Implications of the 1.5°C limit in the Paris Agreement for ... · P. 3 have put forward in relation to the 1.5°C limit in the Paris Agreement and the long term global emission goals

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ClimateAnalyticsgGmbH

Supportingsciencebasedpolicytopreventdangerous

climatechangeenablingsustainabledevelopment

Friedrichstraße231/HausB

10969Berlin/Germany

T/+49(0)30259229520

W/www.climateanalytics.org

ClimateAnalyticsgGmbH

Supportingsciencebasedpolicytopreventdangerous

climatechangeenablingsustainabledevelopment

Friedrichstraße231/HausB

10969Berlin/Germany

T/+49(0)30259229520

W/www.climateanalytics.org

August2016

Implicationsofthe1.5°ClimitintheParisAgreementforclimatepolicyanddecarbonisation

B i l l H a r e , N i k l a s R om in g , M i c h i e l S c h a e f f e r , C a r l - F r i e d r i c h S c h l e u s s n e r

P. 2

SummaryTheParisAgreementcontainsalong-termtemperaturegoal(LTTG)ofholdingtheincreaseintheglobalaveragetemperaturetowellbelow2°Candpursuingeffortstolimitthisto1.5°Cabovepreindustriallevels.The1.5°ClimitintheParisAgreementgoessignificantlyfurtherthantheprevious2°CgoaladoptedundertheUNFCCC.TheParisAgreementalsocontainslong-termglobalemissionsgoalstopeakglobalemissionsassoonaspossibleandthenreduceasrapidlyaspossible,accordingtothebestavailablescience,toreachzeroemissionsgloballyinthesecondhalfofthe21stcentury.

Thelegallybindingarchitectureoftheagreementrequiresthatcountriesmustupdateandincreasetheambitionoftheirnationallydeterminedcontributions(NDCs)onacommonfive-yearcyclefollowingafive-yearlyglobalstocktake.TheParisAgreementobligescountriestoprogressivelyincreasetheirlevelofambitionandaction,asreflectedintheirNDCs,soastomeetthelongtermtemperaturegoaloftheagreement(Mace,2016).

ThisreportprovidesanoverviewofthreeimportantissuesarisingfromtheParisagreement:

• Benefitsofthe1.5°Climit intheParisAgreementintermsofavoidedglobalandnationalclimateimpactsandrisks;

• The global emission pathways needed tomeet the Paris agreement’s long-termtemperaturegoal, including the required timingofpeakglobalemissionsandofzeroglobalemissions;

• Theremainingcarbonbudgetconsistentwiththe1.5°ClimitintheParisAgreement.

Theglobalemissionpathwayissuesandcarbonbudgetareparticularlyimportant,asinordertofulfilitsclimatecommitmentsundertheParisAgreement,Australia’spresentandfutureactionsonemissionreductionswillneedtofitwithintheselimits.Australia'sinitialnationallydeterminedcontribution(INDC)isinadequateevenfortheformer2°Cgoal1.AtpresenttheoverallgloballyaggregatedeffectofINDCsandcurrentpoliciesputtheworldona3°Corcloseto4oCpathwaysrespectively2,muchabovetheprevious2°Climit.Thefirstreview,orstocktake,oftheaggregateeffectofallINDCcommitmentsthatcountries

1http://climateactiontracker.org/countries/australia.html≈2 http://climateactiontracker.org/andhttp://unfccc.int/resource/docs/2016/cop22/eng/02.pdf

P. 3

haveputforwardinrelationtothe1.5°ClimitintheParisAgreementandthelongtermglobalemissiongoalswilloccurin2018aspartoffacilitativedialogueagreedinParis.The2018facilitativedialogueprocesswillbeinformedbyaspecificallyrequestedIPCCSpecialReporton1.5°Cimpactsandonhowtoclosethegapbetweencurrentpolicies,NDCsandemissionpathwaysconsistentwiththeParisAgreementin2025and2030.Basedonthis,by2020atthelatestgovernmentsneedtosubmitupdatedNDCsfor2025and2030thateffectivelyclosethisgap.ThereisthereforeanexpectationthatAustraliaimprovesitsemissionreductionspledgetobeconsistentwiththeParisAgreementanditsenablingdecisions3.Thegovernment's2017reviewofclimatepoliciesanditspromisedconsiderationoflongtermtargetsprovidesanimportantopportunityforstrengtheningsuchcommitments.The2018stocktakewillbeanothercrucialopportunity.

Limitingwarmingto1.5°Cprovideskeybenefitscomparedtoa2°Climit

Atpresentglobalwarmingisabout1°Cabovepreindustriallevels.Theworldisalreadyexperiencingsubstantialimpactsanddamages,includingforagriculture,humanlivelihoodsandnaturalsystemssuchastheGreatBarrierReef.Concernsthatsustainedglobalwarmingof2°Cabovepre-industrialwouldleadtoverylargeimpacts,damagesandrisksledmanyvulnerablecountriestoexpressconcernthattheformer2°Climitwastoounsafe.TheseandotherconcernsledtheworldtoadopttheParisAgreement’slong-termtemperaturegoal,whichincludesa1.5°Climit,asopposedtotheearlier2°Cgoaladoptedbytheinternationalcommunity.Whyhasthiscomeabout?

Recentscientificliteratureshowsthatthereisasignificantincreaseinimpactsandrisksasglobal-meanwarmingmovesfrom1.5°Cto2°Cabovepre-industriallevels.Thisseeminglysmalldifferenceinglobalaveragetemperaturerepresentsalargeadditionofenergyintotheglobalclimatesystemandturnsouttocarrylargeconsequences.

Iftheworldwarmsby1.5°C,currentlyrareclimaterelatedextremes(extremeheatwaves,unusualdryspells,extremerainfall,massiveglobalcoralbleachingevents)wouldbecomethenewnormal.Ifglobalmeanwarmingweretoreach2°C,theclimatesystemwouldmoveintounchartedterritory.Forexample,inaworldthatis2°Cwarmerthanpre-industriallevels,thetypicalannuallengthofwarmspellswouldbeupto60dayslongerthantodayacrossnorthernpartsofAustraliaandaround20dayslongeracrosscentralandsouthernregions.Thisincreasewouldbereducedbyatleast30%,ifglobalwarmingislimitedto1.5°C.

3 http://unfccc.int/resource/docs/2015/cop21/eng/10a01.pdf#page=2

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The1.5°Climitwouldmeansmallerincreasesintemperatureoftheannualhottestdays,reducedimpactsonwateravailabilityandasmallerincreasesinlengthofdryspells,inparticularacrosssouthern,south-westernandmid-westernpartsofAustralia.

Inlightofthe2014-2016massiveglobalcoralbleachingevent(Eakinetal.,2016)thathasdrasticallyaffectedtheGreatBarrierReef4,itisespeciallycriticaltonotethatlimitingglobalwarmingto1.5°Cofferssomechanceforafractionoftheworld’scoralreefstosurvive,while2°Cwouldprovideverylimitedchance,ifanyatall.

Globalsealevelriseat1.5°Cwarmingwouldreach40cmabovecurrentlevelsby2100,comparedto50cmat2°C.Howeverthisdoesnottakeintoaccountthelonger-termpost-2100commitmenttofurthersealevelrise,nordoesitincludetheriskofcrossingathresholdtoirreversiblemeltofpartsoftheAntarcticandGreenlandicesheets,withmulti-metresealevelriseasaconsequence.Thisriskisestimatedtobemuchlowerat1.5°Cthanat2°C.

Insummarythedifferencebetween1.5°Cand2°Cmarksthedifferencebetweentheupperendofpresentdayclimatevariabilityandanewclimaticregimeoftemperatureandwaterrelatedextremes.Inacountryalreadyexperiencingtemperaturesexceeding40°Cinitsurbancentresthisisaveryseriousreasonforconcernforhumanhealth(Smithetal.,2014).AlongwithasubstantialdryingtrendprojectedinparticularforsouthernpartofAustraliathiswillnegativelyaffectagriculturalproductivity(Reisingeretal.,2014)aswellasleadtoincreasedrisksofforestfires(Pitman,Narisma,&McAneney,2007).Risksundera1.5°Cwarmingwouldstillbesubstantialbutwouldalleviateseveralofthemostprofoundrisksforcoralreefdegradation,extremetemperaturesanddryingprojectedfor2°C.

Withearlyaction,globalemissionreductionsneededtomeetthe1.5°Climitaretechnicallyandeconomicallyfeasible

Limitingwarmingtobelow1.5°Crequiresqualitativelysimilartransformationsintheproductionanduseofenergytothoseneededtoholdwarmingbelow2°C,howeverthedecarbonisationoftheenergysystemneedstobefasterandmorepronounced.

For1.5°Cscenarios,zeroCO2emissionsgloballyareneededbyabout2050,andabouttenyearslaterfor2°Cscenarios,withglobalemissionspeakingnolaterthanabout2020beforerapidlydeclining.

Fortheworldtogetontoa1.5°Cpathway,thefirstandmosturgentmeasuresincludetherapidscalingupofrenewableenergysystems,energyefficiency,electrificationoftransportsystems,andimprovementsofindustrialandbuildingefficiency.Increasedenergyefficiencyatalllevelsisneededtoreducethegrowthinprimaryenergydemand, 4https://www.coralcoe.org.au/media-releases/coral-death-toll-climbs-on-great-barrier-reef#

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andrenewablesandotherlow-carbonenergysystemsareneededtodecarbonisetheprimaryenergysupplysystemrapidly.

Globally,thedirectmitigationcostsof1.5°Cscenariosareabout1.5-2timeshigheroverthewholeofthe21stcenturythantheformer2°Cgoal,butasnotedabovethebenefitsarevast.

Anyglobaltemperaturelimitisassociatedwithacarbonbudget-theallowedtotalofglobalcumulativeCO2emissionsinordertomeetagivenglobalwarminggoal.HistoricalemissionstodateandhighlevelsofatmosphericCO2concentrationsatpresent(above400ppmCO2)meanthattheremaininggreenhousegas(GHG)andcarbonbudgetsareverysmall.Limitingwarmingto1.5°C(or2°C)thereforenowrequiresintroducingnegativeCO2emissionsatasmallscaleinthe2030sandscalingupthereafter.Inaddition,sequestrationofcarboninbiologicalreservoirs,suchasforestsandsoils,willalsobeneeded.Theneedfornegativeemissiontechnologiesbyitselfisnotadifferentiatingelementbetweenthe1.5°Climitandtheprevious2°Climit.

Thissituationmeansthatinthelongerterm(post2030)technologiestoremoveCO2fromtheatmosphereplayakeyrole,evenwithveryrapidGHGreductionsinthenext10to15years.Thescientificliteratureonemissionpathwayspointstobioenergywithcarboncaptureandstorage(BECCS)asthemostviableoptionatpresent(TheClimateInstitute,2014),withthepotentialforlarge-scaledeploymentatlimitedeconomiccosts.Theliteratureandmodelsgenerallyincludeconsiderationsofinterrelatedlanduseissues,suchasfoodproduction,bioenergy,afforestationandreforestation.WhileBECCSwillnotbewithoutsignificantchallenges(Fussetal.2014)withtechnological,sustainability,social,political,andlegaldimensions,thistechnologyhastheuniquecapabilityofremovingCO2fromtheatmosphereandsupplyingenergyatthesametime.

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Introduction Oneoftheoutcomesofthe2009climatesummitinCopenhagen(COP15)wasanagreementonagoaltoholdwarmingbelowa2°Cincreaseabovepre-industrial.However,atCopenhagenmorethan100vulnerablecountrieswerecallingforlimitingwarmingtobelow1.5°C.Recognisingthis,duringthesubsequentclimatesummitinCancun(COP16)in2010,theUNFCCCestablishedareviewprocesstoevaluatewhetherthelong-termglobaltemperaturegoalofholdingwarmingbelow2°Cwasadequatetoavoiddangerousclimatechange,andwhethergoodprogresswasmadetowardsachievingthelong-termgoal.Thereviewprocessfocusedinparticularonthedifferencesinimpactsbetween1.5°Cand2°Cwarmingabovepre-industriallevels.Thisprocessendedin2015withthefinalreportofitsscientificarm(a“StructuredExpertDialogue”)concludingthatawarmingof2°Ccannotbeconsideredsafeandthat1.5°Cisclosertobeingasafe‘guardrail’(UNFCCC,2015).

Thisveryimportantfindingwasreflectedinthelong-termtemperaturegoaloftheParisAgreementofholdingtheincreaseintheglobalaveragetemperaturewellbelow2°Candpursuingeffortstolimitthisincreaseto1.5°Cabovepreindustriallevels.InParisatCOP21Australiasupportedtheinclusionofthe1.5°ClimitintheParisAgreementlongterm-temperaturegoal.

OnApril22nd2016inNewYork,Australiajoinedmorethan170othernationsinsigningtheParisAgreementtodealwiththeglobalclimatechangeproblemandpledgedtoratifyitin2016.AstheAustraliangovernmentpreparestoratifytheParisAgreement,thuscommittingitselftofulfiltheobligationsitentails,Australia’semissionreductionsandotheractionsneedtobeassessedinlightoftheircompatibilitywiththeParisAgreementtemperaturegoalandotherelementsoftheAgreement.TheanalysisinthispaperoutlinestheglobalemissionenvelopewithinwhichthisassessmentneedstotakeplaceinordertomeettheobligationsoftheParisAgreement.

