Improving the TOPEX/Poseidon calibration procedure by modeling and implementation of glacial isostatic adjustment. AMasterofSciencethesis

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1 Improving the TOPEX/Poseidon calibration procedure by modeling and implementation of glacial isostatic adjustment AMasterofSciencethesis ErikSteenbergen May6,2004

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3 Improving the TOPEX/Poseidon calibration procedure by modeling and implementation of glacial isostatic adjustment AMasterofSciencethesis ErikSteenbergen May6,2004

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5 Improving the TOPEX/Poseidon calibration procedure by modeling and implementation of glacial isostatic adjustment AMasterofSciencethesis ErikSteenbergen Physical,GeometricalandSpaceGeodesy DepartmentofEarthObservationandSpacesytems FacultyofAerospaceEngineering DelftUniversityofTechnology,Netherlands supervisors : Dr.Ir.E.J.O.Schrama Ir.H.H.A.Schotman professor : Prof.Dr.-InghabilR.Klees

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7 Preface TheresearchthatIpresentinthisthesisformstheconcludingtaskoftheMasterofScience programgeodeticengineeringatthedelftuniversityoftechnology. fromjune2003untilmay2004withintheresearchgroupofphysical,geometricaland Itwascarriedout SpaceGeodesywithintheDepartmentofEarthObservationandSpacesystems(DEOS)at thefacultyofaerospaceengineering. ThemainpurposeofthisresearchwastoimplementacalibrationalgorithmfortheTOPEX/ Poseidonaltimeter,toinvestigatetheeectofglacialisostaticadjustmentonthiscalibrationandtomakeanimprovedestimationofthealtimeterdriftparameter.Thisreportis thereforeofinterestforthoseinvolvedinsatellitecalibrationandthoseinterestedinthe inuenceofglacialisostaticadjustmentonseasurfacemeasurements. IamgratefultothesupportofmysupervisorEjoSchramaduringtheresearchandfor inspiringmetoadoptthisresearchtopicintherstplace.secondly,iwouldliketothank HugoSchotmanforthesupervisionandadviceonthesubjectofglacialisostaticadjustment. IwouldalsoliketothankallthepeopleoftheFMRresearchgroupforthegreattimeI hadduringthelastyear. supportedmeduringmyresearchinanyotherway. Finally,Ithankallthosewhoarenotmentionedhere,butwho Delft,May2004 ErikSteenbergen i

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9 Summary Tidegaugeshavebeenusedinthelasttwocenturiestomeasurevariationsinsealevel. Recentinterestinsealevelanditsvariationsisdrivenbyindicationsthatthesesealevel variationsmightbeasensitiveindicatorofglobalclimatevariations[peltierandtushingham,1989].satellitealtimetrywillplayanimportantroleinboththedeterminationofthe meanrateofglobalsealevelrise,andthequanticationofthecontributionsofthevarious physicalcausestothisobservedsealevelchange.tidegaugeshaveapoordistributionover theworldandmeasuresealevelasarelativedierencebetweenlandandseasurfaceand arethereforelesssuitedforlong-termsealevelpredictions. aglobalcoverageandmeasurestheseasurfacewithrespecttotheearth'smasscenter Satellitealtimetrydoeshave butinordertoinvestigatethequalityandstabilityofthealtimetricmeasurements,the altimeterhastobecalibrated.thecalibrationcanbeperformedwiththeuseoftidegauge measurements:thedierencebetweensatelliteandtidegaugemeasurementscanbeused todetermineapossibledriftinthealtimeter. Acrucialpartinthiscalibrationprocedureisthemodelingofverticallandmotionthat ispresentinthetidegaugedata. heightoftheseawithrespecttoalocalbenchmark. Sealevel,asmeasuredbytidegauges,isdenedasthe duetochangesintheheightoftheseasurface,orduetoverticalmovementsoftheland. Changesinsealeveloccureither Twoimportantcausesofverticallandmovementareisostaticadjustmentresultingfrom viscousowofthemantleduetomeltingoflargeicesheets,andtectoniclandmovements byearthquakes,mantleconvectionandsedimenttransport. Thisresearchwillfocusontheeectoftheglacialisostaticadjustment(GIA)onthe calibrationprocedure. TOPEX/Poseidonaltimeter,investigatetheeectofglacialisostaticadjustmentonthis Themaingoalistoimplementacalibrationalgorithmforthe calibrationandmakeanimprovedestimationofthealtimeterdriftparameter. Tidegaugemeasurementsarewidelyusedinthelasttwocenturiesforobtainingsealevel informationinasystematicmanner. madefromdatastoredatthepermanentserviceformeansealevel(psmsl).monthly Long-termestimatesofsealevelchangeareusually andannualvaluesforover500tidegaugescanbefoundinthisdatabase.thecharacteristics oftidegaugemeasurements,especiallytheexistenceofastrongdecennialsignal,makeit necessarytomakeaselectionofthesetidegauges. completenessanddistributionofthetidegauges,andfurthermoreconsistsoftheremoval Thisselectionisbasedonthelength, oftidegaugeslocatedinregionswhichareknowntobetectonicactiveorwerecoveredwith icesheetsduringthelasticeage. tidegaugesfrom500toaround25,whichmakestheestimationofthelong-termsealevel Suchastrongselectionreducesthenumberofsuitable trenddicultandlessreliable. iii

10 TheTOPEX/Poseidonsatellitemissionprovidedalmost10yearsofaltimetricdatawitha precisionofafewcentimeters. theobtaineddataovereverycycleofthesatellite,aperiodof10days,suggestsasealevel Thelong-termsealeveltrend,computedbyaveragingall riseof+1.9mm/year.thedistributionofthissealevelincreaseisnotuniform;regionsof sealevelrisearefoundaroundaustralia,whereregionsofsealevelfallarelocatedinthe northernpartofthepacicocean. Thecomplexrelationshipbetweenicemelt,gravity,solidEarthdeformationandsealevel changescanbedescribedbythesealevelequation. redistributionofthemantlematerial,leadingtobothachangeinthegeoidandaradial Themeltoficewillresultinamass displacementofthesurface. eectofgiaonpresent-daysealevelcanbecalculatedusingapseudo-spectralapproach, Thesetwoeectswillinuencetheobservedsealevel. The whenadeglaciationmodelandearth'srheologymodelareselected. eectofgiaiscalculatedusingtheice-3gdeglaciationmodelandfourdierentearth's Inthisresearchthe rheologymodels.therstmodelisthemodeladoptedbypeltierandtushingham[1991], theotherthreemodelsdescribedtheresultsofahigherviscosityoftheuppermantle, thepresenceofalowviscosityzone,andadecreaseinlithospherethicknessonthesea levelchange.theeectoftheradialdisplacement,importantforaltimetercalibration,is verysmallforoceanicregionswheremosttidegaugesarelocated,intheorderof mm/year. onlychangesintheviscosityofthelowermantlehaveasignicanteect. Dierencescausedbythechangesintherstmodelareevensmaller,and Thealtimetercalibrationconsistofselectingtidegaugeandsatellitedata,andevaluates theirdierencesovertime.thetidegaugedataisselected,correctedfortheequilibriumtide andsmoothedtoreduceoutliers.thesatellitedataisselectedusingtoseparatemethods. The'block-method'selectsallmeasurementsina2x2areaaroundthetidegauge,whereas the'pass-method'selectsupto8passesforeachtidegaugeandusesthedataalong3 latitudeclosesttothetidegauge.theselectionofusefulcombinationsoftidegaugesand satellitedataisbasedoncorrelationbetweentidegaugeandsatellitemeasurementsand RMSofthedierences.Boththe'block-method'andthe'pass-method'showcomparable results,suggestingthatnodriftisoneofthealtimetersispresentbutanosetbetween thetwoaltimetersisclearlypresent.implementationofglacialisostaticadjustmentresults inthefadingofthesmalllineardriftinthemeasurement,andandecreaseoftheoset parameter. implementationofgiais4mm. TheestimatedosetbetweentheTOPEX-AandTOPEX-Baltimeterafter long-termsealevelchangeresultsinanestimatedrateof+2.5mm/year.theinuenceof Usingthisestimatedosettocomputetherateofthe the4mmosetbetweenthetopex-aandtopex-baltimeteristhusconsiderableand cannotbeomitted.

11 Samenvatting Indicatiesdatzeespiegelveranderingeneenbelangrijkeindicatorkunnenzijnvaneenglobaleklimaatsveranderingheefttotvernieuwdeaandachtvoorhetonderwerpgezorgd[Peltier andtushingham,1989].zeespiegelveranderingenwordenalmeerdan100jaarmetbehulp vangetijdestationsgemeten. overdeaarde,enmetenbovendienderelatievezeespiegelverandering,hetverschiltussen Echter,dezegetijdestationshebbeneenslechteverdeling hetlandenzeespiegel.hierdoorzijngetijdestationsmindergeschiktomdelangetermijn zeespiegelveranderingtebepalen. dekkingenmeetdezeespiegeltenopzichtvanhetmassamiddelpuntvandeaarde.hierdoor Satellietaltimetriebeschiktwelovereenwereldwijde zalsatellietaltimetrieeenbelangrijkerolspelenindebepalingvandeglobalezeespiegelstijgingaandeenekant,enbepalingvandebijdragenvandeverschillendegeofysische oorzakenvandezewaargenomenzeespiegelstijgingaandeanderekant. vandealtimeterisvereistomdekwaliteitvandezemetingentekunnenwaarborgen.deze Echter,calibratie calibratiekanwordenuitgevoerdmetbehulpvandegetijdestations: satellitemetingenengetijdedatakanwordengebruiktomeenmogelijkedriftindealtimeter hetverschiltussen tebepalen. Eencruciaaldeelindezecalibratieprocedureisdemodeleringvandeverticalelandbewegingendiewordenwaargenomenindegetijdestations.Hetzeeniveaugemetendoorgetijdestationsisdenieerdalsdehoogtevandezeespiegeltenopzichtvaneenlokaalreferentiepunt ophetland. dezeespiegel,maarookdoorverticalebewegingenvanhetland.tweebelangrijkeoorzaken Veranderingeninhetzeeniveaukunnendusontstaandoorveranderingenin vanzulkeverticalelandbewegingenzijnglacialeisostasieentectonischebewegingendoor aardbevingen,mantelconvectieensedimenttransport. Ditonderzoekzalzichrichtenopheteectvanglacialeisostasieopdecalibratieprocedure. HetdoelvanditonderzoekiseencalibratiealgoritmevoordeTOPEX/Poseidonaltimeter teimplementeren,vervolgensheteectvanglacialeisostasieopdezecalibratietebepalen entenslottehiermeeeenverbeterdedriftparameterteschattenvoordealtimeter. Metingenmetbehulpvangetijdestationswordenal100jaarwereldwijdgebruiktvoorhet systematischverzamelenvangegevensoverdezeespiegel. vanzeespiegelveranderingenwordenvoornamelijkgemaaktmetbehulpvandegetijdedata Langetermijnvoorspellingen dieisopgeslagenbijdepermanentserviceformeansealevel(psmsl).maandelijkse enjaarlijksegemiddeldenvoormeerdan500getijdestationswordenopgeslagenindeze database. een10-jaarlijkssignaal,maakthetnoodzakelijkomgetijdestationsteonderwerpenaaneen Verschillendeeigenschappenvangetijdedata,metnamedeaanwezigheidvan strengeselectie.dezeselectieisgebaseerdopdelengte,volledigheidenruimtelijkeverdeling vandegetijdestations. activiteitaanwezigis,evenalsgebiedendietijdensdelaatsteijstijdbedektwarenmetijsniet Bovendienwordengetijdestationsingebiedenwaartectonische meegenomenindeberekening.zo'nselectiezorgtvooreenuitdunningvandeoriginele500 getijdestationstotongeveer25bruikbarestations.hierdooriseenbepalingvaneenlange termijntrendvandezeespiegelmetbehulpvangetijdedataeenstukminderbetrouwbaar. v

12 DeTOPEX/Poseidonsatellietmissieheeftinmiddels10jaaraanaltimetriedatavoortgebrachtmeteenprecisievanenkelecentimeters.Delangetermijntrendvandezeespiegel, berekenddoorperperiodevan10dageneenglobaalgemiddeldeteberekenen,laateen zeespiegelstijgingzienvan+1.9mm/jaar.despreidingvandezezeespiegelstijgingisalles behalveuniform;gebiedenmeteensterkezeespigelstijgingtot+15mm/jaarzijntevinden rondaustralieengebiedenmeteensterkezeespiegeldalingzijntevindeninthenoordelijke gebiedenvandegroteoceaan. Degecompliceerdesamenhangtussenhetafsmeltenvanijs,zwaartekracht,dedeformatie vandevasteaardeenderesulterendezeespiegelstijgingkanwordenbeschrevenmetbehulp vandezeespiegelvergelijking. ingindegeoidealseenveranderingvanhetaardoppervlak. Hetafsmeltenvanijsresulteertineenzoweleenverander- beidehuninvloedophetzeespiegelniveau. Heteectvanglacialeisostasieopdehuidige Dezetweeeectenhebben zeespiegelkanwordenberekendmetbehulpvaneenpseudo-spectralebenadering,zodra eendeglaciatiemodeleneenrheologiemodelvandeaardeisbepaald. isheteectvanglacialeisostasieberekendmetbehulpvanhetice-3gijsmodelenvier Inditonderzoek verschillenderheologymodelen.heteerstemodeliseenmodeldatisgebruiktdoorpeltier andtushingham[1991],deanderedriemodellenbeschrijvenderesultatenvaneenveranderendeviscosityvandemantelvandeaarde,deaanwezigheidvaneenlaagviscosezone, endeverdunningvandelithosfeer.deimpactvanderadialeverplaatsing,diebelangrijkis indealtimetercalibratie,iskleinoverdeoceanen:waardentussen0.0en0.4mm/jaar.de verschillenveroorzaaktdoorveranderingeninhetrheologiemodelzijnnogkleinerenalleen eenveranderingindeviscositeitvandelagemantelheefteensignicanteect. Dealtimetercalibratieprocedurebestaatuithetselecterenvangetijdestationensatellietdata,enhetevaluerenvandeonderlingeverschillenindetijd.Degetijdestationdata wordtgecorrigeerdvoordeequilibriumgetijenoutlierswordenverwijdert.desatellietdata wordtgeselecteerdoptweeverschillendemanieren. tingenbinneneengebiedvan2x2ronddegetijdestation.thepass-methodeselecteert Theblok-methodeselecteerdalleme- passage.deselectievanbruikbarecombinatiesvangetijdestationsensatellietdatagebeurt tot8passagesvooriedergetijdestationengebruiktdedatalangs3latitudevandeze opbasisvandermsvandeverschiltijdseriesendecorrelatietussenbeidemetingen.beide methodengevenaandatergeensignicantedriftaanwezigisineenvandealtimeters,maar eenosettussentopex-aentopex-baltimeterisduidelijkaanwezig. vanglacialeisostasieleidttothetverdwijnenvandekleinelineairedrifteneenverlaging Implementatie vandeosetparameter.deosetparametertussendetopex-aentopex-baltimeter naimplementatievanglacialeisostasiebedraagt4mm.delangetermijnzeespiegelstijging isnaimplementatievandezeosetbepaaltop+2.5mm/jaar.

