Winter sea ice in the Arctic has failed to reform fully for the third year in a row. Scientists said yesterday that the area of ocean covered by Arctic ice at the end of the winter months was lower only in March 2006.

Researchers fear that the floating sea ice is now on a downward spiral of shrinkage that cannot recover fully even during winter because of warmer temperatures.

Walt Meier of the US National Snow and Ice Data Centre in Boulder, Colorado, which released the satellite data yesterday, said: "We're seeing near-record lows and higher-than-normal temperatures. We expect the downward trend to continue in future years."

The maximum area of the northern hemisphere covered by sea ice in March 2007 was 5.7 million square miles, compared to the 5.6 million square miles recorded in March 2006.

The long-term average for March sea ice, as determined by Nasa measurements from 1979 to 2000, is 6.1 million square miles, according to the centre.

Sea ice in the Arctic oscillates naturally during the seasons, with the area of ocean covered by ice shrinking to its peak minimum extent during September and expanding to its annual maximum during March.

Since 1979, when satellite measurements began, summer sea ice had declined significantly. In September 2005 it reached an all-time record low, with September 2006 the second-lowest.

Scientists fear that the winter failure of the ice to recover fully will mean there is less ice to start with at the beginning of the summer melting period, leading to more rapid shrinkage with each subsequent year.

This is bad news for polar bears, which rely on floating sea ice to hunt for seals. With little ice, polar bears have to swim further in open water, burning much-needed body fat in the process.

One of the greatest concerns is that the melting sea ice will lead to greater areas of open, darker ocean being exposed to sunlight during the summer. Instead of 90 per cent of the heat of the sun being reflected by a cap of sea ice, the heat will be absorbed by the open water, which will exacerbate the trend towards regional warming.

"Low winter recovery means that the ice is freezing up later in the fall [autumn] and growing at a slower pace in the winter," said a spokesman for the centre.

"September usually marks the end of the summer melting season. Low summer extent means that ice is melting faster during the summer and leaving less ice to build on during the winter recovery," he said.

Computer models predict that the summer sea ice will be totally gone by the end of the century. Some scientists, however, believe that this could occur as early as 2040.

If the sea ice of the Arctic disappears completely in summer, regional temperatures could increase faster than in the past. Some scientists also believe that the change in the regional climate could have far-reaching impacts on other parts of the northern hemisphere, perhaps by altering ocean currents such as the Gulf Stream.

© 2007 Independent News and Media Limited