About 34 million years ago, Antarctica began to freeze. For decades, scientists believed a single geological event set that transformation in motion: tectonic plates pulling apart to open ocean gateways around the continent, unleashing a massive current that sealed Antarctica in cold. A study published in April 2026 in the Proceedings of the National Academy of Sciences tells a different story. The Antarctic Circumpolar Current, or ACC, did not switch on like a faucet when seaways opened. It built up gradually, driven by falling carbon dioxide levels over millions of years.
The ACC is the mightiest current in the modern ocean. It loops endlessly around Antarctica, connecting the Atlantic, Pacific, and Indian ocean basins. According to the PNAS study led by Hanna Knahl of the Alfred Wegener Institute, the current carries roughly 100 times the combined flow of every river on Earth. That volume makes it a central engine in how the planet distributes heat and cycles carbon through the deep ocean.
A tectonic story, rewritten
The old explanation was elegant and intuitive. When the Drake Passage between South America and Antarctica widened, and the Tasman Gateway between Australia and Antarctica deepened, water could finally flow unimpeded around the continent. That ring of current thermally isolated Antarctica, triggering rapid ice sheet growth. The idea, rooted in plate tectonics, dominated textbooks for a generation.
Knahl’s team tested it with coupled high-resolution ocean and climate model simulations, constrained against geological reconstructions of what the continents and seafloor looked like roughly 33.5 million years ago. The results, detailed in the PNAS paper, showed that early ACC circulation took shape gradually during the Eocene-Oligocene transition, the period when Earth shifted from a warm greenhouse climate into a cooler icehouse state. The current strengthened as atmospheric CO2 declined and the Southern Ocean cooled, not as an immediate mechanical response to continental drift.
That distinction reorders the cause-and-effect chain. Atmospheric chemistry, not just geography, appears to have governed when and how powerfully the ACC developed.
The simulations used a CO2 concentration of about 600 parts per million, a level that falls within the range projected for the late 21st century under high-emission pathways such as SSP5-8.5, which in some estimates exceeds 1,000 ppm by 2100. The team built on a 2024 study in Science that combined sediment drill cores with climate and ice-sheet modeling to estimate the extent and timing of early Antarctic ice growth. Those estimates served as direct inputs for the new ocean simulations, allowing the models to treat ice sheets and ocean currents as interacting parts of the same system rather than separate problems.
Communications from the Alfred Wegener Institute, summarized in a public briefing, emphasize that the ACC’s emergence was tightly linked to long-term CO2 declines. The current, once established, helped lock Antarctica into a deep freeze by blocking warmer waters from reaching the continent’s shores.
What remains uncertain
The study shifts the scientific narrative, but open questions remain. The exact timing of when the ACC reached something resembling its modern strength is still debated. A separate peer-reviewed study published in Nature Geoscience has argued that full ACC development came even later than the Eocene-Oligocene boundary, raising the possibility that the current took millions of years longer to mature than any single model currently captures. (The original article does not name the authors or publication year of that study, and it has not been independently verified for this report.)
Separately, research published in Nature using drilled sediment cores spanning the Pliocene to the present has reconstructed ACC strength over the past 5.3 million years, showing the current has varied considerably even in geologically recent times. Those fluctuations suggest the ACC is sensitive to shifting climate conditions, though the mechanisms behind shorter-term variability are not yet well understood. (As with the Nature Geoscience work, the specific authors and publication year of this Nature paper have not been confirmed independently.)
The model simulations themselves carry caveats. While described as high-resolution and constrained by geological data, the full details of grid resolution and paleogeographic boundary conditions have not been independently assessed outside the peer-review process. That means some aspects of the simulated circulation, such as the exact path of individual current jets, remain more tentative than the broader conclusion about gradual onset.
There are also uncertainties baked into the proxy records. The sediment cores used in the 2024 Science study provide a powerful but indirect window into past ice volume and ocean conditions. Translating layers of mud and microfossils into precise estimates of ice thickness requires assumptions about how chemical and physical signals in the sediment relate to climate. Small shifts in those assumptions can alter the inferred timing of ice-sheet expansion, and therefore the boundary conditions fed into the ocean models.
Finally, feedbacks between the ACC and the broader climate system are not fully resolved. As the current strengthened, it likely changed how heat and carbon moved between the deep ocean and the atmosphere, potentially reinforcing Antarctic cooling. But how much of that cooling came from the current itself versus the direct effect of falling CO2 is an open question. Different models handle processes like vertical mixing and eddy transport in distinct ways, which can shift how strongly the simulated ACC responds to the same forcing.
Why it matters now
For researchers tracking how ocean circulation might respond to rising greenhouse gases, the practical implication is pointed. If the ACC’s strength depends heavily on atmospheric CO2 and ocean temperatures rather than on the fixed positions of continents, then climbing emissions could weaken or redirect the current in ways that ripple through marine ecosystems, weather patterns, and heat transport across the Southern Hemisphere and beyond.
The 600 ppm CO2 level used in the study sits below the upper range of concentrations projected under the highest-emission shared socioeconomic pathway, SSP5-8.5. That makes the deep past a surprisingly relevant testing ground for understanding the near future.
No single study can deliver a forecast for what the ACC will do as the planet warms. But by demonstrating that the current emerged gradually in tandem with falling CO2, rather than snapping into existence when seaways opened, Knahl’s team has sharpened a key point: the ocean’s great circulation systems are more responsive to atmospheric change than the old tectonic narrative suggested. In a world now pushing CO2 in the opposite direction, that responsiveness is less an academic detail than a reason to pay closer attention to what is happening in the waters around Antarctica.
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*This article was researched with the help of AI, with human editors creating the final content.
