Climate change: A Summary of the Science/media/Royal_Society_Content/policy/... · This is the amount of climate change (as measured by the change in globally-averaged surface temperature)

Climate change: a summary of the scienceSeptember 2010

GlossaryWm-2 (watts per metre squared)The amount of energy that falls on a square metre in one second. Sometimes known as a flux.

Carbon cycleThe term used to describe the flow of carbon, in its various forms, between the atmosphere, oceans, plants, soils and rocks. In the atmosphere, carbon mostly exists as carbon dioxide, but it exists in different forms in other components, such as organic carbon in the soil.

Climate forcing (also known as radiative forcing)The imbalance in the Earth’s energy budget resulting from, for example, changes in the energy received from the Sun, changes in the amounts or characteristics of greenhouse gases and particles, or changes in the nature of the Earth’s surface. Hence climate forcing can result from both human activity and natural causes. It is given in units of Wm-2.

Climate sensitivityThis is the amount of climate change (as measured by the change in globally-averaged surface temperature) for a given amount of climate forcing. It is often quoted (as will be the case here) as the temperature change that eventually results from a doubling in CO2 concentrations, which is calculated to cause a climate forcing of about 3.6 Wm-2.

Internal climate variability:Climate change that occurs in the absence of natural or human-induced climate forcing as a result of interactions within and between the various components of the climate system (such as the atmosphere, the oceans and the frozen world).

Introduction1 Changesinclimatehavesignificantimplicationsforpresentlives,forfuturegenerationsandforecosystemsonwhichhumanitydepends.Consequently,climatechangehasbeenandcontinuestobethesubjectofintensivescientificresearchandpublicdebate.

2 ThereisstrongevidencethatthewarmingoftheEarthoverthelasthalf-centuryhasbeencausedlargelybyhumanactivity,suchastheburningoffossilfuelsandchangesinlanduse,includingagricultureanddeforestation.Thesizeoffuturetemperatureincreasesandotheraspectsofclimatechange,especiallyattheregionalscale,arestillsubjecttouncertainty.Nevertheless,therisksassociatedwithsomeofthesechangesaresubstantial.Itisimportantthatdecisionmakershaveaccesstoclimatescienceofthehighestquality,andcantakeaccountofitsfindingsinformulatingappropriateresponses.

3 Inviewoftheongoingpublicandpoliticaldebatesaboutclimatechange,theaimofthisdocumentistosummarisethecurrentscientificevidenceonclimatechangeanditsdrivers.Itlaysoutclearlywherethescienceiswellestablished,wherethereiswideconsensusbutcontinuingdebate,andwherethereremainssubstantialuncertainty.Theimpactsofclimatechange,asdistinctfromthecauses,arenotconsideredhere.ThisdocumentdrawsuponrecentevidenceandbuildsontheFourthAssessmentReportofWorkingGroupIoftheIntergovernmentalPanelonClimateChange(IPCC),publishedin2007,whichisthemostcomprehensivesourceofclimatescienceanditsuncertainties.

Climate change: a summary of the science I September 2010 I 1The Royal Society

The greenhouse effect4 TheSunistheprimarysourceofenergyfortheEarth’sclimate.Satelliteobservationsshowthatabout30%oftheSun’senergythatreachestheEarthisreflectedbacktospacebyclouds,gasesandsmallparticlesintheatmosphere,andbytheEarth’ssurface.Theremainder,about240Wattspersquaremetre(Wm-2)whenaveragedovertheplanet,isabsorbedbytheatmosphereandthesurface.

5 Tobalancetheabsorptionof240Wm-2fromtheSun,theEarth’ssurfaceandatmospheremustemitthesameamountofenergyintospace;theydosoasinfraredradiation.Onaveragethesurfaceemitssignificantlymorethan240Wm-2,buttheneteffectofabsorptionandemissionofinfraredradiationbyatmosphericgasesandcloudsistoreducetheamountreachingspaceuntilitapproximatelybalancestheincomingenergyfromtheSun.Thesurfaceisthuskeptwarmerthanitotherwisewouldbebecause,inadditiontotheenergyitreceivesfromtheSun,italsoreceivesinfraredenergyemittedbytheatmosphere.Thewarmingthatresultsfromthisinfraredenergyisknownasthegreenhouseeffect.

6 Measurementsfromthesurface,researchaircraftandsatellites,togetherwithlaboratoryobservationsandcalculations,showthat,inadditiontoclouds,thetwogasesmakingthelargestcontributiontothegreenhouseeffectarewatervapourfollowedbycarbondioxide(CO2).Therearesmallercontributionsfrommanyothergasesincludingozone,methane,nitrousoxideandhuman-madegasessuchasCFCs(chlorofluorocarbons).