Comparisonofclimateimpactsbetweena1.5°Canda2°CworldAlreadywithanobservedwarmingofaround1°C,theimpactsofanthropogenicclimatechangearefeltglobally.Temperaturesandsealevelareontherise(IPCC,2013),extremeweathereventsareincreasinginintensityandfrequency(IPCC,2012),withlargelydetrimentaleffectsonglobalagriculture(Lesk,Rowhani,&Ramankutty,2016;Porteretal.,2014)andatmosphericCO2levelsarehighestinmillionsofyears,leadingtograduallyworseningoceanacidification(IPCC,2014).Severeimpactsonmarinelife,includingerosionandmassbleachingoftropicalcoralreefs,aretheconsequenceofalready

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observedoceanacidificationandincreasingtemperatures(J.-P.Gattusoetal.,2015).Duringthe2015-2016ElNiñoevent,massbleachinghasaffectedvastpartsoftheGreatBarrierReef(Normille,2016).Inaddition,thereisgrowingevidencethatpartsoftheWestAntarcticicesheet,a“tippingelement”oftheEarthSystem(Lentonetal.,2008),mayalreadybeinirreversibleretreat,implyingadditionalsealevelriseofatleastonemeterovercenturiestocome(Feldmann&Levermann,2015;Joughin,Smith,&Medley,2014;Rignot,Mouginot,Morlighem,Seroussi,&Scheuchl,2014).

Ocean,cryosphereandglobalsealevelriseAsexplainedbelow,physicalimpactsonoceansandthecryosphere(AntarcticandGreenlandicesheets,smallicecapsandmountainglaciers)differconsiderablybetween1.5°Cand2°Cwarminginrelationtobothgradualimpactsandtotheriskofabruptshiftsofso-called“tippingelements”(Lentonetal.,2008).TippingelementsintheEarthSystemarecharacterisedbyinternalself-amplifyingdynamicsthatmayleadtoacompletechangeinitsstate.Suchdynamicsaretriggeredaboveacertainglobalmeantemperatureincrease,orthetippingpoint.

ArecentstudyanalysingtippingelementsintheEarthSystem,includingoceancirculationandseaicepatternsinclimatemodels,foundthat2°Cwarmingwouldalreadycrossabout50%ofalltippingpointsidentifiedinthesemodelsforanylevelofwarming(Drijfhoutetal.,2015).Thisnumberisreducedtoabout20%ifwarmingislimitedto1.5°C.Besidesthoseassociatedwithsealevelrise,othertippingelementsassessedincludelarge-scaleforestdiebackoftheAmazonandboreal(highlatitude)forests,permafrostcollapse,vegetationshiftsinAfrica’sEasternSahelregion,anddisappearanceofArcticseaice.

MostrelevantforthissectionarepotentialtippingpointsforlargescaleicesheetdisintegrationoftheAntarcticandGreenlandicesheets.TheWestAntarctic(Feldmann&Levermann,2015)aswellaspartsoftheEastAntarcticicesheet(Mengel&Levermann,2014)areatsubstantialriskofdisintegrationduetoocean-icesheetinteractions.Thiscouldresultinadditionalsealevelriseof6-8metersovertimescalesofseveralcenturiestomillennia(Deconto&Pollard,2016).AlthoughoceanicwarmingaroundAntarcticaisprojectedtoincreasewithincreasingglobalwarming(Hellmer,Kauker,Timmermann,Determann,&Rae,2012),ourcurrentunderstandingofthesechangesdoesnotallowforaquantificationofthepreciselevelsofglobalmeantemperatureincreasetowhichthepotentialtippingpointscanbelinked.

TheGreenlandicesheetismoredirectlyvulnerabletoincreasesinatmospherictemperaturesandresearchershaveidentified1.6°CwarmingasthebestestimateforacriticalthresholdforatippingoftheGreenlandicesheet(Robinson,Calov,&Ganopolski,2012).AdisintegrationoftheGreenlandicesheetcouldleadtoupto7msealevelrise

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overthousandsofyears.Assessmentsofpastsealevelevidencefromearthhistoryandstate-of-the-artmodellingresultsindicateanaveragemulti-millennialaveragesealevelriseofabout2.3mper°Cofwarming(Levermannetal.,2013).

Projectionsfor21stcenturysealevelrisearedisplayedinFigure1(Schleussneretal.,2016).Thebestestimatefor21stcenturysealevelriseundera2°Cscenarioisabout50cm,whichis10cmlessundera1.5°Cscenario.Possiblyevenmoreimportantaretheratesofsealevelrisein2100,astheywilllargelyinfluenceapost-2100commitmenttolong-termsealevelrise.Onlyundera1.5°Cscenarioaretheseratesindeclineby2100,reachingpresentdaylevelsbythattime.Note,however,thattheseassessmentsdonotaccountyetfortheeffectofpotentialrapidicesheetdisintegrationdiscussedabove,nordotheyidentifyvulnerabilitiesrelatedtodifferentratesofriseregionallyorincreasesin

sealevelvariabilitythatmaybeinfluencedbyglobalwarming(Widlansky,Timmermann,&Cai,2015).

TropicalcoralreefsTropicalcoralreefsareparticularlyvulnerabletoclimatechange.Theyarethreatenedbyoceanacidification(Pandolfietal.,2003)andintensecoralbleachingasaresultofoceanicwarming(Meissner,Lippmann,&SenGupta,2012).Figure2displaysthefractionofglobaltropicalcoralreefsprojectedtobeatriskoflong-termdegradation,understoodasaneventuallossofthereefecosystem,duetoseverebleachingeventsoccurringatleast

Figure1:Probabilisticprojectionsofglobalsealevelriserelativeto1986-2005levels(lowerpanels)fora1.5°Ctemperaturescenario(left)anda2°Cscenario(rightpanels).Thicklinesindicatemedianestimateswhereasdarkcolouredrangesindicatethe66%likelihoodrangeandlightcolouredrangesthe90%likelihoodrange.Medianestimatesfor2100sealevelriseunder1.5°Cisabout10cmlessthanthe50cmprojectedfor2°C.FromSchleussneretal.(2016).

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everyfiveyearsfor1.5°Cand2°C.Unlessextremelyoptimisticscenariosofcoralreefadaptationareassumed,virtuallyalltropicalcoralreefswillbeatsevereriskofdegradationunder2°Cwarming.Thewarmingdifferencebetween1.5°Cand2°Cislikelytobedecisiveforthefuturesurvivaloftropicalcoralreefsandonlythe1.5°Cscenarioofferssomepotentialfortheseecosystemstoadapt.ThealreadyobservedcoralbleachingillustratesthemagnitudeandscaleofthisriskfortheGreatBarrierReef,aWorldHeritage

site,thathasalreadylostmorethan50%ofitscoralcoversince1985(Gattuso,Hoegh-Guldberg,&Pörtner,2014).Thislosswillhavewidespreadanddetrimentalconsequencesforlivelihoodsofcommunitiesdependingonit.Coralreefsprovidecoastalprotection,accountformorethan10%offishcaughtintropicalcountries(20%ofdevelopingcountries)andtheGreatBarrierReefitselfgeneratesaboutA$5.4billionintourismrevenueannuallytotheAustralianeconomy(Gattusoetal.,2014).

