Washington Rising temperatures caused ice to melt in Greenland at a record rate this year, climate scientists reported Monday.
"The amount of ice lost by Greenland over the last year is the equivalent of two times all the ice in the Alps or a layer of water more than one-half-mile deep covering Washington, D.C.," said Konrad Steffen, an Arctic expert at the University of Colorado in Boulder.
Greenland is about one-quarter as big as the continental United States, and 80 percent of it is covered by a massive ice sheet. It got its name because, from about A.D. 1000 to 1300, during the so-called "Medieval Warm Period," it was warm enough to support forests and thriving colonies of Viking settlers.
This year's melt lifted global sea levels by about two one-hundredths of an inch, Steffen said. If the entire ice cap melted, it could raise the sea by 21 feet, swamping coastal cities and low-lying islands, but such a catastrophe isn't expected for at least a thousand years, if ever.
Steffen, who has spent 18 seasons working on the Greenland ice cap, attributed the accelerated melting to an air temperature increase of about 7 degrees Fahrenheit since 1991.
Ten percent more ice melted this year than in 2005, the previous record year. Since 1979, when satellite data over Greenland began, melting has increased by a total of 30 percent, he said.
Increases in snowfall thicken the ice at higher elevations in the interior of Greenland, but glaciers around the coast have been thinning and sliding more rapidly toward the sea. The gain in the center is more than offset by the loss around the margins, Steffen said.
This acceleration is partly caused by water trickling down through huge tunnels in the ice, known as moulins. The water lubricates the bases of glaciers and speeds their flow toward the sea.
"The more lubrication there is under the ice, the faster that ice moves to the coast," Steffen said. "We know the number of moulins is increasing."
For example, the massive Jacobshavn glacier on the west coast of Greenland has sped up nearly twofold in the last decade.
Data on the recent melting trends were collected by a Defense Department meteorology satellite program that checks the weather for military purposes. In addition, Steffen's team maintains a set of 22 observation posts on the ice cap.
He reported his latest findings at the fall meeting of the American Geophysical Union in San Francisco.
Along with melting glaciers, scientists are concerned about the shrinking of the ice covering the Arctic Ocean. In October, scientists reported another record loss of sea ice in the far north. Floating ice covered 39 percent less area since satellite observations began in 1979. The loss exposes more dark-colored water, which absorbs heat rather than reflecting it as snow and ice do, thereby contributing to global warming.