The West Antarctic bedrock lies below sea level

Changes in the PIG could be important indicators of any larger changes in the ice sheet's interior.

The PIG is up to 2,500 metres thick, and flows over bedrock which lies over 1,500 m below sea level.

The scientists used high-precision radar measurements from the European Remote-Sensing Satellite altimeter, which is accurate to within 20 cm, to map the height of the WAIS at regular intervals since 1992.

The data showed a clear pattern of ice thinning within a 5,000 sq km section of the PIG drainage basin. Using ERS satellite radar interferometry, the team constructed a map of glacial velocity, much of it previously uncharted, to clarify which regions were actually thinning.

Combining the two datasets, they established that the ice had thinned by as much as 10 m. They estimate that the PIG's mass is shrinking by about four Gt (four billion tonnes) annually, the rough equivalent of a sea level rise of .01 mm.

The scientists also found that the grounding line - the point where glacier, sea and seabed meet - had retreated five km inland between 1992 and 1996.

Speculation ended

If the present rate of thinning of the WAIS continues, they believe, the PIG will be floating within 600 years, as the sea pours in to fill the space it has vacated.

Their data suggest that the thinning is the consequence of changes in the ice flow within the PIG, not of changes in snow accumulation.

The thinning is spreading inland from the coast

One of the team, Dr Andrew Shepherd, said: "For the past 25 years, there has been speculation about whether a retreat of a West Antarctic glacier could accelerate ice flow from its interior, producing a rise in sea level. We have shown for the first time that such a retreat is indeed occurring.

"It is of paramount importance to determine whether the thinning is accelerating. Our present theoretical understanding is not sufficient to firmly predict the future evolution of the PIG."

'Uniquely sensitive'

Dr Shepherd told BBC News Online: "The glacier is in a remote part of Antarctica, and there is no in situ weather data from the area.

"What we can say is that the pattern of thinning we've seen is in line with the patterns that climate change theory predicts.

"It peaks at the grounding line and spreads 150 km inland, in line with theories that perturbations at the margins of an ice sheet can transmit a signal inland.

"The WAIS is uniquely sensitive to climate forcing, because its bedrock is one and a half kilometres below sea level.

"The thinning could have profound effects. It could be a kind of feedback mechanism."

Other photos courtesy of British Antarctic Survey