observations, les causes et les projections
Dans les chapitres suivants nous pr´esentons nos travaux de th`ese et les ´el´ements de r´eponse qu’ils apportent aux questions scientifiques n˚6, 7, 8, 9, 10, 11, 13, 14, 20, 21, 22, 23, 28, 29 et 30 de la liste ci-dessus. Ce sont les questions scientifiques qui portent sur la variabilit´e r´egionale du niveau de la mer, ses causes, ses origines et les impacts associ´es. Auparavant nous proposons un article de synth`ese sur les variations pass´ees et actuelles du niveau de la mer et ses causes, que nous avons publi´e dans le journal ”Journal of Geodynamics” en Mars 2012. Cet article s’intitule ”Sea level : a review of present-day and recent-past changes and variability”. Il r´esume le chapitre 1.
JournalofGeodynamics58 (2012) 96–109
ContentslistsavailableatSciVerseScienceDirect
Journal ofGeodynamics
j ou rna l h o me pa g e :h t t p : / / w w w . e l s e v i e r . c o m / l o c a t e / j o g
Review
Sealevel: Areviewofpresent-day andrecent-past changesand variability
BenoitMeyssignac∗,AnnyCazenave
LEGOS-CNES,Toulouse,France
a r t i c l e i n f o
Articlehistory:
Received12December2011 Receivedinrevisedform9March2012 Accepted10March2012
Available online 19 March 2012
Keywords:
Sealevel Altimetry
Globalmeansealevel Regionalsealevel Sealevelreconstruction Climatechange
a b s t r a c t
Inthisreviewarticle,wesummarizeobservationsofsealevelvariations,globallyandregionally,during
the20thcenturyandthelast2decades.Overtheseperiods,theglobalmeansealevelroseatratesof
1.7mm/yrand3.2mm/yrrespectively,asaresultofbothincreaseofoceanthermalexpansionandland
iceloss.Theregionalsealevelvariations,however,havebeendominatedbythethermalexpansionfactor overthelastdecadeseventhoughotherfactorslikeoceansalinityorthesolidEarth’sresponsetothe
lastdeglaciationcanhaveplayedarole.Wealsopresentexamplesoftotallocalsealevelvariations
thatincludetheglobalmeanrise,theregionalvariabilityandverticalcrustalmotions,focusingonthe tropicalPacificislands.Finallyweaddressthefutureevolutionoftheglobalmeansealevelunder on-goingwarmingclimateandtheassociatedregionalvariability.Expectedimpactsoffuturesealevelrise arebrieflypresented.
© 2012 Elsevier Ltd. All rights reserved.
Contents
1. Introduction.......................................................................................................................................... 96
2. Paleosealevel(sincethelastglacialmaximumandlast2000years).............................................................................. 97
3. Thetidegauge-basedinstrumentalrecord(20thcentury).......................................................................................... 97
4. Thealtimetryera..................................................................................................................................... 98
5. Causesofpresent-dayGMSLchanges............................................................................................................... 99
5.1. Globalmeanrise.............................................................................................................................. 99
5.1.1. Oceanwarming..................................................................................................................... 99
5.1.2. Glaciersmelting.................................................................................................................... 99
5.1.3. Icesheets........................................................................................................................... 99
5.2. Interannualvariabilityoftheglobalmeansealevel......................................................................................... 99
6. Causesofpresent-dayandpast-decaderegionalvariability........................................................................................ 100
7. Nonstationarityofspatialtrendpatternsandinternalvariabilityoftheocean–atmospheresystem............................................. 104
8. Localsealevelchangesinafewselectedregions................................................................................................... 104
9. Globalwarmingandfuturelarge-scalesealevelchanges.......................................................................................... 104
10. Outlook............................................................................................................................................. 106
Acknowledgements.................................................................................................................................. 106
References........................................................................................................................................... 106
1. Introduction
Sea level variations spread over a very broad spectrum.
