WASHINGTON - Arctic sea ice melted this summer to the second-lowest level since record-keeping began more than 50 years ago, scientists reported Thursday, mostly blaming global warming.

"This is not a random event," said oceanographer James Overland of the U.S. National Oceanic and Atmospheric Administration. "It's a long-term change in Arctic climate."

The new measurements were taken by the National Snow and Ice Data Center in Boulder, Colo. It reported that the amount of ice covering the Arctic hit its lowest point late last week, covering just 1.67 million square miles. Only in 2007 was there less summer sea ice, which has been dramatically declining since scientists began using satellites to monitor melt in 1979. Other records go back to 1953.

Each summer, sea ice melts and then refreezes starting in the fall. The summer minimum is a key measurement for scientists monitoring man-made global warming. This year's level is 36 percent below the average minimum of 2.59 million square miles.

Since the 1980s, summer Arctic ice has shrunk from something the size of the Lower 48 United States to an area that covers just the region west of the Mississippi River, said senior scientist Walt Meier.

Scientists have predicted the Arctic will eventually be free of sea ice in the summer by mid-century. Sea ice is crucial for polar bears and walruses. It also reflects the sun's heat, so when the ice melts, Earth retains more warmth, Overland said.

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