UPPER LIMIT FOR SEA ICE ALBEDO FEEDBACK CONTRIBUTION TO GLOBAL WARMING

Two versions of the National Center for Atmospheric Research Community Climate Model (CCM) are used to calculate the increase in solar energy absorbed by the Earth-atmosphere system if all sea ice on the planet were to melt. The increase in solar energy is determined at several time points in the seasonal cycle by brief integrations of the models with the surface albedo of sea ice changed to that of open ocean; temperature, cloudiness, and other climate parameters are unchanged during the short integrations, so that our results isolate the climatic effect of sea ice albedo changes in the absence of other processes and feedbacks. (In particular, we do not include the effects of removing the insulation between ocean underneath sea ice and the atmosphere above it.) We find that the globally and annually averaged enhancement of absorbed solar flux due to removal of sea ice is 2-3 W m-2; a simple calculation indicates that most of the difference between model versions is due to differences in the surface albedo of sea ice. About half the albedo reduction at the surface is masked at the top of the atmosphere by clouds, even though the CCM versions we use tend to underestimate cloudiness. Our upper limit is significant compared to the direct radiative forcing of a doubling of atmospheric carbon dioxide, but it suggests that for greenhouse gas warming equivalent to doubling of CO2 or greater, the sea ice albedo feedback is likely to be smaller than from water vapor and potentially that from clouds.