The Antarctic Circumpolar Current is the strongest ocean current in the world and has a significant influence on the global climate. But how did the huge ring current come about? A study now shows that this would require more than opening the sea passages between Antarctica, South America and Australia. Only when strong westerly winds blew through the Tasmanian Seaway did the current get going, removing CO2 from the atmosphere and thereby cooling the climate. The results can help to more accurately predict how the Antarctic Circumpolar Current will respond to current climate change.
Around 34 million years ago, at the transition from the Eocene to the Oligocene, the Earth changed dramatically: tectonic shifts opened up ocean passages between Antarctica, South America and Australia. New mountains emerged, the CO2 content in the atmosphere fell and the poles began to freeze over. At this time, what is now the strongest ocean current in the world, the Antarctic Circumpolar Current (ACC), came into being. It surrounds Antarctica and shields it from warmer ocean currents. It also removes CO2 from the atmosphere by transporting it into the depths of the ocean. Together with other processes, the ACC helped reduce the CO2 content from around 600 ppm to 280 ppm – a value that remained stable for millions of years and has only increased again since industrialization.
Opening of the sea passages
“In order to be able to predict the possible future climate, it is necessary to look into the past using simulations and data and to understand our earth in warmer and more CO2-rich climate conditions than today,” says Hanna Knahl from the Alfred Wegener Institute in Bremerhaven. Together with her team, she modeled how the Antarctic Circumpolar Current emerged around 33.5 million years ago and how the ocean, atmosphere, and land and ice masses interacted with each other.
“Until now, the opening of the ocean passages around Antarctica was usually considered the trigger for the emergence of the Antarctic Circumpolar Current and the driving force behind the climate change at the time,” explain the researchers. “However, our simulations paint a more complex picture: opening the sea passages alone was not enough.” Instead, the Southern Ocean was probably initially divided into two halves, with strong currents already developing in the Atlantic and Indian sectors, while the Pacific part remained calm.
Wind as a trigger
“It was only when Australia moved further away from Antarctica and strong westerly winds blew directly through the Tasman Seaway that the current was able to fully develop there and develop its climate-cooling effect,” explains Knahl. In this way, the Antarctic Circumpolar Current promoted the transition from the greenhouse climate of the time to the icehouse climate in which most of the species living today, including humans, developed over the following millions of years.
“This understanding is crucial because the formation of the circumpolar current strongly drove carbon uptake by the ocean,” says Knahl’s colleague Johann Klages. “This reduction in the greenhouse gas concentration in the Earth’s atmosphere had the potential to usher in 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 greatly help us to classify current changes in the flow dynamics of the Southern Ocean more reliably.”
Source: Hanna Knahl (Alfred Wegener Institute, Bremerhaven) et al., Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2520064123