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8/8/2019 Ultra Elitist Environmental Group Forced to Admit Doubts on Global Warming Science
1/19
Climate change:a summary of the scienceSeptember 2010
8/8/2019 Ultra Elitist Environmental Group Forced to Admit Doubts on Global Warming Science
2/19
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 cycle
The 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 Earths 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 Earths surface. Hence climate forcing can result from both human
activity and natural causes. It is given in units of Wm-2.
Climate sensitivity
This 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).
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Introduction1 Changesinclimatehavesignicantimplicationsforpresentlives,forfuturegenerations
andforecosystemsonwhichhumanitydepends.Consequently,climatechangehas
beenandcontinuestobethesubjectofintensivescienticresearchandpublicdebate.
2 ThereisstrongevidencethatthewarmingoftheEarthoverthelasthalf-centuryhas
beencausedlargelybyhumanactivity,suchastheburningoffossilfuelsandchanges
inlanduse,includingagricultureanddeforestation.Thesizeoffuturetemperature
increasesandotheraspectsofclimatechange,especiallyattheregionalscale,arestill
subjecttouncertainty.Nevertheless,therisksassociatedwithsomeofthesechanges
aresubstantial.Itisimportantthatdecisionmakershaveaccesstoclimatescienceof
thehighestquality,andcantakeaccountofitsndingsinformulatingappropriate
responses.
3 Inviewoftheongoingpublicandpoliticaldebatesaboutclimatechange,theaimofthis
documentistosummarisethecurrentscienticevidenceonclimatechangeandits
drivers.Itlaysoutclearlywherethescienceiswellestablished,wherethereiswide
consensusbutcontinuingdebate,andwherethereremainssubstantialuncertainty.The
impactsofclimatechange,asdistinctfromthecauses,arenotconsideredhere.This
documentdrawsuponrecentevidenceandbuildsontheFourthAssessmentReportof
WorkingGroupIoftheIntergovernmentalPanelonClimateChange(IPCC),publishedin
2007,whichisthemostcomprehensivesourceofclimatescienceanditsuncertainties.
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The greenhouse effect
4
TheSunistheprimarysourceofenergyfortheEarthsclimate.Satelliteobservationsshowthatabout30%oftheSunsenergythatreachestheEarthisreectedbackto
spacebyclouds,gasesandsmallparticlesintheatmosphere,andbytheEarths
surface.Theremainder,about240Wattspersquaremetre(Wm-2)whenaveragedover
theplanet,isabsorbedbytheatmosphereandthesurface.
5 Tobalancetheabsorptionof240Wm-2fromtheSun,theEarthssurfaceand
atmospheremustemitthesameamountofenergyintospace;theydosoasinfrared
radiation.Onaveragethesurfaceemitssignicantlymorethan240Wm-2,butthenet
effectofabsorptionandemissionofinfraredradiationbyatmosphericgasesandclouds
istoreducetheamountreachingspaceuntilitapproximatelybalancestheincoming
energyfromtheSun.Thesurfaceisthuskeptwarmerthanitotherwisewouldbe
because,inadditiontotheenergyitreceivesfromtheSun,italsoreceivesinfrared
energyemittedbytheatmosphere.Thewarmingthatresultsfromthisinfraredenergyis
knownasthegreenhouseeffect.
6 Measurementsfromthesurface,researchaircraftandsatellites,togetherwith
laboratoryobservationsandcalculations,showthat,inadditiontoclouds,thetwo
gasesmakingthelargestcontributiontothegreenhouseeffectarewatervapour
followedbycarbondioxide(CO2).Therearesmallercontributionsfrommanyother
gasesincludingozone,methane,nitrousoxideandhuman-madegasessuchasCFCs
(chlorouorocarbons).
Climate change
7 Climatechangeonaglobalscale,whethernaturalorduetohumanactivity,canbe
initiatedbyprocessesthatmodifyeithertheamountofenergyabsorbedfromtheSun,
ortheamountofinfraredenergyemittedtospace.
8 Climatechangecanthereforebeinitiatedbychangesintheenergyreceivedfromthe
Sun,changesintheamountsorcharacteristicsofgreenhousegases,particlesand
clouds,orchangesinthereectivityoftheEarthssurface.Theimbalancebetweenthe
absorbedandemittedradiationthatresultsfromthesechangeswillbereferredtohere
asclimateforcing(sometimesknownasradiativeforcing)andgiveninunitsof
Wm-2.Apositiveclimateforcingwilltendtocauseawarming,andanegativeforcinga
cooling.Climatechangesacttorestorethebalancebetweentheenergyabsorbedfrom
theSunandtheinfraredenergyemittedintospace.
