast800,000yearsbothCO2andmethanehaveactedasimportantamplifiersoftheclimatechanges triggeredbyvariationsinEarth’sorbitaroundtheSun.AsEarthwarmedfromthelasticeage,temperature continued Figure3.Datafromicecoreshave beenusedtoreconstructAntarctic temperaturesandatmospheric CO2concentrationsoverthepast 800,000years.Temperatureis basedonmeasurementsofthe isotopiccontentofwaterinthe DomeCicecore.CO2ismeasuredinair trapped in ice, and is a composite oftheDomeCandVostokicecore. ThecurrentCO2concentration (bluedot)isfromatmospheric measurements.Thecyclical patternoftemperaturevariations constitutestheiceage/interglacial andCO2 startedtoriseatapproximatelythesametimeandcontinuedtoriseintandemfromabout18,000 to11,000yearsago.Changesinoceantemperature,circulation,chemistry,andbiologycausedCO2tobe releasedtotheatmosphere,whichcombinedwithotherfeedbackstopushEarthintoanevenwarmerstate. For earlier geological times, CO2 concentrations and temperatures have been inferred from less direct methods.ThosesuggestthattheconcentrationofCO2lastapproached400ppmabout3to5million years ago, a period when global average surface temperature is estimated to have been about 2 to 3.5°C higherthaninthepre-industrialperiod.At50millionyearsago,CO2mayhavereached1000ppm,and global average temperature was probably about 10°C warmer than today. Under those conditions, Earth hadlittleice,andsealevelwasatleast60metreshigherthancurrentlevels. 450 400 cycles.Duringthesecycles,changesinCO2concentrations(inblue)track closelywithchangesintemperature (inorange).Astherecordshows, therecentincreaseinatmospheric CO2concentrationisunprecedented inthepast800,000years. AtmosphericCO2concentration surpassed400ppmin2016,andthe averageconcentrationin2019was morethan411ppm.Source:Based onffgurebyJeremyShakun,datafrom AtmosphericCO2concentration 4Antarctic 0temperature -4 - Lüthietal.,2008andJouzeletal.,2007. - Years ago(inthousands) Isthereapointatwhichaddingmore CO2willnotcausefurtherwarming? OurunderstandingofthephysicsbywhichCO2affectsEarth’senergybalanceisconfirmedbylaboratory measurements, as well as by detailed satellite and surface observations of the emission and absorption of infrared energy by the atmosphere. Greenhouse gases absorb some of the infrared energy that Earth emits in so-called bands of stronger absorption that occur at certain wavelengths. Different gases absorb energyatdifferentwavelengths.CO2 hasitsstrongestheat-trappingbandcentredatawavelengthof15 micrometres(millionthsofametre),withabsorptionthatspreadsoutafewmicrometresoneitherside. There are also many weaker absorption bands. As CO2 concentrations increase, the absorption at the cen- treofthestrongbandisalreadysointensethatitplayslittleroleincausingadditionalwarming.However, more energy is absorbed in the weaker bands and away from the centre of the strong band, causing the surfaceandloweratmospheretowarmfurther. Figure4.Theclimatesystemvaries naturallyfromyeartoyearandfrom decadetodecade.Tomakereliable inferencesabouthuman-induced climatechange,multi-decadaland longerrecordsaretypicallyused. Calculating a “running average” over theselongertimescalesallowsone tomoreeasilyseelong-termtrends. Fortheglobalaveragetemperature fortheperiod-(usingthe datafromtheUKMetOfficeHadley Centrerelativetothe1961-90 EvenasCO2isrisingsteadilyintheatmosphere,leadingtogradualwarmingofEarth’ssurface,manynatural factorsaremodulatingthislong-termwarming.Largevolcaniceruptionsincreasethenumberofsmall particlesinthestratosphere.Theseparticlesreflectsunlight,leadingtoshort-termsurfacecoolinglasting typicallytwotothreeyears,followedbyaslowrecovery.Oceancirculationandmixingvarynaturallyonmany time scales, causing variations in sea surface temperatures as well as changes in the rate at which heat is transported to greater depths. For example, the tropical Pacific swings between warm El Niño and cooler La Niña events on timescales of two to seven years. Scientists study many different types of climate variations, suchasthoseondecadalandmulti-decadaltimescalesinthePacificandNorthAtlanticOceans.Each typeofvariationhasitsownuniquecharacteristics.Theseoceanicvariationsareassociatedwithsignificant regionalandglobalshiftsintemperatureandrainfallpatternsthatareevidentintheobservations. Warmingfromdecadetodecadecanalsobeaffectedbyhumanfactorssuchasvariationsinemissionsof greenhouse gases and aerosols (airborne particles that can have both warming and cooling effects) from coal-firedpowerplantsandotherpollutionsources. Thesevariationsinthetemperaturetrendareclearlyevidentintheobservedtemperaturerecord [Figure4].Short-term natural climate variations could also affect the long-term human-induced climate changesignalandvice-versa,becauseclimatevariationsondifferentspaceandtimescalescaninteract withoneanother.Itispartlyforthisreasonthatclimatechangeprojectionsaremadeusingclimate models(seeinfobox,p.20)thatcanaccountformanydifferenttypesofclimatevariationsandtheir interactions.Reliableinferencesabouthuman-inducedclimatechangemustbemadewithalongerview, using records that cover many decades. 0.5°C 0°C average)theplotsshow(top)the averageandrangeofuncertaintyforannuallyaverageddata;(2ndplot) theannualaveragetemperature forthetenyearscentredonany givendate;(3rdplot)theequivalent picturefor30-year;and(4thplot) the60-yearaverages.Source:Met OfffceHadleyCentre,basedonthe HadCRUT4datasetfromtheMetOfffce andClimaticResearchUnit(Moriceet al.,2012). −0.5°C 0.5°C 0°C −0.5°C -°C 0°C −0.5°C 0.5°C 0°C −0.5°C 10 Didtheslowdownofwarmingduringthe 2000stoearly 2010smeanthatclimatechangeisnolongerhappening? Decades of slow warming as well as decades of accelerated warming occur naturally in the climate system. Decadesthatarecoldorwarmcomparedtothelong-termtrendareseenintheobservationsofthepast 150 years and are also captured by climate models. Because the atmosphere stores very little heat, surface temperaturescanberapidlyaffectedbyheatuptakeelsewhereintheclimatesystemandbychangesin externalinfluencesonclimate(suchasparticlesformedfrommaterialloftedhighintotheatmosphere fromvolcaniceruptions). More than 90% of the heat added to the Earth system in recent decades has been absorbed by the oceans and penetrates only slowly into deep water. A faster rate of heat penetration into the deeper ocean will