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article image'Extreme' changes in eight of Antarctic's largest glaciers

By Karen Graham     Apr 2, 2018 in Science
According to a new study, the grounding line, where Antarctica's glaciers become floating ice shelves, is receding as much as 600 feet per year, bolstering fears of worst-case sea level rise.
Researchers at the UK Centre for Polar Observation and Modelling (CPOM) at the University of Leeds have produced the first complete map of how the Antarctic ice sheet's submarine edge, or "grounding line", is shifting.
The research was published today in the journal Nature Geoscience and shows that the Southern Ocean melted 1,463 square kilometers (565 square miles) of Antarctica's underwater ice between 2010 and 2016 - an area the size of Greater London.
Lead by Dr. Hannes Konrad from the University of Leeds, the team found grounding line retreat was extreme at eight of the ice sheets 65 largest glaciers. Since the last Ice Age, the pace of deglaciation has been about 25 meters (82 feet) a year. The retreat of the grounding line or deglaciation is now roughly five times that speed - or about 500 or more feet a year.
Dr. Konrad said: "Our study provides clear evidence that retreat is happening across the ice sheet due to ocean melting at its base, and not just at the few spots that have been mapped before now. This retreat has had a huge impact on inland glaciers, because releasing them from the sea bed removes friction, causing them to speed up and contribute to global sea level rise."
Researchers from the University College London and the Helmholtz Centre for Polar and Marine Research in Germany measured how the grounding lines were shifting using data from the European Space Agency's (ESA) CryoSat-2 satellites. The above animation shows how this was done.
What is the glacial grounding line?
Antarctica's three-mile thick ice sheet is constantly being pulled by gravity into the ocean along deep ocean-bottom canyons. The grounding point is where the ice sheet begins to float. At this point, the glaciers and ice sheet turn into ice shelves.
When the ice shelves loosen from the sea floor, gravity again takes over, accelerating the movement of the ice flow. Ice leaving the land and flowing into the ocean where it melts will cause sea levels to rise.
Besides satellite altimeter observations, the scientists used measurements of ice geometry to track grounding-line movement around the entire continent, tripling the coverage of previous surveys. The team found that grounding lines of some of the glaciers emptying into the Amundsen Sea have actually been retreating faster, by as much as 600 feet a year.
Antarctica s glaciers carry ice from the interior of the continent to the ocean. This NASA illustrat...
Antarctica's glaciers carry ice from the interior of the continent to the ocean. This NASA illustration shows where the ice is moving fastest; areas in red have the fastest flow, followed by those in pink and purple.
NASA Goddard Space Flight Center
Fears of a worst-case Antarctic meltdown scenario
The worst-case scenario looks at the possibility of a 10-foot rise in ocean levels by 2100. This latest study that shows warm ocean waters causing melting below the ice does not bode well with a 2015 study that showed how global warming is melting ice shelves from above by causing more surface melting.
The 2015 study surmised that melting water penetrates deep down into the ice sheets and shelves. When it refreezes, it fractures the ice sheet from within.
In the west Antarctic  Adelie penguins are in decline due to sea ice melt blamed on global warming
In the west Antarctic, Adelie penguins are in decline due to sea ice melt blamed on global warming
Today's study says that climate models suggest that the current rate of retreat could lead to "centennial-scale collapse of the inland catchment areas." This means that huge areas of ice far from the ocean could collapse within 100 years, leading to an unexpected and sudden sea level rise.
Study co-author Professor Andy Shepherd, from the School of Earth and Environment at Leeds, said: "We were delighted at how well CryoSat-2 is able to detect the motion of Antarctica's grounding lines."
"They are impossible places to access from below, and usually invisible on the ground, so it's a fantastic illustration of the value of satellite measurements for identifying and understanding environmental change."
More about Antarctic glaciers, grounding lines, warm ocean waters, melting at base, Environment