LOS ANGELES (Reuters) - The Arctic’s sea ice pack thawed to its third-lowest summer level on record, up slightly from the seasonal melt of the past two years but continuing an overall decline symptomatic of climate change, U.S. scientists said on Thursday.

The island of Big Diomede sits in the morning mist on the Russian side of the Bering Strait as seen from the Russian research vessel Professor Khromov August 28, 2009. REUTERS/Jeffrey Jones

The range of ocean remaining frozen over the northern polar region reached its minimum extent for 2009 on September 12, when it covered 1.97 million square miles (5.1 million square km), and now appears to be growing again as the Arctic starts its annual cool-down, the National Snow and Ice Data Center reported.

That level falls 20 percent below the 30-year average minimum ice cover for the Arctic summer since satellites began measuring it in 1979, and 24 percent less than the 1979-2000 average, the Colorado-based government agency said.

This summer’s minimum represents a loss about about two-thirds of the sea ice measured at the height of Arctic winter in March. By comparison, the Arctic ice shelf typically shrank by a little more than half each summer during the 1980s and 1990s, ice scientist Walt Meier said.

The lowest point on record was reached in September 2007, and the 2009 minimum ranks as the third smallest behind last year’s level. But scientists said they do not consider the slight upward fluctuation again this summer to be a recovery.

The difference was attributed to relatively cooler temperatures this summer compared with the two previous years. Winds also tended to disperse the ice pack over a larger region, scientists said.

“The long-term decline in summer extent is expected to continue in future years,” the report said.

The U.S. government findings were in line with measurements reported separately by the Nansen Environmental and Remote Sensing Center in Norway, which reported this summer’s minimum ice extent at just under 5 million square km (1.93 million square miles).

Scientists regard the Arctic and its sea ice as among the most sensitive barometers of global warming because even small temperature changes make a huge difference.

“If you go from a degree below freezing to 2 degrees above freezing, that’s a completely different environment in the polar region,” Meier said. “You’re going from ice skating to swimming. Whereas if you’re on a tropical beach and it’s 3 degrees warmer, you probably wouldn’t even notice it.”

World leaders will meet at the United Nations in New York on Tuesday to discuss a climate treaty due to be agreed on in December.

MARITIME IMPLICATIONS

The shrinking polar cap poses a loss of crucial habitat for polar bears and has implications for maritime shipping, opening up new routes to navigation.

Once again this year, the Northern Sea Route through the Arctic Ocean along the coast of Siberia opened, enabling two German ships to navigate the passage with Russian icebreaker escorts.

Russian vessels have traversed the passage many times over the years, but the maritime fleets of other nations are showing more interest in the route as the summer thaw expands.

This year, the Amundsen’s Channel through the Northwest Passage also opened briefly, as it did in 2008, but the deeper Parry’s Channel did not. Both opened in 2007.

Scientists have voiced concern for years about the alarming decline in the size of the Arctic ice cap, which functions as a giant air conditioner for the planet’s climate system as it reflects sunlight back into space.

As a greater portion of the ice melts, larger expanses of darker sea water are exposed, absorbing more sunlight and adding to the global warming effect attributed to rising levels of heat-trapping greenhouse gases emitted into the atmosphere by human activity.

Scientists also have measured a thinning of the frozen seas, as older, thicker ice more resilient to warming temperatures gives way to younger, thinner layers that melt more easily in summer.

Scientists monitor Antarctic sea ice as well, but the Arctic is considered a more critical gauge of climate change because more of the northern sea ice remains frozen through the summer, playing a bigger role in cooling the planet.