Thirty-four million years ago, Antarctica was not yet the frozen continent we know. Forests still clung to its margins, and warm ocean water lapped against its coasts. Then something shifted. Tectonic plates pulled apart, seaways widened, and a massive ring of ocean current began to wrap around the continent, cutting it off from the planet’s warmth. A new study published in April 2026 in the Proceedings of the National Academy of Sciences uses high-resolution ocean simulations to reconstruct how that current, the Antarctic Circumpolar Current, first took shape during one of Earth’s most dramatic climate transitions.
The Antarctic Circumpolar Current is the largest ocean current on the planet, moving roughly 130 million cubic meters of water per second through the Southern Ocean. It acts as a thermal wall, blocking warmer waters from reaching Antarctica and helping preserve the ice sheets that hold enough frozen water to raise global sea levels by nearly 60 meters if fully melted. How and when that wall first formed has been debated for decades, and the answer matters now more than ever as rising ocean temperatures raise questions about the current’s long-term stability.
Reconstructing an ancient ocean
The new study, led by a team at the Alfred Wegener Institute, modeled Southern Ocean circulation using a paleogeographic reconstruction dated to approximately 33.5 million years ago, a period straddling the Eocene-Oligocene Transition. At that time, proxy-derived estimates place atmospheric carbon dioxide concentrations near 600 parts per million, roughly 40 percent higher than today’s levels of about 425 ppm, though such reconstructions carry significant uncertainty. Two critical tectonic gateways were in the process of opening: the Drake Passage between South America and Antarctica, and the Tasman Gateway between Australia and Antarctica.
The simulations found that as these gateways widened, ocean circulation around Antarctica reorganized dramatically. Warm subtropical water that had previously reached the continent’s shores was redirected, and a nascent circumpolar flow began to develop. Critically, the researchers concluded that neither tectonic changes nor declining CO2 alone could account for the shift. Both forces worked in tandem, with gateway opening redirecting currents and falling greenhouse gas levels cooling the surface ocean enough to allow ice sheets to nucleate and grow. “Our results show that you need both the tectonic trigger and the CO2 decline acting together to reproduce the circulation changes we see in the geological record,” said lead author Dr. Isabel Sauermilch of the Alfred Wegener Institute, in an institutional summary accompanying the paper.
That finding builds on earlier work that had already pointed toward a proto-current predating full Antarctic glaciation. A study published in Scientific Reports drew on intermediate-depth flow patterns recorded in ocean sediment drift deposits to show that some form of circumpolar water movement was already stirring around Antarctica during the late Eocene, before the continent froze over. Separately, simulations tied to the University of Leicester and the International Ocean Discovery Program, described in an institutional release, showed Antarctic surface waters cooling by up to approximately 5 degrees Celsius as Southern Ocean gateways widened. That figure, drawn from a press release rather than the underlying journal paper, should be treated as approximate, but the magnitude is significant: a regional temperature drop large enough to trigger continental-scale ice growth.
A question of timing
The central dispute in this field is not whether the Antarctic Circumpolar Current exists or matters, but when it matured into the deep, fast-moving flow observed today. The Alfred Wegener Institute simulations place the current’s formation squarely around the 34-million-year mark. But a paper published in January 2024 in Nature Geoscience by Adriane Starr and colleagues challenged that timeline, arguing that a strong, modern-like Antarctic Circumpolar Current did not emerge until the late Miocene, millions of years after glaciation began.
These two positions are not necessarily contradictory. The early proto-current and the full modern current may represent different stages of the same process, separated by millions of years of tectonic and climatic evolution. A shallow, wind-driven flow circling Antarctica at 34 million years ago could have gradually deepened and strengthened as the Drake Passage and Tasman Gateway continued to widen and deepen over geologic time. But the distinction carries real scientific weight: if the current caused Antarctic cooling, it is a primary driver of the planet’s shift into an ice age. If it merely amplified a process already underway from falling CO2, the implications for future climate sensitivity are different.
Resolving the question is complicated by the nature of the evidence. Researchers reconstruct ancient ocean speeds and directions using physical tracers: sedimentary grains sorted by current strength, chemical isotopes in fish debris, and the geometry of sediment drifts on the ocean floor. According to an Imperial College London summary of the Nature Geoscience research, different groups interpret the same sediment records in different ways, and proxy measurements are inherently indirect. The new PNAS simulations gain credibility where their outputs align with these independent geological records, as appears to be the case with the cooling signal. But without access to the full model parameters and datasets, independent researchers must rely on published descriptions to evaluate the results.
Why it matters now
The Antarctic Circumpolar Current’s origin story is not just a question for paleoceanographers. The current today functions as a climate regulator, isolating Antarctica’s ice sheets from the warming ocean to the north. Some climate projections suggest that continued greenhouse gas emissions could weaken or shift the current, potentially allowing warmer water to reach the base of ice shelves in West Antarctica, a region already losing ice at an accelerating rate.
The new simulations suggest that the current originally required both tectonic reorganization and CO2 decline to reach meaningful strength. That dual-driver finding complicates simple predictions about the current’s future. Tectonic geography is not going to change on human timescales, but atmospheric CO2 is rising fast. If the current’s strength depends partly on the temperature gradient between Antarctic and subtropical waters, and that gradient narrows as the planet warms, the thermal barrier protecting Antarctica’s ice could erode in ways that are difficult to reverse.
For now, the research sharpens the scientific picture without fully settling the debate. What it does establish is that the birth of the world’s most powerful ocean current was not a single event but a process, one shaped by the slow grinding of tectonic plates and the chemistry of an atmosphere in transition. Understanding that process is one of the clearest windows science has into how Earth’s climate system responds when its boundary conditions change, a question with obvious relevance as humanity continues to alter those conditions today.
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*This article was researched with the help of AI, with human editors creating the final content.
