The Arctic icecap is now shrinking at record rates in the winter as well as summer, adding to evidence of disastrous melting near the North Pole, according to research by British scientists.

They have found that the widely reported summer shrinkage, which this year resulted in the opening of the Northwest Passage, is continuing in the winter months with the thickness of sea ice decreasing by a record 19% last winter.

Usually the Arctic icecap recedes in summer and then grows back in winter. These findings suggest the period in which the ice renews itself has become much shorter.

Dr Katharine Giles, who led the study and is based at the Centre for Polar Observation and Modelling at University College London (UCL), said the thickness of Arctic sea ice had shown a slow downward trend during the previous five winters but then accelerated.

She said: “After the summer 2007 record melting, the thickness of the winter ice also nose-dived. What is concerning is that sea ice is not just receding but it is also thinning.”

The cause of the thinning is, however, potentially even more alarming. Giles found that the winter air temperatures in 2007 were cold enough that they could not have been the cause.

This suggests some other, longer-term change, such as a rise in water temperature or a change in ocean circulation that has brought warmer water under the ice.

If confirmed, this could mean that the Arctic is likely to melt much faster than had been thought. Some researchers say that the summer icecap could vanish within a decade.

The research, reported in Geophysical Research Letters, showed that last winter the average thickness of sea ice over the whole Arctic was 26cm (10%) less than the average thickness of the previous five winters.

However, sea ice in the western Arctic lost about 49cm of thickness. This region saw the Northwest Passage become ice-free and open to shipping for the first time in 30 years during the summer of 2007.

The UCL researchers used satellites to measure sea-ice thickness from 2002 to 2008. Winter sea ice in the Arctic is about 8ft thick on average.

The team is the first to measure ice thickness throughout the winter, from October to March, over more than half of the Arctic, using the European Space Agency’s Envisat satellite.

Giles’s findings confirm the more detailed work of Peter Wadhams, professor of ocean physics at Cambridge University, who has undertaken six voyages under the icecap in Royal Navy nuclear submarines since 1976 and has gathered data from six more voyages.

The vessels use an upward-looking echo-sounder to measure the thickness of sea ice above the vessel. The data gathered can then be compared with previous years to find changes in thickness.

Wadhams published his first paper in 1990, showing that the Arctic ice had grown 15% thinner between 1976 and 1987.

In March 2007 he went under the Arctic again in HMS Tireless and found that the winter ice had been thinning even more quickly; it was now 50% of the 1976 thickness.

“This enormous ice retreat in the last two summers is the culmination of a thinning process that has been going on for decades, and now the ice is just collapsing,” Wadhams said.

The scale of the ice loss has also been shown by other satellite-based observations that are used to measure the area of the Arctic icecap as it grows and shrinks with the seasons.

In winter it normally reaches about 5.8m square miles before receding to about 2.7m square miles in summer.

In 2007, however, the sun shone for many more days than normal, raising water temperatures to 4.3C above the average. By September the Arctic icecap had lost an extra 1.1m square miles, equivalent to more than 12 times the area of Britain.

That reduced the area of summer ice to 1.6m square miles, 43% smaller than it was in 1979, when satellite observations began.

At the heart of the melting in the Arctic is a simple piece of science. Ice is white, so most of the sunlight hitting it is reflected back into space. When it melts, however, it leaves open ocean, which, being darker, absorbs light and so gets warmer. This helps to melt more ice. It also makes it harder for ice to form again in winter. The process accelerates until there is no more ice to melt.

Wadhams said: “This is one of the most serious problems the world has ever faced.”

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