Thelargestglobal-scale sealevel changes(100-200min ampli-tude)occurred ongeologicaltime scales(ontheorder of∼100
million years) and depended primarily on tectonic processes
∗Correspondingauthor.
E-mailaddress:benoit.meyssignac@legos.obs-mip.fr(B.Meyssignac).
(e.g.large-scalechangeintheshape ofocean basinsassociated withseafloorspreadingandmid-oceanridgesexpansion)(e.g.Haq andSchutter,2008;Muelleretal.,2008;Milleretal.,2011).With theformationoftheAntarcticaicesheetabout34millionyears ago,globalmeansealeveldroppedbyabout50m.Morerecently, cooling of the Earthstarting about3 million years ago, led to glacial/interglacialcyclesdrivenbyincominginsolationchanges inresponsetovariationsoftheEarth’sorbitandobliquity(Berger,
1988).Correspondinggrowthanddecayofnorthernhemisphere
icecapsontimescalesoftensofthousandyearsproducedlarge 0264-3707/$–seefrontmatter© 2012 Elsevier Ltd. All rights reserved.
(e.g.Lambecketal.,2002;Rohlingetal.,2009;YokoyamaandEsat, 2011).On shorter (decadal tomulticentennial)time scalessea levelfluctuationsaremainlydrivenbyclimatechangeinresponse tonaturalforcingfactors(e.g.solarradiationvariations,volcanic eruptions)andtointernalvariabilityoftheclimatesystem(related forexampletoatmosphere–oceanperturbationssuchasElNi ˜no-Southern Oscillation – ENSO, North Atlantic Oscillation – NAO, PacificDecadalOscillation–PDO).Sincethebeginningofthe indus-trialera,abouttwocenturiesago,meansealevelisalsoresponding toanthropogenicglobalwarming.Ineffect,sealevelisavery sen-sitiveindexofclimatechangeandvariability.Forexample,asthe
oceanwarmsinresponsetoglobalwarming,seawatersexpand,
andthussealevelrises.Asmountainglaciersmeltinresponseto increasingairtemperature,sealevelrisesbecauseoffreshwater massinput totheoceans. Similarly,ice mass lossfromthe ice sheetscausessealevelrise.Correspondingincreaseoffreshwater intotheoceanschangeswatersalinity,henceseawaterdensityas wellasoceancirculationthatinturnaffectssealevelataregional scale.Modificationofthelandhydrologicalcycleduetoclimate variabilityanddirectanthropogenicforcingleadstochangesin pre-cipitation/evaporationregimesandriverrunoff,henceultimately tosealevelchanges.Thusglobal,regionalandlocalclimatechanges affectsealevel(e.g.Bindoffetal.,2007).
Inthisarticle,wereviewobservationsofsealevelvariations, globallyandregionally,focusingonthe20thcenturyandthelast 2decades(Sections2–4).Wealsoexaminethecausesofsealevel variations,anddiscusssuccessivelycomponentsoftheglobalmean sealevelriseoverthepasttwodecades,andthecontributionstothe interannualglobalmeansealevel(Section5).Regionalvariability insealevelisaddressedinSections6and7.InSection8,weshow examplesoftotalsealevelchangesmeasuredoverthepast5–6 decades.Inthelastsection(Section9)webrieflyaddressthefuture evolutionoftheglobalmeansealevelunderwarmingclimateand associatedregionalvariability.Concludingremarksareproposed inSection10.
