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This satellite im shows the Arctic sea ice spread on September 21, 2005, when it dropped to the lowest extent yet recorded. The yellow outline indicates where the concentration of ice was as of September 21, 1979.

Global warming in the Arctic might be accelerating out of control, scientists have warned, as new data revealed the floating cap of sea ice has shrunk to probably its smallest in at least a century.

Experts at the US National Snow and Ice Data Center 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 20 per cent below the long-term average for September - melting an extra 1.3 million square kilometers - an area about the size of the Northern Territory. If current trends continue, the summertime Arctic Ocean will be ice-free well before the end of this century.

The head scientist at the Colorado center, Ted Scambos, said melting sea ice accelerates warming because dark-colored water absorbs heat from the sun that was previously reflected back into space by white ice.

"Feed backs 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," Dr Scambos said

The findings are consistent with recent computer simulations showing that a build-up of greenhouse gases could lead to a profoundly transformed Arctic later this century. The North Pole ice cap always grows in winter and shrinks in the summer. The average minimum area from 1979, when precise satellite mapping began, until 2000 was 11 million square kilometers. The new summer low, measured 11 days ago, was 20 per cent below that.

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 per cent, up from 6.5 per cent in 2001.

Walt Meier, also at the Colorado center, 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 over most of the Arctic Ocean often have been 2-3 degrees higher this year than from 1955 to 2004.

The notorious north-west passage through the Canadian Arctic from Europe to Asia was completely open this summer, except for a 95-kilometer swathe of scattered ice floes. The north-east 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, especially polar bears. It is also the latest in a series of discoveries that have raised the spectra of environmental tipping points: critical thresholds beyond which the climate would be unable to recover.