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The Climate Commission produced a report 'The Critical Decade'. This new Climate Commission report argues that the next 10 years requires critical policy decisions like putting a price on carbon if we are to stabilise carbon pollution and avoid dangerous levels of global warming in the future.
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THE CRITICAL DECADEClimate science, risks and responses
2 Purpose
3 Introduction
5 Chapter 1. Developments in the science of climate change6 1.1Observationsofchangesintheclimatesystem13 1.2Whyistheclimatesystemchangingnow?17 1.3Howisthecarboncyclechanging?19 1.4Howcertainisourknowledgeofclimatechange?
22 Chapter 2. Risks associated with a changing climate23 2.1Sea-levelrise27 2.2Oceanacidification32 2.3Thewatercycle38 2.4Extremeevents48 2.5Abrupt,non-linearandirreversiblechangesintheclimatesystem
52 Chapter 3. Implications of the science for emissions reductions53 3.1Thebudgetapproach55 3.2Implicationsforemissionreductiontrajectories56 3.3Relationshipbetweenfossilandbiologicalcarbonemissionsanduptake
61 References
Contents
PublishedbytheClimateCommissionSecretariat(DepartmentofClimateChangeandEnergyEfficiency)
www.climatecommission.gov.au
ISBN: 978-1-921299-50-6(pdf) 978-1-921299-51-3(paperback)
©CommonwealthofAustralia2011
ThisworkiscopyrightCommonwealthofAustralia.AllmaterialcontainedinthisworkiscopyrighttheCommonwealthofAustralia,exceptwhereathirdpartysourceisindicated.
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TheDepartmentofClimateChangeandEnergyEfficiencyassertstherighttoberecognisedasauthoroftheoriginalmaterialinthefollowingmanner:
or
©CommonwealthofAustralia(DepartmentofClimateChangeandEnergyEfficiency)2011.
Permissiontousethirdpartycopyrightcontentinthispublicationcanbesoughtfromtherelevantthirdpartycopyrightowner/s.
IMPORTANT NOTICE – PLEASE READ
ThisdocumentisproducedforgeneralinformationonlyanddoesnotrepresentastatementofthepolicyoftheCommonwealthofAustralia.Whilereasonableeffortshavebeenmadetoensuretheaccuracy,completenessandreliabilityofthematerialcontainedinthisdocument,theCommonwealthofAustraliaandallpersonsactingfortheCommonwealthpreparingthisreportacceptnoliabilityfortheaccuracyoforinferencesfromthematerialcontainedinthispublication,orforanyactionasaresultofanyperson’sorgroup’sinterpretations,deductions,conclusionsoractionsinrelyingonthismaterial.
ScienceUpdate2011 1
Preface
With critical decisions to be made in 2011 on responses to climate change, I hope that this report provides useful information from the scientific community to a wide range of audiences – our political leaders, the general public, the private sector, NGOs, and the media. More specifically, the aim of this report is to provide up-to-date information on the science of climate change and the implications of this knowledge for societal responses, both for mitigation strategies and for the analysis of and responses to risks that climate change poses for Australia.
Overthepasttwoyears,alargenumberofexcellentreviewsandsynthesesofclimatechangesciencehavebeenproducedbyacademiesofscience,bygroupsofexperts,andasoutcomesofmajorinternationalmeetings.Muchofthisinformationisstillcurrent,andIhavedrawnheavilyonitforthisreport;alistofthesedocumentsisgivenbelow.Scientificknowledgeonclimatechangeiscontinuouslyevolving,however,andIhavealsoincludedinformationfromkeypaperspublishedinrecentmonths.
Ihavetriedtobeasbriefaspossibleintreatingthevarioustopicscoveredinthisreport,withtheaimofprovidingthekeypointsonly.Forinterestedreaders,furtherinformationonthetopicscoveredcanbefoundinthereportslistedbelow,and,ofcourse,fromtheoriginalsourcesofinformationinthepeer-reviewedliterature.Thesearepresentedinthereferencelistattheendofthisreportandinsimilarlistsattheendofthereportsbelow.
Atseveralplacesinthisreport,Ihavemademyownsynthesesandjudgementsbasedonlargebodiesofworkwherethereisnoclearconsensusinthepeer-reviewedliterature.Asanexample,onmyreadingoftheliteraturetodateandondiscussionswithexperts,Iexpectthemagnitudeofglobalaveragesea-levelrisein2100comparedto1990tobeintherangeof0.5to1.0metre.Forthisandothersuchjudgements,Itakefullandsoleresponsibility.
Thisreporthasbeenextensivelyreviewedby15-20colleaguesfromCSIRO,theBureauofMeteorologyandtheuniversitysector.Theyareallwidelyrecognisedexpertsintheirfieldsofclimatescience,andIamgratefulforthecareandthoroughnesswithwhichtheyreadandcommentedonearlierdraftsofthereport.
IalsothankmycolleaguesontheScienceAdvisoryPaneloftheClimateCommission,whocriticallyrevieweddraftsofthereport.
Duringthepreparationofthisreport,IhaveworkedcloselywithProfessorRossGarnautandhisteamastheyhaveundertakentheirupdateofclimatescience.Iappreciatedtheavailabilityofearlierdraftsoftheirupdate,andfortheopportunityforfrequentdiscussionsonparticularlytopicalandcontentiousareasofclimatescience.
IamgratefultomanycolleaguesintheDepartmentofClimateChangeandEnergyEfficiencyformanyusefuldiscussionsandforongoingsupportofvariouskinds.
Will Steffen Climate Commissioner CanberraMay2011
Climate Change 2011: Update of science, risks and responses
2 ClimateCommission
Sources that were drawn upon in the compilation of this report
AustralianAcademyofScience(AAS).(2010).The Science of Climate Change: Questions and Answers.August2010.www.science.org.au/policy/climatechange2010.html
Canadell,J.(ed).(2010).Carbonsciencesforanewworld.Current Opinion in Environmental Sustainability(specialissue)2:209-311.
Garnaut,R.(2008).The Garnaut Climate Change Review: Final Report.Cambridge,UK:CambridgeUniversityPress,634pp.
Garnaut,R.(2011).Garnaut Climate Change Review – Update 2011. Update paper five: The science of climate change.www.garnautreview.org.au
IntergovernmentalPanelonClimateChange(IPCC).(2007).Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change,(eds).Solomon,S.,Qin,D.,Manning,M.,Chen,Z.,Marquis,M.,Averyt,K.,Tignor,M.M.B.,Miller,JrH.L.,andChen,Z.Cambridge,UKandNewYork,NY:CambridgeUniversityPress,996pp.
Richardson,K.,Steffen,W.,Schellnhuber,H.-J.,Alcamo,J.,Barker,T.,Kammen,D.M.,Leemans,R.,Liverman,D.,Munasinghe,M.,Osman-Elasha,B.,Stern,N.andWaever,O.(2009).Synthesis Report. Climate Change: Global Risks, Challenges & Decisions.SummaryoftheCopenhagenClimateChangeCongress,10-12March2009.UniversityofCopenhagen,39pp.
Richardson,K.,Steffen,W.,Liverman,D.,Barker,T.,Jotzo,F.,Kammen,D.,Leemans,R.,Lenton,T.,Munasinghe,M.,Osman-Elasha,B.,Schellnhuber,J.,Stern,N.,Vogel,C.,andWaever,O.(2011).Climate Change: Global Risks, Challenges and Decisions.Cambridge:CambridgeUniversityPress,501pp.
RoyalSociety(UK).(2010).Climate change: a summary of the science.London:TheRoyalSociety,September2010,17pp.
Steffen,W.(2009).Climate Change 2009: Faster Change & More Serious Risks.DepartmentofClimateChange,AustralianGovernment,52pp.
TheCopenhagenDiagnosis.(2009).Updating the World on the Latest Climate Science.Allison,I.,Bindoff,N.L.,Bindschadler,R.A.,Cox,P.M.,deNoblet,N.,England,M.H.,Francis,J.E.,Gruber,N.,Haywood,A.M.,Karoly,D.J.,Kaser,G.,LeQuéré,C.,Lenton,T.M.,Mann,M.E.,McNeil,B.I.,Pitman,A.J.,Rahmstorf,S.,Rignot,E.,Schellnhuber,H.J.,Schneider,S.H.,Sherwood,S.C.,Somerville,R.C.J.,Steffen,K.,Steig,E.J.,Visbeck,M.,Weaver,A.J.Sydney,Australia:TheUniversityofNewSouthWalesClimateChangeResearchCentre(CCRC),60pp.
WBGU(GermanAdvisoryCouncilonGlobalChange).(2009).Solving the Climate Dilemma: The Budget Approach.SpecialReport.Berlin:WBGUSecretariat,54pp.
Purpose
THIs upDATE REvIEws wHAT THE sCIEnCE Is TELLIng us AbouT THE nEED To ACT on CLImATE CHAngE, AnD THE RIsks of A CHAngIng CLImATE To AusTRALIA.
–
ScienceUpdate2011 3
Introduction
Over the past two or three years, the science of climate change has become a more widely contested issue in the public and political spheres. Climate science is now being debated outside of the normal discussion and debate that occurs within the peer-reviewed scientific literature in the normal course of research. It is being attacked in the media by many with no credentials in the field. The questioning of the Intergovernmental Panel on Climate Change (IPCC), the “climategate” incident based on hacked emails in the UK, and attempts to intimidate climate scientists have added to the confusion in the public about the veracity of climate science.
Bycontrasttothenoisy,confusing“debate”inthemedia,withintheclimateresearchcommunityourunderstandingoftheclimatesystemcontinuestoadvancestrongly.Someuncertaintiesremainandwillcontinuetodoso,giventhecomplexityoftheclimatesystem,andtheimpossibilityofknowingthefuturepathwaysofhumanpolitical,socialandtechnologicalchanges.Meanwhilethereismuchclimatechangesciencethatisnowwellandconfidentlyunderstood,andforwhichthereisstrongandclearevidence.
–THE EvIDEnCE THAT THE EARTH’s suRfACE Is wARmIng RApIDLy Is now ExCEpTIonALLy sTRong, AnD bEyonD DoubT. EvIDEnCE foR CHAngEs In oTHER AspECTs of THE CLImATE sysTEm Is ALso sTREngTHEnIng. THE pRImARy CAusE of THE obsERvED wARmIng AnD AssoCIATED CHAngEs sInCE THE mID-20TH CEnTuRy – HumAn EmIssIons of gREEnHousE gAsEs – Is ALso known wITH A HIgH LEvEL of ConfIDEnCE.
–
However,thebehaviourofseveralimportantcomponentsorprocessesoftheclimatesystem,includingsomeassociatedwithseriousriskssuchassea-levelriseandchangesinwaterresources,aremuchlesswellunderstoodandarethesubjectofintensescientificresearchanddebate.
Thepurposeofthisupdateistoreviewthecurrentscientificknowledgebaseonclimatechange,particularlywithregardto(i)theunderpinningitprovidesfortheformulationofpolicyand(ii)theinformationitprovidesontherisksofachangingclimatetoAustralia.
Thefirstchapteroftheupdatefocusesonthefundamentalunderstandingoftheclimatesystem,whichisanimportantelementinframingandinformingtheformulationofpolicy.Theanalysisstartswithobservationsoftheclimatesystemandhowitischanging,followedbythereasonsfortheseobservedchanges.Itthenfocusesonthebehaviourofthecarboncycle,whichistheprimaryprocessintheclimatesystemthatpolicyaimstoinfluence.Finally,theoftenconfusingissueofcertaintyinclimatescienceisexplored,withanemphasisonwhatisknownwithahighdegreeofcertaintybutalsowhereconsiderableuncertaintyremainsaboutimportantfeaturesoftheclimatesystem.
Chapter2describessomeofthemostsignificantrisksthatareassociatedwithachangingclimate.Thesectionisfocussedstronglyontheimplicationsofourunderstandingoftheclimatesystemforriskassessments,butdoesnotattempttoundertakethesector-by-sectorriskassessmentsthemselves.Whileitisclearthatclimatechangehaspotentialimpactsonawiderangeofsectors–humanhealth,agriculture,settlementsandinfrastructure,tourism,biodiversityandnaturalecosystems,andothers–manyothernon-climaticfactorsaffecttherisksthatthesesectorsfaceandtheoutcomesthateventuallyoccur.
4 ClimateCommission
Introduction (continued)
Forexample,thereislittledoubtthatextremeweathereventssuchasbushfiresandfloodshavesignificantimpactsonhumanhealthandwell-being.The2011Queenslandfloodshaveledtolong-term,mentalhealthandrelatedproblems,suchasdepression,bereavement,post-traumaticstressdisordersandothermoodandanxietydisorders;andthe2009Victorianbushfiresalsoledtoconsiderablepsychologicaldistress,someofitprolonged,tothosewhoexperiencedthefiresandsurvived(A.J.McMichael,personalcommunication).Suchextremeweathereventshaveoccurredbeforetheadventofhuman-inducedclimatechange,andthedegreetowhichclimatechangeaffectsrisksassociatedwithextremeeventsisaveryactiveareaofresearch.
Inadditiontotheintensityoftheweathereventitself,theseverityoftheQueenslandfloodswereaffectedbyseveralotherfactors,manyofwhicharenotrelatedtoclimate.Theseincludethelandcoverandconditionofcatchmentsandtheefficacyofprotectivestructuressuchasdams.Theultimatehealthoutcomeswereadditionallyinfluencedbythevulnerabilityofindividualsandcommunitiesandtheeffectivenessofwarningsandofemergencymanagementactions.
TheGreatBarrierReef,anoft-citedexampleofaniconicnaturalecosystemvulnerabletothepotentialimpactsofclimatechange,illustratestheimportanceofconsideringmultiple,interactingstressesandnotclimatechangeinisolation.Whileclimate-relatedrisksforcoralreefs,suchastemperatureextremesandincreasingoceanacidity,havebeenwidelydocumentedandsomearewellunderstood(e.g.,Hoegh-Gulbergetal.2007),othernon-climaticfactorsarealsocriticalformaintainingcoralreefsaswell-functioningecosystems.Theseincludeoverfishinganddeclinesinwaterquality(Bryantetal.1998),aswellaspollutants,lowsalinity,turbidity,sedimentationandpathogens,whichputfurtherpressureonreefs(Anthonyetal.2007).Thus,riskassessmentsforparticularsectors,suchashumanhealthandnaturalecosystems,arebestundertakenbyexpertsfortheparticularsector,drawingontheinsightsthatclimatesciencecanofferaswellasonnon-climateknowledgeandinformation.
Thischapteraimstoprovideanup-to-datesynthesisofourscientificunderstandingoffivemajoraspectsoftheclimatesystemthatareimportantforriskassessmentsacrossmanysectors.Theseare:(i)sea-levelrise;(ii)oceanacidification;(iii)thewatercycle;(iv)extremeevents;and(v)abrupt,non-linearandirreversiblechangesintheclimatesystem.
Finally,Chapter3providesalinkbetweenthescienceofclimatechangeandthepolicyoptionsforreducingemissionsofgreenhousegases,particularlycarbondioxide(CO
2).Theapproachtakenherecircumvents
thecomplexityandconfusionofthetargets/timetables/baselinesapproachbyturningtoamuchsimplerbudget–oraggregateemissions–analysis.Theapproachdirectlyrelatesthefurtheramountofemissions,inbillionsoftonnesofCO
2,thatglobalsocietycanemittoachieve
aparticulartemperaturelimit,suchas2°Cabovethepre-industriallevel.
Eachchapteropenswithabriefintroductoryparagraphpresentingthemainthrustofthesection,followedbyaseriesofshortbulletedstatements.Thesearebrief,summarystatementswithoutreferences.Eachofthesestatementsissupportedbymoredetailedtext,withreferencesandfigures,inthemainbodyofeachsection.
moRE THAn 85% of THE ADDITIonAL HEAT DuE To THE EnERgy ImbALAnCE AT THE EARTH’s suRfACE Is AbsoRbED by THE oCEAn (IpCC 2007a).
foR THE mosT RECEnT 10-yEAR pERIoD (2001-2010), gLobAL AvERAgE TEmpERATuRE wAs 0.46 °C AbovE THE 1961-1990 AvERAgE, THE wARmEsT DECADE on RECoRD.
gLobAL sEA LEvEL HAs RIsEn by AbouT 20 Cm sInCE THE 1880s, wHEn THE fIRsT gLobAL EsTImATEs CouLD bE mADE.
CHApTER 1: DEvELopmEnTs In THE sCIEnCE of CLImATE CHAngE
DID you know...
6 ClimateCommission
Chapter 1. Developments in the science of climate change
1.1 Observations of changes in the climate system
Recent observations of changes in the climate system strengthen the conclusions of the IPCC (Intergovernmental Panel on Climate Change) Fourth Assessment Report (2007a) and the Garnaut Review (2008) that contemporary climate change is indeed real, and is occurring at a rapid rate compared with geological time scales. From a human perspective, the rate of climate change is already discernible to the present generation, and will be even more prominent in the lives of our children and grandchildren. It is leading to significant risks today, and more serious risks in the coming decades, as described in Chapter 2.
Inthissectionthefocusisonevidenceforthelong-termwarmingtrend,whileotherchangesintheclimatesystem–extremeevents,thewatercycleandabruptchanges,forexample–aretreatedinthediscussionofclimaterisksinChapter2.Themainconclusionsofthissectionare:
Surface air temperature
TheaveragetemperatureattheEarth’ssurfacehascontinuedtoincrease.Theglobalcombinedlandandseasurfacetemperature(SST)for2010was0.53°Cabovethe1961-1990average(WMO2011)andthus2010ranksamongstthethreewarmestyearsonrecord.
–foR THE mosT RECEnT 10-yEAR pERIoD (2001-2010), gLobAL AvERAgE TEmpERATuRE wAs 0.46 °C AbovE THE 1961-1990 AvERAgE, THE wARmEsT DECADE on RECoRD.
–However,timeseriesofatleastthreedecades–andpreferablymuchlonger–arerequiredtodifferentiatewithconfidencealong-termclimatictrendfromshortertermvariability.Figure1showstheglobalaveragetemperaturerecordfromthelate19thcenturytothepresent.Overthelastthreedecades,therateofwarminghasbeen0.17°Cperdecade,averyhighratefromageologicalperspective.
Therehasbeenconsiderableconfusioninthemediaandinthepublicbetweenshortertermpatternsofvariabilityinclimateandweatherandlongerterm(multi-decadal)trendsinclimate.Theproblemistheinappropriatetendencytoexpectweatherpatternsandchangesovershorttimeperiodsandinparticularregionstoalwaysfollowchangesinglobaltrendsoverlongperiodsoftime.AnexampleistheextremelycoldandsnowyweatherexperiencedbypartsofEuropeandNorthAmericaintheNovember-December2010period.Doesthissignalanendtoglobalwarming?Absolutelynot.Asnotedabove,2010wasoneofthethreewarmestyearsonrecord.Furthermore,themonthofNovember2010wasexceptionallywarm,withextremelyhightemperaturesaroundthenorthernhighlatitudesmorethancompensatingforthecold,snowyweatherinwesternEuropeandpartsofNorthAmerica(Figure2).
– TheaverageairtemperatureattheEarth’ssurfacecontinuesonanupwardtrajectoryatarateof0.17°Cperdecadeoverthepastthreedecades.
– Thetemperatureoftheupper700moftheoceancontinuestoincrease,withmostoftheexcessheatgeneratedbythegrowingenergyimbalanceattheEarth’ssurfacestoredinthiscompartmentofthesystem.