2. Paleosealevel(sincethelastglacialmaximumandlast 2000years)
Quaternaryiceagescausedlarge-scalefluctuationsoftheglobal meansealevel,of>±100mamplitude,asaresultofthegrowingand decayofnorthernhemisphereicecaps(Rohlingetal.,2009).Since about800,000years,thecharacteristicperiodicityofthese fluc-tuationsis∼100,000years.Atthelastglacialmaximum,∼20,000 yearsago,globalmeansealevelwas−130mbelowpresentlevel (Lambecketal.,2002).Subsequentmeltingofthenorthern hemi-sphereicecapscausedbyinsolationchangesledtosustainedsea levelriseduringmorethan10,000years,asillustratedinFig.1 (from Lambecket al.,2002). Duetothecomplex history of ice capmelting(e.g.Peltier,2004),therateofsealevelrisewasnot constant,asevidencedbyseveralpaleosealevelindicatorsof geo-logical andbiological origin(e.g.coral data,micro-atolls, beach rocks,notches,etc.).Forexample,episodesofrapidrise(>1mper century)havebeenreportedatabout−14,000years(Bardetal., 2010).AtthebeginningoftheHolocene(11,000years ago),the rateofrisedecreasedsignificantlyandsealevelstabilizedbetween −6000yearsand−2000yearsago(Lambecketal.,2010).
Thereisnoevidenceoflargefluctuationsoftheglobalmean sealevelduringthepasttwomillennia.Datingofmicrofossilsin salt-marshenvironments(Lambecketal.,2010;Kempetal.,2011) andarchaeologicalevidence(e.g.fromRomanfishtanks,Lambeck etal.,2004)indicatethatsealevelrisedidnotexceed0.05–0.07m percenturyoverthepast2000years(seealsoMilleretal.,2009). Fig.2(from Kempetal.,2011)illustratesthisfact.Itshowsthe
-150 -100 -50 5 10 15 20 25 esl_Barbados esl_Tahiti esl_Huon esl_Christchurch esl_Sunda esl_Bonaparte
time (x1000 cal. years)
Ice volume equivalent sea level (m)
18.5
39
57.5
Difference in ice volume (x10
6
km
2
)
Fig.1.Changesinglobalicevolumeandsealevelequivalentfromthelastglacial maximumtothepresent.Thefigureshowsice-volumeequivalentsealevelforthe past20kyrbasedonisostaticallyadjustedsea-leveldatafromdifferentlocalities (updatedfromLambecketal.,2002withareviseddatasetfortheSundashelffrom
Hanebuthetal.,2009).
sealevelevolutionofthelasttwomillenniabasedonsalt-marsh
microfossils analyses along the eastern coast of North
Amer-ica. According to this study, sea level was a few decimeters
higher/lowerduringtheMiddleAge(12th–14thcentury)/LittleIce Age(16th–18thcentury),butratesofriseremainedverylowuntil thebeginningoftheindustrialera(late18thtoearly19th
cen-tury)whenalargeupwardtrendofthemeansealevelbecomes
wellapparent(Kempetal.,2011;alsoGehrelsetal.,2005,2006; Woodworthetal.,2011a).Thisepochcorrespondstothebeginning oftheinstrumentalerathatalloweddirectsealevelmeasurements withtidegauges(Woodworthetal.,2008,2011b)andnow satel-lites(e.g.FuandCazenave,2001;Churchetal.,2010),unlikeduring thepreviouscenturies/millenniaforwhichsealevelvariationsare deducedindirectlyfromproxyrecords.
3. Thetidegauge-basedinstrumentalrecord(20thcentury)
Theveryfirsttidegaugeswereinstalledinportsofnorthwestern Europetoprovideinformationonoceantides(e.g.Mitchumetal.,
2010). Tidegauges recordsfrom Amsterdam(the Netherlands),
Stockholm(Sweden)andLiverpool(UK)extendbacktotheearlyto mid-18thcentury,whilethosefromBrest(France)andSwinoujscie (Poland)startedintheearly19thcentury.Inthesouthern hemi-sphere,tidegaugerecordsatSydneyandFreemantle(Australia)are amongthelongest(startinginthelate19thcentury).Progressively, thetidegaugenetworkextended(seeforexampleFig.5.2from Mitchumetal.,2010)butforlongtermsealevelstudies,thenumber ofrecordsremainsneverthelessverysmallandthegeographical spreadisquiteinhomogeneous.Besides,tidegaugerecordsoften sufferfrommulti-year-orevenmulti-decade-longgaps.Thesparse
andheterogeneouscoverageoftidegaugerecords,both
tempo-rallyandgeographically,isclearlyaproblemforestimatingreliable historicalmeansealevelvariations.