Climate change7 Climatechangeonaglobalscale,whethernaturalorduetohumanactivity,canbeinitiatedbyprocessesthatmodifyeithertheamountofenergyabsorbedfromtheSun,ortheamountofinfraredenergyemittedtospace.

8 ClimatechangecanthereforebeinitiatedbychangesintheenergyreceivedfromtheSun,changesintheamountsorcharacteristicsofgreenhousegases,particlesandclouds,orchangesinthereflectivityoftheEarth’ssurface.Theimbalancebetweentheabsorbedandemittedradiationthatresultsfromthesechangeswillbereferredtohereas“climateforcing”(sometimesknownas“radiativeforcing”)andgiveninunitsofWm-2.Apositiveclimateforcingwilltendtocauseawarming,andanegativeforcingacooling.ClimatechangesacttorestorethebalancebetweentheenergyabsorbedfromtheSunandtheinfraredenergyemittedintospace.

9 Inprinciple,changesinclimateonawiderangeoftimescalescanalsoarisefromvariationswithintheclimatesystemdueto,forexample,interactionsbetweentheoceansandtheatmosphere;inthisdocument,thisisreferredtoas“internalclimate

Climate and climate change: some background science


variability”.Suchinternalvariabilitycanoccurbecausetheclimateisanexampleofachaoticsystem:onethatcanexhibitcomplexunpredictableinternalvariationsevenintheabsenceoftheclimateforcingsdiscussedinthepreviousparagraph.

10 Thereisverystrongevidencetoindicatethatclimatechangehasoccurredonawiderangeofdifferenttimescalesfromdecadestomanymillionsofyears;humanactivityisarelativelyrecentadditiontothelistofpotentialcausesofclimatechange.

11 TheshiftsbetweenglacialandinterglacialperiodsoverthepastfewmillionyearsarethoughttohavebeenaresponsetochangesinthecharacteristicsoftheEarth’sorbitaroundtheSun.WhiletheseledtoonlysmallchangesinthetotalenergyreceivedfromtheSun,theyledtosignificantchangesinitsgeographicalandseasonaldistribution.Thelargechangesinclimate,inmovinginandoutofglacialperiods,provideevidenceofthesensitivityofclimatetochangesintheEarth’senergybalance,whetherattributabletonaturalcausesortohumanactivity.

Mechanisms of global climate change12 Onceaclimateforcingmechanismhasinitiatedaclimateresponse,thisclimatechangecanleadtofurtherchanges;forexample,inresponsetoawarming,theamountofwatervapourisexpectedtoincrease,theextentofsnowandiceisexpectedtodecrease,andtheamountandpropertiesofcloudscouldalsochange.SuchchangescanfurthermodifytheamountofenergyabsorbedfromtheSun,ortheamountofenergyemittedbytheEarthanditsatmosphere,andleadtoeitherareductionoramplificationofclimatechange.

13 Theoveralleffectofthechangesresultingfromclimateforcingdetermineakeycharacteristicoftheclimatesystem,knownasthe“climatesensitivity”–thisistheamountofclimatechange(asmeasuredbytheequilibriumchangeinglobally-averagedsurfacetemperature)causedbyagivenamountofclimateforcing.Itisoftenquoted(aswillbethecasehere)asthetemperaturechangethateventuallyresultsfromadoublinginCO2concentrationssincepre-industrialtimes,andiscalculatedtocauseaclimateforcingofabout3.6Wm-2.

14 Thenatureoftheclimatesystemisdeterminedbyinteractionsbetweenthemovingatmosphereandoceans,thelandsurface,thelivingworldandthefrozenworld.Therateatwhichheatismovedfromthesurfacetotheoceandepthsisanimportantfactorindeterminingthespeedatwhichclimatecanchangeinresponsetoclimateforcing.

15Sincevariationsinclimatecanresultfrombothclimateforcingandinternalclimatevariability,thedetectionofforcedclimatechangeinobservationsisnotalwaysstraightforward.Furthermore,thedetectionofclimatechangeinobservations,beyondtheexpectedinternalclimatevariability,isnotthesameastheattributionofthatchange


toaparticularcauseorcauses.Attributionrequiresadditionalevidencetoprovideaquantitativelinkbetweentheproposedcauseandtheobservedclimatechange.

Modelling the climate system16 Currentunderstandingofthephysics(andincreasinglythechemistryandbiology)oftheclimatesystemisrepresentedinamathematicalforminclimatemodels,whichareusedtosimulatepastclimateandprovideprojectionsofpossiblefutureclimatechange.Climatemodelsarealsousedtoprovidequantitativeestimatestoassisttheattributionofobservedclimatechangetoaparticularcauseorcauses.

