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Page 1: “Because the Ocean” – achieving the Paris Agreement 1.5°C ... · The analysis underscores the need to limit warming below 1.5°C to limit impacts on ocean systems. Exceeding

www.climateanalytics.org 9November2017

Oceansystemsareparticularlyvulnerabletoclimatechange,andthereisalreadyclearevidenceforlossanddamageinflictedbyclimatechangeonoceansystems.Thisbriefingprovidesanoverviewofthelatestscienceonkeyrisksforoceansystemsincludingfromsea-levelrise,oceanacidificationandimpactsoncoralreefsandothermarineandcoastalecosystems.Theanalysisunderscorestheneedtolimitwarmingbelow1.5°Ctolimitimpactsonoceansystems.Exceedingthatwarminglevelwillfundamentallyaffectoceansystemsandundermineanyotherattemptstoprotectthem.Limitingwarmingto1.5°Cisofparamountimportanceforfutureoceanpathways.ThismeansthatthemosturgentsteptoprotectoceansistoreduceCO2andothergreenhousegasemissionsinlinewithwhatisneededtomeettheParisAgreement1.5°Climit.

KeyclimaterisksforoceansystemsSealevelrise

Sealevelriseasaresultofanthropogenicclimatechangeisakeyimpactaffectingoceanandcoastalsystems.Underwarmingof2°C,wecouldbelookingatsea-levelriseof5mandmoreoverthecomingcenturies1,2.Onlylimitingwarmingtobelow1.5°Cwillkeepsea-levelriseunder1minthelongrun.Warmingexceeding1.5°CwillgreatlyincreasetheprobabilityofreachingcriticaltippingpointsfortheGreenlandandAntarcticicesheets1,3.Thesetippingpointsareuncertain,butestimatedtobebelow2°C.Reachingsuchtippingpointscouldleadtounstoppablemulti-metersea-levelriseovercenturiesandmillenniatocome.Ariseinsealevelswillleadtoasteepincreaseincoastalfloodingrisks.Underawarmingscenarioof2˚Cby2100,wecanexpectsealevelriseof50cm,andpresent“oneinfiftyyears”coastalfloodswillbeoccurringeveryyear.4Risksarereducedunder1.5°C,butarestillverysubstantial.Drasticincreasesincoastalfloodingrisksdemonstratethelossanddamageinflictedbyclimatechangeevenwhenwarmingislimitedto1.5°C.

“BecausetheOcean”–achievingtheParisAgreement1.5°Ctemperaturelimit

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Figure1:Projectionsof2100globalmeansealevelrise2.

Tippingpoints

Warmingbeyond1.5°Cstronglyincreasestherisksofreachingcriticaltippingpointsofoceansystems.Abouttwo-thirdsofcriticalthresholdsinoceansystemsidentifiedinclimatemodelsmaybecrossedundera2°Cwarming-comparedtoaboutone-thirdunder1.5°C.5

Arcticseaice

CriticalthresholdsforArcticsea-icewillbeexceededforwarmingbeyond1.5°C.Asummerice-freeArcticisprojectedforwarmingabove1.5°C6,7withveryprofoundimpactsonmarineandcoastalecosystemsaswellasonthelivelihoodsofindigenouscommunities.

Oceanacidification

AsaresultofanthropogenicCO2emissions,oceanacidificationoverthecomingcenturiescouldbehigherthananytimeinthepast300millionyears8.Theimpactsofoceanacidificationoverthenextdecadeswillleaveasubstantiallegacyinthemarineenvironmentforcenturies.9Onlybylimitingwarmingto1.5°Ccanoceanacidificationbehaltedandreversed.

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www.climateanalytics.org 9November2017

Figure2:Panela:GlobaldeoxygenationunderIPCCscenariosforhighemissions(RCP8.5)abouta4oCglobalwarmingandinaholdbelow2oCscenario(RCP2.6).Whilstthelatter2oCscenariostabilizesdeoxygenationbutdoesnotappeartoinitiatefullrecovery,a1.5oCpathwayappearslikelytoleadtoinitiationofrecovery.Panelb:ObservedandprojectedoceansurfacepHfordifferentwarmingscenarios.

