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Deep ocean circulation near Antarctica on the brink of collapse

Scientists have sounded the alarm that the deep ocean circulation forming around Antarctica may be heading towards collapse, with potentially severe long-term impacts on climate and marine ecosystems worldwide.

This circulation is part of a vast global network of ocean currents, known as the overturning circulation, which transports heat, carbon, oxygen, and nutrients around the planet. It plays a crucial role in regulating Earth’s climate, sea levels, and the productivity of marine ecosystems. 

Cold water sinking near Antarctica drives the deepest flow of this circulation, with approximately 250 trillion tonnes of cold, salty, and oxygen-rich water sinking in the region each year. This water then travels northwards, carrying oxygen into the deep Indian, Pacific, and Atlantic Oceans.

Describing the importance of this process, Professor Matthew England, Deputy Director of the ARC Centre for Excellence in Antarctic Science (ACEAS) at UNSW Sydney, says: “If the oceans had lungs, this would be one of them.”

In a new study published in the journal Nature, an international team of scientists led by Professor England modeled the production of Antarctic deep water until 2050 under the IPCC’s “high emissions scenario.” The researchers collaborated with co-authors from the Australian National University (ANU) and CSIRO.

The model used in the study captures details of ocean processes that previous models have not been able to represent, including the potential influence of meltwater from ice on the circulation. According to the researchers, this deep ocean current has remained in a relatively stable state for thousands of years. However, due to increasing greenhouse gas emissions, Antarctic overturning is predicted to slow down significantly over the next few decades.

“Our modeling shows that if global carbon emissions continue at the current rate, then the Antarctic overturning will slow by more than 40 per cent in the next 30 years – and on a trajectory that looks headed towards collapse,” warns Professor England.

If this deep ocean current were to collapse, the ocean below 4,000 meters would stagnate, trapping nutrients in the deep ocean and reducing their availability for supporting marine life near the ocean surface. “This would trap nutrients in the deep ocean, reducing the nutrients available to support marine life near the ocean surface,” says Professor England.

The researchers found that melting ice around Antarctica makes the nearby ocean waters less dense, slowing the Antarctic overturning circulation. Furthermore, the melt of the Antarctic and Greenland ice sheets is expected to continue to accelerate as the planet warms.

“Our study shows that the melting of the ice sheets has a dramatic impact on the overturning circulation that regulates Earth’s climate,” notes study co-author Dr. Adele Morrison.

“We are talking about the possible long-term extinction of an iconic water mass,” says Professor England. He warns that such profound changes to the ocean’s overturning of heat, freshwater, oxygen, carbon, and nutrients would have a significant adverse impact on the oceans for centuries to come.

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