Download Implications of the 1.5°C limit in the Paris

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. . . . B. i .l l . H. a.r e
. ,. N
. i.k .l a. s . R. o.m. i n
. g. , . M
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. d. r. i c. h. S. c. h.l e. u. s .s n. e. r . .
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. ClimateAnalyticsgGmbH
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. Supportingsciencebasedpolicytopreventdangerous
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. climatechangeenablingsustainabledevelopment
. . . . . . . . . . . . . . . . . . . . . . . Friedrichstraße231/HausB
. . . . . . . . . . . . . . . . . . .T/+49(0)30259229520
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. . . . . . . . . . . . . . . . . . . . . . . . 10969Berlin/Germany
. . . . . . . . . . . . . . . . . . .W/www.climateanalytics.org
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Implicationsofthe1.5°C
limitintheParis
Agreementforclimate
policyand
decarbonisation
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.
Summary
TheParisAgreementcontainsalong-termtemperaturegoal(LTTG)ofholdingthe
increaseintheglobalaveragetemperaturetowellbelow2°Candpursuingeffortstolimit
thisto1.5°Cabovepreindustriallevels.The1.5°ClimitintheParisAgreementgoes
significantlyfurtherthantheprevious2°CgoaladoptedundertheUNFCCC.TheParis
Agreementalsocontainslong-termglobalemissionsgoalstopeakglobalemissionsas
soonaspossibleandthenreduceasrapidlyaspossible,accordingtothebestavailable
science,toreachzeroemissionsgloballyinthesecondhalfofthe21stcentury.
Thelegallybindingarchitectureoftheagreementrequiresthatcountriesmustupdateand
increasetheambitionoftheirnationallydeterminedcontributions(NDCs)onacommon
five-yearcyclefollowingafive-yearlyglobalstocktake.TheParisAgreementobliges
countriestoprogressivelyincreasetheirlevelofambitionandaction,asreflectedintheir
NDCs,soastomeetthelongtermtemperaturegoaloftheagreement(Mace,2016).
ThisreportprovidesanoverviewofthreeimportantissuesarisingfromtheParis
agreement:
• Benefitsofthe1.5°ClimitintheParisAgreementintermsofavoidedglobaland
nationalclimateimpactsandrisks;
• The global emission pathways needed to meet the Paris agreement’s long-term
temperature goal, including the required timing of peak global emissions and of
zeroglobalemissions;
• Theremainingcarbonbudgetconsistentwiththe1.5°ClimitintheParisAgreement.
Theglobalemissionpathwayissuesandcarbonbudgetareparticularlyimportant,asin
ordertofulfilitsclimatecommitmentsundertheParisAgreement,Australia’spresentand
futureactionsonemissionreductionswillneedtofitwithintheselimits.Australia'sinitial
nationallydeterminedcontribution(INDC)isinadequateevenfortheformer2°Cgoal1.At
presenttheoverallgloballyaggregatedeffectofINDCsandcurrentpoliciesputtheworld
ona3°Corcloseto4oCpathwaysrespectively2,muchabovetheprevious2°Climit.The
firstreview,orstocktake,oftheaggregateeffectofallINDCcommitmentsthatcountries
1
2
http://climateactiontracker.org/countries/australia.html≈
http://climateactiontracker.org/andhttp://unfccc.int/resource/docs/2016/cop22/eng/02.pdf
P. 2
haveputforwardinrelationtothe1.5°ClimitintheParisAgreementandthelongterm
globalemissiongoalswilloccurin2018aspartoffacilitativedialogueagreedinParis.The
2018facilitativedialogueprocesswillbeinformedbyaspecificallyrequestedIPCCSpecial
Reporton1.5°Cimpactsandonhowtoclosethegapbetweencurrentpolicies,NDCsand
emissionpathwaysconsistentwiththeParisAgreementin2025and2030.Basedonthis,
by2020atthelatestgovernmentsneedtosubmitupdatedNDCsfor2025and2030that
effectivelyclosethisgap.ThereisthereforeanexpectationthatAustraliaimprovesits
emissionreductionspledgetobeconsistentwiththeParisAgreementanditsenabling
decisions3.Thegovernment's2017reviewofclimatepoliciesanditspromised
considerationoflongtermtargetsprovidesanimportantopportunityforstrengthening
suchcommitments.The2018stocktakewillbeanothercrucialopportunity.
Limitingwarmingto1.5°Cprovideskeybenefitscomparedtoa2°Climit
Atpresentglobalwarmingisabout1°Cabovepreindustriallevels.Theworldisalready
experiencingsubstantialimpactsanddamages,includingforagriculture,human
livelihoodsandnaturalsystemssuchastheGreatBarrierReef.Concernsthatsustained
globalwarmingof2°Cabovepre-industrialwouldleadtoverylargeimpacts,damagesand
risksledmanyvulnerablecountriestoexpressconcernthattheformer2°Climitwastoo
unsafe.TheseandotherconcernsledtheworldtoadopttheParisAgreement’slong-term
temperaturegoal,whichincludesa1.5°Climit,asopposedtotheearlier2°Cgoaladopted
bytheinternationalcommunity.Whyhasthiscomeabout?
Recentscientificliteratureshowsthatthereisasignificantincreaseinimpactsandrisksas
global-meanwarmingmovesfrom1.5°Cto2°Cabovepre-industriallevels.Thisseemingly
smalldifferenceinglobalaveragetemperaturerepresentsalargeadditionofenergyinto
theglobalclimatesystemandturnsouttocarrylargeconsequences.
Iftheworldwarmsby1.5°C,currentlyrareclimaterelatedextremes(extremeheatwaves,
unusualdryspells,extremerainfall,massiveglobalcoralbleachingevents)wouldbecome
thenewnormal.Ifglobalmeanwarmingweretoreach2°C,theclimatesystemwould
moveintounchartedterritory.Forexample,inaworldthatis2°Cwarmerthanpreindustriallevels,thetypicalannuallengthofwarmspellswouldbeupto60dayslonger
thantodayacrossnorthernpartsofAustraliaandaround20dayslongeracrosscentral
andsouthernregions.Thisincreasewouldbereducedbyatleast30%,ifglobalwarmingis
limitedto1.5°C.
3
http://unfccc.int/resource/docs/2015/cop21/eng/10a01.pdf#page=2
P. 3
The1.5°Climitwouldmeansmallerincreasesintemperatureoftheannualhottestdays,
reducedimpactsonwateravailabilityandasmallerincreasesinlengthofdryspells,in
particularacrosssouthern,south-westernandmid-westernpartsofAustralia.
