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The Antarctic Ice Sheet and Environmental Change: a Three-Dimensional Modelling Study

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The Antarctic Ice Sheet and Environmental Change: a Three-Dimensional Modelling Study The Antarctic ice sheet and environmental change: a three-dimensional modelling study Der antarktische Eisschild und globale Umweltveranderungen: Eine dreidimensionale Modellstudie Philippe Huybrechts Ber. Polarforsch. 99 (1992) ISSN 0176 - 5027 Philippe Huybrechts Geografisch Instituut Vrije Universiteit Brussel Pleinlaan, 2 B-1050 Brussel Belgiurn und Alfred-Wegener-Institut füPolar- und Meeresforschung Postfach 120161 Colurnbusstraß W-2850 Bremerhaven Bundesrepubtik Deutschland This work is the printed version of a Ph.D thesis defended at the Faculty of Sciences, Free University of Brusseis, in June 1991. Die vorliegende Arbeit ist die inhaltlich unverändert Fassung einer Dissertation, die Juni 1991 der Fakultä Wissenschaften der Freien Universitä Brussel vorgelegt wurde. CONTENTS ,. > , . F,OREWORD~, -, , . KURZFASSUNG ABSTRACT 1. INTRODUCTION 2. THE ANTARCTIC ICE SHEET IN THE GLOBAL C 2.1. Configuration and flow 2.1.1. Bedrock 2.1.2. Area and volume 2.1.3 East Antarctic ice sheet 2.1.4. West Antarctbce sheet 2.1 5 Ice shelves 2.1.6. Antarct 2.2. Clirnate of the Anta 2.2.3. Accumulation r 2.3. Interaction with the environment: basic rnechanisrns 2.3.1., Impact of the iee sheet on,clirnate 2.3.2. Ice sheet response rnodes tu changes in environmentai conditions 2.4.Jnstability mechantsrns 2.4.1. West Antarctic ice sheet grounding line instability 3:GLACIAL HISTORY 3.1. Inception and growth es of Antarctic-glaciation , 3.2. Fluctuations of the ice sheet since the Pliocene TABLE OF CONTENTS 3.2.1. The 'Queen Maud' maximum 3.2.2. Further Plio-Pleistocene history 3.3. The last glacial cycle 3.3.1. The Last Interglacial 3.3.2. The Last Glacial Maximum 3.3.2.1. CLIMAP reconstruction 3.3.2.2. data acquired since CLIMAP 3.3.3. Holocene retreat 3.3.4. Present evolution 4. THE ICE SHEET MODEL 79 4.1. Statement of the problem 7 9 4.2. Previous modelling studies 82 4.2.1. Thermodynamic models 8 2 4.2.2. Dynamic ice flow models 82 4.2.3. Thermomechanic models 83 4.2.4. Antarctic models 84 4.3. Structure of the Huybrechts model 87 4.4. Formulation of the ice flow 8 9 4.4.1. General force balance and flow law 9 0 4.4.2. Grounded ice 93 4.4.2.1. ice deformation 93 4.4.2.2. basal sliding 9 5 4.4.2.3. complete velocity field 96 4.4.3. Ice shelf 97 4.4.3.1. boundary conditions 9 9 4.4.4. Stress transition Zone at the grounding line 100 4.5. Heat transfer 103 4.5.1. Ice temperature 104 4.5.2. Boundary conditions 105 4.5.3. Rock temperature 107 4.6. Isostatic bed adjustment 109 4.7. Nurnerical rnethods 113 4.7.1. Nurnerical grid 114 4.7.2. Solution of the continuity equation: ADI scheme 11 8 4.7.3. Determination of the ice mass fluxes 123 4.7.3.1. ice sheet 123 TABLE OF CONTENTS 4.7.3.2. ice shelf: point relaxation scheme 4.7.3.3. grounding Zone 4.7.4. Thermodynamic equation 4.7.5. Bedrock adjustment equation 4.8. Data Sets and model forcing 4.8.1. Geometrie model input 4.8.2. Climatic model input 4.8.3. Model forcing during the last glacial- interglacial cycle 4.8.3.1. temperature 4.8.3.2. sea level 4.9. List of symbols 4.9.1. Constants 4.9.2. Variables 5. BASIC SENSITIVITY EXPERIMENTS WITH FIXED GROUNDING LINE 5.1. Experimental setup 5.2. Results 5.2.1.The present reference state 5.2.2. Effect of basal sliding 5.2.3. Effect of geothermal heat flux 5.2.4. Pole of flow-temperature coupling 5.2.4.1. general effects 5.2.4.2. experimental results 5.2.5. Response to a complete glacial-interglacial cycle 5.3. Summary 6. SENSITIVITY EXPERIMENTS ON THE GLACIAL-INTERGLACIAL CONTRAST 6.1. Experimental setup 6.2. Results 6.2.1. Interglacial 'steady state' reference run 6.2.2. Effect of changes in environmental conditioiis 6.2.2.1. ice sheet geometries 6.2.2.2. temperature distributions 6.2.2.3. response time scales 6.2.3. Cornparison with previous studies TABE OF GONTENTS 6.3. Summa 176 7. ELLING THE LAST GL YCLE 179 7.1. Experimental setup 179 7.2. Results 180 7.2.1. Ice sheet evolution 180 7.2.2. The Antarctic ice sheet during the Last Interglacial 185 7.2.3. Present-day imbalance of ice thick~essand bed 187 189 190 7.3.1. Geomorpho 190 193 7.4. Summary 194 8. RESPONSE T0 FUTURE GREENH 8.1. Experimental setup 8.2. The enhanced greenhouse warming effect 8.3.1. Accumulati 8.4. Response of the ice 21 1 8.4.1. Statte re 21 1 Antarctiqa is a fascinating continent. Ever since it was first sighted in the eariy 19p cenjury, it has caught - theiimagination of adventuruus -discoverers and scientists alike. Also l did not,escape to its attraction. l have really enjoyed my introduction to glaciology, above all owing to the people l have met and yvorked with during my travels. Many ipeople have contributed to