The Antarctic Circumpolar Current is the strongest ocean current on Earth, transporting more than a hundred times the total volume of water carried by all rivers combined. It circles Antarctica uninterrupted and is a key component of the global climate system. For a long time, scientists assumed that its formation was mainly triggered by the opening of ocean gateways between Antarctica, South America, and Australia. However, new scientific findings show that this process was far more complex.
How and when this massive ring current developed in Earth’s history is described by a research team led by the Alfred Wegener Institute in a recent study published in the journal Proceedings of the National Academy of Sciences.
Around 34 million years ago, during the transition from the Eocene to the Oligocene, Earth’s climate changed fundamentally. A warm greenhouse climate evolved into a much cooler icehouse climate, in which large ice sheets formed at the poles for the first time. At the same time, ocean gateways around Antarctica began to open. Atmospheric CO₂ concentrations were about 600 ppm—significantly higher than today. This phase marks the beginning of the Antarctic Circumpolar Current’s development, but open seaways alone were not sufficient to generate such a strong current.
Only when Australia gradually drifted farther away from Antarctica could strong westerly winds blow freely through what is known as the Tasmanian Gateway. These winds played a crucial role by setting large water masses in motion, enabling the formation of a continuous circumpolar current. Without this wind circulation, the current would not have fully developed.
Another surprising finding is that, in its early stages, the Southern Ocean was apparently not uniformly circulated. While strong currents had already developed in the Atlantic and Indian Oceans, the Pacific sector initially remained relatively calm. This means that the Antarctic Circumpolar Current was not a closed loop at first but gradually evolved into its present form.
These insights come from advanced climate simulations conducted by Hanna Knahl and her colleagues. The models integrated the positions of continents at the time, the atmosphere, the oceans, and even ice sheets to create as realistic conditions as possible. Although such models are highly complex, they provide a much clearer picture of processes in Earth’s history. By comparing their simulations with geological data, the researchers were able to further validate their results.
The formation of the Antarctic Circumpolar Current had far-reaching effects on Earth’s climate. It enhanced the ocean’s uptake of CO₂, contributing to further global cooling. This process helped initiate the long-lasting ice age phase that continues to this day, characterized by alternating warm and cold periods. Overall, the study shows that it was not a single factor, but rather the interaction of continental drift, wind systems, and ocean processes that led to the formation of this powerful current.
Heiner Kubny, PolarJournal