LONDON - Antarctica is melting, adding to the inexorable rise in global sea levels, endangering millions of lives and whole economies, leading scientists said on Monday.
But while the effect is well known after years of monitoring from land and space, the reasons for the sea warming are not.
"We know sea levels will rise. We need to know by how much and why," Anthony Payne of the University of Bristol and one of the organisers of a major scientific conference in London, told Reuters on the sidelines of the meeting at the Royal Society, Britain's national academy of science.
"This has implications for the whole world -- most people and industries are in coastal areas," he added.
Payne said there was a net loss of mass in Antarctica as the snowfall in the centre of the frigid landmass was more than offset by sea ice melting around the edges.
The key was to find out whether the process was accelerating, or whether it might stabilise or even reverse.
And the important factor was understanding the complex interaction between ocean and wind currents and how much -- if any -- of the warming of the seas was due to mankind's contribution to global warming.
"We know a lot more about the ice sheets than we did before," Payne said. "We know change is happening and that it is rapid. What we don't know is why or what is causing it -- what proportion is anthropomorphic."
Scientists calculate that average world temperatures -- which have already risen by 0.6 degrees Celsius (1.1 Fahrenheit) since 1900 -- could rise by at least two more degrees this century, due in large part to greenhouse gases from burning fossil fuels.
High Economic Cost
Bob Bindschadler, a glaciologist from U.S. space agency NASA, said the West Antarctic ice sheet was reducing -- albeit patchily -- but that if it all melted it would raise global sea levels by 6 metres.
Putting it in context he said that a 1-metre rise in sea levels would cost the United States alone $400 billion -- roughly twice the estimated cost of the destruction wrought by hurricane Katrina in New Orleans last month.
"We don't want to have too many New Orleans," he told the start of the two-day conference that will pool all Antarctic knowledge and help shape the fourth assessment report of the Intergovernmental Panel on Climate Change that is due in 2007.
Eric Rignot, a fellow NASA scientist, said marine ice on the world's coldest continent was in general retreat due to rising sea temperatures.
"The Antarctic ice sheet is changing at a faster rate than anticipated. The coastal changes are the most significant, with the potential to reach far inland," he told an audience of his peers from around the world.
While the vast East Antarctic ice sheet, which is more than double the size of its western neighbour, was more or less stable except at the coastal fringes, there was no guarantee it would remain so.
"The East Antarctic ice sheet is not immune to change," he said, noting that more than one third of the annual 1.8 millimetre rise in global sea levels came from Antarctica.