How Earth’s most powerful ocean current changed over the past 5.3 million years

The Antarctic Circumpolar Current is the most powerful current system on Earth. It moves 100 times as much water as all the Earth’s rivers combined in a circular current that flows clockwise around Antarctica.

In recent decades the ACC has been speeding up, but scientists have been unsure whether this increase is connected to human-induced global warming.

Now, new research indicates that the ACC slowed down during cold glacial periods in the past and gained speed during warm interglacial times – these speedups also correlate with major losses of Antarctica’s ice.

“The ACC has a major influence on heat distribution and CO2 storage in the ocean,” says Dr Frank Lamy, a researcher in the Marine Geology Division of the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) in Germany.

“Until recently, it remained unclear how the ACC responds to climate fluctuations, and whether changes in the ACC offset or amplify the effects of warming,” says Lamy, who is co-author of the paper published in the journal Nature.

“To improve forecasts of our future climate and the stability of the Antarctic Ice Sheet using computer simulations, we need paleo-data that can tell us something about the intensity of the ACC in past warm phases in Earth’s history.”

Lamy is part of an international research team that used sediment cores taken from the South Pacific to reconstruct the flow speed of the ACC over time.

They were able to do this by drawing on the layers in the cores, which correspond to different epochs, and analysing the size distribution of the sediment particles. For example, smaller particles tend to settle to the sea floor during times when the current is sluggish, while larger ones tend to settle when it is moving faster.

They then charted changes in the ACC’s speed over the past 5.3 million years and compared it to changes in climate and changes in the West Antarctic Ice Sheet.

Their findings revealed that during colder phases the ACC slowed and the ice sheet advanced, whereas in warmer phases the ACC accelerated, and the ice sheet pulled back.

“This loss of ice can be attributed to increased heat transport to the south. A stronger ACC means more warm deep water reaches the ice-shelf edge of Antarctica,” says Lamy.

Study coauthor Gisela Winckler, a geochemist at Columbia University’s Lamont-Doherty Earth Observatory, adds that the ACC “is the mightiest and fastest current on the planet. It is arguably the most important current of the Earth climate system.”

The study “implies that the retreat or collapse of Antarctic ice is mechanistically linked to enhanced ACC flow, a scenario we are observing today under global warming,” she says.

Due to anthropogenic climate change, the study concludes, the ACC could grow stronger in the future, and this could lead to accelerated melting of Antarctic ice.