View source for Atlantic meridional overturning circulation

{{Short description|System of surface and deep currents in the Atlantic Ocean}}
{{Redirect|AMOC}}
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{{Use dmy dates|date=April 2022}}
[[File:OCP07 Fig-6.jpg|thumb|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]].<ref name="IPCC_AR6_AnnexVII">IPCC, 2021: [https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_AnnexVII.pdf Annex VII: Glossary] {{Webarchive|url=https://web.archive.org/web/20220605175306/https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_AnnexVII.pdf |date=5 June 2022 }} [Matthews, J.B.R., V. Möller, R. van Diemen, J.S. Fuglestvedt, V. Masson-Delmotte, C.  Méndez, S. Semenov, A. Reisinger (eds.)]. In [https://www.ipcc.ch/report/ar6/wg1/ Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change] {{Webarchive|url=https://web.archive.org/web/20230526182346/https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter09.pdf |date=26 May 2023 }} [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.</ref>{{rp|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]].<ref name="NOAA2023">{{cite web |date=29 March 2023 |title=NOAA Scientists Detect a Reshaping of the Meridional Overturning Circulation in the Southern Ocean |url=https://www.aoml.noaa.gov/noaa-scientists-detect-reshaping-of-the-meridional-overturning-circulation-in-southern-ocean/ |publisher=[[NOAA]] |access-date=28 January 2024 |archive-date=28 January 2024 |archive-url=https://web.archive.org/web/20240128145728/https://www.aoml.noaa.gov/noaa-scientists-detect-reshaping-of-the-meridional-overturning-circulation-in-southern-ocean/ |url-status=live }}</ref>

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]].<ref name="Buckley2016">{{Cite journal |last1=Buckley |first1=Martha W. |last2=Marshall |first2=John |date=2016 |title=Observations, inferences, and mechanisms of the Atlantic Meridional Overturning Circulation: A review |journal=Reviews of Geophysics |language=en |volume=54 |issue=1 |pages=5–63 |doi=10.1002/2015RG000493 |bibcode=2016RvGeo..54....5B |s2cid=54013534 |issn=8755-1209|doi-access=free |hdl=1721.1/108249 |hdl-access=free }}</ref><ref name="Lozier2019">{{Cite journal |last1=Lozier |first1=M. S. |last2=Li |first2=F. |last3=Bacon |first3=S. |last4=Bahr |first4=F. |last5=Bower |first5=A. S. |last6=Cunningham |first6=S. A. |last7=de Jong |first7=M. F. |last8=de Steur |first8=L. |last9=deYoung |first9=B. |last10=Fischer |first10=J. |last11=Gary |first11=S. F. |date=2019 |title=A sea change in our view of overturning in the subpolar North Atlantic |journal=Science |language=en |volume=363 |issue=6426 |pages=516–521 |doi=10.1126/science.aau6592 |pmid=30705189 |bibcode=2019Sci...363..516L |s2cid=59567598 |issn=0036-8075|doi-access=free }}</ref> Changes in the strength of the AMOC can affect multiple elements of the climate system.<ref name="IPCC_AR6_AnnexVII" />{{rp|2238}}

