A large part of the West Antarctic ice sheet appears to be in an "unstoppable" decline meaning global sea level rise predictions will have to be revised, scientists have found.
Glaciers in the Amundsen Sea that contain enough ice to raise sea levels by four feet (1.2 metres) are thawing faster than expected and there is nothing to stop them melting into the ocean, according to a study based on 40 years of observations by researchers from the Nasa space agency and the University of California, Irvine.
"The collapse of this sector of West Antarctica appears to be unstoppable," said glaciologist Eric Rignot, lead author of the study. "It's passed the point of no return."
The fact that the retreat is happening simultaneously over a large sector suggests it was triggered by a common cause, such as an increase in the amount of ocean heat beneath the floating sections of the glaciers, he said. The changes were thought to be linked to climate change.
Some scientists have previously warned that increased concentrations of man-made greenhouse gases in the atmosphere may be affecting wind patterns around Antarctica and causing other changes that are pushing warmer waters towards the continent.
It could take several centuries for all the ice to flow into the sea, Professor Rignot said, adding that the sector would be a "major contributor to sea level rise in the decades and centuries to come".
Projections of global sea levels, which have already risen by nearly 20cm since 1900, will have to be revised as a result of the study's findings, he said.
The glaciers studied by the researchers flow out from land to the ocean, where their leading edges float on the seawater. The point on a glacier where it first loses contact with land is called the grounding line.
The Antarctic glaciers have thinned so much they are floating above places where they used to sit solidly on land, meaning their grounding lines are retreating inland.
As glaciers flow faster, they stretch out and get thinner which makes them lighter and lifts them further off the bedrock. As the grounding line retreats and more of the glacier becomes waterborne, it means there is less resistance below and the flow of ice accelerates.
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In order to stop or slow this process, there needs to be hills or bumps coming up from the glacier bed that can snag the ice from below, but the researchers found there are no such "pinning points" upstream from the present grounding lines in five of the six glaciers in the region.
Only the Haynes Glacier has major obstructions upstream, but it only drains a small section and is retreating as fast as the other glaciers.
The new study is to be published in the Geophysical Research Letters journal.