A huge 19 square mile (55 square km) ice shelf in Canada's northern Arctic broke away last month and the remaining shelves have shrunk at a "massive and disturbing" rate, the latest sign 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 totaling 47 square miles had broken off the nearby Serson Ice Shelf, reducing it in size by 60 percent.

"The changes ... were massive and disturbing," said 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," said Derek Mueller, an Arctic ice shelf specialist at Trent University in Ontario.

"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 in an e-mailed statement from the research team sent late on Tuesday.

Mueller said the total amount of ice lost from the shelves along Ellesmere Island this summer totaled 83 square miles -- more than three times the area of Manhattan island.

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

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

BLEAK FUTURE

"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.

The first sign of serious recent erosion in the five shelves came in late July, when sheets of ice totaling almost eight square miles broke off the Ward Hunt shelf. Since then that shelf has lost another 8.5 square miles

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Ellesmere Island was once home to a single enormous ice shelf totaling around 3,500 square miles. All that is left of that shelf today are the four much smaller shelves that together cover little more than 300 square miles

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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 worries Ottawa, which fears 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 United States and other countries.

David Ljunggren