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, helping shape the climate and weather we experience. However, scientists are concerned that the AMOC is under pressure and may weaken as the planet warms, potentially leading to more extreme swings in the UK's weather, including colder winters, even as average global temperatures continue to rise.
The AMOC includes the Gulf Stream and helps explain why Britain and north-west Europe are milder than their latitude would suggest. It 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, such as 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.
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. But if the AMOC weakens significantly, it could shift storm tracks, alter rainfall, 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, affecting harvests, water supplies, and the livelihoods of hundreds of millions of people. Scientists are closely monitoring the ocean to detect any changes. Historical evidence suggests that the AMOC can change rapidly, as seen during the Younger Dryas, a sudden cooling period that lasted for over a thousand years.
The AMOC's stability is influenced by the density of seawater, which is affected by temperature and salinity. As warm, salty surface water travels north through the Atlantic, evaporation makes it saltier, and further north, it cools, making the water dense enough to sink into the deep ocean. This process is crucial for the AMOC's overturning flow.
Prof. Stefan Rahmstorf, a leading expert on the AMOC, warns that the system could reach a tipping point where the weakening becomes self-amplifying. Warmer surface water is lighter, and extra rainfall and meltwater from ice make the ocean fresher, which is also lighter. If the water in the high North Atlantic becomes too light, less of it sinks, weakening the AMOC further.
Rahmstorf points to a patch of cold water south of Greenland as a clear sign of AMOC weakening. The region is becoming less salty, and other studies indicate changes in the Gulf Stream and signs that parts of the North Atlantic are being renewed more slowly. However, the evidence is complex, and the future of the system cannot yet be reduced to a simple story of imminent collapse.
The UK Met Office's study concluded that a total collapse this century was unlikely, but even a weaker AMOC could reshape weather patterns across Europe and beyond. Prof. Andrew Watson, another leading ocean scientist, argues that the AMOC's behavior is uncertain and that the system may not simply stop if deep water formation weakens in the North Atlantic. The circulation may reorganize, and warm water may still flow north, even if less of it reaches the places where it now gives up heat and sinks.
The disagreement among scientists highlights the uncertainty surrounding the AMOC's future. The argument is over how close the system is to a threshold and how much weight to give to different types of evidence, including ancient climate records, modern observations, statistical warnings, and complex climate models. The stakes are high, and the science does not offer a neat ending, but it is clear that the AMOC is being stressed by the warming of the planet, and cutting emissions is crucial to reducing the pressure on this vital system.
