SAN FRANCISCO, December 7 — The melting of Greenland glaciers and Arctic Ocean sea ice this past summer reached levels not seen in decades, scientists reported today.
This year's summertime melt, which provides more evidence of recent quick warming in the Arctic, is in part driven by natural climate oscillations, the researchers said. But they added that human-driven changes to the environment like the destruction of ozone and the emission of carbon dioxide could well have accelerated and enlarged the effect.
In September, the end of summer, ice coverage of the Arctic Ocean dipped to two million square miles before it started to grow again. Since 1978, when direct satellite measurements of sea ice started, the average summertime minimum has been 2.4 million square miles. Of the sea ice that survived, most was thinner than usual.
"That was probably the craziest summer I've ever seen up there," said Dr. Mark Serreze, a researcher at the National Snow and Ice Data Center in Boulder, Colo., and one of the scientists who presented the findings at a news conference at a meeting of the American Geophysical Union here.
Weather in the Arctic was unusually warm and stormy this year, which broke up ice and melted it more readily. The shrinking fits in with the trend since the late 1970's and general predictions of global warming. "It's the kind of change we'd expect to see," said Dr. James Morrison of the University of Washington in Seattle.
Other data, including some gathered from airplane flights, indicate that the Arctic has not been this ice-free at least since the 1950's. If the shrinking continues at current rates, year-round average sea ice coverage may drop by 20 percent by 2050, and the Arctic may be almost ice-free during summer months, Dr. Serreze said. "I believe we will continue to see reductions in sea ice cover, because I think we are having an effect on the climate," he said. "The rate of that change is debatable."
Other evidence, like melting permafrost and the northward spread of trees, has also suggested that the recent warming is quick and unusual.
The reduction of sea ice is expected to affect global ocean currents. Fresh water from melting ice is less dense than salty sea water and could prevent water from the deep ocean from rising. Open water should also be warmer than ice-covered ocean, because it absorbs more than 80 percent of sunlight that hits it, while ice reflects about 80 percent.
Satellite instruments also observed melting ice over 265,000 square miles of the Greenland ice sheet, exceeding the previous maximum melt area by 9 percent. Ice was melting in areas up to 6,560 feet in altitude that had never shown melting before.
Melted water flows to the base of glaciers, acting as a lubricant that speeds the sliding of the glaciers into the ocean.
In an article in the current issue of the journal Science, scientists at NASA's Jet Propulsion Laboratory in Pasadena, Calif., and the Royal Observatory of Belgium reported that melting glaciers have even changed the shape of the planet. Since 1997, the Earth has become slightly more oblate, like a pumpkin. That reverses a trend since the end of the last Ice Age when, relieved of the crushing ice sheets at the poles, the Earth had been bouncing back into a more spherical shape.
The authors of the Science article wrote that the addition of water melting from glaciers, which then shifted toward the equator, accounted for the squashing of the planet.
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