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The Arctic ice cap thinned significantly between 1976 and 1987, a "provocative" finding that may be an early consequence of global warming, a study based on submarine data showed Wednesday.

Using sonar measurements taken under the ice by British submarines, the University of Cambridge's Peter Wadhams found a 120,000-square-mile region of Arctic ice north of Greenland had lost at least 15 percent of its volume between the first scientific voyage in October 1976 and a second one in May 1987.Between the 1976 and 1987 trips, the average thickness of the ice sheet below sea level fell from about 19.8 feet to 17.5 feet in a north-south sampling route and from about 21 feet to about 13.5 feet in an east-west sampling route.

"Thinning of the ice and a decrease in its extent are expected to be an early consequence of `greenhouse warming,' " Wadhams wrote in a study published in the journal Nature.

However, the British researcher emphasized his study is insufficient to confirm or deny an effect from greenhouse warming because no measurements were taken between October 1976 and May 1987 - and the thickness of the ice has not been rechecked since the 1987 voyage.

The greenhouse effect occurs when carbon dioxide and other gases, acting like the glass windows of a greenhouse, trap heat in Earth's atmosphere and prevent it from escaping back into space. Many scientists think pollutants, like car exhaust, will aggravate the greenhouse effect and trigger an increase in worldwide temperatures.

In an editorial accompanying the British study, scientists from the University of Colorado-Boulder and the Naval Postgraduate School in Monterey, Calif., agreed that Wadham's work shows the average thickness of the Arctic ice cap north of Greenland was "significantly less" in 1976 than it was in 1987.

"The observation is provocative, but more data are required before one can hope to discern overall trends with any confidence," wrote the University of Colorado's Alfred McLaren and his colleagues.