Thermomechanical modelling of Northern Hemisphere ice sheets with a two-level mass-balance parameterization

A 3-D time-dependent thermomechanical ice sheet model was used together with a two-level (snow accumulation/ runoff) mass-balance model to investigatethe Quaternary ice sheets of the northern hemisphere. The model freely generates the ice sheet geometry in response to specified changes of surfacetemperature and mass balance, and includes bedrock adjustment, basal sliding and a full temperature calculation within the ice. The mass balanceparameterisation makes a distinction between snowfall and melting. Yearly snow-fall rates depend on the present precipitation distribution, and are variedproportional to changes of surface temperature and the moisture content of the air. The ablation model is based on the positive degree-day-method, anddistinguishes between ice and snow melting. This paper discusses steady-state characteristics, conditions for growth and retreat and response time scales ofice sheets as a function of a prescribed lowering of summer temperature. Most notably, the modelled extents of the Eurasian ice sheet for a summertemperature lowering of 6-7 K and of the Laurentide ice sheet for a cooling of 9-10 K are in reasonable agreement with most reconstructions based ongeological evidence, except for the presence of a large ice sheet stretching from Alaska across the Bering Strait to most of eastern Siberia. In addition, wetbasal conditions turned out to be always confined to the margin, whereas central areas in these reconstructions remained always cold-based. This is ofrelevance for processes involving reduced basal traction.