How a critical Antarctic current threatens fish stocks

In the 2004 disaster flick The Day after Tomorrow, New York is snap-frozen in a climactic catastrophe caused by awry ocean currents.

The blockbuster – though horribly scientifically inaccurate – boosted the public profile of the Atlantic Meridional Overturning Circulation (AMOC), a system that stabilises parts of the northern hemisphere’s climate.

Antarctic meltwater is slowing key deep water movements.Credit:Getty

Now Australian oceanographers are evoking the sci-fi thriller to draw attention to a dramatic slowdown in AMOC’s Antarctic equivalent. A critical system of currents at the bottom of the ocean powered by freezing, superdense water is breaking down twice as fast as its Hollywood-famous northern sister.

The oceanic slowdown could lead to the extinction of a centuries-old water mass that ventilates the entire ocean, sending tropical rain bands spiralling away from the equator and devastating fish stocks that rely on the nutrients it drags up from the deepest parts of the sea – the abyssal ocean.

“We’ve had it in a relatively stable state for thousands of years,” UNSW ocean circulation expert Professor Matthew England said. “With our ongoing greenhouse gas emissions, we’re changing that system.”

England is part of the team that found the critical Antarctic current could slow by 42 per cent by 2050.

A disruption of that scale could shift tropical rain bands northward by 1000 kilometres and start depleting fisheries within the next 30 years, the researchers said.

The system, called the Antarctic Meridional Overturning Circulation, begins on the fringe of Antarctica’s ice shelf, where 250 trillion tonnes of cold, salty, dense water per year cascades down underwater canyons to the abyssal ocean.

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.

“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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