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Friday, May 23, 2008

Greenland's Ice Sheet predicted to melt faster

by Larry O'Hanlon, Discovery News, May 8, 2008

The idea that Greenland's ice melts sluggishly in response to global warming has long been one hedge against rapid global sea level rise -- but the idea may be wrong, say researchers.

New geologic evidence from the seafloor off the southern tip of Greenland shows that during the two past periods of global warming, the melting of Greenland glaciers was right in synch with rising global temperatures -- rather than lagging behind as models have predicted.

In other words, the ice is very sensitive to recent losses of ice there could be the beginning of a much larger melt than expected.

"People had thought that there was this thermal lag," said Anders Carlson of the University of Wisconsin in Madison.

But offshore sediments that record the debris that is washed out from valleys when glaciers recede tell another story. As Carlson said, "As soon as it starts warming, it starts to melt."

The evidence, which Carlson and his colleagues present in the May issue of the journal Geology, comes in the form of titanium and iron found in cores of sediments offshore at what's called Eirik Drift.

The only sources for the two elements are land-based rocks which wash out of glacial valleys and into the sea once glaciers melt back and expose the ground to the weather. That process, the scientists figure, is a pretty good sign of a warmer climate.

Those two warmer periods were about 120,000 to 132,000 years ago and 10,000 to nearly 15,000 years ago.

"The last very big change was the one that ended the last ice age," explained Kurt Cuffey, a professor in the Geography and Earth and Planetary Sciences departments at the University of California at Berkeley. "What they found pretty clearly is that the melt…began immediately."

By immediately, Cuffey explained, he means geologically speaking -- within a few centuries or less.

The new method fills in a void of information caused by the glaciers themselves. Each new glacial advance tends to obliterate the evidence of past surges and retreats, unless it's left piled up at what was once the end of the glacier.

In the case of Greenland, however, the glaciers end in the sea, which complicates the job of locating and studying them.

As for how it all relates to the recent accelerated melt and net loss of ice mass over much of Greenland, Carlson said their data could herald a lot more of the same.

"What we can say is that (the Greenland Ice Sheet) responded very fast," said Carlson. "Our data suggest that the current negative mass balance is the beginning of a long-term trend."

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