Thinning and retreat of the West Antarctic Ice Sheet in response to 60 years of presently observed ice-shelf melting. Video credit: Matthias Mengel.

"If the presently observed instability in the Amundsen region continues, then it is likely that sea level will rise by more than 3 metres from the contribution of West Antarctica alone, which poses a threat to coastal ecosystems and society worldwide," Anders Levermann of the Potsdam Institute for Climate Impact Research told environmentalresearchweb. "We might be witnessing the beginning of a multi-millennial period of self-sustained ice loss into the ocean that will require a long-term adaptation of coastal protection such as the building or rebuilding or raising of dykes, the construction of seawalls, or the realization of land fills in the hinterland."

It’s likely that further emissions of carbon into the atmosphere will increase the risk of triggering similar instabilities in other regions of Antarctica, according to Levermann.

In West Antarctica, the bedrock on which the ice sheet sits tends to slope downwards away from the coast so that it lies below sea-level. As a result, as the grounding line – the point at which the ice leaves the bedrock to become an ice shelf floating on the ocean – moves inland, this so-called marine ice becomes unstable.

Together with Potsdam colleague Johannes Feldmann, Levermann used a parallel ice sheet model with a horizontal resolution of 5 km covering the entire West Antarctic Ice Sheet, the Antarctic Peninsula and part of East Antarctica.

For simulations where ice-melt in the Amundsen Sea lasted for at least 60 years, an abrupt increase in ice loss followed. The catchment of the Pine Island and Thwaites glaciers, which terminate in the Amundsen Sea, drained in less than 3000 years in the models, and the grounding line retreated inland into the Byrd Subglacial Basin. Although the steep bed of the Ellsworth Subglacial Highlands stopped further retreat into the interior of the West Antarctic Ice Sheet, movement of the grounding line continued laterally into the marine parts of Ellsworth Land and Marie Byrd Land, and out to the Filchner-Ronne and Ross Ice Shelves.

The ice sheet regained its equilibrium in the model after around 13,000 years, with 0.96 x 106 square km of ice that was originally several kilometres thick and grounded subsequently existing as an ice shelf with an average thickness less than 500 m. The previously unconnected Amundsen, Weddell and Ross seas also became linked.

"It was speculated whether a retreat of the West Antarctic Ice Sheet, once triggered, might be stopped at some point by topographic obstacles but we find that this is not the case," said Levermann. "The West Antarctic Ice Sheet would be the first element of the Earth system to tip. Other potential tipping elements are, for example, the Amazonian Rain Forest or the Indian Monsoon Circulation."

The Amundsen Sea sector of West Antarctica is currently causing the bulk of the continent’s ice loss, with Pine Island Glacier and Thwaites Glacier thinning and retreating over the last 40 years. This coincides with an increase in the amount and temperature of relatively warm circumpolar deep water reaching into the ice-shelf cavities in the sector, the researchers say. This water is melting the underside of the ice shelves and causing grounding lines to retreat.

"A collapse of the West Antarctic Ice Sheet has been hypothesized already four decades ago," said Levermann. "Now, with satellite observations of present-day Antarctic ice and bed topography, and the powerful tool of computer simulations available, we could tackle the stability question in our modeling study. Recent modeling studies found that the glaciers' retreat is likely to continue in the near future, pointing to a triggered instability. What these studies that focused on small parts of the West Antarctic Ice Sheet could not answer, is how a triggered instability would affect the ice sheet on the long term."

Now the team is interested in investigating the stability of other Antarctic regions. The researchers, who reported their findings in PNAS, would also like to understand better which factors determine the speed of an unfolding ice-sheet instability.

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