Figure2:Probabilisticprojectionsoftheshareofglobaloceangridcellswithtropicalcoralreefsatriskoflong-termdegradationundera1.5°Cscenario(top)anda2°Cscenario(bottom,forthetemperaturetrajectories,see(Schleussneretal.,2016).Thisisassessedoveralloceangridcellscurrentlycontainingcoralreefs.Thefiguredisplaystheshareforassumptionsaboutfutureevolutionoftemperatureresilienceofcoralreefs.Whereasthe“Constant”-caseassumesobservedlevelsofcoralsusceptibilitytoincreasedwatertemperatures,the“ThermalAdaptation”caseassumesunprecedentedandrapidadaptationoftheseecosystems.Asthisappearstobeveryoptimisticgiventhedetrimentaleffectsofoceanacidificationtotropicalcoralreefcalcificationrates(therebyweakeningthesesystemsconsiderably),theseprojectionsrepresenttheabsolutelowerboundoffuturetropicalcoralreefrisk.FromSchleussneretal.(2016).

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Impactsof1.5°Cand2°CwarmingonAustraliaAustraliaisexposedtoclimatechangeimpacts,includingsealevelriseandcoralreefloss,butalsotoextremeweathereventsanddryingtrends(Reisingeretal.,2014).Theanalysisbelowbuildsonarecentglobalstudythatinvestigateddifferencesbetween1.5°Cand2°Cwarmingforarangeofclimateindicators(Schleussneretal.,2016).

ExtremeWeatherEvents

Inrecentyearstheworldhasexperiencedaprofoundincreaseinfrequencyandintensityofextremeweathereventsduetoanthropogenicclimatechange(IPCC,2012).Describedbelowaretheresultsforfourdifferentextremeweathereventindicators:extremetemperature,warmspells,dryspells(meteorologicaldroughts)andextremeprecipitation(Zhangetal.,2011):

• Intensityofhotextremes(TXx):Howmuchhotterwouldthehottestdayinayearbecome?Theindicatorusedhereistheannualmaximumvalueofdailymaximumtemperature.Thisisagoodindicatorfortheincreaseinextremehightemperatures.

• Warmspelldurationindicator(WSDI):Howmuchlongerwouldacurrentlytypicalseriesofhotdaysbecome?Theindicatorhereistheannualcountofthelongestconsecutiveperiodinwhichthedailymaximumtemperatureforeachdayexceedsthe90%quantileforthisdayoverthereferenceperiod.Theminimumlengthissixconsecutivedays.Thisisagoodindicatorforheat-wavesoccurrence.

• Dryspelllengthorconsecutivedrydays(CDD):Howmuchlongerwoulddryperiodsbecome?Theindicatorhereisthemaximumnumberofconsecutivedaysforwhichtheprecipitationisbelow1mmperdayinayear.Thisisagoodindicatorformeteorologicaldrought.

• Heavyprecipitationintensity(RX5day):Howmuchheavierwouldheavyrainfallperiodsbecome?Theindicatorhereistheannualmaximumofrainfalloveraconsecutive5-dayperiod.Thisisagoodindicatorforfloodingrisk.

Theanalysishasbeenperformedonensemblesofstate-of-the-artclimatemodelsfromthefifthClimateModelIntercomparisonProject(CMIP5,11modelsfortemperatureextremes,14modelsforprecipitationextremes;formoreinformationaboutthemethodologysee(Schleussneretal.,2016)).ThemodelmedianresultsforeachoftheseindicatorsaredisplayedinFigure3for1.5°Cand2°Cwarming,aswellasthedifferencebetweenthetwowarminglevels.

Awarmingof2°Cwouldimplyasubstantialincreaseintemperaturerelatedextremes:annualextremetemperatureswouldexceed3°Cabovevaluestypicallyexperiencedduringtherecentpast(1986-2005)andtheannualmeanlengthofwarmspells(alsorelativetothe1986-2005period)wouldlastaround20daysforcentralandsouthernpartofAustraliaandupto60daysinthenorthernpartofAustralia.

Under1.5°Cwarming,thisincreaseinintensityandlengthofextremetemperatureeventsislesspronouncedcomparedto2°C.Increaseintheintensityofannualhotextremes(TXx)

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wouldbelimitedtoabout2°Cabovethe1986-2005referenceperiodandwarmspellswouldbeupto15daysforthecentralandsouthernpartsofAustraliaanduptoabout30-40daysforthenorthernpartofAustralia.

NotethattheextremeincreasesindicatedinthenorthernpartofAustraliaandcoastalgrid-cellsarisefromthefactthatmostoftheunderlyinggridcellscontaindominantlyoceanandnotland.Thiscouldbeovercomebydownscalingglobalclimatemodelresultsfortheseregionstoachievehigherresolution,e.g.inadynamicaldownscalingapproach(AustralianBureauofMeteorologyandCommonwealthScientificandIndustrialResearchOrganisation(CSIRO),2011).Asnaturaltemperaturevariabilityovertheoceansismuchsmallerthanoverland,theresultingincreaseinwarmspellduration,anextremeeventindexdevelopedforland,isparticularlypronouncedandshouldbeinterpretedwithcaution.

Figure3:Projectionsforchangesinextremeweathereventindicatorsrelativetothe1986-2005referenceperiod.Greymarkedareasindicateregions,wherelessthan66%ofthemodelsintheensembleinvestigatedagreeonthesignofchange.

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Projectionsforprecipitation-relatedindicesindicatesubstantialuncertaintyinfuturechangesofprecipitation-relatedextremes,forbothextremewetanddryevents(greyareasinFigure3).However,significantdifferencesexistinparticularrelatedtoextremeprecipitationeventsinthenorthernpartofAustraliathatareprojectedtointensifyby7-10%comparedtolessthan5%undera1.5°Cwarming.Inspiteofsuchheavierrainfallevents,arobustincreaseindryingisprojectedforlargepartsofAustraliaundera2°Cwarming.Thecurrentlylongestannualdryspellwouldbecomeupto2weekslonger(compareFigure3,bottompanel).Thedifferencebetween1.5°Cand2°Cinthelengthofdryspellsisparticularlypronouncedinthesouthern,south-westernandmid-westernpartsofAustralia.Anoverviewofthiscomparisonofchangesinextremeeventsat1.5°Cand2°Caregivenin

Table1.