– ThealkalinityoftheoceanisdecreasingsteadilyasaresultofacidificationbyanthropogenicCO2
emissions.
– RecentobservationsconfirmnetlossoficefromtheGreenlandandWestAntarcticicesheets;theextentofArcticseaicecovercontinuesonalong-termdownwardtrend.Mostland-basedglaciersandicecapsareinretreat.
– Sea-levelhasrisenatahigherrateoverthepasttwodecades,consistentwithoceanwarmingandanincreasingcontributionfromthelargepolaricesheets.
– ThebiosphereisrespondinginaconsistentwaytoawarmingEarth,withobservedchangesingenepools,speciesranges,timingofbiologicalpatternsandecosystemdynamics.
ScienceUpdate2011 7
Chapter 1. Developments in the science of climate change(continued)
Figure 1. Surface air temperature trend from the 1880s to the present. Thebaselinefortheanalysisisthe1951-1980average.
Source:NASAGISSSurfaceTemperatureAnalysis.
Figure 2. Global map of surface temperature anomalies for November 2010 showing the unusually cold conditions in parts of Europe and North America but the extreme warmth in other parts of the northern hemisphere.
Source:NASAGISSSurfaceTemperatureAnalysis.
0.6
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NOAA National Climatic Data Center
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November 2010 Anomaly vs 1951 – 1980
8 ClimateCommission
Chapter 1. Developments in the science of climate change(continued)
Ocean temperature
Althoughthereisaverystrongfocusonairandseasurfacetemperatureinboththeclimateresearchcommunityandthegeneralpublic,oceantemperatureisabettermeasureofchangesintheclimatesystem.
–moRE THAn 85% of THE ADDITIonAL HEAT DuE To THE EnERgy ImbALAnCE AT THE EARTH’s suRfACE Is AbsoRbED by THE oCEAn (IpCC 2007a).
– Sincethe1960smeasurementsoftheheatcontentoftheupper700moftheoceanhavebeenavailable,andsince2004,measurementstolowerdepths(upto2km)havebecomewidelyavailablewiththedeploymentofArgofloats(GouldandtheArgoScienceTeam2004).
Figure3showstherecordofoceanthermalexpansionfrom1950through2008,showingtheclearlong-termtrendofwarming(Dominguesetal.2008,andupdates).TheDominguesetal.updatedcurveinthisfigure,whichusesthecarefullycheckedandcorrectedArgodataofBarkeretal.(2011),indicatesthatmulti-decadalwarminghascontinuedtotheendoftherecordinDecember2008(Churchetal.2011).Thisrecordisquantitativelyconsistentwiththeobservedrateofsea-levelriseoverthepasthalf-century.Althoughmostoftheadditionalheatstoredintheoceanisfoundintheupper700m,recentobservationsshowthatwarmingofthedeeperoceanwatersinboththeSouthernandAtlanticOceansisnowoccurring(PurkeyandJohnson2010).
Ocean acidification
IncreasingtheatmosphericconcentrationofCO2leads
tomoredissolutionofCO2insurfaceoceanwaters,
increasingtheiracidity(decreasingtheiralkalinity)viatheformationofcarbonicacid.Throughaseriesofchemicalreactions,increasingtheconcentrationofcarbonicacidreducestheconcentrationofcarbonateionsinseawater(Kleypasetal.2006).Thisprocesshasimplicationsformarineorganismsthatformcalciumcarbonateshells(seeSection2.2).Observationsoftheacidityoftheocean’ssurfacewatersshowtheexpecteddecreaseofabout0.1pHunitsincethepre-industrialera(Guinotteetal.2003).
Sea ice and polar ice sheets
Evidencefromthecryosphere(snow,iceandfrozenground)isconsistentwiththewarmingofthesurfaceoftheEarth.Themoststrikingevidencecomesfromthenorthernhighlatitudes,wheretheseaicecoveringtheArcticOceanhasdecreasedsignificantlyoverthelastseveraldecades(Figure4).ChangestotheseaicesurroundingAntarcticaaremorecomplex,withnoappreciablechangeinoverallextentoverthepastseveraldecades.
ThelargepolaricesheetsonGreenlandandWestAntarctica,whichareimportantfactorsinfluencingsea-levelrise,arecurrentlylosingmasstotheoceanthroughbothmeltinganddynamicaliceloss;thatis,bybreak-upandcalvingofblocksofice.However,thereisconsiderableuncertaintyabouttherateatwhichthelatterprocessisoccurring.Inaddition,thesetrendsareoftenbasedonshortertermobservationalrecords(oftenthelast10-15years),anditisnotentirelyclearwhetherthesearelong-termtrendsthatwillbemaintainedintothefutureorareatleastpartlytheresultofnaturaldecadal-scalevariability(e.g.,Rignotetal.2008).
OverthepastdecadeoneofthemostcommonobservationaltoolsforestimatingchangesinicemassistheGRACEsatellitegravitytechnique,whichestimatesthelossofmassfromchangesinthegravityfield.GRACEmeasurementshavebeenprominentinconfirmingatrendoficelossfrompolaricesheets,especiallyGreenland(Figure5).However,theGRACEobservationaltechniqueitselfiscomplexwithsignificantuncertainties;arecentre-analysisoftheGreenlandgravitychangedatasuggeststhattherateoficelosshasbeenoverestimatedbyafactoroftwo(BromwichandNicolas2010;Figure6).
Nevertheless,asynthesisofallobservationsshowsthatthereisanetlossofmassfromtheGreenland(andWestAntarctic)icesheets;theuncertaintyreferstotherateatwhichthisicelossisoccurring,withsomeevidencethatthisrateoflossmaybeaccelerating(Rignotetal.2011).
ScienceUpdate2011 9
Chapter 1. Developments in the science of climate change(continued)
Figure 3. Updated estimates of ocean thermal expansion relative to 1961. TheupdatedDominguesetal.(2008)timeseriesisshownasaredbrokenline,andonestandarddeviationuncertaintyestimatesareindicatedbythegreyshading.TheestimatesforIshiiandKimoto(2009)andLevitusetal.(2009)areshownaspurpleandbrokenpurplelinesrespectively.Theestimatedstratosphericaerosolloading(arbitraryscale)fromthemajorvolcaniceruptionsisshownatthebottom.
Source:Churchetal.(2011).
Figure 4. Observed (red line) and modelled September (end of summer) Arctic sea ice extent in millions of square kilometres. Thesolidpurplelineistheensemblemeanofthe13IPCCAR4modelsandtheedgesofthegreyshadedarearepresenttherangeofmodelprojections.
Source:Stroeveetal.(2007),updatedtoincludedatafor2008.
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Ishii & KimotoLevitus et al.Domingues et al. (updated)
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Observations
Mean and range of IPCC models
10 ClimateCommission
Chapter 1. Developments in the science of climate change(continued)
Figure 5. Results from the recent large area total mass balance measurements of the Greenland ice sheet, placed into common units and displayed versus the time intervals of the observations. Theheightsoftheboxescoverthepublishederrorbarsorrangesinmasschangerateoverthoseintervals.
Source:AMAP(2009),whichincludesreferencestotheindividualdatasetsshowninthefigure.
Net
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Johannessen et al.Science 2005
Zwally et al.JGlac. 2005 SRALT
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Luthke et al.Science 2006- GRACE/MASCON
Chen et al.Science 2006 Grace
Velicogna and WahrNature 2006 GRACE
Slobbe et al.GJI 2009 GRACE(CNES/CSR/DEOS/GFZ range)
Ramillien et al.GBC 2006 GRACE
Wouters et al.GRL 2008 GRACE/EOFfilter
Krabill et al.GRL 2004 ATMa b a bc
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ScienceUpdate2011 11
Chapter 1. Developments in the science of climate change(continued)
Figure 6. Cumulative mass loss of the Greenland ice sheet. TheestimatebyWuetal.(2010)ofmasslosssince2003(red)isconsiderablylowerthananearlierpredictedvalue(Velicogna2009)(purple),owinginparttolargerthanpreviouslyestimatedsubsidenceratesofunderlyingbedrock.Thecurvesandtheirdifferencescanthusbeinterpretedintermsofcontributiontoglobalmeansealevel(right-handverticalaxis).Theshadedareasreflectuncertainties.
Source:BromwichandNicolas(2010).
Land-based glaciers and ice caps
Mostglaciersandmountainice-capsaroundtheworldhavebeeninretreatthepastcenturyandareestimatedtobecontributingabout0.8mmperyeartosealevelriseatthebeginningofthiscentury(IPCC2007a).Glaciersandicecapsarenotyetinequilibriumwiththepresentclimate,andthatadjustmentwouldleadtoamasslossequivalenttoanother18cmsea-levelrise(Bahretal.2009).However,theclimateisstillwarmingandwillalmostsurelycontinuetowarmthroughthiscentury,withcurrentwarmingtrendsleadingtoanestimatedmasslossequivalenttoabout55cmofsea-levelrisebytheendofthecentury(Pfefferetal.2008).
Sea-level rise
Globalsealevelhasrisenbyabout20cmsincethe1880s,whenthefirstglobalestimatescouldbemade.Therateofincreasehasrisentoabout3.2mmyr-1forthe1993-2009period,basedonsatellitealtimeterdata(Cazenaveetal.2009;Dominguesetal.2008;ChurchandWhite2011),comparedtoarateof1.7mmyr-1forthe1900-2009period(ChurchandWhite2011).Figure7showsacomparisonoftheobservedsea-levelrisetoprojectionsfromclimatemodels,whichfirstbecameavailablein1990andweresummarisedinthetwomostrecentIPCCreports(2001;2007a).Observedsealevelsince1993istrackingneartheupperendofthemodelprojections,pointingtowardssignificantrisksofsea-levelrelatedimpactsinthe21stcentury(seeSection2.1).
Figure 7. Sea-level change from 1970 to 2008. Thethinredlinefrom1970to2002isbasedontidegaugedata,andthejaggedpurplelinefrom1993to2008isbasedonsatellitedata.Thethickpurpleandredlinesarerunningmeans.TheenvelopeofIPCCprojections(brokenlinesfrom1990)areshownforcomparison.
Source:afterRahmstorfetal.(2007),basedondatafromCazenaveandNarem(2004);ChurchandWhite(2006),Cazenave(2006)andA.Cazenavefor2006–08data.
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12 ClimateCommission
Severalprocessescontributetosea-levelrise,andaquantitativebudgetoftheirrelativecontributionsforthe1961-2003periodhasbeengenerated(Dominguesetal.2008).Thebudgetshowsthatabout40%oftheriseoverthisperiodcanbeattributedtothethermalexpansionoftheoceanasitwarms,about35%tothemeltingofcontinentalglaciersandicecaps(e.g.,theAndeanandHimalayanmountainglaciers)andabout25%fromthelargepolaricesheetsonGreenlandandAntarctica.Estimatesoftheseindividualtermsaggregatetoatotalof1.5+/-0.4mmyr-1,whichisnotsignificantlydifferentfromtheobservedrateforthesameperiodof1.6+/-0.2mmyr-1.
Figure 8. Local sea-level rise (mm/year) around Australia from the early 1990s to 2008.
Source:NTC2008
Globalaveragevaluesofsea-levelrisemasklargeregionaldifferences.Forexample,aroundAustralia(Figure8)recentsealevelrise(fromtheearly1990sto2008)hasbeenbelowtheglobalaveragealongtheeastcoast,neartheglobalaveragealongthemuchofthesoutherncoast,butatleastdoubletheglobalaveragealongmuchofthenortherncoastline.Suchregionaldifferencesareimportantinassessingtherisksposedbysea-levelriseatparticularlocations.
Terrestrial and marine biosphere
Thebiosphererespondstosignificantchangesintheabioticenvironment,sothelong-termincreaseintemperatureshouldbeevidentinbiosphericresponses.
–InDEED, A CLImATE CHAngE (wARmIng) sIgnAL Is now CLEAR In An InCREAsIng numbER of AusTRALIAn AnD gLobAL obsERvATIons of THE REsponsEs of bIoLogICAL spECIEs AnD ECosysTEms (E.g., pARmEsAn 2006; RooT ET AL. 2005; IpCC 2007b).
–Australianobservationsthatshowaclearresponsetoaclimatesignal,distinguishablefromtheresponsestootherstressorsonecosystems,includegeneticshiftsinthepopulationsoffruitflies(Uminaetal.2005),migrationofbothnativeandferalmammalstohigherelevationsinalpineregions(GreenandPickering2002;Pickeringetal.2004),thesouthwardexpansionofthebreedingrangeofblackflyingfoxes(Welbergenetal.2007),earlierarrivalandlaterdeparturetimesofmigratorybirds(Chambers2005,2008;Chambersetal.2005)andtheearliermatingandlongerpairingofthelargeskinkTiliqua rugosa(BullandBurzacott2002).
Inthemarinerealmresponsestoawarmingclimatehavealsobeenobserved.Theseincludesouthernrangeextensionofthebarrens-formingseaurchinfromthemainlandtoTasmania(Lingetal.2008,2009),significantchangesinthegrowthratesoflong-livedPacificfishspecies(Thresheretal.2007),andasouthwardshiftinthedistributionofoverhalfoftheintertidalspeciesalongtheeastcoastofTasmania(Pitt2008).
PerhapsthebestknownmarineexampleofresponsetoclimatechangeistheincreaseinbleachingeventsontheGreatBarrierReef;therehavebeeneightmassbleachingeventsontheGBRsince1979withnoknownsucheventspriortothatdate(Doneetal.2003).
Chapter 1. Developments in the science of climate change(continued)
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1.2 Why is the climate system changing now?
The evidence for a long-term warming trend in Earth’s climate is overwhelming. The critical question is: what is causing this warming trend, and the associated changes in the climate system?
Thissectionexploresthepossiblereasonsfortheobservedwarmingtrendbyfirstplacingcontemporaryclimatechangeinalongertermperspectiveandthenexaminingthevariouspotentialcausesforthewarming.Themainconclusionsare:
The longer-term context
Earthhasbeenmuchwarmerandmuchcolderinthedistantpast,butforhumanstheperiodofrelevanceisthelasthalf-millionyears,duringwhichfullymodernhumansevolved.Inparticular,thelast12,000years–theHolocene–isimportant;duringthisperiodagriculture,villagesandcities,andmorecomplexsocietiesandcivilisations,includingourcurrentcivilisation,havedeveloped.Comparedtothepatternoflongiceagesandmuchshorterinterglacialperiodsoverthepast420,000years(Petitetal.1999),theHolocenehasbeenanunusuallylongandstablewarmperiod,facilitatingthedevelopmentofhumansocietybeyondthehunter-gathererstage.ThebehaviouroftheclimatesystemduringtheHoloceneprovidesauseful,human-relevantbaselineagainstwhichtotestpossibleexplanationsforcontemporarywarming.
Changes in solar radiation
VariationintheamountofsolarradiationreachingtheEarthhasbeenimplicatedintemperaturefluctuationsearlierintheHolocene,forexample,asapossiblefactorintheMedievalClimateAnomaly,oftencalledtheMedievalWarmPeriod(Mannetal.2009).Variationsinsolarradiationhavebeenknownwithbetteraccuracysincethelate1800s,andespeciallyinthelastthreetofourdecades,andcouldhavecontributedatmost10%totheobservedwarmingtrendinthe20thcentury(LeanandRind2008).Inparticular,therehasbeennosignificantchangeinsolarradiationoverthepast30years(IPCC2007a),whenglobalaveragetemperaturehasrisenatabout0.17°Cperdecade.Furthermore,patternsofwarmingoverrecentdecadesareinconsistentwithsolarforcing.Inparticular,solarforcingwouldproduceawarmingofthestratosphere,inadditiontothatofthetroposphere.Infactstratosphericcoolinghasbeenobserved,inconsistentwithsolarforcingbutconsistentwithCO
2-dominatedforcing(IPCC2007a).
Chapter 1. Developments in the science of climate change(continued)
– Thereisnocredibleevidencethatchangesinincomingsolarradiationcanbethecauseofthecurrentwarmingtrend.
– Neithermulti-decadalorcentury-scalepatternsofnaturalvariability,suchastheMedievalWarmPeriod,norshortertermpatternsofvariability,suchasENSO(ElNiño-SouthernOscillation)ortheNorthAtlanticOscillation,canexplainthegloballycoherentwarmingtrendobservedsincethemiddleofthe20thcentury.
– Thereisaverylargebodyofinternallyconsistentobservations,experiments,analyses,andphysicaltheorythatpointstotheincreasingatmosphericconcentrationofgreenhousegases,withcarbondioxide(CO2
)themostimportant,astheultimatecausefortheobservedwarming.
– ImprovedunderstandingofthesensitivityoftheclimatesystemtotheincreasingatmosphericCO
2
concentrationhasprovidedfurtherevidenceofitsroleinthecurrentwarmingtrend,andprovidedmoreconfidenceinprojectionsoftheleveloffuturewarming.
14 ClimateCommission
Modes of natural variability
TheMedievalClimateAnomaly(MCA),asomewhatwarmerperiodfromabout1000toabout1250or1300AD,hassometimesbeeninvokedtoinferthatthecontemporarywarmingisnothingunusualintheHoloceneandthatitisthuslikelyduetonaturalvariability.However,thebulkofevidencefortheMCAcomesfromthenorthernhemisphere,whichmakesitdifficulttodeterminewhethertheMCAwastrulyglobalinscale.Furthermore,aspatiallyexplicitsynthesisofallavailabletemperaturereconstructionsaroundtheglobesuggeststhattheMCAwashighlyheterogeneous,eveninthenorthernhemisphere,withgloballyaveragedwarmingmuchbelowthatobservedoverthelastcentury(Mannetal.2009;Figure9).Thus,theMCAisdifferentinmagnitudeandextentfromcontemporarywarming(Figure10).
Shorter-termmodesofnaturalvariability,suchasENSOandtheNAO(NorthAtlanticOscillation),areveryimportantinfluencesontheweatherthatpeopleexperiencefromyeartoyear,buttheycannotexplainrecentmulti-decadal,globallysynchronoustrendsintemperature.Rather,suchmodesofnaturalvariabilityaredrivenbychangesincoupledoceanicandatmosphericcirculation;ingeneral,theyredistributeheatbetweenoceanandatmosphereaswellasredistributeitgeographicallyaroundtheplanet.
Greenhouse gas forcing
ThephysicsbywhichgreenhousegasesinfluencetheclimateattheEarth’ssurfaceisnowverywellestablishedandaccepted;itwasfirstproposedin1824byJosephFourier,experimentallyverifiedin1859byJohnTyndall(Crawford1997)andquantifiedneartheendofthe19thcenturybySvanteArrhenius(Arrhenius1896).Muchresearchinthe20thcentury(e.g.,Weart2003;Fleming2007;RevelleandSuess1957)hasstrengthenedthescientificbasisforthetheoryaswellassharpenedourunderstandingofit.Forexample,theverylargedifferencesinsurfacetemperatureamongEarth,VenusandMarscanonlybeexplainedbytheverydifferentamountsofCO
2in
theiratmospheres(Lacisetal.2010).Also,thedifferenceingloballyaveragedtemperaturebetweenaniceageandawarmperiod,about5-6°C,canonlybeexplainedbychangesingreenhousegasconcentrationsandinthereflectivity(albedo)oftheEarth’ssurfacethatamplifytheoriginalmodestchangesintemperatureduetovariationsinincomingsolarradiationcausedbycyclicalchangesintheEarth’sorbitaroundthesun(RahmstorfandSchellnhuber2006).
