WASHINGTON - The dramatic decline of Arctic ice in recent summers greatly accelerated this year, a sign that some alarmed scientists worry could mean global warming is picking up speed.

Greenland's ice sheet shrank far below what scientists had previously seen, and the volume of Arctic sea ice at summer's end was half what was recorded four years earlier, according to new NASA satellite data.

"The Arctic is screaming," said Mark Serreze, senior scientist at the government's snow and ice data center in Boulder, Colo.

Just last year, two top scientists had surprised their colleagues by projecting the possible disappearance of summer Arctic sea ice by 2040.

After reviewing his own new data this week, NASA climate scientist Jay Zwally said: "At this rate, the Arctic Ocean could be nearly ice-free at the end of summer by 2012, much faster than previous predictions."

As is often the case with global warming, however, more data also raise more questions for researchers. Chief among them: Was 2007's record melt a blip in a steadier trend or is the impact of global warming being felt faster even than most computer models' worst-case scenarios?

"The Arctic is often cited as the canary in the coal mine for climate warming," Zwally said. "Now, as a sign of climate warming, the canary has died."

The new data, based on satellite images, came amid a U.N.-led conference on the Indonesian island of Bali that is aimed at laying the groundwork for international consensus on tackling climate change. But there is wide disagreement among nations about how to proceed.

What happens in the Arctic has implications worldwide. Because so much water is frozen in the form of ice, the melting eventually will cause a rise in sea levels.

A decline of sea ice also will affect weather patterns, with less rain and snow in some regions and more in others.

The extent of the 2007 retreat in summer ice surprised many scientists.

"I don't pay much attention to one year," said Waleed Abdalati, NASA's chief of cyrospheric sciences, "but this year the change is so big, particularly in the Arctic sea ice, that you've got to stop and say, 'What is going on here?' You can't look away from what's happening."

Greenland, which is mostly covered by ice, is of particular concern to scientists. If it completely melted - likely over a period of centuries, not decades - it could add more than 22 feet to the world's sea level.

Over nearly three decades of satellite observations, the extent of Greenland's ice melt has increased and then decreased every couple of years. This summer did not fit the pattern.

"I'm quite concerned," said Konrad Steffen of the University of Colorado, which gathered the latest data. "Now I look at 2008. Will it be even warmer than the past year?"

Melting of ice on Greenland and in the Arctic Ocean also alarms scientists because it becomes part of a spiral.

White sea ice reflects about 80 percent of the sun's heat off Earth, NASA's Zwally said. Without sea ice, about 90 percent of the heat goes into the ocean, which then warms everything else. Warmer oceans then lead to more melting.

NASA scientist James Hansen is expected to tell a meeting of researchers in San Francisco tomorrow that the Greenland data suggest that the warming of Earth has reached a tipping point in some ways.

"We have not passed a point of no return," Hansen wrote in an e-mail. "We can still roll things back in time - but it is going to require a quick turn in direction."

For satellite images and details on measuring Arctic ice, plus Bali conference background, go to: