Friday, February 19, 2010

Trends in Northern Hemisphere Snow Cover

Having recently returned from three days of skiing in New Hampshire, I was interested to learn that according to the Rutgers University Global Snow Lab, last week marked the largest Northern Hemisphere snow extent since 1978 (a year that left an indelible mark on the memory of residents of the Boston area like me). On the day we returned home, my kids and I built an 11' Snowman Of Unusual Size (SOUS) from the wet snow that fell the night before we returned.

The excellent climate science blog of Anthony Watts,, recently had a guest post by Steven Goddard calling attention to the trends in Northern Hemisphere snow cover, as recorded by scientists at Rutgers University. I taught myself enough advanced Microsoft Excel skills to make these charts from the Rutgers data:

The first chart shows that since 1965 the Winter extent of northern snow cover varies widely around a fairly steady average of about 45 million square kilometers, while the Summer extent varies less, and has been decreasing by an average of 41 thousand square kilometers per year, which is about 1% per year.

The second chart shows how 2009-10 compares with the weekly average snow cover plus or minus 2 standard deviations, or a 95% confidence interval. Note that of the last 59 weeks of data, only the last point, week #7 of 2010, is "unusual" in the sense of falling outside the 2-sigma channel.

Mr. Goddard makes the excellent point that climate models used by the purveyors of the anthropogenic global warming (AGW) theory predicted that Winter snow extents would be decreasing for the past 20 years, rather than holding steady, and perhaps slightly increasing. I don't agree with Mr. Goddard that the recent apparent increases in Winter snow cover represent a significant trend, but there certainly is no suggestion of a decrease.

The Rutgers snow data supports the theory that the Northern Hemisphere is getting as much if not more snow than ever on average, but that it is dirtier due to anthropogenic soot which darkens the snow and is very potent at making it melt faster when exposed to the sun. If true, this would remove any need to invoke a warming climate explanation for the downward trend in Summer snow extent. However, the leading AGW proponent at NASA, James Hansen, with his coauthor advanced the claim in 2003 that sooty snow could itself contribute to AGW by decreasing the albedo of snow. It seems to me that the albedo of snow, given its high latitude, is far less important than the albedo of the tropical oceans, which is very sensitive to changes in cloud cover, and may be modulated by the interaction of the solar magnetosphere with cosmic rays.