There was less sea ice in the Arctic on Friday than ever before on record, and the melting is continuing, the U.S. National Snow and Ice Data Center reported.

"Today is a historic day," said Mark Serreze, a senior research scientist at the centre. "This is the least sea ice we've ever seen in the satellite record and we have another month left to go in the melt season this year."

Satellite measurements showed 5.2 million square kilometres of ice in the Arctic, falling below the Sept. 21, 2005, record minimum of 5.3 million square kilometres, the agency said.

Sea ice is particularly low in the East Siberian side of the Arctic and the Beaufort Sea north of Alaska, the centre said. Ice in the Canadian archipelago is also quite low, it said.

Along the Atlantic side of the Arctic Ocean, the amount of sea ice is not as unusually low, but there is still less than normal, according to the centre, located in Boulder, Colo.

The snow and ice centre is part of the Co-operative Institute for Research in Environmental Sciences at the University of Colorado. It receives support from NASA, the U.S. National Oceanic and Atmospheric Administration and the National Science Foundation.

Scientists began monitoring the extent of Arctic sea ice in the 1970s when satellite images became available.

The polar regions have long been of concern to climate specialists studying global warming because those regions are expected to feel the impact of climate change sooner and to a greater extent than other areas.

Sea ice in the Arctic helps keep those regions cool by reflecting sunlight that might be absorbed by darker land or ocean surfaces.

Arctic snow and ice reflect 80 per cent of the sunlight they receive, compared with only 10 per cent by open ocean water. That, in turn, causes the ocean to heat up and raises Arctic temperatures.

'Very strong evidence' of greenhouse warming

Unusually clear sky conditions have prevailed in the Arctic in June and July, promoting more sunshine at the time when the sun is highest in the sky over the region. The centre said this led to an unusually high amount of solar energy being absorbed by the Arctic ice surface, accelerating the melting process. Fairly strong winds also brought in some warm air from the south.

But, Serreze said in a telephone interview, while some natural variability is involved in the melting, "we simply can't explain everything through natural processes."

"It is very strong evidence that we are starting to see an effect of greenhouse warming," he said.

The puzzling thing, he said, is that the melting is actually occurring faster than computer climate models have predicted.

Several years ago he would have predicted a complete melt of Arctic sea ice in summer would occur by the year 2070 to 2100, Serreze said. But at the rates now occurring, a complete melt could happen by 2030, he said Friday.

There will still be ice in winter, he said, but it could be gone in summer.