The dense water pushes through the deep sea and hugs the coastline of New Zealand before moving into the Pacific and Indian oceans.

It forces warmer water up, pushing particles left by sinking dead sea creatures back to the surface. This “biological pump” underpins critical parts of the ocean’s ecosystems.

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“We know, for example, that nutrients exported from the Southern Ocean in other current systems support about three-quarters of global phytoplankton,” said research lead Dr Steve Rintoul.

“The sinking of dense water near Antarctica will decline by [about] 40 per cent by 2050. And it’ll be sometime between 2050 and 2100 we start to see the impacts of that on surface productivity.”

A supercomputer spent years cranking through temperature, wind and meltwater data to calculate what could happen under our current emissions trajectory.

The researchers, who published their findings in Nature, focussed on how increasing Antarctic meltwater affects ocean circulation. Melting ice freshens the water around Antarctica, so it’s less salty and dense, weakening the main driver of the ocean overturning.

The results on the future of the system shocked them.

“[It’s] been there for centuries and centuries bringing up nutrients. To see the changes that we project play out in a couple of decades, where you get the extinction of a water mass within a few decades is pretty impressive,” said England.

“It’s not like The Day After Tomorrow, within a day or two, but it’s certainly way faster than I would have thought these overturning circulations could slow down. They’re massive volumes of water.”

The addition of meltwater data was key. It’s a hard element to model because much of it forms in scalloped ice shelf cavities and caves.

The result was drastic: just using wind and temperature data showed no predicted change in the Antarctic overturning by 2050. Add meltwater, and the disaster story unfolds.

The 42 per cent slowdown will be hard to prevent. “It’s hard to turn the ship around with this sort of physics,” England said.

But limiting emissions now will stop even more meltwater dumping into the ocean around Antarctica by the mid-21st century, to the point the system could recover.

“It is another wake-up call, as if we needed more wake-up calls,” Rintoul said.

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