Chapter 1. Developments in the science of climate change(continued)
Figure 9. Reconstructed surface temperature pattern for the Medieval Climate Anomaly (MCA, 950 to 1250 C.E., sometimes called the Medieval Warm Period). Shownarethemeansurfacetemperatureanomaly(left)andassociatedrelativeweightingsofvariousproxyrecordsused(indicatedbysizeofsymbols)forthelow-frequencycomponentofthereconstruction(right).Anomaliesaredefinedrelativetothe1961-1990referenceperiodmean.Statisticalskillisindicatedbyhatching(regionsthatpassvalidationtestsatthep=0.05levelwithrespecttoRE(CE)aredenotedby/(\)hatching).Greymaskindicatesregionsforwhichinadequatelong-termmodernobservationalsurfacetemperaturedataareavailableforthepurposesofcalibrationandvalidation.
Source:Mannetal.(2009),whichcontainsfurtherinformationonmethodology.
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ScienceUpdate2011 15
Figure 10. Comparison of observed continental- and global-scale changes in surface temperature with results simulated by climate models using natural and anthropogenic forcings. Decadalaveragesofobservationsareshownfortheperiod1906to2005(blackline)plottedagainstthecentreofthedecadeandrelativetothecorrespondingaveragefor1901–1950.Linesaredashedwherespatialcoverageislessthan50%.Purpleshadedbandsshowthe5–95%rangefor19simulationsfromfiveclimatemodelsusingonlythenaturalforcingsduetosolaractivityandvolcanoes.Redshadedbandsshowthe5–95%rangefor58simulationsfrom14climatemodelsusingbothnaturalandanthropogenicforcings.
Source:IPCC(2007a)
Chapter 1. Developments in the science of climate change(continued)
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Asknowledgeofthegreenhouseeffectimproves,changesintheclimatesystemmoresubtlethangloballyaveragedtemperatureyieldpatternsofchangeconsistentwiththeinfluenceofCO
2andothergreenhousegases,
andinconsistentwithchangesinsolarradiation.Such“fingerprintsofgreenhousegasforcing”include,forexample,theobservationthatwintersarewarmingmorerapidlythansummersandthatovernightminimumtemperatureshaverisenmorerapidlythandaytimemaximumtemperatures(IPCC2007a).Anapparentinconsistencybetweenobservationswithgreenhousetheorywastheallegedfailuretofindaso-called“tropicalhotspot”,awarminginthetropicalatmosphereabout10-15kmabovetheEarth’ssurface.Inreality,therewasnoinconsistencybetweenobservedandmodelledchangesintropicaluppertropospherictemperatures,allowingforuncertaintiesinobservationsandlargeinternalvariabilityintemperatureintheregion.Furthermore,recentthermalwindcalculationshaveindeedshowngreaterwarmingintheregion(AllenandSherwood2008),confirmingthatthereisnoinconsistencyandprovidinganotherfingerprintofenhancedgreenhouseforcing.
Climate sensitivity
Theriseingloballyaveragedtemperatureatequilibriumduetoagivenchangeinradiativeforcingisknownasthe“climatesensitivity”.Inthecontextofhuman-drivenclimatechange,climatesensitivityusuallyreferstotheequilibriumtemperatureriseresultingfromadoublingofCO2
concentration(from280to560ppm;ppm=partspermillion).Withnootherresponsesoftheclimatesystem(e.g.changesinwatervapour,albedoorclouds),adoublingofCO
2alonewouldresultinaround1°Cwarming–a
numbereasilyderivedfromwellestablishedradiationcalculations.Importantly,watervapouramountsarecloselytiedtotemperature,increasingwithwarming,andtrappingextraheat.Theory,modellingstudiesandobservationsallstronglysupporttherebeingastrongpositive(reinforcing)watervapourfeedback,whichroughlydoublestheinitialwarmingfromCO
2.Otherfeedbacks,
duetoresponsesfromsurfacealbedo(positive),temperature‘lapserate’(negative)andcloudsalsocontribute,withcloudfeedbacksbeingthemostuncertain.
somE of THE mosT ImpoRTAnT REsEARCH In RECEnT yEARs HAs REDuCED THE unCERTAInTy suRRounDIng EsTImATEs of CLImATE sEnsITIvITy.
–Multiplesimulationsbyclimatemodelsdrivenbya560ppmCO
2atmospherehavegeneratedaprobability
densityfunctionwithmostofthevaluesforsensitivityfallingbetween2and4.5°Candapeaknear3°C(IPCC2007a).AnanalysisofthetransitionoftheEarthfromthelasticeagetotheHolocene,whichinfersclimatesensitivityfromtheobservedchangeintemperatureandthecorrespondingchangesinthefactorsthatinfluenceradiativeforcing,alsoestimatesavalueofabout3°C(Hansenetal.2008).Muchoftheuncertaintyonthemagnitudeofclimatesensitivityisassociatedwiththedirectionandstrengthofcloudfeedbacks.Recentobservationalevidencefromshort-termvariationsincloudssuggeststhatshort-termcloudfeedbacksarepositive,reinforcingthewarming,consistentwiththecurrentmodel-basedestimatesofcloudfeedbacks(Clementetal.2009;Dessler2010).
Arecentmodelstudycomparingtherelativeimportanceofvariousgreenhousegasesfortheclimateestimatesasensitivityofapproximately4°CforadoublingofCO2
(Lacisetal.2010).Inaddition,thestudypointstotheimportanceofCO
2astheprincipal“controlknob”
governingEarth’ssurfacetemperature.AlthoughCO2
accountsforonlyabout20%ofEarth’sgreenhouseeffect(otherlong-livedgreenhousegasesaccountfor5%andwatervapourandcloudsaccountfor75%viatheirfastfeedbackeffects),itistheonethateffectivelycontrolsclimatebecauseofitsverylonglifetimeintheatmosphere.Watervapouramountsaredeterminedbyatmospherictemperatures,whichinturnaregovernedbyconcentrationsofthelong-livedgreenhousegasessuchasCO
2.Infact,withouttheselong-livedgreenhouse
gases,theEarth’stemperaturewoulddroprapidlyanddrivetheplanetintoanice-boundstate.
Chapter 1. Developments in the science of climate change(continued)
ScienceUpdate2011 17
1.3 How is the carbon cycle changing?
The analysis in the previous two sections shows that (i) the Earth’s surface is warming at a relatively rapid rate, and (ii) the primary reason for this warming, at least since the middle of the 20th century, is the increase in CO2 in the atmosphere. These conclusions focus attention strongly on the carbon cycle – both how the natural carbon cycle operates and how human activities are modifying the cycle.
Thissectionsummarisesthemostrecentresearchonchangesinthebehaviourofthecarboncycleandpotentialchangestothecycleinthefuture:
Human emissions of CO2
TheGlobalFinancialCrisisledtoadropin2009of1.3%intheglobalemissionsofCO
2fromfossilfuelcombustion,
insharpcontrasttotheaverageannualriseinfossilfuelCO
2emissionsof3.2%forthe2000-2008period
(Friedlingsteinetal.2010).Thegrowthrateofemissionsisexpectedtoresumeitsupwardtrendof3%orgreaterin2010andsubsequentyears,barringafurthersharpeconomicdownturnorrapidandvigorousreductionsinemissionsinresponsetoclimatechange(Figure11;RaupachandCanadell2010).Giventhestrongriseinemissionsoverthepastdecade,currentemissionsareabout37%largerthanthosein1990,sometimesusedasthebaselineagainstwhichtomeasureemissionreductions.ThecurrentrateofemissionsliesnearthetopoftheenvelopeofIPCCprojections(RaupachandCanadell2010).Overthelastfewyears,coalhasovertakenoilasthelargestsourceofCO
2fromfossilfuelcombustion
(LeQuéréetal.2009:CDIAC2010).Despitethedropintheabsoluteamountofemissionsin2009,theatmosphericconcentrationofCO
2stillroseby1.6ppmduringtheyear,
comparedtoanaveragegrowthrateof1.9ppmperyearforthe2000-2008period(TansandConway2010).
Figure 11. Global CO2 emissions since 1997 from fossil fuel and cement production. EmissionswerebasedontheUnitedNationsEnergyStatisticsto2007,andonBPenergydatafrom2007onwards.CementCO
2
emissionsarefromtheUSGeologicalSurvey.Projectionfor2010isincludedinred.
Source:Friedlingsteinetal.(2010),andreferencestherein.
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Chapter 1. Developments in the science of climate change(continued)
– Despitethedipinhumanemissionsofgreenhousegasesin2009duetotheGlobalFinancialCrisis,emissionscontinueonastrongupwardtrend,onaveragetrackingnearthetopofthefamilyofIPCCemissionscenarios.
– Oceanandlandcarbonsinks,whichtogethertakeupmorethanhalfofthehumanemissionsofCO
2,
appeartobeholdingtheirproportionalstrengthscomparedtoemissions,althoughsomerecentevidencequestionsthisconclusionandsuggestsalossofefficiencyinthesenaturalsinksoverthepast60years.
– Ifglobalaveragetemperaturerisessignificantlyabove2°C(relativetopre-industrial),thereisanincreasingriskoflargeemissionsfromtheterrestrialbiosphere,themostlikelysourcebeingmethanestoredinpermafrostinthenorthernhighlatitudes.
18 ClimateCommission
Figure 12. Terms in the global CO2 budget for the period 1850-2008 inclusive. AnthropogenicCO2emissions,
shownaspositivefluxesintotheatmosphere,comprisecontributionsfromfossilfuelcombustionandotherindustrialprocesses,andlandusechange,mainlydeforestation.ThefateofemittedCO
2,includingtheaccumulationof
atmosphericCO2,thelandCO
2sinkandtheoceanCO
2sink,isshownbythebalancingnegativefluxes.Valuesof
averagefluxesfor2000-2008(shownatright)includeasmallresidualbecausealltermswereestimatedindependentlyfrommeasurementsormodels,withoutaprioriapplicationofamass-balanceconstraint.
Source:RaupachandCanadell(2010),basedonLeQuereetal.(2009).
Chapter 1. Developments in the science of climate change(continued)
Ocean and land carbon sinks
Naturalsinksofcarbononlandandinoceans(e.g.,uptakeofCO2
bygrowingvegetation;dissolutionofCO2in
seawater)havehistoricallyremovedoverhalfofthehumanemissionsfromtheatmosphere–forexample,57%forthe1958-2009period(LeQuéréetal.2009).Thecarbonistakenupinapproximatelyequalproportionsbylandandocean(RaupachandCanadell2010;Figure12),althoughthereisconsiderablevariabilityinthestrengthofthesenaturalsinksfromyeartoyear,largelyinresponsetoclimatevariability.Overthepasthalf-centurythecapabilityofthesenaturalsinkshasgenerallykeptpacewiththeincreasinghumanemissionsofCO
2.However,
thereisevidencethattheefficiencyofthesesinksisdeclining(Canadelletal2007;Raupachetal.2008;LeQuéréetal.2009),particularlyintheSouthernocean(LeQuéréetal.2007).Thereareuncertaintieswithsomeoftheseresults,andsomescientificcontroversyoverthedecliningtrendparticularlyinrecentyears(Franceyetal.2010;Knorr2009;Poulteretal.2011).
Theongoingstrengthofthesenaturalsinksiscruciallyimportantforthelevelofeffortthatwillberequiredtolimitclimatechangetonomorethana2°Criseabovepre-industrial,oftenreferredtoasthe2°Cguardrail(CounciloftheEuropeanUnion2005;IPCC2007a;CopenhagenAccord2009).Thistarget,oftenquotedasdefiningtheboundaryof“dangerous”climatechange,isbasedonvaluejudgements,informedbyscientificunderstanding,andhasbeendevelopedthroughapoliticalprocess.Thereisconsiderablescientificevidence(e.g.,Smithetal.2009;Richardsonetal.2011)thatvaluesoftemperatureriseabove2°Care“dangerous”bymostdefinitions,butthisevidencealsoshowsthattherearesignificantrisksofseriousimpactsinvarioussectorsandlocationsattemperatureincreasesoflessthan2°C.Nevertheless,the2°Cguardrailhasbeenawidelyacceptedandquotedpoliticalgoal.
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Vulnerable new sources of carbon
Inadditiontothepotentialweakeningofthecurrentnaturalcarbonsinksastemperatureincreases,thereisthepotentialforactivatingnewnaturalsourcesofcarbonemissionsfrompoolsthatarecurrentlyinactive.Examplesincludemethanehydratesstoredundertheseafloor,organicmaterialstoredintropicalpeatbogsandorganicmaterialstoredinpermafrostinthenorthernhighlatitudesandtheTibetanplateau.Ofthesepotentialsources,thepermafrostcarbonisgenerallyconsideredtobethemostimportant.
–THERE ARE ovER 1,700 bILLIon TonnEs of CARbon sToRED In pERmAfRosT (TARnoCAI ET AL. 2009), wHICH Is AbouT TwICE THE AmounT sToRED In THE ATmospHERE AT pREsEnT.
–Thereisuncertaintyaboutthevulnerabilityofthispotentialnewsourceofcarbon(e.g.,LawrenceandSlater2005;Lawrenceetal.2008),butthereisalreadyevidenceofsomelossofmethanefromthenorthernhighlatitudes(e.g.,Dorrepaaletal.2009).Oneanalysisoffuturevulnerabilityassessesthatabout100billionofthe1,700billiontonnesarevulnerabletothawingthiscentury(Schuuretal.2009).
1.4 How certain is our knowledge of climate change?
Following criticisms of the IPCC, the so-called “climategate” incident in the UK (the hacking of emails of climate scientists at the University of East Anglia), and numerous attacks in the media and elsewhere on climate science, there has been much focus on the veracity of climate science and on the level of certainty or uncertainty surrounding knowledge of climate science. The key question is: Are we confident enough about (i) our understanding of the climate system, (ii) the human influence on climate, and (iii) the consequences of contemporary climate change for societies and ecosystems to provide a reliable knowledge base on which to base policy and economic responses?
Thissectionwillbrieflyexplorethelevelofcertaintyanduncertaintysurroundingtheknowledgebaseonwhichscientificunderstandingofcontemporaryclimatechangerests.Themainmessagesare:
Chapter 1. Developments in the science of climate change(continued)
– TheIPCC’sFourthAssessmentReporthasbeenintensivelyandexhaustivelyscrutinisedandisvirtuallyerror-free.
– TheEarthiswarmingonamulti-decadaltocenturytimescale,andataveryfastratebygeologicalstandards.Thereisnodoubtaboutthisstatement.
– Humanemissionsofgreenhousegases–andCO2
isthemostimportantofthesegases–istheprimaryfactortriggeringobservedclimatechangesinceatleastthemid20thcentury.TheIPCCAR4(2007a)reportattached90%certaintytothatstatement;researchoverthepastfewyearshasstrengthenedourconfidenceinthisstatementevenmore.
– Manyuncertaintiessurroundprojectionsoftheparticularrisksthatclimatechangeposesforhumansocietiesandnaturalandmanagedecosystems,especiallyatsmallerspatialscales.However,ourcurrentlevelofunderstandingprovidessomeusefulinsights:(i)somesocial,economicandenvironmentalimpactsarealreadyobservablefromthecurrentlevelofclimatechange;(ii)thenumberandmagnitudeofclimateriskswillriseastheclimatewarmsfurther.
20 ClimateCommission
Figure 13. The process by which the IPCC carries out an assessment, including the careful and exhaustive, two-tiered review process.
Source:IPCC(2011).
The IPCC
Astheprimarysourceofinformationonclimatechangeforthepolicycommunity,theIPCCproducesperiodicassessmentsoftheliteraturebyscientificexpertsapproximatelyeverysixyears,aswellasinterimspecialreports.Therearethreeworkinggroups–oneeachforthefundamentalclimatescience;impacts,adaptationandvulnerability;andmitigationofclimatechange.TheFourthAssessmentReport(AR4),publishedin2007,involvedabout1,250expertauthorsand2,500reviewers,whoproducedabout90,000commentsondrafts,eachoneofwhichwasaddressedexplicitlybytheauthors.Thisexhaustive,thoroughprocessisshownschematicallyinFigure13.
TheIPCCAR4hasbeenintensivelyandexhaustivelyscrutinised,includingformalreviewssuchasthatbytheInterAcademyCouncil(2010),andonlytwoperipheralerrors,bothofthemintheWG2reportonimpactsandadaptation,haveyetbeenfound(inapublicationcontainingapproximately2.5millionwords!).Noerrorshavebeenfoundinanyofthemainconclusions,norhaveanyerrorsbeenfoundinthe996-pageWG1report,whichdescribesourunderstandingofhowandwhytheclimatesystemischanging.TheIPCCAR4WG1reportprovidesthescientificinputtothedevelopmentofclimatepolicy.Severalofficial“assessmentsoftheassessment”haveconcludedthattheconclusionsoftheAR4aresound(InterAcademyCouncil2010;RoyalSociety2010;NationalResearchCouncil2010).
Chapter 1. Developments in the science of climate change(continued)
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Insummary,despiteintensive,andultimatelyunsuccessful,attemptstofindimportanterrorsintheassesments,theIPCChasbeenconfirmedasasourceofreliablescientificinformationonclimatechange.
Certainty of warming
TheevidencethattheEarthiswarmingonamulti-decadaltimescale,andataveryfastratebygeologicalstandards,isnowoverwhelming.Someofthisevidencehasbeenpresentedabove.TheIPCCusedtheword“unequivocal”todescribeourconfidenceintheobservationsofawarmingEarth.Observationssince2007havestrengthenedourconfidenceinthisstatement.
Human causation
Basedonitsthoroughassessmentoftheevidence,theIPCCin2007statedthat:
–“mosT of THE obsERvED InCREAsE In gLobAL AvERAgE TEmpERATuREs sInCE THE mID-20TH CEnTuRy Is vERy LIkELy DuE To THE obsERvED InCREAsE In AnTHRopogEnIC gREEnHousE gAs ConCEnTRATIons”.
–Thetermvery likelyintheIPCCdefinitionsofuncertaintyisassociatedwithagreaterthan90%certaintythatthestatementiscorrect(IPCC2007a).ResearchoverthepastfewyearshasfurtherstrengthenedourconfidenceintheIPCC’sassessmentofattribution.Suchresearch,describedearlier,includesbetterestimatesofclimatesensitivity,moreobservationsofthepatternsofclimaticchanges–“fingerprints”characteristicofgreenhousegasforcing,andimprovedunderstandingofthelong-termroleofCO
2intheclimatesystem.
Large uncertainties
Althoughthefundamentalfeaturesofclimatechange,asdescribedabove,areverywellknown,significantuncertaintiessurroundourunderstandingofthebehaviourofimportantpartsoftheclimatesystem.Forexample,thewaysinwhichthelargepolaricesheetsonGreenlandandAntarcticaarerespondingandwillrespondinfuturetowarmingarenotwellknown,andaregeneratingintensediscussionandfurtherresearchinthescientificcommunity.Similarly,althoughconsiderableevidencepointstowardanaccelerationofthehydrologicalcycleastheclimatewarms–increasedevaporation,morewatervapourintheatmosphere,andincreasedprecipitation–thistrendisstillbeingdebatedintheresearchcommunity,asistheinfluenceofclimatechangeonspatialpatternsofprecipitationacrosstheEarth’ssurfaceandonthetemporalpatternsofprecipitation–droughtsandintenserainfallevents.Theseuncertainties,however,innowaydiminishourconfidenceintheobservationthattheEarthiswarmingandinourassessmentthathumanemissionsofgreenhousegasesaretheprimaryreasonforthiswarming.
