Earth's strongest ocean current is slowing down due to climate change - and the consequences could be disastrous

Scientists have warned that Earth's strongest ocean current is slowing down due to climate change, with potentially disastrous consequences.

The Antarctic Circumpolar Current (ACC) transports about 173 million cubic metres of water every second in an unbroken ring around the entire Antarctic continent.

However, new research shows that the ACC could become 20 per cent slower by 2050 thanks to human-caused climate change.

If that were to happen, it could lead to a 'vicious cycle' of rapidly melting ice, rising sea levels, and spiralling global temperatures.

More than four times stronger than the Gulf Stream, the ACC is a key part of the 'ocean conveyor belt' that pushes water, heat, and nutrients around the planet.

But as fresh water from melting sea ice is dumped into some areas of the Antarctic, the processes driving that conveyor belt will start to weaken.

Lead researcher Dr Bishakhdatta Gayen, associate professor at the University of Melbourne, says: '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.'

Much like its more famous northern cousin, the Atlantic Meridional Overturning Circulation (AMOC), the ACC is driven by cold, salty water.

As ice forms in the ocean, the water which is left behind becomes extremely salty and cold, and so becomes exceptionally dense.

That cold, dense water sinks quickly into the deepest 'abyssal' zones of the ocean and sweeps northwards, carrying oxygen and CO2 and churning up the nutrient-rich sediments at the ocean floor.

The water is then pulled to the surface through a process called upwelling in areas such as the Southern Ocean south of Australia, bringing nutrients to the surface and driving the cycle of the ocean current.

However, as the climate warms, the ice surrounding Antarctica is melting at an ever-faster rate.

Recent research has shown that sea ice around the southern continent plunged to a record low last year.

According to data gathered by the US National Snow and Ice Data Center (NSIDC), the Antarctic sea ice extent was 801,548 square miles (2.07 million square km) as of February 15, 2025.

This is more than 200,000 square miles below the February 15 average for the historical reference period between 1981 and 2010 - 1.13 million square miles (2.932 million square km).

Dr Gayen says: '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.'

Using Australia's most powerful supercomputer, Dr Gayen and his colleagues modelled how this process would affect the ACC in a 'high-emissions' scenario - one in which greenhouse gas emissions continue to rise.

The simulations showed that ice melting alone would cause a 20 per cent slowdown in the ACC by 2050.

Although they do not model the climate impacts of this change in their paper, current research suggests that such a slowdown would have a widespread impact on the climate.

This is because the ACC acts as a physical and oceanographic barrier, protecting the Southern Ocean from the rest of the world.

Co-author Dr Taimoor Sohail, a postdoctoral researcher at the University of Melbourne, told MailOnline: 'One way that the ACC is important in regulating our climate is that it controls how much heat gets onto Antarctica and onto the Antarctic Ice Shelf.

'So, a slowdown in the ACC could allow more heat to transfer south from the warmer North, and that would enable ice melting to accelerate.'

Dr Sohail explains that this creates a 'vicious cycle' in which ice melt weakens the ACC which, in turn, allows ice to melt faster and further weaken the protective currents.

The Antarctic Ice Sheet holds around 90 per cent of all the fresh water on Earth and has the potential to cause massive increases in sea levels should it melt.

While this paper does not suggest that the Antarctic Ice Sheet will vanish anytime soon, even modest accelerations in sea level rises could be catastrophic for the 230 million people who live within three feet of the high tide line today.

This vicious cycle would only be worsened by a reduction in the ACC's ability to moderate CO2 and heat levels in the atmosphere.

In some parts of the ocean surface waters which are in contact with the atmosphere warm up and absorb CO2 before being pulled rapidly down into the deep ocean in a 'ventilation process'.

The resulting 'Antarctic bottom water' traps a huge amount of the world's CO2 and heat.

However, research suggests that the processes which pull surface water down to the depths are controlled by the strength of the ACC.

The atmosphere currently absorbs more than 90 per cent of the heat trapped in the climate system but, were the ACC to weaken, that process might falter, leading to faster warming.

A slowing of the ACC would also accelerate changes to the water cycle, leading to more extreme weather all around the globe.

Additionally, the slowing of the ACC would be a disaster for Antarctica's already fragile ecosystem.

The rapidly circulating current acts as a barrier which prevents invasive species from crossing onto the continent, particularly drifting rafts of southern bull kelp which ride on ocean currents.

Dr Sohail says: 'A slowdown in the ACC would allow the creatures that live on those rafts to get onto the continent at greater frequency and in concentrated regions.

'So, it prevents a threat to all the ecosystems and food webs on the Antarctic continent.'

According to their paper, published in the journal Environmental Research Letters, the only way to prevent these changes is to stop the rapid melting of Antarctic sea ice.

Dr Sohail concludes: 'The key point of this study is that there is a clear link between ice melting and a slowdown of the ACC.

'So, if we can try to reduce how much ice melting happens, through climate mitigation, then there's a good chance these projections might not actually be borne out.'