A 25-square-mile shelf of floating ice that jutted into the Arctic Ocean for 3,000 years from Canada’s northernmost shore broke away abruptly in the summer of 2005, apparently freed by sharply warming temperatures and jostling wind and waves, scientists said yesterday.

The Ayles ice shelf, as the ancient 100-foot-thick slab was called, drifted out of a fjord along the north coast of Ellesmere Island when the jumbled sheath of floating sea ice that tended to press against the coast there even in summers was replaced by open waters because of the warming, the scientists said.

The change was first noticed by Laurie Weir of the Canadian Ice Service as she examined satellite images taken of Ellesmere and surrounding ice on and after Aug. 13, 2005. In less than an hour, around midday that day, a broad crack opened and the ice shelf was on its way out to sea.

The shelf is one of the few remnants of a broad expanse of floating shelves of ice that once protruded along much of the Ellesmere coast, somewhat like the brim on a hat.

Such shelves are far thicker and older than the milling cloak of sea ice that drifts atop the Arctic Ocean. The sea ice consists of floes ranging from 3 to 9 feet thick or so that are built up over just a few years.

The Arctic sea ice has experienced sharp summertime retreats for several decades, adding to evidence of significant warming near the North Pole. (Neither melting ice shelves nor sea ice contribute to rising sea levels because they sit in the sea already, like ice cubes in a drink.)

Ninety percent of the 3,900 square miles of ice shelves that existed in 1906 when the Arctic explorer Robert Peary first surveyed the region are gone, said Luke Copland, the director of the University of Ottawa’s Laboratory for Cryospheric Research.

In a paper summarizing the event but not yet published, Dr. Copland and other researchers said that the transformation of the Ayles ice from a shorebound shelf to a drifting ice island appeared to be a result of unusual Arctic warmth in 2005 on top of a longer-term warming trend.

He said that it was premature to attribute the breakaway to human-caused climate change, although he said that it was a clear sign the warming in the region was producing significant and abrupt changes, and more were likely in coming years. “The quick pace of these changes right now is what stands out,” he said.

The age of the Ayles ice shelf was estimated by using chemical means to date driftwood found behind it, said Derek Mueller, one of those who helped write the paper, from the University of Alaska in Fairbanks.
(Por Andrew C. Revkin, The N.Y.Times, 30/12/2006)