Tuesday, May 13, 2014

Western Antarctic ice sheet collapse has already begun, scientists warn

The collapse of the Western Antarctica ice sheet is already under way and is unstoppable, two separate teams of scientists said on Monday.
The glaciers' retreat is being driven by climate change and is already causing sea-level rise at a much faster rate than scientists had anticipated.
The loss of the entire western Antarctica ice sheet could eventually cause up to 4 metres (13ft) of sea-level rise, devastating low-lying and coastal areas around the world. But the researchers said that even though such a rise could not be stopped, it is still several centuries off, and potentially up to 1,000 years away.
The two studies, by Nasa and the University of Washington, looked at the ice sheets of western Antarctica over different periods of time.
The Nasa researchers focused on melting over the last 20 years, while the scientists at the University of Washington used computer modelling to look into the future of the western Antarctic ice sheet.
But both studies came to broadly similar conclusions – that the thinning and melting of the Antarctic ice sheet has begun and cannot be halted, even with drastic action to cut the greenhouse gas emissions that cause climate change.
They also suggest that recent accumulation of ice in Antarctica was temporary.
“A large sector of the western Antarctic ice sheet has gone into a state of irreversible retreat. It has passed the point of no return,” Eric Rignot, a glaciologist at Nasa and the University of California, Irvine, told a conference call. “This retreat will have major consequences for sea level rise worldwide.”
The two studies between them suggest sea-level rise will be far greater than envisaged by the United Nations’ IPCC report earlier this year. The IPCC forecast on sea-level rise did not factor in the melting of the western Antarctica ice sheet.
The Nasa study, to be published in Geophysical Research Letters, studied the retreat of six glaciers in western Antarctica that are already the major drivers of global sea-level rise.
One of those glaciers, Pine Island, retreated 31km at its centre from 1992-2011. Rignot said all six glaciers together contained enough ice to add an additional 1.2m (4ft) to sea levels around the world.
In the University of Washington study, which will be published in the journal Science, researchers used detailed topography maps, airborne radar and computer modelling to reach greater certainty about the projected timeline of the ice sheet collapse.
The study honed in on the Thwaites glacier – a broad glacier that is part of the Amundsen Sea. Scientists have known for years that the Thwaites glacier is the soft underbelly of the Antarctic ice sheet, and first found that it was unstable decades ago.
The University of Washington researchers said that the fast-moving Thwaites glacier could be lost in a matter of centuries. The loss of that glacier alone would raise global sea level by nearly 2ft.
Thwaites also acts as a dam that holds back the rest of the ice sheet. Once Thwaites goes, researchers said, the remaining ice in the sheet could cause another 10 to 13ft (3-4m) of global sea-level rise.
“The thinning we are seeing is not just some temporary trend. It is really the beginning of a larger scale collapse that is likely to play out over a two to 10-century range,” Ian Joughin, a University of Washington glaciologist, told The Guardian.
He said the retreat would begin slowly, resulting in sea-level rise of less than 1mm a year for a couple of hundred years. But “then boom, it just starts to really go,” Joughin said.
Even under the worst-case scenario currently envisaged, the collapse of the entire ice sheet is about 200 years off – and the collapse could be as far away as 1,000 years, depending on future warming.
But collapse is inevitable, the scientists said. Joughin put the most likely timeframe at between 200 and 500 years.. read more: