Portrait of a meltdown: many factors led to 2007's record low in Arctic sea ice.

A variety of climatological factors converged this year in a perfect storm that dramatically melted the Arctic Ocean's ice cover to a record low. The abrupt downturn could be a harbinger of ice-poor summers for decades to come.

In late summer, scientists reported that Arctic sea ice had shrunk to cover only about 4.2 million square kilometers (SN: 10/13/07, p. 238). That area is about 38 percent below the long-term average for late-summer ice coverage. Moreover, it's a striking 23 percent below the previous record low, set just 2 years ago. An adverse combination of factors contributed to this year's steep decline, researchers noted last week at a meeting of the American Geophysical Union in San Francisco.

First, a long-term trend in thinning and shrinkage of Arctic ice set the stage for this year's meltdown, says Jinlun Zhang, an oceanographer at the University of Washington in Seattle. End-of-summer ice coverage has been declining by about 11.4 percent per decade since 1979. Also, average ice thickness decreased by about 1.13 meters, or 22 percent, between 1981 and 2000.

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Second, Zhang notes, unusually strong summer winds pushed much of the ice out of the central Arctic, leaving a large area of thin ice and open water. Third, a decrease in cloud cover in the Arctic--a trend suspected but not confirmed earlier this year (SN: 6/16/07, p. 382)--allowed more sunlight to reach the ocean. Because open water absorbs more of the sun's radiation than snow-covered ice, it significantly boosts warming trends both for the ocean and for the atmosphere above it (SN: 11/12/05, p. 312). This so-called ice/albedo feedback accelerated this year's melting, says Zhang.

In parts of the Arctic Ocean this year, sea surface temperatures were 3.5[degrees]C warmer than average and a full 1.5[degrees]C warmer than previously recorded highs, says Michael Steele, also of the University of Washington in Seattle. All that warm water chewed away at Arctic ice from below. In some parts of the Beaufort Sea, north of Alaska and western Canada, ice that started the summer 3.3 m thick ended up measuring just 50 centimeters, says Donald K. Perovich, a geophysicist at the U.S. Army Cold Regions Research and Engineering Laboratory in Hanover, N.H.

About 70 cm of that shrinkage resulted from melting of the ice's upper surface--a typical amount for the summer, says Perovich. However, a whopping 2 in or so of that erosion, about five times the normal summer loss, occurred from below.

The thinning conceals the true extent of ice loss, says Perovich. "There's a lot less ice there than we think," he notes. "And the farther we go down this path, the harder it is to get back."

Indeed, the Arctic meltback may be self-perpetuating, says Steele. In some areas, the average date for winter freeze-up is now 2 months later than usual. The extra heat absorbed during summer months will suppress ice thickness by as much as 75 cm, about half the growth in thickness during an average winter.

Has the meltdown in the Arctic reached a point of no return? Many scientists, including Perovich, speculate that it has. "Years from now, we'll look back at 2007 and be amazed," he says.