Inlightofthe2014-2016massiveglobalcoralbleachingevent(Eakinetal.,2016)thathas
drasticallyaffectedtheGreatBarrierReef4,itisespeciallycriticaltonotethatlimiting
globalwarmingto1.5°Cofferssomechanceforafractionoftheworld’scoralreefsto
survive,while2°Cwouldprovideverylimitedchance,ifanyatall.
Globalsealevelriseat1.5°Cwarmingwouldreach40cmabovecurrentlevelsby2100,
comparedto50cmat2°C.Howeverthisdoesnottakeintoaccountthelonger-termpost2100commitmenttofurthersealevelrise,nordoesitincludetheriskofcrossinga
thresholdtoirreversiblemeltofpartsoftheAntarcticandGreenlandicesheets,with
multi-metresealevelriseasaconsequence.Thisriskisestimatedtobemuchlowerat
1.5°Cthanat2°C.
Insummarythedifferencebetween1.5°Cand2°Cmarksthedifferencebetweenthe
upperendofpresentdayclimatevariabilityandanewclimaticregimeoftemperatureand
waterrelatedextremes.Inacountryalreadyexperiencingtemperaturesexceeding40°Cin
itsurbancentresthisisaveryseriousreasonforconcernforhumanhealth(Smithetal.,
2014).Alongwithasubstantialdryingtrendprojectedinparticularforsouthernpartof
Australiathiswillnegativelyaffectagriculturalproductivity(Reisingeretal.,2014)aswell
asleadtoincreasedrisksofforestfires(Pitman,Narisma,&McAneney,2007).Risks
undera1.5°Cwarmingwouldstillbesubstantialbutwouldalleviateseveralofthemost
profoundrisksforcoralreefdegradation,extremetemperaturesanddryingprojectedfor
2°C.
Withearlyaction,globalemissionreductionsneededtomeetthe1.5°Climitare
technicallyandeconomicallyfeasible
Limitingwarmingtobelow1.5°Crequiresqualitativelysimilartransformationsinthe
productionanduseofenergytothoseneededtoholdwarmingbelow2°C,howeverthe
decarbonisationoftheenergysystemneedstobefasterandmorepronounced.
For1.5°Cscenarios,zeroCO2emissionsgloballyareneededbyabout2050,andaboutten
yearslaterfor2°Cscenarios,withglobalemissionspeakingnolaterthanabout2020
beforerapidlydeclining.
Fortheworldtogetontoa1.5°Cpathway,thefirstandmosturgentmeasuresincludethe
rapidscalingupofrenewableenergysystems,energyefficiency,electrificationof
transportsystems,andimprovementsofindustrialandbuildingefficiency.Increased
energyefficiencyatalllevelsisneededtoreducethegrowthinprimaryenergydemand,
4
https://www.coralcoe.org.au/media-releases/coral-death-toll-climbs-on-great-barrier-reef#
P. 4
andrenewablesandotherlow-carbonenergysystemsareneededtodecarbonisethe
primaryenergysupplysystemrapidly.
Globally,thedirectmitigationcostsof1.5°Cscenariosareabout1.5-2timeshigherover
thewholeofthe21stcenturythantheformer2°Cgoal,butasnotedabovethebenefits
arevast.
Anyglobaltemperaturelimitisassociatedwithacarbonbudget-theallowedtotalof
globalcumulativeCO2emissionsinordertomeetagivenglobalwarminggoal.Historical
emissionstodateandhighlevelsofatmosphericCO2concentrationsatpresent(above
400ppmCO2)meanthattheremaininggreenhousegas(GHG)andcarbonbudgetsare
verysmall.Limitingwarmingto1.5°C(or2°C)thereforenowrequiresintroducingnegative
CO2emissionsatasmallscaleinthe2030sandscalingupthereafter.Inaddition,
sequestrationofcarboninbiologicalreservoirs,suchasforestsandsoils,willalsobe
needed.Theneedfornegativeemissiontechnologiesbyitselfisnotadifferentiating
elementbetweenthe1.5°Climitandtheprevious2°Climit.
Thissituationmeansthatinthelongerterm(post2030)technologiestoremoveCO2from
theatmosphereplayakeyrole,evenwithveryrapidGHGreductionsinthenext10to15
years.Thescientificliteratureonemissionpathwayspointstobioenergywithcarbon
captureandstorage(BECCS)asthemostviableoptionatpresent(TheClimateInstitute,
2014),withthepotentialforlarge-scaledeploymentatlimitedeconomiccosts.The
literatureandmodelsgenerallyincludeconsiderationsofinterrelatedlanduseissues,
suchasfoodproduction,bioenergy,afforestationandreforestation.WhileBECCSwillnot
bewithoutsignificantchallenges(Fussetal.2014)withtechnological,sustainability,
social,political,andlegaldimensions,thistechnologyhastheuniquecapabilityof
removingCO2fromtheatmosphereandsupplyingenergyatthesametime.
P. 5
Introduction
Oneoftheoutcomesofthe2009climatesummitinCopenhagen(COP15)wasan
agreementonagoaltoholdwarmingbelowa2°Cincreaseabovepre-industrial.However,
atCopenhagenmorethan100vulnerablecountrieswerecallingforlimitingwarmingto
below1.5°C.Recognisingthis,duringthesubsequentclimatesummitinCancun(COP16)in
2010,theUNFCCCestablishedareviewprocesstoevaluatewhetherthelong-termglobal
temperaturegoalofholdingwarmingbelow2°Cwasadequatetoavoiddangerousclimate
change,andwhethergoodprogresswasmadetowardsachievingthelong-termgoal.The
reviewprocessfocusedinparticularonthedifferencesinimpactsbetween1.5°Cand2°C
warmingabovepre-industriallevels.Thisprocessendedin2015withthefinalreportofits
scientificarm(a“StructuredExpertDialogue”)concludingthatawarmingof2°Ccannotbe
consideredsafeandthat1.5°Cisclosertobeingasafe‘guardrail’(UNFCCC,2015).
Thisveryimportantfindingwasreflectedinthelong-termtemperaturegoaloftheParis
Agreementofholdingtheincreaseintheglobalaveragetemperaturewellbelow2°Cand
pursuingeffortstolimitthisincreaseto1.5°Cabovepreindustriallevels.InParisatCOP21
Australiasupportedtheinclusionofthe1.5°ClimitintheParisAgreementlongtermtemperaturegoal.
OnApril22nd2016inNewYork,Australiajoinedmorethan170othernationsinsigning
theParisAgreementtodealwiththeglobalclimatechangeproblemandpledgedtoratify
itin2016.AstheAustraliangovernmentpreparestoratifytheParisAgreement,thus
committingitselftofulfiltheobligationsitentails,Australia’semissionreductionsand
otheractionsneedtobeassessedinlightoftheircompatibilitywiththeParisAgreement
temperaturegoalandotherelementsoftheAgreement.Theanalysisinthispaper
outlinestheglobalemissionenvelopewithinwhichthisassessmentneedstotakeplacein
ordertomeettheobligationsoftheParisAgreement.
