Record heat sweeps Arctic Sea, ice in 2007

This graphic shows in red the area of arctic ice that was present in the summer of 1980 but missing this summer. At lower right is the equivalent surface area in terms of the size of the United States.

(Credit: Credit: Don Perovich, U.S. Army Cold Regions Research and Engineering Laboratory)

SAN FRANCISCO--Warmth may not be an attribute you associate with a place where the sun doesn't shine in the winter and the sea freezes over, but all things are relative. And compared to earlier years, the Arctic was downright sweltering this year.

According to new research presented here at the the American Geophysical Union conference, the Arctic Ocean reached record high temperatures, arctic ice diminished to a record low, and ice melted on Greenland for a record number of days.

"In 2007, we had off-the-charts warming" of the Arctic Sea in the summer, said Mike Steele, an oceanographer with the Polar Science Center at the University of Washington.

Specifically, he said the Arctic Sea surface temperature was 3.5 degrees Celsius (6.3 degrees Fahrenheit) warmer than the 100-year historical average and 1.5 degrees Celsius (2.7 degrees Fahrenheit) warmer than the historical maximum. Two factors were at play in the heating: the sun and, to a lesser degree, warmer ocean currents, he said. In one area north of Russia, temperatures were 5 degrees Celsius (9 degrees Fahrenheit) above average.

This comparison shows the 'off-the-charts' relative warmth of the Arctic Ocean in the summer of 2007.

(Credit: Mike Steele, University of Washington)

There's a feedback loop that connects the ocean temperature and the melting of sea ice. "The ocean absorbs heat, which melts the ice, which means there's more open ocean, which means more heat is absorbed," said Don Perovich, an arctic ice scientist at the U.S. Army Cold Regions Research and Engineering Laboratory. "It's a classic positive feedback."

It's no surprise that the arctic ice is shrinking. But the new data gives a specific measurement for how much is gone.

Between September 1980 and September 2007, the Arctic Sea ice dropped from 7.8 million square kilometers to 4.2 million square kilometers, Perovich said. "You can see the Northwest Passage, the shortcut across the top of the world, was ice-free at the end of the summer."

For comparison, the area of ice is the same as all the states east of the Mississippi River and a broad swath of those to its west, he said.

The math behind the feedback loop involves a property called albedo, which measures the fraction of sunlight that's reflected. The albedo of ice is about 85 percent, compared to 7 percent for the ocean.

What got the feedback loop started is a subject of some debate. "The ice-albedo feedback needs a trigger," Perovich said.

Culprits include a surge of warm water from the Pacific Ocean and anomalous winds that may have pushed ice to create more open ocean. And global warming in general means warmer air, which means a later start to winter ice freezing and less freezing when it does begin, Steele said.

Steele estimates that 2007's warm summer will reduce ice thickness by about a meter, Steele added.

With thinner ice, it's easier to start the feedback loop again. "The ice is more vulnerable to a short-term wind event," Perovich said.

Greenland, too, is showing signs of warming.

Red areas here show areas of Greenland where ice melting in 2007 lasted unusually long--the darkest being 30 days more than the average of the years from 1988 through 2006.

(Credit: Credit Marco Tedesco, University of Maryland)

"2007 set a new record, with melting occurring for 25 to 30 days longer than the average of 1980 to 2006," said Marco Tedesco of the University of Maryland.

The rate of increase in melting since 1988 is about 19,000 additional square kilometers each year, about 1.5 times the size of Maryland, Tedesco said.

Greenland, too, has an albedo-related feedback loop. When less snow falls, older and darker snow is more exposed, and this older snow absorbs more heat, Tedesco said. That albedo effect, combined with unusually high temperatures, were responsible for the increased melting, Tedesco said.

Arctic Sea ice melting doesn't increase sea level, but Greenland is another matter: all its water is on land today, so thawing will increase oceans.