13 Contents 1 Introduction 1 2 Sealevelvariations Deningrelativesealevel Physicalcausesofrelativesealevelchange Tidegaugemeasurementtechniques PermanentServiceforMeanSeaLevel Estimationofglobalsealeveltrend Measurementprincipleofsatellitealtimetry Altimetricsatellitemissions Errorsourcesinaltimetermeasurements Satellitealtimetrymeasurements AltimeterCalibration Redistributionofoceanwaterduetodecayoficesheets Derivingthesealevelequation SolutionoftheGreen'sfunctions Schematicoverviewofthesealevelequation Pseudo-spectralapproach ICE-3Gdeglaciationmodel Earth'srheologymodels ResultsfrommodelingGIA vii 3 Tidegaugemeasurements 9 4 SatellitealtimetryfromTOPEX/Poseidon 21 5 Modelingsealevelvariations 33 6 Computingglacialisostaticadjustment 41

14 7 Altimetercalibration: Selectionofsatelliteandtidegaugedata Principlesofthecalibrationalgorithm Selectingandpreparationoftidegaugedata Selectingthesatellitedata Selectingsatellitedatausingthe'block-method'... Selectingsatellitedatausingthe'pass-method' Analysisoftheexistenceofannualsignal Driftparameterestimationusingthe'block-method' Driftparameterestimationusingthe'pass-method'... ImplementationofGIAintothecalibrationalgorithm Altimetercalibration: Estimationofthedriftparameter 67 9 Conclusionsandrecommendations 77 Bibliography 79 A AcronymsandAbbreviations 83 B Sealevelmeasuredbytidegauges 85 C AnalysisoftheWOCEtidegauges 87 D Pseudo-spectralapproach 89 E RadialdisplacementsfortheappliedGIAmodels 93

15 Chapter 1 Introduction Foroverhundredyearssealevelvariationshavebeenmeasuredusingtidegauges. urbanareasallovertheworldareconcentratedalongtheshorelines,whichexplainsthe Major growinginterestinsealevelvariations. furthermoredrivenbyindicationsofclimatechangeoverthelastdecades.sinceanincrease Recentinterestinsealevelanditsvariationsis oftheearth'stemperaturewillaectthesealevel,theresultingsealevelvariationsmightbe asensitiveindicatorofglobalclimatevariations[peltierandtushingham,1989].scientic researchinsealevelvariationsisfocusedontwosubjects:thedeterminationofthemean rateofglobalsealevelriseoverthelastcenturyandrecentchangesinthisrateontheone hand,andthefurtherunderstandingofthevariousphysicalcausesofthisobservedsealevel changeontheotherhand.quanticationofthecontributionsofoceanthermalexpansion, behaviorofglobalicereservoirs,glacialisostaticadjustmentandvariationsingroundwater storagetothisobservedsealevelchangewillbeessentialforfuturepredictions. Satellitealtimetrywillplayanimportantroleintheseresearchtopics. globalcoverageoftheoceanicregionsandthepossibilityfordirectmeasurementofthe Duetoanearly seasurface,comparedtoarelativemeasurementusingtidegauges,satellitealtimetryhas twolargeadvantagesovertidegaugemeasurementsinthedeterminationofglobalsea levelchange.moreimportant,satellitealtimetrygivesopportunitiesforinvestigatinglocal changesincurrentpatterns,thenon-uniformredistributionofmeltwaterfromantarctica orgreenland,andinterannualphenomenasuchaselni~no.thismakessatellitealtimetry alsoimportantforfurtherresearchonthequanticationoftheseprocesses. Inordertoinvestigatethequalityandstabilityofthealtimetricmeasurements,thealtimeter itselfhastobecalibrated.thiscanbedoneusingaspecialdesignedcalibrationplatform orbycomparingthesatellitemeasurementswiththemeasurementsmadeatatidegauge atthetimethesatellitepassed. measurementsmightleadtothedetectionofapossibledriftinthealtimeter. Monitoringthedierencebetweenthosetwoseasurface thesecalibrationproceduresproveditsvaluebydetectinganerrorinthealtimetersoftware Theuseof calibrationprocedureisthetreatmentofverticallandmotion. afewyearsafterthelaunchofthesatellite[christensenetal.,1994].acrucialpartinthis thesealevelrelativetothelandandsatellitealtimetryisadirectmeasurementofthesea Tidegaugeswillmeasure surface.thereforeaverticallandmotionofthetidegaugestationwill,besidesapossible driftinthealtimeter,alsoaectthedierencebetweenthosetwomeasurements. Verticallandmotionsattidegaugescanbecausedbylocaltectoniceects,sedimentation andinstability,orbyglacialisostaticadjustment(gia).duetomeltingoflargeareasof iceduringthelasticeage,theearthisnotinisostaticequilibrium,whichresultsina redistributionofmantlematerial,surfacewaterandthesurfaceitself. relativesealevelmeasuredattidegaugescanreach-12mm/yearsealevelriseatsites Theeectonthe 1

16 thatwereonceice-coveredandupto+2.5mm/yearinareasimmediatelyperipheralto theiceloadregions.theeectongeoidchangesmeasuredbythesatellitewillbebetween -0.1mm/yearand+1.0mm/year[MitrovicaandPeltier,1991]. thoseratesistheverticallandmotionatthetidegauges.sincedouglas[1991]estimated Thedierencebetween therateoftheglobalsealevelchangebasedontidegaugedatatobe+1.8mm/year,and Neremetal.[1997]calculatedaratebasedonsatellitedataof+2.3mm/year,theinuence ofgiashouldbeaccuratelymodeledandlteredfromthetidegaugedatainordertouse thenasabasisforthecalibrationofsatellitealtimetry. Thisresearchwillfocusontheeectoftheglacialisostaticadjustment(GIA)onthecalibrationprocedure.Themaingoalofthisreasearchcanbedescribedas: ImplementacalibrationalgorithmfortheTOPEX/Poseidonaltimeter,investigatetheeectofglacialisostaticadjustmentonthiscalibrationandmakean improvedestimationofthealtimeterdriftparameter. Inordertogiveanimprovedestimationofthedriftparameterthefollowingstepsare performed. Thecalibrationalgorithmcombinesthesetwodatasourcesbyselectionofsuitedcouples First,tidegaugedataandsatellitedataarecarefullyselectedandanalyzed. ofmeasurements,andcalculatesaglobaldierenceforeachtimeperiod.glacialisostatic adjustmentismodeledandbyvaryingseveralparametersanindicationoftherangeofthe eectonthecalibrationalgorithmisobtained.thecomputedgiaisthenimplementedin thecalibrationalgorithmandanestimationofthedriftparametercanbecalculated. Thepurposeofthisresearchisnottogiveanewestimationoftheglobalsealeveltrendfrom eithertidegaugeorsatellitedata,butpurelytoevaluatethealtimetercalibrationprocedure. Therefore,onlysatellitedatafromtheT/Pmissionisusedandallothersatellitemissionare notinvestigated.thetidegaugedataisusedfromtheworldoceancirculationexperiment database(woce);otherdatasourcesforbothmeasurementswillnotbetreated. modelingofgiaisonlydonetogiveanindicationoftherangeoftheeect. Theused computationtechniquesaswellasthedeglaciationandearth'srheologymodelsaretherefore takenfrompreviousresearchesbyotherauthors.thecharactericsandassumptionsforeach ofthesemodelswillbetreatedbutthecorrectnessandjustifcationfortheuseofthesemodels isnotprimarilythesubjectofthisresearch. Thestructureofthisreportisasfollows. dened. Furthermore,thevariousphysicalcausesoftheobservedsealevelvariationsare Inchapter2sealevelanditsvariationsare treated. scribedinchapter3and4,respectively.foreachofthesetwomeasurementtechniquesthe Themeasurementprinciplesofbothtidegaugesandsatellitealtimetryarede- obeservationprinciples,dataacquisition,errorsources,andtherecentestimationsofthe globalsealeveltrendaretreated.inthenexttwochaptersthetheoryofgiaisexplained. Inchapter5thecalculationofsealevelvariationsduetoachangeiniceloadisexplained. Inchapter6analgorithmforthecalculationofthesevariationsisdescribed,afterwhich thedeglaciationandearth'srheologymodelsaredenedand,forseveralvariationsofthese models,thegiaiscalculated. bycombiningtidegaugeandaltimeterdata. Inchapter7and8thealtimetercalibrationisperformed variousassumptionsaretreatedcomprehensively.chapter7describestheselectionofthe Twodierentapproachesareusedandthe tidegaugesandsatellitedata,whereaschapter8ismorefocussedontheestimationof thedriftparameterfromthetimeseriesdierences. implementedandanimprovedestimationofthedriftparameterisobtained. Finally,thecalculationsofGIAare sionsandrecommendationsofthisreseachcanbefoundinchapter9. Theacronymsand Theconclu- abbreviationsusedinthisreasearchcanbefoundinappendixa. 2

