THE Arctic ice cap could disappear completely well before the end of the century under the impact of global warming, according to observations released yesterday.

Ice cover in the Arctic Ocean has, this month, shrunk to its lowest extent in more than a century, American scientists have found. If the shrinking trend continues at its present rate of 8 per cent a year, there could be no ice at all at the pole as early as the summer of 2060.

The extent of Arctic sea ice varies naturally, but researchers from the US National Snow and Ice Data Centre (NSIDC) and Nasa, who produced the new data, said that there are strong indications that human-induced global warming is at least partially responsible.

“It’s still a controversial issue, and there’s always going to be some uncertainty because the climate system does have a lot of natural variability, especially in the Arctic,” said Mark Serreze, of NSIDC, who led the research, “but I think the evidence is growing very, very strong that part of what we’re seeing now is the increased greenhouse effect.”

Sea ice cover in the Arctic, which does not boast a continental land mass like Antarctica, always shrinks to its lowest point in September, at the end of the northern hemisphere summer. For each of the past four years, satellite data has shown a substantial reduction in ice extent during this period.

On September 21 the average size of the cap had dropped to 2.05 million square miles, the lowest in satellite records that stretch back to 1978. Less accurate records indicate that the latest low is unprecedented for more than a century. The current area covered by ice is 20 per cent less than the average from 1978 to 2000.

“September 2005 will set a new record minimum in the amount of Arctic sea ice cover,” Dr Serreze said. “It’s the least sea ice we’ve seen in the satellite record, and continues a pattern of extreme low extents of sea ice which we’ve seen for the past four years.”

Julienne Stroeve, one of his colleagues, said: “Considering the record low amounts of sea ice this year leading up to the month of September, 2005 will almost certainly surpass 2002 as the lowest amount of ice cover in more than a century.”

The decline has the potential to create a feedback effect that accelerates global warming. Sea ice reflects much of the Sun’s heat back into space, while the ocean beneath absorbs it, so shrinking cover will raise temperatures still further.

“Feedbacks in the system are starting to take hold,” Ted Scambos, of NSIDC, said. “These could change our estimate of the rate of decline of sea ice. Right now, our projections use a steady linear decline, but when feedbacks are involved the decline could pick up speed.”

As sea ice is already floating, it does not raise global sea levels when it melts. Changes in its extent, however, can have a major effect on the habitats of species such as polar bears. “Polar bears must wait out the summer melt season on land, using their stored fat until they can return to the ice,” Roger Barry, head of NSIDC, said, “but if winter recovery and sea ice extent continue to decline, how will these beasts survive?”

Average surface air temperatures in the Arctic are up to 3C higher than 50 years ago.

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