Wateravailability

Inadditiontochangesinextremeweatherevents,Australiaisparticularlypronetolong-termdryingtrends.Here,weanalysechangesinannualwateravailability(totalrunoff,Qtot)basedontheclimateimpactmodelintercomparisonprojectISI-MIP(Scheweetal.,2014).Theprojectionspresentedarebasedon5bias-correctedclimateprojectionswithstate-of-the-artclimatemodelsand11hydrologicalmodelscomprising,intotal,amodelensembleof55models.

Undera1.5°Cscenario,wefindareductioninannualwateravailabilityofabout10%formostpartsofAustralia,withreductioninannualwateravailabilityinwesternpartsofAustraliaexceeding15%(Figure4).Under2°C,wateravailabilityisprojectedtocontinuetodecreaseacrossthesouthernhalfofthecontinentandismostpronouncedinthesouth-easternandsouth-westernandPilbararegions.Reductionsexceed20%forwesternpartsofAustralia.Inthenorthernthirdofthecontinentourresultsindicateareversalintheprojectedtrendtowardsaslightwetting,probablyrelatedtomonsoonintensification.Itis,however,importanttounderscorethattheseprojectionsareuncertainandeven

2°C 1.5°C 2°C - 1.5°C

Qto

t [%

]

Figure4:Projectionsforchangesinannualwateravailability(Qtot)relativetothe1986-2005referenceperiod.Gridcellswherelessthan66%ofallGCM-GHMpairsagreewiththemediansignofchangearemaskedgrey.Gridcellswithanannualmeanrunoffoflessthan0.05mm/dayaremaskedwhite.

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reductionsexceeding40%(30%)arestillwithinthe‘likely’(66%probability)rangeofprojectionsfora2°C(1.5°C)warming.

Table1:Changesintemperatureandprecipitationrelatedextremeeventindicesprojectedat1.5°Cand2°CwarmingfromSchleussneretal.(2016).Themedianchangeexperiencedby50%oftheland-areaisgivenforthenorthernpartofAustralia(Northof30°S)andthesouthernpartAustralia(Southof30°CandincludingTasmaniaandNewZealand).Thelikelyrangeoverthemodelensemble(66%likelihood)isindicatedinsquarebrackets.ChangesinWSDIareexpressedindaysandRX5day,CDD,aswellasQtotin%change.Allchangesareassessedrelativetothe1986-2005referenceperiod.

Indexofchange NorthernpartofAustralia SouthernpartofAustralia

1.5°C 2°C 1.5°C 2°C

Increaseinwarmspellduration–[WSDI,days]

36.5

[27.7,44.4]

52.3

[40.0,62.0]

12.7

[8.3,15.4]

19.8

[13.6,25.9]

Increaseinheavyprecipitationintensity[RX5Day,%]

4.3

[0.4,7.4]

5.1

[1.2,9.8]

2.0

[-2.0,5.1]

2.7

[-1.2,6.7]

Increaseindryspelllengthorconsecutivedrydays[CDD,%]

6.7

[-1.2,11.4]

8.2

[1.2,15.3]

3.5

[0.4,8.2]

5.9

[1.2,12.9]

Changeinannualwateravailability[Qtot,%]

-10.4

[-36.2,-2.5]

-6.8

[-40.5,7.6]

-7.2

[-33.9,-1.4]

-12.7

[-44.4,6.8]

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Comparisonofglobalmitigationpathwaysbetweena1.5°Canda2°CworldIntegratedAssessmentModelsofClimateChange(IAMs)provideinformationonthecomplexinterrelationbetweentheeconomy,energyuseandtheglobalclimate.SomeofthemostimportantinsightsIAMsofferatasystemlevelrelatetotheenergysystemtransformationnecessaryforachievingaspecificclimatetarget–suchaslimitingglobalmeantemperatureincreaseto1.5°Corholdwarmingbelow2°Cabovepreindustriallevels.Energysystemtransformationmeans,inessence,ashiftawayfromtherelianceonfossilfuels,towardsrenewablesontheenergysupplyside,combinedwithenergyefficiencyimprovementsonthedemandside.IAMsprovideestimatesofthedirectcostofclimatechangemitigation,comparedtoabaselinescenariowithoutclimatepolicy.Muchresearchshowsthatthesedirectmitigationcostsareexpectedtobepartially,orfullybalancedbythebenefitsofmitigation,inparticulareconomicco-benefitsofmitigation(e.g.reducedairpollutioneffectsonhumanhealthandagricultureduetoreduceduseoffossilfuels)andavoidedclimatechange.IAMsingeneraldonotaccountforco-benefitsordamagesfromclimatechange.

Theenergysystemtransformationsnecessarytolimitwarmingtolessthaneither1.5°Cor2°Careverysimilar,butdeploymentoflow,zeroandnegativecarbonenergysupplyoptionsneedstohappenfasterinthe1.5°Climitcasesothatemissionreductioncanberealisedearlier(Rogelj,Luderer,etal.,2015;Schaefferetal.,n.d.).Thisisassociatedwith1.5-2timeshighercostsoverthecenturyasawhole,withcostshigherinthenextfewdecades,ratherthanlaterinthecentury(Rogelj,Luderer,etal.2015).

Baseline,1.5°Cand2°Cscenarios

ThissectionassessesglobaltotalCO2andGHGemissionsforthreescenariosfromtheMESSAGEmodel5(base,1.5°Cand2°C)andcomparestheseforGHGswithhistoricaldataandtwoprojectionsoftheeffectsofcurrentpolicies(CAThighandCATlow)fromtheClimateActionTracker(CAT).TheCAThighandCATlowscenariosdescribetherangeofemissionpathwaysthatcanbeexpectedtoresultfromacontinuationofcurrentclimatepolicies,inabottom-upglobalaggregationofcountry-levelassessments.ItisimportanttonotethatanassessmentoftheeffectsreductionspledgesandINDCs(Intended

5 http://www.iiasa.ac.at/web/home/research/researchPrograms/Energy/MESSAGE.en.html

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NationallyDeterminedContributions)thatcountriesputforwardin2015duringtheParisAgreementnegotiationsresultinmostcasesinloweremissionsthancurrentpolicies.

IntheMESSAGEbaselinescenario,noglobaltemperaturegoalisassumed.

