NESDIS/NOAA weekly snow charts 1966-2000 (produced by David Robinson) and NSIDC/University of Colorado microwave retrieved 1978-1999 weekly snow analyses (produced by Richard Armstrong) are used to evaluate observed climatic trends and variability of Northern Hemisphere snow cover extent. A new statistical technique has been developed and applied to estimate seasonal variations in normals, standard deviations, and linear trends from the two data sets. The analysis reveals significant disagreements between normals, standard deviations, and trends estimated from the two data sets. These disagreements will be discussed. Both data sets show a downward trend in Northern Hemisphere snow cover extent during the past few decades. The GFDL/NOAA R30, Canadian CGCM2, and Hadley Centre/UK HadCM3 climate models forced by observed anthropogenic CO2 and aerosols realistically reproduce the microwave-observed contemporary climatic trend and variability of Northern Hemisphere snow cover extent.

Using very long climate model control runs to examine internal unforced climate variability, we find that observed decrease in the Northern Hemisphere snow cover extent is likely related to contemporary global warming, and is consistent with anthropogenic climate change. The models predict continued substantial snow cover and sea ice extent decreases in the current century.