Comparisonofclimateimpactsbetween
a1.5°Canda2°Cworld
Alreadywithanobservedwarmingofaround1°C,theimpactsofanthropogenicclimate
changearefeltglobally.Temperaturesandsealevelareontherise(IPCC,2013),extreme
weathereventsareincreasinginintensityandfrequency(IPCC,2012),withlargely
detrimentaleffectsonglobalagriculture(Lesk,Rowhani,&Ramankutty,2016;Porteret
al.,2014)andatmosphericCO2levelsarehighestinmillionsofyears,leadingtogradually
worseningoceanacidification(IPCC,2014).Severeimpactsonmarinelife,including
erosionandmassbleachingoftropicalcoralreefs,aretheconsequenceofalready
P. 6
observedoceanacidificationandincreasingtemperatures(J.-P.Gattusoetal.,2015).
Duringthe2015-2016ElNiñoevent,massbleachinghasaffectedvastpartsoftheGreat
BarrierReef(Normille,2016).Inaddition,thereisgrowingevidencethatpartsoftheWest
Antarcticicesheet,a“tippingelement”oftheEarthSystem(Lentonetal.,2008),may
alreadybeinirreversibleretreat,implyingadditionalsealevelriseofatleastonemeter
overcenturiestocome(Feldmann&Levermann,2015;Joughin,Smith,&Medley,2014;
Rignot,Mouginot,Morlighem,Seroussi,&Scheuchl,2014).
Ocean,cryosphereandglobalsealevelrise
Asexplainedbelow,physicalimpactsonoceansandthecryosphere(Antarcticand
Greenlandicesheets,smallicecapsandmountainglaciers)differconsiderablybetween
1.5°Cand2°Cwarminginrelationtobothgradualimpactsandtotheriskofabruptshifts
ofso-called“tippingelements”(Lentonetal.,2008).TippingelementsintheEarthSystem
arecharacterisedbyinternalself-amplifyingdynamicsthatmayleadtoacompletechange
initsstate.Suchdynamicsaretriggeredaboveacertainglobalmeantemperature
increase,orthetippingpoint.
ArecentstudyanalysingtippingelementsintheEarthSystem,includingoceancirculation
andseaicepatternsinclimatemodels,foundthat2°Cwarmingwouldalreadycrossabout
50%ofalltippingpointsidentifiedinthesemodelsforanylevelofwarming(Drijfhoutet
al.,2015).Thisnumberisreducedtoabout20%ifwarmingislimitedto1.5°C.Besides
thoseassociatedwithsealevelrise,othertippingelementsassessedincludelarge-scale
forestdiebackoftheAmazonandboreal(highlatitude)forests,permafrostcollapse,
vegetationshiftsinAfrica’sEasternSahelregion,anddisappearanceofArcticseaice.
Mostrelevantforthissectionarepotentialtippingpointsforlargescaleicesheet
disintegrationoftheAntarcticandGreenlandicesheets.TheWestAntarctic(Feldmann&
Levermann,2015)aswellaspartsoftheEastAntarcticicesheet(Mengel&Levermann,
2014)areatsubstantialriskofdisintegrationduetoocean-icesheetinteractions.This
couldresultinadditionalsealevelriseof6-8metersovertimescalesofseveralcenturies
tomillennia(Deconto&Pollard,2016).AlthoughoceanicwarmingaroundAntarcticais
projectedtoincreasewithincreasingglobalwarming(Hellmer,Kauker,Timmermann,
Determann,&Rae,2012),ourcurrentunderstandingofthesechangesdoesnotallowfor
aquantificationofthepreciselevelsofglobalmeantemperatureincreasetowhichthe
potentialtippingpointscanbelinked.
TheGreenlandicesheetismoredirectlyvulnerabletoincreasesinatmospheric
temperaturesandresearchershaveidentified1.6°Cwarmingasthebestestimatefora
criticalthresholdforatippingoftheGreenlandicesheet(Robinson,Calov,&Ganopolski,
2012).AdisintegrationoftheGreenlandicesheetcouldleadtoupto7msealevelrise
P. 7
overthousandsofyears.Assessmentsofpastsealevelevidencefromearthhistoryand
state-of-the-artmodellingresultsindicateanaveragemulti-millennialaveragesealevel
riseofabout2.3mper°Cofwarming(Levermannetal.,2013).
Projectionsfor21stcenturysealevelrisearedisplayedinFigure1(Schleussneretal.,
2016).Thebestestimatefor21stcenturysealevelriseundera2°Cscenarioisabout50
cm,whichis10cmlessundera1.5°Cscenario.Possiblyevenmoreimportantarethe
ratesofsealevelrisein2100,astheywilllargelyinfluenceapost-2100commitmentto
long-termsealevelrise.Onlyundera1.5°Cscenarioaretheseratesindeclineby2100,
reachingpresentdaylevelsbythattime.Note,however,thattheseassessmentsdonot
accountyetfortheeffectofpotentialrapidicesheetdisintegrationdiscussedabove,nor
dotheyidentifyvulnerabilitiesrelatedtodifferentratesofriseregionallyorincreasesin
Figure1:Probabilisticprojectionsofglobalsealevelriserelativeto1986-2005levels(lowerpanels)for
a1.5°Ctemperaturescenario(left)anda2°Cscenario(rightpanels).Thicklinesindicatemedian
estimateswhereasdarkcolouredrangesindicatethe66%likelihoodrangeandlightcolouredranges
the90%likelihoodrange.Medianestimatesfor2100sealevelriseunder1.5°Cisabout10cmlessthan
the50cmprojectedfor2°C.FromSchleussneretal.(2016).
sealevelvariabilitythatmaybeinfluencedbyglobalwarming(Widlansky,Timmermann,
&Cai,2015).
