Parts of the ice sheets covering Antarctica are melting faster than predicted, with the net loss of ice probably accelerating in recent years because of global warming, a study has found.
A satellite survey between 1996 and 2006 found that the net loss of ice from Antarctica rose by about 75 per cent as the movement of glaciers towards the sea speeded up.
Scientists estimate that that the West Antarctic Ice Sheet lost about 132 billion tons of ice in 2006, compared with a loss of 83 billion tons in 1996. In addition, the Antarctic peninsula lost about 60 billion tons of ice in 2006.
"To put these figures into perspective, 4 billion tons of ice is enough to provide drinking water for the whole UK population for one year," said Professor Jonathan Bamber, of the University of Bristol. "We think the glaciers of the Antarctic are moving faster to the sea. The computer models of future sea-level rise have not really taken this into account."
Sea levels are estimated to have risen by 1.8mm a year on average during the 20th century, but data from the past decade or so suggest that the average rise is now about 3.4 mm per year.
Computer models used by the Intergovernmental Panel on Climate Change (IPCC), which predict that sea levels will rise by no more than about 50cm by 2100, are based largely on the stability of the Antarctic ice sheets. But many scientists now believe this forecast is too restrained. "I agree with a number of scientists who feel the IPCC is likely to have underestimated the upper bound of predicted sea-level rise by the end of the century – 50 cm is probably too conservative," Professor Bamber added.
There are two key factors in estimating the net loss of Antarctic ice. The first is the flow of glaciers towards the sea; the second is the build-up of snow over the vast landmass of the frozen continent. The IPCC models imply that global warming will increase the moisture content of the atmosphere and so may actually increase snowfall over Antarctica, much of which is too cold to be affected by rising global temperatures. This would suggest a net build-up of ice. However, Professor Bamber believes the IPCC's models have not taken into account the complex, dynamic interaction between the ocean and the ice shelves of West Antarctica and the Antarctic Peninsula, which are warmer than East Antarctica.
Eric Rignot, who led the latest study published in the journal Nature Geoscience, said the findings indicated a rapid loss of ice to the sea rather than a net gain. "We have determined that the loss is increasing with time, quite rapidly at 75 per cent in ten years," Dr Rignot said. "We have also established that most of this loss, if not its entirety, is caused by glacier acceleration. The IPCC focussed on the surface mass balance component. We find this component is not indicative of the true mass balance."
The acceleration in ice loss over the past 10 years could increase in coming decades, he added. "As some of these glaciers reach deeper beds, their speeds could double or triple, in which case the contribution to sea-level rise from Antarctica could increase quite significantly beyond what it is now. Many people suspect Antarctic ice to be immune from changes. We are finding this is not the case.
"The future is the big question. The potential exists for ice speed to increase two or three times, which will result in a doubling of the mass deficit from Antarctica."
Melting into history
* July 1985: UK scientists detect hole in ozone layer
* January 1995: Larsen A ice shelf disintegrates
* July 1998: Evidence suggests future collapse of West Antarctic ice sheet
* March 2000: An iceberg 183 miles long and 22 miles wide breaks adrift
* February 2002: Larsen B ice shelf collapses
* October 2003: World's largest iceberg splits
* March 2006: Research shows shrinking ice has raised sea levels by 1.2mm
* September 2007: Sea ice covering Antarctica melts back to record lowReuse content