This winter saw the fifth lowest maximum ice extent on record since monitoring by satellite began in 1979, said the Colorado-based National Snow and Ice Data Center (NSIDC).

The period between 2004 and 2009 saw the lowest ice extent, said Charles Fowler, a University of Colorado (CU) glaciologist who led a team of scientists for the research.

The researchers found that the maximum sea ice extent for 2008 and 2009, reached on February 28, was 5.85 million square miles (15.2 million square kilometers), 278,000 square miles (720,000 square kilometers) less than the average extent between 1979 and 2000.

"Ice extent is an important measure of the health of the Arctic, but it only gives us a two-dimensional view of the ice cover," said Walter Meier, research scientist at the NSIDC and CU.

"Thickness is important, especially in the winter, because it is the best overall indicator of the health of the ice cover. As the ice cover in the Arctic grows thinner, it grows more vulnerable to melting in the summer."

Until recently, most Arctic sea ice survived at least one summer and often several. But the situation has changed dramatically, according to the scientists.

Thin seasonal ice, which melts and refreezes every year, now makes up about 70 percent of Arctic sea ice in wintertime, up from 40 to 50 percent in the 1980s and 1990s.

Thicker ice (9 feet, or 2.74 meters) that survives at least two summer seasons now only accounts for 10 percent of wintertime ice cover, down from 30 to 40 percent, the researchers said.

Last year, a team of researchers led by Ron Kwok of NASA's Jet Propulsion Laboratory in Pasadena, California, mapped for the first time sea ice thickness for the entire Arctic basin.

Kwok said that older, thicker sea ice is declining and being replaced with newer, thinner ice more vulnerable to melting in the summer, the US space agency noted in a statement.

A team of three British explorers on February 28 set out on an 85-day, 850-kilometer (530-mile) trek to the North Pole to measure the thickness of sea ice along the way.

Global warming is believed to be the main culprit in the rapidly melting north polar ice cap that is freeing up new sea routes and untapped mineral resources on the ocean bottom.

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