Melting ice could put Antarctica’s deep currents on path to ‘collapse’

The melting of Antarctica’s ice sheets could put surrounding deep currents on a path to collapse, scientists say, with dramatic consequences for our planet’s ocean and climate.

Their just-published landmark study warns that continuing high emissions could send enough meltwater into the ocean to slow the globally-critical circulation of deep water around the frozen continent.

“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 co-ordinator Professor Matthew England said.

Such changes would profoundly alter ocean overturning of heat, freshwater, oxygen, carbon and nutrients, with impacts felt for centuries to come.

The implications of our heating planet for deep ocean currents have been among the biggest questions facing scientists - and something that inspired the 2004 sci-fi disaster blockbuster The Day After Tomorrow.

While many studies have focused on climate-induced slowing of vertical overturning circulation deep in the North Atlantic Ocean, the latest study, published in the journal Nature, is the first to shed light on causes of warming at the bottom of the Southern Ocean, deep below New Zealand.

Importantly, it also explored the mechanisms causing that warming: namely meltwater coming from Antarctica.

The healthy circulation of the deep current around the continent – called Antarctic Bottom Water – couldn’t be more important for our planet.

Each year, about 250 trillion tonnes of cold, salty, oxygen-rich water sinks near Antarctica, before spreading northward and carrying oxygen into the deep Indian, Pacific and Atlantic oceans.

“If the oceans had lungs, this would be one of them,” said England, deputy director of the University of New South Wales’ ARC Centre for Excellence in Antarctic Science.

Particularly, this sinking cold water drove the deepest flow of the overturning circulation – a network of currents that spans the world’s oceans.

It was this overturning that ferried heat, carbon, oxygen and nutrients around the globe, influencing climate, sea level and the productivity of marine ecosystems.

“Some people call it an engine room, some people call it a flywheel... but no matter what, Antarctic Bottom Water production drives global ocean circulation,” Victoria University climate scientist Professor Tim Naish said.

Antarctica's vast ice sheets store an equivalent 60m of potential sea level rise. Photo / Jamie Morton

While scientists had known this circulation was slowing down, Naish said the new study told us much more about why.

“We’ve always suspected [what’s been driving the slow-down], but these scientists have used all the data and modelling to put it all together.”

That included modelling the volume of Antarctic deep water that would be produced under a high, “business-as-usual” global warming scenario, out to 2050.

This was able to capture detail of ocean processes that previous models hadn’t been able to, including how predictions for meltwater from ice might influence the circulation.

That melting was essentially confirmed as the main driver of slowing the overturning circulation, with the changing winds and the surface warming playing only minor roles.

The extra meltwater decreased the ocean salinity and the ocean density, and, in turn, affected the strength of the vertical overturning circulation.

If this deep ocean current slowed to the point of collapse, oceans below 4000m would effectively stagnate.

“This would trap nutrients in the deep ocean, reducing the nutrients available to support marine life near the ocean surface,” England said.

Co-author Dr Steve Rintoul, of the Commonwealth Scientific and Industrial Research Organisation and the Australian Antarctic Program Partnership, said the model simulations showed a slowing of the overturning, leading to rapid warming of the deep ocean.

“Direct measurements confirm that warming of the deep ocean is indeed already under way,” Rintoul said.

The melting of the Antarctic and Greenland ice sheets was expected to continue to accelerate as the planet warmed.

“Our study shows that the melting of the ice sheets has a dramatic impact on the overturning circulation that regulates Earth’s climate,” said co-author Dr Adele Morrison, of the Australian National University’s Research School of Earth Sciences.

England added: “We are talking about the possible long-term extinction of an iconic water mass.”

University of New South Wales climate scientist Professor John Church said many uncertainties remain about the impact beyond 2050, including the potentially larger loss of mass from Antarctica postulated in some studies.

“But it seems almost certain that continuing on a high greenhouse gas emission pathway will lead to even more profound effects on the ocean and the climate system,” Church said.

“The world urgently needs to drastically reduce our emissions to get off the high emission pathway we are currently following.”

Niwa ocean modeller Dr Erik Behrens pointed to another fundamental part of the picture: the oceans’ absorption of carbon.

Measurements suggest that 40 per cent of the human-produced CO2 found in oceans worldwide was originally absorbed from the atmosphere into the Southern Ocean - making it one of the planet’s most important carbon sinks.

“If this deep overturning circulation slows that will likely reduce the current rate of carbon uptake from the atmosphere associated with this circulation and is putting more pressure on lowering ‘our’ emissions,” Behrens said.

Unfortunately, the Intergovernmental Panel on Climate Change’s recently-issued synthesis of its Sixth Assessment Report noted that, even under the lowest of emissions scenarios, the symbolic warming threshold of 1.5C mark would be “more likely than not” met because of the increased, cumulative levels of CO2 we’ve already pumped into the atmosphere.

According to a mid-range scenario – in which CO2 levels peaked at around 2040, but then hit the 2C point sometime between 2041 and 2060 – this century might finish up being 2.7C hotter than pre-industrial years.