U.S. Snow Cover Shrinking (Yes, Really)

It may not seem so in many places today, but North America and Eurasia's snow cover has shrunk, according to study of more than 40 years of weather satellite-based snow cover maps.

The discovery of a sharp decline in late winter and early spring snow cover starting in the 1980s until 1990 was revealed after researchers made overdue adjustments to decades of daily snow cover maps.

After the fine tuning the maps, New Jersey State Climatologist and Rutgers University professor David Robinson found a 1990 decline in spring snow extent that we still see today.

"It has remained lower over the past 20 years compared to the previous 20 years in North America and Eurasia," he said.

Put another way, the snow cover is leaving earlier but is just as extensive in winter and fall. This, said Robinson, is just how models have suggested climate should be changing snow cover and matches other research that has found earlier warming and greening up of the land in spring.

"It all fits together," said Robinson, who is scheduled to present his work on Jan. 20 at the meeting of the American Meteorological Society in Atlanta.

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As for why the snow cover maps needed adjusting, it has to do with how they were made in the first place, Robinson said.

The maps are based on satellite images that are divided into grids. Squares in the grids are either judged as snow-covered or not, depending on how much they are filled with white. Just how much they must be filled to qualify as snow-covered varies in different places. So to just take the resulting maps from all corners of the continent, as is, and use them to study climate can lead to problems.

"If they are misused they can result in faulty research," Robinson told Discovery News. "What we've done is not changed it but tightened it up a little to develop better confidence limits. This has hardly needed cleaning up in some areas." But in others, like some mountainous places, the data needed to be looked at again, he said.

The revised snow cover maps are already proving useful to researchers working on a better understanding of how snow cover and the harder-to-measure matter of snow depth fit into the larger climate change picture.

"The prevailing wisdom for the past 15 or 20 years is that snow cover extent is more important than snow depth," said hydroclimatologist Gavin Gong of Columbia University. That's because snow is bright and reflects sunlight back into space, rather than being absorbed into the ground and emitted back into the atmosphere as heat.

So places that have snow on the ground tend to be colder than nearby places without snow, said Gong. This is why good snow cover maps are so important for climatologists.

Gong, for his part, is taking the matter further and trying to see if depth of snow is also playing a role in cooling things down over the much wider, more northern areas that are solid white on the maps for the longest.

"The question is how big a role that can play when spread out over North America." said Gong. "It will have to be considered in conjunction with snow extent."