9 Inprinciple,changesinclimateonawiderangeoftimescalescanalsoarisefrom
variationswithintheclimatesystemdueto,forexample,interactionsbetweenthe
oceansandtheatmosphere;inthisdocument,thisisreferredtoasinternalclimate
Climate and climate change:some background science
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variability.Suchinternalvariabilitycanoccurbecausetheclimateisanexampleofa
chaoticsystem:onethatcanexhibitcomplexunpredictableinternalvariationsevenin
theabsenceoftheclimateforcingsdiscussedinthepreviousparagraph.
10 Thereisverystrongevidencetoindicatethatclimatechangehasoccurredonawide
rangeofdifferenttimescalesfromdecadestomanymillionsofyears;humanactivityis
arelativelyrecentadditiontothelistofpotentialcausesofclimatechange.
11 Theshiftsbetweenglacialandinterglacialperiodsoverthepastfewmillionyearsare
thoughttohavebeenaresponsetochangesinthecharacteristicsoftheEarthsorbit
aroundtheSun.Whiletheseledtoonlysmallchangesinthetotalenergyreceivedfrom
theSun,theyledtosignicantchangesinitsgeographicalandseasonaldistribution.Thelargechangesinclimate,inmovinginandoutofglacialperiods,provideevidence
ofthesensitivityofclimatetochangesintheEarthsenergybalance,whether
attributabletonaturalcausesortohumanactivity.
Mechanisms of global climate change
12 Onceaclimateforcingmechanismhasinitiatedaclimateresponse,thisclimatechange
canleadtofurtherchanges;forexample,inresponsetoawarming,theamountof
watervapourisexpectedtoincrease,theextentofsnowandiceisexpectedto
decrease,andtheamountandpropertiesofcloudscouldalsochange.Suchchanges
canfurthermodifytheamountofenergyabsorbedfromtheSun,ortheamountof
energyemittedbytheEarthanditsatmosphere,andleadtoeitherareductionor
amplicationofclimatechange.
13 Theoveralleffectofthechangesresultingfromclimateforcingdetermineakey
characteristicoftheclimatesystem,knownastheclimatesensitivitythisisthe
amountofclimatechange(asmeasuredbytheequilibriumchangeinglobally-averaged
surfacetemperature)causedbyagivenamountofclimateforcing.Itisoftenquoted(as
willbethecasehere)asthetemperaturechangethateventuallyresultsfromadoublinginCO2concentrationssincepre-industrialtimes,andiscalculatedtocauseaclimate
forcingofabout3.6Wm-2.
14 Thenatureoftheclimatesystemisdeterminedbyinteractionsbetweenthemoving
atmosphereandoceans,thelandsurface,thelivingworldandthefrozenworld.The
rateatwhichheatismovedfromthesurfacetotheoceandepthsisanimportantfactor
indeterminingthespeedatwhichclimatecanchangeinresponsetoclimateforcing.
15Sincevariationsinclimatecanresultfrombothclimateforcingandinternalclimatevariability,thedetectionofforcedclimatechangeinobservationsisnotalways
straightforward.Furthermore,thedetectionofclimatechangeinobservations,beyond
theexpectedinternalclimatevariability,isnotthesameastheattributionofthatchange
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toaparticularcauseorcauses.Attributionrequiresadditionalevidencetoprovidea
quantitativelinkbetweentheproposedcauseandtheobservedclimatechange.
Modelling the climate system
16 Currentunderstandingofthephysics(andincreasinglythechemistryandbiology)of
theclimatesystemisrepresentedinamathematicalforminclimatemodels,whichare
usedtosimulatepastclimateandprovideprojectionsofpossiblefutureclimatechange.
Climatemodelsarealsousedtoprovidequantitativeestimatestoassisttheattribution
ofobservedclimatechangetoaparticularcauseorcauses.
17Climatemodelsvaryconsiderablyincomplexity.Thesimplestcanbedescribedbyafew
equations,andmayrepresenttheclimatebyglobal-averagesurfacetemperaturealone.
