Submarines provide the data for thinning

"This came out of measurements which I did (on both occasions) from British submarines - Sovereign in 1976 and Trafalgar in 1996.

"We covered the same regions of the Arctic and used the same equipment, so the results are fully compatible.

"The amount of thinning, which is very large, agrees with results published last year by Drew Rothrock, of the University of Washington, for thinning rates on the other side of the Arctic.

"That region, from the North Pole to the Bering Strait, was measured by US submarines over approximately the same interval. So this confirms that the thinning of sea ice is an Arctic-wide effect."

Sonar systems

Dr Wadhams and Dr Davis report their work in Geophysical Research Letters, published by the American Geophysical Union.

The data on ice thinning were collected by two upward-looking sonar systems mounted on the submarines.

The thinning of the ice between Fram Strait and the Pole was first reported by Dr Wadhams in 1990.

He had found a 15% decrease in ice thickness between his voyage in 1976 on Sovereign and another in 1987 on a third UK submarine, Superb.

Disputed cause

The authors write: "We note that this loss of ice thickness, although at the time it appeared large, was actually an underestimate of the thinning which must have taken place between 1976 and 1987."

The cause of the thinning remains uncertain

This, they say, was because the 1987 experiment was conducted at a different time of year and using a different sort of sonar.

They add: "Comparing our present results with those reported in 1990, we speculate that a substantial part of the thinning that occurred in the experimental region between 1976 and 1996 took place during the first of those two decades."

Many scientists are convinced that human influences are responsible for the thinning of the Arctic sea ice, though not all.

Climate implications

Some point to the influence of a natural climate phenomenon known as the Arctic Oscillation (AO).

This is an erratic see-saw that alternately raises and lowers atmospheric pressure over the North Pole while lowering and raising it in a ring around the polar region.

Dr Wadhams and Dr Davis end their report on a cautionary note. They write of "the very rapid changes which are occurring in the ocean structure of the Arctic, with a greater influx of heat into the Atlantic layer".

And they say: "We feel that this additional support for the dramatic thinning rates reported by Drew Rothrock and others has serious implications for the future of high-latitude climates."