Short answers, pulled from the story.
The Atlantic meridional overturning circulation is the main current system in the Atlantic Ocean, carrying warm, salty water north near the surface and cold, deep water back south. It forms half of the global thermohaline circulation, with the Southern Ocean overturning circulation making up the other half. It plays a major role in Earth's climate system.
The Atlantic meridional overturning circulation carries up to 25 percent of the total heat moving into the northern hemisphere. This heat, delivered partly through the North Atlantic Current, keeps northern and western Europe between 4 and 10 degrees Celsius warmer than it would otherwise be.
Willi Dansgaard and Hans Oeschger identified 25 abrupt temperature oscillations, now called Dansgaard-Oeschger events, by analyzing Greenland ice cores in the 1980s. These swings occurred during the Late Pleistocene, between about 126,000 and 11,700 years ago.
One statistical analysis of lower-complexity climate models projects a possible collapse around 2065, an estimate revised from 2057 in August 2025, with 95 percent confidence of collapse between 2037 and 2109. The IPCC's 2021 assessment held only medium confidence that a full collapse would be avoided before 2100, and many scientists remain skeptical of the 2065 projection.
A collapse of the Atlantic meridional overturning circulation would trigger sharp cooling across Britain, Ireland, France, and the Nordic countries, lower rainfall and reduce farmland in Great Britain, and intensify winter storms. It could also shift the El Nino-Southern Oscillation toward dominant La Nina conditions and cut North Atlantic plankton populations to less than half their normal levels.
The oceanographer Henry Stommel modeled the Atlantic meridional overturning circulation in the 1960s using what became known as the Stommel Box model. His work introduced the Stommel bifurcation, the idea that the circulation could exist in a strong state or collapse into a much weaker one that would not easily recover.