slowthewarmingseenatthesurfaceandintheatmosphere,butbyitselfitwillnotchangethelong-term warmingthatwilloccurfromagivenamountofCO2.Forexample,recentstudiesshowthatsomeheat comesoutoftheoceanintotheatmosphereduringwarmElNiñoevents,andmoreheatpenetratesto ocean depths in cold La Niñas. Such changes occur repeatedly over timescales of decades and longer. An example is the major El Niño event in 1997–98 when the globally averaged air temperature soared to the highestlevelinthe20thcenturyastheoceanlostheattotheatmosphere,mainlybyevaporation. Evenduringtheslowdownintheriseofaveragesurfacetemperature,alonger-termwarmingtrend was still evident (see Figure 4). Over that period, for example, record heatwaves were documented in Europe (summer 2003), in Russia (summer 2010), in the USA (July 2012), and in Australia (January 2013). Each of the last four decades was warmer than any previous decade since widespread thermometer measurements were introduced in the 1850s. The continuing effects of the warming climate are seen in theincreasingtrendsinoceanheatcontentandsealevel,aswellasinthecontinuedmeltingofArcticsea ice,glaciersandtheGreenlandicesheet. TheBasicsof ClimateChange GreenhousegasesaffectEarth’senergybalanceandclimate. TheSunservesastheprimaryenergysourceforEarth’sclimate.Someoftheincoming sunlight is reflected directly back into space, especially by bright surfaces such as ice and clouds,andtherestisabsorbedbythesurfaceandtheatmosphere.Muchofthisabsorbed solarenergyisre-emittedasheat(longwaveorinfraredradiation).Theatmosphereinturn absorbs and re-radiates heat, some of which escapes to space. Any disturbance to this balanceofincomingandoutgoingenergywillaffecttheclimate.Forexample,smallchanges intheoutputofenergyfromtheSunwillaffectthisbalancedirectly. Ifallheatenergyemittedfromthesurfacepassedthroughtheatmospheredirectlyinto space,Earth’saveragesurfacetemperaturewouldbetensofdegreescolderthantoday. Greenhouse gases in the atmosphere, including water vapour, carbon dioxide, methane, andnitrousoxide,acttomakethesurfacemuchwarmerthanthisbecausetheyabsorband emitheatenergyinalldirections(includingdownwards),keepingEarth’ssurfaceandlower atmospherewarm[FigureB1].Withoutthisgreenhouseeffect,lifeasweknowitcouldnot haveevolvedonourplanet.Addingmoregreenhousegasestotheatmospheremakesit evenmoreeffectiveatpreventingheatfromescapingintospace.Whentheenergyleavingis lessthantheenergyentering,Earthwarmsuntilanewbalanceisestablished. figureb1.Greenhousegasesin theatmosphere,includingwater vapour, carbon dioxide, methane, andnitrousoxide,absorbheat energyandemititinalldirections THEGREENHOUSEEFFECT (includingdownwards),keeping Earth’ssurfaceandloweratmo- sphere warm. Adding more green- housegasestotheatmosphere enhancestheeffect,makingEarth’s Somesolarradiationis reflected by Earthandtheatmosphere Someoftheinfraredradiationpasses through the atmosphere.Some is absorbed by greenhousegases and re-emitted in all directionsby the atmosphere. The effect of surface and lower atmosphere even warmer.Imagebasedonafigure from US Environmental Protection Agency. Atmosphere thisistowarmEarth’ssurface and theloweratmosphere. Someradiationisabsorbed byEarth’ssurfaceandwarmsit Infraredradiationis emitted byEarth’ssurface basicsofclimatechange Greenhouse gases emitted by human activities alter Earth’s energy balance and thus its climate. Humans also affect climate by changing the nature of the land surfaces (for example by clearing forests for farming) and through the emission of pollutants that affecttheamountandtypeofparticlesintheatmosphere. Scientistshavedeterminedthat,whenallhumanandnaturalfactorsareconsidered, Earth’sclimatebalancehasbeenalteredtowardswarming,withthebiggestcontributor beingincreasesinCO2. Humanactivitieshaveaddedgreenhousegasestotheatmosphere. The atmospheric concentrations of carbon dioxide, methane, and nitrous oxide have increased significantly since the Industrial Revolution began. In the case of carbon dioxide, theaverageconcentrationmeasuredattheMaunaLoaObservatoryinHawaiihasrisen from316partspermillion(ppm)1in1959(thefirstfullyearofdataavailable)tomore than411ppmin2019[FigureB2].Thesameratesofincreasehavesincebeenrecorded atnumerousotherstationsworldwide.Sincepreindustrialtimes,theatmospheric concentrationofCO2hasincreasedbyover40%,methanehasincreasedbymorethan 150%, and nitrous oxide has increased by roughly 20%. More than half of the increase in CO2hasoccurredsince1970.IncreasesinallthreegasescontributetowarmingofEarth, withtheincreaseinCO2playingthelargestrole.SeepageB3tolearnaboutthesourcesof humanemittedgreenhousegases. Scientists have examined greenhouse gases in the context of the past. Analysis of air trappedinsideicethathasbeenaccumulatingovertimeinAntarcticashowsthattheCO2 1thatis,foreverymillionmoleculesintheair,31GofthemwereCO2 FigureB2.Measurements of atmosphericCO2since1958from theMaunaLoaObservatoryin Hawaii(black)andfromtheSouth Pole(red)showasteadyannual increaseinatmosphericCO2concentration.Themeasurements aremadeatremoteplaceslike these because they are not greatly influenced by local processes, so therefore they are representativeofthebackgroundatmosphere. Thesmallup-and-downsaw-tooth patternreflectsseasonalchanges inthereleaseanduptakeofCO2by 420 400 380 360 340 320 plants.Source:ScrippsCO2Program- Year B2ClimateChange basicsofclimatechange concentrationbegantoincreasesignificantlyinthe19thcentury[FigureB3],afterstaying intherangeof260to280ppmfortheprevious10,000years.Icecorerecordsextending back 800,000 years show that during that time, CO2concentrations remained within the range of 170 to 300 ppm throughout many “ice age” cycles—see infobox, pg. B4 to learn abouttheiceages—andnoconcentrationabove300ppmisseeninicecorerecords untilthepast200years. 