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dinsdag 23 mei 2017

Antarctic warming threatens vast ice shelf

Antarctic warming
threatens vast ice shelf


The world’s second largest ice shelf is in danger of shrinking dramatically within a few decades as the Southern Ocean continues to warm.


By Tim Radford
LONDON, 22 May, 2017 – German scientists have worked out the process that could destroy an Antarctic ice shelf the size of Iraq.
They predict that, in a few decades, the oceanographic machinery that keeps the Ronne-Filchner ice shelf in the Weddell Sea will fail. Warm ocean will begin to eat away at the 450,000 sq km sheet of floating ice.
And the insulating zone of cold, very salty water that had for centuries served as a natural protection for the world’s second largest ice shelf will have gone. The Southern Ocean will have been irrevocably changed.
The ice will lose its anchorage on the sea floor. This had served as a brake on glacial movement from the Antarctic mainland, so the flow of glacial ice from the continent to the sea will increase.

Less sea ice forming

“We can see the first signs of this trend today,” says Hartmut Hellmer, an oceanographer at the Alfred Wegener Institute’s Helmholtz Centre for Polar and Marine Research in Bremerhaven, Germany.
“First of all, less sea ice is forming in the region and, secondly, oceanographic recordings from the continental shelf break confirm that warm water masses are already moving closer and closer to the ice shelf in pulses”
He and colleagues report in the Journal of Climate that the self-amplifying feedback cycle that they have identified in the Weddell Sea could be irreversible − which means that the world’s second largest ice shelf would shrink dramatically.
Other research teams have predicted that the melting of the south polar ice shelves could double by 2050, that processes below the ice are causing fractures that result in huge icebergs, and that lessons from 15,000 years ago confirm that worldwide sea levels rose three metres as a consequence of a sudden warming of Antarctica.

“ There will be no turning back once the warm
water masses find their way under the ice shelf ”

So the German research is not a shock. But it does help climate scientists understand just how a region that had maintained a stable sheath of ice around itself for all human history could suddenly melt.
The change begins in the sea itself. Ice melts in summer and freezes again in autumn and winter, releasing salt below the ice shelf to support a layer of insulating water as cold as minus 2°C. This has buffered the Ronne-Filchner ice shelf from inflowing water that is 0.8°C − almost three degrees warmer.
But rising air temperatures – and they are rising worldwide, thanks to the prodigal use of fossil fuels and the build-up of the greenhouse gas carbon dioxide – mean that less sea ice will form. And this will be the beginning of the transformation of the Weddell Sea. By 2070, changes will be visible.
“Our simulations show that there will be no turning back once the warm water masses find their way under the ice shelf, since their heat will accelerate the melting at its base,” Dr Hellmer says.

Carbon dioxide levels

“In turn, the resulting meltwater will produce an intensified overturning, which will suck even more warm water from the Weddell gyre under the ice. As such, according to our calculations, the hope that the ocean would some day run out of heat won’t pan out in the long run.”.
The scientists start from the assumption that  if humans go on burning fossil fuels at an ever-accelerating rate, carbon dioxide levels in the atmosphere will reach 700 parts per million. Through most of human history and prehistory, they have bobbed along at 280 ppm.
Right now, they stand at 400 ppm, and climates have already begun to change, with a corresponding climb in average global temperatures. Something comparable has already started to happen to the Amundsen Sea on the other side of the Antarctic Peninsula.
“When it comes to the Amundsen Sea, where warm water has already reached the continental shelf and even the grounding line of some ice shelves, we can safely say that this inflow of heat cannot be stopped; the climate regime change has already taken place,” Dr Hellmer says.
“In other words, the losses of mass of the West Antarctic Ice Sheet will intensify – just like the models predict.” – Climate News Network

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