Melting snow hastens warming in the Arctic
STUDY: Expansion of snow-free days is accelerating climate change
Anchorage Daily News, Sept. 24, 2005
Melting snow has triggered the warmest summers across Arctic Alaska in at least 400 years, setting in motion tree and shrub growth that will accelerate warming by two to seven times as the century unfolds.
The slow expansion of the tundra's snow-free season by about 2.5 days per decade since the 1960s explains 95 percent of the recent rise in summer temperatures, and is far more influential than changes in vegetation, sea ice, atmospheric circulation or clouds, according to a report published this week in Science Express.
Those few extra days when the sun bakes brown tundra instead of
getting reflected back into space by snow produces a surprising impact, wrote University of Alaska Fairbanks ecologist Terry Chapin and 20 co-authors. They have warmed the tundra by three watts for every square meter -- as much heating as you'd get from doubling the concentration of the greenhouse gas carbon dioxide in the atmosphere.
"There's been a long-term interest in why it is that high latitude
climate seems to be warming more rapidly than the rest of the world,"
said Chapin, a professor at the Institute of Arctic Biology and the
first Alaska member of the National Academy of Sciences. "Basically, I
thought that maybe vegetation would be having a large influence, but the bottom line of that paper is that snowmelt swamps the vegetation."
Even small increases in the time the landscape spends dark rather than white make a huge difference in how much solar energy gets absorbed, explained snow researcher Matthew Sturm, with the U.S. Army Cold Regions Research and Engineering Laboratory at Fort Wainwright.
"If you sort of think about the short summer period, there's just a
certain number of days when we have that nice dark tundra exposed," he said. "If we add a couple days where we don't have snow cover, we have a big impact. Just peeling that back a couple days per decade, and there's a lot of warming."
The paper, the "Role of Land-Surface Changes in Arctic Summer
Warming," arose from a project sponsored by the National Science
Foundation. Coordinated by Chapin and Sturm, it drew on a decade of work by 21 ecologists and biologists, snow and ice experts, climate
researchers and supercomputer jockeys. It crunched a hemisphere of data -- shifts in temperature, cloud cover, solar energy, snow cover and vegetation.
"We argue that recent changes in the length of the snow-free season have triggered a set of interlinked feedbacks that will amplify future rates of summer warming," the authors wrote.
The study is only one of several new reports describing how climate
change appears to be accelerating across the Arctic. Tundra has been greening up with more shrubs that, in turn, trap more solar energy, according to new papers published by scientists at Woods Hole Research Center and the Army research lab. At the same time, Interior spruce forests have declined, under stress from drought and wildfires.
Scientists say there's no question that overall Arctic warmth has been influenced both by increases in greenhouse gas concentrations and natural cycles, though the relative contributions are still not clear.
This newest study suggests that policy-makers should take Alaska's
warming climate as a spur to action, regardless of the causes, said
Chapin, the lead author. That means people ought to find ways to cut back on fossil fuel consumption while preparing for big changes in the landscape.
"It's a chance for policy-makers and industry to look for innovative
ways to maximize the societal benefit of the fuels that we do use," he said. "I think there's lots that can be done to reduce fossil fuels
that would have modest or even positive impacts on the economy."
The study found summer warming in Arctic Alaska and western Canada sped up over time, resulting in an increase of almost three-quarters of a degree Fahrenheit per decade over the past 40 years. But explaining why was complicated.
Changes in ocean cycles influence winter temperatures and don't fully explain summer warmth. Shrinking sea ice also has the biggest impact on fall and winter conditions. More summer cloudiness tends to "dampen" the amount of sun beating down over the seasons, the scientists said.
Vegetation has spread, too, with tall shrubs advancing into the tundra and the tree line slipping north. Spring leaf-out has come 10 to 12 days earlier in Alaska over the past half century. But all these
shifts, while moving faster and faster, account for only about 2
percent of the summer warming observed so far, the scientists said.
"The summer warming in Alaska is best explained by a lengthening of
the snow-free season, causing sensible warming of the lower atmosphere to begin earlier," they concluded.
But as the shrubs expand in the tundra, their influence will grow --
catching more solar heat, trapping more insulating snow, enriching the soil with nutrients. Eventually vegetation will take over.
"Because of these feedbacks, there are lots of reasons to think that
this warming will continue," Chapin said.
Understanding what factors are pushing the shrub expansion "would
reduce the likelihood of unexpected surprises" in future summer
warming, the scientists wrote.
Chapin, one of the most influential scientists in Alaska, said he
hopes to begin looking into what factors might pushing climate changes in other areas of Alaska.
"I'm interested in asking similar questions for the boreal forest,
where there's an increase in forest fires," he said.
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