West Antarctic ice sheet collapse 'unstoppable'
Big melt Ice is melting in the western Antarctic at an unstoppable pace, say NASA scientists.
The discovery holds major consequences for global sea level rise in the coming decades, they warn.
The speedy melting means that prior calculations of sea level rise worldwide made by the Intergovernmental Panel on Climate Change will have to be adjusted upwards, say the scientists.
"A large sector of the West Antarctic ice sheet has gone into a state of irreversible retreat. It has passed the point of no return," says Eric Rignot, professor of Earth system science at the University of California Irvine.
"The retreat of ice is unstoppable," he says, noting that surveys have shown there is no large hill at the back of these glaciers that could hold back the melting ice.
"This retreat will have major consequences for sea level rise worldwide," he adds, anticipating the melting will take place largely in the next two centuries.
"It will raise sea level by 1.2 metres," says Rignot, whose paper has been accepted for publication in the peer-reviewed Geophysical Research Letters, a journal of the American Geophysical Union.
A previous study published in the same journal documents observational changes in the Antarctic in recent years, and predict the future behaviour of the melting ice through computer models.
Rignot, a glaciologist at NASA's Jet Propulsion Laboratory, says his study compiles data from satellites, airplanes, ships and ground surveys in the West Antarctic ice sheet.
Retreat of the Pine Island Glacier has slowed in recent years, but scientists say that was likely to due to a very rapid retreat it went through at first.
The nearby Thwaites glacier has been speeding up since 2006.
"From year to year things can change a little bit, so it is really important when looking at this observational evidence to look at the long term trend of these glaciers," says Rignot.
Thwaites Glacier melt
A separate study published in the journal Science found that Thwaites Glacier is melting fast and that its collapse could raise global sea level by more than half a metre.
That study was based largely on computer modelling of the future, in addition to airborne radar measurements of the West Antarctic ice sheet that allowed scientists to map the underlying bedrock.
Study author Ian Joughin, a glaciologist at the University of Washington, says the process is now expected to take between 200 and 1000 years.
He also found that such a collapse may be inevitable.
"All of our simulations show it will retreat at less than a millimetre of sea level rise per year for a couple of hundred years, and then, boom, it just starts to really go," says Joughin.
Current projections of sea level rise, agreed upon by international surveys, do not account for the Antarctic ice sheet melting.
Sridhar Anandakrishnan, professor of geosciences at Pennsylvania State University, says that studies like these will cause the United Nations to revisit their projections.
"The number for 2100 will almost certainly be revised and revised upwards, and my guess is toward what is now their upper limit of something like 90 centimetres," says Anandakrishnan, who was not involved in the new reports.
He says that the rise in sea level is widely accepted to be a result of human-caused climate change, driven by the burning of fossil fuels.
"As glaciers and ice sheets flow faster, that water has nowhere to go but in the ocean and once in the ocean this results in a rise in sea level around the globe," he says.
Even though in the past, Greenland has been the source of most melting glaciers, that appears to be changing, with more and more coming from the South Pole.
"Thus far the loss of ice from Antarctica has not been of the same scale. This appears to be changing," he says.