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A catastrophic collapse of the Arctic sea ice could lead to radical climate changes in the northern hemisphere according to scientists who warn that the rapid melting is at a "tipping point" beyond which it may not recover.

The scientists attribute the loss of some 38,000 square miles of sea ice - an area the size of Alaska - to rising levels of carbon dioxide in the atmosphere as well as to natural variability in Arctic ice.

Ever since satellite measurements of the Arctic sea ice began in 1979, the surface area covered by summer sea ice has retreated from the long-term average. This has increased the rate of coastal erosion from Alaska to Siberia and caused problems for polar bears, which rely on sea ice for hunting seals.

However, in recent years the rate of melting has accelerated and the sea ice is showing signs of not recovering even during the cold, dark months of the Arctic winter. This has led to even less sea ice at the start of the summer melting season.

Mark Serreze, a senior glaciologist at the University of Colorado at Boulder, said the world was heading towards a situation where the Arctic will soon be almost totally ice-free during summer, which could have a dramatic impact on weather patterns across the northern hemisphere.

"When the ice thins to a vulnerable state, the bottom will drop out and we may quickly move into a new, seasonally ice-free state of the Arctic," Dr Serreze said.

"I think there is some evidence that we may have reached that tipping point, and the impacts will not be confined to the Arctic region," he said.

Some studies have linked the loss of sea ice in the Arctic to changes in atmospheric weather patterns that influence such things as rainfall in southern and western Europe and the amount of snow in the Rocky Mountains of the American Midwest.

The Arctic is one of the fastest warming regions on Earth and scientists fear that temperatures could rise even faster once sea ice melts to expose dark ocean, which absorbs heat more easily without its reflective cap of ice.

"While the Arctic is losing a great deal of ice in the summer months, it now seems that it also is regenerating less ice in the winter. With this increasing vulnerability, a kick to the system just from natural climate fluctuations could send it into a tailspin," Dr Serreze said.

During the late 1980s and early 1990s, changing wind patterns flushed much of the thick sea ice out of the Arctic Ocean and into the North Atlantic, where it drifted south and melted away.

A thinner layer of young ice formed in its place, which more readily melts during the warmer, summer months - leading to the appearance of a greater area of open water that absorbs sunlight and heat. The summer sea ice reached an all-time minimum in September 2005, with September 2006 the second lowest.

"This ice-flushing even could be a small-scale analogue of the sort of kick that could invoke rapid collapse, or it could have been the kick itself. At this point, I don't think we really know," Dr Serreze said.

Julienne Stroeve from the US National Snow and Ice Data Centre in Colorado said that the winter sea ice failed again this year to recover fully.

"The freeze-up this year was again delayed, and ice extents from October through to December set new record lows during the satellite era," she said.

Computer models suggest that summer sea ice could disappear altogether by 2080. Some forecasts even predict an ice-free summer by 2040.

By Steve Connor, Science Editor of The Independent

Published: 16 March 2007

http://news.independent.co.uk/environment/climate_change/article2362744.ece