Themostcomplicatedandcomputer-intensivemodelsrepresentmanydetailsofthe
interactionsbetweencomponentsoftheclimatesystem.Thesemorecomplexmodels
representvariationsinparameterssuchastemperature,windandhumiditywith
latitude,longitudeandaltitudeintheatmosphere,andalsorepresentsimilarvariations
intheocean.Incomplexclimatemodelsclimatesensitivityemergesasanoutput;inthe
simplercalculationsitisspeciedeitherasaninputoritemergesasaconsequenceof
simplied(butplausible)assumptions.
18Byapplyingestablishedlawsofuiddynamicsandthermodynamics,themorecomplex
climatemodelssimulatemanyimportantweatherphenomenathatdeterminethe
climate.However,limitationsofcomputerpowermeanthatthesemodelscannot
directlyrepresentphenomenaoccurringatsmallscales.Forexample,individualclouds
arerepresentedbymoreapproximatemethods.Sincetherearevariouswaystomake
theseapproximations,therepresentationcanvaryinclimatemodelsdevelopedat
differentclimateinstitutes.Theuseofthesedifferentapproximationsleadstoarangeof
estimatesofclimatesensitivity,especiallybecauseofdifferencesbetweenmodelsinthe
responseofcloudstoclimatechange.Thereareintensiveeffortstocomparethemodelswithobservationsandwitheachother.Thespreadofresultsfromthesemodels
givesusefulinformationonthedegreeofcondenceinthereliabilityofprojectionsof
climatechange.
19Unlikeweather-forecastmodels,climatemodelsdonotseektopredicttheactual
weatheronaparticulardayataparticularlocation.Themorecomplexmodelsdo
howeversimulateindividualweatherphenomena,suchasmid-latitudedepressionsand
anticyclones,andaimtogivesimulationsofpossibleweathersequencesmuchfarther
intothefuturethanweatherforecastmodels.Fromsuchsimulations,onecanderive
thecharacteristicsofclimatelikelytooccurinfuturedecades,includingmean
temperatureandtemperatureextremes.
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Changes in global-average surface temperature
20
Measurementssuitableforshowinghowsurfacetemperaturehaschangedwithtimeacrosstheworldbecameavailablearound1850.Analysesofthesedata,inanumberof
institutes,trytotakeintoaccountchangingdistributionsofmeasurements,changing
observationtechniques,andchangingsurroundingsofobservingstations(e.g.some
stationsbecomemoreurbanwithtime,whichcanmakemeasurementsfromthemless
representativeofwiderareas).
21Measurementsshowthataveragedovertheglobe,thesurfacehaswarmedbyabout
0.8oC(withanuncertaintyofabout0.2oC)since1850.Thiswarminghasnotbeen
gradual,buthasbeenlargelyconcentratedintwoperiods,fromaround1910toaround
1940andfromaround1975toaround2000.Thewarmingperiodsarefoundinthree
independenttemperaturerecordsoverland,overseaandinoceansurfacewater.Even
withinthesewarmingperiodstherehasbeenconsiderableyear-to-yearvariability.The
warminghasalsonotbeengeographicallyuniformsomeregions,mostmarkedlythe
high-latitudenortherncontinents,haveexperiencedgreaterwarming;afewregions
haveexperiencedlittlewarming,orevenaslightcooling.
22Whenthesesurfacetemperaturesareaveragedoverperiodsofadecade,toremove
someoftheyear-to-yearvariability,eachdecadesincethe1970shasbeenclearly
warmer(givenknownuncertainties)thantheoneimmediatelyprecedingit.Thedecade
2000-2009was,globally,around0.15oCwarmerthanthedecade1990-1999.
23Localtemperaturesaregenerallyapoorguidetoglobalconditions.Forexample,a
colder-than-averagewinterintheUKdoesnotmeanthatcolder-than-average
conditionsareexperiencedglobally.Similarly,observedvariationsinglobaltemperature
overaperiodofjustafewyearscouldbeamisleadingguidetounderlyinglonger-term
trendsinglobaltemperature.
