By Michael McCarthy, Environment Editor
Published: 30 December 2006

A vast ice shelf in the Canadian Arctic has broken up, a further sign of the astonishing rate at which polar ice is now melting because of global warming.

The Ayles ice shelf, more than 40 square miles in extent - over five times the size of central London - has broken clear from the coast of Ellesmere Island, about 500 miles south of the North Pole in the Canadian Arctic, it emerged yesterday.

The broken shelf has formed an ice island, in what a leading scientist described as a "dramatic and disturbing event", citing climate change as the cause.

The news caps a dramatic year of discovery about just how quickly the polar ice is disappearing.

It comes as America's leading climate scientist, James Hansen, warns in today's Independent that the Earth is being turned into "a different planet" because of the continuing increase in man-made emissions of greenhouse gases.

The break-up of the Ayles shelf occurred 16 months ago, in an area so remote it was not at first detected. "This is a dramatic and disturbing event," said Professor Warwick Vincent of Laval University in Quebec City. "It shows that we are losing remarkable features of the Canadian North that have been in place for many thousands of years."Ice shelves float on the sea, but are connected to land (as opposed to ice sheets, which are wholly land-based). In the past five years, several ice shelves along the fringes of the Antarctic peninsula have started to become unstable or break up. The most spectacular was the 2002 collapse of the Larsen B ice shelf, the size of Luxembourg.

Until now, there had not been a similar event among the six major shelves remaining in Canada's Arctic, which are packed with ancient ice that is more than 3,000 years old.

Professor Vincent, who studies Arctic ecosystems, travelled to the newly formed ice island and was amazed at what he saw. "It's like a cruise missile has come down and hit the ice shelf," he said. "Unusually warm temperatures definitely played a major role. It is consistent with climate change." The collapse was picked up by the Canadian Ice Service, which notified Luke Copland, head of the new global ice laboratory at the University of Ottawa. Using US and Canadian satellite images, as well as seismic data - the event registered on earthquake monitors more than 150 miles away - Professor Copland discovered that the ice shelf collapsed in the early afternoon of 13 August 2005. Scientists were surprised at the speed of the event, Professor Copland said - it took less than an hour.

There have already been several disturbing indications this year that the Arctic ice is melting at a much faster rate than expected. In September, two Nasa reports showed a great surge in the disappearance of the winter sea ice over the past two years, with an area the size of Turkey disappearing in 12 months.