17Climatemodelsvaryconsiderablyincomplexity.Thesimplestcanbedescribedbyafewequations,andmayrepresenttheclimatebyglobal-averagesurfacetemperaturealone.Themostcomplicatedandcomputer-intensivemodelsrepresentmanydetailsoftheinteractionsbetweencomponentsoftheclimatesystem.Thesemorecomplexmodelsrepresentvariationsinparameterssuchastemperature,windandhumiditywithlatitude,longitudeandaltitudeintheatmosphere,andalsorepresentsimilarvariationsintheocean.Incomplexclimatemodelsclimatesensitivityemergesasanoutput;inthesimplercalculationsitisspecifiedeitherasaninputoritemergesasaconsequenceofsimplified(butplausible)assumptions.

18Byapplyingestablishedlawsoffluiddynamicsandthermodynamics,themorecomplexclimatemodelssimulatemanyimportantweatherphenomenathatdeterminetheclimate.However,limitationsofcomputerpowermeanthatthesemodelscannotdirectlyrepresentphenomenaoccurringatsmallscales.Forexample,individualcloudsarerepresentedbymoreapproximatemethods.Sincetherearevariouswaystomaketheseapproximations,therepresentationcanvaryinclimatemodelsdevelopedatdifferentclimateinstitutes.Theuseofthesedifferentapproximationsleadstoarangeofestimatesofclimatesensitivity,especiallybecauseofdifferencesbetweenmodelsintheresponseofcloudstoclimatechange.Thereareintensiveeffortstocomparethemodelswithobservationsandwitheachother.Thespreadofresultsfromthesemodelsgivesusefulinformationonthedegreeofconfidenceinthereliabilityofprojectionsofclimatechange.

19Unlikeweather-forecastmodels,climatemodelsdonotseektopredicttheactualweatheronaparticulardayataparticularlocation.Themorecomplexmodelsdohoweversimulateindividualweatherphenomena,suchasmid-latitudedepressionsandanticyclones,andaimtogivesimulationsofpossibleweathersequencesmuchfartherintothefuturethanweatherforecastmodels.Fromsuchsimulations,onecanderivethecharacteristicsofclimatelikelytooccurinfuturedecades,includingmeantemperatureandtemperatureextremes.


Changes in global-average surface temperature20 Measurementssuitableforshowinghowsurfacetemperaturehaschangedwithtimeacrosstheworldbecameavailablearound1850.Analysesofthesedata,inanumberofinstitutes,trytotakeintoaccountchangingdistributionsofmeasurements,changingobservationtechniques,andchangingsurroundingsofobservingstations(e.g.somestationsbecomemoreurbanwithtime,whichcanmakemeasurementsfromthemlessrepresentativeofwiderareas).

21Measurementsshowthataveragedovertheglobe,thesurfacehaswarmedbyabout0.8oC(withanuncertaintyofabout±0.2oC)since1850.Thiswarminghasnotbeengradual,buthasbeenlargelyconcentratedintwoperiods,fromaround1910toaround1940andfromaround1975toaround2000.Thewarmingperiodsarefoundinthreeindependenttemperaturerecordsoverland,overseaandinoceansurfacewater.Evenwithinthesewarmingperiodstherehasbeenconsiderableyear-to-yearvariability.Thewarminghasalsonotbeengeographicallyuniform–someregions,mostmarkedlythehigh-latitudenortherncontinents,haveexperiencedgreaterwarming;afewregionshaveexperiencedlittlewarming,orevenaslightcooling.

22Whenthesesurfacetemperaturesareaveragedoverperiodsofadecade,toremovesomeoftheyear-to-yearvariability,eachdecadesincethe1970shasbeenclearlywarmer(givenknownuncertainties)thantheoneimmediatelyprecedingit.Thedecade2000-2009was,globally,around0.15oCwarmerthanthedecade1990-1999.

23Localtemperaturesaregenerallyapoorguidetoglobalconditions.Forexample,acolder-than-averagewinterintheUKdoesnotmeanthatcolder-than-averageconditionsareexperiencedglobally.Similarly,observedvariationsinglobaltemperatureoveraperiodofjustafewyearscouldbeamisleadingguidetounderlyinglonger-termtrendsinglobaltemperature.