410 390 370 350 330 310 290 270 - Year FigureB3.CO2variationsduring thepast1,000years,obtained fromanalysisofairtrappedinan icecoreextractedfromAntarctica (redsquares),showasharprisein atmosphericCO2startinginthelate 19thcentury.Modernatmospheric measurementsfromMaunaLoaare superimposedingray.Source:ffgure byEricWolff,datafromEtheridgeetal., 199G; MacFarling Meure et al., 200G; Scripps CO2 Program. Learnaboutthesourcesofhuman-emittedgreenhousegases: Carbon dioxide (CO2)hasboth naturalandhumansources,but CO2levelsareincreasingprimarily becauseofthecombustionoffossil fuels,cementproduction,defor- estation(whichreducestheCO2taken up by trees and increases the CO2releasedbydecompositionof thedetritus),andotherlanduse changes.IncreasesinCO2arethe single largest contributor to global warming. Methane (CH4) hasbothhuman andnaturalsources,andlevels haverisensignificantlysince pre-industrialtimesduetohuman activitiessuchasraisinglivestock, growingpaddyrice,fillinglandfills, andusingnaturalgas(whichis mostlyCH4,someofwhichmay bereleasedwhenitisextracted, transported,andused). Nitrousoxide(N2O)concentra- tions have risen primarily because ofagriculturalactivitiessuchasthe useofnitrogen-basedfertilisersand landusechanges. Halocarbons, including chloro- fluorocarbons(CFCs),arechem- icalsusedasrefrigerantsandfire retardants.Inadditiontobeing potent greenhouse gases, CFCs alsodamagetheozonelayer.The productionofmostCFCshasnow beenbanned,sotheirimpactis startingtodecline.However,many CFC replacements are also potent greenhouse gases and their concen- trationsandtheconcentrations ofotherhalocarbonscontinueto increase. Evidence&Causes2020B3 basicsofclimatechange Measurements of the forms (isotopes) of carbon in the modern atmosphere show a clear fingerprintoftheadditionof“old”carbon(depletedinnaturalradioactive14C)coming fromthecombustionoffossilfuels(asopposedto“newer”carboncomingfromliving systems).Inaddition,itisknownthathumanactivities(excludinglandusechanges) currentlyemitanestimated10billiontonnesofcarboneachyear,mostlybyburningfossil fuels,whichismorethanenoughtoexplaintheobservedincreaseinconcentration. These and other lines of evidence point conclusively to the fact that the elevated CO2concentrationinouratmosphereistheresultofhumanactivities. Climaterecordsshowawarmingtrend. Estimatingglobalaveragesurfaceairtemperatureincreaserequirescarefulanalysisof millionsofmeasurementsfromaroundtheworld,includingfromlandstations,ships, andsatellites.Despitethemanycomplicationsofsynthesisingsuchdata,multiple independent teams have concluded separately and unanimously that global average surface airtemperaturehasrisenbyabout1°C(1.8°F)since1900[FigureB4].Althoughtherecordshowsseveralpausesandaccelerationsintheincreasingtrend,eachofthelastfour decades has been warmer than any other decade in the instrumental record since 1850. Goingfurtherbackintimebeforeaccuratethermometerswerewidelyavailable, temperatures can be reconstructed using climate-sensitive indicators “proxies” Learnabouttheiceages: Detailedanalysesofoceansediments, icecores,andotherdatashowthatforat leastthelast2.6millionyears,Earthhas gonethroughextendedperiodswhen temperaturesweremuchlowerthan todayandthickblanketsoficecovered large areas of the Northern Hemisphere. Theselongcoldspells,lastinginthe mostrecentcyclesforaround100,000 years,wereinterruptedbyshorterwarm ‘interglacial’periods,includingthepast 10,000years. Throughacombinationoftheory, observations, and modelling, scientists havededucedthattheiceages*are triggeredbyrecurringvariationsin Earth’sorbitthatprimarilyalterthe regionalandseasonaldistributionof solarenergyreachingEarth.These relativelysmallchangesinsolarenergy arereinforcedoverthousandsofyearsby gradualchangesinEarth’sicecover(the cryosphere),especiallyovertheNorthern Hemisphere,andinatmospheric composition,eventuallyleadingtolarge changesinglobaltemperature. Theaverageglobaltemperaturechange duringanice-agecycleisestimatedas5 °C±1°C(9°F±2°F). *NotethatingeologicaltermsEarthhas beeninaniceageeversincetheAntarctic IceSheetlastformedabout3Gmillion yearsago.However,inthisdocumentwe haveusedtheterminitsmorecolloquial usageindicatingtheregularoccurrenceof extensiveicesheetsoverNorthAmerica andnorthernEurasia. B4ClimateChange basicsofclimatechange inmaterialssuchastreerings,icecores,andmarinesediments.Comparisonsofthe thermometerrecordwiththeseproxymeasurementssuggestthatthetimesincetheearly 1980shasbeenthewarmest40-yearperiodinatleasteightcenturies,andthatglobal temperatureisrisingtowardspeaktemperatureslastseen5,000to10,000yearsagointhe warmestpartofourcurrentinterglacialperiod. Manyotherimpactsassociatedwiththewarmingtrendhavebecomeevidentinrecent years.Arcticsummerseaicecoverhasshrunkdramatically.Theheatcontentoftheocean hasincreased.Globalaveragesealevelhasrisenbyapproximately16cm(6inches)since 1901,duebothtotheexpansionofwarmeroceanwaterandtotheadditionofmeltwaters from glaciers and ice sheets on land. Warming and precipitation changes are altering the geographicalrangesofmanyplantandanimalspeciesandthetimingoftheirlifecycles. Inadditiontotheeffectsonclimate,someoftheexcessCO2intheatmosphereisbeing takenupbytheocean,changingitschemicalcomposition(causingoceanacidification). - -0.2 -0.4 -0.6 - -0.2 -0.4 -0.6 Annualaverage HadleyCentre(UKMet) FigureB4. Earth’sglobalaverage surfacetemperaturehasrisen,as showninthisplotofcombinedlandandoceanmeasurements from1850to2019derivedfrom three independent analyses of the availabledatasets.Thetoppanel showsannualaveragevaluesfrom thethreeanalyses,andthebottom panelshowsdecadalaverage values,includingtheuncertainty range(greybars)forthemaroon (HadCRUT4)dataset.Thetem- peraturechangesarerelativetothe global average surface temperature,averaged from 1961−1990. Source:NOAA Climate.gov, based on IPCC AR5. DatafromUKMetOfffceHadleyCentre (maroon),USNationalAeronauticsand Space Administration Goddard Institute forSpaceStudies(red),andUSNational OceanicandAtmosphericAdministration NationalCentersforEnvironmentalInfor- Year mation(orange). Evidence&Causes2020B5 basicsofclimatechange B6ClimateChange Manycomplexprocessesshapeourclimate. BasedjustonthephysicsoftheamountofenergythatCO2absorbsandemits,adoublingof atmospheric CO2 concentration from pre-industrial levels (up to about 560 ppm) would by itselfcauseaglobalaveragetemperatureincreaseofabout1°C(1.8°F).Intheoverallclimate system,however,thingsaremorecomplex;warmingleadstofurthereffects(feedbacks)that either amplify or diminish the initial warming. The most important feedbacks involve various forms of water. A warmer atmosphere generally containsmorewatervapour.Watervapourisapotentgreenhousegas,thuscausingmore warming;itsshortlifetimeintheatmospherekeepsitsincreaselargelyinstepwithwarming. Thus,watervapouristreatedasanamplifier,andnotadriver,ofclimatechange.Higher