The Atlantic Meridional Overturning Circulation (AMOC) is a large system of ocean currents that carry warm water from the tropics northwards into the North Atlantic, and colder water in the deeper ocean southwards toward the tropics. It acts like a conveyor belt, driven by differences in temperature and the water’s density (in part caused by changes in the salt content). As warm water flows northwards on the surface (the Gulf Stream), it cools and some evaporation occurs, which increases the concentration of salt. Eventually, this colder, deeper water gets pulled back to the surface to replace the warm water flowing northwards, in a process called “upwelling” and the circulation is complete. This global process makes sure that the world’s oceans are continually mixed and oxygenated, and that heat and energy are distributed around the planet1.
The RAPID is a series of arrays of moorings which have been in operation for over 20 years. It is designed to measure the variability of the AMOC2. This RAPID system has observed an AMOC weakening of about 1.0 Sverdrup* per decade from 2004 to 20233, although the variation is large. Despite this variation, the trend can be clearly seen in published data2,3. While the values obtained from RAPID have not reached ‘unfamiliar’ values, they are consistent with the predicted weakening of AMOC from climate modelling. This suggests that ‘unfamiliar’ values would appear by 2040, and that previously ‘unknown’ values would appear by 20603.
A recent study has suggested that the weakening of the AMOC can be explained by the meltwater from the Greenland Ice Sheet and Canadian glaciers, and indicates that the AMOC is likely to continue to weaken. Greenland has lost 5,900 billion tonnes of ice in the last 24 years. That amount is equivalent to the whole of the state of New South Wales covered in ice 8 metres thick4,5.
This fresh meltwater flowing into the subarctic ocean is less dense than salty seawater, so less cooling waters descend into the ocean depths. This reduces the southward flow of deep, cold waters in the Atlantic, and thereby weakening the Gulf Stream, which is the main northward flow of warm waters at the surface4.
Recent climate modelling suggests that the AMOC will weaken by another 30% by 2060. However these estimates do not take into account the meltwater flowing into the subarctic ocean. If this meltwater is added to the climate projections, that 30% weakening could happen in the next 14 years, i.e. by 20404. This will disrupt the current climatic system, leading to harsher winters in Europe, and drier conditions in the northern tropics. In the southern hemisphere summers could be hotter and wetter4.
What would the collapse of the AMOC, unlikely though it may be, mean for the planet? It would be terrifying.
*Sverdrup: This is a non-SI unit, named to honour Norwegian oceanographer and meteorologist Harald Ulrik Sverdrup (1888-1957), and is used for quantifying large-scale volume transport by oceanographers, meteorologists, and atmospheric physicists. In this case it refers to the volumetric rate of transport of water in ocean currents. Its symbol is Sv and 1.0 Sv is equivalent to volume transport of one million cubic metres per second6.
Sources
- https://www.blotreport.com/2022/11/19/running-amoc/
- https://rapid.ac.uk/#:~:text=The%20Atlantic%20meridional%20overturning%20circulation,from%201.2%20million%20power%20plants.
- https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2025GL115055
- https://www.unsw.edu.au/newsroom/news/2024/11/meltwater-greenland-arctic-weakening-ocean-circulation
- https://www.nature.com/articles/s41561-024-01568-1
- https://ui.adsabs.harvard.edu/abs/2018PhTea..56..377A/abstract