Other changes in climate24Notallaspectsoftheclimatesystemhavebeenobservedoveraslongaperiodassurfacetemperatures–forexample,thosebasedonsatelliteobservationsdateback,atbest,tothe1970s.Norarethemeasurementsofotheraspectsoftheclimatesystemalwaysofthesamequality.Collectively,however,theyprovidemuchevidenceofclimatechangeconsistentwiththesurfacetemperaturechanges.Thisincludesincreasesintheaveragetemperatureofboththeupper700moftheoceanandthetroposphere(theatmosphereupto10-18km),widespread(thoughnotuniversal)decreasesinthelengthofmountainglaciersandincreasesinaveragesealevel.Therehasbeenanoveralldeclineintheareacoveredbysea-icefloatingontheArcticOceanoverthepast30years(althoughtherehasbeenasmallincreaseintheareacoveredbysea-icearoundAntarctica).

Aspects of climate change on which there is wide agreement


Changes in atmospheric composition25Global-averageCO2concentrationshavebeenobservedtoincreasefromlevelsofaround280partspermillion(ppm)inthemid-19thcenturytoaround388ppmbytheendof2009.CO2concentrationscanbemeasuredin“ancientair”trappedinbubblesinice,deepbelowthesurfaceinAntarcticaandGreenland;theseshowthatpresent-dayconcentrationsarehigherthananythathavebeenobservedinthepast800,000years,whenCO2variedbetweenabout180and300ppm.Variouslinesofevidencepointstronglytohumanactivitybeingthemainreasonfortherecentincrease,mainlyduetotheburningoffossilfuels(coal,oil,gas)withsmallercontributionsfromland-usechangesandcementmanufacture.TheevidenceincludestheconsistencybetweencalculationsoftheemittedCO2andthatexpectedtohaveaccumulatedintheatmosphere,theanalysisoftheproportionsofdifferentCO2isotopes,andtheamountofoxygenintheair.

26TheseobservationsshowthatabouthalfoftheCO2emittedbyhumanactivitysincetheindustrialrevolutionhasremainedintheatmosphere.Theremainderhasbeentakenupbytheoceans,soilsandplantsalthoughtheexactamountgoingtoeachoftheseindividuallyislesswellknown.

27Concentrationsofmanyothergreenhousegaseshaveincreased.Theconcentrationofmethanehasmorethandoubledinthepast150years;thisrecentandrapidincreaseisunprecedentedinthe800,000yearrecordandevidencestronglysuggeststhatitarisesmainlyasaresultofhumanactivity.

Climate forcing by greenhouse gas changes28Changesinatmosphericcompositionresultingfromhumanactivityhaveenhancedthenaturalgreenhouseeffect,causingapositiveclimateforcing.Calculations,whicharesupportedbylaboratoryandatmosphericmeasurements,indicatethattheseadditionalgaseshavecausedaclimateforcingduringtheindustrialeraofaround2.9Wm-2,withanuncertaintyofabout±0.2Wm-2.Otherclimatechangemechanismsresultingfromhumanactivityaremoreuncertain(seelater);calculationsthattakeintoaccounttheseotherpositiveandnegativeforcings(includingtheroleofatmosphericparticles)indicatethattheneteffectofallhumanactivityhascausedapositiveclimateforcingofaround1.6Wm-2withanestimateduncertaintyofabout±0.8Wm-2.

29Applicationofestablishedphysicalprinciplesshowsthat,evenintheabsenceofprocessesthatamplifyorreduceclimatechange(seeparagraphs12&13),theclimatesensitivitywouldbearound1oC,foradoublingofCO2concentrations.Aclimateforcingof1.6Wm-2(seepreviousparagraph)would,inthishypotheticalcase,leadtoaglobally-averagedsurfacewarmingofabout0.4oC.However,aswillbediscussedinparagraph


36,itisexpectedthattheactualchange,afteraccountingfortheadditionalprocesses,willbegreaterthanthis.

Carbon dioxide and climate30EvidencefromicecoresindicatesanactiveroleforCO2intheclimatesystem.Thisisbecausetheamountofcarbonheldinoceans,soilsandplantsdependsontemperatureandotherconditions.Inotherwords,changesinCO2 can lead to climate change and climatechangecanalsoaltertheconcentrationsofCO2.


Aspects of climate change where there is a wide consensus but continuing debate and discussionThe carbon cycle and climate31 OnceatmosphericCO2concentrationsareincreased,carboncyclemodels(whichsimulatetheexchangeofcarbonbetweentheatmosphere,oceans,soilsandplants)indicatethatitwouldtakeaverylongtimeforthatincreasedCO2todisappear;thisismainlyduetowell-knownchemicalreactionsintheocean.CurrentunderstandingindicatesthateveniftherewasacompletecessationofemissionsofCO2todayfromhumanactivity,itwouldtakeseveralmillenniaforCO2concentrationstoreturntopre-industrialconcentrations.

Other drivers of global climate change32Inadditiontochangesingreenhousegasconcentrations,therearealargenumberoflesswellcharacterisedcontributionstoclimateforcing,bothnaturalandhuman-induced.