[[Climate change]] may weaken the AMOC through increases in [[ocean heat content]] and elevated flows of freshwater from melting [[ice sheet]]s.<ref>{{Cite web |date=2024-05-30 |title=Historic iceberg surges offer insights on modern climate change |url=https://news.ucsb.edu/2024/021493/historic-iceberg-surges-offer-insights-modern-climate-change |access-date=2024-05-30 |website=The Current |language=en |archive-date=30 May 2024 |archive-url=https://web.archive.org/web/20240530213828/https://news.ucsb.edu/2024/021493/historic-iceberg-surges-offer-insights-modern-climate-change |url-status=live }}</ref> Studies using oceanographic reconstructions suggest that {{as of|2015|alt=as of 2015}}, the AMOC was weaker than before the [[Industrial Revolution]].<ref name="Rahmstorf2015" /><ref name="Caesar2021">{{Cite journal |last1=Caesar |first1=L. |last2=McCarthy |first2=G.D. |last3=Thornalley |first3=D. J. R. |last4=Cahill |first4=N. |last5=Rahmstorf |first5=S. |date=25 February 2021 |title=Current Atlantic Meridional Overturning Circulation weakest in last millennium |journal=Nature Geoscience |volume=14 |issue=3 |pages=118–120 |bibcode=2021NatGe..14..118C |doi=10.1038/s41561-021-00699-z |s2cid=232052381 |url=https://mural.maynoothuniversity.ie/16046/1/LS_current.pdf }}</ref> 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.<ref name="Latif2022">{{Cite journal |last1=Latif |first1=Mojib |last2=Sun |first2=Jing |last3=Visbeck |first3=Martin |last4=Bordbar |date=25 April 2022 |title=Natural variability has dominated Atlantic Meridional Overturning Circulation since 1900 |journal=Nature Climate Change |volume=12 |issue=5 |pages=455–460 |bibcode=2022NatCC..12..455L |doi=10.1038/s41558-022-01342-4 |s2cid=248385988 |doi-access=free}}</ref><ref name="Kilbourne2022">{{Cite journal |last1=Kilbourne |first1=Kelly Halimeda |last2=et |first2=al. |date=17 February 2022 |title=Atlantic circulation change still uncertain |journal=Nature Geoscience |volume=15 |issue=3 |pages=165–167 |bibcode=2022NatGe..15..165K |doi=10.1038/s41561-022-00896-4 |s2cid=246901665 |hdl=2117/363518 |url=https://dro.dur.ac.uk/35682/ |hdl-access=free }}</ref> [[Climate model]]s predict the AMOC will further weaken during the 21st century.<ref name="SROCC_SPM_20190925">IPCC, 2019: [https://www.ipcc.ch/site/assets/uploads/sites/3/2022/03/01_SROCC_SPM_FINAL.pdf Summary for Policymakers] {{Webarchive|url=https://web.archive.org/web/20221018131710/https://www.ipcc.ch/site/assets/uploads/sites/3/2022/03/01_SROCC_SPM_FINAL.pdf |date=18 October 2022 }}. In: [https://www.ipcc.ch/srocc/ IPCC Special Report on the Ocean and Cryosphere in a Changing Climate] {{Webarchive|url=https://web.archive.org/web/20210712081923/https://www.ipcc.ch/srocc/ |date=12 July 2021 }} [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}}.</ref>{{rp|19}} This weakening would reduce average air temperatures over [[Scandinavia]], [[Great Britain]], and [[Ireland]], because these regions are warmed by the [[North Atlantic Current]].<ref name="Lenton2008">{{cite journal |last1=Lenton |first1=T. M. |last2=Held |first2=H. |last3=Kriegler |first3=E. |last4=Hall |first4=J. W. |last5=Lucht |first5=W. |last6=Rahmstorf |first6=S. |last7=Schellnhuber |first7=H. J. |year=2008 |title=Inaugural Article: Tipping elements in the Earth's climate system |journal=Proceedings of the National Academy of Sciences |volume=105 |issue=6 |pages=1786–1793 |bibcode=2008PNAS..105.1786L |doi=10.1073/pnas.0705414105 |pmc=2538841 |pmid=18258748 |doi-access=free}}</ref> Weakening of the AMOC would also accelerate [[sea level rise]] around North America and reduce [[primary production]] in the North Atlantic.<ref name="Schmittner2005" /> 

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]].<ref name="CarbonBrief" /> A collapse would substantially lower the average temperature and amount of rain and snowfall in Europe.<ref name="ArmstrongMcKay2022" /><ref name="Phys2020" /> It may also raise the frequency of extreme weather events and have other severe effects.<ref name="GTPR2023" /><ref name="Hansen2015" />

==Overall structure==

[[File:thermohaline_circulation.svg|lang=am|thumb|link={{filepath:thermohaline_circulation.svg}}|AMOC in relation to the global thermohaline circulation [{{filepath:thermohaline_circulation.svg}} (animation)] ]]

The Atlantic meridional overturning circulation (AMOC) is the main current system in the Atlantic Ocean<ref name="IPCC_AR6_AnnexVII" />{{rp|2238}} and is also part of the global [[thermohaline circulation]], which connects the world's oceans with a single "conveyor belt" of continuous water exchange.<ref name="Broecker1991" /> Normally, relatively warm, less-saline water stays on the ocean's surface while deep layers are colder, denser and more-saline, in what is known as [[ocean stratification]].<ref>{{cite journal |last1=Yamaguchi |first1=Ryohei |last2=Suga |first2=Toshio |date=12 December 2019 |title=Trend and Variability in Global Upper-Ocean Stratification Since the 1960s |journal=[[Journal of Geophysical Research: Oceans]] |volume=124 |issue=12 |pages=8933–8948 |doi=10.1029/2019JC015439 |bibcode=2019JGRC..124.8933Y }}</ref> Deep water eventually gains heat and/or loses salinity in an exchange with the mixed ocean layer, and becomes less dense and rises towards the surface. Differences in temperature and salinity exist between ocean layers and between parts of the [[World Ocean]], and together they drive the thermohaline circulation.<ref name="Broecker1991" /> The [[Pacific Ocean]] is less saline than the other oceans because it receives large quantities of fresh rainfall.<ref>{{Cite journal |last1=Craig |first1=Philip M. |last2=Ferreira |first2=David |last3=Methven |first3=John |date=8 June 2017 |title=The contrast between Atlantic and Pacific surface water fluxes |journal=Tellus A: Dynamic Meteorology and Oceanography |volume=69 |issue=1 |page=1330454 |doi=10.1080/16000870.2017.1330454 |bibcode=2017TellA..6930454C |doi-access=free }}</ref> Its surface water is insufficiently saline to sink lower than several hundred meters, meaning deep ocean water must come from elsewhere.<ref name="Broecker1991">{{cite journal |last1=Broecker |first1=Wallace |title=The great ocean conveyor |journal=Oceanography |date=1991 |volume=4 |issue=2 |pages=79–89 |url=https://tos.org/oceanography/assets/docs/4-2_broecker.pdf |doi=10.5670/oceanog.1991.07 |doi-access=free}}</ref>

