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Fears that global sea levels this century may rise faster and further than expected are supported by a study showing that 300 glaciers in Antarctica have begun to move more quickly into the ocean.
Scientists believe that the accelerated movement of glaciers in the Antarctic Peninsula indicates a
dramatic shift in the way melting ice around the world contributes to overall increases in global sea levels.
Instead of simply adding huge volumes of meltwater to the sea, scientists have found that rising temperatures are causing glaciers as far apart as Alaska, Greenland and now Antarctica to break up and slip into the ocean at a faster rate than expected.
The findings are likely to raises concerns within the UN's Intergovernmental Panel on Climate Change (IPCC) which earlier this year downplayed the so-called "dynamic" nature of melting glaciers - when rising temperatures cause them to break up quickly rather than simple melt away slowly.
Using radar images taken between 1993 and 2003, scientists at the British Antarctic Survey in Cambridge mapped a 12 per cent increase in the average rate of movement of more than 300 glaciers in the Antarctic Peninsula over the 10-year period.
The scientists believe that their findings are among the first to suggest that as glaciers being to melt they experience a physical transformation that causes an acceleration in their movement into the sea.
"We're only just now getting to grips with just how big these dynamic processes may be.
There are still a lot of surprises out there," said David Vaughan, a glaciologist at the British Antarctic Survey and a co-author of the study.
"It is yet another example of how subpolar glaciers are responding very quickly to climate change because they are close to the temperature transition from ice to water," Dr Vaughan said.
"Scientists want to know why these things are happening because that's the route to the prediction of future sea levels," he said.
In its fourth report published in February, the IPCC said that sea levels this century could rise by between 20 and 43 centimetres, but it accepted that this could be a serious underestimate if ice sheets and glaciers undergo the sort of dynamic changes that existing computer models do not take fully into account.
"Dynamical processes related to ice flow not included in current models but suggested by recent observations could increase the vulnerability of the ice sheets to warming, increasing future sea level rise," the IPCC report warned.
"Understanding of these processes is limited and there is no consensus on their magnitude," it said.
Hamish Pritchard, a co-author of the study published in the Journal of Geophysical Research, said that the latest findings from Antarctica demonstrate how melting glaciers can change in a way that speeds up their eventual disappearance into the sea.
The study showed that rising temperatures cause glaciers to become thinner, which makes them more buoyant when resting on submerged bedrock, so allowing them to slip faster into the ocean, Dr Pritchard said.
"The Antarctic Peninsula has experienced some of the fastest warming on Earth, nearly 3C over the last half century.
Eighty seven per cent of its glaciers have been retreating during this period and now we see these glaciers are also speeding up," Dr Pritchard said.
"They are speeding up in a steady, progressive way.
Warming causes widespread thinning, which causes widespread acceleration due to an increase in buoyancy.
They speed up and the fronts of the glaciers break off and float away," he said.
"What we're seeing is a very important feature of these dynamic effects. "Chris Rapley, director of the British Antarctic Survey, said: "Without doubt we are seeing a striking global picture of ice on the retreat."
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