"When Does the Antarctic Ice Sheet Surge?"... 3-Million-Year Simulation Identifies the Threshold [Reading Science]

"Rapid Expansion of Antarctic Ice Sheet Below 236 ppm CO2": Threshold Response Identified

"Clue to Improving Accuracy of Future Sea Level Rise Projections"—Published in Nature Geoscience

The research team at the Institute for Basic Science (IBS) has revealed, through an ultra-long-term simulation reproducing Earth’s climate over the past 3 million years, that the Antarctic ice sheet exhibits a "threshold response," undergoing rapid and dramatic growth when atmospheric carbon dioxide (CO₂) concentrations fall below a certain level. The researchers expect that these findings will provide important clues for improving the accuracy of future sea level rise projections.

On May 28, the research team led by Axel Timmermann, Director of the IBS Climate Physics Research Center and Distinguished Professor at Pusan National University, announced that they have confirmed the Antarctic ice sheet began responding much more sensitively to changes in carbon dioxide around the Middle Pleistocene Transition (MPT), approximately 1 million years ago.

Relationship between Antarctic Ice Sheet Changes and Atmospheric Carbon Dioxide (CO₂) Concentration over the Past 3 Million Years. The upper right graph shows the Antarctic ice sheet volume changes reproduced by climate simulations. The lower right graph illustrates the nonlinear relationship between CO₂ concentration and Antarctic ice sheet volume, indicating a critical response where the ice sheet rapidly expands when CO₂ levels drop below a certain threshold after about 1 million years ago (blue line). The left map compares Antarctic ice sheet elevation changes under high CO₂ conditions, transitional state, and low CO₂ conditions. Provided by the research team

In particular, the team identified that a nonlinear threshold response occurs when atmospheric carbon dioxide concentration drops below approximately 236 ppm, causing the Antarctic ice sheet to expand rapidly. Currently, the atmospheric CO₂ concentration stands at about 420 ppm—significantly higher than the pre-industrial level of approximately 285 ppm.

The Middle Pleistocene Transition refers to the period around 1 million years ago when glacial cycles became longer and more intense. However, due to a lack of long-term climate data, details regarding how the Antarctic ice sheet changed during this period have remained unclear.

"Rapid Growth Below 236 ppm"... Changes in Antarctic Ice Sheet Sensitivity

The researchers combined the Community Earth System Model (CESM) from the U.S. National Center for Atmospheric Research (NCAR) with the Penn State University Ice-Sheet–Ice-Shelf Model (PSUIM) to simulate how the Antarctic ice sheet has grown and collapsed over the past 3 million years.

Analysis showed that during the early Pleistocene, the Antarctic ice sheet primarily fluctuated according to the expansion and contraction of floating ice shelves. However, since around 1 million years ago, simultaneous ocean cooling and sea level drop driven by declining carbon dioxide began to fundamentally alter the structure of the ice sheet.

Schematic diagram of the response of the Ross Sea ice shelf in Antarctica to climate change and sea level variations. The left side shows ice shelf changes due to climate cooling, the center shows changes due to sea level decline, and the right side illustrates the outcome when both factors act simultaneously. When sea level declines and ocean cooling occur together, the ice shelf more easily grounds on the seabed, leading to more stable growth of the Antarctic ice sheet. Provided by the research team

In particular, as some parts of the floating ice shelf became anchored to the seabed, the flow of glaciers into the ocean was suppressed, and the Antarctic ice sheet transitioned into a much more stable and rapidly growing state compared to before.

The team confirmed that, during this process, a threshold response occurs when atmospheric carbon dioxide concentrations fall below about 236 ppm, leading to a sudden and rapid expansion of the Antarctic ice sheet.

They also found that not only changes in temperature, but also the combined effects of sea level drop and land uplift, further accelerate ice sheet growth.

"Climate Does Not Only Change Gradually"

The researchers explained that this study demonstrates how the climate system does not simply change gradually, but can undergo abrupt changes when certain thresholds are crossed.

Axel Timmermann, Director of the IBS Climate Physics Research Division (left), and Kyungsook Yoon, Research Fellow of the IBS Climate Physics Research Division (first author). Courtesy of IBS

First author Kyungsook Yoon stated, "Since about 1 million years ago, the Antarctic ice sheet has become much more sensitive to changes in carbon dioxide and temperature," adding, "This shows that when the climate system crosses a certain threshold, it can lead to sudden and significant changes."

Director Axel Timmermann commented, "This research shows that the Antarctic ice sheet was more sensitive to changes in carbon dioxide concentration than previously thought," and added, "It is expected to contribute to more accurate predictions of future sea level rise."

The results of this study were published on May 28 in the international journal Nature Geoscience.

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