THE CO2-INDUCED THICKENING THINNING OF THE GREENLAND AND ANTARCTIC ICE SHEETS AS SIMULATED BY A GCM (CCM1) AND AN ICE-SHEET MODEL

Scaling analysis shows that the mean thickness of an ice sheet depends on the product of two poorly known quantities, the ice viscosity and the net snow accumulation rate. We adjust the viscosity of an ice shee t in order to get a consistent value of this product for the present-d ay ice sheet volume and area given the net snow accumulation rate calc ulated by an atmospheric general circulation model (GCM). We then hold this artificial theology constant in further numerical experiments. W e hope that in doing so we can partially compensate for systematic GCM errors in simulating the snow accumulation rate, and, therefore, thic kening/thinning of ice sheets will depend mostly on the tendency in th e net accumulation change rather than on its absolute value. Using thi s approach, the response of the Greenland and Antarctic ice sheets to doubling CO2 concentration is simulated and the horizontal distributio n of possible thickening/thinning of polar ice obtained. We find that, initially, the region of thickening ice is close to the area of incre ased snowfall rate, but later it significantly changes under the influ ence of internal ice flow dynamics. The sea-level changes predicted by our experiments agree with some empirical estimates. The sensitivity experiment with assigned basal sliding does not show significant chang es in the large-scale ice topography, meaning, for example, that there is no indication of a possible disintegration of the West Antarctic i ce sheet. At the same time, the regional thickening/thinning of ice (a nd consequently the sea-level change) depends strongly on processes at the ice sheet bottom.