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Ice in Antarctica (symbolic image): Sea ice is formed from frozen ocean water, while shelf is fed by an ice sheet, glacier or ice stream

Photo: Henryk Sadura / Tetra images / Getty Images

Even if greenhouse gas emissions are reduced rapidly and comprehensively, worryingly large areas of ice shelves in West Antarctica could melt. A team of researchers came to this conclusion with the help of model calculations.

According to the journal Nature Climate Change, warm ocean currents in the Amundsen Sea will dramatically thin out the ice shelf from below in the course of the 21st century – with serious effects on the glaciers stabilized by this ice and thus on global sea level rise.

One should prepare for a significant increase, advises the team led by Kaitlin Naughten from the British Antarctic Survey. If the West Antarctic Ice Sheet were to melt completely, the sea level would rise by up to five metres.

Four scenarios

However, the group only studied the development of the ice shelf – the outer part of the ice sheet that lies on the sea. While the ice shelf in eastern Antarctica is stable or partially growing, it has been thinning out in West Antarctica for years, mainly due to warm ocean currents.

Just recently, a study in the journal Science Advances showed that the Antarctic Ice Shelf has lost 25.7 trillion tons of meltwater within 5 years, a large part of it in the Amundsen Sea, which borders West Antarctica.

Naughten's team simulated the development in this marine area using four different climate scenarios: These ranged from the optimistic scenario in which the Earth warms up by a maximum of only 1.5 degrees Celsius compared to pre-industrial times, to the extreme scenario RCP8.5 of unchecked climate change.

Rapid warming in all scenarios

"All scenarios show a clear and widespread future warming of the Amundsen Sea and an increased melting of its ice shelf," the authors note. "Even in the most ambitious scenario, the Amundsen Sea is warming three times faster than it did in the 20th century." This warming applies to all four scenarios. The extreme scenario RCP8.5 will have an additional impact in this regard from around 2045.

The reason for this trend lies in the water column, the group writes, and there above all in the so-called thermocline. This layer separates cold water at a depth of 100 to 400 metres in the region from a deeper zone with slightly warmer water. In all simulated climate scenarios, this separating layer shifts upwards and with it the layer of warmer water that hollows out the ice shelf from below.

With the thinning of the ice shelf, its support function is likely to decrease, the research group emphasizes. As a result, large ice masses behind it, such as the Thwaites or Pine Island glaciers, could drain more quickly and cause sea levels to rise.

"Our simulations show a sobering outlook for the Amundsen Sea," the team concludes. By the year 2100, it could be up to two degrees Celsius warmer than before industrialization. "For Antarctic water masses, an increase of two degrees is astonishing," it says.

Researchers support the thesis, but also point out the limitations of the study

The team itself admits weaknesses of the study: it did not take into account any interactions with a changed ice geometry and focuses only on the influence of ocean currents, not on atmospheric factors. These, however, could become more important in the long term.

The study offers the most comprehensive projections to date on the warming of the Amundsen Sea, writes Taimoor Sohail of the University of New South Wales in Sydney in a commentary on the paper. "While the window of opportunity to prevent the melting of ice shelves in West Antarctica has likely passed, the actual consequences of climate change for sea level will depend on a variety of factors."

For example, climate measures influence the stability of the East Antarctic Ice Sheet, which is much larger. Nevertheless, in view of the study, it seems clear that a rise in sea level is inevitable. Political decision-makers must now find ways to prepare for the consequences.

Reinhard Drews of the University of Tübingen, who was not involved in the study, told the Science Media Center that CO₂ emissions in Germany are also leading to ice loss in Antarctica. "This process has already begun, and an inherent inertia means that we can't stop some of the ice loss, no matter what measures are taken," he says.

He continued: "This does not call into question the necessity of climate policy, quite the contrary. It is a further indication of how serious the changes are and how local action can have a global impact. For better or for worse."