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A satellite image shows the separation of a large piece of the Ward Hunt Island Ice shelf last week. The island, which lies west of the break, is one of Canada's northernmost points of land and a base for Arctic research. ((NASA))

The fracture of a four-square-kilometre chunk of ice from the largest remaining ice shelf in the Arctic last week is a one-way change brought on by warming temperatures, says a scientist who has studied the process.

Derek Mueller, a polar scientist and research fellow at Trent University, in Peterborough, Ont., said the ice breaking off from the Ward Hunt Island Ice Shelf, just north of Ellesmere Island, marks the continuation of a process that has been years in the making.

"Ice shelves don't just break up. There's no karate chop," he said. "This is the result of a gradual weakening over time as a result of warming temperatures."

Mueller said the changes in the shelf, which surrounds Ward Hunt Island off the north coast of Ellesmere, provide further evidence the planet, and in particular the North, is warming due to climate change.

An expedition by U.S. explorer Robert Peary in 1906 put the size of the Ellesmere Island Ice Shelf at just under 9,000 square kilometres. The ice shelf has since broken up into smaller pieces, the largest of which is the Ward Hunt Island Shelf. The total area of all of these pieces is now less than 900 square kilometres.

No winter renewal of ice shelf

Mueller said if year-to-year warming wasn't occurring, Arctic researchers would have been able to detect some renewal of ice during the winter months, either through thickening of the ice or the spread of the shelves. But researchers haven't seen any sign of renewal, he said.

"This is a one-way change," he said.

Mueller was part of the team of polar researchers that first discovered in 2002 a large central crack in the ice shelf, which had occurred between 2000 and 2002. An expedition alongside Canadian Rangers on a patrol around Ellesmere Island in April 2008 found an 18-kilometre crack along the shelf, a further sign that fragmentation was likely.

"Ice shelf integrity was lost in 2002," said Mueller.

The 2002 fissure was blamed for the draining of an epishelf lake in Disraeli Fjord. Epishelf lakes are a unique ecosystem with both lighter freshwater and denser saltwater, with the freshwater component kept away from the ocean by the ice shelf while the bottom layer of seawater can mix with ocean water. The ecosystem is home to a unique mix of zooplankton and other tiny life forms.

Mueller said the next major epishelf lake potentially under threat in the region is the Milne epishelf lake, which is bound in the Milne fjord by the Milne Ice Shelf, the second-largest remnant of the Ellesmere Island Ice Shelf.

The ice island that formed last week is the largest to form off Canada's northern coast since the 2005 collapse of the Ayles Ice Shelf, which lay between the Milne and Ward Hunt Island shelves. The ice island that subsequently formed had a surface area of about 66 square kilometres.