33Volcaniceruptionsareexamplesofanaturalclimateforcingmechanism.Anindividualvolcaniceruptionhasitslargesteffectsontheclimateforonlyafewyearsaftertheeruption;theseeffectsaredependentonthelocation,sizeandtypeoftheeruption.

34NaturalforcingduetosustainedvariationsintheenergyemittedbytheSunoverthepast150yearsisestimatedtobesmall(about0.12Wm-2);however,directobservationsoftheenergyemittedbytheSunonlybecameavailableinthe1970sandestimatesoverlongerperiodsrelyonobservationsofchangesinothercharacteristicsoftheSun.Anumberofmechanismshavebeenproposedthatcouldreduceoramplifytheeffectofsolarvariations;theseremainareasofactiveresearch.

35Humanactivityresultsinemissionsofmanyshort-livedgases(suchascarbonmonoxideandsulphurdioxide)andparticlesintotheatmosphere.Theseaffecttheatmosphericconcentrationsofotherclimate-importantgasessuchasozone,andotherparticleswhichleadtoaclimateforcing.Calculations,coupledtoavarietyofatmosphericobservations,indicatethatparticleshavecausedanegativeclimateforcingofaround0.5Wm-2withanuncertaintyof±0.2Wm-2.Theparticlesalsodirectlyinfluencecloudproperties;thismoreuncertaineffectisdiscussedinparagraph47.

Climate sensitivity36Themorecomplexclimatemodels,supportedbyobservations,allowclimatesensitivitytobecalculatedinthepresenceofprocessesthatamplifyorreducethesizeoftheclimateresponse.Increasesinwatervapouralone,inresponsetowarming,areestimatedtoapproximatelydoubletheclimatesensitivityfromitsvalueintheabsence


ofamplifyingprocesses.Thereneverthelessremainuncertaintiesinhowmuchwatervapouramountswillchange,andhowthesechangeswillbedistributedintheatmosphere,inresponsetoawarming.Climatemodelsindicatethattheoverallclimatesensitivity(forahypotheticaldoublingofCO2intheatmosphere)islikelytolieintherange2oCto4.5oC;thisrangeismainlyduetothedifficultiesinsimulatingtheoveralleffectoftheresponseofcloudstoclimatechangementionedearlier.

Attribution of climate change37Thesizeandsustainednatureoftheobservedglobal-averagesurfacewarmingondecadalandlongertimescalesgreatlyexceedstheinternalclimatevariabilitysimulatedbythecomplexclimatemodels.Unlessthisvariabilityhasbeengrosslyunderestimated,theobservedclimatechangemustresultfromnaturaland/orhuman-inducedclimateforcing.

38Whenonlynaturalclimateforcingsareputintoclimatemodels,themodelsareincapableofreproducingthesizeoftheobservedincreaseinglobal-averagesurfacetemperaturesoverthepast50years.However,whenthemodelsincludeestimatesofforcingsresultingfromhumanactivity,theycanreproducetheincrease.Thesameresultisfound,albeitwithagreaterspreadbetweendifferentmodels,forthesimulationofobservedsurfacetemperaturechangesforeachofthehabitablecontinentsseparately.

39Whenknownuncertaintiesinbothobservedtrendsandclimatemodelsaretakenintoaccount,theobservedverticalandlatitudinalvariationsoftemperaturechangearealsobroadlyconsistentwiththoseexpectedfromadominantroleforhumanactivity.Thereisanongoingcontroversyconcerningwhetherornottheincreasedwarmingwithheightinthetropicalregionsgivenbyclimatemodelsissupportedbysatellitemeasurements.

Future climate change40Aswithalmostanyattemptstoforecastfutureconditions,projectionsoffutureclimatechangedependonanumberoffactors.Futureemissionsduetohumanactivitywilldependonsocial,technologicalandpopulationchangeswhichcannotbeknownwithconfidence.Theunderlyinguncertaintiesinclimatescienceandtheinabilitytopredictpreciselythesizeoffuturenaturalclimateforcingmechanismsmeanthatprojectionsmustbemadewhichtakeintoaccounttherangeofuncertaintiesacrossthesedifferentareas.

41The2007assessmentbytheIntergovernmentalPanelonClimateChange(IPCC)madeprojectionsoffuturechangesusinganumberofpossiblescenariosoffutureemissions,


basedonadiverserangeofassumptions.TheIPCC’sbestestimatewasthatglobally-averagedsurfacetemperatureswouldbebetween2.5-4.7oChigherby2100comparedtopre-industriallevels.Thefullrangeofprojectedtemperatureincreasesby2100wasfoundtobe1.8-7.1oCbasedonthevariousscenariosanduncertaintiesinclimatesensitivity.