the compietion of-this thesis. Some of them I would like to thank in particular. First of all, Hans Oerlernans, who introduced me to the field of ice sheet modelling at the Rijksuniversiteit Utrecht and whose knowledge of ice and climate dynamics provided on rnany an occasion a good starting-point for lengthy and absorbing talksi Jhese always took place in a friendly alrnosphere. l arti grateful for the confidence he offered me and the effort he took to comment on my model results when they became available. Many thanks are also due to Hugo Decleir, the prornotor of this thesis at the Vrije Universiteit Brussel, for his support and valuable advice to place my results in a wider context. l appreciate the freedorn he has given me to carry out rny research independently and am grateful for the possibility l had to participate in international rneetings whenever there was a need for it. But above all, l thank hirn, and the Japanese host expedition (JARE 31), for the opportunity I was offered to visit that part of Antarctica Hugo knows so well, namely the Ser Rondane Mountains in Dronning Maud Land. My first encounter with the 'real thing' certainly was an instructive experience. I am also deeply indebted to the Alfred Wegener Institut füPolar- und Meeresforschung, Brernerhaven, and to the head of the geophysics departrnent, Heinz Miller, for the - it seemed unlimited - access l was granted to iheir CRAY-2 computing facilities at the University of Stuttgart. Without these facilities, this research would simply not have been possible. In total, l stayed more than one year in Brernerhaven and was able to work in a pleasant and stirnulating environment. l thank my colFegues of the glaciology section for their hospitality: Jürge Deterrnann, Hans Oerter and my room-mate Sepp Kipfstuhl for their interest, and Anne Letreguilly and Niels Reeh for the fruitful cooperation which resulted. Acknowledgements are furtherrnore due to David Drewry and Paul Cooper, at that time associafed with Ihe Scott Polar Research Institute, Cambridge, fŸ providing their data set on accumulation rate and surface temperature; to W,F. Budd and his CO-workersat the University of Melbourne for communicating a digitized version of basic SPRI-maps; to Hans Pfeiffenberger and Jens Schlüteof the AWI-Rechnergruppe for logistic support in dealing with yet another graphics device and Computer link; to Tony Payne at the University of Edinburgh for proof-reading and commenting on the manuscript; to Jan Van Mieghem at the Geografisch Instituut of the VUB for drafting many of the figures; and to all those that expressed interest in my work. Last but not least, l thank Monik for sharing and enduring the hardships of a scientist's life, which turned out to include almost rnore time abroad than at home. Brussel, March 15th 1991 Philippe Huybrechts During this research, l was financially supported by the Belgian National Fund for Scientific Research (NFWO) and the Belgian Scientific Research Prograrn on Antarctica (Science Policy Office,Services of the Prime Minister) under contract ANTAR 11/04. KURZFASSUNG Diese Arbeit befaßsich mit den Reaktionen des antarktischen Eisschildes auf verändert Umweltbedingungen, sowohl füdie länger paleoklimatische Zeitskala (104-1 05 a), als auch fü die kürzere mit dem künftige Treibhauseffekt in Verbindung stehenden Zeitskala (102 a). Die Reaktionen des antarktischen Inlandeises sind von große Interesse, weil Änderunge seiner Mächtigkei und Ausdehnung eine wichtige Rolle spielen füglobale atmospherische und ozeanographische Prozesse. Diese Änderunge sind auch entscheidend verantwortlich füweltweite Meeresspiegeländerungen In deren Zusammenhang wird oft ein schneller Zusammenbruch der Westantarktis, eines marinen Eisschildes, in Erwägun gezogen. Der erste Teil gibt eine qualitative Beschreibung der Rolle des Eisschildes fü die globale Umwelt und diskutiert möglich gegenseitige Einflüsse Außerde wird ein Überblicübe die glaziale Geschichte des Inlandeises gegeben und anhand von verfügbare Feldmessungen dessen Ausdehnung währen des letzten Glazials rekonstruiert. Darauffolgend wird der antarktische Eisschild einschließlic der Schelfeise mit einem hochauflösende dreidimensionalen Fließmodeluntersucht. Das Modell berücksichtigdie Dynamik der Aufsetzlinie, basales Gleiten sowie isostatische Reaktionen des Untergrundes. Eisfluà und Temperaturfeld sind gekoppelt, so da spezifizierte Umweltbedingungen die Geometrie des Eisschildes kontrollieren. Meeresspiegelschwankungen sowie Änderunge der Massenbilanz und Oberflächentemperatu treiben das Modell an. Die Simulation des heutigen Eisschildes zeigt zunächst da das Modell realistische Ergebnisse liefert. Anschließen erfolgt eine Reihe klimatischer Experimente, in denen das Modell benutzt wird, das Inlandeis währen des letzten Glazial-Interglazial-Zyklusses zu untersuchen.
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