1) Recent advances allow global climate models to realistically simulate surface mass balance of ice sheets, through explicit representation of snow processes, sub-grid elevation effects, and coupling with ice sheet models. 2) When forced by a high emissions scenario, the model projects a doubling of surface melt and a negative surface mass balance over Greenland by 2100, contributing 0.55 meters to sea level rise. 3) This is due to a 4-5°C warming over Greenland, increased cloudiness reducing sunlight while enhancing downwelling longwave radiation, and a 500m average rise in the equilibrium line altitude.