Global sea level rise figures may have to be revised upwards say scientists, as pace of melt increases
The calving front of the Thwaites ice shelf in Antarctica (Pic: NASA)
By Gerard Wynn
West Antarctic glaciers have started an unstoppable slide into the sea, as a result of warmer seas, which will in turn drain vast ice sheets and add several metres to global sea levels, two studies concluded.
The studies investigated a particular part of West Antarctica where the retreat of glaciers is fastest.
The speed of retreat and discharge of ice had increased almost continuously over the past forty years, and faster than previously thought, implying that expected sea level rise would have to be revised upwards.
The speed of the retreat shocked scientists writing in the journal Geophysical Research Letters.
“We’ve passed the point of no return,” said lead author Eric Rignot, from the University of California and also a glaciologist at NASA’s Jet Propulsion Laboratory.
“It’s just a matter of time before these glaciers completely disappear into the sea.”
The investigation was into six glaciers draining into the Amundsen Sea, including the large Pine Island and Thwaites glaciers.
Together, all six contained enough ice to raise global sea levels by 1.2 metres. They drained ice sheets which held ice equivalent to several more metres.
“This retreat will have major consequences for sea level rise worldwide,” said Rignot. “It will raise sea levels by 1.2 metres. Its retreat will also influence adjacent sectors of the West Antarctic ice sheet which could triple this contribution to sea level.”
The critical factor leading to the speed of melt was the fact that the bellies of the glaciers were travelling underwater along a sea bed sloping inland in deep valleys.
As a result, as they melt, more of the ice is detached from the sea bed, and so the faster the glaciers slide into the sea.
The authors of the Geophysical Research Letters article measured the point at which the end of the glaciers leave the sea bed, called the grounding line.
They found that the ground line was retreating by an average rate of about one kilometre a year. If they maintained that rate of retreat, the glaciers would disappear entirely in a couple of centuries.
The only way the retreat would slow is if there were an obstacle on the bed, or if the glaciers were retreating over land which was rising upwards, instead of downwards.
“But we don’t see that in any of these glaciers,” said Rignot.
“This system is evolving very fast and is progressing exactly as you would expect if it was about to collapse to the sea. There’s a bunch of feedbacks that makes this retreat unstoppable.”
Warmer seas were responsible for the retreat: climate change was the ultimate cause, not so much because of an observed global warming of seas, however, but because of stronger winds which were driving warmer seas polewards, said Rignot.
This is your earth on climate change: Collapse of Antarctic ice sheet “almost certainly unstoppable," scientists say. http://t.co/rCsuRj6KAA
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A separate study, also published this week, made similar conclusions regarding ice sheet collapse in the same part of West Antarctica, focusing on the Thwaites Glacier.
The study, published in the journal Science, concluded that the glacier would disappear in a matter of centuries.
As in the NASA study, the authors concluded that because of the particular, local circumstances, including topography, the ice sheet collapse may be inevitable.
The edge of the Thwaites glacier sits on a bay which will get rapidly deeper as the glacier retreats further, hastening the collapse of ice into the sea.
The authors took a modelling approach, using various assumptions for melt rates to estimate how long it would take for the glacier to collapse.
“Our simulations provide strong evidence that the process of marine ice-sheet destabilization is already under way on Thwaites Glacier. The similarity between our highest melt rates and present observations suggests that collapse may be closer to a few centuries than to a millennium.”