Other changes in climate24Notallaspectsoftheclimatesystemhavebeenobservedoveraslongaperiodas
surfacetemperaturesforexample,thosebasedonsatelliteobservationsdateback,at
best,tothe1970s.Norarethemeasurementsofotheraspectsoftheclimatesystem
alwaysofthesamequality.Collectively,however,theyprovidemuchevidenceof
climatechangeconsistentwiththesurfacetemperaturechanges.Thisincludes
increasesintheaveragetemperatureofboththeupper700moftheoceanandthe
troposphere(theatmosphereupto10-18km),widespread(thoughnotuniversal)
decreasesinthelengthofmountainglaciersandincreasesinaveragesealevel.Therehasbeenanoveralldeclineintheareacoveredbysea-iceoatingontheArcticOcean
overthepast30years(althoughtherehasbeenasmallincreaseintheareacoveredby
sea-icearoundAntarctica).
Aspects of climate change onwhich there is wide agreement
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Changes in atmospheric composition
25
Global-averageCO2concentrationshavebeenobservedtoincreasefromlevelsofaround280partspermillion(ppm)inthemid-19thcenturytoaround388ppmbythe
endof2009.CO2concentrationscanbemeasuredinancientairtrappedinbubblesin
ice,deepbelowthesurfaceinAntarcticaandGreenland;theseshowthatpresent-day
concentrationsarehigherthananythathavebeenobservedinthepast800,000years,
whenCO2variedbetweenabout180and300ppm.Variouslinesofevidencepoint
stronglytohumanactivitybeingthemainreasonfortherecentincrease,mainlydueto
theburningoffossilfuels(coal,oil,gas)withsmallercontributionsfromland-use
changesandcementmanufacture.Theevidenceincludestheconsistencybetween
calculationsoftheemittedCO2andthatexpectedtohaveaccumulatedintheatmosphere,theanalysisoftheproportionsofdifferentCO2isotopes,andtheamount
ofoxygenintheair.
26TheseobservationsshowthatabouthalfoftheCO2emittedbyhumanactivitysincethe
industrialrevolutionhasremainedintheatmosphere.Theremainderhasbeentakenup
bytheoceans,soilsandplantsalthoughtheexactamountgoingtoeachofthese
individuallyislesswellknown.
27Concentrationsofmanyothergreenhousegaseshaveincreased.Theconcentrationof
methanehasmorethandoubledinthepast150years;thisrecentandrapidincreaseis
unprecedentedinthe800,000yearrecordandevidencestronglysuggeststhatitarises
mainlyasaresultofhumanactivity.
Climate forcing by greenhouse gas changes
28Changesinatmosphericcompositionresultingfromhumanactivityhaveenhancedthe
naturalgreenhouseeffect,causingapositiveclimateforcing.Calculations,whichare
supportedbylaboratoryandatmosphericmeasurements,indicatethattheseadditional
gaseshavecausedaclimateforcingduringtheindustrialeraofaround2.9Wm-2
,withanuncertaintyofabout0.2Wm-2.Otherclimatechangemechanismsresultingfrom
humanactivityaremoreuncertain(seelater);calculationsthattakeintoaccountthese
otherpositiveandnegativeforcings(includingtheroleofatmosphericparticles)indicate
thattheneteffectofallhumanactivityhascausedapositiveclimateforcingofaround
1.6Wm-2withanestimateduncertaintyofabout0.8Wm-2.
29Applicationofestablishedphysicalprinciplesshowsthat,evenintheabsenceof
processesthatamplifyorreduceclimatechange(seeparagraphs12&13),theclimate
sensitivitywouldbearound1o
C,foradoublingofCO2concentrations.Aclimateforcingof1.6Wm-2(seepreviousparagraph)would,inthishypotheticalcase,leadtoaglobally-
averagedsurfacewarmingofabout0.4oC.However,aswillbediscussedinparagraph
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36,itisexpectedthattheactualchange,afteraccountingfortheadditionalprocesses,
willbegreaterthanthis.
Carbon dioxide and climate
30EvidencefromicecoresindicatesanactiveroleforCO2intheclimatesystem.Thisis
becausetheamountofcarbonheldinoceans,soilsandplantsdependsontemperature
andotherconditions.Inotherwords,changesinCO2 can lead to climate change and
climatechangecanalsoaltertheconcentrationsofCO2.
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Aspects of climate change wherethere is a wide consensus butcontinuing debate and discussionThe carbon cycle and climate
31 OnceatmosphericCO2concentrationsareincreased,carboncyclemodels(which
simulatetheexchangeofcarbonbetweentheatmosphere,oceans,soilsandplants)
indicatethatitwouldtakeaverylongtimeforthatincreasedCO2todisappear;thisis
mainlyduetowell-knownchemicalreactionsintheocean.Currentunderstanding
indicatesthateveniftherewasacompletecessationofemissionsofCO2todayfrom
humanactivity,itwouldtakeseveralmillenniaforCO2concentrationstoreturntopre-
industrialconcentrations.