Ocean water in the North Atlantic is more saline than that in the Pacific, partly because extensive evaporation on the surface concentrates salt within the remaining water and partly because [[sea ice]] near the [[Arctic Circle]] expels salt as it freezes during winter.<ref>{{cite web|url=https://nsidc.org/cryosphere/seaice/characteristics/brine_salinity.html|title=Salinity and Brine|publisher=NSIDC|access-date=12 April 2022|archive-date=18 December 2009|archive-url=https://web.archive.org/web/20091218093622/http://nsidc.org/seaice/characteristics/brine_salinity.html|url-status=live}}</ref> Even more importantly, evaporated moisture in the Atlantic is swiftly carried away by atmospheric circulation before it can fall back as rain. [[Trade winds]] move this moisture across Central America and to the eastern North Pacific, where it falls as rain.<ref>{{Cite journal |last1=Wang |first1=Chunzai |last2=Zhang |first2=Liping |last3=Lee |first3=Sang-Ki |date=15 February 2013 |title=Response of Freshwater Flux and Sea Surface Salinity to Variability of the Atlantic Warm Pool |journal=Journal of Climate |volume=26 |issue=4 |pages=1249–1267 |doi=10.1175/JCLI-D-12-00284.1 |doi-access=free |bibcode=2013JCli...26.1249W }}</ref> Major mountain ranges such as the [[Tibetan Plateau]], the [[Rocky Mountains]] and the Andes prevent any equivalent moisture transport back to the Atlantic.<ref>{{Cite journal |last1=Yang |first1=Haijun |last2=Jiang |first2=Rui |last3=Wen |first3=Qin |last4=Liu |first4=Yimin |last5=Wu |first5=Guoxiong |last6=Huang |first6=Jiangping |date=23 March 2024 |title=The role of mountains in shaping the global meridional overturning circulation |journal=Nature Communications |volume=15 |issue=1 |page=2602 |doi=10.1038/s41467-024-46856-x |pmid=38521775 |pmc=10960852 |bibcode=2024NatCo..15.2602Y }}</ref>

Due to this process, Atlantic surface water becomes salty and therefore dense, eventually [[downwelling]] to form the [[North Atlantic Deep Water]] (NADW).<ref name="Marshall2012" /> NADW formation primarily occurs in the [[Nordic Seas]] and involves a complex interplay of regional water masses such as the [[Denmark Strait]] Overflow Water (DSOW), Iceland-Scotland Overflow Water (ISOW) and Nordic Seas Overflow Water.<ref>{{Cite journal |last1=Rhein |first1=Monika |last2=Kieke |first2=Dagmar |last3=Hüttl-Kabus |first3=Sabine |last4=Roessler |first4=Achim| last5=Mertens |first5=Christian |last6=Meissner |first6=Robert |last7=Klein |first7=Birgit |last8=Böning |first8=Claus W. |last9=Yashayaev |first9=Igor |date=10 January 2009 |title=Deep water formation, the subpolar gyre, and the meridional overturning circulation in the subpolar North Atlantic |journal=Deep Sea Research Part II: Topical Studies in Oceanography |volume=58 |issue=17–18 |pages=1819–1832 |doi=10.1029/2008GL036162 |bibcode=2009GeoRL..36.1606Y |s2cid=56353963 |doi-access=free }}</ref> [[Labrador Sea Water]] may play an important role as well but increasing evidence suggests water in Labrador and [[Irminger Sea]]s primarily recirculates through the [[North Atlantic Gyre]] and has little connection with the rest of the AMOC.<ref name="Lozier2019" /><ref name="AR6_WG1_Chapter92" /><ref name="ArmstrongMcKay2022" />

