The Antarctic Circumpolar Current (ACC), a major conduit connecting ocean basins, is slowing due to melting ice sheets, potentially compounding climate change-related concerns about sea level rise, ocean warming, and marine ecosystem instability.
University of Melbourne and Norway Research Centre (NORCE) researchers discovered the trend, which they estimate could lead to a 20 percent slowing of the ACC if carbon emissions remain high through 20250. Ocean salinity, circulation patterns, and other oceanic properties are all subject to change with the incoming deluge of fresh water into the Southern Ocean as the Antarctic ice sheet loses amass at an accelerating pace.
Investigating the Current
Fluid mechanist Associate Professor Bishakhdatta Gayen and climate scientist Dr. Taimoor Sohail of the University of Melbourne joined NORCE oceanographer Dr. Andreas Klocker for the project. The international team analyzed advanced simulations of ocean and sea ice currents, incorporating heat transport and other factors, to identify how changes to temperature, saltiness, and wind conditions would affect the ocean.
University of New South Wales researchers originally ran the simulations on Australia’s fastest supercomputer, GADI, using the ACCESS-OM2-01 model developed by Australian researchers across various institutions. Crucially, the model resolved the small-scale processes at play in the ACC, which have long been a mystery to scientists.
“The ocean is extremely complex and finely balanced. If this current ‘engine’ breaks down, there could be severe consequences, including more climate variability, with greater extremes in certain regions, and accelerated global warming due to a reduction in the ocean’s capacity to act as a carbon sink,” Gayen said.
The Antarctic Circumpolar Current
While the Gulf Stream gets a lot of attention for its effect on weather in the eastern US and western Europe, the ACC is four times stronger. Together, these and other currents make up the “ocean conveyor belt,” the combination of currents that push water through the Atlantic, Pacific, and Indian Oceans, linking the globe. Heat, carbon dioxide, chemicals, and biology exchange between the world’s oceans through this network.
One of the ACC’s most important functions is maintaining marine ecosystems by acting as a natural barrier to invasive species. The current carries along rafts of southern bull kelp and prevents marine creatures like shrimp or mollusks from reaching Antarctica.
As the ACC slows, such species could begin to drift into Antarctica, upending its ecosystem by changing the available food supply to indigenous species like Antarctic penguins.
Antarctic ice and the Ocean’s Future
The projections found ocean water’s journey from the surface to the deep potentially slowing in the future. Ice melt has met such a level that, even if emissions lower, a similar slow-down may be inevitable if current melting projections hold. While the Paris Agreement sought to limit global warming to 1.5°C above preindustrial levels, many experts agree that level has already been reached, with increasing heat impacting Antarctic melting. Previously, ACC studies identified the current accelerating due to increased temperature disparities in different ocean latitudes due to climate change, which the new results dispute.
“The melting ice sheets dump vast quantities of fresh water into the salty ocean. This sudden change in ocean ‘salinity’ has a series of consequences – including the weakening of the sinking of surface ocean water to the deep (called the Antarctic Bottom Water), and, based on this study, a weakening of the strong ocean jet that surrounds Antarctica,” Gayen said.
“Ocean models have historically been unable to adequately resolve the small-scale processes that control current strength,” Gayen concluded. “This model resolves such processes, and shows a mechanism through which the ACC is projected to actually slow down in the future. However, further observational and modeling studies of this poorly-observed region are necessary to definitively discern the current’s response to climate change.”
The paper “Decline of Antarctic Circumpolar Current Due to Polar Ocean Freshening” appeared on March 3, 2025 in Environmental Research Letters.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.