A long-term retreat in snow and ice cover in the Northern Hemisphere is weakening the ability of these seasonal cloaks of white to reflect sunlight back into space and cool global climate, according to a study published this week.
Indeed, over the past 30 years, the cooling effect from this so-called cryosphere – essentially areas covered by snow and ice at least part of the year – appears to have weakened at more than twice the pace projected by global climate models, the research team conducting the work estimates.
The study, which appeared online Sunday in the journal Nature Geoscience, represents a first cut at trying to calculate from direct measurements the impact of climate change on the Northern Hemisphere's cryosphere. The study was conducted by a team of federal and university scientists who examined data gathered between 1979 and 2008.
Of particular interest is a self-reinforcing process, or feedback, though which warming reduces snow and ice cover. Those reductions expose more ocean and landscape to sunlight during spring, summer and fall. After absorbing the sunlight, these exposed features radiate the heat back into the atmosphere. This accelerates the loss of snow and ice already triggered by global warming.
Pinning down the size of this effect – one of three major feedbacks in the global climate process – is important in understanding how much the global climate could warm in response to rising concentrations of greenhouse gases that human activities have pumped into the atmosphere, explains Mark Serreze, director of the National Snow and Ice Data Center in Boulder, Colo.
Over the past decade or more, other teams have tracked the decline of snow and ice cover in the Northern Hemisphere, the northward march at high latitudes of vegetation typically found farther south, and other changes that suggest the feedback has kicked in.
This latest study says "yes, the feedback is working as we suspected it would be," says Dr. Serreze, who was not part of the team conducting the research. "But it also argues that maybe the feedback is stronger than we thought it would be."
That last point will likely be challenged, he adds, as part of the scientific process.
"Putting numbers to these feedbacks is a tough thing to do," he says. Still, "this is an important paper. I see this as a significant advance in climate science."
As one of the world's two deep-freeze thermostats, Antarctica is still chiller-in-chief. But because the Arctic is warmer than its southern counterpart, small changes in temperatures at the top of the world have a relatively larger effect on ice and snow cover, researchers say.
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.