Sea ice thickness sensitivity to longwave radiation: the albedo in the closet

A recent study of 11 Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4) coupled Global Climate Models (GCMs) by Parkinson et al (2006) reports that the simulated Northern Hemisphere sea ice covers are very similar to each other and to observations. But the variance between the models in Arctic cloud cover is large, producing an intermodel range of approximately 40 W m-2 in the downwelling longwave radiation which is the most important direct flux reaching the surface of the sea ice. Hence a large disparity in the modeled sea ice cover due to these differences in cloudiness might have been expected. Here we use a simplified thermodynamic sea ice model to show that the equilibrium sea ice thickness that one expects to result from such forcing variations is an extremely sensitive function of the ice albedo. We conclude that this sensitivity underlies the approach such GCMs must use to compute their sea ice covers.