On the Persistent Spread in Snow - Albedo Feedback

Snow-albedo feedback (SAF) is examined in 25 climate change simulations 5 participating in the Coupled Model Intercomparison Project version 5 (CMIP5). SAF 6 behavior is compared to the feedback’s behavior in the previous (CMIP3) generation of 7 global models. SAF strength exhibits a fivefold spread across CMIP5 models, ranging 8 from 0.03 to 0.16 W m−2 K−1 (ensemble-mean = 0.08 W m−2 K−1). This accounts for 9 much of the spread in 21st century warming of Northern Hemisphere land masses, and 10 is very similar to the spread found in CMIP3 models. As with the CMIP3 models, there 11 is a high degree of correspondence between the magnitudes of seasonal cycle and climate 12 change versions of the feedback. Here we also show that their geographical footprint 13 is similar. The ensemble-mean SAF strength is close to an observed estimate of the 14 real climate’s seasonal cycle feedback strength. SAF strength is strongly correlated 15 with the climatological surface albedo when the ground is covered by snow. The inter16 model variation in this quantity is surprisingly large, ranging from 0.39 to 0.75. Models 17 X. Qu, A. Hall Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, PO BOX 951565, CA 90095-1565 E-mail: [email protected] 2 with large surface albedo when these regions are snow-covered will also have a large 18 surface albedo contrast between snow-covered and snow-free regions, and therefore a 19 correspondingly large SAF. Widely-varying treatments of vegetation masking of snow20 covered surfaces are probably responsible for the spread in surface albedo where snow 21 occurs, and the persistent spread in SAF in global climate models. 22