Other drivers of global climate change
32Inadditiontochangesingreenhousegasconcentrations,therearealargenumberof
lesswellcharacterisedcontributionstoclimateforcing,bothnaturalandhuman-
induced.
33Volcaniceruptionsareexamplesofanaturalclimateforcingmechanism.Anindividual
volcaniceruptionhasitslargesteffectsontheclimateforonlyafewyearsafterthe
eruption;theseeffectsaredependentonthelocation,sizeandtypeoftheeruption.
34NaturalforcingduetosustainedvariationsintheenergyemittedbytheSunoverthe
past150yearsisestimatedtobesmall(about0.12Wm-2);however,directobservations
oftheenergyemittedbytheSunonlybecameavailableinthe1970sandestimatesover
longerperiodsrelyonobservationsofchangesinothercharacteristicsoftheSun.A
numberofmechanismshavebeenproposedthatcouldreduceoramplifytheeffectof
solarvariations;theseremainareasofactiveresearch.
35Humanactivityresultsinemissionsofmanyshort-livedgases(suchascarbon
monoxideandsulphurdioxide)andparticlesintotheatmosphere.Theseaffecttheatmosphericconcentrationsofotherclimate-importantgasessuchasozone,andother
particleswhichleadtoaclimateforcing.Calculations,coupledtoavarietyof
atmosphericobservations,indicatethatparticleshavecausedanegativeclimateforcing
ofaround0.5Wm-2withanuncertaintyof0.2Wm-2.Theparticlesalsodirectly
inuencecloudproperties;thismoreuncertaineffectisdiscussedinparagraph47.
Climate sensitivity
36Themorecomplexclimatemodels,supportedbyobservations,allowclimatesensitivity
tobecalculatedinthepresenceofprocessesthatamplifyorreducethesizeofthe
climateresponse.Increasesinwatervapouralone,inresponsetowarming,are
estimatedtoapproximatelydoubletheclimatesensitivityfromitsvalueintheabsence
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ofamplifyingprocesses.Thereneverthelessremainuncertaintiesinhowmuchwater
vapouramountswillchange,andhowthesechangeswillbedistributedinthe
atmosphere,inresponsetoawarming.Climatemodelsindicatethattheoverallclimate
sensitivity(forahypotheticaldoublingofCO2intheatmosphere)islikelytolieinthe
range2oCto4.5oC;thisrangeismainlyduetothedifcultiesinsimulatingtheoverall
effectoftheresponseofcloudstoclimatechangementionedearlier.
Attribution of climate change
37Thesizeandsustainednatureoftheobservedglobal-averagesurfacewarmingon
decadalandlongertimescalesgreatlyexceedstheinternalclimatevariabilitysimulated
bythecomplexclimatemodels.Unlessthisvariabilityhasbeengrosslyunderestimated,theobservedclimatechangemustresultfromnaturaland/orhuman-inducedclimate
forcing.
38Whenonlynaturalclimateforcingsareputintoclimatemodels,themodelsare
incapableofreproducingthesizeoftheobservedincreaseinglobal-averagesurface
temperaturesoverthepast50years.However,whenthemodelsincludeestimatesof
forcingsresultingfromhumanactivity,theycanreproducetheincrease.Thesame
resultisfound,albeitwithagreaterspreadbetweendifferentmodels,forthesimulation
ofobservedsurfacetemperaturechangesforeachofthehabitablecontinentsseparately.
39Whenknownuncertaintiesinbothobservedtrendsandclimatemodelsaretakeninto
account,theobservedverticalandlatitudinalvariationsoftemperaturechangearealso
broadlyconsistentwiththoseexpectedfromadominantroleforhumanactivity.There
isanongoingcontroversyconcerningwhetherornottheincreasedwarmingwith
heightinthetropicalregionsgivenbyclimatemodelsissupportedbysatellite
measurements.
