Antarctic ice sheet destabilized within a decade | Polarjournal
During the warming after the last glacial maximum, there were repeated periods when masses of icebergs broke off in Antarctica, leading to global sea level rise over many hundreds of years. Photo: Dr. Michael Wenger

Since the last ice age about 20,000 years ago, there have been several periods in which masses of icebergs broke off from the Antarctic ice sheet. Using seafloor samples, an international team of researchers led by the University of Bonn has now been able to show that within just one decade the tipping point was reached, after which the mass loss of ice continued for many centuries. Similar to past events, the current accelerating ice mass loss could also represent such a tipping point with irreversible and prolonged ice retreat associated with global sea level rise.

In the new study, published in Nature Communications, the scientists searched the seafloor of the Southern Ocean, more specifically of the so-called “Iceberg Alley” – the iceberg route to the north – specifically for debris that was trapped in the icebergs thousands of years ago and landed on the seafloor as they melted. The team led by Dr. Michael Weber from the Institute of Geosciences at the University of Bonn used the sediment cores to date the natural climate archive and counted the debris left behind by the icebergs.

The red dots mark the routes of large icebergs that broke off from the Antarctic ice sheet. In general, they follow the Antarctic Coastal Current in a counterclockwise direction (brown arrows). Many of them continue northward along the “Iceberg Alley” between the Weddell Sea and the Scotia Sea. The white stars mark the sampling points. Map: Weber et al. 2021. An animation of the iceberg drift is available at the following link:

The results show that since the last ice age about 19,000 to 9,000 years ago, there have been eight phases with large amounts of debris, which scientists interpret as retreat phases of the Antarctic ice sheet. During this time, temperatures rose and the climate warmed, causing repeated mass ice loss into the Southern Ocean in Antarctica. An accompanying data model study showed that each of the eight phases destabilized the ice sheet within ten years, leading to global sea-level rise that lasted for several centuries to a millennium. The re-stabilization then occurred just as quickly within a decade.

According to the authors, the results are relevant to the ice retreat that is currently observed. “Our findings are consistent with a growing body of evidence suggesting the acceleration of Antarctic ice-mass loss in recent decades may mark the begin of a self-sustaining and irreversible period of ice sheet retreat and substantial global sea level rise,” said lead author Dr Michael Weber.

Combining the sediment records with computer models of ice sheet behavior provided evidence that each of the eight calving phases also reflects increased ice loss within the ice sheet. “We found that iceberg calving events on multi-year time scales were synchronous with discharge of grounded ice from the Antarctic Ice Sheet,” explains Prof Nick Golledge, co-author of the study from the University of Wellington, New Zealand.

Dr. Michael Weber of the University of Bonn, lead author of the study, fears that we are witnessing “the tipping” of the Antarctic ice sheet. Photo: Dr. Michael Weber/University of Bonn

Subsequent application of statistical methods revealed that there are indeed tipping points in the ice sheet system. “If it just takes one decade to tip a system like this, that’s actually quite scary because if the Antarctic Ice Sheet behaves in future like it did in the past, we must be experiencing the tipping right now,” Weber said.

The researchers cannot predict when the ice sheet will re-stabilize, as this depends largely on how strong future climate warming will be.

Julia Hager, PolarJournal

Link to the study: Michael E. Weber, Nicholas R. Golledge, Chris J. Fogwill, Chris S. M. Turney, Zoë A. Thomas. Decadal-scale onset and termination of Antarctic ice-mass loss during the last deglaciation. Nature Communications, 2021; 12 (1) DOI: 10.1038/s41467-021-27053-6

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