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Parts of the ice sheets covering Antarctica are melting faster than predicted, and the loss of ice has probably accelerated in recent years because of global warming, a study has found.
A satellite survey between 1996 and last year 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 say the West Antarctic ice sheet lost about 132 billion tonnes of ice last year, compared with a loss of 83 billion tonnes in 1996.
As well, the Antarctic peninsula lost about 60 billion tonnes of ice last year.
"To put these figures into perspective, four billion tonnes 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 3.4mm a year.
Computer models used by the Intergovernmental Panel on Climate Change, 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 - 50cm is probably too conservative," Professor Bamber said.
There are two main 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 climate change panel's models imply that global warming will increase the moisture content of the atmosphere and so may 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 models have not taken into account the complex, 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 10 years," Dr Rignot said.
"We have also established that most of this loss, if not its entirety, is caused by glacier acceleration. The IPCC focused 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 potential exists for ice speed to increase two or three times, which will result in a doubling of the mass deficit from Antarctica."
BACK TO WATER
Ice loss on the West Antarctic Ice Sheet:
* 1996: 83 billion tonnes
* 2006: 132 billion tonnes
- Independent
