A new study shows that the vast Antarctic ice sheet fundamentally changed the way it responded to Earth's climate change and became significantly more sensitive after it crossed a hidden climate threshold about 1 million years ago. The research team pointed out that this discovery helps explain the evolution of the Antarctic ice sheet in geological history and may also provide a new reference for future sea level rise predictions.
Antarctica currently stores the largest amount of ice on Earth and is extremely critical in regulating global sea levels. About 1 million years ago, the Earth's climate experienced a significant transition into the so-called "Mid-Pleistocene Transition", during which ice ages began to become longer, colder, and more intense.
Although the scientific community has long noticed this change, it has long been difficult to accurately determine how the Antarctic ice sheet responded to climate change due to limited ancient temperature and precipitation records.
To solve this problem, the researchers used a newly developed paleoclimate simulation model from the Center for Climate Physics of the Korea Institute of Basic Sciences, which can reconstruct global climate conditions over the past 3 million years.
The research team then input the simulated temperature and precipitation data into the ice sheet-ice shelf model developed by Penn State University to track changes in ice sheet thickness, flow and temperature in Antarctica and the Northern Hemisphere, while simulating the behavior of floating ice shelves in areas such as the Ross Sea and Weddell Sea.
Powered by South Korea's most advanced basic science supercomputer, the model paints a coherent picture of the physical mechanisms of how the world's major ice sheets evolve in a changing climate.
The results show that after the mid-Pleistocene transition, the Antarctic ice sheet entered a distinct dynamical state. The researchers identified a key carbon dioxide threshold, about 240 parts per million; when atmospheric CO2 concentrations fall below this level, the sensitivity of Antarctic ice mass to changes in ocean and atmospheric temperatures increases significantly, and the ice sheet size also experiences more dramatic fluctuations.
Kyung-Sook Yun, the first author of the paper and a researcher at the Center for Climate Physics of the Korea Institute of Basic Sciences, said that after the transformation, the Antarctic ice sheet's response to climate forcing was significantly enhanced, which shows that the ice sheet system does not evolve slowly and linearly, but becomes more susceptible to external influences after crossing a certain critical point.
The simulations also show that a combination of factors made it easier for the Antarctic ice sheet to expand after about 1 million years ago. One is that ocean temperatures were lower during the ice age, which weakened the melting of parts of the bottom of the ice below sea level. The other is that the global sea level is about 50 to 100 meters lower than it is now. The lower sea level reduces the pressure on the bedrock beneath the Antarctic ice shelf. Over time, the bedrock slowly rises, which in turn promotes further thickening of the ice in coastal areas.
The researchers believe that these mechanisms combined to shape the larger and longer-lasting Antarctic ice sheet during later glacial cycles.
The authors also caution that the findings mean Antarctica's response to climate change may be more difficult to predict than previously thought. Co-author Axel Timmermann, director of the Center for Climate Physics at the Korea Institute of Basic Sciences, pointed out that the Antarctic ice sheet may be more sensitive to external forcing than previously expected, which also raises an important question: how it will change in the future in the context of global warming.
The research team emphasized that ice sheets do not always respond to environmental changes in a smooth and gradual manner. They may suddenly change their behavioral patterns after crossing a threshold and significantly change their sensitivity to external influences. Understanding when and why these transitions occur is critical to improving the accuracy of predictions of future sea level rise.