temperaturesinthepolarregionsmeltseaiceandreduceseasonalsnowcover,exposinga darkeroceanandlandsurfacethatcanabsorbmoreheat,causingfurtherwarming.Another importantbutuncertainfeedbackconcernschangesinclouds.Warmingandincreasesinwater vapour together may cause cloud cover to increase or decrease which can either amplify or dampentemperaturechangedependingonthechangesinthehorizontalextent,altitude,and propertiesofclouds.Thelatestassessmentofthescienceindicatesthattheoverallnetglobal effectofcloudchangesislikelytobetoamplifywarming. Theoceanmoderatesclimatechange.Theoceanisahugeheatreservoir,butitisdifficultto heat its full depth because warm water tends to stay near the surface. The rate at which heat is transferred to the deep ocean is therefore slow; it varies from year to year and from decade to decade, and it helps to determine the pace of warming at the surface. Observations of the sub-surface ocean are limited prior to about 1970, but since then, warming of the upper 700 m (2,300 feet) is readily apparent, and deeper warming is also clearly observed since about 1990. Surface temperatures and rainfall in most regions vary greatly from the global average because of geographical location, in particular latitude and continental position. Both the average values of temperature, rainfall, and their extremes (which generally have the largest impacts onnaturalsystemsandhumaninfrastructure),arealsostronglyaffectedbylocalpatternsof winds. Estimating the effects of feedback processes, the pace of the warming, and regional climate changerequirestheuseofmathematicalmodelsoftheatmosphere,ocean,land,andice (the cryosphere) built upon established laws of physics and the latest understanding of the physical, chemical and biological processes affecting climate, and run on powerful computers. Modelsvaryintheirprojectionsofhowmuchadditionalwarmingtoexpect(dependingonthe type of model and on assumptions used in simulating certain climate processes, particularly cloud formation and ocean mixing), but all such models agree that the overall net effect of feedbacksistoamplifywarming. basicsofclimatechange Humanactivitiesarechangingtheclimate. Rigorous analysis of all data and lines of evidence shows that most of the observed global warming over the past 50 years or so cannot be explained by natural causes and instead requiresasignificantrolefortheinfluenceofhumanactivities. Inordertodiscernthehumaninfluenceonclimate,scientistsmustconsidermanynatural variations that affect temperature, precipitation, and other aspects of climate from local to globalscale,ontimescalesfromdaystodecadesandlonger.OnenaturalvariationistheEl Niño Southern Oscillation (ENSO), an irregular alternation between warming and cooling (lastingabouttwotosevenyears)intheequatorialPacificOceanthatcausessignificant year-to-yearregionalandglobalshiftsintemperatureandrainfallpatterns.Volcanic eruptionsalsoalterclimate,inpartincreasingtheamountofsmall(aerosol)particlesinthe stratospherethatreflectorabsorbsunlight,leadingtoashort-termsurfacecoolinglasting typicallyabouttwotothreeyears.Overhundredsofthousandsofyears,slow,recurring variations in Earth’s orbit around the Sun, which alter the distribution of solar energy receivedbyEarth,havebeenenoughtotriggertheiceagecyclesofthepast800,000years. Fingerprintingisapowerfulwayofstudyingthecausesofclimatechange.Differentinfluences onclimateleadtodifferentpatternsseeninclimaterecords.Thisbecomesobviouswhen scientistsprobebeyondchangesintheaveragetemperatureoftheplanetandlookmore closely at geographical and temporal patterns of climate change. For example, an increase intheSun’senergyoutputwillleadtoaverydifferentpatternoftemperaturechange(across Earth’ssurfaceandverticallyintheatmosphere)comparedtothatinducedbyanincreasein CO2concentration.Observedatmospherictemperaturechangesshowafingerprintmuch continued Learnmoreaboutotherhumancausesofclimatechange: Inadditiontoemittinggreenhouse gases,humanactivitieshavealso altered Earth’s energy balance through, forexample: Changesinlanduse.Changesinthe waypeopleuseland—forexample, forforests,farms,orcities—can leadtobothwarmingand coolingeffectslocallyby changingthereflectivityof Earth’ssurfaces(affectinghowmuch sunlightissentbackintospace)and bychanginghowwetaregionis. Emissionsof pollutants (other thangreenhouse gases).Someindustrial andagriculturalprocessesemit pollutantsthatproduceaerosols (smalldropletsorparticles suspendedintheatmosphere). MostaerosolscoolEarthby reflectingsunlightbackto space.Someaerosolsalsoaffect theformationofclouds,which canhaveawarmingorcooling effectdependingontheirtypeand location.Blackcarbonparticles(or “soot”)producedwhenfossilfuels orvegetationareburnedgenerally haveawarmingeffectbecausethey absorbincomingsolarradiation. Evidence&Causes2020B7 basicsofclimatechange closertothatofalong-termCO2 increasethantothatofafluctuatingSunalone.Scientists routinelytestwhetherpurelynaturalchangesintheSun,volcanicactivity,orinternalclimate variabilitycouldplausiblyexplainthepatternsofchangetheyhaveobservedinmanydifferent aspectsoftheclimatesystem.Theseanalyseshaveshownthattheobservedclimatechanges ofthepastseveraldecadescannotbeexplainedjustbynaturalfactors. FigureB5.Theamountandrateof warmingexpectedforthe21stcen- turydependsonthetotalamountof greenhouse gases that humankind emits. Models project the tempera- tureincreaseforabusiness-as-usual emissionsscenario(inred)and aggressiveemissionreductions, fallingclosetozero50yearsfrom now(inblue).Blackisthemodelled estimateofpastwarming.Each solidlinerepresentstheaverage of different model runs using the same emissions scenario, and the shadedareasprovideameasureof thespread(onestandarddeviation) between the temperature changes projected by the different models. Alldataarerelativetoareference period(settozero)of-. Source: Based on IPCC AR5 B8ClimateChange Howwillclimatechangeinthefuture? Scientistshavemademajoradvancesintheobservations,theory,andmodellingofEarth’s climatesystem,andtheseadvanceshaveenabledthemtoprojectfutureclimatechange withincreasingconfidence.Nevertheless,severalmajorissuesmakeitimpossibletogive precise estimates of how global or regional temperature trends will evolve decade by decade