More than four times stronger than the Gulf Stream, the Antarctic Circumpolar Current (ACC) 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. Scientists from the University of Melbourne and NORCE Norway Research Centre have now shown the ACC slowing by around 20% by 2050 in a high carbon emissions scenario. This influx of fresh water into the Southern Ocean is expected to change the properties, such as density (salinity), of the ocean and its circulation patterns.

Sohail et al. analyzed a high-resolution ocean and sea ice simulation of ocean currents, heat transport and other factors to diagnose the impact of changing temperature, saltiness and wind conditions. Image credit: Sohail et al., doi: 10.1088/1748-9326/adb31c.

“The ocean is extremely complex and finely balanced,” said study co-author Dr. Bishakhdatta Gayen, a fluid mechanist at the University of Melbourne.

“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.”

The ACC works as a barrier to invasive species, like rafts of southern bull kelp that ride the currents, or marine-borne animals like shrimp or mollusks, from other continents reaching Antarctica.

As this current slows and weakens, there is a higher likelihood such species will make their way onto the fragile Antarctic continent, with a potentially severe impact on the food web, which may, for example, change the available diet of Antarctic penguins.

The ACC is a crucial part of the world’s ocean conveyor belt, which moves water around the globe — linking the Atlantic, Pacific and Indian Oceans — and is the main mechanism for the exchange of heat, carbon dioxide, chemicals and biology across these ocean basins.

In their research, the authors used Australia’s fastest supercomputer and climate simulator, GADI, located at Access National Research Infrastructure.

They found that the transport of ocean water from the surface to the deep may also slow in the future.

“It is predicted that the slow-down will be similar under the lower emissions scenario, provided ice melting accelerates as predicted in other studies,” Dr. Sohail said.

“The 2015 Paris Agreement aimed to limit global warming to 1.5 degrees Celsius above pre-industrial levels.”

“Many scientists agree that we have already reached this 1.5 degree target, and it is likely to get hotter, with flow-on impacts on Antarctic ice melting.”

“Concerted efforts to limit global warming (by reducing carbon emissions) will limit Antarctic ice melting, averting the projected ACC slowdown.”

The research reveals that the impact of ice melting and ocean warming on the ACC is more complex than previously thought.

“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,” Dr. Gayen said.

The study was published in the journal Environmental Research Letters.

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Taimoor Sohail et al. 2025. Decline of Antarctic Circumpolar Current due to polar ocean freshening. Environ. Res. Lett 20, 034046; doi: 10.1088/1748-9326/adb31c