LOS ANGELES - The Arctic ice pack melted this summer to its third-smallest size on record, up slightly from the low points of the past two years but continuing an overall shrinking trend symptomatic of climate change, U.S. scientists said on Thursday.

In this undated photo provided by the U.S. Fish and Wildlife Service, walrus congregate on a beach in Alaska. Thousands of walrus are congregating on Alaska's northwest coast because of receding sea ice in the Arctic. An environmental group is seeking to list walrus as an endangered or threatened because of disappearing summer sea ice. (AP Photo/ U.S. Fish and Wildlife Service) Northern sea ice retreated to 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 sea 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.

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, with Chukchi and Beaufort seas especially chilly by comparison with 2007. 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 kilometers (1.93 million square miles).

Many scientists regard shrinking Arctic ice as among the most obvious signs of global warming. World leaders will meet at the United Nations in New York on September 22 to discuss a U.N. climate treaty due to be agreed 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 Arctic ice cover, as older, thicker ice more resilient to warming temperatures gives way to a younger, thinner layer that melts more easily in summer.

(Additional reporting by Alister Doyle in Oslo; Editing by Mary Milliken and Cynthia Osterman)