intothefuture.Firstly,wecannotpredicthowmuchCO2humanactivitieswillemit,asthis dependsonfactorssuchashowtheglobaleconomydevelopsandhowsociety’sproduction andconsumptionofenergychangesinthecomingdecades.Secondly,withcurrent understandingofthecomplexitiesofhowclimatefeedbacksoperate,thereisarangeof possibleoutcomes,evenforaparticularscenarioofCO2emissions.Finally,overtimescales ofadecadeorso,naturalvariabilitycanmodulatetheeffectsofanunderlyingtrendin temperature. Taken together, all model projections indicate that Earth will continue to warm considerablymoreoverthenextfewdecadestocenturies.Iftherewerenotechnologicalor policy changes to reduce emission trends from their current trajectory, then further globally- averagedwarmingof2.6to4.8°C(4.7to8.6°F)inadditiontothatwhichhasalready occurredwouldbeexpectedduringthe21stcentury[FigureB5].Projectingwhatthoseranges willmeanfortheclimateexperiencedatanyparticularlocationisachallengingscientific problem,butestimatesarecontinuingtoimproveasregionalandlocal-scalemodelsadvance. Global average surface temperature change 6.0 4.0 2.0 0.0 −- 11 Iftheworldiswarming,whyaresomewintersandsummersstillverycold? Climatechangemeansnotonlychangesingloballyaveragedsurfacetemperature,butalsochangesin atmosphericcirculation,inthesizeandpatternsofnaturalclimatevariations,andinlocalweather.La Niña events shift weather patterns so that some regions are made wetter, and wet summers are generally cooler. Stronger winds from polar regions can contribute to an occasional colder winter. In a similar way, thepersistenceofonephaseofanatmosphericcirculationpatternknownastheNorthAtlanticOscilla- tion has contributed to several recent cold winters in Europe, eastern North America, and northern Asia. AtmosphericandoceancirculationpatternswillevolveasEarthwarmsandwillinfluencestormtracks andmanyotheraspectsoftheweather.Globalwarmingtiltstheoddsinfavourofmorewarmdaysand seasons and fewer cold days and seasons. For example, across the continental United States in the 1960s there were more daily record low temperatures than record highs, but in the 2000s there were more than twiceasmanyrecordhighsasrecordlows.Anotherimportantexampleoftiltingtheoddsisthatover recent decades heatwaves have increased in frequency in large parts of Europe, Asia, South America, and Australia.Marineheatwavesarealsoincreasing. WhyisArcticseaicedecreasingwhileAntarcticseaicehaschangedlittle? Figure5.The Arctic summer seaiceextentin2012,(measured in September) was a record low, shown(inwhite)comparedtothe mediansummerseaiceextentfor 1979to2000(inorangeoutline).In 2013,Arcticsummerseaiceextent rebounded somewhat, but was still thesixthsmallestextentonrecord. In2019,seaiceextenteffectively tiedforthesecondlowestminimum inthesatelliterecord,alongwith 2007and2016—behindonly2012, whichisstilltherecordminimum. The13lowesticeextentsinthe satelliteerahavealloccurredinthe last13years.Source: National Snow and Ice Data Center SomedifferencesinseasonalseaiceextentbetweentheArcticandAntarcticareduetobasicgeography anditsinfluenceonatmosphericandoceaniccirculation.TheArcticisanoceanbasinsurroundedlargely bymountainouscontinentallandmasses,andAntarcticaisacontinentsurroundedbyocean.Inthe Arctic, sea ice extent is limited by the surrounding land masses. In the Southern Ocean winter, sea ice can expandfreelyintothesurroundingocean,withitssouthernboundarysetbythecoastlineofAntarctica. Because Antarctic sea ice forms at latitudes further from the South Pole (and closer to the equator), less ice survives the summer. Sea ice extent in both poles changes seasonally; however, longer-term variabilityinsummerandwintericeextentisdifferentineachhemisphere,dueinparttothesebasicgeographical differences. Sea ice in the Arctic has decreased dramatically since the late 1970s, particularly in summer and autumn. Since the satellite record began in 1978, the yearly minimum Arctic sea ice extent (which occurs in September) has decreased by about 40% [Figure5]. Ice cover expands again each Arctic winter, but the ice is thinner than it used to be. Estimates of past sea ice extent suggest that this decline may be unprecedented in at least the past 1,450 years. Because sea ice is highly reflective, warming is amplified as theicedecreasesandmoresunshineisabsorbedbythedarkerunderlyingoceansurface. SeaiceintheAntarcticshowedaslightincreaseinoverallextentfrom1979to2014,althoughsome areas, such as that to the west of the Antarctic Peninsula experienced a decrease. Short-term trends in theSouthernOcean,suchasthoseobserved, canreadilyoccurfromnaturalvariabilityofthe atmosphere,oceanandseaicesystem.Changes insurfacewindpatternsaroundthecontinent contributedtotheAntarcticpatternofseaice change; ocean factors such as the addition of cool freshwaterfrommeltingiceshelvesmayalso have played a role. However, after 2014, Antarctic iceextentbegantodecline,reachingarecordlow (withinthe40yearsofsatellitedata)in2017,and remaininglowinthefollowingtwoyears. 13 Howdoesclimatechangeaffectthestrengthandfrequencyoffloods, droughts,hurricanes,andtornadoes? AsEarth’sclimatehaswarmed,morefrequentandmoreintenseweathereventshavebothbeenobserved aroundtheworld.Scientiststypicallyidentifytheseweathereventsas“extreme”iftheyareunlike90%or95% of similar weather events that happened before in the same region. Many factors contribute to any individual extremeweatherevent—includingpatternsofnaturalclimatevariability,suchasElNiñoandLaNiña— making it challenging to attribute any particular extreme event to human-caused climate change. However, studies can show whether the warming climate made an event more severe or more likely to happen. Awarmingclimatecancontributetotheintensityofheatwavesbyincreasingthechancesofveryhotdaysand nights.Climatewarmingalsoincreasesevaporationonland,whichcanworsendroughtandcreateconditions more prone to wildfire and a longer wildfire season. A warming atmosphere is also associated with heavier precipitationevents(rainandsnowstorms)throughincreasesintheair’scapacitytoholdmoisture.ElNiño eventsfavourdroughtinmanytropicalandsubtropicallandareas,whileLaNiñaeventspromotewetter conditionsinmanyplaces.Theseshort-termandregionalvariationsareexpectedtobecomemoreextremein awarmingclimate. Earth’swarmerandmoisteratmosphereandwarmeroceansmakeitlikelythatthestrongesthurricaneswill