17 Chapter 2 Sea level variations Theannualmeansealevelisvaryingconstantlyandthesechangesarecausedbyanumber ofphysicalphenomena.beforecontributingsealevelvariationstothesedierentcauses,a cleardenitionofsealevelchangeisneeded.insection2.1thedistinctionbetweensealevel andseasurfaceheightismade.furthermore,relativesealevelchangeanditsinuenceis treated.insection2.2thephysicalcausesofsealevelvariationsaredescribedwiththefocus ontheirinuenceonrecentsealevelvariations. discussthosecomponentsindetail,onlyabriefdescriptionofthevariouscomponentswill Itisnotthepurposeofthisresearchto begiven.insection2.3someremarkswillbemadeaboutmeasuringsealevelvariations,a topicwhichwillbemoreextensivelydescribedinlaterchapters. 2.1 Deningrelativesealevel Inordertoinvestigatepresent-daysealevelchangeonehastomakeacleardistiction betweensealevelandseasurface. denedreferencepoint.satellitealtimetryisameasurementtechniquefromwhichthesea Seasurfaceisdenedastheheightoftheseaabovea surfaceheightcanbederived. respecttoalocalbenchmarkandisthereforeonlydenedattheboundaryofoceanand Sealevelisusuallydenedastheheightoftheseawith land.themostwellknowntechniqueformeasuringsealevelisatidegaugemeasurement. Thetidegaugebenchmarkisaverticalreferencepointonland,towhichthetidegauge isconnectedusingtraditionalsurveyingtechniques. periodandonaregularbasisthenameansealevelcanbedened.meansealevelisdened Ifsealevelismeasuredoveralonger assealevelaveragedoveraperiodoftimesuchasamonthorayear,longenoughthat uctuationscausedbywind,wavesandtidesarelargelyremoved.sincesealevelisdened asthedierencebetweenlandandsea,thesemeasurementsarerelativemeasurements. Changesinsealeveloccurduetochangesintheheightoftheseasurfaceorduetovertical movementsofthelandonwhichthebenchmarkissituated[emeryandaubery,1991].in generaltherearetwocausesoflandmovementsthatmightplayanimportantroleinsea levelmeasurements: isostaticadjustmentresultingfromslowviscousresponseofthemantletomeltingof largeicesheetsandtheadditionaloftheirmasstotheocean. tectoniclandmovements,bothrapidbyearthquakesandslowmovementsthrough mantleconvectionandsedimenttransportation. 3

18 Relativesealevelchangeisprimarilyaresultofanongoingclimatechange,sincethelast iceage. 120metersandonatimescaleofafewthousandyearssealevelshowsmajortemporaland Sincethislasticeage,about18.000yearsago,sealevelhasalreadyrisenover spatialvariations[chappellandshackleton,1986]. forthelast yearsisgiven,showingthesemayordierences. Ingure2.1thehistoryofsealevel sealevelisrisingforsuchalongtime,thepresentsealevelriseisperhapsnotdisturbing, Duetothefactthat butapossibleaccelerationofthissealevelrisecausedbythe'greenhouse-eect',certainly wouldbemoredisturbing.anaccurateestimationofthepresent-daysealevelriseandthe possibleaccelerationisthereforecrucial.besidestheprecisemeasurementofthissealevel change,theunderstandingofthephysicalcausescreatingthesealevelchangeisanother majorresearchtopic.inthenextsectionthevariousphysicalcausesofsealevelchangeare brieydescribed Sea level fluctuations over the past 160,000 years 0 Differences from - 40 present day sea level [meters] Years before present (x1000) Figure2.1:Historyofsealevelvariationsduringthelast years. Thesealevelvariations arebasedongeologicaldatafromnewguinea.asteepincreaseof120meterssincethelastglacial maximumapproximately18.000yearsagoisclearlyvisible[chappellandshackleton,1986]. 2.2 Physicalcausesofrelativesealevelchange Inthissectionseveralcausesofsealevelchangewillbediscussed. separatechangesinlandlevelfromchangesinleveloftheoceansurface,ataskthatcan Thechallengeisto beonlypartlysuccessfulgiventhebroadspectrumofprocesses. thevariouseects,attentionwillbegiventotheinuenceonpresent-daysealevelchange. Besidesadescriptionof Sincetheseprocessesareallcomplicatedandmuchresearchintheseareasisstillgoingon, thissectiononlyintendstogiveashortintroductioninthevarioustopicsandtogivean indicationoftheirmagnitude. Platetectonics Oneofthelong-timescaleeectsonsealevelisplatetectonics.Aspectssuchascollisions betweentwoormoreplates,changeintheaveragedepthoftheocean,changesinthe sizeofwaterandlandareas,andthedirectionofthepresentplatemotionwillaectsea level. Furthermore,changesinthespreadingrateatmid-oceanridgeswillinuencesea 4

