Assessing Snow Albedo Feedback in Simulated Climate Change

We isolate and quantify the two factors controlling Northern Hemisphere springtime snow albedo feedback in transient climate change based on scenario runs of 17 climate models used in the IPCC 4th Assessment. The first factor is the dependence of planetary albedo on surface albedo, representing the atmosphere’s attenuation effect on surface albedo anomalies. It is potentially a major source of divergence in simulations of snow albedo feedback because of large differences in simulated cloud fields in Northern Hemisphere land areas. To calculate it, we develop an analytical model governing planetary albedo. We show detailed validation of the analytical model for two of the simulations, CCSM3.0 and CM2.0, demonstrating that it facilitates highly accurate calculation of the dependence of planetary albedo on surface albedo given readily-available simulation output. We find in all simulations surface albedo anomalies are attenuated by approximately half in Northern Hemisphere land areas as they are transformed into planetary albedo anomalies. The intermodel standard deviation in this factor is surprisingly small, less than 10% of the mean. Moreover, when we calculate an observational estimate of this factor by applying the same method to the satellite-based ISCCP data, we find most simulations agree with ISCCP values to within about 10%, in spite of further disagreements between observed and simulated cloud fields. This suggests even large relative errors in simulated cloud fields do not result in significant error in this factor, enhancing confidence in climate models. The second factor, related exclusively to surface processes, is the change in surface albedo associated with an anthropogenically-induced temperature change in Northern Hemisphere land areas. It exhibits much more intermodel variability. Its standard deviation is about 1/3 of the mean, with the largest value being approximately three times larger than the smallest. Therefore this factor is unquestionably the main source of the large