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The Antarctic Circumpolar Current (ACC) is Earth's largest oceanic current, circling around Antarctica from west to east in alignment with Earth's rotation. This cold ocean current is driven primarily by the westerly wind drift. Connecting the Atlantic, Pacific and Indian Oceans, the ACC is critical for global heat transport, the carbon cycle and the interoceanic exchanging of nutrients. The ACC thus influences the regional and the global climate, and impacts biodiversity.

A recent appearing in the journal Nature Communications documented a study by an international research team of 36 scientists from five countries led by Prof. Xufeng Zheng of Hainan University in Haikou, China.

Using core samples taken from a depth of 3,000 to 4,000 meters, the researchers determined the flow velocity of the ACC. The drilling vessel JOIDES Resolution was deployed in the Scotia Sea north of Antarctica in 2019 to collect the samples within the framework of the Integrated Ocean Discovery Program (IODP). The expedition was led by Dr. Michael Weber of the University of Bonn Institute of Geosciences.

Measurements made of grain size distribution in the sediments now allow conclusions to be drawn on changes in flow velocity. Put simply, at higher velocities, fine particles are carried along by the current and only settle on the seabed when velocity decreases accordingly. Knowing the size distribution of the particles in the core allows determining variations in flow velocity over differing periods of time. This is especially the case given a relatively fine-grained silt fraction of 0.1 to 0.063 millimeters, which was the researchers' focus.

Flow velocity was three times greater

"Accordingly, the velocity in the second-to-last warm period, roughly 130,000 years ago, was more than three times greater than in the last millennia comprising the current warm period," Weber reports. While this finding contradicts expectations given a largely similar climate, researchers attribute the difference to varying radiation resulting from changes in Earth's orbit around the sun.

Earth circles the sun in an elliptical orbit cycle that repeats approximately every 100,000 years. Additionally, Earth's axis changes in tilt and rotation every 21,000 years. "Both parameters showed a simultaneous, mutually reinforcing maximum exclusively during the last warm period," says Weber, which could have altered the westerly winds that drive the Antarctic Circumpolar Current.

Drawing upon other data as well, the researchers have concluded that there is evidence that the ACC shifted poleward in the last interglacial period by at least five degrees of latitude (approximately 600 kilometers). "This brought warmer waters closer to the Antarctic ice sheets, which may have contributed to sea level being 6 to 9 meters higher in the last interglacial," Weber explains. Given the current constellation of orbital factors, the researchers believe that the natural climate system should shift the ACC northward in the coming centuries or millennia, counteracting the predicted southward shift due to climate change.

The researchers conclude, however, that gauging the relative significance of natural climate variability versus human influence—a complex and uncertain endeavor—is crucial in order to accurately predict shifts in the ACC in the context of climate change scenarios. Project leader Xufeng Zheng: "In future research it will be essential to combine geological records of the past and climate modeling."