Future climate change
40Aswithalmostanyattemptstoforecastfutureconditions,projectionsoffutureclimate
changedependonanumberoffactors.Futureemissionsduetohumanactivitywill
dependonsocial,technologicalandpopulationchangeswhichcannotbeknownwith
condence.Theunderlyinguncertaintiesinclimatescienceandtheinabilitytopredict
preciselythesizeoffuturenaturalclimateforcingmechanismsmeanthatprojections
mustbemadewhichtakeintoaccounttherangeofuncertaintiesacrossthesedifferent
areas.
41The2007assessmentbytheIntergovernmentalPanelonClimateChange(IPCC)made
projectionsoffuturechangesusinganumberofpossiblescenariosoffutureemissions,
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basedonadiverserangeofassumptions.TheIPCCsbestestimatewasthatglobally-
averagedsurfacetemperatureswouldbebetween2.5-4.7oChigherby2100compared
topre-industriallevels.Thefullrangeofprojectedtemperatureincreasesby2100was
foundtobe1.8-7.1oCbasedonthevariousscenariosanduncertaintiesinclimate
sensitivity.
42Evenintheextremelyunlikelyeventthatthereisnofurtherincreaseinclimateforcing,a
furtherwarmingwouldbeexpectedtooccurastheoceansslowlyrespondtothe
existingclimateforcing,amountingtoafurtherfewtenthsofadegreecentigradeby
theyear2100.
43Theuncertaintyinthepredictedwarmingasaresultofhumanactivityoverthenexttwodecadesissmaller,therangebeing0.2to0.4oCperdecade.Ontheseshortertime-
scales,theactualchangecouldhoweverbereducedorenlargedsignicantlybyinternal
climatevariabilityandnaturalclimateforcings.
44 Increasesintemperaturearepredictedtobelargeronland,particularlyonthenorthern
continentsinwinter.Lesswarmingispredicted,forexample,overtheNorthAtlantic
Ocean.Climatemodelstendtopredictthatprecipitationwillgenerallyincreaseinareas
withalreadyhighamountsofprecipitationandgenerallydecreaseinareaswithlow
amountsofprecipitation.45Becauseofthethermalexpansionoftheocean,itisverylikelythatformanycenturies
therateofglobalsea-levelrisewillbeatleastaslargeastherateof20cmpercentury
thathasbeenobservedoverthepastcentury.Paragraph49discussestheadditional,
butmoreuncertain,contributiontosea-levelrisefromthemeltingoflandice.
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46 Observationsarenotyetgoodenoughtoquantify,withcondence,someaspectsof
theevolutionofeitherclimateforcingorclimatechange,orforhelpingtoplacetight
boundsontheclimatesensitivity.Observationsofsurfacetemperaturechangebefore
1850arealsoscarce.
47 Asnotedabove,projectionsofclimatechangearesensitivetothedetailsofthe
representationofcloudsinmodels.Particlesoriginatingfrombothhumanactivitiesand
naturalsourceshavethepotentialtostronglyinuencethepropertiesofclouds,with
consequencesforestimatesofclimateforcing.Currentscienticunderstandingofthis
effectispoor.
48Additionalmechanismsthatinuenceclimatesensitivityhavebeenidentied,includingtheresponseofthecarboncycletoclimatechange,forexamplethelossoforganic
carboncurrentlystoredinsoils.Theneteffectofchangesinthecarboncycleinall
currentmodelsistoincreasewarming,byanamountthatvariesconsiderablyfrom
modeltomodelbecauseofuncertaintiesinhowtorepresenttherelevantprocesses.
ThefuturestrengthoftheuptakeofCO2bythelandandoceans(whichtogetherare
currentlyresponsiblefortakingupabouthalfoftheemissionsfromhumanactivity
seeparagraph26)isverypoorlyunderstood,particularlybecauseofgapsinour
understandingoftheresponseofbiologicalprocessestochangesinbothCO2
concentrationsandclimate.
49Thereiscurrentlyinsufcientunderstandingoftheenhancedmeltingandretreatofthe
icesheetsonGreenlandandWestAntarcticatopredictexactlyhowmuchtherateof
sealevelrisewillincreaseabovethatobservedinthepastcentury(seeparagraph45)
foragiventemperatureincrease.Similarly,thepossibilityoflargechangesinthe
circulationoftheNorthAtlanticOceancannotbeassessedwithcondence.Thelatter
limitstheabilitytopredictwithcondencewhatchangesinclimatewilloccurin
WesternEurope.
