SurfacemeltdominatesAlaskaglaciermassbalanceC.F.Larsen,E.Burgess,A.A.A - PDF document

SurfacemeltdominatesAlaskaglaciermassbalanceC.F.Larsen,E.Burgess,A.A.Arendt,S.O,A.J.Johnson,andC.KienholzGeophysicalInstitute,UniversityofAlaskaFairbanks,Fairbanks,Alaska,USA,AlaskaScienceCenter,U.S.GeologicalSurvey,Anchorage,Alaska,USA,PolarScienceCenter,AppliedPhysicsLaboratory,UniversityofWashington,Seattle,Washington,USAMountainglacierscompriseasmallandwidelydistributedfractionoftheworldsterrestrialice,yettheirrapidlossespresentlydrivealargepercentageofthecryospherescontributiontosealevelrise. LARSENETAL.ALASKAGLACIERMASSBALANCE5902 PUBLICATION GeophysicalResearchLettersRESEARCHLETTERKeyPoints:Alaskamassbalanceis75±11/16Gtyr1(1994 rangeoftheentireAlaskaregion(FigureS6).Ourdatadonotincludeanyglacierswithareas3km,whichaccountfor~16%ofAlaskaglacierarea(FigureS5).Thisomissionhaspotentialtobiasourregionalmassbalanceestimateaswediscussinsection4.Oursurveysarescheduledclosetotheannualmassmaximuminspringortheannualmassminimuminautumn,whentherateofseasonalmasschangeisnearzero.Repeatsurveysoccurwithinanaverageof8daysoftheprevioussurveydate(themaximumis30calendardays).Intervalsbetweenrepeatsurveysusedhereinarerequiredtobeaminimumof5years,withanaverageof10years(FigureS1).Theintervalsweusearecenteredon2008(FigureS2),owingtoasteadyincreaseinthenumberofsurveysperformedannually.Weincorporateonlythelongestintervalavailableforeachglacierinouranalysis,asthisminimizestheimpactsofseasonalelevationchange,interannualvariability,anduncertaintiesassociatedwithsnowandrndensication[,2013].Weestimatethetotalmassbalance(,expressedasanaverageannualmassbalancerateinwaterequivalentunits)of116glaciers(TableS1),representing41%ofAlaskasglacierizedarea.Weintegratethemeasuredratesofcenterlineelevationchangeovereachsurveyedglaciershypsometry[Johnsonetal2013]usingtheRandolphGlacierInventory(RGI;[Pfefferetal.,2014;Kienholzetal.,2015]).Wemitigatepotentialbiasesresultingfromtheuseofcenterlineelevationchangesasrepresentativeofglacier-widechanges[Berthieretal.,2010]followingJohnsonetal.[2013](seethesupportinginformation).Volumechangeisconvertedtotheglacier-widemassbalance()usingaconstantdensityof850kgmtheassumptionofSorgeslaw[,2013;Johnsonetal.,2013].Oursurveyscover18tidewaterglaciers,accountingfor81%ofAlaskastidewaterglacierarea,and32lake-terminatingglaciers,representing68%ofthetotallake-terminatingglacierarea.Weextrapolatetounsurveyedglaciersaftersubdividingtheelevationchangeobservationsbyterminustype(land-,lake-,andtidewater-terminating)andthendeterminingthemeanelevationchangeprolesforthesethreedynamicalclasses(Figure2).Wechoosetoparameterizeelevationchangeasafunctionofelevationbecauseofthedominantcontrolofelevation-dependentclimategradientsonmassbalancedistribution.Otherregionalizationapproachesemphasizetheroleofglaciergeometryasquantiedbysurfaceareaorslopeincontrollingthecollectiveresponseofglacierstoclimate[Harrison,2013;etal.,2015],butthewayinwhichtheseapproachesapplytoextrapolationofaltimetrydatahasnotyetbeenexplored. Figure1.Estimatedmassbalance(19942013)forsurveyedandunsurveyedglaciersinthemostdenselyglacierizedsubregionofAlaska.Theinsetshowstheentireregion.Blacklinesindicatesurveyightlines. GeophysicalResearchLetters10.1002/2015GL064349 