Some of those areas are about a mile thick, so they've still got plenty of ice to burn through. But the drop in thickness is speeding up. In parts of Antarctica, the yearly rate of thinning from 2003 to 2007 is 50 percent higher than it was from 1995 to 2003.
These new measurements, based on 50 million laser readings from a NASA satellite, confirm what some of the more pessimistic scientists thought: The melting along the crucial edges of the two major ice sheets is accelerating and is in a self-feeding loop. The more the ice melts, the more water surrounds and eats away at the remaining ice.
"To some extent it's a runaway effect. The question is how far will it run?" said the study's lead author, Hamish Pritchard of the British Antarctic Survey. "It's more widespread than we previously thought."
The study doesn't answer the crucial question of how much this worsening melt will add to projections of sea level rise from man-made global warming. Some scientists have previously estimated that steady melting of the two ice sheets will add about 3 feet, maybe more, to sea levels by the end of the century. But the ice sheets are so big it would probably take hundreds of years for them to completely disappear.
As scientists watch ice shelves retreat or just plain collapse, some thought the problem could slow or be temporary. The latest measurements eliminate "the most optimistic view," said Penn State University professor Richard Alley, who wasn't part of the study.
The research found that 81 of the 111 Greenland glaciers surveyed are thinning at an accelerating, self-feeding pace. The key problem is not heat in the air, but the water near the ice sheets, Pritchard said. The water is not just warmer but its circulation is also adding to the melt.
"It is alarming," said Jason Box of Ohio State University, who also wasn't part of the study.
Worsening data, including this report, keep proving "that we're underestimating" how sensitive the ice sheets are to changes, he said.
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