The Atlantic Meridional Overturning Circulation (AMOC) is a vast system of ocean currents that influences the climate far beyond the Atlantic. It carries warm surface water towards the Arctic and returns colder water thousands of miles south through the deep ocean. The AMOC is under pressure due to global warming, and scientists predict it will weaken as the planet warms. This could lead 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, roughly 50 times the total energy humanity uses. Some researchers say the warning signs are already visible: a strange patch of cooling in the North Atlantic and changes in how salty the water is.
The AMOC could reach a tipping point, where the weakening could begin to feed on itself. 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, and the AMOC weakens. This could lead to a self-sustaining system, where the current helps keep the North Atlantic salty enough for the current to keep running.
The AMOC has shifted before, and its behavior has changed in the past. The AMOC is part of the same planetary heat-moving system that drives weather itself. If deep water formation weakens in the North Atlantic, the system may not simply stop. The sinking may shift, and the circulation may reorganize. Warm water may still flow north, even if less of it reaches the places where it now gives up heat and sinks.
The AMOC depends on deep water forming in a handful of places in the North Atlantic, including the Labrador Sea, the Greenland Sea, and waters south of Iceland. Climate models cannot resolve the detail directly, and they have to approximate it. The AMOC is being stressed by the warming of the planet, and the more greenhouse gases humanity releases, the greater that stress becomes.
Cutting emissions does not remove every uncertainty, but it reduces the pressure on a system no society can afford to push too far. 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 safest response is already clear: act on the uncertainty and reduce greenhouse gas emissions.