LARSENETAL.ALASKAGLACIERMASSBALANCE5903 Toconstructthemeanelevationchangelesforeachdynamicalclassofglacier(Figure2),wenormalizeeachglacierelevationchangeprolebythemaximumandminimumelevationofthatglacier:wherearetheelevationsoftheglacierterminusandhead[Johnsonetal.,2013].Thisnormalizationallowscomparisonandaveragingofelevationchangesacrossallglaciersizesandelevationdistributions[Johnsonetal.,2013].UsingthenonparametricMann-Kendalltest,wendnostatisticallysignicantrelationshipbetweenglacierminimumglacierelevationandmassbalance(=0.78).However,wedoobserveacharacteristicpatternofeleva-tionchangesovereachglacierselevationrange.Theseobservationsencouragestack-ingofthenormalizedelevationchangelestocharacterizeaverageelevationchanges(FiguresS8S10inthesupportinginformation).Priortoaveragingtheproles,aterminuscorrectionisappliedforallretreatingglaciers(FigureS11).WeomitColumbiaGlacier,YakutatIceeldglaciers,andallsurge-typeglacierswhenderivingmeanprolestominimizetheinuenceofanomalousdynamicsontheextrapolation.OurextrapolationextendstoallglaciersintheAlaskaregionofversion4.0oftheRGIIKienholzetal.,2015].Weestimatethemassbalanceofeachunmeasuredglacierby(i)selectingthesoutlinefromtheRGIinventoryinventoryKienholzetal.,2015],(ii)choosingthenor-malizedmeanelevationchangeproassociatedwiththatglaciersdynamicaltype,(iii)extractingthehypsometryforthatglacierandnormalizingitbyelevationasdenedabove,and(iv)integratingthemeanelevationchangeproleoverthenormalizedglacierhypsometry.Wetreateachglacierindividuallyinthisextrapola-tion,avoidingtheuseofaggregatehyp-sometriesofunsurveyedglacierareaaswasdoneinpreviousstudies[etal.,2013;Dasetal.,2014].ThistreatmentallowsustoproperlyaccountforindividualglaciergeometriesandthedistributionofelevationchangesalongtheirproTheimportanceofdoingthissummationcorrectlyovertheextrapolatedareais Figure2.Surveyedcenterlineelevationchangerateasafunctionofnormalizedelevationwherezeroistheglacierterminusandonetheglacierhead(greylines).Thickandthincoloredlinesrepresentmeanelevationchangeandstandarderrorofthemean,respectively.Shadedareasindicate1variability.Horizontalboxplotsshowthedistributionofintervallengths,includingthemedian(black),interquartilerange(boxsize),and595percentilesoftheintervals.(a)Land-terminatingglaciers.DashedlineisGulkanaGlacier,anddotdashisWolverineGlacier.(b)Lake-terminatingglaciers.DashedlineisYakutatGlacier.(c)Tidewaterglaciers.DashedlineisColumbiaGlacier,anddot-dashisHubbardGlacier.TheexpandedaxisforFigure2crelativetoFigures2aand2bisrequiredtocapturevariabilityneartheterminioftheseglaciers.TherangeofFigures2aand2bisindicatedwiththegreyshadinginFigure2c. GeophysicalResearchLetters10.1002/2015GL064349 LARSENETAL.ALASKAGLACIERMASSBALANCE5904 