海角大神

Arctic sea ice, a key indicator of climate change, melted to its lowest level on record this year before beginning its autumnal freeze, researchers at the听U.S. National Snow and Ice听Data Center听said on Wednesday.

The extent of ice听probably hit its low point on Sept. 16, when it covered 1.32 million square miles (3.42 million square km) of the Arctic听Ocean, the smallest amount since satellite records began 33 years ago.

Changing weather conditions could further shrink the extent, the center said. A final analysis is expected next month.

The record was broken on Aug. 26, when the ice听shrank below the record set in 2007. After that, it kept melting for three more weeks, bringing the ice听extent - defined by NSIDC as the area covered by at least 15 percent ice听- to nearly half of the 1979-2000 average.

"We are now in uncharted territory,"听Mark Serreze, the center's director, said in a statement. "While we've long known that as the planet warms up, changes would be seen first and be most pronounced in the Arctic, few of us were prepared for how rapidly the changes would actually occur."

The summer听ice isn't just dwindling. It is also thin, relatively fragile seasonal ice听instead of the hardier multi-year ice听that can better withstand bright sunlight.

"The strong late-season decline is indicative of how thin the ice听cover is," said NSIDC's Walt Meier. "Ice has to be quite thin to continue melting away as the sun goes down and fall approaches."

The Arctic听is a potent weather-maker for the temperate zone, and is sometimes dubbed Earth's air conditioner for its cooling effects. However, as ice听wanes and temperatures rise in the far north, the Arctic听could add more heat and moisture to the climate system.

More extreme weather?

"What happens in the Arctic听doesn't stay in the Arctic," said听Dan Lashof, a climate scientist at the nonprofit Natural Resources Defense Council. "This has a real impact on Americans where they live and work."

Melting Arctic听ice changes the shape and position of the jetstream, allowing tropical air to penetrate further north and Arctic听air to penetrate further south, Lashof said in a telephone interview, leading to more extreme weather.

"That is a truly staggering rate of melting, far beyond what scientists thought would happen a few years ago,"听Bob Ward听of the听London School of Economics听and Political Science said in a statement. "Policy-makers need to wake up to the scale and pace of the impacts from climate change."

Recent climate models suggest the Arctic听could be free of听ice听before 2050. But the observed rate of melting is faster than what is shown in many of the models, according to NSIDC scientist听Julienne Stroeve.

Both the听Northwest Passage听along听Canada's coast and the Northern Sea听Route along听Russia听were open to traffic this summer, and investors gathered in听Alaska听last month to discuss commercial and transportation opportunities for the Arctic.

The environmental group听Greenpeace International听took issue with that approach.

"Rather than dealing with the root causes of climate change, the current response from our leaders is to watch the听ice听melt and then divide up the spoils," the group's executive director,听Kumi Naidoo听said in a statement.