Tocharacterisepoliciesconsistentwiththe2°CgoaladoptedintheCopenhagenAccordsin2009wedrawfromexistingemissionspathwaysthatholdtheincreaseinglobalaveragetemperaturebelow2°Cwithatleastalikelyprobability(>66%).Atpresentmostemissionpathwaysthatcomeclosetothe1.5°Climit,exceedanincreaseof1.5°Cabovepre-industriallevelsduringthe21stcentury,beforedroppingtothe1.5°Climitby2100.Inthisstudywehaveusedoneofthelowestscenariosavailablethatlimitwarmingto1.5°Cby2100withaprobabilityofmorethan50%(andholdswarmingbelow2oCwithaboutan85%probability).Thisshouldnotbeseenasaninterpretationofthe1.5°ClimitintheParisAgreement.Anewgenerationofscenariosarebeingdevelopedwhichmaynotexceed1.5°C,howeverthesewerenotyetavailableforthisreport.

Probabilitiesofstayingbelowthegivenwarmingthresholdarecomputedusingthereducedcomplexityglobalclimate-carboncyclemodelMAGICC(Meinshausen,Raper,&Wigley,2011),inthesamemethodologyaswasappliedforIPCC’sFifthAssessmentReport,andaccountsforuncertaintiesinimportantclimate-relatedparameters–e.g.climatesensitivityandcarbon–cyclecharacteristics–byreturningprobabilistictemperatureresponses.

TheMESSAGEscenariosweretakenfromanensembleofscenariosusedinpeer-reviewedpublications(Rogelj,McCollum,Reisinger,Meinshausen,&Riahi,2013;Rogelj,Mccollum,&Riahi,2013)andselectedfortheirinter-scenariocomparability:

• Scenarios share the same assumptions regarding substantial improvements in energyefficiency–energydemand iscomparatively lowalready in thebaselinescenario. In theshort tomedium term, GHG emissions arewell in linewith the current policy CAT lowscenario,whichgivesthelowerestimateofglobalemissionsforaworldinwhichcurrently(2015) implementedclimatepolicieswherecontinued.Thiswould leadtoabout3.3°Cofglobalwarminguntil21006andinthelongtermtendtobeconsiderablyhigherthanthis.Globalwarmingresultingfromsuchabaselinecanthereforebeconsideredtobewellabove3.3°Cin2100.

• Climatepolicyfullycompatiblewiththerespectivetemperaturegoalsonlybeginsfrom2020inthesescenarios.Thisisinlinewiththecurrentstateofaffairsregardingclimatepolicy.Acceleratedpoliciesleadingtohighermitigationpre-2020remainfeasibleandwouldresultin slightly lower rates of reduction post 2020 required to still achieve the long-termtemperaturegoal.

TheMESSAGEbaselinescenario,whichassumestheabsenceofglobalclimatepolicytolimitwarmingtoanycertainlevel,showsadeclineinCO2emissionstowardstheendof 6 http://climateactiontracker.org/global.html

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thecentury.Thisismainlyduetoassumedconstraintsinfossilfuelavailabilityatprices/costthatcancompetewithprojecteddecreasingpricesforrenewableenergysupply,howeverbythattimewarminghasreachedaround3.5°C.Table2showsrelativechangesinglobalCO2andGHGemissionslevelsin2030and2050,relativeto2005levels,andpercapitaemissionslevels.ThisemphasisesagaintheneedforglobalzeroCO2emissionsandmajorreductionsinGHGemissionsbyaround2050inordertolimitwarmingto1.5°C.ItalsoshowsthevastdiscrepancybetweenthegoalslaidoutintheParisAgreementandwhatcountriesaroundtheworldarecurrentlydoing–thelattercanbeseenforGHGemissionsintheCAThighandlowscenarios.Also,theMESSAGEbaselinescenarioassumesamoreoptimisticdevelopmentcomparedtotheCAThighscenarioandisrelativelyclosetotheCATlowscenario.

Figure5:GlobalCO2andGHGemissionsforbaseline,1.5°Cand2°CscenariosfromtheMESSAGEIAM,GlobalcurrentpolicyGHGpathways,Source:IIASA/JoeriRogelj,CAT

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Table2:Globalchangesandglobal-meanpercapitavaluesofCO2andGHGemissions

2030 2050

Scenario ChangeinCO2emissions(on2005levels)

[%]

ChangeinGHGemissions(on2005levels)

[%]

PercapitaCO2emissions

[tCO2/yr]

PercapitaGHGemissions

[tCO2e/yr]

ChangeinCO2emissions(on2005levels)

[%]

ChangeinGHGemissions(on2005levels)

[%]

PercapitaCO2emissions

[tCO2/yr]

PercapitaGHGemissions

[tCO2e/yr]

1.5°C -29.4 -18.8 2.9 4.6 -99.2 -70.5 <0.1 1.5

2°C -2.7 2.7 4.0 5.8 -66.9 -44.5 1.2 2.8

Currentpolicies-CATHigh1)2)

- 40.3 - 7.2 - 76.0 - 7.91)

Currentpolicies-CATLow1)2)

- 33.0 - 6.8 - 48.2 - 6.71)

Baseline 16.0 20.8 4.8 6.8 31 37.8 4.9 7.01)TheCATglobalassessment(http://climateactiontracker.org/global.html)doesnotspecifyglobalpopulationdata.Therefore,theMediumscenariofromtheUnitedNationsWorldPopulationprospects2015(https://esa.un.org/unpd/wpp/)wasusedforglobalpopulationnumbersin2030and2050.Giventhelong-termemissionpathwaysofCATarederivedfromIPCCFifthAssessmentReportscenarios,whicharealsooftenassociatedwithUNpopulationprojections,thisisareasonableapproach,butitmustbenotedthatpopulationprojectionsarehighlyuncertainandalternativeassumptionswouldgivedifferentper-capitaemissionvalues.2)CATglobalassessmentconsidersonlytotalGHGs.Therefore,noCO2couldbecomputed.

GlobalemissionreductionsfortheParisAgreement

Withclimatepolicyinplace,totalGHGemissions(thesumofCO2emissionsandglobalwarmingpotential(GWP)7weightednon-CO2GHGemissions)needtodeclinerapidlytoreachgloballyaggregatedzeroemissionsandthenbecomenegativeinthelatterpartofthe21stcentury.NegativetotalGHGsemissionsresultfromnegativeCO2emissionsoutweighingtheremainingnon-CO2GHGemissions:CO2emissionsneedtobecome

7 Applies 100 year GWPs based on IPCC Fourth Assessment Report (AR4) GWPs. Comparison of AR4 GWPs with IPCC SAR can be found here: https://www.ipcc.ch/publications_and_data/ar4/wg1/en/tssts-2-5.html. IPCC AR4 based GWPs are gradually replacing IPCC SAR based estimates as new emission reporting guidelines take effect. The overall picture of a rapid decline towards zero global GWP GWP weighted emissions does not however change.