OceandeoxygenationOceandeoxygenationisalreadyoccurringasaresultofclimatechangewithprofoundimplicationsforoceanproductivityandmarinehabitat.10,11Underwarmingscenariosexceeding4°C,oceanoxygenlevelsincoastalseascouldreducebymorethan40%.Limitingwarmingtobelow1.5°Cwouldlimitthistolessthan10%.12

Tropicalcyclones

Tropical cyclones represent an existential threat to tropical island and coastalcommunities - causing loss and damage. The climate hazards posed by tropicalcyclonesareintensifiedbyincreasingrisksoffloodingthroughheavierrainfallandsea level rise as a result of climate change. Attribution of tropical cyclones toclimate change is difficult. However, a robust increase of the most devastatingstormswith climate change is evident. Under 2.5°C of global warming, themostdevastatingstormsareprojectedtooccuruptotwiceasoftenastoday.13

Marinefisheries

Underawarmingpathwayimpliedbypoliciescurrentlyinplace,Indo-Pacificmaximumcatchpotentialwouldbealmosthalved.IntheIndo-Pacific,reductioninmaximumcatchpotentialdoublesbetweenawarmingof1.5°Cand2°C.Asaconsequencetherearelargebenefitsintermsofavoideddamagestomarinefisheriesofmeetingthe1.5°Cglobalwarmingtarget.14Benefitsarelargestfortropicalregions.

Tropicalcoralreefs

Unprecedentedmarineheatwavescriticallyaffectingcoastalandoceanecosystemsarealreadybeingobservedtoday,andasteepriseintheirintensityandfrequencyisprojectedwithincreasingwarming.Forexample,aheatwaveinearly2016inAustraliaandtheSouthPacificledtowidespreadcoralbleachingintheCoralSeaandextensivedie-backofmangrovesinnorthernAustralia.Under2°Cwarming,suchheatwaveswouldbethenewnormal,occurringin9outof10years.15

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Tropicalcoralreefsarealreadybeingverysubstantiallyaffectedbycoralbleachingandoceanacidification,andmostwillnotsurviveawarmingof2°C.16,17Limitingwarmingto1.5°Cwillleavesomechanceforecosystemstoadapt,butlossesincoralcover,alreadyoccurringtoday,willstillbeextensive.PresentlevelsofwarmingarealreadyinflictinglossandDamagetotheseuniqueandprecioussystemsandthelivelihoodsdependingonthem.

Mangroveandseagrassecosystems

Risingsealevelscouldbedetrimentalformangroveecosystemsinsomelow-lyingregions,forexampleintheSolomonIslandsandSumatra.Mangrovesmaybecompletelysubmergedbeforetheendofthecenturyinareaswheresedimentinputisnotsufficientformangrovestogrowinpacewithsealevelrise,18andtheimportantecosystemservicestheyprovide–includingcoastalprotection–willbelost.

Mangrovesandseagrassesarevulnerabletoamultitudeofclimateimpacts,includingoceanwarming,oceanacidification,sealevelriseandincreasedstorminess,19andrecoveryratesofthesefragileecosystemsareslow.20TherecentextremeElNiñoeventasledtoaveryextensivediebackofmangrovesinnorthernAustralia.

Warmingbeyond1.5°Cwillfundamentallyaffectcoastalandmarineorganismsandecosystemservices.Awarmingbeyond1.5˚CimpliessubstantialrisksforalmostallkeymarineorganismsandecosystemservicesidentifiedinthelatestIPCCassessmentreport(AR5).21

Figure3:ExampleofcoralreefmortalityinAmericanSamoa2014-2016PhotobyR.Vevers,XLCatlinSeaviewSurvey|CoralReefWatch

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www.climateanalytics.org 9November2017

References:

1. Deconto,R.M.&Pollard,D.ContributionofAntarcticatopastandfuturesea-levelrise.Nature531,591–597(2016).2. Nauels,A.,Rogelj,J.,Schleussner,C.&Meinshausen,M.Linkingsealevelriseandsocioeconomicindicatorsunderthe

SharedSocioeconomicPathways.Environ.Res.Lett.12,11(2017).3. Robinson,A.,Calov,R.&Ganopolski,A.MultistabilityandcriticalthresholdsoftheGreenlandicesheet.Nat.Clim.

