Scientists have challenged the long-held belief that the Antarctic Circumpolar Current (ACC) formed simply because ocean gateways opened between continents. New research published in the Proceedings of the National Academy of Sciences indicates that the world’s strongest ocean current required a specific combination of tectonic movement and intense wind patterns to ignite.
Researchers at the Alfred Wegener Institute (AWI) used advanced climate models to track the development of the ACC roughly 34 million years ago. This period marked the transition from a warm, greenhouse climate to the icehouse conditions that created Antarctica's massive ice sheets.
The mechanics of a climate shift
For years, geologists believed that as Australia and South America drifted away from Antarctica, the resulting gaps allowed the current to flow freely. The new study confirms that while these gateways were necessary, they were not sufficient on their own to create the current.
"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," said Hanna Knahl, a climate modeller at AWI and lead author of the study.
Before this alignment, the Southern Ocean functioned differently. The simulations show that while strong currents existed in the Atlantic and Indian sectors, the Pacific remained relatively calm. It was not until the continents reached a specific configuration that the current became a continuous, global loop.
This shift in ocean circulation helped pull carbon dioxide out of the atmosphere, contributing to a global cooling event. During this era, atmospheric CO2 levels stood at approximately 600 ppm—a concentration scientists warn could be reached again by the end of this century.
Knahl emphasized that while the past cannot be used to perfectly predict the future, the study provides vital context for understanding how the Earth system behaves under high-CO2 conditions. The research team combined Antarctic ice sheet models with ocean and atmospheric data to create a high-resolution reconstruction of the deep past.
By comparing these simulations with geological evidence, the researchers identified how the interactions between ice, wind, and land surfaces shaped the modern climate. This coupled-modeling approach allowed the team to capture complex feedback loops that previous, simpler studies missed.
This study represents a significant step in understanding how the Earth's most powerful current transitioned into its current state. The findings illustrate that the ACC did not just appear, but switched on at a critical point in history to fundamentally alter the planet's temperature.
