glacier fluctuations

178 Pages·2012·10.42 MB·English

GLACIER FLUCTUATIONS
GLOBAL TEMPERATURE AND SEA-LEVEL CHANGE
Paul Willem Leclercq
Beoordelingscommissie: Prof. dr. M.R.vandenBroeke
Prof. dr. W.Haeberli
Prof. dr. B.J.J.M.vandenHurk
Prof. dr. C.Schneider
ISBN: 978-90-393-5717-0
printing: Ipskamp
InstituteforMarineandAtmosphericresearchUtrecht(IMAU)
DepartmentofPhysicsandAstronomy
FacultyofScience
UniversiteitUtrecht
Financial support was provided by the Netherlands Organisation for Scientific Research
NWOthroughresearchgrant816.01.016.
Cover: falsecolourLandsatimageofSukkertoppen,WestGreenland. CourtesyofUSGS.
GLACIER FLUCTUATIONS
GLOBAL TEMPERATURE AND SEA-LEVEL CHANGE
GLETSJER FLUCTUATIES,
MONDIALE TEMPERATUUR EN ZEESPIEGEL VERANDERING
(meteensamenvattinginhetNederlands)
PROEFSCHRIFT
terverkrijgingvandegraadvandoctoraande
UniversiteitUtrechtopgezagvanderectormagnificus,
prof. dr. G.J.vanderZwaan,ingevolgehetbesluitvan
hetcollegevoorpromotiesinhetopenbaarte
verdedigenopwoensdag8februari2012desmiddags
te12.45uur
door
PaulWillemLeclercq
geborenop17juli1979teTilburg
Promotor: Prof. dr. J.Oerlemans
Contents
Summary iii
1 Introductionandconclusions 1
1.1 Reconstructingclimateofthepast . . . . . . . . . . . . . . . . . . . . . . . 2
1.2 Glaciers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3 Observedglacierlengthfluctuations . . . . . . . . . . . . . . . . . . . . . . 16
1.4 Climatereconstructionsfromglacierlengthfluctuations . . . . . . . . . . . . 19
1.5 Glaciercontributiontosea-levelrise . . . . . . . . . . . . . . . . . . . . . . 23
1.6 Climate versus mass balance – the influence of adjustment of the glacier
geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
1.7 Mainconclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
1.8 Discussionandoutlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
2 Adatasetofworld-wideglacierlengthfluctuations 31
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
2.2 Includeddata . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
2.3 Datasetdescriptives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
2.4 Concludingsummary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
3 Climate reconstruction from the glacier length record of Glaciar Frías, North
Patagonia 47
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.2 Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
3.3 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
3.4 Resultsanddiscussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
3.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
i
ii Contents
4 Globalandhemispherictemperaturereconstructionfromglacierlengthfluctu-
ations 73
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
4.2 Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
4.3 Theoryandmethods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
4.4 Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
4.5 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
4.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
4.7 Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
5 Theglaciercontributiontosea-levelrisefortheperiod1800-2005 101
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
5.2 Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
5.3 Regionalandgloballengthrecords . . . . . . . . . . . . . . . . . . . . . . . 106
5.4 Proxyforglaciervolume . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
5.5 Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
5.6 Conclusionanddiscussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
6 Theinfluenceofglacierretreatonmassbalance,estimatedfromlengthchanges121
6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
6.2 Climaticinterpretationofmassbalance . . . . . . . . . . . . . . . . . . . . 122
6.3 Reference-surfaceversusconventionalmassbalance . . . . . . . . . . . . . 123
6.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
A Appendix: Overviewofglacierlengthrecordswithreferences 127
Bibliography 147
Samenvatting 161
CurriculumVitea 165
Publications 167
Acknowledgements/Dankwoord 169
Summary
Thecurrentworld-wideglacierretreatisaclearsignofglobalwarming. Inaddition,glaciers
contributetosea-levelriseasaconsequenceofthecurrentretreat.Inthisthesisweuserecords
ofpastglacierfluctuationstoreconstructpastclimatevariationsandtheglaciercontribution
tosea-levelchange.