Tidegaugesmeasuresealevelrelativelytotheground,hence
monitor also ground motions. In active tectonic and volcanic
regions,orinareassubjecttostronggroundsubsidencedueto natu-ralcauses(e.g.sedimentloadinginriverdeltas)orhumanactivities (groundwater pumpingandoil/gasextraction),tide gaugedata aredirectlyaffectedbythecorrespondinggroundmotions.Post
98 B.Meyssignac,A.Cazenave/JournalofGeodynamics58 (2012) 96–109
Fig.2.Relativesealevelreconstructionforthelast2000yearsfromsalt-marshdataanalyses.(FromKempetal.,2011.)
glacialrebound, thevisco-elasticresponse oftheEarthcrustto lastdeglaciation(alsocalledGlacialIsostaticAdjustment–GIA)is anotherprocessthatgivesrisetoverticallandmovement.While
verticalgroundmotionsneedtobeconsideredwhenestimating
totallocal(relative)sealevelchange(seeSection8),tocompare observedsealevelvariationswithclimate-relatedcomponents,the groundmotionsneedtobesubtracted.
Toprovideareliablehistoricalsealeveltimeseriesbasedon tidegaugerecords,variousstrategieshavebeendeveloped.Some authorsonlyconsideredafewtensoflong(>60years)goodquality tidegaugesrecordsfromtectonicallystablecontinentalandisland coasts, andcorrected thedatafor GIAonly(e.g.Douglas,1991, 2001;Peltier,2001;Holgate,2007).Otherauthorsusedalarger setofrecordsfromavarietyofregions,covering differenttime periods,anddevelopeddifferentapproachestoderivethemean sealevelcurve.Forexample,Jevrejevaetal.(2006,2008)useda regionalcoherencycriteriumamongtidegaugerecordsinorder toexcludeoutliers(e.g.tidegaugeaffectedbylargelocal verti-calgroundmotions).Churchetal.(2004)andChurchandWhite (2006,2011)developeda‘reconstruction’method(seeSection6) todeterminea‘globalmean’sealevelcurvefromsparsetidegauge recordssince1870.Inthesestudies,theonlyverticalmotion cor-rectedforisGIA.Sinceafewyears,theavailabilityofGPS-based precisepositioningatsometidegaugesiteshasalloweddirect mea-surementsof verticalground motion.Thisistheapproachused by Woppelmannet al.(2007, 2009). GPS-basedvertical ground motionsarestillbasedonshortrecords(10–15year-longonly)but itisgenerallyassumedthatthesearerepresentativeoflong-term trends.Inspiteofavarietyofapproaches,theresultsbasedfrom thesestudiesareratherhomogeneousandgiveameanrate20th centuryriseintherangeof1.6–1.8mm/yr.TheChurchandWhite (2011)’stidegauge-basedmeansealevelcurvesince1870isshown inFig.3.Accordingtothisfigure,20thcenturysealevelrisewas notlinear.Infact,interannualtodecadalvariability(inadditionto shorter-termfluctuationsnotconsideredhere)aresuperimposed onthemeantrend.ThesewillbediscussedinSection5.2.
4. Thealtimetryera
Since the early1990s, sea level variations are measured by altimetersatellites(Cheltonetal.,2001;FuandCazenave,2001). The satellite altimetry measurement is derived as follows (see Fig.4):theonboardradaraltimetertransmitsmicrowaveradiation towardstheseasurfacewhichpartlyreflectsbacktothesatellite. Measurementoftheround-triptraveltimeprovidestheheightof thesatelliteabovetheinstantaneousseasurface.Theseasurface
heightmeasurementisdeducedfromthedifferencebetweenthe
satellitedistancetotheEarth’scentreofmass(deducedfrom pre-ciseorbitography)andthesatellitealtitudeabovetheseasurface
Fig.3. 20thcenturysealevelcurve(inblackandassociateduncertaintyinlight gray)basedonpastsealevelreconstructionusingtidegaugedataand addi-tionalinformation(fromChurchandWhite,2011).Inthebox:altimetry-basedsea levelcurvebetween1993and2011(datafromAVISO;http://www.aviso.oceanobs. com/en/data/products/sea-surface-height-products/global/msla/index.html)(blue points:dataat10-dayinterval;theredcurveisbasedona3-monthsmoothing ofthebluedata).
