THE PRESENT EVOLUTION OF THE GREENLAND ICE-SHEET - AN ASSESSMENT BY MODELING

The present state of balance of the Greenland ice sheet is an important initial condition to assess the ice sheet's contribution to future sea levels. In this paper this problem is investigated by first simulating the evolution of the ice sheet during the last glacial-interglacial cycle and subsequently analyzing the local thickness change which results for the present time. The 3-D, time-dependent ice sheet model includes thermomechanical coupling and the effect of the stiffness contrast between Holocene and Wisconsin ice. The model forcing consists of a temperature record for central Greenland which resolves features down to a period of 100 years. The calculations indicate that the ice sheet as a whole is at present thickening slightly at a mean rate of almost 1 cm/yr, corresponding to a world-wide sea-level lowering of only a few mm during the last hundred years. However, large spatial differences occur. Central and northern parts high up in the accumulation area show a slight thinning which is most likely due to a slow but consistent basal warming in response to the last glacial-interglacial transition. Marginal thinning, most notably in the northeast and along major parts of the west coast, on the other hand, is probably due to increased melting caused by higher temperatures subsequent to the Little Ice Age. The most striking feature is a rather important thickening in the southwestern part of the ice sheet, which appeared to be robust against all changes in environmental and ice-dynamic factors tried. It is suggested that this represents a long-term trend caused by a purely dynamic reaction to the geometry which came out of the last glacial-interglacial transition. A comparison of the calculated response pattern with all of the independent observations which exist for the southern part of the ice sheet shows a remarkably good agreement, in particular with respect to the sign of the evolution.