A simple 1 1/2-dimensional continental ice sheet model is presented. The model is based on\ud Nye's (1959) proposal to express the vertical mean horizontal ice-velocity as u= Bt m/b, where\ud tb is the basal shear stress and B and m are constants. Essentially, the spread of ice is governed\ud by a nonlinear diffusion equation for the ice thickness. The diffusivity increases with both\ud ice thickness and surface slope. In one direction (y) a typical scale is prescribed that governs\ud the lateral ice-mass discharge, whereas in the other direction (x) the ice-sheet evolution is\ud computed explicitly on a grid with a spacing of 70 km.\ud A series of experiments has been carried out with various boundary conditions and\ud parameterizations of the annual mass balance. It appears that the boundedness of continents\ud and bedrock elevations creates a strongly nonlinear response of ice sheets to climatic variations.\ud The behaviour of Northern Hemisphere ice sheets as computed with the numerical model is\ud compared to that predicted by a perfect-plasticity model. It is found that those models give\ud qualitatively the same results.\ud Including bedrock sinking in a simple way reveals that this causes Northern Hemisphere\ud ice sheets to disappear spontaneously within 15,000 years, after about 50,000 years of growth\ud (initiated by a cold period)
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