The Antarctic Circumpolar Current carries more than 100 times the total flow of all the world’s rivers combined. It circles Antarctica without being blocked by land, making it one of the most important drivers of the global climate system. New research published in the journal Proceedings of the National Academy of Sciences explores how and when this immense current first developed. The findings reveal that simply opening ocean gateways between Antarctica, South America, and Australia was not enough to create it.
Around 34 million years ago, Earth experienced a dramatic transformation during the transition into the Oligocene — shifting from a warm greenhouse world with little ice to a cooler icehouse climate marked by expanding polar ice sheets. During this period, ocean passages between Antarctica, Australia, and South America widened and deepened. At the same time, the Antarctic Circumpolar Current (ACC) began to take shape, and the Antarctic Ice Sheet started forming.
Atmospheric CO2 levels were about 600 ppm back then. This level has not been reached again since, although some future climate scenarios suggest it could be surpassed by the end of this century. “In order to predict the possible future climate, it is necessary to look into the past with simulations and data to understand our Earth in warmer and more CO2-rich climate states than today,” says Hanna Knahl, climate modeller at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) and lead author of the study, which now appears in the Proceedings of the National Academy of Sciences (PNAS). “But careful, the climate of the past can of course not be projected 1:1 onto the future. Our study shows that the circumpolar current in its ‘infancy’ influenced the climate very differently than today’s fully developed ACC does.”
Reconstructing the Birth of the Antarctic Circumpolar Current
To understand how the ACC formed, Knahl and her team ran detailed climate simulations based on Earth’s geography about 33.5 million years ago, when Australia and South America were positioned much closer to Antarctica. The researchers combined these simulations with an Antarctic Ice Sheet model from a 2024 Science study, linking it with ocean, atmosphere, and land systems to track how currents evolved.
The modeled results were then compared with geological reconstructions from the same time period, allowing the team to test how well their simulations matched real-world evidence.
The Key Role of Winds and Shifting Continents
The results highlight the importance of the Tasman Gateway, a seaway between Antarctica and Australia. “There were already indications that the wind in the Tasman Gateway played an important role in the formation of the ACC. Our simulations can clearly confirm this: Only when Australia had moved further away from Antarctica and the strong westerly winds blew directly through the Tasman Gateway, the current could fully develop,” Knahl explains.
The study also suggests that the Southern Ocean looked very different during this early stage. Even though ocean passages were already open, the current did not yet form a continuous loop. Instead, strong flow developed in the Atlantic and Indian regions, while the Pacific sector remained relatively calm.
Advanced Simulations Reveal New Insights
Coupling climate and ice sheet models is still a relatively new and complex approach, but it allows scientists to capture interactions between different parts of the Earth system more realistically. For this work, researchers from AWI’s Palaeoclimate Dynamics and Marine Geology divisions collaborated with international partners, including the Australian Centre of Excellence in Antarctic Science and the Antarctic Research Centre Wellington.
“With this PNAS study, we are showing — for the first time — how helpful and important it is to carry out these coupled and relatively high-resolution model simulations for the climate of the deep past. Even though they are very demanding, they provide novel insights into the interaction of ice, atmosphere, land surface, and ocean,” explains AWI palaeoclimate modeller Prof Dr Gerrit Lohmann, co-author of the study.
Why the Antarctic Current Matters for Today’s Climate
By reconstructing the formation of the ACC, the researchers were able to show how global ocean circulation was reorganized in Earth’s past. This shift had major consequences for the planet’s climate system. According to AWI geoscientist Dr Johann Klages, “This understanding is crucial, as the formation of the ACC has strongly driven carbon uptake by the ocean. This reduction in the concentration of greenhouse gases in Earth’s atmosphere thus had the potential to initiate the cooler climate of the so-called Cenozoic Ice Age, which continues to this day with permanently ice-covered polar ice caps, in which warm and cold periods alternate. This new knowledge will therefore help us to more reliably interpret recent changes in Southern Ocean circulation more reliably.”
These findings provide a clearer picture of how ocean currents, atmospheric conditions, and shifting continents worked together to reshape Earth’s climate, offering valuable context for understanding future changes.