19 levelvariations.asthespreadingratesincreasesmorecrustiscreated.thisextraoceanic crustwillleadtoanincreaseofthethicknessoftheoceanoorandthusadecreaseofthe averagewaterdepth.ingure2.2thevariousprocessesleadingtodeformationareshown, alongwiththetimeanddistancetheyinuencedeformationoftheearth.alloftheeects areveryslowintimeandtheinuenceontheannualsealevelratewillbequitesmall.for instance,themaximumrateofsealevelchangeduetochangesinthegeometryofoceanic ridgesisabout0.01mm/year[emeryandaubery,1991] Mantle convection Plate tectonics Time scale (years) Regional and local tectonic strain accumulation Earthquake displacements Glacial rebound Core motions Solid and ocean tides Atmospheric loading Length scale (km) Figure2.2:Thespace-timespectrumoftectonicprocessesleadingtodeformationoftheEarth.On thex-axisthedistanceuptowhichtheprocesseshaveeectisshown. They-axisshowsthetime scaleinyearsonwhichtheprocesseshaveeect.reproducedfrom[lambeck,1988]. Isostaticeects IsostaticeectsoccurbecausetheEarthtendstoremainingravitationalequilibrium. animbalanceoccurs,thecrustrisesorsinkstocorrectthisimbalance.suchanimbalance If couldbecausedbychangesintheearth'srotationaxisorbythemeltoficesheetsforcedby temperaturechanges.thiscorrectionwillresultinaglacialorhydro-isostaticadjustment. Theglacialisostasydealswithtimescalesof years,seealsogure2.2. these yearssmallerperiodscanbeidentiedthatarelinkedwithother,smaller Within phenomena.duringglacialloadingthecentralmassbelowtheicesheetdeformsdownwards whilethesurroundingarearisesduetomantleowandformstheso-calledperipheralbulge. Astheicesheetsmeltagainduringthedeglaciationperiod,thedepressedareareboundsand theperipheralbulgesinksagain.anothertypeofisostasythatwilloccurishydro-isostasy. Asglaciersmeltandmeltwaterowsintotheoceans,theoceanoorhasagreaterload onit. isostasyeectsformacomplexsystemofrelativesealevelchangesdependingonboththe ThiswillalsoresultintoaredistributionoftheEarth'sinterior. Altogetherthese timehistoryofloadingandunloadingandthedistancefromicesheettoshoreline. wholeprocessiscalledglacialisostaticadjustment(gia).althoughthemeltingofthe This icesheetshasalreadynishedmorethen5000yearsago,inlargepartsofcanadaand Scandinaviathisreboundingisstillanongoingprocess,andiscalledpost-glacialrebound (PGR). 5

20 Thermalexpansionofoceanwater Oneofthemajorcontributorstoglobalsealevelchangeisthermalexpansionoftheoceans. Thermalexpansionoccurswhenwateroftheoceanwarmsup,resultinginadecreaseofthe waterdensityandthusavolumeincrease.thisphenomenonoccursatalltemperaturesand resultsinwhatisknownasthestericsealevelrise.theredistributionofwaterasaresult ofthissealevelchangewilltakemanydecadestoevencenturiestoarriveatitsnalstate. Overthelastcenturythemeanseasurfacetemperaturehasincreasedbyapproximately whichissurroundedwithalotofdiscussionanduncertainties.levitusetal.[2000]suggests 0:5oC.Therateatwhichtheseasurfacerisesasaresultfromthermalexpansionisatopic aglobaltrendintheorderof0:3 0:7mm/year. GlaciersandIce-caps Alltheexistingglaciersintheworld,excludingtheGreenlandandAntarticaareascovera totalareaof km2.togetherwiththeicecapstheseglacierscontainwaterthatis equivalentto0.5meterofglobalsealevel[churchetal.,2001].thepossibleeectofmelting iceofgreenlandandantarcticamightbeevengreater.glaciershaveasmallereectonthe globalsealevelthangreenlandandantarcticabutaremoresensitivetoclimatechange. Rapidchangesinmassarepossibleandthereforeglacierscanplayanimportantrolein sealevelchange. changeisknown. Unfortunatelyonlyforasmallnumberofglaciersthepresentrateof indicatethatglaciersandicecapsaremetingintherecentpast,resultinginacontribution Observationsofthemassbalanceofglaciersandtemperaturechanges totheglobalsealevelof0:2 2001]. TheGreenlandandAntarcticaicesheetscontainenoughwatertoraisetheglobal 0:4mm/yearoverthelasthundredyears[Church et al., sealevelbyalmost70m.evenasmallchangeofoneofthemwouldhaveasignicanteect. Estimationsfortheyears suggestacontributiontoglobalsealevelrisebetween +80and+100mmforGreenlandand Cryosat,plannedtobelaunchedattheendof2004,shouldgainmoreinsightintothe 80mmforAntarctica. Thesatellitemissionof behavioroficecapsandglaciers. Contributionofthevariouseects Apartfromthelargereectsmentionedinthepreviousparagraphs,someminoreects needtobementionedhere.oneoftheseeectsissurfaceandgroundwaterstorage.this includeseectsthathavetodowiththeamountofwaterstoredintheground,onthe surfaceinlakesandreservoirs,andchangesinsurfacecharacteristicswhicheectruno orevatransportation. eectofthesevariouseectscanbeofeithersign,andtherateoftheindividualeectshas Eachoftheseeectsmaynotbeconsiderableinsize,butthenet- increasedoverthelastdecade. localoodsandchangesinthesalinityoftheocean. OthersmallereectsareclimaticeectssuchasElNi~no, Theinuenceofthemajoreectscanbeseenintable2.1.Fromthisoverviewthedistinction canbemadebetweenprocesseswithacauserelatedtoarecentclimatechangeandprocesses asgiathatareaectingrelativesealevelforoverthousandyears.theintergovernmental PanelonClimateChange(IPCC)calculatedforseveralscenarios,theimpactofpollution ofgreenhousegasses,industryandriseoftemperatureontheglobalsealevelforthenext centrury.duetothegreatuncertaintiesintheeectofthevariouscomponentsfromtable 2.1thepredictionsvarybetween+0:09and+0:81meterforthenextcentury[Church betterunderstood. al.,2001]. Inordertoimprovethesepredictionsthevariouscomponentsneedtobe importantrole. Foroveracenturytidegaugemeasurementsaremadeandfurthermore Forthispurpose,precisemeasurementsofsealevelvariationsplayan 6

21 duringthelasttenyearssatellitemeasurementshavebecomeaccurateenoughtoprovide anothersourceofseasurfacemeasurements. Table2.1:Estimatedcontributionofvariouscomponentsonglobalsealevelvariationsinmm/year. Minimumandmaximumratesoftheinuenceiscalculatedfrombothobservationsandmodelsand averagedovertheperiod1910to1990[churchetal.,2001]. Component ThermalExpansion 0.3mm/yr Minimum 0.7mm/yr Maximum GlaciersandIcecaps Greenland Antartica GlacialIsostaticAdjustment Terrestrialstorage Total -0.8mm/yr mm/yr 7

22 8

23 Chapter 3 Tide gauge measurements Sealevelanditsvariationshavebeenmeasuredforovertwocenturiesusingtidegauges. Therstsealevelmeasurementsweremadeinordertopredicttidalmovementsinharbor areas. andinscandinaviain1704. TherstrecordedmeasurementsintheNetherlandsweredoneasearlyas1682, tidegaugesitselfandthemeasurementaccuracyhavechanged.insection3.1thehistorical Itisobviousthatsincethestartofthesemeasurementsthe changesinmeasurementtechniquesarediscussed.insection3.2thecollectingoftidegauge databythepermanentserviceformeansealevel(psmsl)istreated. recentestimationsoftheglobalsealeveltrendaretreatedandputinperspective. Insection Tidegaugemeasurementtechniques Tidegaugemeasurementsarewidelyusedduringthelasttwocenturiestoobtainina systematicmannerinformationaboutsealevel. types:themostusedtypeoftidegaugesisthetidepole,whichafterseveralimprovements Tidegaugescanbedividedintoseveral iscalledstillingwell.othertypesoftidegaugesarebasedonmeasurementprinciplessuch aspressureorreectionofthesurface.inthissectionaglobaloverviewofthevarioustypes oftidegaugesisgiven,largelybasedonpugh[1987]andvanonselen[2001]. Stillingwellwithoatsystem Oneoftherstsystemsformeasuringsealevelvariationsconsistedofaso-calledtidepole, verticallymountedontheareaofinterest. heightrelativetothispolecanbereadatregularintervals.thetidepoleisconnectedto Atthetidepoletheinstantaneoussealevel apermanentbenchmark,theso-calledtidegaugebenchmark. markedpointlocatedonastablesurfacesuchasexposedrock,aquaywallorasubstantial Abenchmarkisaclearly building. toinstallandcanbeusedalmostanywhere. Theadvantageofthistidepolesystemisthatitischeapandrelativesimple harbors.nowadaysthistidepoleisusedonlyasasimplemethodtocheckthereadingsof Thereforemanytidepolesareintegratedin othermoresophisticatedtidegaugesystems.theprecisionofthetidepolemeasurements isdeterminedbyarandomreadingerrorofapproximately2cm,systematicerrorsofthe tidepoleitselfortheenvironment,andblundersinthereadingorregistrationoftheheight bytheoperator. 9

