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The Antarctic Circumpolar Current is the world's strongest ocean current and plays a disproportionate role in the climate system due to its role as a conduit for major ocean basins. This vast current system is linked to the ocean's vertical overturning circulation, and is thus pivotal to the uptake of heat and CO 2 in the ocean. The strength of the Antarctic Circumpolar Current has varied substantially across warm and cold climates in Earth's past, but the exact dynamical drivers of this change remain elusive. This is in part because ocean models were not able to adequately resolve the eddies and dense shelf water formation that control current strength. Here, we use a global ocean model which resolves such processes to diagnose the impact of future thermal, haline and wind conditions on the strength of the Antarctic Circumpolar Current. By 2050, our model suggests the strength of the Antarctic Circumpolar Current will decline by ∼ 20% in an extreme scenario. This decline is further supported by simple scaling theory, and is driven by ice shelf melting around Antarctica, which weakens the zonal density stratification historically supported by surface temperature gradients. Such a strong decline in transport would have critical implications for the entire global ocean circulation, and hence the Earth's climate system. Southern Ocean | Antarctic Circumpolar Current | Ocean Freshening | Antarctic Bottom Water