The change also appears to be becoming increasingly self-sustaining: The additional open water absorbs solar energy that would otherwise be reflected back into space by bright white ice, says Ted A. Scambos, a scientist at the National Snow and Ice Data Center in Boulder, Colo., which compiled the data along with NASA.
The findings are consistent with recent computer simulations indicating that a buildup of greenhouse gases from smokestack and tailpipe emissions could lead to a profoundly transformed Arctic later this century, when much of the once ice-locked ocean would routinely become open water in the summers.
Expanding areas of open water in the summer could be a boon to whales and cod stocks, and the ice retreat could create summertime shipping shortcuts between the Atlantic and the Pacific.
But scientists say a host of troubles may lie ahead as well. One of the most important consequences of Arctic warming will be an increased flow of meltwater from glaciers and ice sheets, and thus an accelerated rise in sea levels, threatening to flood coastal areas. Loss of sea ice could also hurt both polar bears and Eskimo seal hunters.
In the last century, the Earth's average temperature has increased by one degree Fahrenheit, according to the Environmental Protection Agency, and it's now clear that such a change has profound consequences in a delicate region like the Arctic. The big question for scientists is whether this warming is part of a natural cycle or is caused by human activities.
"The polar regions are our 'canary in the coal mine' for global warmingthey respond first," says Dork Sahagian, director of the Environmental Initiative at Lehigh University. "This is why observed reduction in seasonal ice cover in the Arctic Ocean is a harbinger of global warming, and scientists are taking it very seriously."
A record summer
The Arctic ice cap always grows in the winter and shrinks in the summer. The average minimum area from 1979, when precise satellite mapping began, until 2000 was 2.69 million square miles, similar in size to the contiguous area of the U.S. The new summer low, measured on September 19, was 20 percent below that. The difference between the average ice area and the area this summer was about 500,000 square miles over the years, an area roughly twice the size of Texas, the scientists say.
This summer was the fourth in a row during which the ice-cap areas were sharply below the long-term average, says Mark C. Serreze, a scientist at the snow and ice center and a professor at the University of Colorado at Boulder.
Serreze says the role of accumulating greenhouse-gas emissions has become increasingly apparent in recent years with rising air and sea temperatures. "With all that dark open water, you start to see an increase in Arctic Ocean heat storage," he says. "Come autumn and winter, that makes it a lot harder to grow ice, and the next spring you're left with less and thinner ice. And it's easier to lose even more the next year."
Still, many scientists say it is not yet possible to determine what portion of Arctic change is being caused by emissions from human sources and how much is just the climate's usual wiggles.
Compared with climate changes that have occurred over the past 4,000 to 7,000 years, the current changes are nothing alarming, according to Patrick Michaels, a professor of environmental sciences at the University of Virginia. "We had a large temperature rise in the Arctic 75 years ago that was faster than the one that is occurring now," he says.
A different arctic
Other experts are also expressing caution. William L. Chapman, a sea-ice researcher at the University of Illinois at Urbana-Champaign, notes that the size of the ice cap can vary tremendously, in part because of changes in wind patterns, which can cause the ice to heap up against one Arctic shore or drift away from another.
When current conditions are examined in the context of the broad historical context, says Michaels, the notion "of apocalypse becomes somewhat dulled."
In any case, Serreze, of the University of Colorado, says the Arctic is "becoming a profoundly different place than we grew up thinking about."