24 Themostproblematicpartofthetidepoleisthereadingofthesealevel.Theminimization ofthisreadingproblemisreachedbysupplyingthetidegaugewithastillingwell.astilling wellisatransparenttubeverticallyplacedalongsidethetidepoleinthewater,longenough tocoveranypossiblerangeoftidesattheparticularsite.thebottomofthewellisclosed exceptforanarrowoutletforthewatertoowinandout.thecompletesystemistherefore calledatidepolewithoatinstillingtube. environmentconditions,suchaswind,andthesmalldiameteroftheinletprovidesalter Thestillingtubeformsaprotectionagainst ofhighfrequencymovementsofwaves. structureofpole,mechanicalproblemssuchasthefrictionoftheoatwithstillingtube, Theaccuracyofthissystemisdependingonthe dierenceintemperatureandsalinitybetweenwaterinthetubeandopensea,andthe visibilityofthereading[vanonselen,2001]. Asecondimprovementtothetraditionallytidepoleistheautomaticrecordingofsealevel. Bothprevioussystemsrequiredthatapersonmadetheobservationsofthesealevelatthe tidegauge. observationswithanhourlyfrequencyorevenmorefrequentmeasurementswereimpossible. Apartfromthesystematicalerrorsintroducedbymanuallyreadingsealevel, Sincethemid-nineteenthcenturyself-recordingtidegaugeswereoperated.Themechanical recordingprovidedaconstantmeasuringdevicebutintroducedalsoadeviationinthe measurementscausedbychangesinthedrumorpaper. recordingdeviceisexpensiveandmorediculttoinstall.ingure3.1thecompletesystem Furthermoresuchamechanical ofamechanicalrecordingstillingwelltidegaugeisshown. singlemeasurementusingastillingtubetidegaugewillnotbebetterthanabout5cm. Thestandarddeviationofa Hourlymeasurementsgeneratedfromamorefrequentsignalarecapableofreachingan accuracyof1or2cm,whereasdailymeanvalueswillhaveastandarddeviationinthe orderof1mmbyreducingrandomerrors[vanonselen,2001]. Figure3.1:Mechanicalrecordingtidepolewithoatinstillingtube[Pugh,1987]. 10

25 Acousticreectiontidegauges Furtherimprovementintheaccuracyofthestillingwelltidegaugecanbeestablishedwhen theoatofthesystemisreplacedbyaremotesensor. thatnophysicalcontactwiththeactualseasurfaceisnecessary. Theadvantageofthissystemis measuredbymeanofthetimetakenbyanacousticpulsetotravelbetweentheacoustic Thesealevelheightis sensor,theinstantaneousseasurfaceandback.fromthistwo-waytraveltimetheheightof theacousticsensorrelativetothetidegaugebenchmarkcanbedetermined.theaccuracy forthemeasurementtechniqueis1-2mmafteraveringthemeasurementsover6minutes [Pugh,1987]. Subsurfacepressuretidegauges Insteadofmeasuringthesealeveldirectly,analternativeapproachistomeasurethepressure ofaxedpointbelowtheseasurface.thispressurecanbeconvertedintoasealevelusing thebasichydrostaticrelationship;themeasuredpressureistheatmosphericpressureacting onseasurfacepluspressureofwatercolumnabovethesensor.sincethepressurecannotbe measuredpreciselyattheseasurface,themeasuredatmosphericpressurehastobereduced tothesurface.therelationshipbetweenthemeasuredwaterpressureandtheseasurface heightbecomes P=Pa+agha+wgD (3.1) wherepisthemeasuredpressure,patheatmosphericpressureatheighthaabovethesea anddthedepthofwatercolumnabovethesensor.ingure3.2themeasurementprinciple surface,wandathedensitiesofwaterandair,respectivily,gthegravitationalconstant ofapressuretidegaugeisshown. waterpressure,thedensityofwaterandthegravitationalacceleration. Thelargestcontributorstotheerrorarethemeasured thismeasurementsystemisabout1cm[pugh,1987].theadvanceisthatthetechniqueis Theaccuracyof convenienttouse,butitisdiculttocoupletothetidegaugebenchmark. P A Tide Gauge Station ρ g h a a ρ g D w Figure3.2:Subsurfacepressuretidegauge. levelmeasurementusingthebasichydrostaticrelationship.thepressureistheatmosphericpressure Thismeasuredpressurecanbeconvertedintoasea actingonseasurfacepluspressureofwatercolumnabovethesensor. Formeasurementsatlongdistancesfromthecoast,usuallyapressuresensoratthebottom oftheoceanispreferred,whichmeasurespressureofcolumnabove.atthismomentthere arenomethodsdevelopedtotransmitthedata,soafterayearthepressuresensorhas 11 P

26 toberemovedagain. time. Pressuresensorscanbeusedupto4000metersbelowtheseasurfaceandmeasure Thismeansthatitcanonlydealwitharelativeshortperiodsof thesealevelwithanaccuracyof1cm.theaccuracyismainlyaectedbytheestimation oftheatmosphericpressure,rapidchangesinsalinityinthewater,andthemovementof measurementdevice[vanonselen,2001]. 3.2 PermanentServiceforMeanSeaLevel Theglobaldatabaseforlong-termsealevelchangeinformationfromtidegaugesisknown asthepermanentserviceformeansealevel(psmsl).since1933,thepsmslhasbeen responsibleforthecollection,publication,analysisandinterpretationofsealeveldatafrom theglobalnetworkoftidegauges.aroundtheworldover1800tidegaugestationscollector havecoolectedsealeveldataonaregularbasis.thecompletedatasetatpsmslconsistof almost49.000station-yearsofmonthlyandannualmeanvaluesofsealevel[psmsl-website, 2003].Theoldesttimeseries,theoneofBrestinFrance,startsasearlyas1807.Although oldermeasurementsareavailable,thequalityofthesemeasurementsisnotgoodenoughto meetthepsmslstandardsandthesetidegaugestationsaretherefornotadmitedinthe PSMSL-database.Ingure3.3anoverviewofallthePSMSLtidegaugestationsaroundthe worldisgiven.thepsmsl-databasecontainstwodierenttypesofmeasurements,called metricdataandrevisedlocalreferencedata(rlr).themetricdataisdemeasurementthat isactuallydoneatthetidegaugeitself.thismeanspracticallythatthismeasurementisthe dierencebetweenthesealevelandthetidegaugebenchmark,whichisunknowninheight relativetoaglobalreference. themonthlyandannualmeanshavetobereducedtoacommondatum,whichresultsin Inordertoanalyzetimeseriesofsealevelmeasurements, derlr-dataset. sealevel,anarbitrarychoicemadeinordertoavoidnegativenumbersintheresulting TheRLRdatumateachstationisdenedtobe7000mmbelowmean RLRmonthlyandannualmeanvalues.Thedetailedrelationshipsateachsitebetweenthe RLRdatum,thebenchmarkheightsandthetidegaugezeroarenotrequiredbyanalysis ofthedataset.thisrlr-databaseisthebasisforalmosteveryresearchinglobalsealevel variationestimation. locatedinthenetherlandsarestoredinthepsmsl-database.thisisbecausethedutch, Fromgure3.3canbeseenthatnoneofthetidegaugethatare alongwiththegermanrecordsaremeasuredrelativetonationallevellingsytems,normaal AmsterdamschPeil(NAP)andNormalNull(NN),respectively.Ingeneral,measurements thataremaderelativetonationallevellingsystemsmightcontaindatumshifts,which explainswhythepsmslrefusestoobtainthiskindofmeasurements[psmsl-website, 2003]. Formorethan500tidegaugestationsthemonthlyandannualmeansealevelrecordsare storedinthepsmsl-database. valuesforthetidegaugestationinsanfrancisco.inthisguretherelativesealevelsare Figure3.4displaystheRLRmonthlyandannualmean shown,whichmeansthattheoriginisarbitraryandthatchangesinwaterlevelcannotbe distinguishedfromverticalmovementsoftheland.ontheleft,themonthlymeansealevels displayanupwardtrendofafewmillimetersperyear,withanadditionalgeophysicalnoise ofabout100mm.ontheright,theannualmeansealeveliscalculated,andastraightline isttedthoughthedatausingasimpleleastsquaresestimation. Ingure3.5thetrendofsealevelatSanFranciscofora50-yearslidingwindowisshown. Eachpointinthegurerepresentsthetrendofsealevelinmillimetersperyearcomputed fortheentireperiodoftheprecedingandfollowing25years. between 0:7and+2:6mm/yearandevenchangessignatseveralpoints. Theestimatedtrendvaries arbitrarilyselected50-yeartrendofsealevelatsanfranciscodoesnotreecttheunderlying Obviouslyan trend.thelong-termsealeveltrendof+1.42mm/yearisfarbelowthevalueofthetrend 12

