The Atlantic Meridional Overturning Circulation (AMOC), a vast system of ocean currents, is under pressure due to global warming. This system, which carries warm surface water towards the Arctic and returns colder water thousands of miles south, is crucial in shaping the climate and weather patterns in the UK and north-west Europe. Scientists are concerned that the AMOC may weaken or even collapse, leading to more extreme swings in the UK's weather, including colder winters, even as average global temperatures continue to rise.
The AMOC's influence is close to home for the UK, as it helps shape the climate and weather patterns in the region. The AMOC includes the Gulf Stream and helps explain why Britain and north-west Europe are milder than their latitude would suggest. The system carries about one petawatt of heat northwards, which is roughly 50 times the total energy humanity uses.
Some researchers say the warning signs are already visible, including a strange patch of cooling in the North Atlantic and changes in how salty the water is. They point to recent studies suggesting the circulation may be less stable than previously thought, raising the possibility that it could weaken sharply or even shift into a different state. In the most extreme scenario, some scientists warn it could 'collapse'.
However, others urge caution, saying weakening is not the same as collapse, and that the evidence may point to a slower decline or reorganisation rather than a sudden shutdown. But if the AMOC weakens significantly, it could shift storm tracks, alter rainfall and make winters more volatile.
A severe weakening, or collapse, could go further, bringing colder, drier winters to the UK and north-west Europe even as the planet as a whole continues to heat up. The AMOC helps shape rainfall and temperature patterns far beyond Europe, and a major shift could tug at the West African monsoon, tropical rainfall belts and rainfall over the Amazon, affecting harvests, water supplies and the livelihoods of hundreds of millions of people.
The AMOC has shifted before, and the evidence suggests it can change abruptly during climate change. The Gulf Stream shifted hundreds of miles north during the Younger Dryas, a sudden reversal in the warming that followed the last ice age. This cooling appears to have taken hold over just a few decades, pushing Britain and parts of northern Europe back towards colder, more hostile conditions for more than a thousand years.
The AMOC depends on dense water sinking in the high North Atlantic, which is cold and salty. As warm, salty surface water travels north through the Atlantic, evaporation makes it saltier. Further north, it cools. Together, these processes can make the water heavy enough to sink into the deep ocean, helping to drive the AMOC's overturning flow.
The AMOC's weakening could feed on itself, as warmer surface water is lighter, and extra rainfall and meltwater from ice make the ocean fresher. Fresher water is lighter too, and if the water in the high North Atlantic becomes too light, less of it sinks, weakening the AMOC further. This creates a feedback loop, where the current helps keep the North Atlantic salty enough for the current to keep running.
The 'cold blob' in the North Atlantic is one of the clearest fingerprints of AMOC weakening. If the current is carrying less warm water north, that region would be expected to warm less than the rest of the ocean, or even cool. The temperature signal is only part of the story, as the same region is also becoming less salty, and other studies point to changes in the Gulf Stream and signs that parts of the North Atlantic are being renewed more slowly.
The AMOC's future is uncertain, and the evidence is complex. Climate models cannot resolve the detail of deep water formation, and the disagreement is a glimpse of science doing its hardest work: weighing uncertain evidence when the stakes are high. The AMOC is being stressed by the warming of the planet, and cutting emissions does not remove every uncertainty, but it reduces the pressure on a system no society can afford to push too far.
The safest response is already clear: act on our uncertainty. The science does not offer a neat ending, but it offers probabilities, warnings, caveats and arguments between experts who weigh the same clues differently. The AMOC is still moving, still mysterious, and still capable of surprise.
