Seth Borenstein, Associated Press 23 Sep 09;
WASHINGTON – New satellite information shows that ice sheets in Greenland and western Antarctica continue to shrink faster than scientists thought and in some places are already in runaway melt mode.
British scientists for the first time calculated changes in the height of the vulnerable but massive ice sheets and found them especially worse at their edges. That's where warmer water eats away from below. In some parts of Antarctica, ice sheets have been losing 30 feet a year in thickness since 2003, according to a paper published online Thursday in the journal Nature.
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.
Antarctic coastal ice thinning surprises experts
Alister Doyle, Reuters 23 Sep 09;
OSLO (Reuters) - Scientists are surprised at how extensively coastal ice in Antarctica and Greenland is thinning, according to a study Wednesday that could help predict rising sea levels linked to climate change.
Analysis of millions of NASA satellite laser images showed the biggest loss of ice was caused by glaciers speeding up when they flowed into the sea, according to scientists at the British Antarctic Survey (BAS) and Bristol University.
"We were surprised to see such a strong pattern of thinning glaciers across such large areas of coastline -- it's widespread and in some cases thinning extends hundreds of kilometers inland," said Hamish Pritchard of BAS who led the study.
"We think that warm ocean currents reaching the coast and melting the glacier front is the most likely cause of faster glacier flow," he said in a statement.
"This kind of ice loss is so poorly understood that it remains the most unpredictable part of future sea level rise," he added. BAS said the study gave the "most comprehensive picture" of the thinning glaciers so far.
Rising seas caused by a thaw of vast stores of ice on Antarctica and Greenland could threaten Pacific islands, coasts from China to the United States and cities from London to Buenos Aires.
U.N. Secretary-General Ban Ki-moon said earlier this month global warming, blamed mainly on burning fossil fuels, could raise sea levels by 50 cm to 2 meters (20 inches to 6 ft 6 in) this century -- higher than most experts have predicted.
Among findings, Wednesday's study said 81 of 111 fast-moving glaciers in Greenland were thinning at twice the rate of slow-flowing ice at the same altitude.
"Dynamic thinning of Greenland and Antarctic ice-sheet margins is more sensitive, pervasive, enduring and important than previously realized," they wrote. "Dynamic thinning" means loss of ice due to a faster flow.
They said it was too early to determine whether the thinning was a sign that sea level rise would accelerate from a current rate of about 3 mm (0.12 inches) a year.
"Working that out is the next task," David Vaughan, a BAS glaciologist among the authors, told Reuters. Thinning in some areas could be caused by changes in snowfall, for instance, not the slide of ice toward the ocean, he said.