cant.Ifweinsteadintegratethemeanprolesforland-,lake-,andtidewater-terminatingglaciersoveraggregatehypsometriesofunsurveyedareasofeachrespectiveglaciertype,thenthemassbalanceoftheAlaskaregionisoverestimatedby41%.Subdividingtheunsurveyedareaintosubregionswithsimilarhyp-sometriesaswasdoneinpreviousstudies[Johnsonetal.,2013;Dasetal.,2014]likelyreducesuncertaintiesfromusingaggregatehypsometriesbuttowhatdegreeisunclear.3.ResultsOuranalysisyieldsamassbalanceof75±11GtyrfortheAlaskaregion(Figure3).Thetidewaterglaciermassbalanceis5±3Gtyr.ColumbiaGlaciercontributes4±0.3Gtyr,andtheremaining48tidewaterglaciersinAlaskahaveanestimatedmassbalanceofonly1±3Gtyr.Lake-terminatingglacierscomprise20%oftheAlaskaglacierareaandcontribute24%tothetotalmassloss(17±2Gtyr).Theremaining70%ofAlaskaglaciermasslossisattributabletoland-terminatingglaciers(53±6Gtyr),whichtendtoshowsteadychangemoredirectlycoupledandproportionaltovariationsinclimate.Ourtotalmassbudgetagreeswellwithexistingregionalestimates,includingthoseusingIce,Cloud,andlandElevationSatellite(65±12GtyrrArendtetal.,2013]andGravityRecoveryandClimateExperiment(GRACE)(76±4Gtyr69±11GtyrrSasgenetal.,2012;Luthckeetal.,2013],aswellasa20032009consensusestimatethatcombinedGRACE,eldmeasurements,andearlierairbornealtimetry50±17GtyrrGardneretal.,2013].OurestimateissignicantlymorenegativethanoneGRACEestimate42±6GtyryrJacobetal.,2012].OuruncertaintiesaresmallerthanthoseofapreviousUniversityofAlaskaFairbanks(UAF)altimetrystudy(96±35GtyrrArendtetal.,2002]forthe19952001period,whichhadfewersurveyedglaciers(28versus116)andextrapolatedtounsurveyedglaciersusinganincompleteglacierinventory.LimitingtheextrapolationtotheareainArendtetal.[2002]reducesourestimateto71Gtyr Figure3.Massbalancedistributionsandpartitioning.(a)Boxplotsforsurveyedglaciermassbalance,groupedbyclimateregionandterminustype.Boxesrepresenttheinterquartilerange(25thto75thpercentiles),withanannotatedhorizontallineatthemedianvalue.Whiskersextendtothe5thand95thpercentiles.Outliersareshownwithcircles.Diamondsshowsurveyedglacierswithanomalousbehaviorthatareexcludedfromthedistributionsshownbytheboxplots.Brown,green,andblueindicateland-,lake-,andtidewater-terminatingglaciers,respectively.Samplesize()isgivenaboveeachboxplot.TheinteriorclimateregionincludestheWrangellandAlaskaranges.ThesouthcentralclimateregionincludestheKenai,Chugach,andSt.Eliasranges,andthesoutheastregionincludesYakutatIceeld,GlacierBay,andJuneauandStikineiceelds.SeeFigure1forlocations.(b)Regionalmassbalanceofsurveyed(darkershading)andunsurveyedglaciers(lightershading)forthethreeterminustypes.Uncertaintiesare1.Horizontaldashedlineindicatestheestimatedtotalregionalbalance(75Gtyr GeophysicalResearchLetters10.1002/2015GL064349 LARSENETAL.ALASKAGLACIERMASSBALANCE5905 Partitioningthedatasetbydynamicalclassisolatesthelargestobservedsystematicvariationsinsurfaceelevationchange,whilekeepingeachsamplelargeenoughtoprovidearobustmeanprole.Alternativestothisdynamic-basedpartitioning,includinganall-inclusiveextrapolation,geographic[Arendtetal.,2009]andclimate-basedpartitioning,andcombinationsthereofareexaminedinFigureS12.Thesedifferencesinpartitioningmethodsperturbtheregionalmassbalanceestimateby10%(FigureS12A).Thissensitivitytestingalsohighlightsthattidewaterglaciershavemassbalancessignicantlydifferentfromallothergroupingsofglaciersregardlessofthepartitioningapproach(FigureS12B).Wealsotesttheeffectofusingdifferentminimumintervallengthsfrom4to10years(FigureS12).Requiringa10yearintervallengthreducesoursamplesizeby~40%butonlyaffectstheregionalmassbalanceby7%,whichsuggeststhatglacier-to-glacierelevationchangesarehighlystochasticinnature.4.DiscussionWeobservelargeglacier-to-glaciervariationsinelevationchangerates(Figure2),whichleadtolargevariationsinglaciermassbalance(Figure3).Consideringonlysurveyedland-terminating,nonsurgetypeglaciers,wethestandarddeviationofmassbalanceis0.54meterswaterequivalent(mwe)yr,morethanhalfthemagnitudeoftheirregionalaveragemassbalance.Wendsimilarlybroaddistributionsofmassbalancesforland-,lake-,andtidewater-terminatingglaciers.Thegreatestvariabilityinelevationchangeratesisneartidewatertermini,wherethestandarddeviationofelevationchangeexceeds5mweyr,nearlydoublethatoftheland-andlake-terminatingtermini.However,overtheupper80%oftheprole(Figure2),tidewaterglacierelevationchangevariabilityissimilar(within10%)tothealong-prolevariabilityofland-andlake-terminatingglaciersatcorrespondingelevations.Whileourmeasurementintervalsarenotallcoincident,temporalvariabilityisunlikelytoberesponsibleforthelargemassbalancevariabilityobserved(FiguresS3andS4inthesupportinginformation).Rather,wendthatthemajorityofthevariabilityweobserveisnotspatiallyautocorrelatedandresultsfrompersistentglacier-to-glacierdifferencesinmassbalance.Thisvariabilityislargeenoughtoprecludethedetectionofpatternsinmassbalanceresultingfromclimatevariability,continentality,orlatitude,despiteourobservationsspanningroughly1300kmandarangeofclimates.Nonetheless,amidstthislargerandomvariability,wendfasterratesofland-terminatingmasslossintheinteriorthanincoastalsubregions(Figure3a)(=0.05)(seethesupportinginformation).ThisobservationisoppositefromndingsbyBerthieretal.[2010],whofoundslowerratesofmasslossintheinteriorthanonthecoastfortheperiod19622006,butisinagreementwithbasin-scaleeldobservations[etal.,2014].Furthermore,wendland-terminatingglaciermassbalancetobesignicantly(=0.01)correlatedtoglaciersize(FigureS5),exhibitingmorenegativevaluesonsmallerglaciers.Thissupportsarecentglobalconsensusreport[Gardneretal.,2013]thatsuggestedthateldprogramshaveoversampledsmallglaciersandbiasedglobalmountain-glacierSLRestimates.Conversely,ourdatasetpreferentiallytargetslargeglaciersduetotheirgreaterSLRpotential.Weestimatethatthissamplingbiascouldleadustounderestimateregionalmasslossby5Gtyr(seethesupportinginformation).Itisreasonabletoassumethattheobserveddifferencesinglaciermassbalancesaredrivenbyglaciergeometryandlocalclimatevariability,butoverwhatrangeofscalesdotheseparametersbecomeimportant?Meteorologicalstationdatashowsomeregionalhomogeneityintemperatureandprecipitationvariabilitybutwithdistinctdifferencesoneithersideoftopographicdivides[Bienieketal.,2012].Alaskasglaciersarepositionedonthesetopographicdividesandthusoccupytransitionalclimatezoneswithcomplexclimatepatternsthatdeviatefromregionalaverages.Highlyvariableclimateinthesealpinezoneslikelycontributestothelargeglacier-to-glaciervariabilityweobserve,evenamongneighboringglaciers.Inaddition,anindividualglaciersthickness,slope,andhypsometrycombineincomplexwaystoproducedifferentresponsetimestoagivenclimatesignal,andtheslopeandaspectofglaciersandtheirsurroundingtopographycreatespatialvariabilityinradiationbudgetsandsurfacemassbalance[Harrison,2013;Bahretal.,2015].Ifsuchfactorsareindeedtheprincipledriversofthevariabilityobserved,thenmassbalancemodelsmayneedtocarefullyaddressbasin-scaledetailsandparameterization.Dynamicdriversofmasslossassociatedwithcalvingglaciersaddfurthercomplexitytoourassessmentofmassbalancevariability.Manysurveyedlake-terminatingglaciersshowmorerapidthinningneartheirterminithanobservedonland-terminatingglaciers(Figure2a).Medianmassbalanceratesforlake-terminatingglaciersare GeophysicalResearchLetters10.1002/2015GL064349 LARSENETAL.ALASKAGLACIERMASSBALANCE5906 morenegativeincoastalregionswherelarge,well-developedproglaciallakesexist(Figure3a).Theselakesareindicativeofsubstantialoverdeepeningsalongtheglacierbedsnearthesetermini,ageometrythatcanleadtodynamicinstabilitiessimilartothosefoundattidewaterglaciers.Evenintheabsenceofsignicanticebergcalving,largeproglaciallakesimpedetheseglaciersfromachievingequilibriumsimplybecausetheterminusisheldattheelevationofthelakeuntiltheretreatisclearoftheoverdeepening[Mercer,1961].Thecombinedimpactoftheseeffectscanbeverylarge;YakutatGlacierissecondonlytoColumbiaGlacierforthemostnegativemassbalanceamongtheglacierswesurveyed.YakutatGlacierterminatesinanunusuallylongoverdeepening,whichampliestheseeffects[Trüsseletal.,2013].Themajorityoflake-terminatingglacierswesurveyeddoesnotexhibitsuchextremelyrapidmassloss(TableS1).Atpresent,theYakutatIceeldsystemisanoutlierwithinourobservationsoflake-terminatingglaciers.Despitethenegligiblecollectivemasslossfromthetidewaterglaciers(excludingColumbia),14ofthe18surveyedtidewaterglaciersexhibitanegativemassbalance(TableS1).ExcludingColumbia,LeConte,andthosewithpositivemassbalance,massbalancesfortheremaining12surveyedglaciersaresmallinmagnitude,only0.3±0.3mweyrascomparedto0.9±0.1mweyrforland-andlake-terminatingglaciers.Thefourtidewaterglaciersgainingmasshaveacollectivemassbalanceof+0.5±0.9Gtyrthusoffsetsomeoftheregionaltidewatermassloss.Sevenofthesurveyedtidewaterglaciersareadvancing[McNabbandHock,2014],butveofthesehavenegativemassbalancesduetoupstreamthinning,demonstratingthatadvancedoesnotnecessarilyimplymassgain.Overcenturialtimescales,advanceandretreatofthesegrounded,temperateglaciersfollowtheaperiodicandpartiallyunstabletidewaterglaciercyclecyclePostetal.,2011].AlmostallofAlaskastidewaterglaciersarenowinaphaseofthiscycle[Postetal.,2011;McNabbandHock,2014]followingwidespreaddynamicretreatsincetheendoftheLittleIceAge.Thisconguration,whichoccursafterthelossofsignicantportionsoftheablationarea,tendstofavormassgainundersteadyclimateconditions[Postetal.,2011].Despitethis,ourobservationsshowthatthemajorityoftidewaterglaciermassbalancesarenegative,suggestingthattheclimatehaswarmedenoughtoretardtheonsetoftheadvancephaseofthetidewaterglaciercycle.Overtheperiod19942013,Alaskastidewaterglacierscontributedonly6%ofAlaskasmassloss,establishingthatrapidtidewaterglacierretreatisnotaprimarycontrolonregionalmassloss.TidewaterglaciersherearenowlessvulnerabletofuturecatastrophicretreatthantheyhavebeenatanytimesincetheendoftheLittleIceAge.However,severallarge,coastalglacierswithbroadareasoficegroundedbelowsealevel(Malaspina,Hubbard,Bering,andTaku)remainpotentiallysusceptibletofuturecalvinginstability.Thescaleofanysuchretreatcouldbesubstantial,asbestdemonstratedinAlaskabyGlacierspost-LittleIceAge10mmSLRcontribution[Motykaetal.,2007].Withnosuchevolutionimminent,ourresultsnowturnattentiontosurfacemeltasthemorepredictableandultimatelymorecertainmechanismofAlaskasfuturemassloss.Therateofsurfacemasslossweobserveonnontidewaterglaciersisextremelyhigh,comparabletomasslossratesatlowerlatitudes[Gardneretal.,2013].Regionallossesareoccurringatnearlydoubletheratefoundovertheperiod19622006[Berthieretal.,2010].ModelssuggestthatSMBlosseswillnotdecline[andHock,2011].Attheserates,AlaskacontributedasmuchtoSLRevery5years(~1mm)astheentire35yearretreatofColumbiaGlacier[Neeletal.,2005].DespiteGreenlandsicecoveredareabeing20timesgreaterthanthatofAlaska,lossesinAlaskawerefullyonethirdofthetotallossfromtheicesheetduring20052010[Vaughanetal.,2013].EvenifAlaskaslarge-scaletidewaterglacierlossesarenowarelicofthepast,AlaskawillcontinuetobeaprimarycontributortoglobalSLRthroughtheendofthiscentury.AlthoughadiminishedimpactoftidewaterinstabilityonAlaskaglaciermassbalanceimprovesthepredictabilityoffuturechanges,ultimatelythedominanceofsurfacemassbalancewillresultinmorewidespreadwastageoftheseglaciersunderawarmingclimate.Arendt,A.A.(2011),AssessingthestatusofAlaskasglaciers,Science(6033),10441045,doi:10.1126/science.1204400.Arendt,A.A.,K.A.Echelmeyer,W.D.Harrison,C.S.Lingle,andV.B.Valentine(2002),RapidwastageofAlaskaglaciersandtheircontributiontorisingsealevel,Science(5580),382386,doi:10.1126/science.1072497.Arendt,A.,J.Walsh,andW.Harrison(2009),ChangesofglaciersandclimateinNorthwesternNorthAmericaduringthelatetwentiethJ.Clim.(15),41174134,doi:10.1175/2009JCLI2784.1.Arendt,A.,S.Luthcke,A.Gardner,S.ONeel,D.Hill,G.Moholdt,andW.Abdalati(2013),AnalysisofaGRACEglobalmasconsolutionforGulfofAlaskaGlaciers,J.Glaciol.(217),913924,doi:10.3189/2013JoG12J197.AcknowledgmentsThisstudyisdedicatedtoKeithEchelmeyer;pilot,mountaineer,naturalist,pioneerglaciologist,andcofounderoftheAlaskaglacieraltimetryprogram.ProgramcofounderWillHarrisonprovidedhelpfuldiscussionandreviews.DetailedandinsightfulreviewsbyGrahamCogleyandErikIvinsimprovedthemanuscripttremendously.WethankPaulClausandUltimaThuleLodgeforightsupport.C.Larsen,E.Burgess,andA.JohnsonwerefundedbyNASANNX13AD52A.A.ArendtwasfundedbyNASANNX15AG21G.C.KienholzwasfundedbyNASANNX11AF41G.S.ONeelandE.BurgesswerefundedbytheU.S.GeologicalSurveyClimateandLandUseResearchandDevelopmentProgramandtheAlaskaClimateScienceCenter.ForaccesstodataseeNSIDC:http://nsidc.org/data/icebridge/data_summaries.html.TheEditorthanksJ.GrahamCogleyandananonymousreviewerfortheirassistanceinevaluatingthispaper. 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