Tropicalcoralreefs
Tropicalcoralreefsareparticularlyvulnerabletoclimatechange.Theyarethreatenedby
oceanacidification(Pandolfietal.,2003)andintensecoralbleachingasaresultofoceanic
warming(Meissner,Lippmann,&SenGupta,2012).Figure2displaysthefractionofglobal
tropicalcoralreefsprojectedtobeatriskoflong-termdegradation,understoodasan
eventuallossofthereefecosystem,duetoseverebleachingeventsoccurringatleast
P. 8
everyfiveyearsfor1.5°Cand2°C.Unlessextremelyoptimisticscenariosofcoralreef
adaptationareassumed,virtuallyalltropicalcoralreefswillbeatsevereriskof
degradationunder2°Cwarming.Thewarmingdifferencebetween1.5°Cand2°Cislikely
tobedecisiveforthefuturesurvivaloftropicalcoralreefsandonlythe1.5°Cscenario
offerssomepotentialfortheseecosystemstoadapt.Thealreadyobservedcoralbleaching
illustratesthemagnitudeandscaleofthisriskfortheGreatBarrierReef,aWorldHeritage
Figure2:Probabilisticprojectionsoftheshareofglobaloceangridcellswithtropical
coralreefsatriskoflong-termdegradationundera1.5°Cscenario(top)anda2°C
scenario(bottom,forthetemperaturetrajectories,see(Schleussneretal.,2016).This
isassessedoveralloceangridcellscurrentlycontainingcoralreefs.Thefigure
displaystheshareforassumptionsaboutfutureevolutionoftemperatureresilience
ofcoralreefs.Whereasthe“Constant”-caseassumesobservedlevelsofcoral
susceptibilitytoincreasedwatertemperatures,the“ThermalAdaptation”case
assumesunprecedentedandrapidadaptationoftheseecosystems.Asthisappearsto
beveryoptimisticgiventhedetrimentaleffectsofoceanacidificationtotropicalcoral
reefcalcificationrates(therebyweakeningthesesystemsconsiderably),these
projectionsrepresenttheabsolutelowerboundoffuturetropicalcoralreefrisk.From
Schleussneretal.(2016).
site,thathasalreadylostmorethan50%ofitscoralcoversince1985(Gattuso,HoeghGuldberg,&Pörtner,2014).Thislosswillhavewidespreadanddetrimentalconsequences
forlivelihoodsofcommunitiesdependingonit.Coralreefsprovidecoastalprotection,
accountformorethan10%offishcaughtintropicalcountries(20%ofdeveloping
countries)andtheGreatBarrierReefitselfgeneratesaboutA$5.4billionintourism
revenueannuallytotheAustralianeconomy(Gattusoetal.,2014).
P. 9
Impactsof1.5°Cand2°CwarmingonAustralia
Australiaisexposedtoclimatechangeimpacts,includingsealevelriseandcoralreefloss,
butalsotoextremeweathereventsanddryingtrends(Reisingeretal.,2014).Theanalysis
belowbuildsonarecentglobalstudythatinvestigateddifferencesbetween1.5°Cand2°C
warmingforarangeofclimateindicators(Schleussneretal.,2016).
ExtremeWeatherEvents
Inrecentyearstheworldhasexperiencedaprofoundincreaseinfrequencyandintensity
ofextremeweathereventsduetoanthropogenicclimatechange(IPCC,2012).Described
belowaretheresultsforfourdifferentextremeweathereventindicators:extreme
temperature,warmspells,dryspells(meteorologicaldroughts)andextremeprecipitation
(Zhangetal.,2011):
• Intensityofhotextremes(TXx):Howmuchhotterwouldthehottestdayinayearbecome?
Theindicatorusedhereistheannualmaximumvalueofdailymaximumtemperature.Thisisa
goodindicatorfortheincreaseinextremehightemperatures.
• Warmspelldurationindicator(WSDI):Howmuchlongerwouldacurrentlytypicalseriesofhot
daysbecome?Theindicatorhereistheannualcountofthelongestconsecutiveperiodinwhich
thedailymaximumtemperatureforeachdayexceedsthe90%quantileforthisdayoverthe
referenceperiod.Theminimumlengthissixconsecutivedays.Thisisagoodindicatorforheatwavesoccurrence.
• Dryspelllengthorconsecutivedrydays(CDD):Howmuchlongerwoulddryperiodsbecome?
Theindicatorhereisthemaximumnumberofconsecutivedaysforwhichtheprecipitationis
below1mmperdayinayear.Thisisagoodindicatorformeteorologicaldrought.
• Heavyprecipitationintensity(RX5day):Howmuchheavierwouldheavyrainfallperiods
become?Theindicatorhereistheannualmaximumofrainfalloveraconsecutive5-dayperiod.
Thisisagoodindicatorforfloodingrisk.
Theanalysishasbeenperformedonensemblesofstate-of-the-artclimatemodelsfrom
thefifthClimateModelIntercomparisonProject(CMIP5,11modelsfortemperature
extremes,14modelsforprecipitationextremes;formoreinformationaboutthe
methodologysee(Schleussneretal.,2016)).Themodelmedianresultsforeachofthese
indicatorsaredisplayedinFigure3for1.5°Cand2°Cwarming,aswellasthedifference
betweenthetwowarminglevels.
Awarmingof2°Cwouldimplyasubstantialincreaseintemperaturerelatedextremes:
annualextremetemperatureswouldexceed3°Cabovevaluestypicallyexperiencedduring
therecentpast(1986-2005)andtheannualmeanlengthofwarmspells(alsorelativeto
the1986-2005period)wouldlastaround20daysforcentralandsouthernpartof
Australiaandupto60daysinthenorthernpartofAustralia.
Under1.5°Cwarming,thisincreaseinintensityandlengthofextremetemperatureevents
islesspronouncedcomparedto2°C.Increaseintheintensityofannualhotextremes(TXx)
P. 10
wouldbelimitedtoabout2°Cabovethe1986-2005referenceperiodandwarmspells
wouldbeupto15daysforthecentralandsouthernpartsofAustraliaanduptoabout
30-40daysforthenorthernpartofAustralia.
Figure3:Projectionsforchangesinextremeweathereventindicatorsrelativetothe
1986-2005referenceperiod.Greymarkedareasindicateregions,wherelessthan66%of
themodelsintheensembleinvestigatedagreeonthesignofchange.
NotethattheextremeincreasesindicatedinthenorthernpartofAustraliaandcoastal
grid-cellsarisefromthefactthatmostoftheunderlyinggridcellscontaindominantly
oceanandnotland.Thiscouldbeovercomebydownscalingglobalclimatemodelresults
fortheseregionstoachievehigherresolution,e.g.inadynamicaldownscalingapproach
(AustralianBureauofMeteorologyandCommonwealthScientificandIndustrialResearch
Organisation(CSIRO),2011).Asnaturaltemperaturevariabilityovertheoceansismuch
smallerthanoverland,theresultingincreaseinwarmspellduration,anextremeevent
indexdevelopedforland,isparticularlypronouncedandshouldbeinterpretedwith
caution.
