sea ice thinned dramatically between the winters of 2004 and 2008, with thick older ice shrinking by the equivalent of Alaska's land area, a study using data from a Nasa satellite showed Tuesday.

Using information from Nasa's Ice, Cloud and Land Satellite (ICESat), scientists from the US space agency and the University of Washington in Seattle estimated both the thickness and volume of the Arctic Ocean's ice cover.

ICESat allows scientists to measure changes in the thickness and volume of Arctic ice, whereas previously scientists relied only on measurements of area to determine how much of the Arctic Ocean is covered in ice.

Scientists found that Arctic sea ice thinned some 17.8cm a year, or 67cm over four winters, according to the study by Nasa and the University of Washington, published in the Journal of Geophysical Research-Oceans.

They also found that thicker, older ice, which has survived one or more summers, shrank by 42 per cent. 'Between 2004 and 2008, multi-year ice cover shrank 595,000 square miles (1.5 million sq km) - nearly the size of Alaska's land area,' a report of the study's findings said.

The Arctic ice cap grows each winter, when the northerly region grows intensely cold as the sun sets for several months. Then, in the summer, wind and ocean currents cause some of the ice to flow out of the Arctic, while warmer temperatures make much of it melt in place.

Thicker, older ice is less vulnerable than thinner ice to melting in the summer months. But in recent years, the amount of ice replaced in the winter has not been sufficient to offset summer ice losses, the ICESat study showed. That makes for more open water in summer, which absorbs more heat, warming the ocean and further melting the ice, the report of the scientists' findings said.

The research team attributed the changes in the overall thickness and volume of Arctic Ocean sea ice to recent warming and anomalies in patterns of sea ice circulation.

'The near-zero replenishment of the multi-year ice cover, combined with unusual exports of ice out of the Arctic after the summers of 2005 and 2007, have both played significant roles in the loss of Arctic sea ice volume,' said Ron Kwok of Nasa's Jet Propulsion Laboratory in California who led the study.

Data from the study will help scientists to better understand how fast the volume of Arctic ice is decreasing and how soon the region might be 'nearly ice-free in the summer,' said Mr Kwok.

A study published in April by the Colorado-based National Snow and Ice Data Center (NSIDC) also showed that the Arctic ice cap is thinner than ever and the maximum extent of Arctic ice was at an all-time low. The same month, US researchers warned that the Arctic could be almost ice-free within 30 years, not 90 as scientists had previously estimated. -- AFP