Arctic sea ice levels have reached their third-lowest level on record, opening a highway for northern mariners but causing problems for the mammals that live in the North.

Scientists at the U.S. National Snow and Ice Data Center in Colorado released their preliminary assessment Wednesday of minimum sea ice levels across the Arctic this year. The figure dropped to 4.76 million square kilometres, the third-lowest since satellites began keeping track. It is also nearly 40 per cent below the 20-year average.

"This is the third time in the satellite record that we've fallen below five million square kilometres for the minimum," said director Mark Serreze. "All of those have occurred in the last four years."

Last year's minimum was just over that threshold at 5.1 million square kilometres.

The 2010 figure destroys early optimism that Arctic sea ice was recovering.

Prospects dim for those who depend on ice

In April, the centre published data showing that sea ice — defined as ocean with a minimum of 15 per cent ice cover — had almost recovered to the 20-year average. That ignited a flurry of interest from climate change skeptics.

"Last winter, old multi-year ice drifted into Chukchi and Beaufort [seas] north of Alaska," Serreze said. "The thinking was that some of this ice may survive the summer melt season and kind of replenish ice conditions in the Arctic.

"It didn't happen. Most of it just melted away."

The warmest April on record in the Arctic made short work of it.

"While one may have heard in some circles that the Arctic sea ice cover is recovering, I'm afraid that that's not happening," said Serreze. "This is simply sharpening the long-term downward trend."

The Polar Science Center at the University of Washington also reported in March that the total volume of Arctic sea ice was the lowest it has been since 1979 and is 38 per cent below the maximum value.

Some Arctic mammals have noticed.

In northwestern Alaska, tens of thousands of walruses are now resting on shore instead of on the ice shelves they usually use. The same phenomenon occurred in 2007 — the lowest year on record for sea ice — and in 2009.

Serreze said there's no solid ice in the Chukchi Sea until about 80 degrees north latitude, far north of the usual line.

"The prospects for the future are not that good, if you depend on ice," he said.

Late start to Arctic winter

The retreating ice has opened up a new challenge for adventurous mariners.

This summer has been one of the first on record during which both Canada's Northwest Passage and the Northeast Passage over Russia's Arctic coast have been ice-free at the same time. That has opened the door to a single-season Arctic circumnavigation of the globe, a voyage that two vessels are currently attempting.

The Norwegian vessel Northern Passage, a 10-metre Fiberglas-hulled trimaran, is now between the Canadian mainland and Victoria Island off the central Arctic coast.

"We are cruising along with a good tailwind and good speed," wrote expedition leader Børge Ousland in his blog.

"People we have spoken to say that there [is] very little ice this year, and the water temperature is incredibly seven degrees even here ... so we don't expect the passage to freeze up before the beginning of October."

Ousland expects the sailing ships to reach Pond Inlet, Nunavut, by Sept. 23.

"Inuit I spoke to in Point Barrow [Alaska] said that 20 years ago it started to snow in August," he wrote. "People walked around in shorts when we were there recently, and they said that winter nowadays comes in October, sometimes November."

The second vessel attempting the voyage, the Peter 1, an 18-metre steel-hulled vessel out of Russia, is near Cambridge Bay, Nunavut.