be more intense, produce more rainfall, affect new areas, and possibly be larger and longer-lived. This issupportedbyavailableobservationalevidenceintheNorthAtlantic.Inaddition,sealevelrise(see Question14)increasestheamountofseawaterthatispushedontoshoreduringcoastalstorms,which, along with more rainfall produced by the storms, can result in more destructive storm surges and flooding. Whileglobalwarmingislikelymakinghurricanesmoreintense,thechangeinthenumberofhurricanes eachyearisquiteuncertain.Thisremainsasubjectofongoingresearch. Someconditionsfavourableforstrongthunderstormsthatspawntornadoesareexpectedtoincreasewith warming,butuncertaintyexistsinotherfactorsthataffecttornadoformation,suchaschangesinthevertical andhorizontalvariationsofwinds. Howfastissealevelrising? Figure 6.Observationsshow thattheglobalaveragesealevel has risen by about 16 cm (6 inches) sincethelate19thcentury.Sealevel isrisingfasterinrecentdecades; measurementsfromtidegauges (blue)andsatellites(red)indicate thatthebestestimateforthe averagesealevelriseoverthelast decadeiscentredon3.6mmper year(0.14inchesperyear).The shadedarearepresentsthesealeveluncertainty, which has decreased as the number of gauge sites used in calculating the global averages andthenumberofdatapointshave increased.Source:ShumandKuo(2011) This sea level rise has been driven by expansion of water volume as the ocean warms, melting of mountain glaciers in all regions of the world, and mass losses from the Greenland and Antarctic ice sheets. All of theseresultfromawarmingclimate.Fluctuationsinsealevelalsooccurduetochangesintheamountsof water stored on land. The amount of sea level change experienced at any given location also depends on a varietyofotherfactors,includingwhetherregionalgeologicalprocessesandreboundofthelandweighted down by previous ice sheets are causing the land itself to rise or sink, and whether changes in winds and currents are piling ocean water against some coasts or moving water away. The effects of rising sea level are felt most acutely in the increased frequency and intensity of occasional stormsurges.IfCO2andothergreenhousegasescontinuetoincreaseontheircurrenttrajectories,itis projectedthatsealevelmayrise,atminimum,byafurther0.4to0.8m(1.3to2.6feet)by2100,although future ice sheet melt could make these values considerably higher. Moreover, rising sea levels will not stopin2100;sealevelswillbemuchhigherinthefollowingcenturiesastheseacontinuestotakeup heatandglacierscontinuetoretreat.ItremainsdifficulttopredictthedetailsofhowtheGreenlandand AntarcticIceSheetswillrespondtocontinuedwarming,butitisthoughtthatGreenlandandperhaps WestAntarcticawillcontinuetolosemass,whereasthecolderpartsofAntarcticacouldgainmassas they receive more snowfall from warmer air that contains more moisture. Sea level in the last interglacial (warm)periodaround125,000yearsagopeakedatprobably5to10mabovethepresentlevel.Duringthis period,thepolarregionswerewarmerthantheyaretoday.Thissuggeststhat,overmillennia,longperiods ofincreasedwarmthwillleadtoverysignificantlossofpartsoftheGreenlandandAntarcticIceSheetsandtoconsequentsealevelrise. Whatisoceanacidificationandwhy doesitmatter? CO2 dissolves in water to form a weak acid, and the oceans have absorbed about a third of the CO2 resulting fromhumanactivities,leadingtoasteadydecreaseinoceanpHlevels.WithincreasingatmosphericCO2, thischemicalbalancewillchangeevenmoreduringthenextcentury.Laboratoryandotherexperiments showthatunderhighCO2andinmoreacidicwaters,somemarinespecieshavemisshapenshellsand lower growth rates, although the effect varies among species. Acidification also alters the cycling of nutrientsandmanyotherelementsandcompoundsintheocean,anditislikelytoshiftthecompetitive advantage among species, with as-yet-to-be-determined impacts on marine ecosystems and the food web. figure7.AsCO2intheairhas increased,therehasbeenan increaseintheCO2contentofthe surfaceocean(upperbox),anda decrease in the seawater pH (lowerbox).Source:adaptedfromDoreetal. (2009) and Bates et al. (2012). 400 390 380 370 360 350 340 330 320 - Year 8.11 8.10 8.09 8.08 8.07 8.06 16 figure8.If emissions continueon their present trajectory, without eithertechnologicalorregulatory abatement, then the best estimate is that global average temperature willwarmafurther2.6to4.8°C (4.7to8.6°F)bytheendofthe century(right).Landareasare projectedtowarmmorethanocean areasandhencemorethanthe global mean. The figure on the left showsprojectedwarmingwithvery aggressive emissions reductions. Thefiguresrepresentmulti-model estimatesoftemperatureaverages for-comparedto-. Source: IPCC AR5 Howconfidentarescientiststhat Earthwillwarmfurtheroverthecomingcentury? Warmingduetotheadditionoflargeamountsofgreenhousegasestotheatmospherecanbeunderstood intermsofverybasicpropertiesofgreenhousegases.Itwillinturnleadtomanychangesinnatural climateprocesses,withaneteffectofamplifyingthewarming.Thesizeofthewarmingthatwillbe experienced depends largely on the amount of greenhouse gases accumulating in the atmosphere and hence on the trajectory of emissions. If the total cumulative emissions since 1875 are kept below about 900gigatonnes(900billiontonnes)ofcarbon,thenthereisatwo-thirdschanceofkeepingtherisein globalaveragetemperaturesincethepre-industrialperiodbelow2°C(3.6°F).However,two-thirdsofthis amount has already been emitted. A target of keeping global average temperature rise below 1.5 °C (2.7 °F) wouldallowforevenlesstotalcumulativeemissionssince1875. BasedjustontheestablishedphysicsoftheamountofheatCO2absorbsandemits,adoublingof atmospheric CO2 concentration from preindustrial levels (up to about 560 ppm) would by itself, without amplificationbyanyothereffects,causeaglobalaveragetemperatureincreaseofabout1°C(1.8°F). However, the total amount of warming from a given amount of emissions depends on chains of effects (feedbacks) that can individually either amplify or diminish the initial warming. Themostimportantamplifyingfeedbackiscausedbywatervapour,whichisapotentgreenhousegas.As CO2 increases and warms the atmosphere, the warmer air can hold more moisture and trap more heat in the lower atmosphere. Also, as Arctic sea ice and glaciers melt, more sunlight is absorbed into the darker underlyinglandandoceansurfaces,causingfurtherwarmingandfurthermeltingoficeandsnow.The biggestuncertaintyinourunderstandingoffeedbacksrelatestoclouds(whichcanhavebothpositiveand negative feedbacks), and how the properties of clouds will change in response to climate change. Otherimportantfeedbacksinvolvethecarboncycle.Currentlythelandandoceans togetherabsorbabouthalfoftheCO2emitted fromhumanactivities,butthecapacitiesof landandoceantostoreadditionalcarbonare expectedtodecreasewithadditionalwarming, leadingtofasterincreasesinatmospheric CO2andfasterwarming.Modelsvaryintheir projectionsofhowmuchadditionalwarming toexpect,butallsuchmodelsagreethatthe overallneteffectoffeedbacksistoamplify thewarming. Areclimatechangesofafewdegreesacauseforconcern? 