Fig.5.Earth’scoveragebytheTopex/Poseidon,Jason-1andJason-2altimeter satel-litesduringanorbitalcycleof10days.
(deducedfromtheradaraltimetermeasurement).Theseasurface heightmeasurementneedstobecorrectedforvariousfactorsdue toionosphericandtroposphericdelay,andforbiasesbetweenthe meanelectromagneticscatteringsurfaceandtheseasurfaceatthe air-seainterface.Othercorrectionsduegeophysicaleffects,such assolid Earth, poleand ocean tidesare alsoapplied.Altimeter satellitescoverthewholeEarthsurfacewithinafewdays–called orbitalcycle(seeFig.5).Geographicalaveragingofallindividual seasurfaceheightsmeasurementsduringanorbitalcycleallows determiningaglobalmeansealevelvalue,andfurther construct-ingaglobalmeansealeveltimeseries.Asthesatellitefliesover thesameareasfromoneorbitalcycletoanother,itisalsopossible toconstructa‘local’sealeveltimeseries,hencededuceregional variabilityinsealevel.
High-precisionsatellitealtimetrybeganwiththelaunchofthe Topex/Poseidonsatellitein1992anditssuccessors,Jason-1(2001) andJason-2(2008).Theprecisionofanindividualseasurfaceheight measurementbasedonthesemissionshasnowreachedthe1–2cm level(e.g.Neremetal.,2010;Beckleyetal.,2010;Mitchumetal., 2010). Precisionontheglobal meanrateof rise iscurrently of ∼0.4–0.5mm/yr.Thisvalueisbasedonerrorbudgetanalysesof allsourcesoferroraffectingthealtimetrysystemoron compar-isonswithtidegauge-basedsealevelmeasurements(e.g.Ablain etal.,2009).InFig.3,thealtimetry-basedglobalmeansealevel curvesinceearly1993issuperimposedonthetide-gauge-based 20thcenturysealevelcurve.Wenoteanalmostlinearincrease (exceptfortemporaryanomaliesassociatedwiththe1997/1998El Ni ˜noandthe2007/2008and2010/2011LaNi ˜naevents).Overthis 18year-longperiod,therateofglobalmeansealevelriseamounts to3.2±0.5mm/yr(e.g.CazenaveandLlovel,2010;Neremetal., 2010;Mitchumetal.,2010).Thisrateissignificantlyhigherthanthe meanraterecordedbytidegaugesoverthepastdecades,eventually suggestingsealevelriseacceleration(Merrifieldetal.,2009).
5. Causesofpresent-dayGMSLchanges
5.1. Globalmeanrise
Themainfactorscausingcurrentglobalmeansealevelriseare thermalexpansionofseawaters,landicelossandfreshwatermass
exchangebetweenoceansandlandwaterreservoirs.Therecent
trendsofthesecontributionsmostlikelyresultfromglobalclimate changeinducedbyanthropogenicgreenhousegasesemissions.
5.1.1. Oceanwarming
Analysesofinsituoceantemperaturedatacollectedoverthe past50yearsbyshipsandrecentlybyArgoprofilingfloats(Argo
thatoceanheatcontent,andhenceoceanthermalexpansion,has significantlyincreasedsince1950(e.g.Levitusetal.,2009;Ishiiand Kimoto,2009;Dominguesetal.,2008;Churchetal.,2011a).Ocean warmingexplainsabout30%–40%oftheobservedsealevelriseof thelastfewdecades(e.g.Churchetal.,2011b).Asteepincreasewas
observedinthermalexpansionoverthedecade1993–2003(e.g.
