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 US 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 of 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.