18 Both theory and direct observations have confirmed that global warming is associated with greater warming overlandthanoceans,moisteningoftheatmosphere,shiftsinregionalprecipitationpatterns,increasesin extremeweatherevents,oceanacidification,meltingglaciers,andrisingsealevels(whichincreasestheriskof coastal inundation and storm surge). Already, record high temperatures are on average significantly outpacing recordlowtemperatures,wetareasarebecomingwetterasdryareasarebecomingdrier,heavyrainstorms havebecomeheavier,andsnowpacks(animportantsourceoffreshwaterformanyregions)aredecreasing. Theseimpactsareexpectedtoincreasewithgreaterwarmingandwillthreatenfoodproduction,freshwater supplies, coastal infrastructure, and especially the welfare of the huge population currently livinginlow-lyingareas.Eventhoughcertainregionsmayrealisesomelocalbenefitfromthewarming,the long-term consequences overall will be disruptive. It is not only an increase of a few degrees in global average temperature that is cause for concern—the pace atwhichthiswarmingoccursisalsoimportant(seeQuestion6).Rapidhuman-causedclimatechanges meanthatlesstimeisavailabletoallowforadaptationmeasurestobeputinplaceorforecosystemsto adapt, posing greater risks in areas vulnerable to more intense extreme weather events and rising sea levels. Whatarescientistsdoingtoaddresskeyuncertaintiesinour understandingoftheclimatesystem? continued Comparisons of model predictions with observations identify what is well-understoodand,atthesametime,revealuncertaintiesorgapsin ourunderstanding.Thishelpstosetprioritiesfornewresearch.Vigilant monitoring of the entire climate system—the atmosphere, oceans, land,andice—isthereforecritical,astheclimatesystemmaybefullof surprises. Together,fieldandlaboratorydataandtheoreticalunderstandingareused toadvancemodelsofEarth’sclimatesystemandtoimproverepresentation ofkeyprocessesinthem,especiallythoseassociatedwithclouds,aerosols, andtransportofheatintotheoceans.Thisiscriticalforaccuratelysimulating climate change and associated changes in severe weather, especially at the regionalandlocalscalesimportantforpolicydecisions. Simulatinghowcloudswillchangewithwarmingandinturnmayaffect warmingremainsoneofthemajorchallengesforglobalclimatemodels, inpartbecausedifferentcloudtypeshavedifferentimpactsonclimate, andthemanycloudprocessesoccuronscalessmallerthanmostcurrent modelscanresolve.Greatercomputerpowerisalreadyallowingforsome oftheseprocessestoberesolvedinthenewgenerationofmodels. Dozens of groups and research institutions work on climate models, andscientistsarenowabletoanalyseresultsfromessentiallyallofthe world’smajorEarth-SystemModelsandcomparethemwitheachother and with observations. Such opportunities are of tremendous benefit inbringingoutthestrengthsandweaknessesofvariousmodelsand diagnosingthecausesofdifferencesamongmodels,sothatresearch can focus on the relevant processes. Differences among models allow estimates to be made of the uncertainties in projections of future climate change.Additionally,largearchivesofresultsfrommanydifferent modelshelpscientiststoidentifyaspectsofclimatechangeprojections thatarerobustandthatcanbeinterpretedintermsofknownphysical mechanisms. Studyinghowclimaterespondedtomajorchangesinthepastisanother way of checking that we understand how different processes work and that modelsarecapableofperformingreliablyunderawiderangeofconditions. 19 Aredisasterscenariosabouttippingpointslike “turningoffthe GulfStream” andreleaseofmethanefromthe Arcticacauseforconcern? The composition of the atmosphere is changing towards conditions that have not been experienced for millions of years, so we are headed for unknown territory, and uncertainty is large. The climate system involvesmanycompetingprocessesthatcouldswitchtheclimateintoadifferentstateonceathreshold hasbeenexceeded. A well-known example is the south-north ocean overturning circulation, which is maintained by cold salty watersinkingintheNorthAtlanticandinvolvesthetransportofextraheattotheNorthAtlanticviathe GulfStream.Duringthelasticeage,pulsesoffreshwaterfromthemeltingicesheetoverNorthAmerica ledtoslowingdownofthisoverturningcirculation.Thisinturncausedwidespreadchangesinclimate around the Northern Hemisphere. Freshening of the North Atlantic from the melting of the Greenland ice sheet is gradual, however, and hence is not expected to cause abrupt changes. AnotherconcernrelatestotheArctic,wheresubstantialwarmingcoulddestabilisemethane(a greenhousegas)trappedinoceansedimentsandpermafrost,potentiallyleadingtoarapidreleaseof alargeamountofmethane.Ifsucharapidreleaseoccurred,thenmajor,fastclimatechangeswould ensue. Such high-risk changes are considered unlikely in this century, but are by definition hard to predict. Scientists are therefore continuing to study the possibility of exceeding such tipping points, beyond which werisklargeandabruptchanges. Inadditiontoabruptchangesintheclimatesystemitself,steadyclimatechangecancrossthresholdsthat triggerabruptchangesinothersystems.Inhumansystems,forexample,infrastructurehastypicallybeen builttoaccommodatetheclimatevariabilityatthetimeofconstruction.Gradualclimatechangescan causeabruptchangesintheutilityoftheinfrastructure—suchaswhenrisingsealevelssuddenlysurpass sea walls, or when thawing permafrost causes the sudden collapse of pipelines, buildings, or roads. In naturalsystems,asairandwatertemperaturesrise,somespecies—suchasthemountainpikaandmany oceancorals—willnolongerbeabletosurviveintheircurrenthabitatsandwillbeforcedtorelocate(if possible)orrapidlyadapt.Otherspeciesmayfarebetterinthenewconditions,causingabruptshiftsin thebalanceofecosystems;forexample,warmertemperatureshaveallowedmorebarkbeetlestosurvive over winter in some regions, where beetle outbreaks have destroyed forests. 