A groundbreaking study published in the Proceedings of the National Academy of Sciences offers an unprecedented look into the early formation of the Antarctic Circumpolar Current (ACC), a colossal oceanic flow that has shaped Earth’s climate for millions of years. This current, which environmentally sweeps around Antarctica unbounded by continents, circulates more water than all the world’s rivers combined, making it a fundamental driver of global ocean and climate systems. The research, led by climate modeller Hanna Knahl from the Alfred Wegener Institute, employs innovative coupled simulations of ocean, atmosphere, land, and cryosphere interactions from approximately 33.5 million years ago. Their findings challenge prior assumptions that the ACC’s birth was solely dependent on the opening of Southern Ocean gateways, revealing instead the critical role of atmospheric dynamics alongside continental shifts.
During the transition from the late Eocene greenhouse climate to the cooler Oligocene icehouse climate, Earth underwent a radical climate shift roughly 34 million years ago. Before this period, polar ice sheets were virtually absent, but as this transition unfolded, Antarctica began to accumulate vast, permanent ice sheets. This change coincided with the widening and deepening of ocean passages—the Drake Passage between South America and Antarctica, and the Tasman Gateway between Australia and Antarctica. These tectonic movements created the potential for an uninterrupted circumpolar flow, but models demonstrate that this geological restructuring alone did not immediately lead to the ACC’s full development. Instead, the simulations highlight the synergistic influence of ocean gateway geometry and intensifying westerly winds that forced the current to gain strength and continuity.
The atmospheric circulation, particularly the western winds channeled through the Tasman Gateway, was a major catalyst in enabling the current’s vigorous onset. When Australia was still relatively close to Antarctica, the Tasman Gateway was too narrow for the winds to funnel effectively, restricting the ACC’s intensity. Only as Australia migrated further eastward, expanding this passage, could the powerful westerlies initiate and sustain the circumpolar flow in full force. This interaction between shifting continental positions and atmospheric patterns marks a critical nuance, showing that the ACC’s infancy was a complex process regulated by both tectonic evolution and evolving wind systems, rather than a simple “gateway opening” scenario.
Interestingly, the simulations reveal a striking spatial heterogeneity in the Southern Ocean’s character during the ACC’s formation. Despite open passages around Antarctica, the model indicates that the current was robust in the Atlantic and Indian Ocean sectors but remained far weaker in the Pacific sector. This dichotomy suggests an early-stage Southern Ocean divided by oceanographic and atmospheric forces, where circumpolar flow was only partially established. Such findings hint at a dynamic and transitional ocean environment that gradually matured into the powerful, unbroken circulation system recognized today.
This investigation is notable for its methodological novelty. By coupling an Antarctic Ice Sheet model—updated based on 2024 data—with advanced, high-resolution ocean-atmosphere simulations, the team leveraged one of the most comprehensive palaeoclimate modeling frameworks to date. This holistic approach allowed for an integrated perspective on how ice sheet expansion, ocean circulation, atmospheric dynamics, and changing land configurations influenced one another during the critical green- to icehouse climate transition. These advances mark a significant step forward in our ability to reconstruct ancient climates with rigorous fidelity.
Beyond adding depth to our understanding of Earth’s geological past, this research provides essential insights relevant to today’s and future climates. Atmospheric CO₂ concentrations during the studied period hovered around 600 ppm—levels not seen since but projected to be exceeded in some future emission scenarios. “Predicting how current and future climate states could evolve requires learning from these ancient climates with substantially elevated greenhouse gas concentrations,” Knahl emphasizes. Yet, the study cautions that paleoclimate conditions must not be straightforwardly projected onto the present, given the stark differences revealed between the ACC’s initial and fully matured states.
The team’s collaborative efforts united expertise across multiple institutions. Apart from AWI’s Palaeoclimate Dynamics and Marine Geology divisions, the Australian Centre of Excellence in Antarctic Science and Wellington’s Antarctic Research Centre joined forces to produce these high-complexity coupled simulations. This cross-disciplinary synergy was crucial for tackling the immense computational demands and interpreting the nuanced results the simulations yielded. The study exemplifies how merging diverse scientific perspectives can unlock intricate climate processes frozen deep in Earth’s history.
The broader significance of the ACC formation extends to global carbon cycles. The current plays a pivotal role in regulating carbon uptake by the Southern Ocean. As it intensified, it likely contributed to a substantial drawdown of atmospheric greenhouse gases, ushering in a stabilizing feedback that helped cement the onset of the Cenozoic Ice Age. This epoch is characterized by persistent polar ice caps and cyclical warm and cold phases, deeply influencing Earth’s climatic and ecological trajectories.
As highlighted by palaeoclimate modeller Prof Dr Gerrit Lohmann, conducting such advanced coupled simulations—accounting simultaneously for ocean, cryosphere, atmosphere, and land surface dynamics—not only deepens scientific comprehension of past climate states but also enhances the predictive frameworks used to anticipate changes in modern ocean circulation. Understanding how the ACC evolved provides an empirical foundation for interpreting shifts observed in Southern Ocean currents today amid ongoing climate warming.
Lead geoscientist Dr Johann Klages reflects on the implications: “Unlocking the ACC’s developmental history improves our grasp of the Southern Ocean’s climate influence and ocean-atmosphere interactions across geological timescales.” Such knowledge is indispensable for forming robust climate models that better capture feedback mechanisms pivotal to Earth’s climate resilience and sensitivity.
The visualization of the early ACC, modeled around 34 million years ago, illustrates a unique moment in Earth’s climatic evolution—a time when a nascent circumpolar current began shaping global ocean circulation patterns in ways that would ultimately influence atmospheric composition and the establishment of large polar ice sheets. This study unravels the complexity hidden beneath what was once considered a straightforward tectonic trigger for oceanic change.
In summary, the Alfred Wegener Institute-led research elucidates the intricate interplay of Earth’s geological and atmospheric systems during one of the most critical climate transitions in our planet’s history. The findings provide a vital reference point for understanding how ocean currents respond to tectonic and climatic drivers, with important ramifications for predicting future climate dynamics in an era of accelerating greenhouse gas emissions and polar ice mass changes.
Subject of Research: Not applicable
Article Title: Configuration of circum-Antarctic circulation at the last green- to icehouse climate transition
News Publication Date: 6-Apr-2026
Web References: http://dx.doi.org/10.1073/pnas.2520064123
References: Proceedings of the National Academy of Sciences
Image Credits: Alfred Wegener Institute / Hanna Knahl, Patrick Scholz
Keywords: Antarctic Circumpolar Current, paleoclimate modeling, Oligocene transition, Southern Ocean, climate simulation, greenhouse to icehouse climate, ocean-atmosphere interaction, Antarctic ice sheet, Tasman Gateway, westerly winds