50Theabilityofthecurrentgenerationofmodelstosimulatesomeaspectsofregional
climatechangeislimited,judgingfromthespreadofresultsfromdifferentmodels;
thereislittlecondenceinspecicprojectionsoffutureregionalclimatechange,except
atcontinentalscales.
Aspects that are not wellunderstood
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Developments in climate science51Climatechangesciencehasadvancedmarkedlyoverthepast20years,asaresultof
manyfactors.Theseincludeimprovedmethodsforhandlinglong-termclimatedata
sets,theever-lengtheningrecordofclimateobservations,improvedmeasurement
techniques,includingthosefromsatellites,betterunderstandingoftheclimatesystem,
improvedmethodsforsimulatingtheclimatesystem,andincreasedcomputerpower.
52Oneindicationoftheseadvancesistheincreasingdegreeofcondenceinthe
attributionofclimatechangetohumanactivity,asexpressedinthekeyconclusionsof
IPCCWorkingGroup1(WG1)initsassessments.
53Initsrstassessmentpublishedin1990,IPCCWG1concludedthatthesizeof[the
observedsurface]warmingisbroadlyconsistentwithpredictionsofclimatemodels[whichsimulatetheimpactofhumanactivity],butitisalsoofthesamemagnitudeas
naturalclimatevariability.Itssecondassessment,publishedin1995,concludedthat
thebalanceofevidencesuggeststhatthereisadiscerniblehumaninuenceon
climatechange.Itsfourth,andmostrecent,assessment,publishedin2007,concluded
thatmostoftheobservedincreaseingloballyaverage[surface]temperaturessincethe
mid-20thcenturyisverylikelyduetotheobservedincreaseinanthropogenic
greenhousegasconcentrations.
54
Remaininguncertaintiesarethesubjectofongoingresearchworldwide.Someuncertaintiesareunlikelyevertobesignicantlyreduced,becauseof,forexample,the
lackofobservationsofpastchangesrelevanttosomeaspectsofbothclimateforcing
andclimatechange.
55 Otheruncertaintiesmaystarttoberesolved.Forexample,satellitesnowincorporate
improvedtechniquestomeasurecloudcharacteristicsacrosstheglobe.Usingclimate
modelsforday-to-dayweatherpredictionwillenable,forexample,identicationof
errorsintherepresentationofcloudsinmodels;anysucherrorswillleadtoerrorsin
forecastsofmaximumandminimumtemperatures(whichareeasilyobserved).New
high-performancecomputerswillallowclimatemodelstorepresentsomesmaller-scale
phenomena(includingcloudsystemsanddetailsoftropicalstorms)directly,andare
expectedtoimprovecondenceinregionalpredictions.
56 Thereremainsthepossibilitythathithertounknownaspectsoftheclimateandclimate
changecouldemergeandleadtosignicantmodicationsinourunderstanding.
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57Thereisstrongevidencethatchangesingreenhousegasconcentrationsduetohuman
activityarethedominantcauseoftheglobalwarmingthathastakenplaceoverthelast
halfcentury.Thiswarmingtrendisexpectedtocontinueasarechangesinprecipitation
overthelongterminmanyregions.Furtherandmorerapidincreasesinsealevelare
likelywhichwillhaveprofoundimplicationsforcoastalcommunitiesandecosystems.
58ItisnotpossibletodetermineexactlyhowmuchtheEarthwillwarmorexactlyhowthe
climatewillchangeinthefuture,butcarefulestimatesofpotentialchangesand
associateduncertaintieshavebeenmade.Scientistscontinuetoworktonarrowthese
areasofuncertainty.Uncertaintycanworkbothways,sincethechangesandtheir
impactsmaybeeithersmallerorlargerthanthoseprojected.
59Likemanyimportantdecisions,policychoicesaboutclimatechangehavetobemadein
theabsenceofperfectknowledge.Eveniftheremaininguncertaintieswere
substantiallyresolved,thewidevarietyofinterests,culturesandbeliefsinsocietywould
makeconsensusaboutsuchchoicesdifculttoachieve.However,thepotentialimpacts
ofclimatechangearesufcientlyseriousthatimportantdecisionswillneedtobemade.
Climatescienceincludingthesubstantialbodyofknowledgethatisalreadywell
established,andtheresultsoffutureresearchistheessentialbasisforfutureclimate
projectionsandplanning,andmustbeavitalcomponentofpublicreasoninginthis
complexandchallengingarea.
