Arctic Melting Linked to Human Causes, Long-Term Review Finds

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Arctic temperatures have also varied dramatically from decade to decade and were abnormally warm from 1925 to 1945, the data show.

But the most recent reports demonstrate that Arctic temperatures have increased at almost twice the global average rate over the past century.

Natural variations "play a large role" in the Arctic's changing air temperature, Serreze said. But overall the observations are consistent with the melting that climate models have predicted would result from higher greenhouse-gas emissions.

"There has always been uncertainty about whether these observed changes are natural variability," he said.

"But we now have a consensus between observations and what the models are telling us. In my mind, it's very convincing that we're starting to the see the effects of human activity on Arctic ice cover."

Arctic Impact Goes Global

Human-caused warming may change the seasonal ice system and dramatically speed the loss of ice, said Bruno Tremblay of McGill University in Montreal, Canada, who was not involved in the new study.

"The system can recover from a low ice year. But, as warming continues, a critical thickness is reached beyond which the system no longer recovers," he said.

Open water absorbs solar energy that would otherwise have been reflected by ice, he explained, which perpetuates the melting cycle.

"So ocean water warms, and the ice forms later in the fall, and you typically have an earlier melt onset," he said.

Vanishing sea ice might also spur the melting of Greenland's ice cap by warming the ocean waters into which the glaciers spill.

"In a place like Greenland the amount of heat that waters carry in summer is directly related to how much ice there would be," said Ian Howat, a researcher with the University of Washington's Polar Science Center who was not involved with the Science report.

"One can almost imagine sea ice acting like a buffer around the coast of Greenland. Remove that buffer, and the heat could more efficiently get at the edge of the ice sheet."

No data exists to link shrinking sea ice cover with Greenland's glaciers, Howat said, but he sees an intriguing correlation.

"It's interesting that 2005 was the largest single loss of Arctic sea ice," he said.

"The glaciers in Greenland retreated more in 2005 than we've ever seen them retreat. One of the major points of our research is that the glaciers are much more sensitive to short-term variability than we had previously thought."

(Related: "Greenland's Ice Melt Grew by 250 Percent, Satellites Show" [September 20, 2006].)

Weather Effects

Many scientists believe that the loss of sea ice may have significant impacts on the world's climate.

The regions near Earth's equator function as a sort of furnace for the planet's climate system, while the Arctic acts as a kind of air conditioner by reflecting warming sunlight and cooling ocean waters. The two regions are intimately linked.

(See National Geographic magazine's "Global Warning: Signs From Earth.")

A loss of ice will likely change water temperatures and affect the circulation of ocean currents, altering regional climates, Serreze said.

"If you fundamentally change the nature of the Arctic, the rest of the system has to respond. The devil is in the details—what will those changes look like?" he asked.

Some models suggest reduced rainfall and increased droughts in the already arid western regions of North America.

The changes could also result in increased winter precipitation in western and southern Europe, as well as more intense storm tracks in the globe's mid latitudes.

But climate modeling is so complex that no one really knows, Serreze pointed out.

"To my mind that's more worrisome," Serreze said. "What kind of surprises could we be in store for?"

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