Accurate simulation of ice-sheet surface mass balance requires higher spatial resolution than is afforded by typical atmosphereocean general circulation models (AOGCMs),owing, in particular, to the need to resolve the narrowand steep margins where the majority of precipitation and ablation occurs. We have developed a method for calculating massbalance changes by combining ice-sheet average time-series from AOGCM projections for future centuries, both with information from high-resolution climate models run for short periods and with a 20 km ice-sheet mass-balance model. Antarctica contributes negativelyto sea level on account of increased accumulation, while Greenland contributes positively because ablation increases more rapidly. The uncertainty in the results is about 20% for Antarctica and 35% for Greenland. Changes in ice-sheet topography and dynamics are not included, but we discuss their possible effects. For an annual- and area-average warming exceeding 4:5G0:9 K in Greenland and 3:1G0:8 K in the global average, the net surface mass balance of the Greenland ice sheet becomes negative, in which case it is likely that the ice sheet would eventually be eliminated, raising global-average sea level by 7 m.