Firstly,acoherentdatasetofworld-wideglacierfluctuationsoverthepastcenturiesiscom-
piled. Most available information of glacier fluctuations concerns fluctuations in glacier
length. There is currently a large number of sources available, varying from field obser-
vations,satelliteimagesandaerialphotographytoreconstructionsfromhistoricaldocuments
andgeologicalevidence. Thedataset,resultingfromthecompilationofavailabledata,con-
tains374lengthrecordsofglaciersfromallcontinents. Allrecordsstartbefore1950andthe
longestrecordhasitsfirstdatapointin1534. ThedatasetisdescribedinChapter2.
InChapter3,aclimaticinterpretationofthelengthfluctuationsofGlaciarFríasispresented.
This glacier in North Patagonia has the longest detailed length record in southern South
America. Analysis of the Glaciar Frías fluctuations can provide insight in the fluctuations
ofNorthPatagonianclimate. Theglacierbehaviourismodelledwithasimplifiedmassbal-
ancemodelthatiscoupledwithaflowlinemodel. AwarmingofNorthPatagonianclimate
with 1.16 C since the mid 17th century, or a decrease in precipitation of 34%, would best
◦
explain the observed retreat since 1639. In addition, the glacier model is forced with ex-
isting climate reconstructions. It appears that the length fluctuations are mainly driven by
variationsintemperatureratherthanvariationsinprecipitation. Themodelledglacierlengths
serveasatestfortheclimatereconstructions. Thetranslationinglacierlengthshowsthatthe
uncertaintiesinthereconstructionsareratherlarge.
Thedevelopmentofsuchdetailedmodelsisnotfeasibleforallglaciersinthelengthfluctu-
ations data set. In the next chapter a simplified approach is used to reconstruct global and
hemispheric temperature for the period 1600–2000 from world-wide glacier length fluctu-
ations. The reconstructions show that global temperature was more or less constant from
1600 until the middle of the 19th century. Since then, temperature rises until 2000, with a
period of slight cooling from 1940 to 1970. Glacier-based reconstructions are completely
independentfrombothotherproxy-basedreconstructionsandtheinstrumentalrecord. Still,
the reconstructed temperature agrees well with the instrumental record of the 20th century
anditisinbroadagreementwithexistingtemperaturereconstructions. However,according
totheglacierlengthreconstructiontheglobalwarmingstartsinthemiddleofthe19thcentury
insteadofinthebeginningofthe20thcentury,asindicatedbyseveralotherreconstructions.
iii
iv Summary
Thedatasetofglacierlengthchangescanalsobeusedtoestimatetheglaciercontributionto
sea-levelchange. InChapter5,aglobalglacierlengthsignaliscalculatedfromtheavailable
glacierlengthrecords. Thegloballengthsignalisscaledtoglobalvolumechange,whichis
calibratedonmassbalanceandgeodeticobservationsoftheperiod1950–2005. Therecon-
structedglaciercontributionis8.4 2.1cmfortheperiod1800–2005and9.1 2.3cmfor
± ±
theperiod1850–2005.Theseestimatesaresignificantlyhigherthanearlierestimates.Glacier
retreataccountsforhalftheobservedsea-levelrisesincethemiddleofthe19thcentury.
Duetothedynamicresponseofglacierstochangesintheclimaticforcing,theconventional
surfacemassbalanceisnotagoodindicatorofclimatevariationsontimescalesoftheglacier
responsetime. Instead,thereference-surfacemassbalanceshouldbeusedtoaccountforthe
changesinglaciergeometry.Inthelastchapter,afirst-orderestimateismadeofthedifference
betweentheconventionalmassbalanceandthereference-surfacemassbalanceinthespecific
caseoftwelveSwissglaciersfortheperiod1908–2008. Toobtainthisestimatewemadeuse
oftheobservedlengthfluctuations.
Knowledge of historical glacier length fluctuations provides insight in the interaction be-
tween glaciers and climate. Moreover, the compilation of world-wide data set of glacier
length records provides a means to reconstruct the climatic fluctuations over the last four
centuries independently from other reconstructions. This reconstruction confirms that the
currentglobalwarmingof0.94 Csincethemiddleofthe19th isexceptionalinthelastfour
◦
centuries at least. With the glacier length records it also is possible to estimate the glacier
contributiontosea-levelsincethebeginningofthe19th century,extendingtherecordbased
ondirectobservationsofglaciervolume.
1
Introduction and conclusions
Glaciers have retreated world-wide over the last century, and this retreat is continuing at
present. The general retreat of glaciers is an evident sign of global warming. Observations
of glacier fluctuations over the past centuries can be used to learn more about past climate,
and, in addition, can help us to explain the sea-level rise observed over the last 200 years.
