The Critical Decade: Full report

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)

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ISBN: 978-1-921299-50-6(pdf) 978-1-921299-51-3(paperback)

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IMPORTANT NOTICE – PLEASE READ

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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.

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8 ClimateCommission


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


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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10 ClimateCommission


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.

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ScienceUpdate2011 11


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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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).


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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).


– Thereisnocredibleevidencethatchangesinincomingsolarradiationcanbethecauseofthecurrentwarmingtrend.

– Neithermulti-decadalorcentury-scalepatternsofnaturalvariability,suchastheMedievalWarmPeriod,norshortertermpatternsofvariability,suchasENSO(ElNiño-SouthernOscillation)ortheNorthAtlanticOscillation,canexplainthegloballycoherentwarmingtrendobservedsincethemiddleofthe20thcentury.

– Thereisaverylargebodyofinternallyconsistentobservations,experiments,analyses,andphysicaltheorythatpointstotheincreasingatmosphericconcentrationofgreenhousegases,withcarbondioxide(CO2

)themostimportant,astheultimatecausefortheobservedwarming.

– ImprovedunderstandingofthesensitivityoftheclimatesystemtotheincreasingatmosphericCO

2

concentrationhasprovidedfurtherevidenceofitsroleinthecurrentwarmingtrend,andprovidedmoreconfidenceinprojectionsoftheleveloffuturewarming.


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).


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.

-2.5 -.9 -.7 -.5 -.3 -.1 .1 .3 .5 .7 .9 1.4


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)


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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.



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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– Despitethedipinhumanemissionsofgreenhousegasesin2009duetotheGlobalFinancialCrisis,emissionscontinueonastrongupwardtrend,onaveragetrackingnearthetopofthefamilyofIPCCemissionscenarios.

– Oceanandlandcarbonsinks,whichtogethertakeupmorethanhalfofthehumanemissionsofCO

2,

appeartobeholdingtheirproportionalstrengthscomparedtoemissions,althoughsomerecentevidencequestionsthisconclusionandsuggestsalossofefficiencyinthesenaturalsinksoverthepast60years.

– Ifglobalaveragetemperaturerisessignificantlyabove2°C(relativetopre-industrial),thereisanincreasingriskoflargeemissionsfromtheterrestrialbiosphere,themostlikelysourcebeingmethanestoredinpermafrostinthenorthernhighlatitudes.


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).


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:


– 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.


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).


IPCC approves outline

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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 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...


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.


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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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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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.




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

Brisbane

Darwin

Fremantle

Adelaide

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Hobart

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.



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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Source:Hoegh-Guldbergetal.(2007),whichgivesmoreinformationonthefigure.SeealsoFigure20afortheconnectionbetweenaragonitesaturationandcoralreefstate.

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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).


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Source:De’athetal.(2009).

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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).

–

A. B. C.

375 ppm +1 C 450 – 500 ppm +2˚C > 500 ppm > +3 °C

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.



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.



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.



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.



Figure 22. Trend in total annual stream flow into Perth dams 1911-2010.

Source:WesternAustralianWaterCorporation.

Total 1911–1974Average 338 GLTotal 1975–2000Average 117 GLTotal 2001–2005Average 92.4 GLTotal 2006–2010Average 57.7 GL

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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).



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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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:


– Modestchangesinaveragevaluesofclimaticparameters–forexample,temperatureandrainfall–canleadtodisproportionatelylargechangesinthefrequencyandintensityofextremeevents.

– OnaglobalscaleandacrossAustraliaitisverylikelythatsinceabout1950therehasbeenadecreaseinthenumberoflowtemperatureextremesandanincreaseinthenumberofhightemperatureextremes.InAustraliahightemperatureextremeshaveincreasedsignificantlyoverthepastdecade,whilethenumberoflowtemperatureextremeshasdecreased.

– TheseasonalityandintensityoflargebushfiresinsoutheastAustraliaislikelychanging,withclimatechangeapossiblecontributingfactor.Examplesincludethe2003Canberrafiresandthe2009Victoriafires.


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.


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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.



Figure 27. Number of (a) record hot day maximums and (b) record cold day maximums at Australian climate reference stations.


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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.




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).

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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.



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.



Actual PercentAverage Percent5-Year Moving Average

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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.

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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.


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.


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.

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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


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.



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...


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.


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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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.



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.

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35

30

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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



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.



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).



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.



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-sequestrationtothefossilfuelemissionssectorsdoesnotlead toanydelaysintheinvestmentordeploymentoflow-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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