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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

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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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

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