A "dramatic" change to ocean circulation could unfold in the Southern Ocean over the next three decades with wide-reaching effects on weather and fisheries, according to researchers.
The landmark study, published in Nature on Thursday, examined waters at the deepest layers of the ocean that play a crucial role in circulating heat and nutrients around the globe.
Deputy director of the Australian Research Council's Centre for Excellence in Antarctic Science, Matthew England, who coordinated the study, said the results were both significant and "concerning", likening their projecting to the premise of The Day After Tomorrow.
The fictional film, which was based on the real-life slowdown of the Atlantic Meridional Overturning Circulation current, saw polar melting disrupt the North Atlantic current, setting off a chain of events that influenced weather around the globe.
"In our simulation, [the slowing circulation] in the Antarctic outpaces the North Atlantic by two to one," Dr England said.
"We know so much about the Atlantic overturning and it's been such an established part of science, so much so that a film has been made about it.
"And here we have an overturning circulation that's just as important to humanity, where we still don't understand why things are changing, what the drivers are, and what the future is."
Freshening the world's densest waters
The findings of the study all have to do with the production of incredibly dense water, formed around Antarctica, known as Antarctic Bottom Water.
The water is essentially the by-product of sea ice formation around Antarctica, which leaves behind very salty and cold water, which can sink to the deepest layers of the ocean.
It is a key part of the conveyer-belt-like system of underwater currents known as the "overturning circulation", which cycle heat, carbon, and nutrients around the globe.
But oceanographer Adele Morrison, who was one of the authors of the paper, said simulations had shown the overturning circulation would slow down considerably based on a high-emissions scenario.
"By 2050 we're looking at a 40 per cent reduction of the abyssal overturning circulation," Dr Morrison said.
"It's huge. If we shut down this transport of water around the globe, that has a huge impact on lots of things."
Melting ice behind change
Dr Morrison said the slowdown was driven almost exclusively by the melting of ice sheets and shelves.
"So this puts extra fresh water into the ocean around Antarctica," she said.
"And it's this freshwater that reduces the density and lightens the waters around Antarctica.
"Therefore you don't get as much descending of those dense waters into the abyss, and you get a reduction in the overturning circulation."
Melting of ice around Antarctica is a direct consequence of climate change.
Potential 'collapse' of ecosystems
The overturning currents play a key role in bringing nutrients from the bottom layers of the ocean, where dead marine creatures have sunk to, back toward the surface.
Dr Morrison said slowing the overturning process could have dire consequences for marine life.
"Once you shut down this overturning circulation and its resupply of nutrients, we're looking at danger of collapse of some of these ecosystems," she said.
She said this flow-on effect would play out over a much longer time frame.
"We're studying the dense water formed around Antarctica and its descent into the abyssal ocean," she said.
"But most of the impacts are then felt when the waters rise back up.
"And so the timescales are a bit slower, more sort of century-type timescales."
Impacts to rainfall in the Southern Hemisphere
The overturning circulation also has a relationship with climate patterns.
Dr England said a change in overturning in the ocean could lead to less rainfall in the Southern Hemisphere.
He said this was driven by a shift in the position of tropical rainbands.
"Overall, the Southern Hemisphere tends to be a bit drier with this overturning slowing down and the Northern Hemisphere tends to get a bit wetter," Dr England said.
Like the impacts to marine life, this would happen over a much longer timescale.
Lead researcher Dr Qian Li, from Massachusetts Institute of Technology, said one of the things that might limit that change was the countering impact of the overturning current in the Northern Hemisphere.
"It depends which magnitude is larger," she said.
The slowdown of circulation could also promote further ice melt around Antarctica, according to the researchers.
Reducing emissions crucial, say researchers
Dr England said without change to emissions, the downward trajectory would continue.
"That 40 per cent value is not where it stops; we only run the simulations to 2050," he said.
"So there's every chance that's going to be 60 per cent or 80 per cent in the decades that follow 2050."
Dr England said the importance of reversing the trend should not be underestimated.
"All of human civilisation has developed with this overturning circulation," he said.
"If we switch it off, we fundamentally change the cycling of nutrients through the oceans.
"We could see mass extinction of some species of fish, we could see a loss of fisheries that we rely on for food down the track."
