Scientists have determined that Earth's most powerful ocean current, the Antarctic Circumpolar Current, formed through shifting continents and strong winds rather than solely from opening ocean gateways. This development around 34 million years ago helped draw down atmospheric carbon dioxide, contributing to a global cooling that led to the Antarctic Ice Sheet. The findings come from climate simulations published in the Proceedings of the National Academy of Sciences.
The Antarctic Circumpolar Current carries more than 100 times the flow of all the world's rivers combined and circles Antarctica unimpeded by land, driving much of the global climate system. Around 34 million years ago, during the shift from a greenhouse to an icehouse world, ocean passages between Antarctica, South America, and Australia widened. However, researchers at the Alfred Wegener Institute found that these changes alone were insufficient to create the current fully, as detailed in their recent Proceedings of the National Academy of Sciences study led by Hanna Knahl. Knahl's team used high-resolution climate simulations based on Earth's geography 33.5 million years ago, when Australia was closer to Antarctica. They integrated these with an ice sheet model and compared results to geological evidence. The simulations showed that strong westerly winds blowing through the newly widened Tasman Gateway between Antarctica and Australia were crucial for the current's development. Initially, the current did not form a continuous loop; strong flows emerged in the Atlantic and Indian sectors, while the Pacific remained calmer. This reorganization enhanced ocean carbon uptake, reducing atmospheric CO2 levels—which were around 600 ppm at the time—and initiating the Cenozoic Ice Age with permanent polar ice caps, according to AWI geoscientist Johann Klages. Knahl noted, '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.' Co-author Gerrit Lohmann emphasized the value of coupled models: 'They provide novel insights into the interaction of ice, atmosphere, land surface, and ocean.' These insights aid in interpreting modern Southern Ocean changes.