Powerful Antarctic Circumpolar Current expected to slow down

Global carbon emissions are slowing the Antarctic Circumpolar Current (ACC), the world’s most powerful ocean current.

The problem echoes that of the Atlantic Meridional Overturning Circulation (AMOC), which has concerned scientists saying that such a slow-down could produce substantial, possibly catastrophic, flow-on effects.

Now for the first time, researchers from University of Melbourne and NORCE Norway Research Centre, say they expect a 20% slowdown in the Antarctic Circumpolar Current (ACC) by 2050 under a high carbon emissions scenario.

“Our study found a robust link between Antarctic ice melting and Antarctic Circumpolar Current slowdown,” says lead author, Dr Taimoor Sohail.

Their paper appears in Environmental Research Letters.

The ACC keeps Antarctica frozen, moving clockwise around the continent. Sohail says the current is “100 times stronger than the Amazon River and 5 times stronger than the Gulf Stream, it’s a really important component of the climate system, linking the Atlantic, Pacific and Indian Oceans.”

As the ‘global conveyor belt’, the point of mixing for the worlds’ three major oceans, the ACC helps to control oceanic heat distribution and carbon absorption on a global scale, says Sohail, adding that as the global ocean absorbs around 90% of the heat building up in the atmosphere and much of the carbon dioxide, it’s vitally important in mitigating global warming.

The ACC is also a strong physical and oceanographic barrier that keeps Antarctica isolated, he says, sustaining Antarctic biodiversity by excluding invasive species.

To its north, the ACC borders other current systems, like the Leeuwin, flowing south down Australia’s west coast, then around to the Great Australian Bight and the east coast of Tasmania.

Co-author Associate Professor Bishakhdatta Gayen 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.”

The researchers investigated possible future changes to the ACC, by analysing a series of model simulations from 2000 out to 2050 — under high carbon emission scenarios.  Australia’s fastest supercomputer and climate simulator, GADI, was used for the analysis. The projections were conducted by a University of New South Whales research team.

Projections showed a 20% slowdown in the ACC in 2050 under a high emissions scenario, Sohail says, but the deceleration would be similar under lower emissions scenarios, provided ice melting accelerates as predicted in other studies.

Impacts on the Leeuwin and other bordering currents are unknown, but a weakening ACC has impacted the Leeuwin Current, and Australia, in the past. 

As the world warms and Antarctic glacial ice melts, huge amounts of cold fresh water pour into the Southern Ocean, straight into the path of the ACC. And because they spread out, rather than sink, the current slows, says Gayen.

Slowing matters because the ACC normally prevents warm waters from reaching the Antarctic shelf and continent, says Sohail.

Sohail says “If the Antarctic Circumpolar Current slows down, it may enable more warm water to get onto the Antarctic shelf, thereby accelerating Antarctic ice loss. Now this is part of a potentially vicious cycle, where, as the ACC slows down, it enables more warm water to come up onto the shelf, which then accelerates ice melting. As ice melts further, it causes more ACC deceleration, hastening the ACC slowdown further.”

Melting ice also contributes to global sea level rise, he adds.

“What happens to the ACC is really dictated by how much ice melts, so it’s going to really depend on where, when and how that ice melting occurs around Antarctica.

“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.”