Baku, October 8 (AZERTAC). The Arctic Ocean will be completely devoid of summer ice before the 21st century has ended, a NASA study predicts.

The new work shows that the permanent ice cap over the ocean - the cover that survives through the warm summer months - is disappearing far faster than previously thought.

Between 1978 and 2000, 1.2 million square kilometres of apparently permanent ice melted away. That is an area five times the size of Britain and represents a loss of nine per cent per decade.

"At this rate, permanent ice will have disappeared before the end of this century," says NASA ice physicist Josefino Comiso.

Past satellite studies of ice cover have looked at the average ice cover for individual months during the year. They found an average retreat of around three per cent a decade.

But the minimum ice cover, which represents the ice that lasts through the summer, occurs at different times in different regions of the ocean. So to get a clearer estimate of the amount of ice that survives, Comiso reanalysed the data.

He measured the minimum extent of sea ice in each region - and that revealed the much higher rate of loss. The trend is continuing. This past summer, "we had the least amount of permanent ice cover ever observed," says Comiso.

"If the permanent ice cover disappears, the entire Arctic ocean climate and ecology would become very different," says Comiso, at NASA`s Goddard Space Flight Center in Greenbelt, Maryland. It will give an extra kick to global warming and polar bears, which live by hunting on the sea ice, will have nowhere to go.

The ice loss coincides with a summer warming trend of 1.2 °C a decade in the air above the ice. The biggest disappearance of ice so far has been over the Beaufort and Chukchi Seas, north of Canada and Alaska. This is also where air temperatures have warmed most.

Comiso agrees that some of the ice retreat could be due to short-term fluctuations in climate or ocean currents. But he warns that the rate at which ice disappears is likely to accelerate in future.

As the ice disappears, it will speed up the warming of Arctic waters. This is because ice reflects most of the Sun`s rays back into space, while the dark-blue oceans absorb much more heat.

Warmer water will delay autumn freezing, leading to a thinner ice cover in the winter and spring. In turn this will make the sea ice more vulnerable to melting the next summer. This positive feedback is propelling the Arctic ice to a point of no return beyond which, says Comiso, the melting will be become "irreversible".