Unstable ice sheet could cause ocean to rise by 4 metres in an ‘irreversible’ cycle of melting, say scientists
Pic: feserc/Flickr
By Gerard Wynn
East Antarctica may become a large contributor to future sea-level rise beyond this century, with ocean warming of 1C or more, a paper published in the journal Nature Climate Change found this week.
The impact of polar ice melt on global sea level rise has largely focused to date on the Arctic, because of rapid warming there.
In Antarctica, the focus has been in the so-called West Antarctic ice sheet, because of higher temperatures than in the rest of the continent, and because much of the ice there is below sea level.
But a separate, East Antarctic ice sheet, called the Wilkes Ice Sheet, is dammed from sliding and melting into the sea by a small amount of ice, found authors from the Potsdam Institute for Climate Impact Research.
While it would take several thousand years to drain the ice sheet into the sea, this process would become irreversible after sustained warming over just a few centuries.
With the dam removed, enough ice would reach the sea to cause 3 to 4 metres sea level rise over a period of 25,000 years, they calculated.
They calculated that the amount of ice holding back the ice sheet was equivalent to just 8 centimetres of sea level rise.
The amount of ice that reached the sea depended on the amount of sustained warming, between 1 and 2.5C local ocean temperature, and its timescale, from 200 to 800 years.
“We show here that the removal of a specific coastal ice volume equivalent to less than 80mm of global sea-level rise at the margin of the Wilkes Basin destabilizes the regional ice flow and leads to a self-sustained discharge of the entire basin and a global sea-level rise of 3–4 metres.
“Beyond the threshold of 80 mm, only unstable states exist and the ice sheet enters a phase of self-sustained retreat in all of our simulations.”
Historical
The authors based their findings both on a new understanding of the topography under the ice sheets, which suggested the potential for melt water to be carried towards the sea, and also on an understanding of the historical record of ice melt in the region.
Topographical analysis showed that the Wilkes Basin is the largest region with land below sea level in East Antarctica.
The authors reported that there was a precedent for the discharge of melt water from the region, contributing to global sea level rise.
“During the mid- to late Pliocene (4.8-3.5 million years ago) massive ice discharge occurred from the unstable margins of Adélie and Wilkes Land due to ice-stream surges that were linked to rapid grounding-line retreat during a warming climate.
“The Pliocene, featuring temperature and CO2 levels similar to end-of-this-century projections, may have had a significantly smaller ice sheet in East Antarctica’s Wilkes Basin, posing the question of the stability of its present marine-based ice.”
The authors pinned the stability of the ice sheet on a small volume of ice they called an ice plug.
“A small volume of ice (to be referred to as the ‘ice plug’) is the deciding factor for the stability of the marine-based ice.”