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article imageAntarctic glacial melt may cause irreversible sea level rise

By Karen Graham     Jul 10, 2019 in Environment
A NASA-funded study has found that Antarctica faces a tipping point where glacial melting will accelerate and become irreversible even if global heating eases.
Arctic sea ice and mountain glaciers are melting, and it is hard to look at images and videos of what is happening. But their contribution to sea level rise pales in comparison to what we may see now that the Antarctic ice sheet faces a tipping point - a point of no return.
A new study, funded by the National Science Foundation and NASA and published and online by the Proceedings of the National Academy of Sciences has found instability in the Thwaites glacier, meaning there would probably come a point when it was impossible to stop it flowing into the sea and triggering a 50 centimeter (19.6 inches) sea level rise.
This instability is unlikely to be found only in the Thwaites Glacier. Actually, the research shows the rate of ice loss from five Antarctic glaciers had doubled in six years and was five times faster than in the 1990s. And the ice loss is spreading from the coast into the continent's interior, with as much as 100 meters (328 feet) of ice reduction in some places.
The Thwaites Glacier part of the West Antarctica Ice Sheet is undergoing accelerated melt along with...
The Thwaites Glacier part of the West Antarctica Ice Sheet is undergoing accelerated melt along with a number of other glaciers that could see sea levels rise by between 10 and 13 feet.
NASA
Just how much ice the Thwaites glacier will shed in the next 50 or 100 years is difficult to predict because it depends on a number of variables, such as the unpredictable fluctuations in climate and the need for more data.
Ice flow simulations
However, researchers from the Georgia Institute of Technology, NASA Jet Propulsion Laboratory, and the University of Washington have factored the instability into 500 ice flow simulations for Thwaites with refined calculations, according to Phys.org.
All the different scenarios, while diverging strongly - together pointed toward an eventual tipping point. Once this point of no return is reached - and even if global warming were to later stop, the instability would keep pushing ice out to sea at an enormously accelerated rate over the coming centuries.
Ice melt at the grounding line contributes to seawater and thus sea levels  but the larger effect is...
Ice melt at the grounding line contributes to seawater and thus sea levels, but the larger effect is to send more ice above it out into the water, where it also drives up sea level. When sea bottom behind the grounding line, under the ice, slopes downward going inland, it exacerbates the process, which can become unstable, perpetually pushing ice out to sea.
AntargticGlaciers.org.
"If you trigger this instability, you don't need to continue to force the ice sheet by cranking up temperatures. It will keep going by itself, and that's the worry," said Alex Robel, who led the study and is an assistant professor in Georgia Tech's School of Earth and Atmospheric Sciences. "Climate variations will still be important after that tipping point because they will determine how fast the ice will move."
"After reaching the tipping point, Thwaites Glacier could lose all of its ice in a period of 150 years. That would make for a sea level rise of about half a meter (1.64 feet)," said NASA JPL scientist Helene Seroussi, who collaborated on the study. For comparison, the current sea level is 20 cm (nearly 8 inches) above pre-global warming levels and is blamed for increased coastal flooding.
Why is Antarctic ice the big driver of sea level rise?
It is important to remember that Arctic sea ice is already floating on water, and 90 percent of an iceberg's mass is underwater and that when its ice melts, the volume shrinks, resulting in no change in sea level, like a cube of ice in a glass of water.
A massive cavity that is two-thirds the size of Manhattan and nearly the height of the Chrysler Buil...
A massive cavity that is two-thirds the size of Manhattan and nearly the height of the Chrysler Building is growing at the bottom of the Thwaites Glacier, one of the world’s most dangerous glaciers.
NASA/OIB/Jeremy Harbeck
On the other hand, Antarctica holds the most land-supported ice, even if much of that land is seabed holding up just part of the ice's mass, while water holds up part of it. Also, Antarctica is an ice leviathan. "There is almost eight times as much ice in the Antarctic ice sheet as there is in the Greenland ice sheet and 50 times as much as in all the mountain glaciers in the world," Robel said.
The line between where the ice sheet rests on the seafloor and where it extends over water is called the grounding line. In spots where the bedrock underneath the ice behind the grounding line slopes down, deepening as it moves inland, is where instability can kick in, according to the researchers.
"Once the ice is past the grounding line and just over the water, it's contributing to sea level because buoyancy is holding it up more than it was," Robel said. "Ice flows out into the floating ice shelf and melts or breaks off as icebergs."
Will this instability kick in this year or in the next 50 years? In the simulations, Thwaites Glacier colossal ice loss kicked in after 600 years, but it could come sooner. "It could happen in the next 200 to 600 years. It depends on the bedrock topography under the ice, and we don't know it in great detail yet," Seroussi said.
More about Antarctica, glacial ice melt, Sea level rise, irreversible, Nasa study
 
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