There is one possible climate scenario in which the United Kingdom and north-west Europe could face plunging temperatures and severe winters, even as the rest of the world continues to warm.

Although this outcome is not the most likely, scientists are increasingly concerned about its growing probability and the significant consequences it would bring.

The worry stems from the potential weakening—or even collapse—of the Atlantic Meridional Overturning Circulation (Amoc), an ocean current system that transports warm water from the tropics to the North Atlantic. Amoc plays a crucial role in maintaining the relatively mild climate of the UK, making it warmer than other regions at similar latitudes, such as Moscow.

It functions as a vast conveyor belt, carrying warm, salty water northwards, which then cools, sinks, and flows back southward as a deep ocean current. This continuous circulation distributes heat across the planet and influences weather patterns.

However, Amoc appears to be weakening. Direct measurements have only been taken since 2004, which is not enough to confirm a long-term trend. Yet, indirect evidence suggests a potential slowdown of about 15% over the past few centuries, although this is still debated among scientists.

One clue is found in ocean floor sediments; larger grains indicate stronger currents. By dating these grains, scientists can estimate changes in Amoc’s strength over time.

Another indication is the so-called “cold blob” in the North Atlantic—a region that has cooled over recent decades, contrary to global warming trends. This cooling is seen as a possible result of reduced warm water transport due to a weaker Amoc.

The UN’s Intergovernmental Panel on Climate Change (IPCC) predicts that Amoc will weaken this century, but the more concerning possibility is a sudden collapse. Historical evidence suggests that Amoc has shut down multiple times before.

Today, climate change is making the North Atlantic less salty by adding freshwater from the melting Greenland ice sheet and increasing rainfall. Fresher water is less dense and doesn’t sink as easily, potentially slowing the current. If this process reaches a tipping point, it could trigger a runaway collapse of Amoc.

“We really want to avoid a tipping point because then there’s nothing we can do about it,” warns David Thornalley, professor of ocean and climate science at University College London.

The likelihood of an Amoc collapse remains uncertain.

In 2021, the IPCC stated it has a “medium confidence” that Amoc would not collapse this century, though it did expect it to weaken. However, more recent studies suggest the risk of reaching a tipping point may be higher than previously thought.

These studies come with caveats and uncertainties, as climate models yield varying results for such a complex system.

“We don’t believe the idea of an Amoc collapse this century has substantially changed because of these new results,” cautions Dr Laura Jackson, oceanographer at the Met Office.

Nevertheless, growing evidence has prompted over 40 leading ocean and climate scientists to sign an open letter urging greater awareness of the risks.

“I’d say you’re looking at a risk of reaching a tipping point in the coming decades that could be at the 10 or 20% level even if we hold the line at 2C warming [above temperatures of the late 19th Century, before humans started significantly warming the climate],” warns Tim Lenton, professor of Earth system science at the University of Exeter.

Even a continued weakening of Amoc would have serious consequences.

“If the Amoc gradually weakens over the next century, you’re going to get global warming but less warming over Europe,” says Dr Jackson.

The UK could still experience hotter summers due to global warming, but a weaker Amoc might also lead to more intense winter storms.

A full-scale collapse, meanwhile, would be “like a war situation […] something almost unimaginable,” says Prof Lenton.

Over a century or more, temperatures in northern Europe could drop by several degrees per decade, significantly altering regional climates.

In the UK, it could “become horribly, horribly cold … like living in northern Norway,” Prof Thornalley warns.

“Our infrastructure is not set up for that.”

The ripple effects could extend globally, including shifts in tropical rain belts.

“That’s a big story,” warns Prof Lenton.

“If you lost the monsoon or seriously disrupted it, you’d have humanitarian catastrophes, in simple terms, in west Africa [and] probably in India.”

Preparing for this potential future poses complex challenges for governments. The situation is compared to the Covid-19 pandemic—another event foreseen by scientists but difficult to predict in terms of timing.

A recent report warned that the UK faces a “glaring national security blind spot” concerning climate threats linked to Amoc. The government admitted last year that it had not assessed the impact of Amoc changes on economic planning.

Despite these uncertainties, scientists agree that reducing greenhouse gas emissions is the most effective way to mitigate the risk of Amoc collapse. By addressing the root cause of climate change, the probability of reaching a tipping point can be minimised, safeguarding the delicate balance of the Earth’s climate system.

“We’re playing a bit of a Russian roulette game,” warns Prof England.

“The more we stack up the atmosphere with greenhouse gases, the more we warm the system, the more chance we have of an Amoc slowdown and collapse.

“And so I think people need to not give up, because there’s so much to be gained by reducing emissions.

“The scale of change is just so much worse if we do nothing.”