Scientists reconstruct history of world’s most powerful ocean current over last 5.3 million years and its link to global climate

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An ocean current that connects the Indian, Atlantic and Pacific oceans has been speeding up in recent decades with a warming climate. A new study that looked at its speed over the last 5.3 million years has found that it responds to the global climate by picking up pace or slowing down, with far-reaching consequences.

The findings, published in journal Nature, suggested that the current speeding of the Antarctic Circumpolar Current (ACC) does not bode well for the stability of Antarctica’s ice sheets, which are already losing mass at an average rate of about 150 billion tonnes per year, contributing to sea-level rise.

Read more: Antarctica’s sea ice is at its lowest extent ever recorded

ACC plays a key role in regulating global climate and is also the world’s most powerful and primary means of inter-basin exchange of heat, carbon dioxide, chemicals and biology.

Powered by continuous westerly winds, the ocean current circles Antarctica clockwise at about 4 kilometres per hour, carrying 165 million to 182 million cubic metres of water each second.

It reaches from the ocean’s surface to its bottom and measures as much as 2,000 kilometres across. 

So far, scientists know that the winds over the Southern Ocean, which encircles Antarctica, has gained strength by about 40 per cent in the last four decades.

This, in turn, has sped up the ACC, causing relatively warm water from higher latitudes reaching the South Pole. In some parts, especially western Antarctica, these warm waters are melting the undersides of the ice shelves, according to scientists.

Read more: Mega iceberg released 158 billion tonnes of water into ocean

“This loss of ice can be attributed to increased heat transport to the south,” the study’s lead author, Frank Lamy from Germany’s Alfred Wegener Institute, said in a statement. 

“A stronger ACC means more warm, deep water reaches the ice-shelf edge of Antarctica,” the expert added.

Lamy explained that previously, scientists did not have a clear understanding of how the ACC responds to climate fluctuations. This information could help in forecasting our future climate and stability of the Antarctic ice sheet.

To address this gap, the team analysed samples dug through the Earth’s core to learn about the intensity of the ACC in past warm phases of Earth’s history.

In central South Pacific, the research team extracted two core samples at a depth of 3,600 metres. The team then reconstructed the ACC’s flow speed after analysing layers built on core samples over millions of years using an advanced X-ray technique. 

Smaller particles that become a part of the core sediment tend to settle when the current is sluggish and larger ones when it is fast, according to the paper.

Read more: A single Antarctic heatwave or storm can noticeably raise the sea level

During the last 800,000 years, when atmospheric carbon dioxide (CO2) levels varied from 170 to 300 parts per million, the researchers were able to identify a close connection between ACC strength and glacial cycles.

In warm periods and when atmospheric CO2 levels rose, the flow speed increased by up to 80 per cent compared to the present and decreased by up to 50 percent during ice ages. 

In the future, the ACC is likely to grow stronger due to human-caused climate change. This could accelerate melting of Antarctic ice, the researchers warned.

Lamy said more analysis is needed. “We assume that records reflect the overall ACC changes in the Pacific sector. However, there might be major differences in other sectors, like the Indian and Atlantic sectors. So, the might be more spatial heterogeneity. Therefore, we need more sediment records from the vast ACC,” he told Down To Earth.