Antarctic Circumpolar Current Formed Far More Complexly Than Thought, Reshaping 34-Million-Year Climate Story

A new study from the Alfred Wegener Institute has overturned a long-standing explanation for how the world's most powerful ocean current formed — and in doing so, reveals that the forces governing Earth's transition from a warm greenhouse climate to the ice-covered world we inhabit today were far more intricate than scientists had believed.

The Antarctic Circumpolar Current, which encircles Antarctica at latitudes below 40 degrees South, moves more water than all the world's rivers combined. Oceanographers have long attributed its formation primarily to the opening of ocean gateways around Antarctica — the Drake Passage between South America and Antarctica, and the Tasman Gateway between Australia and Antarctica — as tectonic plates slowly drifted apart approximately 34 million years ago. The standard narrative held that widening these passages simply allowed ocean water to flow freely around the continent, setting the current in motion.

But the new research, published in the Proceedings of the National Academy of Sciences and led by Hanna Knahl of the Alfred Wegener Institute, shows that gateway opening alone was insufficient. Using coupled climate-ice sheet simulations based on Earth's geography approximately 33.5 million years ago, the team found that powerful westerly winds had to align with the open gateways before the current could fully develop and produce its climate-cooling effects. "Only when Australia had moved further away from Antarctica and the strong westerly winds blew directly through the Tasman Gateway could the current fully develop," Knahl explained.

The simulations reveal an even more surprising finding: during the current's early formation, the Southern Ocean may not have been a continuous ring of strong flow. Instead, strong currents appear to have dominated the Atlantic and Indian sectors of the Southern Ocean while the Pacific sector remained comparatively calm — a zonally asymmetric structure that persisted for millions of years before the modern pattern emerged. This challenges textbook depictions of the Antarctic Circumpolar Current as having switched on uniformly around the continent at a single point in time.

The climatic consequences of this reappraisal are significant. As the current strengthened, it drove deep ocean water to the surface through a process called upwelling, absorbing atmospheric carbon dioxide and helping pull temperatures down from the greenhouse levels of the Eocene epoch. The research team's ice sheet model shows that this CO2 drawdown was a major trigger for the expansion of the Antarctic Ice Sheet — the event that locked in today's polar climate and set off cascading changes in ocean circulation, weather patterns, and sea levels worldwide. Understanding the timing and mechanism of that transition more precisely helps climate scientists build more accurate models of how today's ice sheet might respond to warming.

The findings carry a cautionary note for the present. The same ocean-atmosphere dynamics that cooled the planet 34 million years ago are sensitive to wind patterns and temperature gradients — both of which are changing as human-caused global warming intensifies. Some recent observational studies have detected signs that the Antarctic Circumpolar Current is shifting southward and changing in strength, raising concerns about the stability of the circulation system that continues to influence global climate today.