P. 11
Projectionsforprecipitation-relatedindicesindicatesubstantialuncertaintyinfuture
changesofprecipitation-relatedextremes,forbothextremewetanddryevents(grey
areasinFigure3).However,significantdifferencesexistinparticularrelatedtoextreme
precipitationeventsinthenorthernpartofAustraliathatareprojectedtointensifyby710%comparedtolessthan5%undera1.5°Cwarming.Inspiteofsuchheavierrainfall
events,arobustincreaseindryingisprojectedforlargepartsofAustraliaundera2°C
warming.Thecurrentlylongestannualdryspellwouldbecomeupto2weekslonger
(compareFigure3,bottompanel).Thedifferencebetween1.5°Cand2°Cinthelengthof
dryspellsisparticularlypronouncedinthesouthern,south-westernandmid-western
partsofAustralia.Anoverviewofthiscomparisonofchangesinextremeeventsat1.5°C
and2°Caregivenin
Table1.
Wateravailability
Inadditiontochangesinextremeweatherevents,Australiaisparticularlypronetolongtermdryingtrends.Here,weanalysechangesinannualwateravailability(totalrunoff,
Qtot)basedontheclimateimpactmodelintercomparisonprojectISI-MIP(Scheweetal.,
2014).Theprojectionspresentedarebasedon5bias-correctedclimateprojectionswith
state-of-the-artclimatemodelsand11hydrologicalmodelscomprising,intotal,amodel
ensembleof55models.
Undera1.5°Cscenario,wefindareductioninannualwateravailabilityofabout10%for
mostpartsofAustralia,withreductioninannualwateravailabilityinwesternpartsof
Australiaexceeding15%(Figure4).Under2°C,wateravailabilityisprojectedtocontinue
todecreaseacrossthesouthernhalfofthecontinentandismostpronouncedinthe
south-easternandsouth-westernandPilbararegions.Reductionsexceed20%forwestern
partsofAustralia.Inthenorthernthirdofthecontinentourresultsindicateareversalin
theprojectedtrendtowardsaslightwetting,probablyrelatedtomonsoonintensification.
Itis,however,importanttounderscorethattheseprojectionsareuncertainandeven
1.5°C
2°C - 1.5°C
Qtot [%]
2°C
Figure4:Projectionsforchangesinannualwateravailability(Qtot)relativetothe1986-2005
referenceperiod.Gridcellswherelessthan66%ofallGCM-GHMpairsagreewiththemediansignof
changearemaskedgrey.Gridcellswithanannualmeanrunoffoflessthan0.05mm/dayaremasked
white.
P. 12
reductionsexceeding40%(30%)arestillwithinthe‘likely’(66%probability)rangeof
projectionsfora2°C(1.5°C)warming.
Table1:Changesintemperatureandprecipitationrelatedextremeeventindicesprojectedat1.5°Cand2°C
warmingfromSchleussneretal.(2016).Themedianchangeexperiencedby50%oftheland-areaisgivenfor
thenorthernpartofAustralia(Northof30°S)andthesouthernpartAustralia(Southof30°Candincluding
TasmaniaandNewZealand).Thelikelyrangeoverthemodelensemble(66%likelihood)isindicatedin
squarebrackets.ChangesinWSDIareexpressedindaysandRX5day,CDD,aswellasQtotin%change.All
changesareassessedrelativetothe1986-2005referenceperiod.
Indexofchange
Increaseinwarmspell
duration–[WSDI,days]
Increaseinheavy
precipitationintensity
[RX5Day,%]
Increaseindryspell
lengthorconsecutivedry
days[CDD,%]
Changeinannualwater
availability[Qtot,%]
NorthernpartofAustralia
SouthernpartofAustralia
1.5°C
2°C
1.5°C
2°C
36.5
52.3
12.7
19.8
[27.7,44.4]
[40.0,62.0]
[8.3,15.4]
[13.6,25.9]
4.3
5.1
2.0
2.7
[0.4,7.4]
[1.2,9.8]
[-2.0,5.1]
[-1.2,6.7]
6.7
8.2
3.5
5.9
[-1.2,11.4]
[1.2,15.3]
[0.4,8.2]
[1.2,12.9]
-10.4
-6.8
-7.2
-12.7
[-36.2,-2.5]
[-40.5,7.6]
[-33.9,-1.4]
[-44.4,6.8]
P. 13
Comparisonofglobalmitigation
pathwaysbetweena1.5°Canda2°C
world
IntegratedAssessmentModelsofClimateChange(IAMs)provideinformationonthe
complexinterrelationbetweentheeconomy,energyuseandtheglobalclimate.Someof
themostimportantinsightsIAMsofferatasystemlevelrelatetotheenergysystem
transformationnecessaryforachievingaspecificclimatetarget–suchaslimitingglobal
meantemperatureincreaseto1.5°Corholdwarmingbelow2°Cabovepreindustrial
levels.Energysystemtransformationmeans,inessence,ashiftawayfromtherelianceon
fossilfuels,towardsrenewablesontheenergysupplyside,combinedwithenergy
efficiencyimprovementsonthedemandside.IAMsprovideestimatesofthedirectcostof
climatechangemitigation,comparedtoabaselinescenariowithoutclimatepolicy.Much
researchshowsthatthesedirectmitigationcostsareexpectedtobepartially,orfully
balancedbythebenefitsofmitigation,inparticulareconomicco-benefitsofmitigation
(e.g.reducedairpollutioneffectsonhumanhealthandagricultureduetoreduceduseof
fossilfuels)andavoidedclimatechange.IAMsingeneraldonotaccountforco-benefits
ordamagesfromclimatechange.
Theenergysystemtransformationsnecessarytolimitwarmingtolessthaneither1.5°Cor
2°Careverysimilar,butdeploymentoflow,zeroandnegativecarbonenergysupply
optionsneedstohappenfasterinthe1.5°Climitcasesothatemissionreductioncanbe
realisedearlier(Rogelj,Luderer,etal.,2015;Schaefferetal.,n.d.).Thisisassociatedwith
1.5-2timeshighercostsoverthecenturyasawhole,withcostshigherinthenextfew
decades,ratherthanlaterinthecentury(Rogelj,Luderer,etal.2015).
Baseline,1.5°Cand2°Cscenarios
ThissectionassessesglobaltotalCO2andGHGemissionsforthreescenariosfromthe
MESSAGEmodel5(base,1.5°Cand2°C)andcomparestheseforGHGswithhistoricaldata
andtwoprojectionsoftheeffectsofcurrentpolicies(CAThighandCATlow)fromthe
ClimateActionTracker(CAT).TheCAThighandCATlowscenariosdescribetherangeof
emissionpathwaysthatcanbeexpectedtoresultfromacontinuationofcurrentclimate
policies,inabottom-upglobalaggregationofcountry-levelassessments.Itisimportant
tonotethatanassessmentoftheeffectsreductionspledgesandINDCs(Intended
5
http://www.iiasa.ac.at/web/home/research/researchPrograms/Energy/MESSAGE.en.html
P. 14
NationallyDeterminedContributions)thatcountriesputforwardin2015duringtheParis
Agreementnegotiationsresultinmostcasesinloweremissionsthancurrentpolicies.