Concluding remarks
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Background readingExtensivebackgroundreferencestothescienticliterature,andsummariesthereof,canbe
foundinthefollowingtwodocuments.
ContributionofWorkingGroupItotheFourthAssessmentReportofthe
IntergovernmentalPanelonClimateChange,2007,Solomon,S.,D.Qin,M.Manning,
Z.Chen,M.Marquis,K.B.Averyt,M.TignorandH.L.Miller(eds.),CambridgeUniversity
Press,Cambridge,UnitedKingdomandNewYork,NY,USA.
ipcc.ch/publications_and_data/ar4/wg1/en/contents.html
AdvancingtheScienceofClimateChange,NationalResearchCouncil,2010.
americasclimatechoices.org/panelscience.shtml
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Acknowledgements
Professor John Pethica FRS (Chair)
PhysicalSecretaryofRoyalSociety
Ms Fiona Fox
Director,ScienceMediaCentre,UK
Sir Brian Hoskins FRS
DirectorGranthamInstituteforClimate
Change,ImperialCollege,UKProfessor Michael Kelly FRS
ProfessorofTechnology,Universityof
Cambridge,UK
Professor John Mitchell FRS
DirectorofClimateScience,MetOfce,UK
Professor Susan Owens
ProfessorofEnvironmentandPolicy,
UniversityofCambridge,UK
Professor Tim Palmer FRS
RoyalSocietyResearchProfessor,
UniversityofOxford,UK
Professor John Shepherd FRS
ProfessorialResearchFellowinEarth
SystemScience,University
ofSouthampton,UK
Professor Keith Shine FRS
ProfessorofPhysicalMeteorology,
UniversityofReading,UK
Professor David Spiegelhalter FRS
ProfessorofthePublicUnderstandingof
Risk,UniversityofCambridge,UK
Royal Society Science Policy Centre
Rachel Garthwaite
SeniorPolicyAdviser
Stuart Leckie
PolicyAdviser
Tony McBride
HeadofStrategy
Working Group
TheRoyalSocietywouldliketoacknowledgethemembersoftheworkinggroupthatproducedthisdocument:
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Dame Jean Thomas FRS (Chair)
BiologicalSecretaryoftheRoyalSociety
Professor Alastair Fitter FRS
Pro-Vice-ChancellorforResearch,
UniversityofYork,UK
Dame Louise Johnson FRSSeniorFellowatDiamondLightSources,UK
Professor Brian Launder FRS
ResearchProfessor,Universityof
Manchester,UK
Professor John Pyle FRS
CoDirector,CentreforAtmospheric
Science,UniversityofCambridge,UK
Professor Chris Rapley
Director,ScienceMuseum,UK
Sir Alan Wilson FRSProfessorofUrbanandRegionalSystems,
UniversityCollegeLondon,UK
Professor David Fowler FRS
CentreforEcologyandHydrology,
Edinburgh,UK
Professor Joanna Haigh
ProfessorofAtmosphericPhysics,Imperial
College,London,UK
Professor Cyril Hilsum FRS
CorporateResearchAdvisor/Visiting
ProfessorofPhysics,UniversityCollege
London,UK
Professor Anthony Kelly FRS
DistinguishedResearchFellowinthe
DepartmentofMaterialsScienceand
Metallurgy,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
FormerChiefScienticAdvisor(UK
Government)
Professor Andrew Watson FRS
RoyalSociety2010AnniversaryResearch
Professor,UniversityofEastAnglia,UK
Sir Arnold Wolfendale FRS
EmeritusProfessorofPhysics,Universityof
Durham,UK
Professor Carl Wunsch ForMemRS
CecilandIdaGreenProfessorofPhysical
Oceanography,MassachusettsInstituteof
Technology,USA
Review Panel
TheRoyalSocietygratefullyacknowledgesthecontributionoftheindependentpanelofreviewers.Thereviewpanelmemberswerenotaskedtoendorsethenaldocuments
conclusions.
Contributors
TheRoyalSocietygratefullyacknowledgesthecontributionofindividualswhohave
commentedonthedocumentatearlierstagesofitspreparation.Theseindividualswere
notaskedtoendorsethedocument.
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Cover: Artists image of CryoSat-2, ESA, C. Vijoux.
The aim of the CryoSat mission is to determine variationsin the thickness of the Earths 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