Scientists concerned Antarctic ice melt threatens to collapse the deep ocean currents which affect global temperature and marine habitats

Century-long impacts to marine ecosystems and global climate beckon if Antarctic deep ocean circulation collapses, say Australian scientists.

Evoking ‘The Day After Tomorrow’, a film where the shutdown of North Atlantic ocean currents shut down and cause widespread and rapid climate change, this new research explores the real-world slowing of similar processes in the Southern Ocean around Antarctica.

The research examines overturning circulation:a natural process by which a web of deep-water ocean currents act like a conveyer belt, transporting nutrients, oxygen, carbon and heat from the depths of the sea around the globe, shaping climate systems and marine life. It is a finely balanced system.

But new modelling, led by Professor Matthew England from UNSW, says this process could slow by more than 40% and collapse by the midpoint of the century, assuming the current global rate of greenhouse gas emissions continues.

This, more than other atmospheric influences, is pulling the handbrake on overturning circulation.

Meltwater diluting density

In Antarctica, hundreds of trillions of tonnes of dense salt water sinks to the bottom of the ocean every year.

This water is rich nutrients, thanks to the deterioration of dead animal matter sinking with it. Deep ocean currents convey this material northward to the Atlantic, Pacific and Indian oceans.

But the process is being altered by meltwater – the term given to freshwater released by thawing ice in Antarctica. Meltwater is increasing, driven by ever-increasing greenhouse gases pushing global temperatures higher.

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The freshwater effectively dilutes the density of deep ocean salt water. With less water sinking to the ocean’s depths, the overturning circulation process will slow, in turn preventing the movement of nutrients, oxygen and heat to other regions.

In addition, England says this process has “knock on” effects that will see the ability of the ocean to absorb carbon reduced, and lead to changes in tropical rainfall bands and the health of marine ecosystem productivity.

These also affect North Atlantic overturning – the central idea to the ‘Day After Tomorrow’ – which is already estimated by scientists to be at a thousand-year minimum.

“Once you change the overturning circulation, you reset the global energy balance,” England says.

“And you can shift the ITCZ – the intertropical convergence zone, the big rainfall bands in the tropics – almost synchronously with those changes.

“It’s almost an instantaneous response in the atmosphere.”

A landmark study, and a call to action for carbon cuts

With the rate of ice melt driven by greenhouse gas rises, the research published by England’s team adds an important, antipodean perspective to add to existing research on North Atlantic overturning slowdown.

Oceanographer Professor Trevor McDougall, also based at UNSW but not involved with the research, described the modelling as a “landmark study”.

“This study is the first to shed light on the causes of the warming at the bottom of the Southern Ocean, and importantly, the mechanisms that are causing this warming,” McDougall says. 

In highlighting the influence of Antarctic waters on global nutrient and energy flows, it marks what the authors describe as another “piece of the puzzle” towards understanding the interconnected impacts of climate change.

Their modelling was performed under the ‘high’ IPCC climate change scenario, which  puts the Earth on a three-degree warming pathway if there is little change to current global carbon emissions.

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Aside from the impacts of slowed overturning processes on climate conditions, the consequences for marine productivity are also severe – directly impacting food chain foundations by diminishing phytoplankton availability and cutting feeding opportunities for larger fish and marine mammals.

“The choices we make today … commit us to a certain level of warming. That certain level of warming goes along with a certain amount of ice melt, so we commit ourselves now to this freshwater input,” says Dr Steve Rintoul, who leads the CSIRO’s Southern Ocean research team.

“What our study does is show what the impacts of that freshwater input are.”

Earlier in March, the IPCC synthesis report forecast current, high levels of carbon emissions would lead to major pressure on ecosystems, fish stock depletion and three to four times the number of extreme climate events within the lifetimes of children born today, compared to Baby Boomers.