IntheMESSAGEbaselinescenario,noglobaltemperaturegoalisassumed.
Tocharacterisepoliciesconsistentwiththe2°CgoaladoptedintheCopenhagenAccords
in2009wedrawfromexistingemissionspathwaysthatholdtheincreaseinglobal
averagetemperaturebelow2°Cwithatleastalikelyprobability(>66%).Atpresentmost
emissionpathwaysthatcomeclosetothe1.5°Climit,exceedanincreaseof1.5°Cabove
pre-industriallevelsduringthe21stcentury,beforedroppingtothe1.5°Climitby2100.In
thisstudywehaveusedoneofthelowestscenariosavailablethatlimitwarmingto1.5°C
by2100withaprobabilityofmorethan50%(andholdswarmingbelow2oCwithaboutan
85%probability).Thisshouldnotbeseenasaninterpretationofthe1.5°Climitinthe
ParisAgreement.Anewgenerationofscenariosarebeingdevelopedwhichmaynot
exceed1.5°C,howeverthesewerenotyetavailableforthisreport.
Probabilitiesofstayingbelowthegivenwarmingthresholdarecomputedusingthe
reducedcomplexityglobalclimate-carboncyclemodelMAGICC(Meinshausen,Raper,&
Wigley,2011),inthesamemethodologyaswasappliedforIPCC’sFifthAssessment
Report,andaccountsforuncertaintiesinimportantclimate-relatedparameters–e.g.
climatesensitivityandcarbon–cyclecharacteristics–byreturningprobabilistic
temperatureresponses.
TheMESSAGEscenariosweretakenfromanensembleofscenariosusedinpeer-reviewed
publications(Rogelj,McCollum,Reisinger,Meinshausen,&Riahi,2013;Rogelj,Mccollum,
&Riahi,2013)andselectedfortheirinter-scenariocomparability:
•
Scenarios share the same assumptions regarding substantial improvements in energy
efficiency – energy demand is comparatively low already in the baseline scenario. In the
short to medium term, GHG emissions are well in line with the current policy CAT low
scenario,whichgivesthelowerestimateofglobalemissionsforaworldinwhichcurrently
(2015)implementedclimatepolicieswherecontinued.Thiswouldleadtoabout3.3°Cof
globalwarminguntil21006andinthelongtermtendtobeconsiderablyhigherthanthis.
Globalwarmingresultingfromsuchabaselinecanthereforebeconsideredtobewellabove
3.3°Cin2100.
Climatepolicyfullycompatiblewiththerespectivetemperaturegoalsonlybeginsfrom2020
inthesescenarios.Thisisinlinewiththecurrentstateofaffairsregardingclimatepolicy.
Acceleratedpoliciesleadingtohighermitigationpre-2020remainfeasibleandwouldresult
in slightly lower rates of reduction post 2020 required to still achieve the long-term
temperaturegoal.
•
TheMESSAGEbaselinescenario,whichassumestheabsenceofglobalclimatepolicyto
limitwarmingtoanycertainlevel,showsadeclineinCO2emissionstowardstheendof
6
http://climateactiontracker.org/global.html
P. 15
thecentury.Thisismainlyduetoassumedconstraintsinfossilfuelavailabilityat
prices/costthatcancompetewithprojecteddecreasingpricesforrenewableenergy
supply,howeverbythattimewarminghasreachedaround3.5°C.Table2showsrelative
changesinglobalCO2andGHGemissionslevelsin2030and2050,relativeto2005levels,
andpercapitaemissionslevels.ThisemphasisesagaintheneedforglobalzeroCO2
emissionsandmajorreductionsinGHGemissionsbyaround2050inordertolimit
warmingto1.5°C.Italsoshowsthevastdiscrepancybetweenthegoalslaidoutinthe
ParisAgreementandwhatcountriesaroundtheworldarecurrentlydoing–thelattercan
beseenforGHGemissionsintheCAThighandlowscenarios.Also,theMESSAGEbaseline
scenarioassumesamoreoptimisticdevelopmentcomparedtotheCAThighscenarioand
isrelativelyclosetotheCATlowscenario.
Figure5:GlobalCO2andGHGemissionsforbaseline,1.5°Cand2°CscenariosfromtheMESSAGEIAM,Global
currentpolicyGHGpathways,Source:IIASA/JoeriRogelj,CAT
P. 16
Table2:Globalchangesandglobal-meanpercapitavaluesofCO2andGHGemissions
2030
Scenario
Changein
CO2
emissions
(on2005
levels)
Changein
GHG
emissions
(on2005
levels)
[%]
[%]
2050
Percapita
CO2
emissions
Percapita
GHG
emissions
[tCO2/yr]
[tCO2e/yr]
Changein
CO2
emissions
(on2005
levels)
Changein
GHG
emissions
(on2005
levels)
[%]
[%]
Percapita
CO2
emissions
Percapita
GHG
emissions
[tCO2/yr]
[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
Current
policies-
CAT
1)2)
High -
40.3
-
7.2
-
76.0
-
7.9
1)
Current
policies-
CAT
1)2)
Low -
33.0
-
6.8
-
48.2
-
6.7
1)
Baseline
16.0
20.8
4.8
6.8
31
37.8
4.9
7.0
1)
TheCATglobalassessment(http://climateactiontracker.org/global.html)doesnotspecifyglobalpopulationdata.Therefore,the
MediumscenariofromtheUnitedNationsWorldPopulationprospects2015(https://esa.un.org/unpd/wpp/)wasusedforglobal
populationnumbersin2030and2050.Giventhelong-termemissionpathwaysofCATarederivedfromIPCCFifthAssessmentReport
scenarios,whicharealsooftenassociatedwithUNpopulationprojections,thisisareasonableapproach,butitmustbenotedthat
populationprojectionsarehighlyuncertainandalternativeassumptionswouldgivedifferentper-capitaemissionvalues.
2)
CATglobalassessmentconsidersonlytotalGHGs.Therefore,noCO2couldbecomputed.
GlobalemissionreductionsfortheParisAgreement
Withclimatepolicyinplace,totalGHGemissions(thesumofCO2emissionsandglobal
warmingpotential(GWP)7weightednon-CO2GHGemissions)needtodeclinerapidlyto
reachgloballyaggregatedzeroemissionsandthenbecomenegativeinthelatterpartof
the21stcentury.NegativetotalGHGsemissionsresultfromnegativeCO2emissions
outweighingtheremainingnon-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.
P. 17
negativeshortlyafter2060inthe2°Cscenariosandaround2050inthe1.5°Cscenarios
(Figure5)8.
