Atlantic meridional overturning circulation

The Atlantic meridional overturning circulation (AMOC) is the "main current system in the South and North Atlantic Oceans".^: 2238 As such, it is a component of Earth's oceanic circulation system and plays an important role in the climate system. The AMOC includes currents at the surface as well as at great depths in the Atlantic Ocean. These currents are driven by changes in the atmospheric weather as well as by changes in temperature and salinity. They collectively make up one half of the global thermohaline circulation that encompasses the flow of major ocean currents. The other half is the Southern Ocean overturning circulation.

The AMOC is characterized by a northward flow of warmer, fresher water in the upper layers of the Atlantic, and a southward flow of colder, saltier and deeper waters. These limbs are linked by regions of overturning in the Nordic Seas and the Southern Ocean. Overturning sites are associated with the intense exchange of heat, dissolved oxygen, carbon and other nutrients. They are very important for the ocean's ecosystems and for its functioning as a carbon sink. Thus, if the strength of the AMOC changes, multiple elements of the climate system would be affected.^: 2238

Climate change has the potential to weaken the AMOC through increases in ocean heat content and elevated freshwater flows from the melting ice sheets. Studies using oceanographic reconstructions suggest that the AMOC is now already weaker than it was before the Industrial Revolution. However, there is debate over the relative contributions of different factors. It is unclear how much of it is due to either climate change or due to the circulation's natural variability over hundreds or thousands of years. Climate models predict that the AMOC will weaken further over the 21st century.^: 19 This would affect average temperature over Scandinavia and Great Britain because these regions are warmed by the North Atlantic drift. Weakening of the AMOC would also accelerate sea level rise around North America and reduce primary production in the North Atlantic.

Severe weakening of the AMOC may lead to an outright collapse of the circulation, which would not be easily reversible and thus constitute one of the tipping points in the climate system. A collapse would substantially lower the average temperature and amount of rain and snowfall in Europe. It would also potentially raise the frequency of extreme weather events and have other severe effects. Gold-standard Earth system models indicate that a collapse is unlikely, and would only become plausible if high levels of warming are sustained well after the year 2100. Some paleoceanographic research seems to support this idea. However, some researchers fear that the complex models are too stable, and lower-complexity projections pointing to an earlier collapse are more accurate. One of those projections suggests that AMOC collapse could happen around 2057, but many scientists are skeptical of the claim. Some research also suggests that the Southern Ocean overturning circulation may be more prone to collapse.