Antarctic Circumpolar Current. Credit: Wikimedia Commons
Earth’s mightiest river has no banks. The Antarctic Circumpolar Current is the world’s largest ocean current, carrying a hundred times more water than all the world’s rivers combined and acting as a planetary refrigerator that locks the southern pole in a perpetual chill.
Scientists always assumed this deep freeze began the second that shifting tectonic plates tore South America and Australia away from Antarctica. Yet, a new virtual reconstruction disagrees.
The simulation reveals that those newly opened oceanic gateways actually sat dormant until Australia drifted far enough north to align with the planet’s howling westerly winds. Only then did those gales finally ignite the current, promoting carbon uptake and plunging our greenhouse world into a permanent ice age.
A World in Transition
Around 34 million years ago, Earth underwent a transition period. The Eocene epoch, a period high in greenhouse gases and with little permanent ice, gave way to the Oligocene.
During this period, the slow drift of plate tectonics widened and deepened passages of water—specifically, the Drake Passage and the Tasman Gateway—between Antarctica, South America, and Australia.
Geologists long suspected that these expanding gaps essentially uncorked the Southern Ocean, allowing water to flow freely around the pole and triggering a deep freeze. Yet, geological evidence shows that even after the seaways parted, the immense circumpolar current remained sluggish and incomplete.
To solve the puzzle, researchers built a digital time machine. They fed a high-resolution climate model with data reflecting the geography of Earth 33.5 million years ago. To complete the picture, they incorporated a detailed model of the early Antarctic ice sheet from a 2024 Science study.
“With this PNAS study, we are showing—for the first time—how helpful and important it is to carry out these coupled and relatively high-resolution model simulations for the climate of the deep past. Even though they are very demanding, they provide novel insights into the interaction of ice, atmosphere, land surface, and ocean,” explains Prof Dr Gerrit Lohmann, an Earth system modeler at the Alfred Wegener Institute and a co-author of the study.
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The Missing Ingredient: Winds
Model simulation of the ACC. Credit: Alfred Wegener Institute/Hanna Knahl, Patrick Scholz
The resulting simulations point to a critical factor that earlier work had already hinted at: the wind.
Early on, the oceanic gaps were simply in the wrong place. The powerful westerly winds, which today whip the Southern Ocean into a frenzy, were blowing too far north to push water through the newly formed Tasman Gateway.
Hanna Knahl, a climate modeler and the study’s lead author, points out the necessity of this atmospheric alignment. “There were already indications that the wind in the Tasman Gateway played an important role in the formation of the ACC. Our simulations can clearly confirm this: 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,” she says.
Before Australia drifted further north, the ocean currents behaved erratically. The researchers found that rather than flowing in a continuous loop, the early current fractured. Strong flows churned through the Atlantic and Indian sectors, but once the water passed the Tasman Gateway, it deflected northward and dissipated. The Pacific sector remained remarkably calm and heavily stratified.
Thermal Insulation
As the continents migrated into positions that aligned the oceanic gateways with the prevailing westerly winds, the Antarctic Circumpolar Current roared to life.
As the current strengthened, it likely increased Antarctica’s thermal isolation. The authors also argue that this circulation change could have boosted ocean carbon uptake, with broader climate consequences.
“This understanding is crucial, as the formation of the ACC has strongly driven carbon uptake by the ocean,” Dr Johann Klages, a geoscientist and study co-author, notes. “This reduction in the concentration of greenhouse gases in Earth’s atmosphere thus had the potential to initiate the cooler climate of the so-called Cenozoic Ice Age, which continues to this day with permanently ice-covered polar ice caps, in which warm and cold periods alternate. This new knowledge will therefore help us to more reliably interpret recent changes in Southern Ocean circulation,” Klages explains.
Echoes of an Ancient Greenhouse
By the Early Oligocene Glacial Maximum, atmospheric CO2 was around 600 parts per million (ppm), after falling from roughly 1,000 ppm in the late Eocene, according to the paper’s summary of earlier reconstructions.
Humanity has not experienced air that carbon-rich, though current emissions trajectories suggest we could reach similar concentrations by the end of the century. Understanding the mechanics of a high-CO2 world helps scientists refine the models they use to forecast our own future.
“In order to predict the possible future climate, it is necessary to look into the past with simulations and data to understand our Earth in warmer and more CO2-rich climate states than today,” Knahl says. “But careful, the climate of the past can of course not be projected 1:1 onto the future. Our study shows that the circumpolar current in its ‘infancy’ influenced the climate very differently than today’s fully developed ACC does.”
Earth’s climate is driven by highly sensitive variables that are currently shifting at record speeds. To accurately forecast future climate scenarios, scientists must first resolve the precise historical conditions that formed the world we inhabit today.
The study was published in the journal Proceedings of the National Academy of Sciences (PNAS).