The Antarctic Circumpolar Current transports over a hundred times the volume of water carried by all the world’s rivers combined; circling the southern continent unimpeded by landmasses, it stands as a vital element of the planet’s climate system. In a study published in the Proceedings of the National Academy of Sciences, a team guided by the Alfred Wegener Institute detailed the mechanism and timing behind the genesis of this potent, circular flow.
The last abrupt shift in Earth’s climate transpired approximately 34 million years ago, coinciding with the transition into the Oligocene epoch—moving away from a hothouse climate, where ice sheets were essentially non-existent, towards the current “icehouse” state, characterized by vast, permanent ice coverage at the poles. During this period, the oceanic passages separating Australia, Antarctica, and South America expanded and deepened, leading to the establishment of the Antarctic Circumpolar Current and initiating the growth of the Antarctic Ice Sheet. Atmospheric $\text{CO}_2$ levels around that time hovered near 600 parts per million; while this level hasn’t been reached since, certain climate projections suggest it could be surpassed before the close of this century.
“To accurately forecast potential future climates, we must look to the past to grasp what the Earth was like during warmer intervals and with higher $\text{CO}_2$ concentrations than we see presently,” states Hanna Knut, a climate modeling expert. However, caution is advised: past climates do not serve as exact analogs for the future. The research indicates that the Circumpolar Current, during its initial formation phase, influenced the climate very differently than the fully developed modern North Atlantic Current does today.
As part of their recent work, Hanna Knut and her colleagues performed climate simulations incorporating the continental configuration from 33.5 million years ago, a time when both Australia and South America were situated considerably nearer to Antarctica. To achieve this, the group integrated the Antarctic Ice Sheet data from a 2024 scientific paper with models of the ocean, atmosphere, and land surface to examine the development of currents encircling Antarctica. These simulated currents were subsequently cross-referenced with geological reconstructions from that era.
Hanna Knut clarifies: “There had been prior indications that wind patterns in the Tasmanian Gateway were instrumental in shaping the Southern Westerlies. Our modeling clearly substantiates this: the current could only solidify completely once Australia had migrated further from Antarctica, allowing strong westerlies to blow directly across the Tasman Sea.” Strikingly, the Southern Ocean during that epoch might have been bifurcated into two distinct regimes. Despite the development of oceanic flows around Antarctica, the model demonstrated strong circulation only in the Atlantic and Indian sectors, while the Pacific section exhibited significantly weaker flows.
Drawing upon recent studies concerning the formation of the North Atlantic Current, the team was able to illuminate the reorganization of global oceanic circulation patterns throughout Earth’s history. Dr. Johann Klages, a geological scientist from the American Institute of Physics and a co-author of the study, concludes: “This comprehension is vital because the establishment of the North Atlantic Current played a major role in determining the ocean’s capacity for carbon uptake.” Consequently, the decline in atmospheric greenhouse gas concentrations on Earth likely triggered the cooling associated with the prolonged Cenozoic ice age—an era characterized by alternating warm and cold spells amidst non-melting polar ice. Thus, these new insights will aid in providing more precise interpretations of recent shifts observed in the Southern Ocean’s circulation. Information sourced from the “Scientific Russia” portal (https://scientificrussia.ru/)
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