Table3showstheremainingbudgetsuntiltheendofthe21stcentury,forbothofthe
climatepolicyscenariosandasanindicationoftheoverallmitigationefforts,alsoforthe
baseline.Thesocalledthresholdavoidancebudgets(Rogeljetal.,2016)inthelasttwo
columnsarethecumulativeCO2orGHGemissionsbetween2015and2100thatlimitthe
globaltemperatureincreaseby2100tobelowacertainvaluewithagivenprobability
(66%for2°C,50%for1.5°C).Asmentionedabove,keyclimate-systemuncertaintiesare
includedintheseprobabilityestimates,suchasuncertaintiesinclimatesensitivityand
carbon–cyclecharacteristics.Inaddition,therangesdrawnfromtheliteratureare
providedtoputthescenarioscentralinthisreportincontext,indicatingsomeofthe
uncertaintiesrelatedto,forexample,relativecostsoftechnologiesacrossmodels,earlyvs
latereductions,CO2vsnon-CO2reductionsetc.
Carbonbudget
Comparedtothebaselinescenario,cumulativeemissionsofover3000GtCO2needtobe
avoidedtoholdwarmingbelow2°C.Forthe1.5°Cby2100scenarios,thisnumberrisesto
nearly3500GtCO2tobeavoided.
Putanotherway,theremainingglobalcarbonbudgetforthe1.5oCovertheperiod2015-
2100scenarioislessthan250GtCO2(245GtCO2).Itiscriticaltounderstandthatthisis
netbudgetoverthecenturyasthe1.5°Cby2100scenarioemits775GtCO2from20152050andthenwithnegativeemissionstechnologiesandbiologicalcarbonsequestration
take530GtCO2outoftheatmospherefrom2051-2100.
BoththespeedandtheoveralldegreeofmitigationarelesspronouncedfortotalGHGs.
Whilstitisgenerallyunderstoodthattheemissionreductionpossibilitiesfromnon-CO2
GHGemissionsarenotasgreatasfromCO2emissionsfromtheenergysystem,itisalso
clearthatthereremainsasubstantialdegreeofuncertaintyabouttheabilitytoreduce
emissionsintheseareas.DifferentIAMmodelsassumeverydifferentemissionreduction
possibilities(Gernaatetal.,2015),andthisissueisreflectedbytherangesincludedinthe
table.Ifreductionpossibilitiesfornon-CO2GHGsaregreaterthanassumedthen
correspondingCO2budgetsarehigher,andviceversa(Rogelj,Reisinger,etal.,2015).
Anadvantageofillustratingbudgetsusingasinglemodelframeworkistherobustlikewith-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.
P. 18
modelparameterslike,forinstance,technologyspecificinvestmentcostwillalwaysaffect
allscenariosandthereforeshifttheresultsofallscenarios–thisisthesocalledbaselineeffect.
Table3:Yearofgloballyzeroemissions(approximate)andemissionsbudgets,Sources:IPCCAR5,UNEPEmissionsGap
Report(2014),IIASA(Rogeljetal2015;2016),owncalculations.
CO2
GHG
CO2
GHG
CO2
GHG
CO2
GHG
Base
1485
2090
2355
3345
3840
5440
Likely
below2°C
Scenariothis
report
2062
2087
1050
1580
-300
340
750
1915
Rangeof
scenariosin
1)2)
IPCCAR5 2055-2070
2080-2100
390-1140
-
-
-
470-1020
-
Scenariothis
report
2050
2075
775
1280
-530
60
245
1340
2045-2050
2060-2080
680-795
-
-655--440
-
45-355
-
Atleast
50%
below
1.5°Cby
2100
Rangeof
scenariosinUNEP
3)
EGR Yearofzeroemissions
Budget[GtCO2]
2016-2050
2051-2100
2016-2100
1)
YearofzeroemissionsasreportedinUNEPEmissionsGapReport2014basedonre-analysisofIPCCAR5emissions
scenariosthatreach2020globalGHGemissionslevelsconsistentwithINDCanalysis.
2)
CO2budgetsinIPCCAR5WGIIIarefor2011onwards,henceadjustedherebysubtracting160GtCO2ofpastemissions
2011-2015(Rogeljetal2016)–theIPCCAR5scenariodatabaseincludesmanyscenariosthatreach2020globalGHG
emissionslevelsconsiderablybelowlevelsconsistentwithNDCsandhencetheCO2budgetsreportedforAR5amount
typicallytolowerlevelsoftotalcumulativeemissionsinthe2016-2050periodthanthescenariosselectedfordetailed
analysisinthisreport.IPCCAR5didnotreportbudgetsfortotalGHGs,norforthe2051-2100period.
3)
Re-analysisofscenariodatafromUNEPEmissionsGapReport2014basedonRogeljetal(2015).UNEPEGRdidnot
reportbudgetsfortotalGHGs
NeedfornegativeCO2emissions
IAMfindingscurrentlypointtotheneedforlarge-scalecarbonuptakeactivitiesand
negativeemissiontechnologiestoachievetheseclimategoals.Majorcontributionsin
existingscenariostocarbonuptake(sequesteringinbiologicalcarbonreservoirs)come
fromafforestation&reforestation.NegativeCO2emissionsinthepresentgenerationof
IAMmodelsderivefromtechnologiescombiningbioenergywithcarboncaptureand
storage(BECCS)(Gough&Upham,2011)whichusesbiomasstofuelthermalpower
plantsforprovisionofelectricityandthenstorestheCO2fromthecombustion
P. 19
underground,andatthisstageisidentifiedinthescientificliteratureasthemostlikely
availablenegativeemissionsoptionwithpotentialtogrowtolarge-scaledeploymentat
limitedeconomiccosts.
LikemostotherIAMsofthecurrentmodelgeneration,theMESSAGEmodelincludesa
coherentrepresentationoftheland-usesectorandbiomassavailability,approximately
consideringinter-relatedlanduseissues,suchasfoodproduction,bioenergy,
afforestationandreforestation.InparalleltonegativeemissionsthroughBECCS,thelow
emissionscenariosfromMESSAGEalsoincludesequestrationthroughafforestationand
reforestation,reachingcumulativesequestrationofaround220-230GtCO2bytheendof
thecenturyintheParisAgreement1.5°CandCancunAgreements2°Cscenarios.While
BECCSwillnotbewithoutitschallenges(Fussetal.,2014)withtechnological,
sustainability,social,political,legalandlegislativedimensions,ithastheuniquecapability
ofremovingCO2fromtheatmospherewhileatthesametimesupplyingenergy.
