Ice at the North Pole has melted to the lowest level since satellite observations began in 1972 in another sign of accelerating global warming, polar scientists noted.

At the rate North Pole ice is melting, the Arctic will be largely ice-free in the northern summer 40 years sooner than projected in the last Intergovernmental Panel on Climate Change assessment report. Arctic ice is now believed to cover its smallest area in 8,000 years.

Bremen University physicists issue daily ice maps, which indicate that the floating ice in the Arctic covered 4.24 million square kilometres last Thursday. The previous one-day minimum was 4.27 million square kilometres on Sept. 17, 2007. Scientists said the record melting was undoubtedly caused by human-induced global warming.

''The sea ice retreat can no more be explained with the natural variability ... caused by weather,'' said the head of the Institute of Environmental Physics at Bremen, Georg Heygster.

''Climate models show that the reduction is related to the man-made global warming, which, due to the albedo effect, is particularly pronounced in the Arctic.''

The albedo effect concerns how a surface reflects the sun's heat. Sea ice reflects more of the sun's heat back into space than darker seawater, which absorbs the sun's heat and gets warmer.

A senior researcher at the Australian Antarctic Division, Tony Worby, said polar scientists generally agree that the Arctic is melting faster than the IPCC models predicted.

''Some of the climate models showed that the ice would disappear in summer in about 70 years, but the observations are tracking well ahead of that,'' Worby said.

Separate research at the Polar Science Center of the University of Washington in Seattle also suggests Arctic ice is in a downward spiral. Their data shows that sea ice last volume in August was half the average and 62 per cent lower than the maximum covering the Arctic in 1979. The research will be published in the Journal of Geophysical Research.

''Ice volume is now plunging faster than it did at the same time last year when the record was set,'' said University of Washington senior oceanographer Axel Schweiger.

The National Snow and Ice Data Center in the U.S. is expected to publish the study results in a few days.

Arctic ice plays a critical role in regulating Earth's climate. Retreating summer sea ice is described by scientists as indicative of global warming.