Manyuncertaintiesalsosurroundourunderstandingoftherisksthatclimatechangeposesforhumansocietiesandnaturalandmanagedecosystems.Theseuncertaintiesstemfromseveralfactors:(i)uncertaintiesintheprojectionsofpotentialimpactsfromfutureclimatechange;(ii)uncertaintiesassociatedwiththedynamicsofsystemsbeingimpactedbyclimate,suchasagriculturalsystems,naturalecosystems,orurbansystems;and(iii)uncertaintiesinthewaysinwhichhumanswillrespondtothethreatsofclimatechangebyreducingtheirvulnerabilityorincreasingtheiradaptivecapacity.Despitetheseseeminglydauntinguncertainties,anumberofsocial,economicandenvironmentalimpactscanbeobservedthatareconsistentwithwhatisanticipatedfromthecurrentlevelofclimatechange.Thenumberandmagnitudeofclimate-relatedriskswillriseconsiderablyastheclimatewarmstowards2°Cabovethepre-industriallevel;andabovethe2°Cguardrail,therisksmayrisedramatically(Smithetal.2009;Richardsonetal.2011).Themostseriousrisksareassociatedwithpotentialabruptorirreversiblechangesinlargefeaturesoftheclimatesystem,suchastheswitchtoadrystateoftheIndianSummerMonsoon(Lentonetal.2008).Decision-makinginthefaceofsuchuncertaintieswillremainabigchallengeforthepolicyandmanagementcommunities.
Chapter 1. Developments in the science of climate change(continued)
CHApTER 2:RIsks AssoCIATED wITH A CHAngIng CLImATE
TEmpERATuRE InCREAsEs of 1 oR 2 °C mAy sEEm moDEsT, buT THEy CAn LEAD To DIspRopoRTIonATELy LARgE CHAngEs In THE fREquEnCy AnD InTEnsITy of ExTREmE wEATHER EvEnTs.
CoRAL-DomInATED ECosysTEms ARE sEnsITIvE To smALL RIsEs In THE TEmpERATuRE of THE wATER In wHICH THEy REsIDE. wHEn THE sEA TEmpERATuRE RIsEs 1-2°C AbovE noRmAL foR A sIx-EIgHT wEEk pERIoD THE CoRALs ARE “bLEACHED”.
wHAT wE CAn sAy wITH CERTAInTy Is THAT RAInfALL pATTERns wILL CHAngE As A REsuLT of CLImATE CHAngE AnD ofTEn In unpREDICTAbLE wAys, CREATIng LARgE RIsks foR wATER AvAILAbILITy.
DID you know...
ScienceUpdate2011 23
Chapter 2. Risks associated with a changing climate
2.1 Sea-level rise
With much of our population and a high fraction of our infrastructure located close to the coast, Australia is vulnerable to the risks posed by sea-level rise. Although sea level will continue to rise for many centuries (Solomon et al. 2009), the more immediate concern is the level of risk associated with sea-level rise out to 2100, when some of our existing infrastructure and much new infrastructure will be at risk. The rate at which sea level will rise through this century is a critical factor in determining the degree of exposure to risk.
ThissectionbuildsonSection1.1onobservationsofsea-levelrisetoexploretherangeofratesatwhichsea-levelcouldrisethiscenturyandtheimplicationsofsuchrisesfortheinundationofpartsofourcoastline.
Thekeymessagesare:
Projections of future sea-level rise
–pRojECTIons of sEA-LEvEL RIsE foR THE REsT of THE CEnTuRy vARy wIDELy, fRom THE ofT-quoTED RAngE of 0.19-0.59 m bAsED on THE IpCC AR4 (2007a) To nEARLy 2 mETREs (vERmEER AnD RAHmsToRf 2009).
–TheIPCCprojectionsareoftenmisquotedastheydonottakeintoaccountthelossoficeduetodynamicalprocessesinthelargepolaricesheets.
Whenestimatesforthisprocessareincluded,therangechangesto0.18–0.76m,andtheIPCCiscarefultonotethathighervaluescannotbeexcluded(IPCC2007a;Figure14).Bycomparison,theThirdAssessmentReportoftheIPCC(2001)projectedasea-levelriseof0.11–0.88mforthiscentury.
Theprojectionsyieldinghigherrangesofsea-levelareoftenbasedonstatisticalorsemi-empiricalmodelsthatrelatetheobservedsea-levelriseoverthepast120yearstotheobservedtemperatureriseoverthatperiod(e.g.,Rahmstorf2007;Hortonetal.2008;Grinstedetal.2009).Theapproachistouseprojectionsoftemperatureriseto2100toestimatethecorrespondingriseinsea-levelbasedontheobservedrelationship.Therangeoftemperatureincreasesthenyieldsarangeofprojectedsea-levelchanges.Projectionsusingsemi-empiricalmodelsaregenerallyhigherthanthoseoftheIPCCbecausetheyincorporatetheobservedaccelerationofsea-levelriseduringthe1990-2009periodandprojectthatfurtheraccelerationwilloccurastheclimatewarms.Figure15showsanexampleofprojectedchangesinsealevelbasedonasemi-empiricalmodel(VermeerandRahmstorf2009).
– Aplausibleestimateoftheamountofsea-levelriseby2100comparedto2000is0.5to1.0m.Thereissignificantuncertaintyaroundthisestimate,thelargestofwhichisrelatedtothedynamicsoflargepolaricesheets.
– Muchmorehasbeenlearnedaboutthedynamicsofthelargepolaricesheetsthroughthepastdecadebutcriticaluncertaintiesremain,includingtherateatwhichmassiscurrentlybeinglost,theconstraintsondynamiclossoficeandtherelativeimportanceofnaturalvariabilityandlonger-termtrends.
– Theimpactsofrisingsea-levelareexperiencedthrough“highsea-levelevents”whenacombinationofsea-levelrise,ahightideandastormsurgeorexcessiverun-offtriggeraninundationevent.Verymodestrisesinsea-level,forexample,50cm,canleadtoveryhighmultiplyingfactors–sometimes100timesormore–inthefrequencyofoccurrenceofhighsea-levelevents.
24 ClimateCommission
Chapter 2. Risks associated with a changing climate (continued)
Figure 14. Projections of sea-level rise from 2100 from the IPCC Third Assessment Report (TAR) and the Fourth Assessment Report (AR4). TheTARprojectionsareindicatedbytheshadedregionsandthebrokenredlinesaretheupperandlowerlimits.TheAR4projectionsarethebarsplottedinthe2090-2100period.Theinsetshowssealevelobservedwithsatellitealtimetersfrom1993to2006(red)andobservedwithcoastalsea-levelmeasurementsfrom1990to2001(purpledashes).
Source:ACECRC2008.
Figure 15. Projection of sea-level rise from 1990 to 2100, based on IPCC temperature projections for three different emission scenarios (labelled on right, see Vermeer and Rahmstorf (2009) for explanation of uncertainty ranges). Thesea-levelrangeprojectedintheIPCCAR4(2007a)(excludingcontributionsfromice-sheetdynamicprocesses)forthesescenariosisshownforcomparisoninthebarsonthebottomright.Alsoshownistheobservations-basedannualglobalsea-leveldata(solidredlineto2003)includingartificialreservoircorrection.
Source:VermeerandRahmstorf(2009),andreferencestherein.
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ScienceUpdate2011 25
Chapter 2. Risks associated with a changing climate (continued)
Anestimateofthelikelymagnitudeofsea-levelrisethiscenturyisusefulinformationforriskassessments.Acontinuationofthecurrentlyobservedrateof3.2mmyr-1wouldgiveariseofabout0.32mby2100,aboutthemid-rangeoftheIPCCscenarios.However,sealeveliscurrentlytrackingneartheupperrangeofthescenarios,anditseemsunlikelythattherateofsea-levelrisewillremainfixedfornearlyacenturyatitscurrentlevelasthetemperaturecontinuestorise.Ontheotherhand,projectionsof1.5or2.0metresseemhighinlightofrecentquestionssurroundingestimatesofthecurrentrateofmasslossfrompolaricesheets(seeFigure6).
Anestimateforthemostlikelymagnitudeofsea-levelrisein2100relativeto2000takingpolaricesheetdynamicsintoaccountisabout0.8m(Pfefferetal.2008),andanexpertassessmentofGreenlandicesheetdynamicssuggeststhatitwillcontributeabout20cmtoglobalsea-levelriseby2100(Dahl-JensenandSteffen2011).Thesearebothconsistentwithanestimateofa0.5–1.0mriseinsealevelby2100.
Dynamics of large polar ice sheets
Thelargestuncertaintyintheprojectionsofsea-levelrisediscussedaboveisthebehaviourofthelargemassesoficeonGreenlandandAntarctica.Projectionsattheupperlevelsoftherangesofsea-levelriseassumeamuchgreatercontributionfromthesepolaricesheets,and,inparticular,fromdynamicalprocessesthatdischargelargeblocksoficeintothesea.Observationsoverthepast20years,eitherbysatelliteoraircraftaltimetersthatmeasurechangesintheheightoftheicesheetsorbysatellitegravitymeasurementsthatinferchangesinmass,showacceleratingdecreasesinthemassoftheGreenlandicesheetoverthepast15years(Figure16)andinthemassoftheAntarcticicesheetoverthepastdecade.Suchobservationsappeartosupporthigherestimatesofsea-levelriseby2100.
Figure 16. Estimates of the net mass budget of the Greenland ice sheet since 1960. Anegativemassbudgetindicatesicelossandsea-levelrise.DottedboxesrepresentestimatesusedbytheIPCC(2007a).Thesolidboxesarepost-IPCCAR4assessment(R=Rignotetal.2008b;VW=VelicognaandWahr2006;L=Luthckeetal.2006;WT=Woutersetal.2008;CZ=Cazenaveetal.2009;V=Velicogna2009).
Source:TheCopenhagenDiagnosis(2009).
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26 ClimateCommission
However,themeasurementsareforveryshortperiodsoftimeandsoaredifficulttoextrapolatetolongertimescales.Forexample,Figure16includesonlyonerecordofmasschangeintheGreenlandicesheetthatislongerthan20years,therecordofRignotetal.(2008b)from1958.Thatobservationalrecordshowsconsiderablevariabilityonadecadaltimescale,makingitmoredifficulttoextrapolatetheobservationsofthelast10-15yearsintothefuturewithahighdegreeofcertainty.Furthermore,arecentanalysisofthegravitymeasurementmethodology(BromwichandNicolas2010)arguesthatestimatesofthecumulativemasslossoftheGreenlandicesheetaretoolargebyafactoroftwoorso(Figure6).ThestudyofPfefferetal.(2008)ofthekinematicconstraintsonrapidicedischargefromthelargepolaricesheetssuggestsanabsolutemaximumsea-levelriseof2metresby2100,butonlyunderextremeclimaticforcing.
–gIvEn THE ImpoRTAnCE of THE LARgE poLAR ICE sHEETs foR THE RATE of sEA-LEvEL RIsE To 2100 AnD bEyonD, THEsE ongoIng unCERTAInTIEs AbouT THE bEHAvIouR of THE ICE sHEETs unDER fuRTHER gLobAL TEmpERATuRE InCREAsEs CompRIsE onE of THE mosT pREssIng sCIEnTIfIC REsEARCH CHALLEngEs THAT REquIRE uRgEnT REsoLuTIon.
–
High sea-level events (inundation)
Manyoftherisksduetosea-levelriseareassociatedwithinundationevents,whichdamagehumansettlementsandinfrastructureinlow-lyingcoastalareas,andcanleadtoerosionofsandybeachesandsoftcoastlines.Whileasea-levelriseof0.5m–lessthantheaveragewaistheightofanadulthuman–maynotseemlikeamatterformuchconcern,suchmodestlevelsofsea-levelrisecanleadtounexpectedlylargeincreasesinthefrequencyofextremehighsea-levelevents.Thesearedefinedasinundationeventsassociatedwithhightidesandstormsurges,amplifiedbytheslowriseinsealevel.Sucheventsareverysensitivetosmallincreasesinsealevel,andtheprobabilityoftheseeventsrisesinahighlynonlinearwaywithrisingsealevel.
Figure17showstheresultsofananalysisexploringtheimplicationsofsea-levelriseforextremesea-leveleventsaroundtheAustraliancoastline(Churchetal.2008).Asea-levelriseof0.5m,atthelowerendoftheestimatesfor2100,wasassumedintheanalysisshowninthefigure,andleadstosurprisinglylargeimpacts.ForcoastalareasaroundAustralia’slargestcities–SydneyandMelbourne–ariseof0.5mleadstoverylargeincreasesintheincidenceofextremeevents,byfactorsof1000or10,000forsomelocations.Amultiplyingfactorof100meansthatanextremeeventwithacurrentprobabilityofoccurrenceof1-in-100–theso-calledone-in-a-hundred-yearevent–wouldoccureveryyear.Amultiplicationfactorof1000impliesthattheone-in-a-hundred-yearinundationeventwouldoccuralmosteverymonth.
Chapter 2. Risks associated with a changing climate (continued)
ScienceUpdate2011 27
Chapter 2. Risks associated with a changing climate (continued)
Figure 17. Estimated multiplying factor for the increase in the frequency of occurrence of high sea-level events caused by a sea-level rise of 0.5 metres. Highsea-leveleventsareverysensitivetosmallincreasesinsealevel.
Source:ACECRC2008.
Theobservedsea-levelriseofabout20cmfrom1880to2000shouldalreadyhaveledtoanincreaseintheincidenceofextremesea-levelevents.Suchincreaseshaveindeedbeenobservedatplaceswithverylongrecords,suchasFremantleandFortDenison,wherea3-foldincreaseininundationeventshasoccurred(Churchetal.2006).ThisisconsistentwiththemethodologyusedtoproduceFigure17.
Amoredetailedassessmentofthepotentialimpactsofsea-levelrisehasbeencarriedoutbythethenDepartmentofClimateChange(DCC2009),providingestimatesofareasofinundationforasea-levelriseof1.1m,justabovetheupperendofourprojectionrangefor2100.TheDepartmenthasrecentlyreleasedmoredetailedmapstohighlightlow-lyingcoastalareasvulnerabletoinundationfromsea-levelrise.
10000 x 1000 x 100 x
Sydney
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2.2 Ocean acidification
Changes in the alkalinity/acidity of the ocean represent a change in a fundamental environmental condition for marine ecosystems. In particular, those marine organisms that form calcium carbonate shells are at risk from decreasing alkalinity of the ocean, which reduces the concentration of carbonate ions in seawater. Corals are probably the most well-know of these organisms, but other calcifying organisms are important for the marine carbon cycle and play fundamental roles in the dynamics of marine ecosystems.
Thissectionprovidesinformationontheprojectedmagnitudeandrateofchangeofoceanalkalinity/aciditythroughthiscentury,andonobservationsoftheimpactsofincreasingacidityonmarineecosystems.Thekeymessagesare:
– Thecontemporaryrateofincreaseinoceanacidity(decreaseinalkalinity)isverylargefromalong-timeperspective.
– Theeffectsofincreasingacidityaremostapparentinthehighlatitudeoceans,wheretheratesofdissolutionofatmosphericCO
2arethegreatest.
– Increasingacidityintropicaloceansurfacewatersisalreadyaffectingcoralgrowth;calcificationrateshavedroppedbyabout15%overthepasttwodecades.
– RisingSSTshaveincreasedthenumberofbleachingeventsobservedontheGreatBarrierReef(GBR)overthelastfewdecades.Thereisasignificantriskthatwithatemperatureriseabove2°Crelativetopre-industriallevelsandatCO2
concentrationsabove500ppm,muchoftheGBRwillbeconvertedtoanalgae-dominatedecosystem.
28 ClimateCommission
Chapter 2. Risks associated with a changing climate (continued)
Ocean acidity in a long-time context
Therateatwhichoceanacidityisincreasingisimportant,especiallyfromanevolutionaryperspective.Figure18ashowsthechangeinoceanacidityoverthepast25millionyearsandprojectedto2100(Turleyetal.2006;2007).
Oceanacidityhasvariedconsiderablyoverthatperiod,butthelevelofaciditytodayisashighasitwas25millionyearsago,thepreviousmostacidicstateintherecord.
–moRE sTRIkIng Is THE RATE of CHAngE In ACIDITy THAT HAs ALREADy oCCuRRED fRom 1800 To 2000 AnD THAT wHICH Is pRojECTED To 2100. THIs Is An ExCEpTIonALLy RApID RATE of CHAngE, LIkELy unpRECEDEnTED In THE 25 mILLIon yEARs of THE RECoRD, AnD wouLD no DoubT pLACE sEvERE EvoLuTIonARy pREssuRE on mARInE oRgAnIsms. (fIguRE 18b)
–
Figure 18a. Ocean acidity (pH) over the past 25 million years and projected to 2100. ThelowerthepH,themoreacidictheoceanbecomes.Prehistoricsurface-layerpHvalueswerereconstructedusingboron-isotoperatiosofancientplanktonicforaminiferashells.FuturepHvalueswerederivedfrommodelsbasedonIPCCmeanscenarios.
Source:Turleyetal.(2006).Marine ecosystems
Theimpactsofincreasingoceanacidityarealreadyevidentinsomemarinespecies(Moyetal.2009).Manyoftheearliestimpactsareexpectedinpolarorsub-polarwaters,asCO
2ismoresolubleincoldwaterthanwarm
andsoacidificationisexpectedtoproceedmorerapidlythere.ObservationalstudiesintheSouthernOceanofacidityandcarbonateionconcentrationshowstrongseasonalminimumsinwinter;conditionsdeleteriousforthegrowthofcalcifyingplanktonspeciescouldoccurasearlyas2030inwinter(McNeilandMatear2008).Experimentsinseawaterwiththeaciditylevelexpectedin2100haveshowna30%reductionincalcificationratesforacommonpteropod(pelagicmarinesnail),animportantcomponentofmarinefoodchains(Comeauetal.2009).Evenlargerreductionsincalcificationratesofaround50%havebeenfoundinexperimentswithadeepwatercoral(Maieretal.2009).Impactsofacidificationfurtherupmarinefoodchains,includingfish,arelargelyunknownasyet(TurleyandFindlay2009).Previousoceanacidificationeventsarelikelytohavebeensignificantfactorsinmassextinctioneventsinmarineecosystems(Veron2008).
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ScienceUpdate2011 29
Figure 18b. Changes in relative aragonite saturation predicted to occur as atmospheric CO2 concentrations (ppm – upper left of panels) increase over shallow-water coral reef locations (pink dots).
Source:Hoegh-Guldbergetal.(2007),whichgivesmoreinformationonthefigure.SeealsoFigure20afortheconnectionbetweenaragonitesaturationandcoralreefstate.
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Chapter 2. Risks associated with a changing climate (continued)
30 ClimateCommission
Coral reefs
TherisksofclimatechangeforcoralreefsareparticularlyimportantforAustralia,giventheiconicstatusandeconomicimportanceoftheGreatBarrierReef(OxfordEconomics2009).
Thereisevidencethatshowsapossibleimpactoftheincreaseinaciditythathasalreadyoccurred,basedonastudyofchangesinthecalcificationrateofthecoralPorites(De’athetal.2009).Theobservationalstudywascarriedoutusing328siteson69reefsandshowedaprecipitousdropincalcificationrate,linearextensionandcoraldensity,allindicatorsofcoralgrowth,inthelast15-20yearsofa400-yearrecord(Figure19).Thesedataaresuggestiveofahighlynonlinearresponseofcoralstooceanacidity(incombinationwithotherstressors),perhapstakingtheformofthreshold-abruptchangebehaviour(cf.Section3.5).
