NY Times: Arctic Ice Cap Shrank Sharply This Summer, Experts Say

Arctic Ice Cap Shrank Sharply This Summer, Experts Say

By ANDREW C. REVKIN
Published: September 28, 2005
The floating cap of sea ice on the Arctic Ocean shrank this summer to what is probably its smallest size in a century, continuing a trend toward less summer ice that is hard to explain without attributing it in part to human-caused global warming, various experts on the region said today.

The findings are consistent with recent computer simulations showing that a buildup of smokestack and tailpipe emissions of greenhouse gases could lead to a profoundly transformed Arctic later this century in which much of the once ice-locked ocean is routinely open water in summers.

It also appears that the change is becoming self sustaining, with the increased open water absorbing solar energy that would be reflected back into space by bright white ice, said Ted A. Scambos, a scientist at the National Snow and Ice Data Center in Boulder, Colo., which compiled the data along with NASA.

"Feedbacks in the system are starting to take hold," Dr. Scambos said. "The consecutive record-low extents make it pretty certain a long-term decline is underway."

The North Pole ice cap always grows in winter and shrinks in the summer, but the new summer low, measured on Sept. 19th, was 20 percent below the average minimum ice extent measured from 2000 back to 1978, when precise satellite mapping of the ice began, the snow and ice center reported.

The difference between the average ice area and the area that persisted this summer was about 500,000 square miles, or twice the size of Texas, the scientists said.

This summer was the fourth in a row with ice extents sharply below the long-term average, said Mark Serreze, a senior scientist at the snow and ice center and a professor at the University of Colorado in Boulder.

A natural cycle in the polar atmosphere, the Arctic Oscillation, that contributed to the reduction in Arctic ice in the past was not a significant factor right now, he said, adding that rising temperatures driven by accumulating greenhouse-gas emissions had to be playing a role.

He and other scientists said that there could be more variability ahead, including some years in which the sea ice will grow. But they have found few hints that other factors, like more Arctic cloudiness in a warming world, might reverse the trend.

"With all that dark open water, you start to see an increase in Arctic Ocean heat storage," Dr. Serreze said. "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."

The result, he said, is that the Arctic is "becoming a profoundly different place than we grew up thinking about."

Other experts on Arctic ice and climate disagreed on details. For example, Ignatius G. Rigor at the University of Washington said that the change was likely due to a mix of factors, including residual influences from the atmospheric cycle.

But he agreed with Dr. Serreze that the influence from greenhouse gases had to be involved.

"The global warming idea has to be a good part of the story," Dr. Rigor said. "I think we have a different climate state in the Arctic now."