27 Figure3.3:Locationoftidegaugestationsfromwhichrevisedlocalreference(RLR)recordsare available. Allofthedatafromthesemorethan500tidegaugesareavailablefromthePermanent ServiceforMeanSeaLevel(PSMSL)[Douglas,1991]. estimatedfromthelast80yearsofdata.possibleexplanationsfortheselargevaraiationsin long-termestimationsaretheinterdecanalsignalsintheseasurface,temporalorremaining changesinthelocalandglobalcurrentpatterns,ortemporallocaldistortionsatthetide gauge. 13

28 7400 Montly mean sea level at San Francisco 7250 Annual mean sea level at San Francisco Estimated trend = 1.42 mm/year Height [mm] 7100 Height [mm] Year Year Figure3.4:ThemonthlyandannualmeansealevelsatthetidegaugestationofSanFrancisco. Fromthemontlymeansealevel(ontheleft)theglobalsealeveltrendishardlyseen,buttheannual meansealevel(ontheright)showsaclearupwardtrendop+1.42mm/yearoverthe140yearsof data.dataisobtainedfromthepsmsl-website[2003]. 3.0 Estimation of the long-term sea level trend at San Francisco based at 50-year of tide gauge data Calculated trend [mm/year] Year Figure 3.5:Fifty-yeartrendsofsealevelatSanFrancisco. Eachpointrepresentsthesealevel trendin[mm/year]calculatedfortheperiodoftheprecedingandfollowing25years.afterdouglas [2001]. Forseveraltidegaugestationstheannualmeansealevelisshowningure3.6,withonthe righthandside,theestimatedlongtermsealeveltrend.itisclearthatthetrendsestimated forstockholmandoslodierfromtheotherestimatedtrends,sincetheestimatedtrend innegative. glacialisostaticadjustment(gia):thetidegaugestationsaremeasuringanincreaseinsea ThereasonisthatbothStockholmandOsloareinuencedbytheeectof levelaswellasaverticallandmotion.thetrendofthelandupliftisatthesetwostations largerthenthesealevelrise,thusthetidegaugeisactuallymeasuringarelativesealevel decrease. stationswithlargelylocalorregionaleects. Inthenextsectionmoreattentionwillbegiventothehandlingoftidegauge 14

29 9000 Oslo Long term sea level trend [mm/year] 8500 Stockholm ± ± 0.16 Sea level (mm) San Fransisco Pensacola Key West Honolulu 1.42 ± ± ± ± Brest 0.98 ± Cascais 0.97 ± Year Figure 3.6:Theannualmeansealevelsforseveraltidegaugestations,basedondatafromthe PSMSLdatabase.Ontherighthandside,theestimatedlongtermsealeveltrendisshown. 3.3 Estimationofglobalsealeveltrend Intheprevioussectionwasshownthatestimatingasealeveltrendfromasingletidegauge stationmightbedicult,seegure3.5.theestimationofagloballong-termtrendfrom theavailabletidegaugesisevenmoredicult,duetothepoordistributionofthetide gauges.since1980,13valuesoftheglobalsealeveltrendhavebeendeterminedfromtide gaugedata,withvaluesintherangeof+1.15upto3.0mm/year.someoftheseresearches arebasedonaglobaldataset,othertrytogiveanestimationforalocalregion.inallthese datatwodierentkindsofgeophysicalnoisecanbeidentied,verticalcrustalmovement andlowfrequentvariationsofthesealevel. sealevelrisebasedonaglobaldatasetaregiven. Intable3.1thedeterminationsoftheglobal Thevaluesintable3.1areallcalculatedfromthesamedatafromthetidegaugedatabase atpsmsl.thedierencesbetweenthevariousestimationscanthereforenotbeexplained bythetidegaugedataorthemeasurementaccuracy,butthedierenceoccurbasedin themethodoftheresearchersthemselves.thedierentwaytodealwiththetwotypesof geophysicalnoise,verticalcrustalmotionsandlowfrequentvariationsofsealevel,determine theestimatedglobalsealevelrate.peltierandtushingham[1989,1991],trupinandwahr 15

