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: Les calottes glaciaires de l'Antarctique et du Groenland jouent un role majeur dans le systeme climatique de la Terre et sont une des principales causes des variations du niveau moyen desmers. La modelisation physique de ces calottes glaciaires fournit un excellent outil pour mieux comprendre la facon dont elles reagissent aux changements environnementaux et permet ainside reconstituer leur geometrie dans les conditions aussi bien passees que futures. Sont presentes ici des resultats de modelisation tridimensionnelle incluant des simulations decomportement sur les deux derniers cycles glaciaire-interglaciaire, ainsi que des predictions de reponse en cas de rechauffement lie a l'effet de serre. Il apparait que ces deux calottes ontcontribue a une baisse d'a peu pres 15 a 20 metres du niveau moyen des mers durant le dernier maximum glaciaire. En les forcant par les scenarios de rechauffement climatique futur lesplus recents issus de l'IPCC, ces modeles montrent que les effets des calottes de l'Antarctique et du Groenland sur le niveau des mers pourraient s'equilibrer jusqu'a la fin du siecleprochain du fait d'amplitudes similaires (10 cm), mais de signes opposes.

Description: The bedrock isostatic response exerts a strong control on ice sheet dynamics and is therefore always taken into account in ice sheet models. This paperreviews the various methods normally used in the ice-sheet modeling community to deal with the bedrock response and compares these with a moresophisticated full Earth model. Each of these bedrock treatments, five in total, are coupled with a three-dimensional thermomechanical ice sheet model underthe same forcing conditions to simulate the Antarctic ice sheet during the last glacial cycle. The outputs of the simulations are compared on the basis of thetime-dependent behavior for the total ice volume and the mean bedrock elevation during the cycle, and of the present rate of uplift over Antarctica. Thiscomparison confirms the necessity of accounting for the elastic bending of the lithosphere in order to yield realistic bedrock patterns. It furthermoredemonstrates the deficiencies inherent to the diffusion equation in modeling the complex deformation within the mantle. Nevertheless, when characteristicparameters are varied within their range of uncertainty, differences within one single method are often of the same order as those between the variousmethods. This overview finally tries to point out the main advantages and drawbacks of each of these methods and to determine which one is mostappropriate depending on the specific modeling requirements.

Description: A high-resolution, three-dimensional thermomechanical ice-sheet model, which includes isostasy, the possibility of ice sheet expansion on the continentalshelf and refined climatic parameterisations, was used to investigate the basal thermal regime of the Greenland ice sheet. The thermodynamic calculations takeinto account the usual terms of heat flow within the ice, a thermal active bedrock layer in transient situations, and all of the effects associated with changes inice thickness and flow pattern. Basal temperature conditions are documented with respect to glacial-interglacial shifts in climatic boundary conditions, both insteady state as during simulations over the last two glacial cycles using the GRIP d180 record. It is found that the basal temperature field shows a largesensitivity in steady state experiments, but that during a glacial cycle, basal temperature variations are strongly damped, in particular in central areas. Acomparison has been made with measured data from deep ice cores and implications are discussed.

Description: We present a series of benchmark experiments designed for testing and comparing numerical ice sheet models. Following the outcome of two EISMINTworkshops organized to intercompare large-scale ice-sheet models currently in operation, model benchmark experiments are described for ice sheets underfixed and moving margin conditions. These address both steady-state and time-dependent behavior under schematic boundary conditions and with prescribedphysics. A comparison was made of each model's prediction of basic geophysical variables such as ice thickness, velocity and temperature. Consensusachieved in the model intercomparison provides reference solutions against which the accuracy and consistency of ice-sheet modeling codes can be assessed.

Description: A standard numerical experiment featuring the Ross Ice Shelf, Antarctica, is presented as a test package for the development and intercomparison of ice-shelfmodels. The emphasis of this package is solution of stress-equilibrium equations for an ice-shelf velocity consistent with present observations. As ademonstration, we compare five independently developed ice-shelf models based on finite-difference and finite-element methods. Our results suggest thatthere is little difference between finite-element and finite-difference methods in capturing the basic, large-scale flow features of the ice shelf. We additionallyshow that the fit between model and observed velocity depends strongly on the ice-shelf temperature field for which there is presently little observationalcontrol. The main differences between model results are due to the equations being solved, the boundary conditions at the ice front and the discretisationmethod (finite element vs. finite difference)

Description: A comparison of the two-dimensional horizontal velocity field obtained on the basis of conservation of mass (balance velocity) with the diagnostic velocityfield calculated with an ice sheet model (dynamics velocity) may yield information on shortcomings in the way the ice sheet model describes the ice flow. Thispaper presents improved geometric boundary conditions (surface elevation and ice thickness) required as input in such calculations. In particular, the surfaceelevation grid is described in detail, as it has been generated specifically for such a study and represents a new standard in accuracy and resolution forcalculating surface slopes. The digital elevation model was generated on a 10 km grid size from over 20,000,000 height estimates obtained from seven 35-dayrepeat cycles of ERS-1 radar altimeter data. For surface slopes less than 0.5¡ the relative elevation accuracy is better than 1 m. In areas of high surface slope(coastal and mountainous regions) the altimeter measurements have been supplemented with data taken from the Antarctic Digital Database. South of 81.5¡,data from the SPRI folio map have been used. The ice thickness grid was produced from a combination of a re-digitisation of the SPRI folio and the originalradio echo sounding flight lines. For areas of grounded ice, the bedrock was estimated from surface elevation and ice thickness. Significant differences (inexcess of 25 percent of ice thickness) were obtained between an earlier digitisation of the folio bedrock map and the dataset derived here. Furthermore, a newvalue of 26.6 x 106 km3 was obtained for the total volume of the ice sheet and shelves, which is a reduction of 12 percent compared with the original estimatederived during the compilation of the SPRI folio. These differences will have an important influence on the results obtained by numerical ice sheet models.