After record heat, could the Atlantic make Britain's weather even more extreme?
The Atlantic Meridional Overturning Circulation (AMOC) is a vast, north-south system of currents that carries warm surface water towards the Arctic and returns colder water thousands of miles south through the deep ocean. It is a key component within the Earth's climate system, and its influence is particularly significant for the UK and north-west Europe, where it helps shape the climate and weather patterns. The AMOC is under pressure due to global warming, and scientists are concerned that it may weaken, potentially leading to more extreme weather conditions in the UK, including colder winters, even as average global temperatures continue to rise.
The AMOC includes the Gulf Stream, which helps explain why Britain and north-west Europe are milder than their latitude would suggest. Some researchers say the warning signs are already visible, such as a strange patch of cooling in the North Atlantic and changes in the salinity of the water. 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, arguing that weakening is not the same as collapse, and that the evidence may point to a slower decline or reorganization rather than a sudden shutdown. If the AMOC weakens significantly, it could shift storm tracks, alter rainfall patterns, and make winters more volatile. A severe weakening or collapse could bring 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 affect the West African monsoon, tropical rainfall belts, and rainfall over the Amazon. This could have significant impacts on harvests, water supplies, and the livelihoods of hundreds of millions of people. Scientists are therefore closely monitoring the ocean to detect any changes in the AMOC.
The AMOC has shifted before, and the evidence suggests that it can change abruptly during climate change. The Younger Dryas, a sudden reversal in the warming that followed the last ice age, is an example of this. The cooling appears to have taken hold over just a few decades, pushing Britain and parts of northern Europe back towards colder, more hostile conditions. This would have been a catastrophic upheaval for the small groups of hunter-gatherers living in Britain at the time.
The AMOC depends on dense water sinking in the high North Atlantic, which is influenced by cold and salty water. As warm, salty surface water travels north through the Atlantic, evaporation makes it saltier, and further north, it cools. This process can make the water heavy enough to sink into the deep ocean, helping to drive the AMOC's overturning flow. However, if the water in the high North Atlantic becomes too light, the AMOC could weaken, leading to a feedback loop where the current helps keep the North Atlantic salty enough for the current to keep running, but less salt makes the water lighter, so it sinks less easily and the circulation weakens further.
One patch of ocean has attracted a lot of attention: a cold blob south of Greenland, which has stubbornly refused to warm up as the rest of the ocean has. This region is 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. These are clear fingerprints of AMOC weakening.
However, the evidence is complex, and the future of the system cannot yet be reduced to a simple story of imminent collapse. Climate models cannot resolve the detail of deep water formation in the North Atlantic, and they have to approximate it. This means that the evidence is uncertain, and the disagreement among scientists is a glimpse of science doing its hardest work: weighing uncertain evidence when the stakes are high.
The safest response is already clear: cutting emissions reduces the pressure on the AMOC and minimizes the risk of extreme weather conditions. However, the AMOC is still moving, still mysterious, and still capable of surprise. Whether it is simply weakening or being pushed towards something more abrupt remains unresolved, but the science does not offer a neat ending. It offers probabilities, warnings, caveats, and arguments between experts who weigh the same clues differently.
Personally, I think that the AMOC is a fascinating and complex system that is under pressure due to global warming. I believe that the evidence suggests that it could weaken, potentially leading to more extreme weather conditions in the UK. However, I also think that the evidence is uncertain, and the disagreement among scientists is a natural part of the scientific process. I believe that the safest response is to cut emissions and minimize the risk of extreme weather conditions, while also closely monitoring the ocean to detect any changes in the AMOC.