[[File:Thermohaline Circulation 2.png|thumb|A summary of the path of the thermohaline circulation. Blue paths represent deep-water currents, while red paths represent surface currents]]

The NADW is not the deepest water layer in the Atlantic Ocean; the [[Antarctic bottom water]] (AABW) is always the densest, deepest ocean layer in any basin deeper than {{convert|4000|m|mi|abbr=out}}.<ref>{{Cite web|title=AMS Glossary of Meteorology, Antarctic Bottom Water|url=https://glossary.ametsoc.org/wiki/Antarctic_bottom_water|publisher=American Meteorological Society|access-date=29 June 2023}}</ref> As the upper reaches of the AABW flow [[upwelling|upwells]], it melds into and reinforces the NADW. The formation of the NADW is also the beginning of the ''lower cell'' of the circulation.<ref name="Broecker1991" /><ref name="Buckley2016" /> The downwelling that forms the NADW is balanced by an equal amount of upwelling. In the western Atlantic, [[Ekman transport]], the increase in ocean-layer mixing caused by wind activity, results in strong upwelling in the [[Canary Current]] and the [[Benguela Current]], which are located on the northwest and southwest coasts of Africa. {{As of|2014}}, upwelling is substantially stronger around the Canary Current than the Benguela Current, though an opposite pattern existed until the closure of the [[Central American Seaway]] during the late [[Pliocene]].<ref>{{Cite journal |last1=Prange |first1=M. |last2=Schulz |first2=M. |date=3 September 2004 |title=A coastal upwelling seesaw in the Atlantic Ocean as a result of the closure of the Central American Seaway |journal=Geophysical Research Letters |volume=31 |issue=17 |doi=10.1029/2007GL030285 |bibcode=2007GeoRL..3413614B |s2cid=13857911 |doi-access=free }}</ref> In the Eastern Atlantic, significant upwelling occurs only during certain months of the year because this region's deep [[thermocline]] means it is more dependent on the state of [[sea surface temperature]] than on wind activity. There is also a multi-year upwelling cycle that occurs in synchronization with the [[El Niño]]/La Niña cycle.<ref>{{Cite journal |last1=Wang |first1=Li-Chiao |last2=Fei-Fei |first2=Jing |last3=Wu |first3=Chau-Ron |last4=Hsu |first4=Huang-Hsiung |date=2 March 2017 |title=Dynamics of upwelling annual cycle in the equatorial Atlantic Ocean |journal=Geophysical Research Letters |volume=44 |issue=8 |pages=3737–3743 |doi=10.1002/2017GL072588 |bibcode=2017GeoRL..44.3737W |s2cid=132601314 |doi-access=free }}</ref>

At the same time, the NADW moves southward and at the southern end of the Atlantic transect, around 80% of it upwells in the Southern Ocean,<ref name="Marshall2012" /><ref>{{Cite journal |last1=Talley |first1=Lynne D. |date=October 2, 2015 |title=Closure of the global overturning circulation through the Indian, Pacific, and Southern Oceans: Schematics and transports |journal=Oceanography |volume=26 |issue=1 |pages=80–97 |doi=10.5670/oceanog.2013.07 |doi-access=free }}</ref> connecting it with the [[Southern Ocean overturning circulation]] (SOOC).<ref name="Morrison2015">{{Cite journal |last1=Morrison |first1=Adele K. |last2=Frölicher |first2=Thomas L. |last3=Sarmiento |first3=Jorge L. |date=January 2015 |title=Upwelling in the Southern Ocean |journal=Physics Today |volume=68 |issue=1 |pages=27 |doi=10.1063/PT.3.2654  |bibcode=2015PhT....68a..27M |doi-access=free }}</ref> After upwelling, the water is understood to take one of two pathways. Water surfacing close to Antarctica will likely be cooled by [[Antarctic sea ice]] and sink back into the lower cell of the circulation. Some of this water will rejoin the AABW but the rest of the lower-cell flow will eventually reach the depths of the Pacific and Indian oceans.<ref name="Broecker1991" /> Water that upwells at lower, ice-free latitudes moves further northward due to [[Ekman transport]] and is committed to the ''upper cell''. The warm water in the upper cell is responsible for the return flow to the North Atlantic, which occurs mainly around the coast of Africa{{clarify|reason=Africa has several coasts; which one(s) is/are relevant here?|date=June 2024}} and through the [[Indonesian archipelago]]. Once this water returns to the North Atlantic, it becomes cooler and denser, and sinks, feeding back into the NADW.<ref name="Morrison2015" /><ref name="Marshall2012">{{Cite journal |last1=Marshall |first1=John |last2=Speer |first2=Kevin |date=26 February 2012 |title=Closure of the meridional overturning circulation through Southern Ocean upwelling |journal=Nature Geoscience |volume=5 |issue=3 |pages=171–180 |doi=10.1038/ngeo1391 |bibcode=2012NatGe...5..171M }}</ref>

