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THE layer of ice that blankets the top of the globe is melting at a startling rate, according to a US scientific survey. Measurements collected by nuclear submarines beneath the Arctic Sea show that the great sheet is some 40 per cent thinner than it was 20-40 years ago.
The researchers concluded that the thinning was continuing at a rate of about four inches a year, far faster than previously thought.
"It's a startling result," said Dr Andrew Rothrock, the scientist who led the study, which is published this week in the journal Geophysical Research Letters.
Dr Rothrock and his team found that where the Arctic Sea ice was about 9ft thick between 1958 and 1976, between 1993 and 1997 it had thinned to an average of 6ft. Over the last century the average surface temperature of the Earth has risen by about one degree Fahrenheit, but in parts of the Arctic the rate of increase is much faster. Scientists disagree over whether climatic changes in the Arctic are the result of natural changes, human activity triggering global warming or a combination of those factors.
"The decrease in sea ice occurs all across the Arctic Ocean and corresponds to previously reported evidence that the Arctic climate is warming," the American Geophysical Union, the publisher of the journal, concluded. Some scientists predict that the emission of heat- trapping gases such as carbon dioxide, if continued at present levels, will cause a marked rise in global temperatures over the next decade.
The evidence for the new study was compiled by nuclear submarines using sonar equipment over two distinct time periods, starting in 1958, when the US submarine Nautilus was first deployed in the Arctic Ocean. The first data period ended in 1976, and a second survey was conducted between 1993 and 1997. The scientists compared the thickness of the ice at 29 points where the two surveys intersected.
At each site the layer had thinned, by more than 6ft in some regions, while the total area of sea ice had also shrunk and the season of ice cover had shortened. Dr Rothrock, an oceanographer at the University of Washington in Seattle, believes that the ice-thinning is the result of a shift in the natural patterns of atmospheric circulation, possibly triggered or accelerated by global warming.
A Greenpeace expedition this year suggested that the climate change is having an impact on the ecology and wildlife in the region. Scientists say that polar bears, seals and seabirds have been affected by the retreating ice, but the most dramatic effect is on the declining walrus population.