GREENLAND AND ANTARCTIC MASS BALANCES FOR PRESENT AND DOUBLED ATMOSPHERIC CO2 FROM THE GENESIS VERSION-2 GLOBAL CLIMATE MODEL

As anthropogenic greenhouse warming occurs in the next century, change s in the mass balances of Greenland and Antarctica will probably accel erate and may have significant effects on global sea level. Recent tre nds and possible future changes in these mass balances have received c onsiderable attention in the glaciological literature, but until recen tly relatively few general circulation modeling (GCM) studies have foc used on the problem. However, there are two significant problems in us ing GCMs to predict mass balance distributions on ice sheets: (i) the relatively coarse GCM horizontal resolution truncates the topography o f the ice-sheet flanks and smaller ice sheets such as Greenland, and ( ii) the snow and ice physics in most GCMs does not include ice-sheet-s pecific processes such as the refreezing of meltwater. Two techniques are described that attack these problems, involving (i) an elevation-b ased correction to the surface meteorology and (ii) a simple a posteri ori correction for the refreezing of meltwater following Pfeffer et al . Using these techniques in a new version 2 of the Global Environmenta l and Ecological Simulation of Interactive Systems global climate mode l, the authors present global climate and ice-sheet mass-balance resul ts from two equilibrated runs for present and doubled atmospheric CO2. This GCM is well suited for ice-sheet mass-balance studies because (a ) the surface can be represented at a finer resolution (2 degrees lat x 2 degrees long) than the atmospheric GCM, (b) the two correction tec hniques are included as part of the model, and (c) the model's mass ba lances for present-day Greenland and Antarctica are realistic. When at mospheric CO2 is doubled, the net annual surface mass balance decrease s on Greenland from +13 to -12 cm yr(-1) and increases on Antarctica f rom +18 to +21 cm yr(-1). The corresponding changes in the ice-sheet c ontributions to global sea level are +1.2 and -1.3 mm yr-l, respective ly, yielding a combined contribution of -0.1 mm yr(-1). That would be a very minor component of the total sea level rise of similar to 5 mm yr-l expected in the next century, mainly from thermal expansion of th e oceans and melting of smaller glaciers. However, biases in the GCM c limate suggest a range of uncertainty in the ice-sheet contribution fr om about -2 to +1 mm yr(-1).