Description: The numerical treatment of the grounding line in dynamic ice-sheet models is crucial because it determines the extent the grounded ice sheet will attain when moving over terrain lying below sea level. The marine ice-sheet problem arises because the grounding zone constitutes the transition between two well-defined ice-deformation regimes for floating and grounded ice, respectively. Here we present the results of four different models tested within the framework of the EISMINT Model Intercomparison exercise. This involved a simple downward sloping bedrock plane overlain by an ice sheet/ ice shelf system subjected to impulse sea-level and accumulation variations. It turned out that no consensus could be reached regarding the geometry of the resulting ice sheets, raising fundamental questions about the state of equilibrium and stability of the ice sheet/ ice-shelf junction and the reversibility of the process of grounding-line migration.

Description: During the last decade, significant progress has been made regarding the study of polar ice sheets and their interaction with the global climate system. This must to a large extent be due to the increased quantity of high-quality geophysical and glacial-geological observations. Equally important, however, is the crucial role being played by numerical ice-sheet models to interprete and link all these various pieces of information. The present-day generation of large-scale three-dimensional ice sheet models typically operate on horizontal grids of 20 to 40 km, have between 10 and 30 layers in the vertical and solve the fully coupled thermomechanic ice-flow equations.The most performant of these models are applied to the Greenland and Antarctic ice sheets, and are being employed to both examine the present state of these ice sheets and their response to changes in environmental conditions on time scales ranging from their inception during the Tertiary, their behaviour during the Quaternary ice ages and their response to future climatic warming.Current developments concentrate on a better description of boundary conditions and the interaction with the atmosphere, lithosphere, and ocean. In terms of ice-sheet variations, the surface mass-balance is often the most critical boundary condition, and here a lot is expected from combining ice-sheet models with General Circulation Models and more sophisticated mass-balance models in one or other way.During the talk, an overview will be given of the structure and physical basis of these ice-sheet models and of the type of problems which are addressed by them. Examples will be discussed of simulations of the Greenland and Antarctic ice sheets during the glacial cycles and of predictions of their future behaviour due to anthropogenic climatic change. This will be done within the context of the cryospheric contribution to global sea level changes.

Description: Ice sheet modelling is an essential tool for estimating the effect of climate change on the Greenland ice sheet. The large spatial and long-term temporal scale ofice sheet models limits the amount of data which can be used to test model results. The geological record is useful because it provides test material on the timescales typical for the memory of ice sheets (millennia). This paper compares modelled ice marginal positions with a geological scenario of ice marginpositions since the Last Glacial Maximum to the present in west Greenland. Morphological evidence of ice margin positions is provided by moraines.Moraine systems are dated by 14C-dated marine shells and terrestrial peat. We compared three Greenland ice sheet models. There are distinct differences inmodelled ice margin positions between the models and between model results and the geological record. Disagreement between models and the geologicalrecord in the near-coastal area are explained by the insufficient treatment of marginal processes in a tidewater environment. A smaller than present ice sheetaround the warm period in the Holocene (Holocene climatic optimum) only pops up when an unsmooth forcing is used. This underlines the importance ofshort-term variations in climatic variables in determining ice margin positions, in the past, but also in the future.

Description: The specific geomorphological problem adressed in this paper is which thermal conditions determined moraine formation in west Greenland during Holocenedeglaciation. Ice sheet modelling and geothermal research are used to delineate boundary conditions for landform formation and hereby improve and evaluategeomorphologiacal hypotheses concerning moraine formation. Marginal thermal conditions are reconstructed from modelled basal temperature and estimatesof Mean Annual ground Surface Temperature (MAST) contemporaneous to moraine formation. In mountainous areas with an altitude above 800m, ice marginmorphology will be characterized by landforms typical for cold conditions owing to the combination of relatively thin ice ice throughout Holocenedeglaciation and pronounced negatice MAST values in the proglacial area. Low lying areas (0 - 250m), with a sufficient areal extension in the direction of iceflow, have relatively thick ice throughout Holocene deglaciation. The combination of basal temperatures at the pressure melting point with positive MASTvalues in the proglacial area is postulated to produce deposits related to temperate beds and margins.

Description: Projections of changes in surface air temperature and global mean sea level over the next century are presented for all IS92 radiative scenarios. A zonal meanenergy-balance climate model is used to estimate temperature changes and thermal expansion, precipitation-dependent sensitivity values are used to estimate thesea-level contribution of glaciers and small ice caps and dynamic ice-sheet models coupled to surface mass balance models are employed with regard to theAntarctic and Greenland ice sheets. A few of the sea-level projections have been included in the IPCC96-report for comparison with the revised IPCC96projections. Here it is demonstrated that the observed inter-model differences are similar for all IS92 radiative forcing scenarios: the projections of global surfaceair temperature change resemble the revised IPCC96 projections. In this paper, the reasons for the inter-model differences in sea-level results are considered. Thelargest inter-model differences in individual sea-level contributions are found for thermal expansion and for the Antarctic ice sheet. Sensitivity experiments arepresented that show the importance of different assumptions about the temperature forcing of the glacier and ice-sheet models and about the weakening of theocean circulation. Furthermore, uncertainties in thermal expansion caused by uncertainties in ocean heat mixing are considered. It is concluded that theinter-model differences in sea-level projections are caused by the use of essentially different models in this paper and in the revised IPCC96 projections.

Description: The landscape of the Transantarctic Mountains is the result of the coupled evolution of the West Antarctic rift system and the East Antarcticice sheet. Studies of this glacial-tectonic system generally assume that the evolving surface elevation of the Transantarctic Mountains is a keydeterminant of the changing East Antarctic ice-sheet dynamics between the Miocene and today. Here, we extend previous work (Huybrechts,Ph., 1993. Glaciological modeling of the Late Cenozoic East Antarctic ice sheet: stability or dynamism? Geografiska Annaler Stockholm,75A (4), 221-238) by using numerical models of the ice sheet and lithospere to examine the impact of different bedrock surface elevations ofthe Transantarctic Mountains on ice-sheet dynamics. There are widely different interpretations of the evolution of the TransantarcticMountains from the available data, so we explore bedrock surface elevations suggested by empirical evidence in recent papers about thesensitivity of the Late Cenozoic ice sheet. The results show that the surface elevation of the individual mountain blocks has only a very localeffect on ice-sheet dynamics. The existing mountain blocks of the Transantarctic Mountains, which force inland ice to drain through troughsadjacent to the mountain blocks, were overridden by inland ice when bedrock elevations were 1 km lower. When the troughs through themountains were less well developed, in the Pliocene or Miocene, inland ice was thicker and ice-surface gradients and ice-velocities across themountains were higher. This led to more active and erosive outlet glaciers through the mountains and further development of these troughs.From these results, the key determinant of East Antarctic ice dynamics appears to be the interplay between the development of major troughsthrough the Transantarctic Mountains and rising mountain elevations. The glacial history of the central Transantarctic Mountains ranges wasvery different to that of more peripheral mountain ranges, such as the Dry Valleys and Victoria Land. The development of independent icecentres in the latter regions and the overriding of these ice centres by the main ice sheet is very sensitive to the timing of surface uplift and theparticular climate profile of the period. Conversely, the ice-surface profile across the central ranges is similar under widely different climates.The limitations of such a study stem from the necessarily schematic bedrock elevations input to the model and simplifications within themodels. At present, insufficiently detailed modelling of the impact of troughs on ice-sheet dynamics means this paper is necessarilyspeculative. However, this work points to the importance of the outlet troughs on ice-sheet dynamics, rather than simply the rising surfaceelevations of the Transantarctic Mountains along the rift margin upwarp.