A huge 55-square-kilometre ice shelf in Canada's northern Arctic broke away last month and the remaining shelves have shrunk at a "massive and disturbing" rate. These are the latest signs of accelerating climate change in the remote region, scientists said on Tuesday.

They said the Markham Ice Shelf, one of just five remaining ice shelves in the Canadian Arctic, split away from Ellesmere Island in early August. They also said two large chunks totalling 120 square km had broken off the nearby Serson Ice Shelf, reducing it in size by 60%.

"The changes ... were massive and disturbing," says Warwick Vincent, director of the Centre for Northern Studies at Laval University in Quebec.

Temperatures in large parts of the Arctic have risen far faster than the global average in recent decades, a development that experts say is linked to global warming.

"These substantial calving events underscore the rapidity of changes taking place in the Arctic," says Derek Mueller, an Arctic ice shelf specialist at Trent University in Ontario.

End of an era

"These changes are irreversible under the present climate and indicate that the environmental conditions that have kept these ice shelves in balance for thousands of years are no longer present," he said.

Mueller said the total amount of ice lost from the shelves along Ellesmere Island this summer totalled 215 square km - more than three times the area of New York's Manhattan island.

The figure is more than 10 times the amount of ice shelf cover that scientists estimated on 30 July would vanish from around the island this summer.

"Reduced sea ice conditions and unusually high air temperatures have facilitated the ice shelf losses," says Luke Copland of the University of Ottawa.

"Extensive new cracks across remaining parts of the largest remaining ice shelf, the Ward Hunt, mean that it will continue to disintegrate in the coming years," he said.

Destructive summer

The first sign of serious recent erosion in the five shelves came in late July, when sheets of ice totalling almost 21 square km broke off the Ward Hunt shelf. Since then the shelf has lost another 22 square km.

Ellesmere Island was once home to a single enormous ice shelf of around 9000 square km. All that is left of that shelf today are the four much smaller shelves that together cover only about 800 square km.

Scientists say the ice shelves, which contain unique ecosystems that had yet to be studied, will not be replaced because they took so long to form.

The rapid melting of ice in the Canadian Arctic archipelago is a cause for concern to authorities in Ottawa, who fear foreign ships might try to sail through the waters without seeking permission first.

Last week Prime Minister Stephen Harper said Canada would toughen reporting requirements for ships entering its waters in the Far North, where some of those territorial claims are disputed by the US and other countries.