Atlantic Ocean's Circulation Could Collapse

A key system in regulating the earth's climate may be on course to a tipping point, scientists have warned.

The Atlantic Meridional Overturning Circulation, or the AMOC, is a large system of ocean currents that circulates water within the Atlantic Ocean, bringing warm water north and cooler water south. The dynamics of this system are thought to be essential for the relative warmth of the Northern Hemisphere, but they are also vulnerable to climate change.

Numerous studies have shown that this ocean system will likely slow down under climate change, but it is uncertain when and how fast these changes might occur. The system has also been identified as a tipping element in the climate system, meaning that it may undergo sudden, rapid change after years of exposure to a slowly developing climate force.

In a new study, published in the journal Science Advances, researchers from Utrecht University in the Netherlands created a mathematical model of this system to investigate how our changing climate might influence this system and the impacts this disruption would have on the rest of the world.

By their models, this collapse would dramatically change the distribution of heat across the planet, resulting in rapid and severe cooling in the Northern Hemisphere and slight warming in the south. This rapid cooling would be particularly prominent across Europe, with temperatures dropping by more than 3 degrees Celsius per decade.

"In comparison with the present-day global mean surface temperature trend of about 0.2 degrees Celsius per decade, no realistic adaptation measures can deal with such rapid temperature changes under an AMOC collapse," the authors wrote.

Luckily, their models have also detected an observable change in the transport dynamics of freshwater in the Atlantic Ocean about 25 years before this tipping point is reached, offering a potential early warning signal for an impending AMOC collapse.

"This study brings further worrisome evidence about the potential for abrupt and disruptive changes to the North Atlantic region," Jon Robson, a professor and research fellow at the National Centre for Atmospheric Science and the University of Reading who was not involved in the study, said in a statement. "Such an AMOC collapse would have profound impacts on society and ecosystems through changes in regional temperatures, rainfall and winds."

However, this collapse isn't predicted any time soon.

"This sounds alarming, but it's important to note that this is not the same as saying collapse is going to happen imminently," Andrew Watson a Royal Society Research Professor and professor at the University of Exeter, said in a statement. "They have to run their model for a long time (1700 years) and push it quite hard to make the collapse happen, so, assuming they are right, the "route" could still be a long one and there may be time for us to change it."

Watson also stressed that it is important to remember that models are only a prediction, not a reality.

"The real system may be more, or less, prone to collapse than this model suggests," he said. "However, this is a very useful study, because with a comparatively realistic model, the mechanism of AMOC collapse is revealed in some detail, and that allows the authors to suggest practical observations we can make that would help predict if a tipping point is close."

Jonathan Bamber, the Director of the Bristol Glaciology Centre and professor at the University of Bristol, also highlighted that the model uses fairly extreme climate scenarios to create these predictions.

"They did this by imposing a huge freshwater forcing to the North Atlantic that is entirely unrealistic for even the most extreme warming scenario over the next century," he said in a statement. "Their freshwater forcing applied to the North Atlantic is equivalent to 6 cm a year of sea level rise by the end of the experiment, which is more than seen during the collapse of the ice sheet that covered North America during the last glaciation.

"To what extent such an experiment can be used to infer robust behaviour in the climate system is difficult to know but it is interesting that they do see a collapse in the AMOC, even in this artificial case."

What is still unclear, however, is how close we actually are to this tipping point.

"The research makes a convincing case that the AMOC is approaching a tipping point based on a robust, physically-based early warning indicator," Tim Lenton, director of the Global Systems Institute and professor at the University of Exeter, said in a statement. "What it cannot (and does not) say is how close the tipping point, because as it shows that there is insufficient data to make a statistically reliable estimate of that."

Irrespective of when this might happen, this study provides important insights into our planet's climate systems and how they might change in the future.

"We have to plan for the worst," Lenton said. "We should invest in collecting relevant data and improving estimation of how close a tipping point is, improving assessment of what its impacts would be, and getting pre-prepared for how we could best manage and adapt to those impacts if they start to unfold."