Researchers in South Korea have found that the Antarctic ice sheet became dramatically more responsive to climate forcing after a key shift in Earth’s ice age cycles roughly 1 million years ago. The discovery has fresh implications for predicting future sea level rise.
Antarctica’s ice sheet — the largest reservoir of frozen water on Earth — did not always respond to climate change the way it does today. A new study published in Nature Geoscience reveals that the ice sheet crossed a critical threshold roughly 1 million years ago, after which it became significantly more sensitive to shifts in atmospheric carbon dioxide and ocean temperatures. The findings could reshape how scientists forecast future sea level rise driven by global warming.
A Turning Point in Earth’s Ice Age History
About 1 million years ago, Earth’s climate underwent a profound reorganization called the Mid-Pleistocene Transition. Ice ages grew longer, colder and more intense — a dramatic departure from the pattern that had dominated for the previous 2 million years. Despite the recognized importance of this shift, scientists had struggled to understand exactly how major ice sheets adapted to it, partly because reliable, long-term climate data for feeding into ice sheet models were scarce.
To close that gap, researchers at the IBS Center for Climate Physics (ICCP) at Pusan National University in South Korea combined two powerful computational tools. First, they drew on a detailed paleoclimate simulation that reconstructs global climate conditions across the last 3 million years. Temperature and precipitation outputs from that simulation were then fed into the Penn State University ice-sheet-ice-shelf model, which tracks changes in ice flow, temperature and elevation for both the Northern Hemisphere ice sheets and Antarctica — including the behavior of floating ice shelves in the Ross and Weddell Seas. The combined model was run on one of South Korea’s most powerful computers dedicated to basic science, producing a physically coherent, spatially detailed picture of how ice sheets evolved under continuously changing climate conditions.
A Critical CO2 Threshold
The simulation identified a striking pattern: once atmospheric CO2 concentrations dropped below roughly 240 parts per million, the amplitude of Antarctic ice fluctuations jumped sharply. In other words, the ice sheet did not gradually grow more reactive over time — it crossed a threshold and entered a fundamentally different mode of behavior.
“After this transition, the Antarctic ice sheet reacts much more strongly to changes in climate forcing. This indicates that the system does not evolve gradually but instead becomes more responsive after crossing a particular threshold in the climate system,” lead author YUN Kyung-Sook, a researcher at the IBS Center for Climate Physics, said in a news release.
The research points to a combination of reinforcing mechanisms that drove this change. Colder ocean temperatures during glacial periods reduced the rate at which seawater melted the underside of ice shelves, allowing more ice to accumulate. At the same time, global sea levels dropped by roughly 50 to 100 meters below present levels, easing the pressure on the bedrock beneath coastal ice shelves. That pressure relief triggered a slow upward rebound of the land, which in turn promoted thickening of ice along Antarctica’s coastline. These factors worked together to sustain the larger, more persistent ice sheets that characterized the later ice age cycles.
Why It Matters for Future Sea Level Rise
Understanding past ice sheet behavior is not just an exercise in geological curiosity — it is directly relevant to projections of how much the oceans could rise as greenhouse gas concentrations climb. The Antarctic ice sheet contains enough frozen water to raise global sea levels by roughly 58 meters if it melted entirely, making its stability one of the most consequential questions in climate science.
“Our findings suggest that the Antarctic ice sheet was more sensitive to external forcings than previously assumed. This also raises important questions about its future response to global warming,” added co-author Axel Timmerman, the director of the IBS Center for Climate Physics.
The study underscores that ice sheets are not passive, linear responders to climate change. Instead, they can undergo abrupt shifts in behavior when key thresholds are crossed — a dynamic that existing sea level projections may not fully capture. If the ice sheet’s sensitivity was underestimated in the past, current models predicting its response to rising CO2 levels may also be missing important nonlinear dynamics.
What This Means for Students and Researchers
For students studying earth sciences, climatology, oceanography or environmental policy, this research is a reminder of how much our understanding of the planet’s climate system is still evolving. The methods used here — coupling high-resolution paleoclimate simulations with ice sheet models and running them on cutting-edge supercomputing infrastructure — represent the frontier of how scientists reconstruct and project large-scale environmental change.
The work also highlights the value of international collaboration and long-term investment in basic science computing. The paleoclimate simulation underpinning the study was developed at the ICCP, but the ice sheet model originated at Penn State University, illustrating how cross-institutional tools are advancing climate research in ways that no single lab could achieve alone.
Source: IBS Center for Climate Physics, Pusan National University