42Evenintheextremelyunlikelyeventthatthereisnofurtherincreaseinclimateforcing,afurtherwarmingwouldbeexpectedtooccurastheoceansslowlyrespondtotheexistingclimateforcing,amountingtoafurtherfewtenthsofadegreecentigradebytheyear2100.

43Theuncertaintyinthepredictedwarmingasaresultofhumanactivityoverthenexttwodecadesissmaller,therangebeing0.2to0.4oCperdecade.Ontheseshortertime-scales,theactualchangecouldhoweverbereducedorenlargedsignificantlybyinternalclimatevariabilityandnaturalclimateforcings.

44 Increasesintemperaturearepredictedtobelargeronland,particularlyonthenortherncontinentsinwinter.Lesswarmingispredicted,forexample,overtheNorthAtlanticOcean.Climatemodelstendtopredictthatprecipitationwillgenerallyincreaseinareaswithalreadyhighamountsofprecipitationandgenerallydecreaseinareaswithlowamountsofprecipitation.

45Becauseofthethermalexpansionoftheocean,itisverylikelythatformanycenturiestherateofglobalsea-levelrisewillbeatleastaslargeastherateof20cmpercenturythathasbeenobservedoverthepastcentury.Paragraph49discussestheadditional,butmoreuncertain,contributiontosea-levelrisefromthemeltingoflandice.


46 Observationsarenotyetgoodenoughtoquantify,withconfidence,someaspectsoftheevolutionofeitherclimateforcingorclimatechange,orforhelpingtoplacetightboundsontheclimatesensitivity.Observationsofsurfacetemperaturechangebefore1850arealsoscarce.

47 Asnotedabove,projectionsofclimatechangearesensitivetothedetailsoftherepresentationofcloudsinmodels.Particlesoriginatingfrombothhumanactivitiesandnaturalsourceshavethepotentialtostronglyinfluencethepropertiesofclouds,withconsequencesforestimatesofclimateforcing.Currentscientificunderstandingofthiseffectispoor.

48Additionalmechanismsthatinfluenceclimatesensitivityhavebeenidentified,includingtheresponseofthecarboncycletoclimatechange,forexamplethelossoforganiccarboncurrentlystoredinsoils.Theneteffectofchangesinthecarboncycleinallcurrentmodelsistoincreasewarming,byanamountthatvariesconsiderablyfrommodeltomodelbecauseofuncertaintiesinhowtorepresenttherelevantprocesses.ThefuturestrengthoftheuptakeofCO2bythelandandoceans(whichtogetherarecurrentlyresponsiblefortakingupabouthalfoftheemissionsfromhumanactivity–seeparagraph26)isverypoorlyunderstood,particularlybecauseofgapsinourunderstandingoftheresponseofbiologicalprocessestochangesinbothCO2 concentrationsandclimate.

49ThereiscurrentlyinsufficientunderstandingoftheenhancedmeltingandretreatoftheicesheetsonGreenlandandWestAntarcticatopredictexactlyhowmuchtherateofsealevelrisewillincreaseabovethatobservedinthepastcentury(seeparagraph45)foragiventemperatureincrease.Similarly,thepossibilityoflargechangesinthecirculationoftheNorthAtlanticOceancannotbeassessedwithconfidence.ThelatterlimitstheabilitytopredictwithconfidencewhatchangesinclimatewilloccurinWesternEurope.

50Theabilityofthecurrentgenerationofmodelstosimulatesomeaspectsofregionalclimatechangeislimited,judgingfromthespreadofresultsfromdifferentmodels;thereislittleconfidenceinspecificprojectionsoffutureregionalclimatechange,exceptatcontinentalscales.

Aspects that are not well understood


Developments in climate science51Climatechangesciencehasadvancedmarkedlyoverthepast20years,asaresultofmanyfactors.Theseincludeimprovedmethodsforhandlinglong-termclimatedatasets,theever-lengtheningrecordofclimateobservations,improvedmeasurementtechniques,includingthosefromsatellites,betterunderstandingoftheclimatesystem,improvedmethodsforsimulatingtheclimatesystem,andincreasedcomputerpower.

52Oneindicationoftheseadvancesistheincreasingdegreeofconfidenceintheattributionofclimatechangetohumanactivity,asexpressedinthekeyconclusionsofIPCCWorkingGroup1(WG1)initsassessments.

