Atlantic meridional overturning circulation

"AMOC" redirects here. For other uses, see AMOC (disambiguation).

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 Atlantic meridional overturning circulation (AMOC) is the main ocean current system in the Atlantic Ocean.[1]: 2238  It is a component of Earth's ocean circulation system and plays an important role in the climate system. The AMOC includes Atlantic currents at the surface and at great depths that are driven by changes in weather, temperature and salinity. Those currents comprise half of the global thermohaline circulation that includes the flow of major ocean currents, the other half being the Southern Ocean overturning circulation.[2]

The AMOC is composed of a northward flow of warm, more saline water in the Atlantic's upper layers and a southward, return flow of cold, salty, deep water. Warm water from the south is more saline ('halocline') because of the higher evaporation rate in the tropical zone. The warm saline water forms the upper layer of the ocean ('thermocline'), but when this layer cools down, the density of the salty water increases, making it sink into the deep. This is an important part of the motor of the AMOC system. The 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, 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, Great Britain and Ireland because these regions are warmed by the North Atlantic Current.[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] In October 2024, 44 climate scientists published an open letter to the Nordic Council of Ministers, claiming that according to scientific studies in the past few years, the risk of AMOC collapse has been greatly underestimated, it can occur in the next few decades, while some changes are already happening. It would have devastating and irreversible impacts especially for Nordic countries, but also for other parts of the world. They called on Nordic countries to ensure the implementation of the Paris Agreement to prevent it.[29][30]

  1. ^ a b IPCC, 2021: Annex VII: Glossary [Matthews, J.B.R., V. Möller, R. van Diemen, J.S. Fuglestvedt, V. Masson-Delmotte, C. Méndez, S. Semenov, A. Reisinger (eds.)]. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 2215–2256, doi:10.1017/9781009157896.022.
  2. ^ "NOAA Scientists Detect a Reshaping of the Meridional Overturning Circulation in the Southern Ocean". NOAA. 29 March 2023.
  3. ^ Buckley, Martha W.; Marshall, John (2016). "Observations, inferences, and mechanisms of the Atlantic Meridional Overturning Circulation: A review". Reviews of Geophysics. 54 (1): 5–63. Bibcode:2016RvGeo..54....5B. doi:10.1002/2015RG000493. hdl:1721.1/108249. ISSN 8755-1209. S2CID 54013534.
  4. ^ Lozier, M. S.; Li, F.; Bacon, S.; Bahr, F.; Bower, A. S.; Cunningham, S. A.; de Jong, M. F.; de Steur, L.; deYoung, B.; Fischer, J.; Gary, S. F. (2019). "A sea change in our view of overturning in the subpolar North Atlantic". Science. 363 (6426): 516–521. Bibcode:2019Sci...363..516L. doi:10.1126/science.aau6592. ISSN 0036-8075. PMID 30705189. S2CID 59567598.
  5. ^ "Historic iceberg surges offer insights on modern climate change". The Current. 30 May 2024. Retrieved 30 May 2024.
  6. ^ Cite error: The named reference Rahmstorf2015 was invoked but never defined (see the help page).
  7. ^ Caesar, L.; McCarthy, G.D.; Thornalley, D. J. R.; Cahill, N.; Rahmstorf, S. (25 February 2021). "Current Atlantic Meridional Overturning Circulation weakest in last millennium" (PDF). Nature Geoscience. 14 (3): 118–120. Bibcode:2021NatGe..14..118C. doi:10.1038/s41561-021-00699-z. S2CID 232052381.
  8. ^ Latif, Mojib; Sun, Jing; Visbeck, Martin; Bordbar (25 April 2022). "Natural variability has dominated Atlantic Meridional Overturning Circulation since 1900". Nature Climate Change. 12 (5): 455–460. Bibcode:2022NatCC..12..455L. doi:10.1038/s41558-022-01342-4. S2CID 248385988.
  9. ^ Kilbourne, Kelly Halimeda; et, al. (17 February 2022). "Atlantic circulation change still uncertain". Nature Geoscience. 15 (3): 165–167. Bibcode:2022NatGe..15..165K. doi:10.1038/s41561-022-00896-4. hdl:2117/363518. S2CID 246901665.
  10. ^ IPCC, 2019: Summary for Policymakers. In: IPCC Special Report on the Ocean and Cryosphere in a Changing Climate [H.-O. Pörtner, D.C. Roberts, V. Masson-Delmotte, P. Zhai, M. Tignor, E. Poloczanska, K. Mintenbeck, A. Alegría, M. Nicolai, A. Okem, J. Petzold, B. Rama, N.M. Weyer (eds.)]. Cambridge University Press, Cambridge, UK and New York, NY, USA. doi:10.1017/9781009157964.001.
  11. ^ Lenton, T. M.; Held, H.; Kriegler, E.; Hall, J. W.; Lucht, W.; Rahmstorf, S.; Schellnhuber, H. J. (2008). "Inaugural Article: Tipping elements in the Earth's climate system". Proceedings of the National Academy of Sciences. 105 (6): 1786–1793. Bibcode:2008PNAS..105.1786L. doi:10.1073/pnas.0705414105. PMC 2538841. PMID 18258748.
  12. ^ Cite error: The named reference Schmittner2005 was invoked but never defined (see the help page).
  13. ^ Cite error: The named reference CarbonBrief was invoked but never defined (see the help page).
  14. ^ a b Cite error: The named reference ArmstrongMcKay2022 was invoked but never defined (see the help page).
  15. ^ Cite error: The named reference Phys2020 was invoked but never defined (see the help page).
  16. ^ a b Lenton, 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.
  17. ^ Cite error: The named reference Hansen2015 was invoked but never defined (see the help page).
  18. ^ Cite error: The named reference Liu2017 was invoked but never defined (see the help page).
  19. ^ Cite error: The named reference Bakker2016 was invoked but never defined (see the help page).
  20. ^ Cite error: The named reference Sigmond2020 was invoked but never defined (see the help page).
  21. ^ Cite error: The named reference He2022 was invoked but never defined (see the help page).
  22. ^ Cite error: The named reference Kim2022 was invoked but never defined (see the help page).
  23. ^ Cite error: The named reference Valdes2011 was invoked but never defined (see the help page).
  24. ^ Cite error: The named reference Lohmann2021 was invoked but never defined (see the help page).
  25. ^ Cite error: The named reference Boers2021 was invoked but never defined (see the help page).
  26. ^ Cite error: The named reference Ditlevsen2023 was invoked but never defined (see the help page).
  27. ^ Cite error: The named reference SMC2023 was invoked but never defined (see the help page).
  28. ^ 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. OSTI 2242376. S2CID 255028552.
  29. ^ "Open Letter by Climate Scientists to the Nordic Council of Ministers" (PDF). Retrieved 31 October 2024.
  30. ^ Pare, Sascha. "Key Atlantic current could collapse soon, 'impacting the entire world for centuries to come,' leading climate scientists war". Live Science. Retrieved 31 October 2024.