Global warming: Impact of receding snow and ice surprises scientists

Reality check on climate models

Mark Flanner, a climate researcher at the University of Michigan who led the team, says the goal of the new study was to provide a reality check on global climate models' representations of the impact that declining snow and ice has on the Earth's so-called radiation budget. The radiation budget is a kind of bookkeeping process that tries to account for all the sunlight Earth receives and either reflects or converts into heat.

Using satellite measurements as well as field measurements of the extent of snow and ice cover, the team teased out details of seasonal patterns in the amount of solar radiation the Northern Hemisphere's snow and ice reflect.

Snow appears to have its maximum cooling effect – reflecting the most sunlight back into space – in late spring, as the light strengthens but snow cover is still near its maximum extent for the year. Sea ice in the Arctic Ocean has its biggest effect in June, before its annual summer melt-back accelerates, explains Don Perovich, a researcher at the US Army Corps of Engineers Cold Regions Research and Engineering Laboratory in Hanover, N.H., and a member of the team reporting the results.

That means "it becomes important when you melt snow and ice," he says. "If you start that melting earlier, you tend to have a lower albedo every day throughout the summer," he says. Albedo is a measure of a surface's ability to reflect light.

Research published by a different team in 2009 showed that at least for the Canadian Archipelago, the melt season grew at a rate of about seven days per decade during the 1979-2008 period. Most of that expansion has come at season's end, the team reported, but the onset of the melt season was coming earlier as well.

But the eyebrow-arching moment for Dr. Flanner and his colleagues came in comparing real-world measurements of the ice-snow feedback with those from models.

Twice the decline in cooling effect

According to the team, the measured decline in the cooling effect of the Northern Hemisphere's shrinking cryosphere associated with a 1-degree Celsius increase in in Northern Hemisphere temperatures was more than twice that predicted by climate models.

"The reduction was somewhat surprising," Flanner says.

The team acknowledges that the study has its limitations.

For instance, the 30-year period "is right on the edge of being long enough" to separate long-term trends from year-to-year changes in conditions that occur naturally, Flanner says.

But Dr. Perovich adds that many of the assumptions the team had to make as it analyzed the data are likely to prove conservative.

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