US scientists have suggested that the gradual melting of the huge West Antarctic ice sheet may not be the fault of humankind - at least not yet.

The work warns that climate change could hasten the total collapse of the sheet.

The research, published in the journal Science, concludes that the melting, which began nearly 10,000 years ago, may well continue until the sheet collapses completely.

The sheet is about 932,000 sq km (360,000 square miles) in area, roughly the size of Texas and Colorado together.

If it did collapse, global sea levels would rise by up to five or six metres, enough to submerge many coastal regions. But collapse is unlikely, on present trends, for another 7,000 years.

Professor Howard Conway of the University of Washington, one of the scientists involved in the research, said there might be nothing anyone could do to slow or reverse the melting.

Accelerated melting

But he said climate change resulting from human activities could bring about the collapse of the sheet sooner than natural causes would.

"Collapse appears to be part of an ongoing natural cycle, probably caused by rising sea levels which were inititated by the melting of the northern hemisphere ice sheets at the end of the last ice age.

"But the process could easily speed up if we continue to contribute to warming the atmosphere and oceans."

Much of the ice sheet on the Antarctic land mass lies below sea level, which makes it very vulnerable to rising sea levels.

The ice has been in retreat for thousands of years

The researchers used evidence gathered from raised beaches - land which rose after the ice pressing it down had melted - and through radar imaging of sub-surface ice structures.

This enabled them to establish that the ice sheet has both thinned and decreased in area since the last glacial maximum, 20,000 years ago.

The ice was once up to 800 metres thick. But melting has relieved its pressure on the land so much that some of the raised beaches are now 30 metres above sea level.

The researchers also found that the boundary between floating ice and ice grounded on the sea floor has receded about 1,280 km (800 miles) since the ice age and has withdrawn an average of about 120 m (400 feet) per year for the last 7,600 years.