An ice shelf almost the size of Manhattan has broken off from an island in the Canadian Arctic, researchers revealed today, warning that the near-record loss of polar ice cover this summer was an indicator of the changes global warming would inflict on mankind.

The region’s ice shelf has lost 83 square miles this summer, the second-biggest retreat of ice cover in the Arctic Ocean since satellite measurements began 30 years ago, Martin Jeffries of the US National Science Foundation and University of Alaska Fairbanks, said.

But unlike retreats in the last century, the current warming of the climate means the ice shelves will not reform.

“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,” said Derek Mueller, an Arctic ice shelf specialist at Trent University in Ontario.

The 4,500-year-old Markham Ice Shelf separated from Ellesmere Island in early August and the 19-square-mile shelf is now adrift in the Arctic Ocean, Mr Mueller said.

“The Markham Ice Shelf was a big surprise because it suddenly disappeared. We went under a cloud for a bit during our research and, when the weather cleared up, all of a sudden there was no more ice shelf. It was a shocking event that underscores the rapidity of changes taking place in the Arctic,” he explained.

Two large sections of ice have also detached from the Serson Ice Shelf, shrinking it by 47 square miles - or 60 per cent. The Ward Hunt Ice Shelf, which lost 8 square miles earlier in the summer, has also continued to break up.

Mr Mueller said that a further 8 square miles had since broken off the shelf - which at around 155 square miles and 40-metres-thick is the largest remaining in the Arctic.

In all, the Arctic ice shelves have lost an area more than three times the size of Manhattan Island this summer – 23 percent of their total mass - the researchers said. The loss is more than 10 times initial predictions, they added.

Warwick Vincent, director of Laval University’s Centre for Northern Studies and a researcher in the program ArcticNet, said extensive cracks in the Ward Hunt shelf meant that it would continue to disintegrate.

“Clearly the long-term viability of that ice shelf is now actually short-term,” he said.

Formed by accumulating snow and freezing meltwater, ice shelves are large platforms of thick, ancient sea ice that float on the ocean’s surface but are connected to land.

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