BECCSisacombinationofalreadyknowntechnologies–thermalpowerplantsfiredwith
biofuel,captureofCO2fromthecombustiongasesandthentransportandstoragein
securegeologicalformations(Metz,Davidson,deConinck,Loos,&Meyer,2005),buthas
notyetbeendeployedonalargescale(Gough&Upham,2011).BECCScanbe
categorisedasanegativeemissionstechnologybecauseitextractsCO2fromtheairasan
integralpartofanenergyproductionsystem.Plantstakeupandstorecarbonduring
growththroughphotosynthesis.Wheneitherforest&agriculturalresidues,ordedicated
bioenergycrops,areharvestedandcombustedinpowerplants,andtheresultingCO2
emissionsarecapturedandstoredunderground,thisresultsinanetextractionofCO2
fromtheatmosphere,whileprovidingenergyservices.
Anotheroptionfornegativeemissionsisdirectaircapture(DAC)(Broehm,Strefler,&
Bauer,2015),whereCO2isextractedfromambientornearambientatmospheric
concentrationsandstoredinageologicalreservoir.Theeconomiccostsofthisare
estimatedtobefargreaterthanBECCS(McLaren,2011),whichiswhythisoptionatthis
stagedoesnotgenerallyplayaroleinIAMscenarios.DAChastheadvantagethatitis
limitedonlybytheamountofenergyandstorageavailable,andnotalsobytheamountof
landthatisrequiredaswithBECCS.
Asisthecasewithdirectaircapture,othermeansoftakingCO2fromtheairand
sequesteringinbiologicalcarbonreservoirs,includingreforestation,afforestationand
increasedCO2uptakebysoils,donotsupplytheadditionalbenefitofenergyoutput.Some
ofthesemay,however,provideotherservices,suchaswaterbasinmanagement,orsoil
improvements.Landuseobjectiveconflictscanalsoarisewhereapriorityoncarbon
storageinforestsmayconflictwithbiodiversity,watermanagement,culturalandland
scapevalues,asisthecasewithbioenergysystems.Asmentionedabove,inmostofthe
scenariosforlimitingwarmingtobelow2°Cand1.5°Csignificantcarbonsequestrationin
P. 20
theterrestrialbiosphereisassumed,andnegativeCO2emissionsfromindustrialprocesses
suchasBECCSarerequiredinadditiontothis.
DiscussionandConclusions
Recentscientificdevelopmentsshowthatthereareconsiderablebenefitstolimiting
warmingincreaseto1.5°Ccomparedto2°C.Thedifferenceinglobalmeantemperature
increasebetween1.5°Cand2°Chassubstantialimplicationsforkeyclimateimpactsand
indicatorsforAustralia.Projectionsoffuturecoralbleachingindicatethatthe0.5°C
temperaturedifferencemightbedecisiveforthesurvivalofthecoralreefsworldwideand
inparticulartheGreatBarrierReef(Frieleretal.,2012).Recentinsituandmodelling
resultsofthethermaltoleranceofreef-buildingcoralsintheGreatBarrierReefhasshown
thatthisprotectivemechanismislikelytobelostunderwarmingscenariosexceeding
1.5°C,furtheramplifyingthedegradationriskofthesystem(Ainsworthetal.,2016).This
concernisfurtherreinforcedbytherecentestimationthatunabatedwarmingislikelyto
rendertherecentextremetemperatureanomaliesthatprevailedduringMarch2016as
newnormaltemperaturesintheCoralSearegioncoveringthenorthernsectorsofthe
GreatBarrierReefbythemid-2030s9.
Asdemonstratedinthisreport,thedifferencebetween1.5°Cand2°Cmarksthe
differencebetweentheupperendofpresentdayclimatevariabilityandanewclimatic
regimeinrelationtotemperatureandwaterrelatedextremes.Inacountryalready
experiencingtemperaturesexceeding40°Cinitsurbancentres,thisisaveryserious
reasonforconcernforhumanhealthaswellaslabourproductivity(Smithetal.,2014).In
combinationwithasubstantialdryingtrendprojectedinparticularforsouthernAustralia
andinparticulartheSouth-WestLandDivisionofWesternAustralia,thiswillnegatively
affectagriculturalproductivity(Reisingeretal.,2014)aswellasleadtoincreasedrisksof
grasslandandforestfires(Pitmanetal.,2007).Risksundera1.5°Cwarmingwouldstillbe
substantialbutwouldalleviateseveralofthemostprofoundrisksforcoralreef
degradation,extremetemperaturesanddryingprojectedfor2°C.
Limitingwarmingtobelow1.5°Crequiressimilarenergysystemtransformationstothose
neededtoholdwarmingbelow2°Cbuttheglobaldecarbonisationoftheenergysystem
needstobefasterandmorepronounced.
Thecriticaldevelopmentsinthenext5to15years,iftheworldistogetontoa1.5°
pathway(orevena2°Cpathway),isamorerapiddeploymentofrenewableenergyand
low,zeroandnegativeemissionenergysupplytechnologies.Thisinvolvesamorerapid
9
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retirementofexistingemission-intensiveenergysystemcomponentsandacorresponding
rapidupscalinginzero-emissionandhighlyefficienttechnologies.
Inthelongerterm(post2030)technologiestoremoveCO2fromtheatmosphereplaya
considerableroleinexistingemissionscenariosforachievingeither1.5°Cor2°C,andthe
needforthesebyitselfdoesnotappeartobeadifferentiatingelementbetweenthe1.5°C
limitandhigherlevelsofwarming.Dependinguponthesuccessindeploymentof
renewableenergy,energyefficiency,electrificationoftransportsystemsandreductionsin
non-CO2GHGemissions,lowscaledeploymentofnegativeemissionstechnologieswould
needtostartinthelate2020searly2030s.Thescientificliteratureatpresentpointsto
BECCSasthemostlikelyavailableoptionwithpotentialtogrowtolarge-scaledeployment
atlimitedeconomiccosts.
Giventheuncertaintiessurroundingthepotentialfornegativeemissionstechnologiesto
bedeployedatscale,itwouldseemprudentthatpolicyaimstominimisetheneedfor
this.Evenwithrapiddeploymentofrenewableenergyandenergyefficiency,withvery
rapidemissionreductionsbetweennowand2050thephysicalpresenceofsomuchCO2in
theatmosphereduetopasthumanactivitiesmeanssomelevelofnegativeemissionsis
virtuallyunavoidableatthisstageiftheParisAgreementlong-termtemperaturegoalisto
bemet.Asaconsequence,itisimportantthattheinitialandmosturgentfocusonParis
Agreementcompatiblepolicyactionsisontherapiddeploymentofrenewableandother
loworzerocarbonenergysystems,combinedwithrapidimprovementsinenergy
efficiencyofindustry,buildingandhousing,andelectrificationoftransportsystems.
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