Wednesday, 29 February 2012

Sea Ice and Snow

The Georgia Institute of Technology has recently released details of a study of the relationship between Arctic Sea Ice and Northern Hemisphere Snow Cover.  The press release summarises it as “The researchers analyzed observational data collected between 1979 and 2010 and found that a decrease in autumn Arctic sea ice of 1 million square kilometers -- the size of the surface area of Egypt -- corresponded to significantly above-normal winter snow cover in large parts of the northern United States, northwestern and central Europe, and northern and central China.”

This goes some way to explaining the fact that whereas Arctic Sea Ice has been tending to decrease (at 53,000 km2/year) snow coverage has declined more slowly (at 22,000 km2/year).  The importance of snow and ice is their role in the albedo feed-back mechanism. Snow reflects almost all the incoming energy, water and land (at least the northern boreal forests where most snow falls) reflect about 10%. So, other things being equal, the influence of snow and ice are equivalent. But, and it’s a big but, other things are not equal. Both sea ice and snow cover vary seasonally. The following chart shows average monthly values for the period 1978 to 2011. This shows that in winter snow covers a much larger area than sea ice.


However the albedo effect is only applicable when the sun is above the horizon. The next chart shows the areas adjusted for solar angle. In this case we have assumed a latitude of 80 °N for sea ice and 70 °N for snow and multiplied areas in the first chart by the sine of the sun angle at midday on the 15th of each month. (Calculated using the tool at This presents a very different picture and suggests that the influence of snow and ice are equivalent – with snow being perhaps more predominant.

This calculation is subject to a large number of caveats. The sea ice area is based on NSIDC ‘sea ice extent’ which owever thse figureshows “the total area of ocean covered with at least 15 percent ice”. This is reasonable as a metric since sea ice is almost 90% below water but of course such ice is not reflecting radiation. The use of 70 °N and 80 °N respectively and mid-day sun angle are also only approximate. Ideally it would be necessary to track areas of ice and snow at different times of the year, at different latitudes and the energy reflected at different times of day. 

The final chart shows the solar-angle adjusted ice and snow area. This was calculated as for the previous chart. It shows that despite the snow area being larger and reducing less than the albedo adjusted ice area has shown a steady decline.

Minor editorial changes on 2 March 2012.

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