53Initsfirstassessmentpublishedin1990,IPCCWG1concludedthatthe“sizeof[theobservedsurface]warmingisbroadlyconsistentwithpredictionsofclimatemodels[whichsimulatetheimpactofhumanactivity],butitisalsoofthesamemagnitudeasnaturalclimatevariability”.Itssecondassessment,publishedin1995,concludedthat“thebalanceofevidencesuggeststhatthereisadiscerniblehumaninfluenceonclimatechange”.Itsfourth,andmostrecent,assessment,publishedin2007,concludedthat“mostoftheobservedincreaseingloballyaverage[surface]temperaturessincethemid-20thcenturyisverylikelyduetotheobservedincreaseinanthropogenicgreenhousegasconcentrations”.

54Remaininguncertaintiesarethesubjectofongoingresearchworldwide.Someuncertaintiesareunlikelyevertobesignificantlyreduced,becauseof,forexample,thelackofobservationsofpastchangesrelevanttosomeaspectsofbothclimateforcingandclimatechange.

55 Otheruncertaintiesmaystarttoberesolved.Forexample,satellitesnowincorporateimprovedtechniquestomeasurecloudcharacteristicsacrosstheglobe.Usingclimatemodelsforday-to-dayweatherpredictionwillenable,forexample,identificationoferrorsintherepresentationofcloudsinmodels;anysucherrorswillleadtoerrorsinforecastsofmaximumandminimumtemperatures(whichareeasilyobserved).Newhigh-performancecomputerswillallowclimatemodelstorepresentsomesmaller-scalephenomena(includingcloudsystemsanddetailsoftropicalstorms)directly,andareexpectedtoimproveconfidenceinregionalpredictions.

56 Thereremainsthepossibilitythathithertounknownaspectsoftheclimateandclimatechangecouldemergeandleadtosignificantmodificationsinourunderstanding.


57Thereisstrongevidencethatchangesingreenhousegasconcentrationsduetohumanactivityarethedominantcauseoftheglobalwarmingthathastakenplaceoverthelasthalfcentury.Thiswarmingtrendisexpectedtocontinueasarechangesinprecipitationoverthelongterminmanyregions.Furtherandmorerapidincreasesinsealevelarelikelywhichwillhaveprofoundimplicationsforcoastalcommunitiesandecosystems.

58ItisnotpossibletodetermineexactlyhowmuchtheEarthwillwarmorexactlyhowtheclimatewillchangeinthefuture,butcarefulestimatesofpotentialchangesandassociateduncertaintieshavebeenmade.Scientistscontinuetoworktonarrowtheseareasofuncertainty.Uncertaintycanworkbothways,sincethechangesandtheirimpactsmaybeeithersmallerorlargerthanthoseprojected.

59Likemanyimportantdecisions,policychoicesaboutclimatechangehavetobemadeintheabsenceofperfectknowledge.Eveniftheremaininguncertaintiesweresubstantiallyresolved,thewidevarietyofinterests,culturesandbeliefsinsocietywouldmakeconsensusaboutsuchchoicesdifficulttoachieve.However,thepotentialimpactsofclimatechangearesufficientlyseriousthatimportantdecisionswillneedtobemade.Climatescience–includingthesubstantialbodyofknowledgethatisalreadywellestablished,andtheresultsoffutureresearch–istheessentialbasisforfutureclimateprojectionsandplanning,andmustbeavitalcomponentofpublicreasoninginthiscomplexandchallengingarea.

Concluding remarks


Background readingExtensivebackgroundreferencestothescientificliterature,andsummariesthereof,canbefoundinthefollowingtwodocuments.

ContributionofWorkingGroupItotheFourthAssessmentReportoftheIntergovernmentalPanelonClimateChange,2007,Solomon,S.,D.Qin,M.Manning, Z.Chen,M.Marquis,K.B.Averyt,M.TignorandH.L.Miller(eds.),CambridgeUniversityPress,Cambridge,UnitedKingdomandNewYork,NY,USA. ipcc.ch/publications_and_data/ar4/wg1/en/contents.html

AdvancingtheScienceofClimateChange,NationalResearchCouncil,2010. americasclimatechoices.org/panelscience.shtml


Acknowledgements

Professor John Pethica FRS (Chair) PhysicalSecretaryofRoyalSociety

Ms Fiona Fox Director,ScienceMediaCentre,UK

Sir Brian Hoskins FRS DirectorGranthamInstituteforClimateChange,ImperialCollege,UK

Professor Michael Kelly FRS ProfessorofTechnology,UniversityofCambridge,UK

Professor John Mitchell FRS DirectorofClimateScience,MetOffice,UK

Professor Susan Owens ProfessorofEnvironmentandPolicy,UniversityofCambridge,UK

Professor Tim Palmer FRS RoyalSocietyResearchProfessor,UniversityofOxford,UK

Professor John Shepherd FRS ProfessorialResearchFellowinEarthSystemScience,University ofSouthampton,UK