Figure 20a. Temperature, atmospheric CO2 concentration and carbonate ion concentrations reconstructed for the past 420,000 years. CarbonateconcentrationswerecalculatedfromVostokicecoredata.Acidityoftheoceanhasvariedby+/-0.1pHunitsoverthepast420,000years.Thethresholdsformajorchangestocoralcommunitiesareindicatedforthermalstress(+2°C)andcarbonateionconcentration(200micro-molkg−1);thelattercorrespondstoanapproximatearagonitesaturationof3.3andanatmosphericCO
2concentrationof480ppm.
RedarrowspointingtowardstheupperrightindicatethepathwaycurrentlyfollowedtowardsatmosphericCO
2
concentrationofmorethan500ppm.
Source:Hoegh-Guldbergetal.(2007),includingdetailsofthereconstructionsandthelocationofthephotosinpart(b).
Chapter 2. Risks associated with a changing climate (continued)
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Figure 19. Variation of (a) calcification (grams per square centimetre per year), (b) linear extension (centimetres per year) and (c) density (grams per cubic centimetre) in Porites over time. Darkgreybandsindicate95%confidenceintervalsforcomparisonbetweenyears,andpurplebandsindicate95%confidenceintervalsforthepredictedvalueforanygivenyear.
Source:De’athetal.(2009).
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ScienceUpdate2011 31
Chapter 2. Risks associated with a changing climate (continued)
CoralsarealsoaffectedbyextremesinSSTs(cf.Section3.4),whichcanleadtocoralbleaching.A21stcenturyriskanalysisforcoralreefsemphasisestheimportanceofbothSSTandacidity/carbonateionconcentrationfortheirfutureviability.Figure20combinesthetemperatureandacidity/carbonateionconcentrationinfluencesinatwo-dimensionalenvironmentalspacediagramthatcontraststhepast,presentandfutureenvironmentsforcoralreefs(Hoegh-Gulbergetal.2007).
Theclusterofreddotsrepresentstheenvelopeofnaturalvariabilitythatreefshaveexperiencedoverthepast420,000years.Presentconditionsofcarbonateionconcentration–butnottemperature–havepushedreefsoutsideofthisenvelope.
mosT EmIssIons AnD CLImATE sCEnARIos foR THE REsT of THIs CEnTuRy (IpCC 2007a) pREDICT THE ConvERsIon of CoRAL REEfs InTo ALgAE-DomInATED ECosysTEms (THE uppER RIgHT quADRAnT of fIguRE 20a AnD THE RIgHT-HAnD pAnEL of fIguRE 20b).
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Source:modifiedfromHoegh-Guldbergetal.(2007).
Figure 20b. Extant examples from the Great Barrier Reef as analogs for the reef states anticipated for the environmental conditions marked A, B and C in part (a) of the figure.
32 ClimateCommission
Chapter 2. Risks associated with a changing climate (continued)
2.3 The water cycle
Australia is the driest of the six inhabited continents, and experiences a high degree of natural climatic variability – the proverbial “land of droughts and flooding rains”. Thus, the link between climate and water resources has been a dominant theme in the lives of all Australians, from the arrival of the first people about 60,000 years ago to the present. The risks of climate change for water resources, and especially the ways in which the longer term trends of human-induced climate change interact with modes of natural variability, is a hotly debated topic, both within and outside of the research community.
–ACHIEvIng A bETTER unDERsTAnDIng of THE nATuRE of THIs RIsk Is An uRgEnT REsEARCH CHALLEngE, THE REsuLTs of wHICH wILL InfoRm mAny mAnAgEmEnT AnD poLICy DECIsIons now AnD InTo THE fuTuRE.
–
Thissectionexploresourcurrentlevelofknowledgeabouttheclimatechange-variabilityrelationshipandtheconsequentrisksforwaterresources.Thekeymessagesare:
– – Observationssince1970showadryingtrendinmostofeasternAustraliaandinsouthwestWesternAustraliabutawettingtrendformuchofthewesternhalfofthecontinent.
– GiventhehighdegreeofnaturalvariabilityofAustralia’srainfall,attributingobservedchangestoclimatechangeisdifficult.Thereisnocleartrend,eitherinobservationsormodelprojections,forhowthemajormodeofvariability,ENSO,isrespondingtoclimatechange.EvidencepointstoapossibleclimatechangelinktoobservedchangesinthebehaviouroftheSouthernAnnularMode(SAM)andtheIndianOceanDipole(IOD).
– ImprovementsinunderstandingoftheclimaticprocessesthatinfluencerainfallsuggestaconnectiontoclimatechangeintheobserveddryingtrendinsoutheastAustralia,especiallyinspring.InsouthwestWesternAustralia,climatechangeislikelytohavemadeasignificantcontributiontotheobservedreductioninrainfall.
– Theconsensusonprojectedchangesinrainfallfortheendofthiscenturyis(i)highforsouthwestWesternAustralia,wherealmostallmodelsprojectcontinuingdryconditions;(ii)moderateforsoutheastandeasternAustralia,whereamajorityofmodelsprojectareduction;and(iii)lowacrossnorthernAustralia.Thereisahighdegreeofuncertaintyintheprojectionsin(ii)and(iii),however.
– Rainfallisthemaindriverofrunoff,whichisthedirectlinktowateravailability.HydrologicalmodellingindicatesthatwateravailabilitywilllikelydeclineinsouthwestWesternAustralia,andinsoutheastAustralia,withlessconfidenceinprojectionsofthelatter.Thereisconsiderableuncertaintyintheprojectionsofamountsandseasonalityofchangesinrunoff.
ScienceUpdate2011 33
Chapter 2. Risks associated with a changing climate (continued)
Observations of rainfall change
AlthoughthecontinentofAustraliahasbecomeslightlywetterintermsoftotalannualrainfalloverthepastcentury,apronouncedpatternhasdevelopedsince1970,withdryinginmuchofeasternAustraliaandinthesouthwestcornerofWesternAustraliaandincreasingrainfallinmuchofthewest(Figure21).Thedevelopmentofthispatternhascoincidedwiththesharpincreaseinglobalaveragetemperature,raisingthequestionofpossiblelinkswithclimatechange.However,Australianaturallyhasahighdegreeofvariabilityinrainfall,withlongperiodsofintensedroughtspunctuatedbyheavyrainfallandflooding,soitisdifficultfromobservationsalonetounequivocallyidentifyanythingthatisdistinctlyunusualaboutthepost-1950pattern(apart,perhaps,fromthedryingtrendinsouthwestWesternAustralia,seebelow).Whiletheinstrumentalrecordgoesbacklittlemorethanacentury,notlongenoughtoclearlydiscernmulti-decadalpatternsofvariabilitythatarerepeatedoncenturytimescales,palaeostudiescouldoffersomeinsightsintotheseverityoftherecentdroughtinalongertimeperspective.Forexample,arecentstudy(GallantandGergis2011)statesthattheverylowstreamflowintheRiverMurrayforthe1998-2008periodisveryrare–abouta1-in-1500yearevent.
Figure 21. Trend in annual total rainfall (mm/10 years) for (a) 1900 – 2010; and (b) 1970 – 2010.
Source:BureauofMeteorology.
34 ClimateCommission
Chapter 2. Risks associated with a changing climate (continued)
The climate change-variability interaction
RainfallpatternsacrossAustraliaareinfluencedincomplexwaysbyseveralmodesofnaturalvariability,themostimportantofwhichareENSO(ElNiño–SouthernOscillation),SAM(SouthernAnnularMode)andIOD(IndianOceanDipole).
Thesemodesareamanifestationofchangesinoceanicandatmosphericcirculationand,inparticular,theircoupling.
Therefore,theirbehaviourmaychangeasoceanicandatmosphericcirculationchangeinresponsetothechangingenergybalanceattheEarth’ssurface.However,forENSOthereisnoclearpatternofchangeinbehaviourthatcanbeobservedintheobservationalrecordoverthepastseveraldecadesandcanbelinkedclearlytoclimatechange,noristhereastrongconsensusinclimatemodelprojectionsofthefuturebehaviourofthismodeofvariability(Collinsetal.2010)
FortheIOD,thenumberof“positive”events,whichinduceareductioninrainfalloversouthernAustraliainwinterandspring,hasbeenincreasingsince1950,reachingarecordhighfrequencyoverthepastdecade(Abrametal.2008;Iharaetal.2008;Caietal.2009a).Bycontrast,thenumberofnegativeIODeventshasbeendecreasing.ThemajorityofclimatemodelsassessedbytheIPCCintheir20thcenturysimulationsproduceanupwardtrendinthefrequencyofpositiveIODevents(Caietal.2009b).Theprojectedpatternofthemeanocean-atmospherecirculationchangeintheIndianOceaninthefutureissimilartothatofapositiveIODphase,implyinganincreaseinpositiveIODfrequencyand/orintensityandthusareductioninrainfalloversouthernAustraliainwinterandspring(Caietal.2011a).
ThesituationisevenclearerfortheSAM.
–THERE Is gooD EvIDEnCE THAT A souTHwARD sHIfT of THE sAm (souTHERn AnnuLAR moDE), wHICH bRIngs RAIn-bEARIng fRonTs In AuTumn AnD wInTER To souTHwEsT wEsTERn AusTRALIA, Is An ImpoRTAnT fACToR In THE obsERvED DRop In RAInfALL THERE ovER THE pAsT sEvERAL DECADEs (TImbAL ET AL. 2010; nICHoLLs 2009).
–CaiandCowan(2006)estimatethatabout50%oftherainfallreductionisattributabletoclimatechange.Thisexplanationisconsistentwithourunderstandingofhowatmosphericcirculationischanginginresponsetoglobalwarming,andisconsistentwithmodelsimulationsofpresentclimateandoffuturechangesintheSAM(FrederiksenandFrederiksen2007;Yin2005;Arblasteretal.2011).
Furthermore,thedryingtrendinthesouthwesthascontinuedinto2011(Figure22),consistentwiththedominantroleoftheSAMthere,incontrasttothemuchwetterperiodintheeast,wheretheothermodesofvariabilityaremoreimportant.
Understanding hydrometeorological processes
Giventheshortobservationalrecord,theimportanceofnaturalvariabilityforAustralia’srainfall,andthelackofconsensusinclimatemodelsimulationsatregionalscales,aprocess-levelunderstandingofthefactorsthatinfluenceAustralianrainfallpatternsoffersonewaytocutthroughthecomplexityanduncertaintyandexplorepossiblelinksbetweenobservedchangesandclimatechange.ThelinkbetweentheSAMandthedecreaseinrainfallinsouthwestWesternAustralia,exploredindepthbytheIndianOceanClimateInitiative(Caietal.2003;CaiandCowan2006;Hendonetal.2007),isanexampleofthesuccessofthisapproach.
ScienceUpdate2011 35
Chapter 2. Risks associated with a changing climate (continued)
Figure 22. Trend in total annual stream flow into Perth dams 1911-2010.
Source:WesternAustralianWaterCorporation.
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ProgresshasalsobeenmadeinunderstandingrecentchangestorainfallinsoutheastAustralia,withtheSEACI(SouthEasternAustralianClimateInitiative,www.seaci.org)playingamajorroleinthisresearch.Severalaspectsoftheobserveddecreaseinrainfall,especiallyinVictoriaandsouthernSouthAustralia,arenowbetterunderstood.First,theproximatecauseoftherainfalldeclineisanincreaseinthesurfaceatmosphericpressureovermuchofthecontinent(Nicholls2009),althoughthecauseoftherisingpressureisnotclear.Inaddition,thesubtropicalridge,aneast-westzoneofhighatmosphericpressurethatoftenliesoverthesouthernpartofthecontinent,hasstrengthenedconsiderablysince1970(Timbaletal.2010;Figure23a).Furthermore,thisstrengtheningofthepressuresystemcorrelatesverywellwiththeriseinglobalmeantemperature(Timbaletal.2010;Figure23b),andisconsistentwithexpectationsfromthebasicphysicsoftheclimatesystem.Inanotherefforttounderstandchangesattheprocesslevel,researchonchangingsouthernhemispherecirculationpatterns,linkedtoanthropogenicincreaseinCO
2concentration,hasshown
aconnectiontoareductioninwinterstormformationandaconsequentreductioninwinterrainfallinsouthernAustralia(Frederiksenetal.2011).
Additionalresearchhasshownthat,whilerainfallvariabilityonyear-to-yeartimescalesisstronglyassociatedwithchangesintropicalSSTs,thisrelationshipexplainslittleoftheobservedrainfalldeclineinthesoutheast(SEACI;Watterson2010).Furthermore,thereisnoevidenceofastronglandcover-rainfallsignaloversoutheastAustralia(NarismaandPitman2003),butthemethodologiesusedtoexplorethisrelationshipareweakandrequirefurtherdevelopment.
StratifyingtheobservedsoutheastAustraliarainfallchangesintoseasons,thereductioninautumnislargest(CaiandCowan2008),followedbythatinspring.Outputsof20thcenturysimulationsby24climatemodelsshowthatonlyoneortwowereabletoreproducetheobservedautumnrainfallreduction.Inthisseason,eventhechangesinthesub-tropicalridgeareunabletoaccountfortherainfallreduction(Caietal.2011b).Themajorityofthesemodelsreproduceareductioninspringrain,asaconsequenceofanupwardtrendinthefrequencyofpositiveIndianOceanDipoleevents(Caietal.2009b).
36 ClimateCommission
Chapter 2. Risks associated with a changing climate (continued)
Figure 23a. Relationship between the May-June-July (MJJ) rainfall in the southwest part of eastern Australia and the sub-tropical ridge intensity during the same three months. Theslopeofthelinearrelationshipandtheamountofexplainedvariance(R2)isshownintheupperrightcorner.
Figure 23b. Long-term (21-year running mean) evolution of the sub-tropical ridge MJJ mean intensity (anomalies in hPa shown on the left-hand Y-axis) compared with the global annual surface temperature.
Source:Timbaletal.(2010),includingfurtherdetailsonmethodology.
Projecting changes in water availability
Althoughtherehasbeenprogressinunravellingsomeofthelinkagesbetweenclimatechangeandobservedshiftsinrainfallpatterns,ourcapabilitytoprojectfuturechangestorainfallpatterns,apartfromthedryingtrendinsouthwestWesternAustralia,remainsuncertain.Doesthe2010-2011extremelywetperiodacrosseasternAustraliarepresentabriefinterruptioninamulti-decadaldryingtrend,orisitashifttoamulti-decadalwetregime,asoneanalysisemphasisingthePacificDecadalOscillation(PDO)asserts(Caietal.2010)?
Modelprojectionsoffuturerainfallchangedonotaddmuchclaritytothesituation.BasedonthemodelsassessedbytheIPCC(2007a),formuchofAustraliathereisnostrongconsensusacrossmodelsonthedirectionofchange(increaseordecreaseinrainfall),oronthemagnitudeorseasonality.ArecentassessmentofclimatesciencebytheAustralianAcademyofScience(2010)notedthatmanyaspectsofclimatechangeremaindifficulttoforesee,and,inparticular,statedthat“howclimatechangewillaffectindividualregionsisveryhardtoprojectindetail,particularlyfuturechangesinrainfallpatterns,andsuchprojectionsarehighlyuncertain.”
Improvingprojectionsoffuturerainfallpatternsisimportant,asrainfallisthemaindriverofrunoff,whichisthelinktoriverflowsandwateravailability.Achangeinannualrainfallistypicallyamplifiedbytwoorthreetimesinthecorrespondingchangeinaverageannualrunoff(Chiew2006).Downscaledprojectionsofrainfallchangefromclimatemodelscanbetranslatedintochangesinwateravailabilitythroughuseofhydrologicalmodellingbasedonpointandcatchmentscaleestimatesofrainfallandpotentialevaporation(Chiewetal.2009).Theresultsofthehydrologicalmodellingreflecttheuncertaintyinthelargerscalerainfallprojectionsbytheclimatemodels,butindicatethatriverflowsinsouthwestWesternAustraliaandinsoutheastAustraliaarelikelytodeclineinthefuture,withhigherconfidenceintheprojectionsfortheformer.Thehydrologicalmodelscanalsoprojectchangesinotheraspectsofwateravailabilitythatareimportantforriskassessment,suchasvariabilityinreservoirinflowsandfloodsandlowflowsthataffectecosystemsandtheenvironment.
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ScienceUpdate2011 37
Chapter 2. Risks associated with a changing climate (continued)
Figure 24. Time series of Sea Surface Temperature (SST) for the month of December from 1900 to 2010 for (a) the Coral Sea and the (b) northern tropical Australian oceanic region.
Source:BureauofMeteorology.
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38 ClimateCommission
ApossiblelinkbetweenclimatechangeandwateravailabilityisviatheriseinSST(Figure24).Basedonthestrongroleofperiodicchangesinocean-atmospherecoupling,suchasENSO,forAustralia’srainfall,thereisaplausibleconnectionbetweentherisingtrendinSSTandthebehaviourofnaturalmodesofvariability.Asnotedinsection3.5(Figure32),observationsintheeasternPacificOceanhaveshownalinkbetweenincreasingSSTs,watervapourcontentintheatmosphere,andheavyprecipitationevents.TheveryhightemperaturesintheCoralSeaandtheNorthernTropicsinlate2010(Figure24)mayhavecontributedtotheverystrongLaNiñaeventinlate2010andearly2011andthustotherecordhighrainfallacrosseasternAustraliainDecember2010(BoM2011a).However,theextentofsuchaninfluence,andeventhedirectionoftheinfluence(towardsstrongerorweakerLaNiñaevents)isunknownatthistime.NocleartrendhasbeenseeninindicesoftheENSO,orineasternAustraliarainfalloverthepastcentury,suggestingthatanylinkbetweenclimatechange,ENSO,andAustralianrainfallissubtle,atleastuptothepresenttime.
ThebottomlineisthatsignificantuncertaintiesstillsurroundtherelationshipbetweenclimatechangeandshiftsinAustralianrainfallpatterns,bothinobservationsoverthepastseveraldecadesandinprojectionsforthefuture.
ItislikelythatthedryingtrendinsouthwestWesternAustraliaislinkedtoclimatechangeandwillcontinue.Formuchoftherestofthecountry,thereisnostrongconsensusoneventhedirectionofchange–moreorless–ofrainfall.Climatechangecould,infact,leadtomoreextremesingeneral–bothindroughtandinrainfall.
–ApART fRom THEsE InsIgHTs, wHAT wE CAn sAy wITH CERTAInTy Is THAT RAInfALL pATTERns wILL CHAngE As A REsuLT of CLImATE CHAngE, AnD ofTEn In unpREDICTAbLE wAys, CREATIng LARgE RIsks foR wATER AvAILAbILITy.
–
Thisdauntinguncertaintynotonlychallengesattemptsatadaptation,butalsoenhances,notdiminishes,theimperativeforrapidandvigorousglobalmitigationofgreenhousegasemissions.
2.4 Extreme events
Many of the impacts of climate change are due to extreme weather events, not changes in average values of climatic parameters. The most important of these are high temperature-related events, such as heatwaves and bushfires; heavy precipitation events; and storms, such as tropical cyclones and hailstorms. The connection between long-term, human-driven climate change and the nature of extreme events is both complex and controversial, leading to intense debate in the scientific community and heated discussion in the public and political arenas.