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negativeshortlyafter2060inthe2°Cscenariosandaround2050inthe1.5°Cscenarios(Figure5)8.Table3showstheremainingbudgetsuntiltheendofthe21stcentury,forbothoftheclimatepolicyscenariosandasanindicationoftheoverallmitigationefforts,alsoforthebaseline.Thesocalledthresholdavoidancebudgets(Rogeljetal.,2016)inthelasttwocolumnsarethecumulativeCO2orGHGemissionsbetween2015and2100thatlimittheglobaltemperatureincreaseby2100tobelowacertainvaluewithagivenprobability(66%for2°C,50%for1.5°C).Asmentionedabove,keyclimate-systemuncertaintiesareincludedintheseprobabilityestimates,suchasuncertaintiesinclimatesensitivityandcarbon–cyclecharacteristics.Inaddition,therangesdrawnfromtheliteratureareprovidedtoputthescenarioscentralinthisreportincontext,indicatingsomeoftheuncertaintiesrelatedto,forexample,relativecostsoftechnologiesacrossmodels,earlyvslatereductions,CO2vsnon-CO2reductionsetc.

Carbonbudget

Comparedtothebaselinescenario,cumulativeemissionsofover3000GtCO2needtobeavoidedtoholdwarmingbelow2°C.Forthe1.5°Cby2100scenarios,thisnumberrisestonearly3500GtCO2tobeavoided.

Putanotherway,theremainingglobalcarbonbudgetforthe1.5oCovertheperiod2015-2100scenarioislessthan250GtCO2(245GtCO2).Itiscriticaltounderstandthatthisisnetbudgetoverthecenturyasthe1.5°Cby2100scenarioemits775GtCO2from2015-2050andthenwithnegativeemissionstechnologiesandbiologicalcarbonsequestrationtake530GtCO2outoftheatmospherefrom2051-2100.

BoththespeedandtheoveralldegreeofmitigationarelesspronouncedfortotalGHGs.Whilstitisgenerallyunderstoodthattheemissionreductionpossibilitiesfromnon-CO2GHGemissionsarenotasgreatasfromCO2emissionsfromtheenergysystem,itisalsoclearthatthereremainsasubstantialdegreeofuncertaintyabouttheabilitytoreduceemissionsintheseareas.DifferentIAMmodelsassumeverydifferentemissionreductionpossibilities(Gernaatetal.,2015),andthisissueisreflectedbytherangesincludedinthetable.Ifreductionpossibilitiesfornon-CO2GHGsaregreaterthanassumedthencorrespondingCO2budgetsarehigher,andviceversa(Rogelj,Reisinger,etal.,2015).

Anadvantageofillustratingbudgetsusingasinglemodelframeworkistherobustlike-with-likecomparisonbetweenthescenarios,becauseanychangeincertainassumed

8 MESSAGE delivers results in 10-year time-steps after 2010. To compute the year in which CO2 emissions need to become zero, linear interpolation between the individual data points in MESSAGE was used. The “Year of net-zero emissions” then refers to the year in which then absolute value of emissions was minimal.

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modelparameterslike,forinstance,technologyspecificinvestmentcostwillalwaysaffectallscenariosandthereforeshifttheresultsofallscenarios–thisisthesocalledbaseline-effect.

Table3:Yearofgloballyzeroemissions(approximate)andemissionsbudgets,Sources:IPCCAR5,UNEPEmissionsGapReport(2014),IIASA(Rogeljetal2015;2016),owncalculations.

1)YearofzeroemissionsasreportedinUNEPEmissionsGapReport2014basedonre-analysisofIPCCAR5emissionsscenariosthatreach2020globalGHGemissionslevelsconsistentwithINDCanalysis.2)CO2budgetsinIPCCAR5WGIIIarefor2011onwards,henceadjustedherebysubtracting160GtCO2ofpastemissions2011-2015(Rogeljetal2016)–theIPCCAR5scenariodatabaseincludesmanyscenariosthatreach2020globalGHGemissionslevelsconsiderablybelowlevelsconsistentwithNDCsandhencetheCO2budgetsreportedforAR5amounttypicallytolowerlevelsoftotalcumulativeemissionsinthe2016-2050periodthanthescenariosselectedfordetailedanalysisinthisreport.IPCCAR5didnotreportbudgetsfortotalGHGs,norforthe2051-2100period.3)Re-analysisofscenariodatafromUNEPEmissionsGapReport2014basedonRogeljetal(2015).UNEPEGRdidnotreportbudgetsfortotalGHGs

NeedfornegativeCO2emissions

IAMfindingscurrentlypointtotheneedforlarge-scalecarbonuptakeactivitiesandnegativeemissiontechnologiestoachievetheseclimategoals.Majorcontributionsinexistingscenariostocarbonuptake(sequesteringinbiologicalcarbonreservoirs)comefromafforestation&reforestation.NegativeCO2emissionsinthepresentgenerationofIAMmodelsderivefromtechnologiescombiningbioenergywithcarboncaptureandstorage(BECCS)(Gough&Upham,2011)whichusesbiomasstofuelthermalpowerplantsforprovisionofelectricityandthenstorestheCO2fromthecombustion

Yearofzeroemissions Budget[GtCO2]

2016-2050 2051-2100 2016-2100

CO2 GHG CO2 GHG CO2 GHG CO2 GHG

Base 1485 2090 2355 3345 3840 5440

Likelybelow2°C

Scenariothisreport 2062 2087 1050 1580 -300 340 750 1915

RangeofscenariosinIPCCAR51)2)

2055-2070 2080-2100 390-1140 - - - 470-1020 -

Atleast50%below1.5°Cby2100

Scenariothisreport 2050 2075 775 1280 -530 60 245 1340

RangeofscenariosinUNEP

EGR3)2045-2050 2060-2080 680-795 - -655--440 - 45-355 -

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underground,andatthisstageisidentifiedinthescientificliteratureasthemostlikelyavailablenegativeemissionsoptionwithpotentialtogrowtolarge-scaledeploymentatlimitedeconomiccosts.

LikemostotherIAMsofthecurrentmodelgeneration,theMESSAGEmodelincludesacoherentrepresentationoftheland-usesectorandbiomassavailability,approximatelyconsideringinter-relatedlanduseissues,suchasfoodproduction,bioenergy,afforestationandreforestation.InparalleltonegativeemissionsthroughBECCS,thelowemissionscenariosfromMESSAGEalsoincludesequestrationthroughafforestationandreforestation,reachingcumulativesequestrationofaround220-230GtCO2bytheendofthecenturyintheParisAgreement1.5°CandCancunAgreements2°Cscenarios.WhileBECCSwillnotbewithoutitschallenges(Fussetal.,2014)withtechnological,sustainability,social,political,legalandlegislativedimensions,ithastheuniquecapabilityofremovingCO2fromtheatmospherewhileatthesametimesupplyingenergy.

