The collapse of a critical Atlantic current can no longer be regarded as a remote possibility, according to a new study, which concludes that drastic reductions in fossil fuel emissions are even more urgent if the catastrophic consequences are to be avoided.

The Atlantic Meridional Overturning Circulation (Amoc) forms a key component of the global climate system. It carries warm tropical waters northwards to Europe and the Arctic, where they cool and sink, driving a deep return flow. The Amoc is already known to be at its weakest for 1,600 years as a result of the climate emergency.

Earlier climate models suggested a collapse before 2100 was unlikely. However, the new research assessed models projected further into the future, to 2300 and 2500. These indicate that the tipping point, after which an Amoc shutdown becomes unavoidable, is likely to be crossed within the next few decades, although the collapse itself may not occur until 50 to 100 years later.

The study found that if carbon emissions continue to increase, 70% of the simulations resulted in collapse. Even with intermediate emissions, 37% of models showed collapse, while a quarter of the low-emission scenarios also ended with shutdown.

Scientists have long warned that an Amoc collapse must be prevented “at all costs”. Such an event would shift the tropical rainfall belt that sustains food production for millions, plunge western Europe into extreme cold winters and drought-ridden summers, and add a further 50cm to already rising sea levels.

The new results are “quite shocking, because I used to say that the chance of Amoc collapsing as a result of global warming was less than 10%”, said Prof Stefan Rahmstorf, at the Potsdam Institute for Climate Impact Research in Germany, who was part of the study team. “Now even in a low-emission scenario, sticking to the Paris agreement, it looks like it may be more like 25%.

“These numbers are not very certain, but we are talking about a matter of risk assessment where even a 10% chance of an Amoc collapse would be far too high. We found that the tipping point where the shutdown becomes inevitable is probably in the next 10 to 20 years or so. That is quite a shocking finding as well and why we have to act really fast in cutting down emissions.”

Warning signs of a tipping point were first identified in 2021, and scientists know that the Amoc has collapsed in Earth’s past.

“Observations in the deep [far North Atlantic] already show a downward trend over the past five to 10 years, consistent with the models’ projections,” said Prof Sybren Drijfhout, at the Royal Netherlands Meteorological Institute, who was also part of the team.

“Even in some intermediate and low-emission scenarios, the Amoc slows drastically by 2100 and completely shuts off thereafter. That shows the shutdown risk is more serious than many people realise.”

The study, published in the journal Environmental Research Letters, examined the standard models used by the Intergovernmental Panel on Climate Change (IPCC). Researchers expressed particular concern that many models indicated the tipping point could be reached within the next decade or two, after which Amoc shutdown would become inevitable due to a self-reinforcing feedback loop.

Rapid warming in the Arctic, driven by the climate crisis, is slowing the cooling of ocean waters there. Warmer water is less dense, causing it to sink more slowly. This slowdown allows rainfall to build up in the salty surface layers, further reducing their density and exacerbating the feedback. Another recent study, using a different method, also suggested the tipping point is likely to be reached around the middle of this century.

As only some IPCC models have been run beyond 2100, the researchers also examined which of those projected to the century’s end already showed the Amoc in terminal decline. This analysis produced the 70%, 37% and 25% collapse figures.

Professor Stefan Rahmstorf warned that the true risks could be even greater, as most models do not account for the influx of meltwater from the Greenland ice sheet, which further freshens the ocean.

Dr Aixue Hu, of the National Center for Atmospheric Research in Colorado, who was not involved in the study, described the findings as important.

“But it is still very uncertain when Amoc collapse will happen or when the Amoc tipping point is going to crossed because of the lack of direct observations [of the ocean] and the varying results from the models.”

By contrast, a previous study led by Dr Jonathan Baker at the Met Office Hadley Centre in the UK suggested that a complete collapse of the Amoc within this century was unlikely.

“This new study highlights that the risk rises after 2100,” he said. “[But] these percentages should be treated with caution – the sample size is small, so more simulations [beyond 2100] are needed to better quantify the risk.”

Nonetheless, Baker said, “the ocean is already changing, and projected shifts in North Atlantic convection are a real concern. Even if a collapse is unlikely, a major weakening is expected, and that alone could have serious impacts on Europe’s climate in the decades to come. But the future of the Atlantic circulation is still in our hands.”

 

——————————————————————————

At Natural World Fund, we are passionate about restoring habitats in the UK to halt the decline in our wildlife.