Is the Atlantic's great conveyor belt slowing down?

Almost everyone agrees the Atlantic's overturning circulation will weaken as the planet warms. The sharp argument is about whether it could abruptly collapse, and nobody can yet say when, or whether, that would happen this century.

First, what it is. The Atlantic meridional overturning circulation, or AMOC, is a northward flow of warm, more saline water in the Atlantic's upper layers and a southward return flow of cold, less salty deep water. Atlantic surface water is salty partly because moisture evaporated from the Atlantic is carried away by the atmosphere before it can fall back as rain. That salty water becomes dense and sinks to form the North Atlantic Deep Water, which keeps the loop turning.

Climate change attacks both ingredients of that sinking. It warms the surface water and freshens it, with meltwater, mainly from Greenland, and extra rain, which makes the sinking harder. In the 1960s Henry Stommel's simple model raised an unsettling idea: the AMOC could exist in a strong state or collapse to a weaker one, and might not recover unless the warming or freshening that caused the collapse is reduced.

Whether it has already slowed is itself argued over. Direct measurements exist only since 2004, from the RAPID mooring array at 26°N. A February 2021 study combining them with reconstructed earlier data found no evidence of an overall decline over the past 30 years, while a November 2024 study in Nature Geoscience found a slowdown of 0.46 sverdrups per decade since 1950.

Whether the real ocean is close to Stommel's edge is disputed even more. The IPCC's Sixth Assessment Report synthesis said the AMOC is very likely to weaken over the 21st century in all scenarios, but that an abrupt collapse is not expected before 2100 (medium confidence). In October 2024, 44 climate scientists signed an open letter claiming the risk of collapse has been greatly underestimated. In February 2025, a Nature study across 34 climate models concluded a collapse is unlikely in the 21st century. A collapse would substantially lower the average temperature, and the amount of rain and snowfall, in Europe.