A British researcher has found preliminary evidence that the North Polar icecap is thinning, possibly as a result of global warming.

Reporting in today's issue of the journal, Nature, the scientist says he believes that about 15 percent of the average thickness of ice pack floating over 115,000 square miles of the Arctic Ocean disappeared between 1976 and 1987. The ice either melted or broke apart and left thinner ice in its place.

Peter Wadhams of the Scott Polar Research Institute of Cambridge University in England bases his conclusions on sonar measurements of ice thickness he took while aboard British navy submarines north of Greenland.

In October 1976 the ice averaged 17.6 feet thick. In May of 1987 it averaged 15 feet.

Disappearance of sea ice in the Arctic is one of the more profound effects predicted by some global warming scenarios. A significant breakup of the ice could open the normally frozen Arctic Ocean to surface ships. A melt-off of ice also could amplify the so-called "greenhouse effect," because snow and ice reflect heat back into space, whereas open water absorbs heat.

The observation of thinning ice surprised Wadhams and others because many researchers have assumed sea ice in the area north of Greenland was stable. Indeed, crude satellite measurements had shown no significant change in ice thickness over a similar period.

Neither Wadhams nor most other researchers is ready to say that today's report proves sea ice is disappearing over the entire Arctic Ocean. The area Wadhams studied represents only a small portion of the total ice pack in the Arctic Ocean, a body of water almost five times as large as the Mediterrean Sea.

Scientists also say the period covered is too short to know whether Wadhams simply observed a natural fluctuation or part of a long-term trend that could be caused by global warming. The work is further complicated by the fact that Wadhams's observations were made in different seasons.

"The ice is constantly breaking apart and coming together. It's not a solid block of ice. It's extremely dynamic," said Alfred McLaren of the University of Colorado, an expert in Arctic climatology and a former commander of a U.S. nuclear attack submarine that operated in Arctic waters.

"In the area north of Greenland, there's a lot going on. To make sense of that particular area, you have to look at submarine data from the same month, over a period of years," McLaren said. "There isn't enough data yet to say anything about the greenhouse effect."

McLaren said the variation could be explained by the effects of two major wind systems that push floating ice, the Trans-Polar Drift Stream and a circular system known as the Beaufort Gyre. As a result, the ice in a given region would change from one year to the next.

Some researchers say the polar regions would be the best places to look for early signs of global warming because predicted temperature increases would be greatest at the poles.

Michael Schlesinger of the University of Illinois, an expert in computer simulations of climate, said scientists have been hindered in studying sea ice thickness because the most complete information is collected by U.S. and Soviet submarines, which patrol under the icecap. The sub commanders consistently use sonar to measure the ice thickness above. Many submarines can crash through thick ice on their way to the surface, but they prefer open water.

Charles Armitage, director of the U.S. Navy's Arctic Submarine Laboratory in San Diego, said that in the last year, the Navy has decided to release ice measurements taken by U.S. submarines over a large portion of the Arctic basin. "It's not a case of the Navy holding back on information," Armitage said.