BECCSisacombinationofalreadyknowntechnologies–thermalpowerplantsfiredwithbiofuel,captureofCO2fromthecombustiongasesandthentransportandstorageinsecuregeologicalformations(Metz,Davidson,deConinck,Loos,&Meyer,2005),buthasnotyetbeendeployedonalargescale(Gough&Upham,2011).BECCScanbecategorisedasanegativeemissionstechnologybecauseitextractsCO2fromtheairasanintegralpartofanenergyproductionsystem.Plantstakeupandstorecarbonduringgrowththroughphotosynthesis.Wheneitherforest&agriculturalresidues,ordedicatedbioenergycrops,areharvestedandcombustedinpowerplants,andtheresultingCO2emissionsarecapturedandstoredunderground,thisresultsinanetextractionofCO2fromtheatmosphere,whileprovidingenergyservices.

Anotheroptionfornegativeemissionsisdirectaircapture(DAC)(Broehm,Strefler,&Bauer,2015),whereCO2isextractedfromambientornearambientatmosphericconcentrationsandstoredinageologicalreservoir.TheeconomiccostsofthisareestimatedtobefargreaterthanBECCS(McLaren,2011),whichiswhythisoptionatthisstagedoesnotgenerallyplayaroleinIAMscenarios.DAChastheadvantagethatitislimitedonlybytheamountofenergyandstorageavailable,andnotalsobytheamountoflandthatisrequiredaswithBECCS.

Asisthecasewithdirectaircapture,othermeansoftakingCO2fromtheairandsequesteringinbiologicalcarbonreservoirs,includingreforestation,afforestationandincreasedCO2uptakebysoils,donotsupplytheadditionalbenefitofenergyoutput.Someofthesemay,however,provideotherservices,suchaswaterbasinmanagement,orsoilimprovements.Landuseobjectiveconflictscanalsoarisewhereapriorityoncarbonstorageinforestsmayconflictwithbiodiversity,watermanagement,culturalandlandscapevalues,asisthecasewithbioenergysystems.Asmentionedabove,inmostofthescenariosforlimitingwarmingtobelow2°Cand1.5°Csignificantcarbonsequestrationin

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theterrestrialbiosphereisassumed,andnegativeCO2emissionsfromindustrialprocessessuchasBECCSarerequiredinadditiontothis.

DiscussionandConclusionsRecentscientificdevelopmentsshowthatthereareconsiderablebenefitstolimitingwarmingincreaseto1.5°Ccomparedto2°C.Thedifferenceinglobalmeantemperatureincreasebetween1.5°Cand2°ChassubstantialimplicationsforkeyclimateimpactsandindicatorsforAustralia.Projectionsoffuturecoralbleachingindicatethatthe0.5°CtemperaturedifferencemightbedecisiveforthesurvivalofthecoralreefsworldwideandinparticulartheGreatBarrierReef(Frieleretal.,2012).Recentinsituandmodellingresultsofthethermaltoleranceofreef-buildingcoralsintheGreatBarrierReefhasshownthatthisprotectivemechanismislikelytobelostunderwarmingscenariosexceeding1.5°C,furtheramplifyingthedegradationriskofthesystem(Ainsworthetal.,2016).ThisconcernisfurtherreinforcedbytherecentestimationthatunabatedwarmingislikelytorendertherecentextremetemperatureanomaliesthatprevailedduringMarch2016asnewnormaltemperaturesintheCoralSearegioncoveringthenorthernsectorsoftheGreatBarrierReefbythemid-2030s9.

Asdemonstratedinthisreport,thedifferencebetween1.5°Cand2°Cmarksthedifferencebetweentheupperendofpresentdayclimatevariabilityandanewclimaticregimeinrelationtotemperatureandwaterrelatedextremes.Inacountryalreadyexperiencingtemperaturesexceeding40°Cinitsurbancentres,thisisaveryseriousreasonforconcernforhumanhealthaswellaslabourproductivity(Smithetal.,2014).IncombinationwithasubstantialdryingtrendprojectedinparticularforsouthernAustraliaandinparticulartheSouth-WestLandDivisionofWesternAustralia,thiswillnegativelyaffectagriculturalproductivity(Reisingeretal.,2014)aswellasleadtoincreasedrisksofgrasslandandforestfires(Pitmanetal.,2007).Risksundera1.5°Cwarmingwouldstillbesubstantialbutwouldalleviateseveralofthemostprofoundrisksforcoralreefdegradation,extremetemperaturesanddryingprojectedfor2°C.

Limitingwarmingtobelow1.5°Crequiressimilarenergysystemtransformationstothoseneededtoholdwarmingbelow2°Cbuttheglobaldecarbonisationoftheenergysystemneedstobefasterandmorepronounced.

Thecriticaldevelopmentsinthenext5to15years,iftheworldistogetontoa1.5°pathway(orevena2°Cpathway),isamorerapiddeploymentofrenewableenergyandlow,zeroandnegativeemissionenergysupplytechnologies.Thisinvolvesamorerapid

9 https://theconversation.com/great-barrier-reef-bleaching-would-be-almost-impossible-without-climate-change-58408

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retirementofexistingemission-intensiveenergysystemcomponentsandacorrespondingrapidupscalinginzero-emissionandhighlyefficienttechnologies.

Inthelongerterm(post2030)technologiestoremoveCO2fromtheatmosphereplayaconsiderableroleinexistingemissionscenariosforachievingeither1.5°Cor2°C,andtheneedforthesebyitselfdoesnotappeartobeadifferentiatingelementbetweenthe1.5°Climitandhigherlevelsofwarming.Dependinguponthesuccessindeploymentofrenewableenergy,energyefficiency,electrificationoftransportsystemsandreductionsinnon-CO2GHGemissions,lowscaledeploymentofnegativeemissionstechnologieswouldneedtostartinthelate2020searly2030s.ThescientificliteratureatpresentpointstoBECCSasthemostlikelyavailableoptionwithpotentialtogrowtolarge-scaledeploymentatlimitedeconomiccosts.

Giventheuncertaintiessurroundingthepotentialfornegativeemissionstechnologiestobedeployedatscale,itwouldseemprudentthatpolicyaimstominimisetheneedforthis.Evenwithrapiddeploymentofrenewableenergyandenergyefficiency,withveryrapidemissionreductionsbetweennowand2050thephysicalpresenceofsomuchCO2intheatmosphereduetopasthumanactivitiesmeanssomelevelofnegativeemissionsisvirtuallyunavoidableatthisstageiftheParisAgreementlong-termtemperaturegoalistobemet.Asaconsequence,itisimportantthattheinitialandmosturgentfocusonParisAgreementcompatiblepolicyactionsisontherapiddeploymentofrenewableandotherloworzerocarbonenergysystems,combinedwithrapidimprovementsinenergyefficiencyofindustry,buildingandhousing,andelectrificationoftransportsystems.

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