A study published in Nature reveals that the Antarctic Circumpolar Current (ACC), the world's largest ocean current, has shown no linear long-term trend in strength over the past 5.3 million years, despite global cooling and increasing ice volume. Instead, the ACC strength reversed on a million-year timescale, with increased strength during Pliocene cooling and subsequent decrease during Early Pleistocene cooling. This shift coincided with a reconfiguration of the Southern Ocean, altering the ACC's sensitivity to atmospheric and oceanic forcings. ACC strength changes are closely linked to 400,000-year eccentricity cycles, likely due to modulation of precessional changes in the South Pacific jet stream linked to tropical Pacific temperature variability. A persistent link between weaker ACC flow, equatorward-shifted opal deposition, and reduced atmospheric CO₂ during glacial periods emerged during the Mid-Pleistocene Transition (MPT). The strongest ACC flow occurred during warmer-than-present intervals of the Plio-Pleistocene, suggesting potential increases in ACC flow with future climate warming. The ACC plays a critical role in the Southern Ocean carbon cycle and atmospheric CO₂ changes. Its strength and position are controlled by wind stress, interaction with deep-ocean bathymetry, and buoyancy forcing. The Southern Westerly Winds drive northward transport of surface water, producing downwelling to the north and upwelling to the south. Buoyancy forcing is controlled by heat and freshwater inputs, affecting the ACC's density structure and strength. During the past decades, warming around Antarctica has been delayed compared with global atmospheric warming, yet the subantarctic ACC has sped up in response to greenhouse-gas forcing, contributing to heat buildup in the subtropics. Sediment records from the Southern Ocean and Drake Passage show a common pattern of reduced ACC flow during glacials, including millennial-scale variations in phase with Antarctic paleotemperature records. These observations highlight potential regional and meridional heterogeneity in ACC flow over Pleistocene glacial–interglacial cycles. The study used sediment records from the pelagic central South Pacific to reconstruct ACC strength over the past 5.3 million years. The results show that ACC strength varied between 50% and 180% of the Holocene mean, with no linear trend. The ACC strength was strongest during warmer-than-present intervals of the Plio-Pleistocene, suggesting that future climate warming may lead to increased ACC flow. The study also found that ACC strength changes are closely linked to orbital forcing, with 400,000-year cycles playing a significant role. The ACC's strength and position are influenced by factors such as the Southern Westerly Winds, oceanic fronts, and the Southern Ocean's biological pump. The study highlights the importance of the ACC in the Southern Ocean carbon cycle and its response to climate changes. The findings have implications for understanding the future of theA study published in Nature reveals that the Antarctic Circumpolar Current (ACC), the world's largest ocean current, has shown no linear long-term trend in strength over the past 5.3 million years, despite global cooling and increasing ice volume. Instead, the ACC strength reversed on a million-year timescale, with increased strength during Pliocene cooling and subsequent decrease during Early Pleistocene cooling. This shift coincided with a reconfiguration of the Southern Ocean, altering the ACC's sensitivity to atmospheric and oceanic forcings. ACC strength changes are closely linked to 400,000-year eccentricity cycles, likely due to modulation of precessional changes in the South Pacific jet stream linked to tropical Pacific temperature variability. A persistent link between weaker ACC flow, equatorward-shifted opal deposition, and reduced atmospheric CO₂ during glacial periods emerged during the Mid-Pleistocene Transition (MPT). The strongest ACC flow occurred during warmer-than-present intervals of the Plio-Pleistocene, suggesting potential increases in ACC flow with future climate warming. The ACC plays a critical role in the Southern Ocean carbon cycle and atmospheric CO₂ changes. Its strength and position are controlled by wind stress, interaction with deep-ocean bathymetry, and buoyancy forcing. The Southern Westerly Winds drive northward transport of surface water, producing downwelling to the north and upwelling to the south. Buoyancy forcing is controlled by heat and freshwater inputs, affecting the ACC's density structure and strength. During the past decades, warming around Antarctica has been delayed compared with global atmospheric warming, yet the subantarctic ACC has sped up in response to greenhouse-gas forcing, contributing to heat buildup in the subtropics. Sediment records from the Southern Ocean and Drake Passage show a common pattern of reduced ACC flow during glacials, including millennial-scale variations in phase with Antarctic paleotemperature records. These observations highlight potential regional and meridional heterogeneity in ACC flow over Pleistocene glacial–interglacial cycles. The study used sediment records from the pelagic central South Pacific to reconstruct ACC strength over the past 5.3 million years. The results show that ACC strength varied between 50% and 180% of the Holocene mean, with no linear trend. The ACC strength was strongest during warmer-than-present intervals of the Plio-Pleistocene, suggesting that future climate warming may lead to increased ACC flow. The study also found that ACC strength changes are closely linked to orbital forcing, with 400,000-year cycles playing a significant role. The ACC's strength and position are influenced by factors such as the Southern Westerly Winds, oceanic fronts, and the Southern Ocean's biological pump. The study highlights the importance of the ACC in the Southern Ocean carbon cycle and its response to climate changes. The findings have implications for understanding the future of the