British researchers used data from a pair of European and American radar satellites to assess the thickness of the ice covering the Arctic Ocean over eight years.

They found that, while the ice has thinned by 40 percent overall since the 1960s, the thickness can in fact fluctuate greatly during the course of the year and also from year to year.

Averaged out over the 1993-2001 duration of the study, the mean ice thickness in winter was just under three metres (10 feet), but over time this figure fluctuated by up to 25 centimetres (two feet) each way.

Within a year, there can be a variation in thickness of as much as 16 percent.

Those fluctuations are 50 percent more than previous computer models have suggested.

Those models have generally assumed that the variations are caused by changes in ocean currents and winds, which stresses the ice.

But the latest study says that those factors are far less important than global warming.

It found that the fluctuations in the ice thickness are mirrored by variations in the length of the summer melt season caused by warmer waters from April to September.

And over time, the records show, the melt season has been getting progressively longer while the ice has thinned.

The shorter winter is potentially disastrous for polar bears, because they depend on the ice to hunt for seals, they say. If the ice breaks up earlier, the bears have less time to find food.

"The regression implies that an increase in the melt season length by one day results in an extra 4.9 centimetres (two inches) of summer ice melt," the authors say.

"Although changes in Arctic circulation patterns may alter the distribution of ice thickness in the Arctic, we conclude that a continuation of the previously observed increase in melt season length will lead to a further overall thinning of the Arctic ice."

Lead author of the research, which appears in Thursday's issue of the British science weekly journal Nature, is Seymour Laxon of the Centre for Polar Observation and Modelling, which is part of University College London.

Last Friday, NASA published satellite images showing that the northern polar ice cap has been shrinking by 10 percent per decade during the past quarter century.

And the period of "perennial sea ice" --- the Arctic sea ice that remains frozen all year round -- reached record lows in 2002 and 2003, NASA said. It blamed global warming, noting that oceans and land masses surrounding the Arctic Ocean had warmed by one degree Celsius (1.8 Fahrenheit) over the past decade.

The shrinkage of the polar ice cap first leapt to prominence a few years ago, when scientists were able to take a ship right up to the North Pole, unencumbered by ice, during the summer melt.

Apart from the impact on wildlife, the melting may have repercussions on other ocean currents, possibly acting as a dampener on the Gulf Stream that provides Western Europe with its warm, wet weather, according to some theories.

However, the melting will not have any effect on sea levels, because Arctic ice is sea ice -- it floats on the sea and thus displaces its own weight.

A much bigger worry would be if the Antarctic ice sheet melted.

Because it lies on rock, the sheet, which in any case is many times thicker than the Arctic one, would disgorge billions of tonnes of water into the sea, with the potential to flood low-lying countries and small islands.

SPACE.WIRE