20 figure9.If global emissions weretosuddenlystop,itwould takealongtimeforsurfaceair temperaturesandtheoceanto begintocoolbecausetheexcess CO2intheatmospherewould remainthereforalongtimeand wouldcontinuetoexertawarming effect.Modelprojectionsshowhow atmospheric CO2concentration (a),surfaceairtemperature(b), andoceanthermalexpansion(c) wouldrespondfollowingascenario of business-as-usual emissions ceasingin2300(red),ascenario ofaggressiveemissionreductions, fallingclosetozero50yearsfrom now(orange),andtwointermediate emissionsscenarios(greenand blue).Thesmalldownwardtick intemperatureat2300iscaused bytheeliminationofemissions of short-lived greenhouse gases, including methane. Source: Zickfeldet al., 2013 Ifemissionsofgreenhousegaseswerestopped, wouldtheclimatereturntotheconditionsof 200 yearsago? IfemissionsofCO2stoppedaltogether,itwouldtakemanythousandsofyearsforatmosphericCO2to return to “pre-industrial” levels due to its very slow transfer to the deep ocean and ultimate burial in ocean sediments. Surface temperatures would stay elevated for at least a thousand years, implying a long-term commitmenttoawarmerplanetduetopastandcurrentemissions.Sealevelwouldlikelycontinue to rise for many centuries even after temperature stopped increasing [Figure9]. Significant cooling would be required to reverse melting of glaciers and the Greenland ice sheet, which formed during past cold climates. The current CO2-induced warming of Earth is therefore essentially irreversible on human timescales. The amount and rate of further warming will depend almost entirely on how much more CO2humankindemits. Scenariosoffutureclimatechangeincreasinglyassumetheuseoftechnologiesthatcanremovegreen- housegasesfromtheatmosphere.Insuch“negativeemissions”scenarios,itassumedthatatsomepoint inthefuture,widespreadeffort willbeundertakenthatutilises suchtechnologiestoremove CO2fromtheatmosphereand loweritsatmosphericconcentra- tion,therebystartingtoreverse CO2-drivenwarmingonlonger timescales.Deploymentofsuch technologiesatscalewould requirelargedecreasesintheir costs.Evenifsuchtechnological fixes were practical, substantial reductionsinCO2emissions wouldstillbeessential. Thisdocumentexplainsthattherearewell-understoodphysicalmechanismsbywhich changes in the amounts of greenhouse gases cause climate changes. It discusses the evidence that the concentrations of these gases in the atmosphere have increased and arestillincreasingrapidly,thatclimatechangeisoccurring,andthatmostoftherecent change is almost certainly due to emissions of greenhouse gases caused by human activities. Further climate change is inevitable; if emissions of greenhouse gases continue unabated,futurechangeswillsubstantiallyexceedthosethathaveoccurredsofar.There remainsarangeofestimatesofthemagnitudeandregionalexpressionoffuturechange, but increases in the extremes of climate that can adversely affect natural ecosystems and humanactivitiesandinfrastructureareexpected. Citizensandgovernmentscanchooseamongseveraloptions(oramixtureofthose options) in response to this information: they can change their pattern of energy productionandusageinordertolimitemissionsofgreenhousegasesandhencethe magnitude of climate changes; they can wait for changes to occur and accept the losses, damage, and suffering that arise; they can adapt to actual and expected changes as much aspossible;ortheycanseekasyetunproven“geoengineering”solutionstocounteract someoftheclimatechangesthatwouldotherwiseoccur.Eachoftheseoptionshas risks,attractionsandcosts,andwhatisactuallydonemaybeamixtureofthesedifferent options.Differentnationsandcommunitieswillvaryintheirvulnerabilityandtheir capacitytoadapt.Thereisanimportantdebatetobehadaboutchoicesamongthese options,todecidewhatisbestforeachgroupornation,andmostimportantlyforthe global population as a whole. The options have to be discussed at a global scale because in many cases those communities that are most vulnerable control few of the emissions, eitherpastorfuture.Ourdescriptionofthescienceofclimatechange,withbothitsfacts anditsuncertainties,isofferedasabasistoinformthatpolicydebate. Authors Thefollowingindividualsservedastheprimarywritingteamforthe2014and2020editionsof thisdocument: Eric Wolff FRS, (UK lead), University of Cambridge Inez Fung (NAS, US lead), University of California,Berkeley BrianHoskinsFRS,GranthamInstitutefor ClimateChange JohnF.B.MitchellFRS,UKMetOffice TimPalmerFRS,UniversityofOxford Benjamin Santer (NAS), Lawrence Livermore NationalLaboratory JohnShepherdFRS,Universityof Southampton KeithShineFRS,UniversityofReading. Susan Solomon (NAS), Massachusetts InstituteofTechnology KevinTrenberth,NationalCenterfor AtmosphericResearch JohnWalsh,UniversityofAlaska, Fairbanks DonWuebbles,UniversityofIllinois Staffsupportforthe2020revisionwasprovidedbyRichardWalker,AmandaPurcell,NancyHuddleston, and Michael Hudson. We offer special thanks to Rebecca Lindsey and NOAA Climate.gov for providing dataandfigureupdates. Reviewers The following individuals served as reviewers of the 2014 document in accordance with procedures approved by the Royal Society and the National Academy of Sciences: RichardAlley(NAS),Departmentof Geosciences,PennsylvaniaStateUniversity AlecBroersFRS,FormerPresidentoftheRoyal AcademyofEngineering Harry Elderfield FRS, Department of Earth Sciences,UniversityofCambridge JoannaHaighFRS,ProfessorofAtmospheric Physics,ImperialCollegeLondon IsaacHeld(NAS),NOAAGeophysicalFluid DynamicsLaboratory John Kutzbach (NAS), Center for Climatic Research,UniversityofWisconsin Jerry Meehl, Senior Scientist, National Center forAtmosphericResearch JohnPendryFRS,ImperialCollegeLondon John Pyle FRS, Department of Chemistry, UniversityofCambridge Gavin Schmidt, NASA Goddard Space Flight Center EmilyShuckburgh,BritishAntarcticSurvey GabrielleWalker,Journalist AndrewWatsonFRS,UniversityofEastAnglia Support The Support for the 2014 Edition was provided by NAS Endowment Funds. We offer sincere thanks to the Ralph J. and Carol M. Cicerone Endowment for NAS Missions for supporting the production of this 2020Edition.