by David Ljunggren

OTTAWA - The incredibly rapid rate at which Canada's Arctic ice shelves are disappearing is an early indicator of the "very substantial changes" that global warming will impose on all mankind, a top scientist said on Wednesday.

Researchers announced late on Tuesday that the five ice shelves along Ellesmere Island in the Far North, which are more than 4,000 years old, had shrunk by 23 percent this summer alone.

The largest shelf is disintegrating and one of the smaller shelves, covering 19 square miles (55 square km), broke away entirely last month.

"Climate models indicate that the greatest changes, the most severe changes, will happen earliest in the highest northern latitudes," said Warwick Vincent, director of the Centre for Northern Studies at Laval University in Quebec.

"This will be the starting point for more substantial changes throughout the rest of the planet.... Our indicators are showing us exactly what the climate models predict," he told Reuters in an interview.

Global warming is forecast to generate more damaging weather extremes such as hurricanes, cyclones and floods.

Vincent, who has visited the ice shelves along Ellesmere Island every year for the past 10 years, said the impact of higher temperatures this year was "staggering".

His team had estimated that the shelves would lose eight square miles this summer. The true figure was 83 square miles.

"What was extraordinary was just the vast quantity of open water ... you could see open water to the horizon in an area that is typically ice-covered throughout the season," he said.

The Markham Ice Shelf split away from Ellesmere Island in early August. Two large chunks totalling 47 square miles have broken off the nearby Serson Ice Shelf, reducing it in size by 60 percent.

The Ward Hunt Ice Shelf, at 155 square miles the largest of the remaining four shelves, is disintegrating.

"Clearly the long-term viability of that ice shelf is now actually short-term," said Vincent.

The peak temperature the team recorded was 67.5 degrees Fahrenheit (19.7 degrees Celsius), far above the average of 46 degrees Fahrenheit.

Vincent said he had no doubt that global warming was caused in part by human activity.

"I think we're at a point where it is not stoppable but it can be slowed down. And if you think about the magnitude of effects on our society, then we really need to buy ourselves more time to get ready for some very substantial changes that are ahead," he said.

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

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

"More and more, we're realizing that it is microscopic life that really dominates the biodiversity of planet Earth ... we really need to understand what that biodiversity is," said Vincent.

Reporting by David Ljunggren; editing by Rob Wilson