Change in mass balance of polar ice sheets and sea level from high-resolution GCM simulations of greenhouse warming

For projecting future sea level, the mass-balance changes on Greenland and Antarctica are considered to be crucial. Promising tools for such estimates are general circulation models (GCM). Until recently, a major impediment was their coarse grid resolution (3 degrees -6 degrees) causing substantial uncertainties in the mass-balance calculations of the poorly resolved ice sheets. The present study is based on a new climate-change experiment of the highest resolution currently feasible (1.1 degrees) performed with the ECHAM4 T106 GCM, thereby increasing confidence in the projected mass-balance and sea-level changes. This new experiment, with doubled CO2 concentration, suggests that the mass gain in Antarctica due to increased accumulation exceeds the melt-induced mass loss in Greenland by a factor of three. The resulting mass-balance change on both ice sheets is equivalent to a net sea-level decrease of 0.6 mm a(-1) under doubled CO2 conditions. This may compensate for a significant portion of the melt-induced sea-level rise from the smaller glaciers and ice caps, thus leaving thermal expansion as the dominant factor for sea-level rise over the next decades. This compensating effect, however, no longer applies should atmospheric CO2 concentration reach levels well above "doubled the present value". On the contrary, under these conditions, the greenhouse warming would become large enough to induce substantial melting also on the Antarctic ice sheet, thereby significantly accelerating global sea-level rise.