Lymanetal.,2010;Levitusetal.,2009;IshiiandKimoto,2009), butsinceabout2003,thermalexpansionhasincreasedlessrapidly (Lymanetal.,2010;Lloveletal.,2010;vonSchuckmannandLe Traon,2011). Therecentslowerrateofstericriselikelyreflects short-termvariabilityratherthananewlong-termtrend.On aver-age,overthesatellitealtimetryera(1993–2010),thecontribution ofocean warmingtoseariseaccountsfor∼30%(Cazenaveand Llovel,2010;CazenaveandRemy,2011;Churchetal.,2011b).
5.1.2. Glaciersmelting
Beingverysensitivetoglobalwarming,mountainglaciersand smallicecapshaveretreatedworldwideduringtherecentdecades, withsignificantaccelerationsincetheearly1990s.Frommass bal-ancestudiesofa largenumberof glaciers,estimateshave been madeofthecontributionofglaciericemelttosealevelrise(Meier etal.,2007;Kaseretal.,2006).Fortheperiod1993–2010,glaciers andicecapshaveaccountedfor∼30%ofsealevelrise(e.g.Cogley, 2009;Steffenetal.,2010;Churchetal.,2011b).
5.1.3. Icesheets
Whilelittlewasknownbeforethe1990sonthemassbalanceof theicesheetsbecauseofinadequateandincompleteobservations, differentremotesensingtechniquesavailablesincethen(e.g. air-borneandsatelliteradarandlaseraltimetry,SyntheticAperture RadarInterferometry–InSAR,andsince2002,spacegravimetry fromtheGRACEmission)haveprovidedimportantresultsonthe changingmassofGreenlandand(west)Antarctica(e.g.Allisonetal., 2009).Thesedataindicatethatbothicesheetsarecurrentlylosing massatanacceleratedrate(e.g.Steffenetal.,2010).Mostrecent massbalanceestimatesfromspace-basedobservations unambigu-ouslyshowicemasslossaccelerationintherecentyears(e.g.Chen etal.,2009;Velicogna,2009;Rignotetal.,2008a,b,2011).Forthe period1993–2003,<15%oftherateofglobalsealevelrisewasdue totheicesheets(IPCCAR4).Buttheircontributionhasincreased upto∼40%since2003–2004.Althoughnotconstantthroughtime, onaverageover2003–2010icesheetsmasslossexplains∼25%of therateofsealevelrise(CazenaveandRemy,2011;Churchetal., 2011a).
Thereismoreandmoreevidencethatrecentnegativeicesheet massbalancemainlyresultsfromrapidoutletglacierflowalong somemarginsofGreenlandandWestAntarctica,andfurther ice-bergdischargeintothesurroundingocean(Alleyetal.,2007,2008; Steffenetal.,2010).Thisdynamicalthinningprocessisgenerally observedincoastalregionswhereglaciersaregroundedbelowsea level(e.g.innortheastandsouthwestGreenland,andAmundsen Seasector,WestAntarctica).Thinningandsubsequentbreak-upof floatingicetonguesoriceshelvesthatbuttressedtheglaciersresult inrapidgroundinglineretreatandacceleratedglacierflow.Several recentobservationshaveshownthatwarmingofsubsurfaceocean waterscouldtriggertheseshort-termdynamicalinstabilities(e.g. Hollandetal.,2008).
Fig.6(updatedfromCazenaveandLlovel,2010)comparesthe observedglobalmeansealevelrisetothedifferentcomponents andtheirsumoverthealtimetryera.
5.2. Interannualvariabilityoftheglobalmeansealevel
Ifthe(linear)globalmeantrendisremovedfromthe altimetry-based sea level curve shown in Fig. 3, significant interannual
100 B.Meyssignac,A.Cazenave/JournalofGeodynamics58 (2012) 96–109
Fig.6.Observedsealevelfromsatellitealtimetryover1993–2010(bluesolidcurve).