BOULDER, Colorado (CNN) -- Ice cover in the Arctic Ocean, long held to be an early warning of a changing climate, has shattered the all-time low record this summer, scientists say.
Satellite image shows the Northwest Passage, marked in yellow, is now fully navigable.
Additionally, the European Space Agency said nearly 200 satellite photos this month taken together showed an ice-free passage along northern Canada, Alaska and Greenland, according to news reports. Ice was retreating to its lowest level since such images were first taken in 1978, according to a report from The Associated Press.
Using satellite data and imagery, the U.S. National Snow and Ice Data Center (NSIDC) now estimates the Arctic ice pack to cover 4.24 million square kilometers (1.63 million square miles) -- equal to just less than half the size of the United States.
That figure is about 20 percent less than the previous all-time low of 5.32 million square kilometers (2.05 million square miles) set in September 2005
Mark Serreze, senior research scientist at NSIDC, termed the decline "astounding."
"It's almost an exclamation point on the pronounced ice loss we've seen in the past 30 years," he said.
Most researchers had anticipated the complete disappearance of the Arctic ice pack during summer months would happen after the year 2070, he said, but now, "losing summer sea ice cover by 2030 is not unreasonable."
Leif Toudal Pedersen of the Danish National Space Center told the AP that Arctic ice has shrunk to some 1 million square miles. The previous low was 1.5 million square miles, in 2005.
"The strong reduction in just one year certainly raises flags that the ice (in summer) may disappear much sooner than expected," Pedersen said in an ESA statement posted on its Web site Friday, according to AP.
Scores of peer-reviewed scientific studies have documented a steady, worldwide decline in ice cover, from the sea-bound ice covering the North Pole to the vast, land-based ice sheets that cover the Antarctic continent. Glaciers, from Greenland to the Alps to Mount Kilimanjaro near the equator, have also been vanishing.
The loss of land-based ice is predicted to lead to a future rise in sea levels. Most estimates predict a rise ranging from a few inches to a meter or more. A substantial rise in sea level could imperil low-lying areas from Bangladesh to Miami to Lower Manhattan, and could magnify the damage from landfalling hurricanes and cyclones.
While the loss of sea ice, like the Arctic ice pack, would not contribute to sea level rise, wildlife experts say it could alter the Arctic ecology, threatening polar bears and other mammals and sea life.
Scientists add that an ice-free Arctic could also accelerate global warming, as white-colored ice tends to deflect heat, while darker-colored water would absorb more heat.
But along with concerns, the melting Arctic also raises possible opportunities on business and political fronts. This summer, both Russia and the United States made efforts to inventory the potential mineral wealth on the ocean floor beneath the declining ice pack. Russia also sent a submarine to the North Pole to stake a symbolic claim to the Arctic as a part of the Russian nation.
The decline in ice also raises the possibility of an ice-free "Northwest Passage," a shipping route north of the Canadian mainland that could provide a shortcut for transit between the Atlantic and Pacific.
It is possible that the Arctic sea ice could decline even further this year before the onset of winter, Serreze said. Ice levels can reach their low point anywhere from mid-September to early October. E-mail to a friend
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