Deep ocean currents heading for collapse: Report

By Tom Housden, Sydney, Mar. 30: Rapidly melting Antarctic ice is causing a dramatic slowdown in deep ocean currents and could have a disastrous effect on the climate, a new report warns.

The deep-water flows which drive ocean currents could decline by 40% by 2050, a team of Australian scientists says.

The currents carry vital heat, oxygen, carbon and nutrients around the globe.

Previous research suggests a slowdown in the North Atlantic current could cause Europe to become colder.

The study, published in the journal Nature, also warns the slowdown could reduce the ocean's ability to absorb carbon dioxide from the atmosphere.

The report outlines how the Earth's network of ocean currents is partly driven by the downward movement of cold, dense saltwater towards the sea bed near Antarctica.

But as fresh water from the ice cap melts, seawater becomes less salty and dense, and the downward movement slows.

These deep ocean currents, or "overturnings", in the northern and southern hemispheres have been relatively stable for thousands of years, scientists say, but they are now being disrupted by the warming climate.

"Our modelling 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," study lead Professor Matthew England said.

"If the oceans had lungs, this would be one of them," Prof England, an oceanographer at Sydney's University of New South Wales, told a news briefing.

Dr Adele Morrison, who contributed to the report, explained that as ocean circulation slowed down, water on the surface quickly reached its carbon-absorbing capacity and was then not replaced by non-carbon-saturated water from greater depths.

The 2018 Atlas Study found the Atlantic Ocean circulation system was weaker than it had been for more than 1,000 years, and had changed significantly in the past 150.

It suggested changes to the conveyor-belt-like Atlantic Meridional Overturning Circulation (Amoc) could cool the ocean and north-west Europe, and affect deep-sea ecosystems.

A sensationalised depiction of the Amoc shutting down was shown in the 2004 climate disaster film The Day After Tomorrow.

But Dr Morrison said a slowdown of the southern overturning would have more of an impact on marine ecosystems and Antarctica itself.

"Overturning brings up nutrients that have sunk to the bottom when organisms die… to resupply nutrients for the global ecosystem and fisheries," she told the BBC.

"The other larger implication that it could have is feedback on how much of Antarctica melt in the future. It opens a pathway for warmer waters which could cause increased melt, which would be further feedback, putting more meltwater into the ocean and slowing down circulation even more," she added.

Scientists spent 35 million computing hours over two years to produce their models, which suggest deep water circulation in the Antarctic could slow at twice the rate of decline in the North Atlantic.

"[It's] stunning to see that happen so quickly," said climatologist Alan Mix from Oregon State University, a co-author of the latest Intergovernmental Panel on Climate Change (IPCC) assessment.

"It appears to be kicking into gear right now. That's headline news," he told Reuters.

The effect of Antarctic meltwater on ocean currents has not yet been factored into IPCC models on climate change, but it is going to be "considerable", Prof England said.