Chang.2,1–4(2012).4. Vitousek,S.etal.Doublingofcoastalfloodingfrequencywithindecadesduetosea-levelrise.Sci.Rep.7,1399(2017).5. Drijfhout,S.etal.CatalogueofabruptshiftsinIntergovernmentalPanelonClimateChangeclimatemodels.Proc.

Natl.Acad.Sci.112,E5777–E5786(2015).6. Screen,J.A.&Williamson,D.Ice-freeArcticat1.5°C?Nat.Clim.Chang.7,230–231(2017).7. Notz,D.&Stroeve,J.ObservedArcticsea-icelossdirectlyfollowsanthropogenicCO2emission.1–9(2016).8. Caldeira,K.&Wickett,M.E.AnthropogeniccarbonandoceanpH.Nature425,2003(2003).9. Mathesius,S.,Hofmann,M.,Caldeira,K.&Schellnhuber,H.J.Long-termresponseofoceanstoCO2removalfromthe

atmosphere.Nat.Clim.Chang.5,1107–1113(2015).10. Keeling,R.F.,Körtzinger,A.&Gruber,N.OceanDeoxygenationinaWarmingWorld.Ann.Rev.Mar.Sci.2,199–229

(2010).11. Long,M.C.,Deutsch,C.&Ito,T.Findingforcedtrendsinoceanicoxygen.GlobalBiogeochem.Cycles30,381–397

(2016).12. Frölicher,T.L.,Rodgers,K.B.,Stock,C.A.&Cheung,W.W.L.Sourcesofuncertaintiesin21stcenturyprojectionsof

potentialoceanecosystemstressors.GlobalBiogeochem.Cycles30,1224–1243(2016).13. Bacmeister,J.T.etal.Projectedchangesintropicalcycloneactivityunderfuturewarmingscenariosusingahigh-

resolutionclimatemodel.Clim.Change1–14(2016).doi:10.1007/s10584-016-1750-x14. Cheung,W.W.L.,Reygondeau,G.&Frölicher,T.L.Largebenefitstomarinefisheriesofmeetingthe1.5°Cglobal

warmingtarget.Science354,1591–1594(2016).15. King,A.D.,Karoly,D.J.&Henley,B.J.Australianclimateextremesat1.5°Cand2°Cofglobalwarming.Nat.Clim.

Chang.7,(2017).16. Frieler,K.etal.Limitingglobalwarmingto2°Cisunlikelytosavemostcoralreefs.Nat.Clim.Chang.3,165–170

(2012).17. Meissner,K.J.,Lippmann,T.&SenGupta,A.Large-scalestressfactorsaffectingcoralreefs:openoceanseasurface

temperatureandsurfaceseawateraragonitesaturationoverthenext400years.CoralReefs31,309–319(2012).18. Lovelock,C.E.etal.ThevulnerabilityofIndo-Pacificmangroveforeststosea-levelrise.Nature526,559–563(2015).19. Ward,R.D.,Friess,D.A.,Day,R.H.&MacKenzie,R.A.Impactsofclimatechangeonmangroveecosystems:aregion

byregionoverview.Ecosyst.Heal.Sustain.2,(2016).20. Macreadie,P.I.etal.Lossesandrecoveryoforganiccarbonfromaseagrassecosystemfollowingdisturbance.

Proceedings.Biol.Sci.282,20151537(2015).21. Gattuso,J.-P.P.etal.ContrastingfuturesforoceanandsocietyfromdifferentanthropogenicCO2emissions

scenarios.Science349,aac4722(2015).22. Nowicki,R.J.,Thomson,J.A.,Burkholder,D.A.,Fourqurean,J.W.&Heithaus,M.R.Predictingseagrassrecovery

timesandtheirimplicationsfollowinganextremeclimateevent.Mar.Ecol.Prog.Ser.567,79–93(2017).

Figure4:Left:MassiveseagrassbedsinWesternAustralia'sSharkBay(aUNESCOWorldHeritageSite)stillhavenotrecoveredfromadevastatingmarineheatwavein2011.22(Credit:SharkBayEcosystemResearchProject).Right:MangrovediebackinnorthernAustraliaobservedinJune2016.(Credit:NCDuke)


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