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The Arctic could lose virtually all its summer sea ice by 2040 - about 40 years earlier than previously thought - according to a study by leading climate scientists.
A rapid acceleration in the loss of sea ice seen in recent years will be dwarfed by the massive melting that could take place within 20 years, the scientists predict.
If nothing is done to curb man-made emissions of greenhouse gases, the Arctic Basin from Siberia and Greenland to Canada and Alaska could be completely open water in summer within the lifetime of today's children.
Previous climate models of the sea ice in the Arctic have suggested that the earliest date for a completely ice-free summer would be around 2080.
An ice-free Arctic would almost certainly lead to the demise of many indigenous people and their way of life, along with the extinction of the polar bear and other species that rely on the year-long sea ice for survival.
But the loss of sea ice could also lead to more serious, long-term climate change, such as the disruption of the warm North Atlantic current that brings mild winters to Britain, or a more rapid loss of the Greenland ice sheet, with a consequent increase in global sea levels.
The area of the Arctic covered by floating sea ice dwindled steadily from 1978, when satellite measurements began, until about 2002.
However, in recent years the loss has accelerated, with a summer minimum recorded in September 2005 and a winter minimum in March 2006.
Scientists estimate that an area of summer sea ice the size of Alaska has been lost over the past 28 years and global warming is now seen as the principal cause.
The latest study analysed past records of sea ice in the Arctic Basin and used powerful supercomputers to model future scenarios of how the summer melting period is likely to develop over the coming decades.
One scenario found that the maximum loss of sea ice in September months could become so abrupt that within 20 years the ice may begin to retreat four times faster than at any time in the observed record.
In this scenario, abrupt changes are highly likely, with late-summer ice dwindling to a third of its original extent within the space of just 10 years.
The findings, which form part of the fourth assessment of the International Panel on Climate Change (IPCC) due next year, are published in the journal Geophysical Research Letters.
Marika Holland of the National Centre of Atmospheric Research in Boulder, Colorado, the report's lead author, said that the rate of loss of Arctic sea ice over the past quarter century might bear little resemblance to the future rate of loss.
"We have already witnessed major losses in sea ice, but our research suggests that the decrease over the next few decades could be far more dramatic than anything that has happened so far. These changes are surprisingly rapid," Dr Holland said.
"As the ice retreats, the ocean transports more heat to the Arctic and the open water absorbs more sunlight, further accelerating the rate of warming and leading to the loss of more ice.
"This is a positive feedback loop with dramatic implications for the entire Arctic region."
Jeff Ridley, a climate scientist at the British Met Office's Hadley Centre for Climate Change, said the prediction of an essentially ice-free Arctic by 2040 was surprising given that other computer models suggested that this was not likely to occur before 2080.
However, Cecilia Bitz of the University of Washington in Seattle, the study's co-author, said the latest assessment was based on a "moderate scenario" of future man-made emissions of carbon dioxide.
"Different models give different results. Naturally we think our model is the finest. Indeed, our sea ice model is more sophisticated than any other," Dr Bitz said.
"For example, some other models have unrealistically thick ice in their simulation of the present-day climate, so they tend to be less sensitive than ours.
"Our model does a very good job at simulating the correct thickness and area of sea ice in the present-day climate."
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