World-wideobservationsofglacierlengthchangesoverthelastfourcenturiesarethetopic
ofthisthesis.
ThisthesisisacompilationofresearcharticlesincludedinChapters2–6. Thesearticlescan
bereadindependentlyfromeachother. Chapter2presentsaglobaldatasetofglacierlength
records, whichisusedinthesubsequentchapters. InChapter3and4, theobservedglacier
length fluctuations are used for the reconstruction of North Patagonian and global climate
overthelastfourcenturies. InChapter5,anestimateoftheglaciercontributiontosea-level
riseisgiven,againbasedontheglacierlengthchanges. Thelastchapterexplorestheeffect
ofchangesintheglaciergeometryontheglaciermassbalanceusingglacierlengthchanges.
This first chapter introduces these topics. The first parts provide a general introduction to
climatereconstructionsandthemainglaciologicalprocesses. Inthefollowingsections, the
topicsofthenextchaptersareintroducedwithsomebackgroundinformation,andasummary
ofthemainresultsandconclusionsispresentedalongwithanoutlookforfutureresearch.
1
2 1.1.Reconstructingclimateofthepast
1.1 Reconstructing climate of the past
Knowledgeofpastclimatevariationsisoneofthekeystotheunderstandingoftheclimate
system. As our climate is not a repeatable experiment, knowledge of the past is needed to
validate climate models. The record of past climate should preferably cover a long period,
suchthatmanydifferenttypesofthe(global)climatevariationsareincluded. Alargerrange
inobservedstatesoftheclimateimprovesthetestingofclimatemodels,whicharecalibrated
onmeasurementsofthepresentclimate. Validationofmodelsoutsidetherangegivenbythe
present climate is of particular importance for models that are used in projections of future
climate. Inthenearfuture,theclimateisexpectedtohaveaverydifferentforcingduetothe
projected enhanced greenhouse effect. Furthermore, the climate system incorporates many
slow processes. The modelling of these processes can only be tested if the record used for
validationisofconsiderablelength.
However, the period of measurements is rather short. The instrumental temperature record
is the longest and starts around the middle of the 19th century. Before this period, there is
noglobalcoverageoftheinstrumentalrecord,althoughsomelongerrecordsofobservations
exist,mainlyinEurope[e.g.Aueretal.,2007]. Fortunately,itisverywellpossibletorecon-
structpastclimatefluctuations. Variationsinclimatehaveanimpactonamanyprocesseson
Earth,likethegrowinganddisseminationoforganisms,erosionandsedimenttransport,and
changesinthecryosphere. Someoftheseprocessesactasnaturalarchives,loggingtheim-
pactofclimaticchange.Themostfrequentlyusednaturalarchivesaretreeringchronologies;
theringwidthandwooddensityareaproxyforvariationsintemperatureorprecipitation.But
manyotherbiologicalproxiesexist(e.g. pollen,corals),aswellasphysicalproxies(suchas
thechemicalcompositionofice,varvedlakesediments,groundtemperature),andhistorical
sources(harvestdatesandotherdocumentaryevidenceofclimaticchange), eachhavingits
ownadvantagesanddisadvantages.
Over recent decades, much progress has been made and an increasing amount of records
of past climate has become available [Jansen et al., 2007]. As an example, a compilation
of several reconstructions of Northern Hemisphere temperature over the last 1300 years is
shown in Figure 1.1. The proxy-based ensemble shows that it is very likely that the recent
decadeswerethewarmestinthepastmillennium. Mostofthesereconstructionsmakeuseof
severaldifferentproxiesandexceptforOerlemans[2005]andPollackandSmerdon[2004]
allreconstructionsincludetree-ringdata.
Thereareseveralproblemstoovercomewhenmakingclimatereconstructionsfromindirect
evidence. Mostproxiesarenotdependentonjustoneclimateparameter(e.g. temperatureor
precipitation), and the proxy signal has to be disentangled. Furthermore, many proxies are
only part of the year sensitive to fluctuations of the climate parameters, or in some seasons
morethaninothers. Thespatialcoverageislimitedformostoftheproxies. Forexample,it
isonlypossibletodrillicecoresontheicesheetsandonalimitednumberofhigh-altitude
glacierbasins[e.g.Thompsonetal.,2003]. TheSouthernHemisphereandinparticularthe