There is a certain amount of disagreement among researchers about whether the rapid climate changes in the Arctic are caused by natural variability or greenhouse gases.

The majority of scientists, however, believe that greenhouse gases are largely responsible for the changes. "If you put this question to the informed science community, there is pretty much of a consensus that this warming that we've seen--like the reductions of sea ice--is due to human activities," says Mark C. Serreze of the National Snow & Ice Data Center at the University of Colorado, Boulder. "There are certainly some naysayers out there," he explains. They argue about how long a record scientists need to be able to say these changes in the Arctic are caused by human activity, he says.

James E. Overland of the National Oceanic & Atmospheric Administration's Pacific Marine Environmental Laboratory in Seattle, for one, believes these changes are permanent and will only intensify. "My view is you can't be 100% sure, but the balance of the evidence is now we're seeing these major changes in the Arctic, and on a 30- to 50-year basis, we're not going to go back to where we were before," he explains. Shipping companies are quite sure the changes in sea ice will only intensify. "They are talking about setting up sea lanes from Europe to Asia across the north of Siberia," he says. In the past, the sea ice was so thick that no shipping firm would have considered using such a route.

One type of evidence that indicates that the Arctic changes are caused by greenhouse gases comes from climate models called general circulation models. When the models are run with elevated levels of CO₂ as one of the inputs, they predict the kinds of large-scale changes in the Arctic that are already under way, Overland says. "Although there are a lot of problems in the detail, the models have gotten a lot better in the last 10 years in getting the large-scale changes that are occurring," he explains.

What is happening in the Arctic is caused by humans, states Jonathan Overpeck, director of the Institute for the Study of Planet Earth at the University of Arizona. In the Arctic, most of what was predicted in the late 1980s as consequences of rising levels of greenhouse gases is happening, he says. "You can't say unequivocably that it's all due to humans, but what's going on is very consistent with the changes we would predict from greenhouse gases in the atmosphere," he explains.

James Morison, principal oceanographer at the Polar Science Center at the University of Washington, says the changes in the Arctic may result from the combined effect of greenhouse gases and natural variability. A lot of the changes seen in the 1990s can be explained by an intensification of the atmospheric circulation of the whole Northern Hemisphere, he says. The counterclockwise west-to-east circulation of wind intensified in the 1990s and brought more warm air into the Greenland Sea. "Consequently, we saw a rise in permafrost temperatures," he says.

Global climate models run with elevated levels of CO₂ as an input show that the atmospheric circulation would intensify in the Arctic, Morison says, but the changes projected by the models are "not as big or as early as the changes we saw in the 1990s." Consequently, he believes that natural variability in addition to CO₂ is involved in the changes.

No matter what their views are about the causes of change, researchers who study the Arctic do not expect it will return to its former climate anytime in the next 50 years or more.

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