As anthropogenic greenhouse warming occurs in the next century, changes in the mass balances of Greenland and Antarctica will probably accelerate and may have significant effects on global sea level. Recent trends and possible future changes in these mass balances have received considerable attention in the glaciological literature, but until recently relatively few general circulation modeling (GCM) studies have focused on the problem. However, there are two significant problems in using GCMs to predict mass balance distributions on ice sheets: (i) the relatively coarse GCM horizontal resolution truncates the topography of 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-specific processes such as the refreezing of meltwater. Two techniques are described that attack these problems, involving (i) an elevation-based correction to the surface meteorology and (ii) a simple a posteriori correction for the refreezing of meltwater following Pfeffer et al Using these techniques in a new version 2 of the Global Environmental and Ecological Simulation of Interactive Systems global climate model, the authors present global climate and ice-sheet mass-balance results 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° lat × 2° long) than the atmospheric GCM, (b) the two correction techniques are included as part of the model, and (c) the model’s mass balances for present-day Greenland and Antarctica are realistic. When atmospheric CO2 is doubled, the net annual surface mass balance decreases on Greenland from +13 to −12 cm yr−1 and increases on Antarctica from +18 to +21 cm yr−1. The corresponding changes in the ice-sheet contributions to global sea level are +1.2 and −1.3 mm yr−1, respectively, yielding a combined contribution of −0.1 mm yr−1. That would be a very minor component of the total sea level rise of ∼5 mm yr−1 expected in the next century, mainly from thermal expansion of the oceans and melting of smaller glaciers. However, biases in the GCM climate suggest a range of uncertainty in the ice-sheet contribution from about −2 to +1 mm yr−1.