The Antarctic Circumpolar Current is accelerating – and it’s not the first time

Ancient ocean sediments have revealed fluctuations in the strength of the Atlantic Circumpolar Current (ACC) over the past 5.3 million years. Comparisons with climatic conditions paint a complicated picture, but recently the current has been strongest when the Earth is at its hottest. It’s no surprise that this is happening again as humans exert our influence on global temperatures.

The ACC is by far the largest water transporter in the world and moves a hundred times as much water as all the world’s rivers combined. There is a deep irony in the latest discovery, as the ACC is thought to be the main reason the Earth is so much cooler now than it was 35 million years ago.

At the time, Tasmania and Patagonia were closer to Antarctica and their presence hindered the development of a current that now perpetually chases its tail around the southern continent. The presence of the ACC prevents warm water from the tropics from reaching the coast of Antarctica and melting the ice in the summer, which would warm the planet by reducing the reflection of light in space.

The ACC does not need to be that strong to fulfill that role, but having shaped Earth’s climate, it is now in turn shaped by it. In recent years the winds have been noticeably accelerating, which is attributed to faster winds in the screaming 60s, the main latitudes of the ACC.

The direct cause of this acceleration is believed to be a 40 percent increase in wind strength over the Southern Ocean. What climatologists want to know is whether those stronger winds are caused by human-induced global warming, or something else. Recently, evidence has emerged that even Mars’ gravity can influence the speed of the ACC, but such a thing would only exert an influence on much longer time scales.

To answer this question, a team from a dozen countries investigated the history of the ACC using sediment cores from the deep ocean. These cores are not easy to collect; In addition to the challenge of drilling into the seabed when the ocean above is miles deep, these driving winds create some of the roughest waters on Earth. Nevertheless, climate scientists were up for the challenge and, from the research vessel JOIDES Resolution, managed to remove five cores in the South Pacific, below the ACC. Two of these were taken near Point Nemo, further from land than anywhere else on Earth.

The ACC is fastest when the card is read or yellow, and slowest for deep blue.  Red dots are locations where sediment cores were collected.

The ACC is fastest when the card is red or yellow, and slowest for deep blue. Red dots are locations where sediment cores were collected.

Image credit: Gisela Winckler

When the ACC is slow, small particles dominate the sediment, but they grow larger as the ACC speeds up, recording the speed over millions of years. Millions of years ago, the ACC grew stronger as the world cooled, but for the past 800,000 years it has been strongest during warmer times. For example, Ice Age velocities were only half the Holocene average.

The current speeds are certainly not as high as the ACC can go, if these cores are to be believed. Some interglacial periods in the past were associated with speeds 80 percent faster than today, suggesting there is plenty of room for growth.

Sediment cores from the JOIDES resolution reveal a complex relationship between climate and the ACC that has become simpler over the past million years.

Sediment cores from the JOIDES resolution reveal a complex relationship between climate and the ACC that has become simpler over the past million years.

Image credit: Gisela Winckler

Despite the role the ACC has played in cooling the planet, you can have too much of a good thing. It is thought that the extra energy could be transferred in a faster flow to the ice at the edges of the continent, speeding up its melting.

“If you leave an ice cube hanging in the air, it takes a long time for it to melt,” study co-author Dr. Gisela Winckler said in a statement. “If you put it in contact with warm water, it goes quickly.” Consequently, the association of heat with a stronger ACC could be a case of one strengthening the other, at least to some extent, rather than a pure case of more heat driving faster flow.

The slowdown of the Atlantic Meridional Overturning Circulation (AMOC) is getting the most attention and will likely pose a threat to us in the short term, but the acceleration of the ACC could be just as important in the long term.

The research has been published in Nature.

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