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On March 31, the last day of the ice-growing season for 2007, ice covered 5.7 million square miles. That's slightly higher that the record low 5.6 million square miles measured last year.

"This year's wintertime low extent is another milestone in a strong downward trend," said researcher Walt Meier. "We're still seeing near-record lows (in sea ice) and higher-than-normal temperatures, and we expect this downward trend to continue in future years."

He also suggested that predictions of sea ice decline expected Friday from the Intergovernmental Panel on Climate Change may be conservative. Actual measurements are showing a steeper decline in the amount of sea ice that what models used by the panel predicted.

"It appears that the models are not capturing something that's going on," Meier said.

Declining sea ice has been blamed on higher winter temperatures in the Arctic, a result of rising greenhouse gases in the atmosphere and strong natural variability in the ice, Meier said.

Researchers track sea ice using satellite data from NASA, the National Oceanic and Atmospheric Administration and the U.S. Department of Defense, as well as data from Canadian satellites and weather observatories.

The annual sea ice minimum is measured in September. As winter begins in the north and temperatures drop, more sea ice is formed and the edge of the ice pack moves south.

The sea ice maximum is measured in March. As temperatures warm, the edge of the ice pack moves north. Off Alaska, the ice pack retreats completely out of the Bering Sea, through the Bering Strait and into the Arctic Ocean.

In the late 1970s and the early 1980s, a typical March measurement or Arctic sea ice would show it covering 6.4 million square miles, Meier said. That's been reduced in recent years by at least 600,000 square miles, an area more than double the size of Texas.

Sea ice is a key component in the habitat of northern marine mammals.

The loss of sea ice has fueled the call in the United States for actions to protect polar bears, which depend on the sea ice platform to hunt their main prey, ringed seals. The U.S. Fish and Wildlife Service, acting on a petition by the Center for Biological Diversity, will decide by January whether polar bears should be listed as "threatened" under the Endangered Species Act.

Females walruses off Alaska use the edge of the retreating sea ice in summer like a conveyor belt, moving north to different feeding areas as the ice moves, leaving their young on the ice platform as they dive to the bottom to feed on clams and other invertebrates. In recent years, including late summer 2004, the edge of the ice sheet moved north beyond the relatively shallow continental shelf to water too deep for walrus cows to dive.

The study noted regional differences in the amount of sea ice. For example, the amount of ice off Alaska in the eastern Bering Sea last month was higher than normal, Meier said. Ice in the Bering Sea is dynamic and highly dependent on wind, appearing farther south if northern winds dominate, he said.

Measurements indicate there was less ice than normal last month in the Barents Sea, off the northern coast of Norway, off Greenland and in the Canadian Atlantic.

Overall, the trend since 1979 shows sea ice decline throughout the Arctic at all longitudes.

"To see that, there really isn't another good explanation for that to happen other than warming air temperatures due to greenhouse gas emissions," he said.

The IPCC uses 20 climate models for predictions of sea ice trends, Meier said.

"All the models show a downward trend," he said.

IPCC models show the Arctic Ocean could be ice free in summers by 2070.

"Our data suggests it could easily happen by 2050 if current trends continue," Meier said.