Driving a high-resolution dynamic ice-sheet model with GCM climate: ice-sheet initiation at 116 000 BP

Most dynamic ice-sheet studies currently use either empirically based parameterizations or simple energy-balance climate models for the surface mass-balance forcing. If three-dimensional global climate models (GCMs) could be used instead, they would greatly improve the potential realism of coupled climate-ice-sheet simulations. However, there are two serious problems in simulating realistic mass balances on ice sheets from GCM simulations: (i) dynamic ice-sheet models and the underlying bedrock topography need horizontal resoltuion of 50-100 km or less, but the finest practical resolution of atmospheric GCMs is currently ~250 km, and (ii) GCM surface physics usually neglects the local refreezing of meltwater on ice sheets. Two techniques are described that address these problems: an elevation correction applied to the atmospheric GCM fields interpolated to the ice-sheet grid, and a refreezing correction involving the annual totals of snowfall, rainfall and local melt at each grid-point. As an example of their use, we have used the GENESUS version 2 GCM at 3.75° × 3.75° resolution to simulate the climate at the end of the last interglaciation at ~116 000 years ago. The atmospheric climate is then used to derive a standard two-dimensional dynamic ice-sheet model for 10 000 years on a 0.5° × 0.5° grid spanning northern North America. The model successfully predicts ice-sheet initiation over the Baffin Island highlands and the Canadian Archipelago, but at a slower rate than observed. A large ice sheet nucleates and grows rapidly over the northwestern Rockies, in conflict with geologic evidence. Possible reasons for these discrepancies are discussed.