Northern Hemisphere Snow Cover Variability and Change, 1915–97
Abstract Historical and reconstructed snow cover data from stations in Canada, the United States, the former Soviet Union, and the People?s Republic of China were used to reconstruct monthly snow cover extent (SCE) fluctuations over midlatitudinal (?40°?60°N) regions of North America (NA) and Eurasia back to the early 1900s using an areal snow index approach. The station distribution over NA allowed SCE to be reconstructed back to 1915 for 6 months (November?April), along with estimates of monthly mean snow water equivalent (SWE) from gridded daily snow depth data. Over Eurasia, SCE was able to be reconstructed back to 1922, but major gaps in the station network limited the approach to 3 months (October, March, and April). The reconstruction provided evidence of a general twentieth century increase in NA SCE, with significant increases in winter (December?February) SWE averaging 3.9% per decade. The results are consistent with an observed increasing trend in winter snow depth over Russia and provide further evidence for systematic increases in precipitation over NH midlatitudes. North American spring snow cover was characterized by rapid decreases during the 1980s and early 1990s with a significant long-term decrease in April SWE averaging 4.4% per decade. Eurasia was characterized by a significant reduction in April SCE over the 1922?97 period associated with a significant spring warming. The snow cover reduction was significant at the hemispheric scale with an estimated average NH SCE loss of 3.1 ? 106 km2 (100 yr)?1 associated with significant warming of 1.26°C (100 yr)?1 over NH midlatitudinal land areas (40°?60°N). The computed temperature sensitivity of NH April SCE was ?2.04 ? 106 km2 °C?1. Since 1950, March SCE decreases have become more important than those in April with significant reductions over both continents averaging 8.5 ? 106 km2 (100 yr)?1. March was also observed to have experienced the largest warming during the November?April snow season with significant post-1950 warming trends in both continents averaging 4.1°C (100 yr)?1. The hemisphere-wide elevated March snow cover?temperature response is consistent with the position of the snowline over continental grassland vegetation zones where snow cover is relatively shallow and the potential snow cover area?albedo feedback is large.