30 [1990],andDouglas[1991,1997]allusedtheICE-3GdeglaciationmodelandthesameEarth model.dierencesinthetrendarecausedbytheselectionandnumberoftidegaugesbased onrecordlength. Lambeck[1990]performedasphericalharmonicdecompositionofthetidegaugerecordsan Forinstance,Douglas[1991]usedonly25tidegauges. Nakibogluand tookthezero-degreetermastheglobalaveragerate. withrecordlengthsasshortas10yearsandstatedthatthegiasignalwouldmakelittle Theyused665tidegaugestations contributiontotheestimatedtrend.theresultsfrompeltierandjiang[1997]weremade basedonanimproveddeglaciationmodel(ice-4g)andanimprovedearth'srheology model. Table researchesthatwerebasedonaglobaldatasetaregiven. 3.1:Recentdeterminationsoftheglobalsealeveltrendbasedontidegaugedata. Only Researcharticle(year) PeltierandTushingham[1989,1991] Estimatedtrend NakibogluandLambeck[1990] TrupinandWahr[1990] Douglas[1991,1997] 0.13 PeltierandJiang[1997] SincethetrendestimatedbyDouglas[1991]isprobablythemostfrequentlyreferredto, somemoreattentionwillbegiventotheselectioncriteriausedinthisresearch.inshort: theselectionoftidegaugeswasbasedontherecordlength,thecompletenessofthetide gaugerecord,andingeneralagreementwithnearbystationtoremovetectonicinuenced stations.furthermore,theice-3gmodelfortheglacialisostaticadjustmentwasusedto removethissignal,andstationsinareasthatwereoncecoveredwithicearenotusedinthe estimation. Length,completenessanddistributionoftidegauges Selectionoftidegaugesbasedonrecordlengthandcompletenesshaseverythingtodowith thelowfrequentvariationsinsealevel.timeseriesofthetidegaugesshow,asseeninthe previoussection,variousperiodicsignals.thelargestofthesignalsthathaveasignicant impactonthetimeserieswillhaveaperiodofapproximately10years.thus,inorderto beabletopredictlong-termsealevelchange,tidegaugedataforaleastseveraldecennia hastobeavailable.althoughgure3.3showsover500stations,thisgivesatoooptimistic lookattheactualsituationoftidegaugemeasurements.althoughtheoldeststationshave recordsofalmost200datayears,mostofthetidegaugestationsareonlyequippedlate afterworldwariiandthusreachabout30-40yearsofdata. Ingure3.7thelengthofthetidegaugesdatasetisvisualized. moststationsonlyhaveaverylimitedamountofdata;80%ofthetimeseriesislessthen Thisgureshowsthat 60yearslong. morethan50yearsofdataisfarlessevenlydistributedthatthecompletedatasetwas,as Besidesthelengthofthedataset,thedistributionofthetidegaugeswith seeningure3.3.almostallstationswithlongertimeseriesarelocatedonthenorthern HemisphereandespeciallyinnorthwesternEuropeandtheeastcoastofNorthAmerica. Tidegaugeswereinitiatedintheselocationsbecausetheevidenceofarelativesealevel changewasclearintheseregions.themeltingoficesheetsincanadaandscandinaviahas adirectandvisualeectontherelativesealevelbyanupliftofthecontinentalcrust.on theotherhand,moststationsinthepacicoceanhaveonlybeenactivesincethe70s,due toagrowinginterestinglobalsealevelchange. 16

31 % % Number of records % Cumulative percentage % More Record length in years Figure3.7:RecordlengthofthetidegaugeinthedatabaseofthePermanentServiceforMeanSea Level.Morethen80%ofthetidegaugetimeseriescontainlessthen50yearsofdatda,whichmake areliableestimationoftheglobalsealevelchangedicult[psmsl-website,2003]. Inordertodealproperlywiththelowfrequentvariationsinsealevel,Douglas[1991]selected onlytidegaugeswithalengthof50yearsandlonger.suchlonglengthisnecessaryinorder todealwiththelowfrequencyvariationswithperiodsuptoadecade.inasecondpaper Douglas[1997]extendedthisrecordlengthevento70years. longerthen50years,douglasfurthermoreonlyselectedtidegaugeforwhichthetimeseries Besidesarecordlengthof wasmorethan80%complete. Verticalcrustalmotions Verticalcrustalmotionsmightoccurduetovariousreasons.Thethreemostcommonforms ofverticalcrustalmotionarecausedbyplatetectonics,glacialisostaticadjustmentoralocal landmotionsuchassedimentationoraweakunderground.thesephenomenawerealready mentionedinchapter2andwillthereforenotbefurtherexplained.anexampleofaregion wereplatetectonicsplaysasignicantroleisindia. andeurasianplateleadstosignicantverticalchangesthatcorruptsealevelmeasurements ThecollisionoftheIndo-Australian attheselocations. estimated.thetableshowslargedierencesintheestimatedtrend,whichcanobviousnot Intable3.2thelong-termsealeveltrendforvestationsinIndiais beexplainedwithpuresealevelrise.thestationstimeseriesshowlargeuctuationsand irregularpatterns,relatedtolocaltectoniceects.otherregionswhereplatetectonicswill polutethetidegaugemeasurementarecalifornia,japan,indonesiaandalaska. [1991]statedthatinordertogetarealiableestimationoftheglobalsealeveltrend,onlytide Douglas gaugestationsthatwerefreeofverticalcrustalmovementduetoplatetectonicsshouldbe selected.thereforealltidegaugestationslocatedatcollisionortectonicplateboundaries wereexcluded. 17

32 Table3.2:Determinationsofrelativesealevelchangefromtidegaugedataavailablefortheregion ofindia. shoslargedierencesthatarecausedbylocaltectoniceects. Althoughalltidegaugesarelocatedrelativelyclosetoeachothertheestimatedtrends Cochin Tidegaugestation Latitude 9.97N Longitude 76.25E Estimatedtrend Vishakhapatnam 17.68N 83.28E 2.2mm/year Bombay 18.92N 72.83E Saugor/Sugar Calucutta 21.65N 22.55N 88.05E 88.30E Theeectofglacialisostaticadjustment(GIA)ontheestimationoftheseasurfacechange wasalreadymentionedinchapter2.theeectontidegaugemeasurementisevenlarger sinceinthiscasetherelativesealevelchangeisimportant. betweentherelativesealevelchange,radialdisplacementoftheoceanoorandthegeoid Moreabouttherelationship changewillbegiveninchapter5.hereitissucienttomentionthattherelativesealevel changecanbeupto-12mm/yearforregionsthatwereonceglaciated.anexamplecould alreadybeseeningure3.6,werethetidegaugestationofstockholmmeasuredalong termsealeveltrendof-11.3mm/year.althoughtheeectofgiacanbemodeledtherate oftherelativesealevelchangeintheregionsthatwereoncecoveredwithiceisuptove timesaslargeastheglobalsealevelrate. thatwerecoveredbyiceduringlastglacialmaximumarealsoexcluded. Thereforestationsthatarelocatedinregions Inordertoexcludetidegaugestationswhicharesueringfromotherlocalverticalland motion,suchassedimentation,harbordevelopment,andinstabilityandweakunderground, anothercriteriaisthatthecalculatedrateforthetidegaugestationshouldhaveareasonable agreementatlowfrequencieswithrecordingsfromnearlygauges[douglas,1991]. Selectingandgroupingthetidegauges Usingtheselectioncriteriaoflength,completeness,tectonicactivities,GIAandlocalland motion,douglasselected21tidegauges,fromtheoriginalalmost500.foreachtidegauge stationthetrendcalculatedfromthepsmsldatabasewascorrectedfortheeectofgia. Inthenextsteptheselectedtidegaugesaregroupedaccordingtheiroceanicregionand foreachregionthemeansealevelchangewascalculated. changeestimationsinthelaststepaglobalmeansealevelchangeiscalculated,andthe Usingtheseregionalsealevel valueof+1.8mm/yearsealevelchangeisobtained. 21tidegaugestationsandtheircalculatedtrendsisshown. InappendixB,thetablewiththe gaugesfromdouglas[1991]alsotheresultsfromdouglas[1997]anddouglas[2001]are Alongwiththeselectedtide shown.itcanbeseenthatdouglasselecteddierentstationsforeachpapers,andevenfor tidegaugesstationsthatrecurred,dierenttrendswereestimateddependingonthetime periodthatwasselected. Duetothefactthattheselectioncriteriaareverystrict,theglobalsealevelestimation isnowbasedononly21tidegaugerecords. willonlygiveanestimationsofthesealevelvariations,andwillnotbeabletogivemore Itisclearthatthisglobalmeanestimation insightininterannualphenomenasuchaselni~noorchangesincurrentpatterns.further understandingofthevariousphysicalcausesofthisobservedsealevelchangecannotbe accomplishedwiththeuseoftidegaugedataonly;thecoverageofthestationsispoor andtheverticallandmotionhavealargeimpactontheresults. quantifyofthecontributionofoceanthermalexpansion,behaviorofglobalicereservoirs Inordertobeableto 18