Thissectionexploresourcurrentlevelofunderstandingabouttherelationshipbetweenclimatechangeandextremeevents,withafocusontypesofextremeeventsthathavealreadyoccurredinAustraliaandarelikelytooccurinfuture.Thekeymessagesare:
Chapter 2. Risks associated with a changing climate (continued)
– Modestchangesinaveragevaluesofclimaticparameters–forexample,temperatureandrainfall–canleadtodisproportionatelylargechangesinthefrequencyandintensityofextremeevents.
– OnaglobalscaleandacrossAustraliaitisverylikelythatsinceabout1950therehasbeenadecreaseinthenumberoflowtemperatureextremesandanincreaseinthenumberofhightemperatureextremes.InAustraliahightemperatureextremeshaveincreasedsignificantlyoverthepastdecade,whilethenumberoflowtemperatureextremeshasdecreased.
– TheseasonalityandintensityoflargebushfiresinsoutheastAustraliaislikelychanging,withclimatechangeapossiblecontributingfactor.Examplesincludethe2003Canberrafiresandthe2009Victoriafires.
ScienceUpdate2011 39
Average-extreme relationship
Temperatureincreasesof1or2°C,orequivalentchangesinotherclimaticparameters,mayseemmodest,buttheycanleadtodisproportionatelylargechangesinthefrequencyandintensityofextremeweatherevents.
Figure25showstherelationshipbetweenachangeinaveragetemperatureandtheincidenceandseverityofextremeevents(IPCC2007a).Amodestshifttohigheraveragetemperaturesleadstoadisproportionatelylargeincreaseinthenumberofextremehightemperatureevents,theareaunderthecurvetotherightofthedashedverticalline.Inaddition,themostextremeeventsbecomemuchmoreintense–thelong“tail”attherightofthedistribution.Correspondingly,extremecoldeventsbecomefewerandlessextreme,asshownintheleft-sideofthefigure.Thissimplepictureassumesthattherewillbenochangeinthevariabilityoftemperaturedistribution.Ifvariabilitywerealsoincreasing,thenthiswouldleadtoamuchlargerimpactinthetails.
AnotherwaytovisualisetherelationshipbetweenaveragesandextremesisshowninFigure26,wheretheleft-handsideofthefigureshowsvariabilityaroundalong-termaveragetemperaturethatdoesnotchange.Thehorizontallinesaboveandbelowthelong-termaveragetemperatureshowthelimitsaboveandbelowwhichextremeeventsaredefinedtooccur.Theright-handsideofthefigureshowsarisingaveragetemperaturewiththesameshorter-termvariabilityimposeduponit.Thefigureagainshowsthatthenumberofextremehightemperatureeventsincreasesandtheintensityofthemostextremeoftheseeventsalsoincreases.
Figure 25. Relationship between means and extremes, showing the connection between a shifting mean and the proportion of extreme events, when extreme events are defined as some fixed threshold related to a significant impact (e.g., heatwave leading to excess deaths).
Source:IPCC(2007a).
Figure 26. The increase in frequency and intensity of extreme events when an underlying, long-term trend is imposed on an existing pattern of natural variability.
Source:AdaptedfromJonesandMearns(2004).
– Thereislittleconfidenceinobservedchangesintropicalcycloneactivityinthepastbecauseofproblemswiththelackofhomogeneityofobservationsovertime.Theglobalfrequencyoftropicalcyclonesisprojectedtoeitherstayaboutthesameorevendecrease.Howeveramodestincreaseinintensityofthemostintensesystems,andinassociatedheavyrainfall,isprojectedastheclimatewarms.
– Onaglobalscale,severalanalysespointtoanincreaseinheavyprecipitationeventsinmanypartsoftheworld,includingtropicalAustralia,consistentwithphysicaltheoryandwithprojectionsofmoreintenserainfalleventsastheclimatewarms.
Chapter 2. Risks associated with a changing climate (continued)
Less cold leather
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40 ClimateCommission
Chapter 2. Risks associated with a changing climate (continued)
Becauseextremeeventsare,bydefinition,relativelyrare,longtimeseriesandlargespatialareasarerequiredtoobtainenoughobservationstodeterminestatisticallywhetherachangeintheirfrequencyandintensityisactuallyoccurring.Inmostpartsoftheworld,theinstrumentalrecordisatmostacenturyorsolong,anddensespatialcoverageofmanyareashasonlybeenachievedforafewdecades,sodeterminingfromobservations,withahighdegreeofconfidence,whetheranychangeinthefrequencyandintensityofextremeeventsisoccurringisverydifficult.
Temperature extremes
AsdescribedinSection2.1,theEarthasawhole,includingtheAustraliancontinent,hasbeenwarmingstronglysincethemiddleofthe20thcentury.Thus,itmightbepossiblefromobservationstodiscernthebeginningsofshiftsinextremesthatareconsistentwithwhatisexpected.ThisisindeedthecaseforAustraliantemperatureextremes(Alexanderetal.2007).
– THE numbER of HIgH TEmpERATuRE ExTREmEs (E.g. HEATwAvEs) In AusTRALIA HAs InCREAsED sIgnIfICAnTLy ovER THE pAsT DECADE, wHILE THE numbER of Low TEmpERATuRE ExTREmEs HAs DECREAsED (fIguRE 27).
–
Anincreaseinwarmnightshasalsooccurredacrossmostofthecontinentandthisisconsistentwithanthropogenicclimatechange(AlexanderandArblaster2009;Figure27).ChangesinMelbournetemperaturesprovideagoodexampleoftheshiftsinthefrequencyandintensityofextremesdepictedschematicallyinFigure25.Thelong-termaverageinthenumberofdaysperyear35°Coraboveis10(BoM2011c).Duringthedecade2000-2009,thenumberofsuchdaysperyearroseto13(BoM2011c).Furthermore,theincreasedintensityofextremeevents–thelongtailtotherightinFigure25–isclearlyevidentinMelbournewiththerecordhightemperatureof46.4°CinFebruary2009,andthethreeconsecutivedaysof43°CoraboveinlateJanuary.
Sea surface temperature
Coral-dominatedecosystemsaresensitivetosmallrisesinthetemperatureofthewaterinwhichtheyreside.Thissensitivityresultsfromthebreakdownofasymbiosisbetweencoralsandtinyorganismscalleddinoflagellates,whicharephotosyntheticallyactiveandprovidecoralswithorganiccarbon.Whentheseatemperaturerises1-2°Cabovenormalforasixtoeightweekperiod,thissymbiosisbreaksdown,thedinoflagellatesareexpelled,andthecoralsare“bleached”.Coralscanrecoverfromableachingevent,butsustainedorrepeatedbleachingcanleadtostarvation,diseaseanddeath(Hoegh-Guldberg1999).
Risingseasurfacetemperature(Figure24)hasincreasedthenumberofbleachingeventsthathavebeenobservedontheGBRoverthelastfewdecades,consistentwithglobaltrendsshowingincreasingincidenceofbleachingeventssince1979(Hoegh-Guldberg1999;Wilkinson2008).TheGBRhasfaredbetterthanmanyreefsaroundtheworld,althoughpartsofthereefhaveexperiencedbleachingeventsin1980,1982,1983,1987,1992,1994,1998,2002and2006–withthe1998and2002eventstheworstonrecordfortheGBR(Berklemansetal.2004).Intheseeventsover50%oftheGBRbleachedintheexceptionallywarmconditions,withanestimatedlossof5-10%ofcoralsineachevent(GBRMPA2009).
Bushfire intensity and frequency
Extremeeventsthatarecloselyrelatedtotemperaturearealsoshowingchangesconsistentwithwhatisexpected.TheintensityandseasonalityoflargebushfiresinsoutheastAustraliaappearstobechanging,withclimatechangeapossiblecontributingfactor(Caietal.2009c).Bushfireshavelongbeenafeatureofecosystemsinthesoutheast;the1939firesinVictoriaareanoften-quotedexampleoflargeandintensivefires.However,inthefirstdecadeofthe21stcentury,twoverylargeandextremelyintensefiresoccurred–the2003Canberrafires,whichdestroyed500housesinsuburbanCanberraandkilledthreepeople,andthe2009Victoriafires,whichkilled173peopleinruralareasofthestate.
ScienceUpdate2011 41
Chapter 2. Risks associated with a changing climate (continued)
Figure 27. Number of (a) record hot day maximums and (b) record cold day maximums at Australian climate reference stations.
Source:BureauofMeteorology.
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42 ClimateCommission
Climatechangeaffectsfireregimesinatleastthreeways(Williamsetal.2009;Lucasetal.2007).First,changingprecipitationpatterns,highertemperaturesandelevatedatmosphericCO
2concentrationsaffectthebiomassand
compositionofvegetation,thefuelloadforfires.Second,highertemperaturestendtodrythefuelload,makingitmoresusceptibletoburning;droughtconditionscansignificantlyexacerbatetheseconditions.Third,climatechangeincreasestheprobabilityofextremefireweatherdays–conditionswithextremetemperature,lowhumidityandhighwinds.
TheseverityofbushfiresinsoutheastAustraliaisstronglypre-conditionedbylowrainfallandhightemperatureinducedbythepositivephaseoftheIndianOceanDipole.Since1950,themajorityoflargebushfiresinsoutheastAustralia,includingtheAshWednesday,Canberra,andBlackSaturdaybushfires,occurredfollowingapositiveIODeventintheprecedingspringseason,whichledtowarmanddryconditions(Caietal.2009c).Since2002,theIndianOceanhasexperiencedfivepositiveIODevents(2002,2004,2006,2007,2008),withclimatechangeacontributortotheincreasingfrequencyoftheseevents(Caietal.2009b).
Tropical cyclones
Therelationshipbetweentropicalcyclonebehaviourandclimatechangeisaparticularlycomplexone,withahighdegreeofuncertaintyinourcurrentunderstanding.Observationalrecordsshownochangesbeyondnaturalvariabilityineitherthefrequencyofcyclonesortheirstormtracks.Withtheadventofsatellitemeasuredintensitiesoftropicalcyclonesin1980,somestudieshavefoundapossiblelinkbetweencycloneintensityandhigherseasurfacetemperatures(e.g.Elsneretal.2008).However,someofthesatellitedataisquestionable(e.g.overtheIndianOcean),andtimeperiodistooshorttoseparateoutdecadalpatternsofnaturalvariabilityfromtheunderlyingtrendofrisingSST(Knutsonetal.2010).Inshort,giventheshortobservationalperiodandthechangingobservationalcapabilitythroughtime,itisnotyetpossibletoattributeanyaspectofchangesincyclonebehaviour(frequency,intensity,rainfall,etc.)toclimatechange;allobservationscurrentlyremainwithintheenvelopeofnaturalvariability(Knutsonetal.2010).
Heavy precipitation events
–THE sEvERE fLooDIng In quEEnsLAnD AnD vICToRIA In EARLy 2011 HAs RAIsED THE quEsTIon of A possIbLE LInk bETwEEn THE fLooDs AnD HumAn-InDuCED CLImATE CHAngE.
–Theseverityofthefloodsisrelatedtoseveralfactors,includingtheintensityoftherainfallevent(s)thattriggeredthefloods,theconditionofthecatchmentsupstreamofandwithinthefloodingarea,theeffectivenessofstructuressuchasdamsdesignedtoameliorateflooding,andthevulnerabilityofpeopleandinfrastructuretoflooding.Herewedealonlywiththepossibleconnectionbetweenclimatechangeandthefrequencyorseverityofextremerainfallevents.
ThefloodsacrosseasternAustraliain2010andearly2011weretheconsequenceofaverystrongLaNiñaevent,andnottheresultofclimatechange.Thatis,theunderlyingcauseofthefloodsisanaturalpartofclimatevariability,whichispartofthereasonwhyAustraliahasalwaysbeena“landofdroughtsandfloodingrains”.Theextent,ifany,oftheinfluenceofthewarmingplanetontheintensityoftheseheavyrainsandfloodsissimplyunknownatthistime.ThereisnoevidencethatthestrengthofLaNiñaeventsisincreasingduetoclimatechange.
Thephysicalconnectionbetweenawarmingclimateandmorerainfallisrelativelystraightforward.Highertemperatures,especiallyofthesurfaceocean,leadtomoreevaporation;thisleadstohigherwatervapourcontentinawarmeratmosphere(whichcanholdmorewatervapour);andthisinturninducesmoreprecipitation.
Chapter 2. Risks associated with a changing climate (continued)
ScienceUpdate2011 43
Chapter 2. Risks associated with a changing climate (continued)
Figure 28. (a) Linear trends in precipitable water (total column water vapour) in % per decade; (b) monthly time series of anomalies relative to the 1988 to 2004 period in % over the global ocean plus linear trend (broken purple line); and (c) monthly time series of global mean (80 °N to 80 °S) anomalies of T2-T12 (an atmospheric radiative signature of upper-trophospheric moistening) relative to 1982 to 2004.
Source:IPCC(2007a),updatedfromTrenberthetal.(2005)andSodenetal.(2005).
c) Global mean T2-T12 (˚C)
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44 ClimateCommission
Figure 29. (a) Observed trends (% per decade) for 1951-2003 in the contribution to total annual precipitation from very wet days (95th percentile). Trendswereonlycalculatedforgridcellswhereboththetotalandthe95thpercentilehadatleast40yearsofdataduringthisperiodandhaddatauntilatleast1999.(b) Anomalies (%) of the global annual time series (relative to 1961-1990) defined as the percentage change of contributions of very wet days from the base period average (22.5%). Thesmoothredcurveshowsdecadalvariations.
Source:IPCC(2007a),basedonAlexanderetal.(2006).
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Chapter 2. Risks associated with a changing climate (continued)
ScienceUpdate2011 45
TheIPCCassessment(2007a)ofobservationsonaglobalscaleshowsanincreaseinatmosphericwatervapourfrom1988to2004(Figure28)aswellasincreasesinprecipitationinmanypartsoftheworld,withasubstantialincreaseinheavyprecipitationevents(Figure29).Arecentstudy(Minetal.2011)comparingobservedandmodel-simulatedpatternsofextremeprecipitationeventsfoundthatovertheNorthernHemispherelandareawithsufficientdatacoverage(abouttwo-thirdsofthetotalarea),human-drivenincreasesingreenhousegasconcentrationshavecontributedtotheobservedintensificationofheavyprecipitationevents.However,thereisnoconsistentevidenceofanobservedincreaseinheavyprecipitationeventsovermostpartsofAustraliaatthistime.
Atthecontinentalscalea100-yearrecordfromtheUnitedStatesshowsasharpincreaseintheareaoftheU.S.experiencingveryheavydailyprecipitationevents(Gleasonetal.2008;Figure30).ArecentanalysisoftemperatureandrainfallextremesinAustraliausingacombinedclimateextremesindexshows,overthewholecontinentandforallseasons,anincreaseintheextentofhotandwetextremesandadecreaseintheextentofcoldanddryextremesannuallyfrom1911to2008atarateofbetween1%and2%perdecade(GallantandKaroly2010).Thesetrendsareprimarilydrivenbychangesintropicalregionsduringsummerandspring.Whilesuchcontinental-scaleanalysesinboththeU.S.andAustraliashowtrendsconsistentwithawarmingplanet,itisdifficulttoattributethemunequivocallytoclimatechangebecauseoftheconsiderablenaturalvariabilityinrainfallpatterns.
Determiningalinkbetweenclimatechangeandextremeeventsbecomesevenmoredifficultforsingleextremeevents,suchastheheavyrainfalleventthattriggeredthefloodsinsoutheastQueenslandinJanuary2011.ThisisespeciallydifficultforeasternAustraliaingeneral,wheremodesofnaturalvariability,suchasENSOandtheIndianOceanDipole(IOD),playaveryimportantroleininfluencingrainfallpatterns.
Duringthesecondhalfof2010astrongLaNiñaeventdevelopedacrossthePacific,withtheSOI(SouthernOscillationIndex)-anindicatorforthestateoftheENSOsystem-showingrecordpositivevaluesforOctoberandDecember2010(BoM2011b).LaNiñaeventsnormallybringheavyrainfalltoeasternAustralia.InthepresentcasethestrongLaNiñaeventwasaccompaniedbyapositivephaseoftheIOD,whichisassociatedwithunusuallywarmoceanwatersaroundIndonesia.ThecombinationofaLaNiñaeventandapositiveIODisrelativelyrare,buttheyreinforceeachothertobringwetter-than-usualconditionsacrossmuchofAustralia.Thus,thesetwomodesofnaturalvariabilityalonecouldhavegeneratedtheheavyprecipitationeventsthatoccurredineasternAustraliainDecember2010andJanuary2011.
However,long-termhuman-inducedclimatechangemayalsobeafactor.
–sEA suRfACE TEmpERATuREs (ssT) HAvE wARmED nEARLy EvERywHERE ovER THE pAsT CEnTuRy, InCLuDIng ARounD AusTRALIA (fIguRE 31).
–Thisadditionalwarmthintheupperocean–SSTsinthenorthernAustralianregionarecurrentlyatornearrecordlevelsandaremuchwarmerforthisLaNiñaeventthanforpreviousstrongLaNiñaevents(Figure24)–maypossiblyhaveenhancedprecipitationandledtoanevenmoreintenseprecipitationeventthanwouldotherwisehaveoccurred,althoughsuchenhancementhasyettobedemonstrated.
Chapter 2. Risks associated with a changing climate (continued)
46 ClimateCommission
Figure 30. Time series of the annual values of the percentage area of the United States with a much greater than normal proportion of precipitation originating from very heavy (equivalent to the highest tenth percentile) 1-day precipitation amounts.
Source:Gleasonetal.(2008),updatedbyNOAAatwww.ncdc.noaa.gov/oa/climate/research/cei/cei.html
Figure 31. Times series of SST for the month of December from 1900 to 2010 for the oceanic region around Australia.
Source:BureauofMeteorology.
Chapter 2. Risks associated with a changing climate (continued)
Actual PercentAverage Percent5-Year Moving Average
1910 1920 1930 1940 1950 1960 1970 1980 1990 2000
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ScienceUpdate2011 47
Chapter 2. Risks associated with a changing climate (continued)
Figure 32. Time series of (a) Niño-3 ENSO index (SST anomalies for 90° to 150 °W, 5 °S – 5 °N region, deseasonalised tropical ocean (30 °S to 30 °N) mean anomalies of SST, and column-integrated water vapour (CWV); and (b) precipitation (P).
Source:AllanandSoden(2008).
ThereisobservationalevidencethatshowsalinkbetweenSSTandrainfallextremes(Figure32).Thetoppanelofthefigureshowsa28-yearrecordofanENSOindex,theseasurfacetemperatureandthecolumn-integratedwatervapourinthetropicalatmosphere.Thethreearecloselyrelated,withENSOdominatingtheinterannualvariability.Thebottompanelshowstwoobservationsofprecipitation,againshowingtheprominentENSOpattern,andalsoshowingthestrongcorrelationbetweenheavyrainfalleventsandperiodsofhighSSTs.However,despiteasubstantialwarmingoftheoceanaroundnorthernAustralia(Figure24),thereisnoevidenceyetofatrendtowardsincreasedprecipitationineasternAustraliaoverthepast50years,although,asnotedabove,GallantandKaroly(2010)foundanincreaseintheextentofhotandwetextremesinthetropicalregionsofAustraliafrom1911to2008atarateofbetween1%and2%perdecade.