Professor Keith Shine FRS ProfessorofPhysicalMeteorology,UniversityofReading,UK

Professor David Spiegelhalter FRS ProfessorofthePublicUnderstandingofRisk,UniversityofCambridge,UK

Royal Society Science Policy Centre

Rachel Garthwaite SeniorPolicyAdviser

Stuart Leckie PolicyAdviser

Tony McBride HeadofStrategy

Working GroupTheRoyalSocietywouldliketoacknowledgethemembersoftheworkinggroupthatproducedthisdocument:


Dame Jean Thomas FRS (Chair) BiologicalSecretaryoftheRoyalSociety

Professor Alastair Fitter FRS Pro-Vice-ChancellorforResearch,UniversityofYork,UK

Dame Louise Johnson FRS SeniorFellowatDiamondLightSources,UK

Professor Brian Launder FRS ResearchProfessor,UniversityofManchester,UK

Professor John Pyle FRS CoDirector,CentreforAtmosphericScience,UniversityofCambridge,UK

Professor Chris Rapley Director,ScienceMuseum,UK

Sir Alan Wilson FRS ProfessorofUrbanandRegionalSystems,UniversityCollegeLondon,UK

Professor David Fowler FRS CentreforEcologyandHydrology,Edinburgh,UK

Professor Joanna Haigh ProfessorofAtmosphericPhysics,ImperialCollege,London,UK

Professor Cyril Hilsum FRS CorporateResearchAdvisor/VisitingProfessorofPhysics,UniversityCollegeLondon,UK

Professor Anthony Kelly FRS DistinguishedResearchFellowintheDepartmentofMaterialsScienceandMetallurgy,UniversityofCambridge,UK

Professor John McWhirter FRS DistinguishedResearchProfessorattheSchoolofEngineering,CardiffUniversity,UK

Sir John Pendry FRS TheBlackettLaboratory,ImperialCollege,London,UK

Sir Alan Rudge FRS Chairman,ERAFoundationLimited,UK

Sir William Stewart FRS FormerChiefScientificAdvisor(UKGovernment)

Professor Andrew Watson FRS RoyalSociety2010AnniversaryResearchProfessor,UniversityofEastAnglia,UK

Sir Arnold Wolfendale FRS EmeritusProfessorofPhysics,UniversityofDurham,UK

Professor Carl Wunsch ForMemRS CecilandIdaGreenProfessorofPhysicalOceanography,MassachusettsInstituteofTechnology,USA

Review PanelTheRoyalSocietygratefullyacknowledgesthecontributionoftheindependentpanelofreviewers.Thereviewpanelmemberswerenotaskedtoendorsethefinaldocument’sconclusions.

ContributorsTheRoyalSocietygratefullyacknowledgesthecontributionofindividualswhohavecommentedonthedocumentatearlierstagesofitspreparation.Theseindividualswerenotaskedtoendorsethedocument.


For further information

The Royal Society Science Policy Centre 6–9 Carlton House Terrace London SW1Y 5AG

T +44 (0)20 7451 2500 F +44 (0)20 7451 2692 E [email protected] W royalsociety.org

The Royal Society

The Royal Society is a Fellowship of more than 1400 outstanding individuals from all areas of science, mathematics, engineering and medicine, who form a global scientific network of the highest calibre. The Fellowship is supported by over 130 permanent staff with responsibility for the day-to-day management of the Society and its activities. The Society encourages public debate on key issues involving science, engineering and medicine, and the use of high quality scientific advice in policy-making. We are committed to delivering the best independent expert advice, drawing upon the experience of the Society’s Fellows and Foreign Members, the wider scientific community and relevant stakeholders.

In our 350th anniversary year and beyond we are working to achieve five strategic priorities:

• Invest in future scientific leaders and in innovation

• Influence policymaking with the best scientific advice

• Invigorate science and mathematics education

• Increase access to the best science internationally

• Inspire an interest in the joy, wonder and excitement of scientific discovery

Issued: September 2010 DES1929

Founded in 1660, the Royal Society is the independent scientific academy of the UK, dedicated to promoting excellence in science

Registered Char ty No 207043

Cover: Artists image of CryoSat-2, ESA, C. Vijoux. The aim of the CryoSat mission is to determine variations in the thickness of the Earth’s marine ice cover and understand the extent to which the Antarctic and Greenland ice sheets are contributing to global sea level rise. The CryoSat-2 satellite was launched on 8 April 2010 and will contribute to improved understanding of how the c imate is changing. © ESA - C. Vijoux.

Documents

Climate change: A Summary of the Science/media/Royal_Society_Content/policy/... · This is the amount of climate change (as measured by the change in globally-averaged surface temperature)