Global warming in the Arctic could be soaring out of control, scientists warned yesterday as new figures revealed that melting of sea ice in the region has accelerated to record levels.
Experts at the US National Snow and Data Centre in Colorado fear the region is locked into a destructive cycle with warmer air melting more ice, which in turn warms the air further. Satellite pictures show that the extent of Arctic sea ice this month dipped some 20% below the long term average for September - melting an extra 500,000 square miles, or an area twice the size of Texas. If current trends continue, the summertime Arctic Ocean will be completely ice-free well before the end of this century.
Ted Scambos, lead scientist at the Colorado centre, said melting sea ice accelerates warming because dark-coloured water absorbs heat from the sun that was previously reflected back into space by white ice. "Feedbacks in the system are starting to take hold. We could see changes in Arctic ice happening much sooner than we thought and that is important because without the ice cover over the Arctic Ocean we have to expect big changes in Earth's weather."
The Arctic sea ice cover reaches its minimum extent each September at the end of the summer melting season. On September 21 the mean sea ice extent dropped to 2.05m square miles, the lowest on record. This is the fourth consecutive year that melting has been greater than average and it pushed the overall decline in sea ice per decade to 8%, up from 6.5% in 2001.
Walt Meier, also at the Colorado centre, said: "Having four years in a row with such low ice extents has never been seen before in the satellite record. It clearly indicates a downward trend, not just a short term anomaly."
Surface air temperatures across most of the Arctic Ocean have been 2-3C higher on average this year than from 1955 to 2004.
The notorious northwest passage through the Canadian Arctic from Europe to Asia - where entire expeditions were lost in earlier centuries as their crews battled thick ice and bitter cold - was completely open this summer, except for a 60 mile swath of scattered ice floes. The northeast passage, north of the Siberian coast, has been ice free since August 15.
Springtime melting in the Arctic has begun much earlier in recent years; this year it started 17 days earlier than expected. The winter rebound of ice, where sea water refreezes, has also been affected. Last winter's recovery was the smallest on record and the peak Arctic ice cover failed to match the previous year's level.
The decline threatens wildlife in the region, including polar bears that spend the summer on land before returning to the ice when it reforms in winter. It is also the latest in a series of discoveries that have raised the spectre of environmental tipping points: critical thresholds beyond which the climate would be unable to recover. Duncan Wingham, an Arctic ice expert at University College London, said: "One has to be a bit careful with the notion of a tipping point because the situation is recoverable.
"If you drop the atmospheric temperature then the ice will come back again. There is a distinction between that and the Greenland ice sheet, which wouldn't reform because the modern climate is far too warm."
Prof Wingham is head of a European project that will launch a new satellite next weekend to monitor the thickness of the Arctic sea ice - and to check on the role global warming plays in its decline. Some had suggested that a periodic weather system called the Arctic oscillation had blown thick sea ice from the Arctic during the 1990s, leaving thin ice more liable to melting in its place.