Lookingtowardsthefuture,RafterandAbbs(2009)usedextremevaluetheorytoexaminechangesintheintensityofextremerainfallassimulatedbyclimatemodels.Theirresultsshowedincreasesinallregionsfor2055and2090formostmodelsconsidered.Thespatialpatternswereconsistentwithpreviousstudies,withsmallerincreasesinthesouthofAustraliaandlargerincreasesinthenorth.Fine-scaleregionalclimatemodelling(e.g.Abbset al.,2007;AbbsandRafter,2009)suggestsincreasesindailyprecipitationextremesonaverage,althoughwithlargefine-scalespatialvariability.Thestudyfoundshortduration(sub-daily)rainfallwillchangemorerapidlythanlongerduration(dailyandmulti-day)rainfall.
ThebottomlineisthatalthoughaconclusivelinkbetweenthesoutheastQueenslandrainfalleventsandclimatechangecannotbemade,suchalinkisplausibleevenifitisnotdiscernibleyet.Fromariskperspective,thisisusefulknowledge,andsuggeststhatitwouldbeprudenttofactorinaclimatechange-inducedincreaseinintenserainfalleventsinurbanandregionalplanning,thedesignoffloodmitigationworks,andanyreviewsofemergencymanagementprocedures.
P (
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48 ClimateCommission
2.5 Abrupt, non-linear and irreversible changes in the climate system
Many projections of future changes in climatic variables are simulated and presented as smooth curves from present values to an altered state at some future point in time. The temperature projections to 2100 highlighted in the IPCC reports are a good example of this. However, smooth changes are not the norm in the climate system. Often the system seems unresponsive to forcing agents until a threshold is reached, after which the system rapidly changes or reorganises into an alternate state. The abrupt drop in rainfall in the mid-1970s in southwest Western Australia is a well-known Australian example. Some changes in the climate system can be irreversible in any timeframe relevant to human affairs, such as the loss of the Greenland ice sheet.
Inthissectionwepresentabriefsummaryofthecurrentknowledgebaseonthepotentialforabrupt,irreversiblechangesintheclimatesystem:
–
The science of abrupt change
Whileitiscommon,eveninmanypartsofthescientificcommunity,toemploycause-effectlogicandlinearthinking(achangeinacausalagentdrivesanappropriatelyscaledresponse),thegrowthofcomplexsystemsciencehasbroughtanewperspectivetoobservingandinterpretingchangesintheclimatesystem.Thephenomenonofabrupt,highlynonlinearchanges,whichoftenoccurwhenanapparentlysmallchangeinaforcingagenttriggersanunexpected,large,complexresponseinthesystem,hasrecentlybeenreviewedinthecontextoftheclimatesystem(Lentonetal.2008;Figure33).
Figure 33. Schematic representation of a system being forced past a tipping point. Thesystem’sresponsetimetosmallperturbations, τ,isrelatedtothegrowingradiusofthepotentialwell.
Source:H.Held,fromLentonetal.(2008).
Figure34isaschematicoftwotypesofabruptchangeinacomplexsystemsuchasclimate–so-called“tippingelements”–oneamono-stablesystemshowingthreshold,abruptchangebehaviourandtheothershowingbistabilitywhenathresholdiscrossed.Animportantfeatureofatippingelementisthatitmustcontainastrongpositive(reinforcing)feedbackprocessinitsinternaldynamics.Inaddition,tippingelementscanhavevaryingdegreesofirreversibility.Forexample,althoughthelargepolaricesheetsonGreenlandandAntarcticahavewaxedandwanedingeologicaltimescales,theyareessentiallyirreversibleontimescalesofrelevancetohumanaffairs.
Chapter 2. Risks associated with a changing climate (continued)
System being forced past a tipping point
τ
– Anumberofpotentialabruptchangesinlargesub-systemsorprocessesintheclimatesystem–so-called“tippingelements”–havebeenidentifiedlargelythroughpalaeo-climaticresearch.Manyofthese,iftriggered,wouldleadtocatastrophicimpactsonhumansocieties.
– ExamplesoftippingelementsincludeabruptchangesintheNorthAtlanticoceancirculation,theswitchoftheIndianmonsoonfromawettoadrystateorviceversa,andtheconversionoftheAmazonrainforesttoagrasslandorasavanna.
– Verylargeuncertaintiessurroundthelikelihood,ornot,ofhuman-drivenclimatechangetriggeringanyoftheseabruptorirreversiblechanges.Expertsagreethattheriskoftriggeringthemincreasesastemperaturerises.
– Abruptshiftsinatmosphericcirculationcanoccurveryquicklyandcanhavelargeimpactsonregionalclimates.Therecentcold,snowywintersinnorthernEurope,andtheirpossiblelinktoclimatechange,compriseagoodexampleofthisrisk.
ScienceUpdate2011 49
Figure 34. Schematic of two types of tipping element that can exhibit a tipping point where a small change in control (δρ) results in a large change in a system feature (∆F), illustrated here in terms of the time-independent equilibrium solutions of the system: (a) A system with bi-stability passing a true bifurcation point. (b) A mono-stable system exhibiting highly non-linear change.
Source:T.M.Lenton,publishedinRichardsonetal.(2011).
Examples of tipping elements in the climate system
Therearemanyexamplesoftippingelementsintheclimatesystem(Figure35);itisusefultoclassifythemintothoseassociatedwiththemeltingoflargemassesofice,thoseinvolvingsignificantchangestouniquebiomes,andthoseassociatedwithlarge-scaleschangesinthecirculationoftheatmosphereandtheocean(Richardsonetal.2011).
–THE gREEnLAnD AnD AnTARCTIC ICE sHEETs mAy noT sEEm LIkE CAnDIDATEs foR TIppIng ELEmEnTs As THEIR RATE of CHAngE Is noT “AbRupT” fRom A HumAn pERspECTIvE, buT THEy ARE DEfInITELy TIppIng ELEmEnTs In THAT bEyonD A RATHER nARRow RAngE of TEmpERATuRE CHAngE, THEy wILL bE CommITTED To IRREvERsIbLE mELTDown (HuybRECHTs AnD DE woLDE 1999).
–TheacceleratingdownwardtrendinthelossofArcticseaiceisindicativeofthreshold-abruptchangebehaviourinwhichthethresholdmayalreadyhavebeencrossed(Perovich2011),althoughthelossofsummerseaiceisnotirreversibleandcouldquicklyrecoverwithareturntoacolderclimate.
Syst
em f
eatu
re (
F)
Control (ρ)
F
a
Syst
em f
eatu
re (
F)
F
Control (ρ)
b
Chapter 2. Risks associated with a changing climate (continued)
50 ClimateCommission
Figure 35. Map of potential policy-relevant tipping elements, adjusted from Lenton et al. (2008) based on further analysis by T.M. Lenton reported in Richardson et al. (2011). Questionmarksindicatesystemswhosestatusaspolicy-relevanttippingelementsisparticularlyuncertain.
Source:V.Huber,T.M.LentonandH.J.Schellnhuber,publishedinRichardsonetal.(2011).
MeltingCirculation ChangeBiome loss No data
Boreal forest
Cold water coral reefs
Tropical Coral Reefs
Marine Biological Carbon Pump?
Boreal Forest
Yedoma Permanfrost
Arctic sea ice
Atlantic Thermohaline Circulation
El Nino-Southern Oscillation
AmazonRainforest
West Antarctic Ice Sheet
Ocean Methane Hydrates?
SW North America?
HimalayanGlaciers?
Indian Summer Monsoon
West Africa Monsoon
Sahara Greening?
Sahel Drying?
Dust SourceShort down
Greenland Ice Sheer
Chapter 2. Risks associated with a changing climate (continued)
TheAmazonrainforestisthemostwidelyquotedexampleofalargebiomeatriskofabruptchangefromawarmingclimate.Theclimate-relatedforcingfactorsincludebothrisingtemperatureandapotentialincreaseinthelengthofthedryseasonandtheintensityofdroughts.Aprominentfeedbackisthewayinwhicharainforeststoresandrecycleswater.Ecologicaldisturbanceprocesses,suchasfiresandinsectinfestations,mayalsobecomeimportantfeedbackprocesses.Simulationsthatincorporatetheseecologicalprocessessuggestthatathresholdexistsarounda2°Ctemperatureincrease,beyondwhichtheareaoftheAmazonforestscommittedtodiebackrisesrapidlyfrom20%toover60%(JonesandLowe2011).SeveredroughtsintheAmazonBasinin2005and2010,alongwiththeobservationthatsuchdroughtsco-occurwithpeaksoffireactivity,supportthisriskassessment(Lewisetal.2011).
PerhapsthearchetypalexampleofatippingelementistheAtlanticthermohalinecirculation(THC),whichinitscurrentmodecontributessignificantlytothemildclimateexperiencedbywesternEuropeandScandinaviabutwhichhasshownthreshold-abruptchangebehaviourinthepast(e.g.,Dansgaard-Oeschgerevents,GanopolskiandRahmstorf2001).AcollapseoftheTHCcouldleadtoareducedlevelofwarminginthenorthAtlanticregioncomparedtotheglobalaverage.CurrentunderstandingoftheTHCsystemsuggeststhatthethresholdforcollapseisstillratherremote(IPCC2007a),butthataweakeningofthestrengthofthecirculationislikelythroughthiscentury(WeberandDrijfhout2011).TheTHCisanexampleofacirculation-relatedtippingelement.OthersincludetheElNiñoSouthernOscillation(ENSO)andtheWestAfricanMonsoon,bothexamplesofcoupledocean-atmospherecirculation.
ScienceUpdate2011 51
Chapter 2. Risks associated with a changing climate (continued)
Likelihood of triggering abrupt changes
Muchoftheinterestintippingelementsderivesfromtheverylargerisksforhumanwell-beingassociatedwithactivationofmanyofthetippingelements.Forexample,lossofsignificantamountsoftheGreenlandandAntarcticicesheetswouldleadtometresofsea-levelrise.TheAsianmonsoon,ormorepreciselytheIndianSummerMonsoon,isatippingelementwhosebehaviourisinfluencedbyboththewarmingoftheIndianOceanandthepresenceofan“atmosphericbrowncloud”overmuchofthesub-continent.Somemodelssuggestatippingpointrelatedtochangesinregionalalbedo,leadingtosuddenswitchesinthestrengthandlocationofmonsoonalrains(Zickfeldetal.2005;Levermannetal.2009).GiventhatoverabillionpeopledirectlydependonthereliablebehaviouroftheIndianSummerMonsoonfortheirfoodproduction,rapidchangesinrainfallcouldhavecatastrophicconsequencesforlargenumbersofpeople.
Table1givesanexampleofhowariskassessmentontippingelementsmightbecarriedout(Richardsonetal.2011).Theassessmentisbasedonthecombinationofthelikelihoodofthetippingelementbeingactivatedandtheimpactonhumanwell-beingofachangeofstateofthetippingelement.Ofthetippingelementsconsidered,itisinterestingthatthehighestrisksareassociatingwiththelossoficefromthelargepolaricesheets.RisksassociatedwiththeAtlanticthermohalinecirculationandthebehaviourofENSOareconsideredtoberatherlowprimarilybecauseofthesmalllikelihoodthatatippingpointwillbepassed.
Abrupt shifts in atmospheric circulation
Tippingelementsassociatedwithchangesinatmosphericcirculation,orcoupledocean-atmospherecirculation,areespeciallyimportantbecauseoftheshorttimescalesonwhichtheycanoperate.Thebi-stabilityoftheIndianSummerMonsoon,notedabove,isanexampleofalargeshiftinatmosphericcirculationthatcanhappenveryquickly,evenonanannualbasis.Therecentcold,snowywinters(2005-06,2009-10,2010-11)inpartsofnorthernEuropeandNorthAmerica(Figure2),andtheirpossiblelinktoclimatechange,compriseanothergoodexampleofrisksassociatedwiththistypeoftippingelement.
Althoughitsoundscounter-intuitive,suchcoldweathermaybelinkedtotheoverallwarmingoftheplanet.Morespecifically,apossiblelinkisviathelossofArcticseaiceinwinterandtheconsequentformationofahighpressurecelloverthepolarregion(PetoukhovandSemenov2010).ThiscellchangespressuregradientsinthenorthAtlanticregion,rearrangingNorthernHemisphereatmosphericcirculationandgeneratingcold,easterlyairflowsovermuchofwesternEurope.Thischangerepresentsanabrupttransitionbetweentwostatesofthecirculation.Interestingly,thethresholdfortheabruptshiftincirculationliesnear40%reductioninseaice,butanothertransition,flippingthecirculationbacktotheearlierregime,isprojectedtoexistatabout80%reductioninseaice.
Table 1. A simple ‘straw man’ example of tipping element risk assessment, by Timothy M. Lenton
Tipping element Likelihoodof passing atipping point(by 2100)
Relative impact** ofchange in state(by 3000)
Risk score(likelihood x impact)
Risk ranking
Arctic summer sea-ice High Low 3 4
Greenland ice sheet Medium-High* High 7.5 1(highest)
West Antarctic ice sheet Medium* High 6 2
Atlantic THC Low* Medium-High 2.5 6
ENSO Low* Medium-High 2.5 6
West African monsoon Low High 3 4
Amazon rainforest Medium* Medium 4 3
Boreal forest Low Low-Medium 1.5 8(lowest)
*Likelihoodsinformedbyexpertelicitation**InitialjudgmentofrelativeimpactsisthesubjectiveassessmentofT.M.L.
HumAnITy CAn EmIT noT moRE THAn 1 TRILLIon TonnEs of Co2 bETwEEn 2000 AnD 2050 To HAvE A 75% CHAnCE of LImITIng TEmpERATuRE RIsE To 2 °C oR LEss.
THE pEAkIng yEAR foR EmIssIons Is vERy ImpoRTAnT foR THE RATE of REDuCTIon THEREAfTER. THE DECADE bETwEEn now AnD 2020 Is CRITICAL.
AbouT 15-20% of nET Co2 EmIssIons gLobALLy HAvE oRIgInATED fRom LAnD ECosysTEms, pRImARILy fRom DEfoREsTATIon.
CHApTER 3:ImpLICATIons of THE sCIEnCE foR EmIssIon REDuCTIons
DID you know...
ScienceUpdate2011 53
Chapter 3: Implications of the science for emission reductions
3.1 The budget approach
Although the targets-and-timetables approach (e.g. an agreed percentage reduction in greenhouse gas emissions by 2020) remains the most common approach to defining trajectories for climate mitigation, the budget, or cumulative emissions, approach is rapidly becoming the favoured approach in analyses in the scientific community. It offers a much simpler, easier-to-understand, transparent and powerful framework to estimate what level of emission reductions is required to meet the 2 °C guardrail.
Thissectionoutlinestheconceptualframeworkforthebudgetapproachanditsimplicationsformitigationstrategies:
Conceptual framework.
ThebudgetapproachavoidstheexplicituseoftargetsforthestabilisationofatmosphericCO
2orCO
2-equivalent
concentrationsbydirectlylinkingtheprojectedriseintemperaturetotheaggregatedglobalemissions(inGtCO
2orGtC)foraspecifiedperiod,usually2000to
2050or2100.Thatis,itisbasedonthedegreeofclimatechangethatwecanexpectinfutureestimateddirectlyfromthesumofadditionalgreenhousegasesthatareemittedtotheatmosphere(e.g.,Allenetal.2009;Meinshausenetal.2009;Figure36).Therelationshipisnotdeterministicbutratherprobabilistic,givenuncertaintiesinourunderstandingofthesensitivityofclimatetoaparticularincreaseintheamountofgreenhousegasesintheatmosphere.
Toapplytheconcept,ifwewishtohavea75%chanceofobservingthe2°Cguardrail,wecanemitnomorethan1000Gt(onetrilliontonnes)ofCO2
intheperiodfrom2000to2050.Ifwanttoachievea50:50chanceofobservingtheguardrail,thenwecanemit1440Gtintheperiod.Inthefirstnineyearsoftheperiod(2000through2008),humanityemitted305GtofCO
2,over30%ofthe
totalbudgetinlessthan20%ofthetimeperiod.
Strategic implications.
Givenanoverallbudgetbetweennowand2050,theapproachdoesnotstipulateanyparticulartrajectory,solongastheoverallbudgetisrespected.Thisapproachallows,inmakingthetransitiontoalow-orno-carboneconomy,aflexibleapproachthatdeliversleastcosttotheeconomy,notonlyacrosssectorsintheeconomyatanyparticulartime,butalsothroughtimefromthepresenttomid-centuryandbeyond.Asitisthecumulativeemissionsovertimethatmustbelimited,ratherthanaseriesofinterimemissionreductiontargetsthatmustbemet,manyemissionreductiontrajectoriesarepossible.However,thelateremissionreductiontrajectoriesareinitiated,themoredifficultandcostlytheybecome(Garnaut2008).
– ThebudgetapproachdirectlylinkstheprojectedriseintemperaturetotheaggregatedglobalemissionsinGtCO
2orGtCforaspecifiedperiod,
usually2000to2050or2100.Forexample,humanitycanemitnotmorethan1trilliontonnesofCO
2
between2000and2050tohaveaprobabilityofabout75%oflimitingtemperatureriseto2°Corless.
– Givenanoverallcarbonbudgetbetween2000and2050,theapproachdoesnotstipulateanyparticulartrajectory,solongastheoverallbudgetisrespected.Thisallowsastrategythatdeliversleastcosttotheeconomyovertimeinmakingthetransitiontoalow-orno-carboneconomy.
54 ClimateCommission
Chapter 3: Implications of the science for emission reductions (continued)
Figure 36. Top: Fossil fuel CO2 emissions for two scenarios: one “business as usual” (red) and the other with net emissions peaking before 2020 and then reducing sharply to near zero emissions by 2100, with the cumulative emission between 2000 and 2050 capped at 1 trillion tonnes of CO2 (purple). Bottom: Median projections and uncertainties of global-mean surface air temperature based on these two emissions scenarios out to 2100. The darkest shaded range for each scenario indicates the most likely temperature rise (50% of simulations fall within this range).
Source:AustralianAcademyofScience(2010),adaptedfromMeinshausenetal.(2009).
Possible future without climate policy
1000 GtCO2 until 2050
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ScienceUpdate2011 55
Chapter 3: Implications of the science for emission reductions (continued)
3.2 Implications for emission reduction trajectories
Although the budget approach allows more flexibility in the economic and technical pathways to emissions reductions than does a targets-and-timetables approach, the fact that we have already consumed over 30% of our post-2000 budget means that much of that flexibility has been squandered if we wish to avoid the escalating risks associated with temperature rises beyond 2 °C. Thus, there is no room for any further delay in embarking on the transition to a low- or no-carbon economy.
Thekeymessagesofthissectionare:
Emissions trajectories
Figure37(WBGU2009)showsthreeofthemultitudeoftrajectoriesforglobalemissionsthatarepossibleunderthebudgetapproachtohavea67%probabilityofmeetingthe2°Cguardrail.Itisclearfromthefigurethatglobalemissionswillneedtobereducedtoveryclosetozeroby2050tomeetthischallenge,thatis,tostabilisetheCO
2concentrationatavaluecompatible
withthe2°Cguardrail.LessambitiousemissionreductionswillslowtheaccumulationofCO
2inthe
atmosphere,butitsconcentrationwillcontinuetorise.
Figure37alsoshowsthatthepeakingyearforemissionsisespeciallyimportantfortherateofreductionthereafter.Forexample,delayingthepeakingyearbyonlynineyears,from2011to2020,changesthemaximumrateofemissionreductionfrom3.7%perannum,whichisverychallengingbutperhapsachievable,to9.0%perannum,whichisimpossibleonanythingbutawartimefooting.
