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John Christy on Snow Cover

John Christy on Snow Cover13-11-2013 16:38
Dr. John Christy has prepared a power point presentation on global warming for Climate Change Roundtable of 30th May 2013 showing a graph with the snow cover for the Northern Hemisphere in the winter.

This graph is part of his internet presentation with a mixed panel (pro and con AGW). The panel quizzed on climate change by Rep David McKinley, a Republican congressman from the coal state of West Virginia was:

Scott Denning..............Colorado State University
Jim Hurrell..................National Center for Atmospheric Research
Joe Casola..................Center for Climate and Energy Solutions
Annie Petsonk.............Environmental Defense Fund
Richard Thomas...........West Virginia University
Marc Morano...............Climate Depot
Dennis Avery...............Hudson Institute
Myron Ebell.................Competitive Enterprise Institute

The unit on the y-axis is millions of square kilometers.

The graph doesn't show any definite trend in the snow cover, but does show a lot of random variation. However, percentage-wise, the variation is not as large, since the y-axis starts as high as 37.0. By only showing the November to April snow cover, the full picture is not presented.

What can be said about snow cover ?
The theory of global warming predicts a rise in temperature and an increase in winter-time precipitation. This means that one would expect

- The share of the winter-time snow-covered area will be growing
- The share of the summer-time snow-covered area will be decreasing

More precipitation in the winter means greater snow cover, but since the global temperature is increasing, it is not obvious which will win out. Here is an excerpt of the IPCC treatment of snow:

Snow cover has decreased in most regions, especially in spring and summer. Northern Hemisphere (NH) snow cover observed by satellite over the 1966 to 2005 period decreased in every month except November and December, with a stepwise drop of 5% in the annual mean in the late 1980s. In the Southern Hemisphere, the few long records or proxies mostly show either decreases or no changes in the past 40 years or more. Where snow cover or snowpack decreased, temperature often dominated; where snow increased, precipitation almost always dominated. For example, NH April snow cover extent is strongly correlated with 40°N to 60°N April temperature, reflecting the feedback between snow and temperature, and declines in the mountains of western North America and in the Swiss Alps have been largest at lower elevations.

Rutgers University Climate Lab Graphs has very good graphs of the development of the snow cover. However, I would like to verify the development myself by downloading data also from Rutgers University Climate Lab Data for the Northern Hemisphere.

The following graphs show the development of the average snow cover in the Northern Hemisphere summer months and winter months in mega square km respectively. Winter is defined as the months of October, November, December this year plus January, February and March of the following year:

The unit on the y-axis is millions of square kilometers.

To the right of each of the above graphs is the corresponding bar graph of the same data. It appears from this graph that the summer-time snow cover is significantly decreasing, whereas the winter-time snow cover is increasing (but not significantly). In the period from 1970 to 2012, summer-time snow has been reduced by about 22%.

Below the graph of the minimum, mean and maximum snow cover for the years 1975-2012 is shown with the best-fit lines:

The unit on the y-axis is millions of square kilometers.

The minimum and mean snow cover are decreasing with -0.0311, -0.036 and the maximum snow cover is increasing with 0.0129 mega km2 per year respectively. Mean and minimum snow cover decrease significantly, but not maximum snow cover.

Monthly data
Snow cover for the Northern Hemisphere can also be analyzed on a monthly basis. I have fitted a model with a linear trend plus 4 cycles with durations of 3, 4, 6 and 12 months for the years 1975 to 2012. Below I have plotted an example for the years 1995 to 1998:

The unit on the y-axis is millions of square kilometers.

This model can explain 99% of the total variation, and the linear trend is estimated to be -0.03311 + / - 0.0094 km2 per year after a prewhitening.

It is remarkable that such a good fit can be made using few parameters.

There is substantial evidence that snow cover is decreasing in the Northern Hemisphere since 1975 to present.

By only showing the snow cover for Northern Hemisphere during winter and forgetting the snow cover during the summer, Christy is not giving the full picture.

Christy has a scientist's knowledge and resources to find, read and understand publications on precipitation inclusive decreasing snow cover. I find it difficult to understand why he is misinterpreting the science to such a degree with respect to snow cover.

See also Record snow cover and Rudgers University Global Snow Lab for more information.

I would like to thank Christy for taking up the discussion; and would urge him in the future to consistently bring in the full view of snow-cover trends.
Join the debate John Christy on Snow Cover:

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