The ice crossed a line and changed character.
A million years ago, Antarctica's ice sheet crossed an invisible line and became a different kind of system — not gradually, but all at once. Researchers at Pusan National University have now identified the threshold at which this happened: 240 parts per million of atmospheric CO2, a number that transformed the ice sheet from a measured responder into an amplifier of climate forces. The discovery, published in Nature Geoscience, carries a quiet warning for the present: a system that has flipped its sensitivity once can flip it again, and the forecasts guiding coastal planning may not be built for that kind of world.
- Antarctica's ice sheet didn't ease into a new climate regime a million years ago — it crossed a threshold and changed character almost instantaneously, swinging far more sharply to every atmospheric and oceanic nudge.
- The missing piece was a number: when CO2 fell below 240 ppm during the Mid-Pleistocene Transition, colder oceans, lower seas, and rebounding bedrock locked the ice into a heavier, more volatile frozen state that defined the ice ages that followed.
- To find it, scientists built a three-million-year reconstruction of global climate from scratch and ran it through an ice sheet model on one of South Korea's fastest supercomputers — watching Antarctica's behavior continuously rather than in isolated snapshots.
- The danger now is that a system capable of flipping toward greater sensitivity in the cold can flip toward greater sensitivity in the warmth — and current sea level rise projections assume the kind of gradual melting this research suggests the ice sheet may not deliver.
- Modelers now have a concrete threshold to work with, but the harder question it raises is whether the forecasts guiding decisions about the world's coastlines are already missing the moment the system lurches.
A million years ago, Antarctica's ice sheet didn't gradually change how it responded to the climate — it flipped. One moment it was reacting to atmospheric and oceanic shifts in a measured, predictable way. The next, it was amplifying every nudge, swinging hard in response to forces that had once moved it gently. Scientists had long known that ice ages grew longer and more severe around that time, stretching from 41,000-year cycles to 100,000-year ones. What they didn't know was how the Antarctic ice itself behaved through that transition.
Kyung-Sook Yun and her team at the Center for Climate Physics at Pusan National University found a way through the missing data. They built a three-million-year reconstruction of global temperature and rainfall patterns and fed it into an ice sheet model from Penn State University, running the simulation on one of South Korea's fastest supercomputers. Rather than recreating isolated moments, they watched Antarctica's ice sheet evolve continuously across that vast stretch of time.
Buried in the results was a threshold: 240 parts per million of atmospheric CO2. When concentrations fell below that number during the Mid-Pleistocene Transition, the ice sheet stopped responding gently and changed character. Three forces appear to have worked together to lock in this new state — colder oceans melting less ice from below, lower sea levels easing pressure on the seafloor and allowing bedrock to rebound upward, and that rising rock letting ice pile thicker along the coast. A heavier, more persistent frozen state took hold.
The reason a million-year-old freeze matters now comes down to one word: thresholds. Co-author Axel Timmermann noted that the ice sheet proved more sensitive to outside forces than scientists had assumed. An ice sheet that flipped its sensitivity in the cold direction can flip it in the warm direction too. Projections built on smooth, gradual melting may be reading the ice wrong — missing the moment the system lurches rather than slides. Identifying this tipping point gives modelers a real target, and reshapes how scientists think about the danger rising along the world's coastlines.
A million years ago, something shifted beneath Antarctica. The ice sheet that covers the continent didn't gradually change how it responded to warming and cooling. It flipped. One moment it was reacting to climate shifts in a measured, predictable way. The next, it was swinging hard—amplifying every nudge from the atmosphere and ocean, becoming far more sensitive to the forces acting on it. Scientists have long known that ice ages grew longer and more severe around that time, stretching from roughly 41,000-year cycles to 100,000-year ones. What they didn't know was exactly how the Antarctic ice itself behaved through that transition. The detailed climate records needed to model that period simply didn't exist.
Kyung-Sook Yun and her team at the Center for Climate Physics at Pusan National University in South Korea found a way around the missing data. They built the climate history themselves—a three-million-year reconstruction of global temperature and rainfall patterns—and then fed it into an ice sheet model from Penn State University that tracks how ice flows, thickens, and spreads across a continent. Running the simulation on one of South Korea's fastest supercomputers, they watched what happened to Antarctica's ice sheet continuously over that vast stretch of time, rather than trying to recreate isolated moments.
Buried in the results was a number that had eluded researchers before: 240 parts per million of atmospheric carbon dioxide. When CO2 dropped below that threshold during what geologists call the Mid-Pleistocene Transition, the Antarctic ice sheet stopped responding gently to changes in air and ocean temperature. Its behavior didn't ease into something new as the planet slowly cooled. It crossed a line and changed character. The response amplified suddenly rather than gradually. "After this transition, the Antarctic ice sheet reacts much more strongly to changes in climate forcing," Yun explained. "This indicates that the system does not evolve gradually but instead becomes more responsive after crossing a particular threshold in the climate system."
Three factors appear to have worked together to lock in this new, more sensitive state. Colder glacial oceans melted less ice from below, where the ice sheet meets the sea, slowing the steady loss that warmer water had been driving along the underside. Lower global sea levels—160 to 330 feet below today's—eased pressure on the seafloor, allowing the bedrock beneath the ice to rebound upward slowly. That uplift let ice pile thicker along the coast. Colder water and rising rock fed each other. A heavier, more persistent frozen state took hold, defining the ice ages that followed.
The reason a million-year-old freeze matters now comes down to one word: thresholds. An ice sheet that flipped its sensitivity in the cold direction can flip it in the warm direction too. Study co-author Axel Timmermann noted that the ice sheet proved more sensitive to outside forces than scientists had assumed, raising hard questions about its future. Other recent modeling has flagged how little ocean warming it might take to push West Antarctica past a tipping point of its own. Antarctica is the largest single wild card in how high the oceans climb this century and beyond.
Projections that assume smooth, predictable melting may be reading the ice wrong. Researchers now have evidence that the Antarctic ice sheet has crossed a sensitivity threshold before—and they have a number attached to it. If ice can switch regimes at a threshold, then forecasts built on gradual change risk missing the moment the system lurches. Identifying this tipping point gives modelers a real target as they sharpen predictions of sea level rise along the world's coastlines. The study, published in Nature Geoscience, reshapes how scientists think about the danger ahead.
Notable Quotes
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.— Kyung-Sook Yun, Center for Climate Physics, Pusan National University
The ice sheet proved more sensitive to outside forces than scientists had assumed, raising hard questions about its future.— Axel Timmermann, study co-author
The Hearth Conversation Another angle on the story
Why does a climate shift from a million years ago matter to us now?
Because it shows us that ice sheets don't always respond to warming the way our models assume. They can flip suddenly from one behavior to another. If it happened before, it can happen again—but in the opposite direction, toward faster melting.
So you're saying the ice sheet is more fragile than we thought?
Not fragile exactly. More like it has a switch. For a long time it responded gently to climate changes. Then something clicked, and it became hypersensitive. We now know that switch exists. We just don't know how close we are to flipping it the other way.
What made it flip a million years ago?
Colder oceans stopped melting the ice from underneath. The seafloor rose as sea levels dropped. Those two things reinforced each other—the ice got thicker, heavier, more stubborn. It locked into a new state.
And that could happen in reverse?
Theoretically, yes. If we warm the oceans enough, if we raise sea levels, we could trigger the opposite flip. The ice could become much more responsive to warming than our current forecasts assume.
So our sea level rise predictions might be too optimistic?
They might be. If the ice can switch regimes at a threshold, and we're pushing toward that threshold, then models built on gradual change could miss the moment everything accelerates.