–In summARy, In TERms of mEETIng THE 2 °C guARDRAIL, THE DECADE bETwEEn now AnD 2020 Is CRITICAL.
–Targets and timetables
Themorefamiliarapproachofconstrictingtheemissionstrajectorytoatimetablewithasetofinterimtargetsbecomeslessimportantinthebudgetapproach.Thestrategicchallengechangesfromwhetherthe2020targetisa5%,25%or40%reductionagainstaparticularbaselinetohowdoweimplementthetransitiontoalow-orno-carboneconomyby2050withtheleasteconomicandsocialcostwhilestayingwithinthebudget?Forexample,themiddletrajectoryofFigure37,withapeakingyearof2015,hasaglobalemissionslevelfor2020ofabout32GtCO
2,whichisaboutthesameasfor
2010,andmuchhigherthanfor1990,theKyotobaseline.ThecurvesofFigure37areforglobalemissions,though,andindustrialisedcountrieswouldbeexpectedtohavemuchlargeremissionreductionsthantheglobalaverage.
– ReducingemissionsofCO2doesnotreduceor
stabiliseitsconcentrationsintheatmosphere;itslowstherateofincreaseofCO
2concentration.
TostabilisetheconcentrationofCO2requires
emissionstobereducedtoverynearzero.
– Thepeakingyearforemissionsisveryimportantfortherateofreductionthereafter.Thedecadebetweennowand2020iscritical.
– Targetsandtimetablesare,inprinciple,lessimportantinthebudgetapproach,buttheurgencyofbendingemissiontrajectoriesdownwardsthisdecadeimpliesthatmoreambitioustargetsfor2020arecriticalinpreventingdelaysinthetransitiontoalow-orno-carboneconomy.
56 ClimateCommission
Chapter 3: Implications of the science for emission reductions (continued)
Figure 37. Three emission trajectories based on the budget approach and giving a 67% probability of meeting the 2 °C guardrail.
Source:WBGU(2009).
Theconnectionbetweenthebudgetapproachandthemorefamiliartargetsandtimetablesapproachisclearonceadesiredtrajectoryisestablishedbasedonanation’soverallcarbonbudget.Thetrajectorytostaywithinthebudget,ineffect,setsaseriesoftargetswithinaspecifictimetablethatdefinethetrajectory.Theflexibilityisassociatedwiththedeterminationofthetrajectoryitself.
–THE buDgET AppRoACH ALso HAs A subTLE buT ImpoRTAnT psyCHoLogICAL ADvAnTAgE ovER THE TARgETs-AnD-TImETAbLE AppRoACH In THAT IT foCusEs ATTEnTIon on THE EnD gAmE – EssEnTIALLy DECARbonIsIng THE EConomy.
–
Thus,investmentdecisionscanbetakenfromalong-termperspective,knowingthatalimitedbudgetismostefficientlyallocatedtoinvestinnewinfrastructurethateventuallydeliversverylowornoemissionsbymid-century,ratherthantoinvestinshorter-termmeasuresaimedatmeetinganinterimtargetthatareperhapslesseffectiveindeliveringlonger-termemissionreductions.
Perhapsthebiggestchallengetoimplementingthebudgetapproachisallocatingtheglobalbudgettoindividualcountries,whereequityissuesbecomeimportant.Thisisapoliticalratherthanascientificquestion,whereastheoverallglobalbudgetismoredirectlyrelatedtothescience.Theproblemisnotuniquetothebudgetapproach,butalsobedevilsnegotiationsunderthetargets-and-timetablesapproachandhasperhapsbeenthesinglemostdifficultissuetoresolvetoachieveaninternationalagreementonaglobalemissionreductionplan.
3.3 Relationship between fossil and biological carbon emissions and uptake
Carbon “offsets”, in which emitters of CO2 from fossil fuel combustion can meet their emission reduction obligations by buying an equivalent amount of carbon uptake by ecological systems, are often proposed as a way of achieving rapid emission reductions at least cost. However, although the immediate net effect on the atmospheric concentration of CO2 is the same for both actions, the nature of the carbon cycle means that the uptake of CO2 from the atmosphere by an ecosystem cannot substitute in the long term for the reduction of an equivalent amount of CO2 emissions from the combustion of fossil fuels. In fact, the offset approach, if poorly implemented, has the potential to lock in more severe climate change for the future.
Glo
bal
em
issi
ons
Gt
CO
2
5
35
30
25
20
15
10
40
2005 20502010 2015 2020 2025 2030 2035 2040 2045
Maximum reduction rate 3.7% per year 5.3% per year 9.0% per year
Peak year 2020
20152011
ScienceUpdate2011 57
Chapter 3: Implications of the science for emission reductions (continued)
AlthoughitisveryimportanttosequesteratmosphericCO
2intolandecosystems,thissectionoutlinesthe
reasonswhyisnotagoodideatoconsidersuchbiologicalsequestrationasanoffsetforfossilfuelemissions.Thekeymessagesare:
Carbon from land ecosystems
Overthepastcenturyabout15%-20%ofCO2emissions
globallyoriginatefromlandecosystems,primarilyfromdeforestation(RaupachandCanadell2010).Thisfractionhasdecreasedoverthepastdecadetoabout11%in2009(Friedlingsteinetal.2010)dueprimarilytothelargeincreaseinfossilfuelemissions.Emissionsfromlandecosystemsrepresenttheremovalofcarbonfromastockintheactiveatmosphere-land-oceancarboncycle.Inessence,deforestationisahuman-drivenredistributionofcarbonamongthethreeactivestocks–fromlandtotheatmosphere,andthen,inpart,totheocean.Itdoesnotintroduceanyadditionalcarbontotheatmosphere-land-oceancycle.Naturalprocessessuchasclimatevariabilityalsoredistributecarbonamongthesethreestocks.AstrongLaNiñaevent,forexample,redistributescarbonfromtheatmospheretothelandthroughincreasedproductivityduetoabove-averageprecipitationinsomepartsoftheworld.However,averagedoverdecades,andintheabsenceofhumanperturbationorlong-termchangesinclimate,landcarbonstocksarerelativelystable.
Fossil fuel combustion
Thecombustionoffossilfuelsrepresentstheinjectionofadditionalcarbonfromaninert,undergroundstockintotheactiveatmosphere-land-oceansystem.Thisadditionalcarbonisredistributedamongthethreemainstocksintheactivecarboncycle,thusaddingtotheamountofatmosphericCO2
.Alittlelessthanhalfoftheadditional,inertcarbonactivatedbythecombustionoffossilfuelsremainsintheatmosphere;therestisredistributedaboutequallytothelandandocean(Canadelletal.2007;Raupachetal.2007).Sothecombustionoffossilfuelisfundamentallydifferentfromdeforestationbecausefossilfuelcombustionintroducesadditionalcarbontotheactivecycle,ratherthanredistributingtheexistingamountofcarbonintheactivecycleamongthethreemajorstocks.
– About15-20%ofnetCO2emissionsgloballyhave
originatedfromlandecosystems,primarilyfromdeforestation.Thisrepresentstheremovalofcarbonfromastockintheactiveatmosphere-land-oceancarboncycle.Itdoesnotintroduceanyadditionalcarbonintotheatmosphere-land-oceansystem,butsimplyredistributesit.
– Thecombustionoffossilfuelsrepresentstheinjectionofadditionalcarbonfromaninert,undergroundstockintotheactiveatmosphere-land-oceancycle.Thisadditionalcarbonisredistributedamongthethreemainstocksintheactivecarboncycle,thusaddingtotheamountofatmosphericCO
2.
– Avoidingemissionsbyprotectingecosystemcarbonstocksisanecessarypartofacomprehensiveapproachtomitigation.SequesteringCO
2into
degradedecosystemsisalsoanimportantmitigationactivitybecauseitreversesanearlieremission.However,sequesteringCO
2intoland
ecosystemsdoesnotremoveitfromtheactiveatmosphere-land-oceancycle.Therefore,thesequesteredcarbonisvulnerabletohumanlanduseandmanagement,whichcanrapidlydepletecarbonstocks,andtomajorchangesinenvironmentalconditions,whichcanchangetheamountofcarbonstoredinthelongterm.
– TheonlywaythatCO2sequesteredintoland
ecosystemscanpermanently“offset”fossilfuelcombustionisifthesequesteredcarbonissubsequentlyremovedfromthelandecosystemandstoredinaninertstateorinastablegeologicalformation,thuslockedawayfromtheactiveatmosphere-land-oceancycle.Anotherapproachtooffsettingistoreplacefossilfuelswithbiofuels.
58 ClimateCommission
Chapter 3: Implications of the science for emission reductions (continued)
Replacing the legacy carbon on land
SequesteringCO2intolandecosystemsdoesnot
removeitfromtheactiveatmosphere-land-oceansystem.Itreturnstheoriginalcarbon,sometimescalled“legacycarbon”,lostfromland-usechangebackintothelandstock,andtheamountthatcanbesequesteredislimitedbytheprevailingenvironmentalconditions.Thatis,atmosphericcarboncannotbesequesteredintolandecosystemsindefinitely.
However,itisveryimportantthatthislegacycarbonbereturnedtolandecosystemsassoonaspossibleforanumberofreasons.First,suchsequestrationisindeedarapidwaytobeginreducingtheanthropogenicburdenofCO2
intheatmosphere.Thus,ityieldssomequickgainswhiletheslowerprocessoftransformingenergyandtransportsystemsunfolds.
–fuRTHERmoRE, If DonE CAREfuLLy, sEquEsTRATIon of CARbon InTo LAnD ECosysTEms CAn LEAD To mAny oTHER Co-bEnEfITs, suCH As EnHAnCED soIL ConDITIon, moRE pRoDuCTIvE AgRICuLTuRAL sysTEms, AnD bETTER bIoDIvERsITy ouTComEs.
–Somegeneralprinciplesprovideaguidefordesigningandimplementinganappropriatelandcarbonmitigationscheme:
1.Thesizeofthestockistheimportantfactorinthecarboncycle,nottherateoffluxfromonecompartment(e.g.atmosphere)toanother(e.g.alandecosystem).Thesetwodifferentaspectsofthecarboncycleareoftenconfused.Althoughafast-growing,mono-cultureplantationforestmayhavearapidrateofcarbonuptakefortheyearsofvigorousgrowth,itwillstorelesscarboninthelongtermthatanoldgrowthforestorasecondaryregrowthforestonthesamesite(Diochonetal.2009;Brownetal.1997;Nepstadetal.1999;CostaandWilson2000;ThornleyandCannell2000).
2.Naturalecosystemstendtomaximisecarbonstorage,thatis,theystoremorecarbonthantheecosystemsthatreplacethemaftertheyareconvertedoractivelymanagedforproduction(Diochonetal.2009;Brownetal.1997;Nepstadetal.1999).AnobservationalstudyoftemperatemoistforestsinsoutheastAustraliaidentifiedtheworld’smostcarbondenseforestanddevelopedaframeworkforidentifyingtheforeststhatarethemostimportantforcarbonstorage(Keithetal.2009).Ingeneral,forestswithhighcarbonstoragecapacitiesarethoseinrelativelycool,moistclimatesthathavefastgrowthcoupledwithlowdecompositionrates,andolder,complex,multi-agedandlayeredforestswithminimalhumandisturbance.Thisframeworkunderscorestheimportanceofeliminatingharvestingofold-growthforestsasperhapsthemostimportantpolicymeasurethatcanbetakentoreduceemissionsfromlandecosystems.RecognitionoftheneedtoprotectprimaryforestshashelpedtocatalyseformulationoftheREDD(ReductionofEmissionsfromDeforestationandforestDegradation)agendaitemundertheUNFCCCnegotiations(http://unfccc.int/methodsandscience/lulucf/items/4123.php).
ScienceUpdate2011 59
Chapter 3: Implications of the science for emission reductions (continued)
3.Ifdesignedcarefully,abio-sequestrationapproachcanyieldsignificantco-benefits.Theseareespeciallyimportantfordeforested,degradedandintensivelycroppedlandswherethepotentialforsequesteringcarbonislarge.Well-conceivedandimplementedbio-sequestrationschemesintheselandscapescanimprovetheproductivityofcroppingsystemsthroughthereplacementofsoilcarbonthatwaslostintillage,candeliveradditionalecosystemservicessuchasimprovedwaterqualityonlandscapes,andcanmaintainorenhancebiodiversity.Therelationshipbetweenbio-sequestrationandbiodiversityisparticularlyimportant,aswell-designedsequestrationschemeshavethepotentialtoyieldpositiveoutcomesforbiodiversity(Steffenetal.2009).Infact,asynthesisoftheinterplayamongforestbiodiversity,productivityandresiliencearguesthatmorediverseforestshavehigherproductivity,storemorecarbon,andaremoreresilienttowardsdisturbancethanthosewithimpoverishedbiodiversity(Thompsonetal.2009).
Therearesomecautionsassociatedwithbio-sequestrationintolandecosystems,however.AsshowninFigure12,thelandsinkishighlyvariableontimescalesofafewyears,varyingbyasmuchas2-3PgCinthosetimeframes.ThestrongfluctuationsaredrivenlargelybymodesofclimatevariabilitysuchasENSOandbyvolcanicactivity,whichinducerapidchangesinsoilrespirationandplantgrowththroughchangesinsolarradiation,rainfall/droughtandtemperature(RaupachandCanadell2010;Kirschbaumetal.2007).
–In THE LongER TERm, CLImATE CHAngE CAn sIgnIfICAnTLy wEAkEn oR EvEn REvERsE THE LAnD sInk THRougH DRougHTs, InCREAsED soIL REspIRATIon AnD DIsTuRbAnCEs suCH As fIRE AnD InsECT ouTbREAks.
–
SimulationsbydynamicglobalvegetationmodelsusingtheIPCCIS92aemissionsscenarioshowalevellingoffofthelandsinkinthesecondhalfofthecenturywithtwomodelsshowingasignificantweakening(Crameretal.2001).Whencoupledtoaclimatemodelininteractivemode,allvegetationmodelsshowaweakeningofthelandsinkby2100withanetreleaseofcarbonbacktotheatmospherecorrespondingtoanadditionalriseinconcentrationfrom20to200ppmCO
2(Friedlingtsteinetal.2006).
Therearealreadyseveralobservationsoftheprocessesinthemodelsthatweakenthelandsinkandultimatelythreatentoreducethesizeofthelandstock.The2003droughtandheatwaveincentralEuropetriggereda30%reductioningrossprimaryproductivityovertheregion,whichresultedinastrongnetsourceof0.5PgCyr-1totheatmosphere,undoingfouryearsofanetcarbonsinkfortheregion(Ciaisetal.2005).Amulti-decadalstudyofthecarbonbalanceofCanadianforestshasdemonstratedthatsince1970theyhavebecomeaweakercarbonsinkdespitealongergrowingseason,owingtoasharpincreaseindisturbancessuchasfireandinsectoutbreakstriggeredbyawarmingclimate(KurzandApps1999).Ascitedearlier,theAmazonrainforest,animportantstockofcarbononagloballevel,hassufferedseveredroughtsandfiresin2005and2010,leadingtoestimatedlossesincarbonstorageof2.2and1.6PgCforthetwodroughtevents,respectively(Lewisetal.2011).Thisanalysissuggeststhatthetwodroughtshaveoffsetadecadeofcarbonsinkactivity,estimatedtobeabout0.4PgCuptakeperannum;suchobservationssupporttheassessmentthat,attemperaturesabovethe2°Cguardrail,theAmazonrainforestisatriskofextensivediebackandconversiontoasavanna,withconsequentlossofcarbontotheatmosphere(cf.Section2.5).Ifthisoccurs,thenAmazonianecosystemswillcontinuetoholdsignificantcarbonstocksbutatlowerthancurrentlevels.
60 ClimateCommission
Chapter 3: Implications of the science for emission reductions (continued)
Insummary,formanyreasonsincreasingcarbonstorageinlandecosystemsisanecessaryanddesirablecomponentofacomprehensiveapproachtogreenhousegasmitigation.However,itisnotequivalenttostoringcarboninasecuregeologicalformation,lockedawayfromtheinfluencesofclimatevariabilityandchangeorfromthedirectimpactsofhumanmanagement.Therelativevulnerabilityofcarbonstoredinlandecosystemstoperturbationscomparedtoinertgeologicalfossilfuelissometimescalledthe“permanence”issueinthedesignofeconomicinstrumentstoreduceemissions.
Geosequestration as an offset
Inprinciple,CO2sequesteredintolandecosystems
canfullyoffsettheemissionsofCO2fromfossilfuel
combustionifthesequesteredcarbonissubsequentlymadeinerttotheimpactsofhumanlandmanagement,environmentaldisturbancesorchangingenvironmentalconditions.Itistheoreticallyconceivablethatbio-sequesteredcarboncouldberemovedandstoredinastablegeologicalformation,lockedawayfromtheactiveatmosphere-land-oceansystem.
Anotherapproach,equivalenttogeosequestration,istoreplacefossilfuelcombustionwithbiofuelcombustiontoproduceenergy.Thegrowthandthencombustionofbiofuelsispotentiallycarbon-neutralasitrepresentsacyclicalprocessesofshiftingcarbonbetweenthelandandtheatmosphericcompartmentsinthefastatmosphere-land-oceancarboncycle.Thefossilfuelsthusreplacedwouldleavethecarboninfossilfuelsinthegroundandthusawayfromtheatmosphere.
Caremustbetaken,however,inthegenerationofthebiofuelstolimittheemissionsassociatedwiththeproductionprocesstolowlevelsrelativetotheamountofenergyproduced,andavoidundesiredsideeffectssuchascompetitionwithfoodproduction,lossofnaturalecosystemsandthusgenerationoflargecarbonemissions(seepoint2onpage60)andlossesofbiodiversity.Ingeneral,biofuelsmadefrom‘waste’biomassfromplantationforestsorfromperennialvegetationgrownonabandonedagriculturallandofferthemostadvantagesandavoidtheundesirablesideeffects.
Focussing on the end game
Thebudgetapproachtomitigationdescribedabove,whichisbecomingmorewidelyusedforanalysesintheresearchcommunity,offerssomescientificallybasedinsightsintomitigationapproaches.Perhapsmostimportantly,itfocusesstronglyonthe“endgame”ratherthaninterimtargets.
–puT sImpLy, If THE 2 °C guARDRAIL Is To bE ACHIEvED, THEn THERE Is no TImE foR DELAy In InvEsTIng In Low AnD no-CARbon TECHnoLogIEs foR EnERgy gEnERATIon, buILT InfRAsTRuCTuRE AnD TRAnspoRT.
–Responsiblyimplementedbio-sequestrationschemesoffersomeearlygains;theycanremovecarbonquicklyfromtheatmosphereandalsoofferanumberofimportantco-benefits.Thechallengeistoensurethatlinkingbio-sequestrationtothefossilfuelemissionssectorsdoesnotleadtoanydelaysintheinvestmentordeploymentoflow-orno-carbontechnologiesinthosesectors.
As you’ve read in this report, we know beyond reasonable doubt that the world is warming and that human emissions of greenhouse gases are the primary cause. The impacts of climate change are already being felt in Australia and around the world with less than 1 degree of warming globally. The risks of future climate change – to our economy, society and environment – are serious, and grow rapidly with each degree of further temperature rise. Minimising these risks requires rapid, deep and ongoing reductions to global greenhouse gas emissions. We must begin now if we are to decarbonise our economy and move to clean energy sources by 2050. This decade is the critical decade.
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