The ancient frozen dome cloaking Greenland is so vast that pilots have crashed into what they thought was a cloud bank spanning the horizon. Flying over it, you can scarcely imagine that this ice could erode fast enough to dangerously raise sea levels any time soon.

Along the flanks in spring and summer, however, the picture is very different. For a lengthening string of warm years, a lacework of blue lakes and rivulets of meltwater have been spreading ever higher on the ice cap. The melting surface darkens, absorbing up to four times as much energy from the sun as unmelted snow, which reflects sunlight. Natural drainpipes called moulins carry water from the surface into the depths, in some places reaching bedrock. The process slightly, but measurably, lubricates and accelerates the grinding passage of ice toward the sea.

Most important, many glaciologists say, is the breakup of huge semisubmerged clots of ice where some large Greenland glaciers, particularly along the west coast, squeeze through fjords as they meet the warming ocean. As these passages have cleared, this has sharply accelerated the flow of many of these creeping, corrugated, frozen rivers.

All of these changes have many glaciologists “a little nervous these days — shell-shocked,” said Ted Scambos, the lead scientist at the National Snow and Ice Data Center in Boulder, Colo., and a veteran of both Greenland and Antarctic studies.

Some fear that the rise in seas in a warming world could be much greater than the upper estimate of about two feet in this century made by the Intergovernmental Panel on Climate Change last year. (Seas rose less than a foot in the 20th century.) The panel’s assessment did not include factors known to contribute to ice flows but not understood well enough to estimate with confidence. All the panel could say was, “Larger values cannot be excluded.”

A scientific scramble is under way to clarify whether the erosion of the world’s most vulnerable ice sheets, in Greenland and West Antarctica, can continue to accelerate. The effort involves field and satellite analyses and sifting for clues from past warm periods, including the last warm span between ice ages, which peaked about 125,000 years ago and had sea levels 12 to 16 feet higher than today’s.

The Arctic Council, representing countries with Arctic territory, has commissioned a report on Greenland’s environmental trends, to be completed before the 2009 climate-treaty talks in Copenhagen, at which the world’s nations have pledged to settle on a long-term plan for limiting human-caused global warming.

Konrad Steffen, a University of Colorado glaciologist who has camped on the shoulders of Greenland’s ice sheet each year since 1990, is the lead author of the chapter in the report on Greenland’s climate. Last August, he and a team focusing on the ways meltwater might affect ice movement dropped a camera 330 feet down a water-filled moulin to explore whether the plumbing system can be mapped.

Research on alpine glaciers shows that as more water flows through such apertures, ice can shift more quickly. But eventually large sewerlike conduits form, limiting the lubrication effect. The camera drop was only an initial test.

Alberto Behar, a NASA engineer who designed the camera, said some unconventional methods were being considered to chart the flow of such water. “We had ideas to send rubber ducks down and see if they pop out in the ocean,” he said. “They’d have a little note saying, ‘Please call this number if you find me.’”

The changes seen in Greenland may turn out to be self-limiting in the short run; surging glaciers can flatten out and slow, for instance. Or they may be a sign that the island’s ice — holding about the same volume of water as the Gulf of Mexico — is poised for a rapid discharge. Scientists are divided on that question, and on whether there is a near-term risk from a Texas-size portion of West Antarctica’s ice sheet that is also showing signs of instability. This split divides those foreseeing a rise in the sea level of a couple of feet this century from water added by Greenland, West Antarctica and mountain glaciers, and a few experts who speak of a couple of yards in that time.

Those holding a more conservative view of Greenland’s near-term fate include Richard Alley of Pennsylvania State University, who noted that ice cores and tests of organic material from beneath the ice implied that the main mass of the Greenland ice sheet clearly endured thousands of years of warming in the past without vanishing.

“It’s basically a big lump of ice sitting on this bedrock,” Dr. Alley said in describing Greenland’s behavior in warm conditions. “What it tries to do is snow more in the middle and melt more on the edges. If it pulls its edges back, then there’s less area to melt, and that helps it survive. That’s why you can have a stable ice sheet in a warmer climate.”

But there is no significant debate on the long-term picture anymore. Should greenhouse-gas emissions follow anything close to a “business as usual” rise, the resulting warming and ice loss at both ends of the earth would cause coasts to retreat for centuries. While it was circumspect about near-term changes, the intergovernmental panel was confident about that long view.

The prospect of having no “normal” coastline for the foreseeable future has many scientists deeply concerned.

“What is at stake is the stability we have always taken for granted” both for coasts and climate itself, said Jason E. Box, an associate professor of geography at Ohio State University. Dr. Box presented fresh findings at the American Geophysical Union meeting last month showing that several Greenland glaciers accelerated sharply in direct response to warming, both in a warm spell starting in the 1920s and now.

Eric Rignot, a longtime student of ice sheets at both poles for NASA’s Jet Propulsion Laboratory, said he hoped the public and policymakers did not interpret uncertainty in the 21st-century forecast as reason for complacency on the need to limit risks by cutting emissions.

Dr. Rignot recently proposed that unabated warming could result in three feet of global sea rise just from water flowing off Greenland, three feet from Antarctica and 18 inches as the remaining alpine glaciers shrivel away.

This is similar to projections by the most prominent NASA climate scientist, James E. Hansen, but more than twice the three-foot rise that many glaciologists seem to agree on as an outer bound for what is possible by the end of the century.

“It is too early to reassure that all will stabilize, and similarly there is no way to predict a catastrophic collapse,” Dr. Rignot said. “But things are definitely far more serious than anyone would have thought five years ago.”

This article may be found at the following link: http://www.nytimes.com/2008/01/08/science/earth/08gree.html?ref=science