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Climatememoryofcorals

Recordsofseatemperatureshavebeenkeptsincethemiddleofthe19thcentury.Theclimatedatastoredincoralsgoesmuchfurtherback.Thishelpsustounderstandcurrentextremeweatherphenomena,suchastheongoingdroughtinAustralia.

Diver under water
AnneSheppard,UniversittWarwick,UK

DuringadiveatthereefoffDiegoGarcia,anislandintheChagosArchipelago(BritishIndianOceanTerritory),MiriamPfeifferobtainsadrillcoreofstonycorals.

Thelightgreycoraldrillcore,whichMiriamPfeifferfetchesfromthecupboardinherofficeattheInstituteofGeosciences,lookslikeaslimcolumnofconcrete.Ithasnothingincommonwiththecolourful,finely-branchedstructuresintheocean,forwhichcoralsarewellknown.Thisisbecausethecolony-formingcnidariansoccurinmanydifferentspeciesandforms.Pfeiffer'scorecomesfromstonycoralsofthegenusPorites,whichgrowslowly,mostlyinsolidstructures,andcanbecomeverylargeandold.

Forthepast20years,theKielprofessorofpalaeontologyandhistoricalgeologyhasinvestigatedcurrentandfossilisedcoralreefsinconnectionwithclimatechange,becausethecoralsareakindoflivingclimatearchive.Theycanrecordthetemperature,justliketreeswiththeirannualgrowthrings.Withcorals,theannualgrowthringsareontopofeachother,explainedPfeiffer,whoreinforcesthepriorityresearchareaKielMarineScience(KMS).Thecoralsgrowapproximatelyonecentimetreperyear,and-dependingonthewatertemperature-incorporatedifferentamountsoftraceelementsintheircalciumcarbonateskeletons.Basedontheanalysisofthesetraceelements,andincombinationwithfurtherstudies,wecandrawconclusionsregardingthewatertemperature.Incontrastwiththeexperimentally-measuredseatemperatures,theclimatedatastoredincoralshasspecialadvantages:itgoesmuchfurtherbackintime,isavailableinoneplace,andislesspronetomeasurementerror.Whenweworkwithlivingcorals,wecaneasilyinvestigatethelast400years.Onlythencanwesayaphenomenonalwaysrecurs,oritoccursonceinawhileandthennotagain.Fossilisedcoralsenableanevenfurtherlookbackintoclimatehistory-upto5000yearsago.Theyprovideimportantcomparativedata,forexampleaboutnaturalclimaticandenvironmentalvariability,andtherebyhelptoplacethecurrently-observedenvironmentalchangesinthecontextofnaturalfluctuations,andevaluatethemaccordingly.

Struktures of corals
Photo:Prof.MiriamPfeiffer

Inthex-rayimageofthecoralcore,theannualgrowthringsareclearlyvisible.Thestonycoral(genus:Poritessp.)growsabout1cmperyear.

 

TheresearchofPfeiffer'sworkinggroupcentresaroundtheIndianOceananditsspecialclimatephenomenon,theIndianOceanDipole,anaturalanomalyinseasurfacetemperaturesinthewesternandeasternIndianOcean.Forexample,itcausestheextremedroughtinAustralia.Duringthecourseofthe20thcentury,theIndianOceanwarmedupthefastestofalltheoceanbasins,andstronglyfollowsglobalclimatewarming,reportedthescientist.ThiswarmingstrengthensthedipoleintheIndianOcean,whichiscausedbytemperaturedifferencesbetweenthewesternandtheeasternpartsoftheIndianOcean.

SimilartoElNioandLaNiainthePacific,thedipolealsogoesthroughdifferentphasesinacycleofthreetoeightyears,inwhichthewatertemperaturechangesdrastically.Inthecurrentpositivephase,thetemperatureishigherinthewesternIndianOceanandlowerintheeasternpart,whichcausesdroughtsinEastAsiaandAustralia,andontheotherhand,torrentialraininpartsofIndiaandEastAfrica.Pfeiffersaid2019wasthemostextremeIndianOceanDipoleeverrecorded,atleastintermsofthetemperaturemagnitude.InDecember2019,thisledtotorrentialrainsandfloodinginEastAfrica,aswellasextremedrynessandthedevastatingbushfiresinAustralia.

Ontheonehand,coralsarevictimsofclimatechange,becauserisingseatemperaturesdamagethereefssobadly,thatsomeofthemwillneverrecover.Atthesametime,coralsalsocontributesignificantlytodeterminingtheclimateofpastcenturies,andthusunderstandingtheprocessesofclimatechange.Withthetemperaturesrecordedbythecorals,IcancalculatethetemperaturegradientsoftheIndianOcean,andthusunderstandwhythisdipoleintheIndianOceanhasstrengthened,saidPfeiffer.Amongotherthings,thisdatawillbeusedtofurtherrefinetheclimatemodels.

Author:KerstinNees

Dyingcoralreefs

Togetherwithotherfactors(oceanacidification,environmentalpollution),theincreaseinseatemperaturesputsthesurvivalofcoralreefsatrisk.Thefrequencyandintensityofthemassdeathofcoralshasreachedpreviouslyunheardofproportionsinthepastthreedecades,andisadirectconsequenceofglobalwarming.Evenatanalmost-inevitableglobalwarmingof1.5degrees,in2018theIntergovernmentalPanelonClimateChange(IPCC)predictedcorallossesof70to90percent.Atanincreaseof2degrees,99percentofcoralreefswouldprobablybelost(accordingtotheIPCCSpecialReport:GlobalWarmingof1.5C).Coralsdopossessacertaindegreeofresilienceandadaptability.Butifheatwavesfollowoneanothereverfaster,theyfinditdifficulttorecover.EvenreefssuchastheGreatBarrierReefoffAustraliamaydisappearcompletely,emphasisedtheKielcoralresearcherMiriamPfeiffer.

Althoughcoralreefsonlycoverabout0.1percentoftheglobaloceans,theyprovideahabitatformorethan25percentofknownfish,andarethereforeanimportantsourceoffoodforthelocalpopulation.Theyarealsoanimportanteconomicfactorintourismformanycountries,andserveascoastalprotection.Ifwehadtoreplacethecoastalprotectionfunctionofthecoralreefswithartificialbreakwaters,thiswouldcostagreatdealofmoney.PfeiffersaidEvenifitseemssoexoticandfarawaytous:halfoftheworld'spopulationlivesinthetropics.Around275millionpeoplelivedirectlyinthesphereofinfluenceofacoralreef.(ne)