Topographic map of the Nordic Seas and subpolar basins with surface currents (solid curves) and deep currents (dashed curves) that form a portion of the Atlantic meridional overturning circulation. Colors of curves indicate approximate temperatures.
The AMOC is composed of a northward flow of warm, less-saline water in the Atlantic's upper layers and a southward, return flow of cold, salty, deep water. These limbs are linked by regions of overturning in the Nordic Seas and the Southern Ocean. Overturning sites are associated with intense exchanges of heat, dissolved oxygen, carbon and other nutrients, and very important for the ocean's ecosystems and its function as a carbon sink.[3][4] Changes in the strength of the AMOC can affect multiple elements of the climate system.[1]: 2238
Climate change may weaken the AMOC through increases in ocean heat content and elevated flows of freshwater from melting ice sheets.[5] Studies using oceanographic reconstructions suggest as of 2015[update], the AMOC is weaker than it was before the Industrial Revolution.[6][7] There is debate over the relative contributions of different factors and it is unclear how much of this weakening is due to climate change or the circulation's natural variability over millennia.[8][9]Climate models predict the AMOC will further weaken during the 21st century;[10]: 19 this weakening would affect average air temperatures over Scandinavia and Great Britain because these regions are warmed by the North Atlantic drift.[11] Weakening of the AMOC would also accelerate sea level rise around North America and reduce primary production in the North Atlantic.[12]
Severe weakening of the AMOC may lead to a collapse of the circulation, which would not be easily reversible and thus constitutes one of the tipping points in the climate system.[13] A collapse would substantially lower the average temperature and amount of rain and snowfall in Europe.[14][15] It may also raise the frequency of extreme weather events and have other severe effects.[16][17] High-quality Earth system models indicate a collapse is unlikely and would only become probable if high levels of warming (≥4 °C (7.2 °F))[14] are sustained long after 2100.[18][19][20] Some paleoceanographic research seems to support this idea.[21][22] Some researchers fear the complex models are too stable[23] and that lower-complexity projections pointing to an earlier collapse are more accurate.[24][25] One of those projections suggests AMOC collapse could happen around 2057[26] but many scientists are skeptical of the projection.[27] Some research also suggests the Southern Ocean overturning circulation may be more prone to collapse than the AMOC.[28][16]
^Cite error: The named reference Schmittner2005 was invoked but never defined (see the help page).
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^ abLenton, T. M.; Armstrong McKay, D.I.; Loriani, S.; Abrams, J.F.; Lade, S.J.; Donges, J.F.; Milkoreit, M.; Powell, T.; Smith, S.R.; Zimm, C.; Buxton, J.E.; Daube, Bruce C.; Krummel, Paul B.; Loh, Zoë; Luijkx, Ingrid T. (2023). The Global Tipping Points Report 2023 (Report). University of Exeter.
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^Liu, Y.; Moore, J. K.; Primeau, F.; Wang, W. L. (22 December 2022). "Reduced CO2 uptake and growing nutrient sequestration from slowing overturning circulation". Nature Climate Change. 13: 83–90. doi:10.1038/s41558-022-01555-7. OSTI2242376. S2CID255028552.