The Antarctic Circumpolar Current carries more than 100 times the total flow of all the world’s rivers combined. It circles Antarctica unobstructed by land and is one of the most important drivers of the global climate system. New research published in journal Proceedings of the National Academy of Sciences Find out when and how this huge flow first occurred. The results of this study revealed that simply opening a maritime gateway between Antarctica, South America and Australia is not enough to create a gateway.
Approximately 34 million years ago, Earth underwent dramatic changes during the transition to the Oligocene. It is a transition from a warm greenhouse environment with little ice to a cooler icehouse climate characterized by expanding polar ice sheets. During this period, the sea route between Antarctica, Australia, and South America became wider and deeper. At the same time, the Antarctic Circumpolar Current (ACC) began to form and the Antarctic ice sheet began to form.
The level of CO2 in the atmosphere at that time was about 600 ppm. Although this level has not been reached again since then, future climate scenarios suggest it could be exceeded by the end of this century. “To predict possible future climates, we need to use simulations and data to look into the past and understand a planet with a warmer, more carbon-dioxide-rich climate than today,” says Hanna Kunar, a climate modeler at the Alfred Wegener Institute’s Helmholtz Center for Polar and Ocean Research (AWI) and lead author of the study. Proceedings of the National Academy of Sciences (PNAS). “But be careful, of course we cannot project past climate one-to-one into the future. Our study shows that the influence of that ‘early’ circumpolar current on climate was very different from today’s fully developed ACC.”
Reenacting the birth of the Antarctic Circumpolar Stream
To understand how the ACC formed, Kunal and her team ran detailed climate simulations based on the Earth’s geography about 33.5 million years ago, when Australia and South America were much closer to the Antarctic. The researchers combined these simulations with a model of the Antarctic ice sheet in 2024. science The research tracks how ocean currents have evolved by linking them with ocean currents, the atmosphere, and land systems.
The modeled results were then compared to geological reconstructions of the same time period, allowing the team to test how well the simulations matched real-world evidence.
The important role of wind and continental movement
The results highlight the importance of the Tasman Gateway, the sea route between Antarctica and Australia. “There were already indications that Tasman Gateway winds played an important role in the formation of the ACC. Our simulations clearly confirm this. Only when Australia moves further away from the South Pole and strong westerly winds blow directly through the Tasman Gateway can the ocean currents fully develop,” Kunar explains.
The study also suggests that the Southern Ocean looked very different at this early stage. The sea route was already open, but the ocean currents had not yet formed a continuous loop. Instead, strong currents developed in the Atlantic and Indian regions, while the Pacific sector remained relatively calm.
Advanced simulation provides new insights
Coupling climate and ice sheet models is still a relatively new and complex approach, but it allows scientists to get a more realistic picture of the interactions between different parts of the Earth system. The study saw researchers from AWI’s Paleoclimate Dynamics and Marine Geology departments collaborate with international partners including the Australian Antarctic Science Center and the Wellington Antarctic Research Centre.
“Now PNAS With this study, we demonstrate for the first time how useful and important it is to perform these coupled, relatively high-resolution model simulations for climate in the distant past. Although they are very demanding, they provide new insights into the interactions of ice, atmosphere, land surface, and ocean,” explains Dr. Gerrit Roman, AWI professor of paleoclimate modeler and co-author of the study.
Why the Southern Ocean Current is important to 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 change had a major impact on the Earth’s climate system. According to AWI geoscientist Dr. Johann Klages, “This understanding is crucial because the formation of the ACC strongly drove carbon uptake by the oceans. This reduction in greenhouse gas concentrations in the Earth’s atmosphere therefore led to the cold climate of the so-called Cenozoic Ice Age. This ice age continues to this day in permanently ice-covered polar ice sheets, with alternating warm and cold periods. Therefore, this new knowledge will help us more reliably interpret recent changes in Antarctica’s ocean circulation. ”
These discoveries provide a clearer picture of how ocean currents, atmospheric conditions, and moving continents have worked together to reshape Earth’s climate, providing valuable context for understanding future changes.
