3-D modelling of ice sheets, climate and global sea level changes

phuybrechts [ at ] awi-bremerhaven.de


This report addresses aspects of the interaction of large continental ice sheets with the global climate system. Results are discussed from studies with a detailed 3-dimensionalthermomechanical ice-sheet model and a 3-dimensional climate model applied to the global continental cryosphere. The experiments focus on the Quaternary ice sheets of the northernhemisphere during the ice ages and on investigating the response of the presently existing ice sheets of Greenland and Antarctica to future climatic warming.On the Quaternary time scale, the research concentrated in the first place on an inverse modeling study, in which an attempt was made to reconstruct the climatic conditions that could haveproduced the Last Glacial Maximum ice sheets as documented in the geological record. A simulation was also made of the northern hemisphere ice sheets during the last two glacial cyclesusing the GRIP temperature record. These experiments document a number of basic characteristics of these ice sheets such as ice volume, ice sheet geometry, response time scales, andthermal regime. In addition, a sensitivity study was performed to assess the influence of climatic and ice-dynamic factors. At the same time, a quasi-three-dimensional climate modeldeveloped by Sellers, in effect a simplified GCM, was implemented in the light of future coupling with the ice sheet model. This climate model was extensively tested under Last GlacialMaximum boundary conditions and results were compared with those obtained with more sophisticated GCM's.Calculations were also performed to assess the impact of the Greenland and Antarctica ice sheets on global sea levels as a result of anthropogenic climate changes. These experiments wereinitialized with simulations over the last two glacial cycles to estimate the present evolution, and subsequently forced with climatic scenarios derived from the ECHAM-LSG generalcirculation model and from the IPCC96 work. This brought to light that the most important effect is to be expected from melting on the Greenland ice sheet. A likely estimate for theGreenland contribution to global sea-level rise is around 10 cm by the year 2100, with the possibility of a sea-level rise of up to 4 m by the year 2500 if greenhouse warming conditions aresustained after that. The Antarctic ice sheet, on the other hand, would in all scenarios tested remain almost stationary or grow slightly, basically because increased precipitation rates woulddominate over increased melting rates and dynamic effects in West Antarctica remain small. A likely estimate for the Antarctic contribution to global sea-level lowering is around 10 cm bythe year 2100, which would largely balance the Greenland contribution. However, large uncertainties remain concerning the temperature forcing, mass-balance changes, the present state ofthe ice sheets, and aspects of ice dynamics on scales inferior to the model resolution.

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Huybrechts, P. and T'siobbel, S. (1997): 3-D modelling of ice sheets, climate and global sea level changes , Final report of the Global Change programme, Federal Office for Scientific, Technical, and Cultural Affairs, Services of the Prime Minister, 100 p. .

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