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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.”