Next tipping point? Atlantic circulation could stop this century

Danish researchers have calculated an essential ocean circulation process could grind to a halt this century, pushing the Earth closer to an irreversible climate change tipping point.

Professors Peter and Susanne Ditlevsen – a brother-sister duo at the University of Copenhagen in Denmark – have collaborated on an analysis of statistical early warning signals to estimate how quickly the Atlantic Meridional Overturning Circulation – or AMOC – is slowing down.

AMOC is one of the planet’s major ocean mixing processes, often described as a conveyer belt that relays cold salt water from the North Atlantic towards the South Atlantic while simultaneously dragging warm water from the south to the north.

This process results in the cooling of warm water near the poles to aid sea ice formation. In turn, residual salt left out of this freezing process causes the surrounding water to become dense and sink to the depths of the ocean where it joins this massive, liquid conveyer.

But the advance of global warming due to industrial greenhouse gas emissions has changed this dynamic.

With warming oceans, sea ice is melting, causing fresh meltwater to dilute this dense saline bottom water.

That’s a problem, because the conveyer process inherent to the AMOC also involves currents ‘upwelling’ cold water to the ocean’s surface over time, dragging nutrients released from dead animals near the sea floor. This process creates nutrient-rich environments for smaller organisms like phytoplankton to thrive, in turn providing a foundation for many marine ecosystems.

If the brakes were hit on AMOC, these vital processes would be severely impacted, and that’s a concern for scientists across the globe.

It’s been known for some time that the AMOC is probably slowing, but even the most recent reports from the Intergovernmental Panel on Climate Change believed such a process would eventuate until next century. Now, the Ditlevesens’ research throws a curve ball at that hypothesis by forecasting this circulation will collapse sometime in the next 65 years.

Modelling the AMOC collapse

The AMOC is considered a climate tipping point connected to the loss of ice in the Arctic and Greenland and influencing the same in Antarctica. Such tipping points could form part of a cascade effect which leads to other of these climate safeguards being breached.

Passing these tipping points – whether in three years or 60 – has researchers like the Ditlevsens concerned.

Their modelling of the AMOC collapse draws on 150 years of Atlantic sea surface temperatures from 1870 to 2020, as a “fingerprint” for the process. With AMOC having been tracked continuously from 2004, their use of proxy temperatures dating to pre-industrial times gave the pair a “baseline, pre-climate change” to assess.

“That’s exactly what enables us to say that, well, now we’re starting to push towards the bifurcation point, a tipping point,” says Peter Ditlevsen, who is a physicist and climate researcher at the University of Copenhagen’s (KU) Niels Bohr Institute.

“It has been known by oceanographers that the Atlantic overturning is weakening and increasing the risk of this [collapse] happening, and two years back, a study was published that showed that we had these early warnings that something is about to happen.

“But what was not established and could not be established was, how significant are these findings, and when was it going to happen?”

After reviewing the emerging research on the AMOC slowdown, he collaborated with his sister Susanne – a professor in statistics at KU – to determine a timescale for the predicted collapse.

“I did it first with some simple statistical means and found really worrisome results,” Peter says.

“So we pulled out the big statistical tools and developed a statistical framework that could do this [calculation] much more precisely, and it turned out to completely confirm what I found.”

Neither speculate on the cause of the slowdown, but note that the process likely began around the 1920s and has continued since.

“So it’s actually only 100 years in which it has changed,” Susanne Ditlevsen says.

“We can speculate about what will happen after the tipping, but our study only deals with the time up to the tipping… we do have confidence that if we continue as business as usual then the tipping will happen within this century.”

“A really important study”

Last year, researchers from the University of New South Wales studied the prospect of an AMOC collapse, finding the potential of more La Niña-like conditions like flooding and higher rainfall across the east coast of Australia, and increased bushfire seasons over the southwestern US, not unlike the conditions these regions experienced during La Niñas in recent years.

The research group, led by Scientia Professor Matthew England, this year modelled a similar oceanic conveyer – in Antarctica – finding the potential for overturning in the region to slow by 40% by the midpoint of the century.

While he wasn’t involved in the Ditlevsens’ research, he says theirs is an important study.

“What’s new here is trying to get exactly how close we are [to the AMOC collapse],” says England.

A change to circulation in the Atlantic, England says, would likely have a knock-on effect on the planet’s climate system – shifting tropical rain belts that have remained stable for millennia.

While slowdowns in one circulation system might be matched by a speeding up of the other over thousand-year time scales, his concern is that the melting of Greenland and Antarctic ice is slowing both systems simultaneously.

“We’ve got a situation where both of these water masses, both of these overturning circulations, are sitting near ice that’s melting,” England says.

“We know both those ice sheets are melting at an unprecedented rate due to human emissions, we know that’s adding meltwater, and so in this situation, all the evidence we have points to a slowdown of both the overturning circulation of the North Atlantic and in the Antarctic.

“It’s a permanent change to our textbook description of the ocean circulation.”

Climate scientists, including those involved in ocean studies like these, agree that reducing the concentration of greenhouse gases like carbon dioxide and methane is essential to avert runaway changes in AMOC and Antarctic circulation.