Enormous Antarctic glacier on brink of collapse could raise sea levels by half a metre alone, scientists warn

Ice sheet may reach tipping point where it becomes unstoppable driver of sea level rise

Harry Cockburn
Tuesday 09 July 2019 13:13
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18 years of satellite imagery shows receding of Thwaites Glacier

An enormous glacier the size of Florida may be on the brink of melting so quickly it could cause catastrophic global sea level rises, scientists have warned.

While the climate crisis has seen temperatures soar and rapidly reduce ice levels in the Arctic, down in the Antarctic, far larger ice sheets containing much more water are now believed to be at significant risk of collapse, despite previously being considered stable.

The Thwaites Glacier is one of five recently identified unstable Antarctic glaciers which have doubled their rate of ice loss in just six years.

Covering 70,000 square miles, it is likely to accelerate its flow into the ocean, a new study into Antarctic ice sheet stability has suggested.

Once it passes a tipping point, it will become an unstoppable process which could raise sea levels at a much higher rate than previously estimated, researchers from the Georgia Institute of Technology, Nasa Jet Propulsion Laboratory, and the University of Washington said after using computer models to forecast how the glacier’s instability could affect the rate of ice it sheds.

All of the models pointed towards the glacier reaching a tipping point and the ice sheet’s subsequent collapse, they said.

In the worst-case scenario the ice from the Thwaites Glacier alone could raise global sea levels by as much as half a metre (20 inches) in as little as 150 years.

The team warned that once a tipping point is reached, even if global temperatures were reduced, the glacier’s flow into the sea would already be unstoppable.

“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 the study’s leader Alex Robel, 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.”

Nasa scientist Helene Seroussi added: “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 metre (1.64 feet).”

For comparison, current sea level is 20cm (nearly 8 inches) above pre-global warming levels and is blamed for increased coastal flooding.

For about 2,000 years until the late 1800s, global sea levels remained almost static with small fluctuations, but then it began to climb, according to the Smithsonian Institution.

The annual rate of sea level rise has roughly doubled since 1990.

The extent of the instability of the Antarctic ice sheet is difficult to study, but the computer models predicting its future collapse provide problems for forecasters and climate modelling teams as it offers a broad range of potential scenarios.

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This is particularly relevant to the challenge of engineering against flood dangers.

“You want to engineer critical infrastructure to be resistant against the upper bound of potential sea level scenarios a hundred years from now,” Dr Robel said. “It can mean building your water treatment plants and nuclear reactors for the absolute worst-case scenario, which could be two or three feet of sea level rise from Thwaites Glacier alone, so it’s a huge difference.”

Antarctica holds vast quantities of ice – around 90 per cent of all ice in the world - and unlike in the Arctic, most of the ice is out of the water and on land. The average thickness of the ice is 1.6 miles deep. At its thickest point the ice sheet is almost three miles deep.

As a result the collapse of its ice sheets presents more serious future scenarios for sea level rise – while in the Arctic, 90 per cent of an iceberg’s mass is already underwater. When its ice melts, the volume shrinks, resulting in little change in sea level.

The instability beneath the Thwaites Glacier is thought to be related to the topography of the area where the ice sheet meets the sea.

The “grounding line”, where the bottom of the ice sheet rests on the sea floor can retreat beneath the glacier due to warming ocean water hollowing out the ice.

In deeper areas, the ice over the sea floor can move faster due to the water giving it more lift, and this can speed up the movement of the glacier.

“Once ice is past the grounding line and just over water, it’s contributing to sea level because buoyancy is holding it up more than it was,” Dr Robel said. “Ice flows out into the floating ice shelf and melts or breaks off as icebergs.”

Dr Seroussi added: “The process becomes self-perpetuating.”

The study is published in the Proceedings of the National Academy of Sciences.

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