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Shrubs are relatively sparse on the tundra and are just beginning to spread, but they have already caused dramatic change, at least in part by absorbing more solar radiation than they reflect.

Why the tundra is transforming

A new study reveals mechanisms behind summer warming of the Arctic tundra

By Deane Morrison

From M, winter 2006

For years, summer has been strengthening its grip on the vast expanse of Arctic tundra. Scientists have documented how years of warming have led to thinning and retreat of ice in the Arctic Ocean--an ominous omen for polar bears, walruses and other marine animals--but no one knew why temperatures were rising even faster over the grassy tundra. Now, a study of the tundra shows how warming of the land can "snowball" into profound ecological changes. The culprits are a longer snow-free season and a shift in vegetation, including shrubs as well as coniferous trees. It was already known that the loss of sea ice allows the ocean to absorb more solar heat. The ocean then warms both the atmosphere and the ice, causing even more melting in a cycle of positive feedbacks. But the melting sea ice can't explain the extensive warming that has happened over land during the summer. Instead, the explanation lies in a different feedback loop that also started with a little warming, says Joe McFadden, an assistant professor of ecology, evolution and behavior in the College of Biological Sciences.

"We can consider the Arctic a bellwether," says McFadden, who includes Minnesota among the regions where climate and ecosystems are sensitive to the timing of snowmelt. "We show that the global warming models' predictions were right that the Arctic would respond earlier and more strongly. This is a wake-up call. Something's really happening."

The key is that a little warming causes small changes that amplify the warming of Arctic lands, McFadden explains. In a paper published in last week's Science magazine, McFadden and co-authors from several other institutions showed--using real data, not computer simulations--that the snow-free period is lengthening in the Arctic and the spread of shrubs and trees is contributing to the change. "We can consider the Arctic a bellwether," says McFadden, who includes Minnesota among the regions where climate and ecosystems are sensitive to the timing of snowmelt. "We show that the global warming models' predictions were right that the Arctic would respond earlier and more strongly. This is a wake-up call. Something's really happening." In the same issue, the magazine also ran a commentary piece praising the work and calling for more attention to Arctic warming in general and the role of terrestrial vegetation in particular. The timing of snowmelt varies greatly around the tundra, but on average the date of snowmelt is being pushed back 2.5 days per decade, the researchers concluded. The loss of snow cover contributes to the growth of trees, which are starting to invade more northern areas. But what surprised the scientists was how the spread of shrubs influences local climate. Shrubs are relatively sparse on the tundra and are just beginning to spread, but they have already caused dramatic change, at least in part by absorbing more solar radiation than they reflect. "The effect of shrubs is almost as great as that of tree expansion," McFadden says. This factor is missing from nearly all models of Arctic climate change, and including it will only intensify the predicted impact of warming on the region. The changes in date of snowmelt and the spread of vegetation have a huge proportional effect because the growing season is so short and the amount of shrub and tree cover is so small. The effects will be felt by all kinds of tundra dwellers. For example, the caribou herds time their northward migrations to the summer calving grounds to coincide with the emergence of tender shoots of vegetation. With earlier snowmelt comes an increased risk of mistiming their arrival, which could have a significant impact on the health of the herds because older shoots are less palatable. Another risk from earlier melting is that the snow will refreeze and coat the vegetation with thick ice that the caribou can't break through. Higher temperatures may also affect the Inuit. Negative impacts on subsistence resources like marine mammals and caribous will diminish their food supplies or at least make it harder to hunt them. Also, snow machines and skis are harder to use when snow is warmer and mushier, and people who have to cross frozen lakes will be at risk from thinner ice. But despite the worrisome climate trends, McFadden finds his niche in the new science of global ecology exciting. He finds the study of previously unknown feedback systems fascinating, and he enjoys having colleagues who share his enthusiasm. "The University has made a big commitment to growing in the area of global ecology," he says. "There are faculty in multiple colleges and departments, including soil, water and climate [Agricultural, Food and Environmental Sciences], geography [Liberal Arts], geology and geophysics [Institute of Technology] and my home department." Doing research in the Arctic was a source of excitement in itself. McFadden set up instruments in 24 research sites between the treeline on the north slope of Alaska and the Arctic Ocean. The instruments measured the exchanges of heat, water vapor and carbon dioxide between the vegetation and the atmosphere. McFadden was quite familiar with the instruments; as a graduate student at the University of California, Berkeley, he had devised a way to package them compactly. Good thing, because he had to be flown to the Alaskan sites by helicopter. In his current work, McFadden is using computer models to examine the interactions between shrubs and snow, in particular whether they contribute to the release of carbon dioxide under the snowpack in winter. The published work was funded by the National Science Foundation's Arctic System Science program.