==Role in the climate system==

[[File:Wang 2022 strong AMOC role.png|thumb|Heat transfer from the ocean to atmosphere (left) and an increase in Atlantic Ocean heat content (right) observed when the AMOC is strong<ref name="Wang2022" />]]

Equatorial areas are the hottest part of the globe; due to [[thermodynamics]], this heat moves towards the [[Geographical pole|poles]]. Most of this heat is transported by [[atmospheric circulation]] but warm, surface [[ocean current]]s play an important role. Heat from the equator moves either northward or southward; the Atlantic Ocean is the only ocean in which the heat flow is northward.<ref name="Bryden2001" /> Much of the heat transfer in the Atlantic occurs due to the [[Gulf Stream]], a surface current that carries warm water northward from the [[Caribbean]]. While the Gulf Stream as a whole is driven by winds alone, its northern-most segment, the [[North Atlantic Current]], obtains much of its heat from thermohaline exchange in the AMOC.<ref name="Buckley2016" /> Thus, the AMOC carries up to 25% of the total heat toward the northern hemisphere,<ref name="Bryden2001">{{Cite journal |last1=Bryden |first1=Harry L. |last2=Imawaki |first2=Shiro |date=2001 |title=Ocean heat transport |journal=International Geophysics |volume=77 |pages=455–474 |doi=10.1016/S0074-6142(01)80134-0 }}</ref> and plays an important role in the climate around northwest Europe.<ref>{{Cite book |last1=Rhines |first1=Peter |last2=Häkkinen |first2=Sirpa |last3=Josey |first3=Simon A. |chapter=Is Oceanic Heat Transport Significant in the Climate System? |date=2008 |title=Arctic–Subarctic Ocean Fluxes |chapter-url=https://link.springer.com/chapter/10.1007/978-1-4020-6774-7_5 |pages=87–109 |doi=10.1007/978-1-4020-6774-7_5 |isbn=978-1-4020-6773-0 |access-date=3 October 2022 |archive-date=8 October 2022 |archive-url=https://web.archive.org/web/20221008165111/https://link.springer.com/chapter/10.1007/978-1-4020-6774-7_5 |url-status=live }}</ref>

Because atmospheric patterns also play a large role in heat transfer, the idea the climate in northern Europe would be as cold as that in northern North America without heat transport via ocean currents (i.e. up to {{convert|15–20|C-change|F-change}} colder) is generally considered incorrect.<ref name="Seager2002">{{Cite journal|last1=Seager|first1=R.|last2=Battisti|first2=D. S.|last3=Yin|first3=J.|last4=Gordon|first4=N.|last5=Naik|first5=N.|last6=Clement|first6=A. C.|author-link6=Amy C. Clement|last7=Cane|first7=M. A.|title=Is the Gulf Stream responsible for Europe's mild winters?|year=2002|journal=Quarterly Journal of the Royal Meteorological Society|volume=128|issue=586|pages=2563–2586|url=http://www.ldeo.columbia.edu/res/div/ocp/gs/pubs/Seager_etal_QJ_2002.pdf|access-date=25 October 2010|doi=10.1256/qj.01.128|bibcode=2002QJRMS.128.2563S|s2cid=8558921|archive-date=5 March 2012|archive-url=https://web.archive.org/web/20120305203208/http://www.ldeo.columbia.edu/res/div/ocp/gs/pubs/Seager_etal_QJ_2002.pdf|url-status=live}}</ref><ref>{{Cite journal |last1=Seager |first1=Richard |date=2006 |title=The Source of Europe's Mild Climate: The notion that the Gulf Stream is responsible for keeping Europe anomalously warm turns out to be a myth |url=https://www.jstor.org/stable/27858802 |journal=American Scientist |volume=94 |issue=4 |pages=334–341 |doi=10.1511/2006.60.999 |jstor=27858802 |access-date=3 October 2022 |archive-date=3 October 2022 |archive-url=https://web.archive.org/web/20221003231931/https://www.jstor.org/stable/27858802 |url-status=live }}</ref> While one modeling study suggested collapse of the AMOC could result in Ice Age-like cooling, including sea-ice expansion and mass glacier formation, within a century,<ref name="van Westen2024" /><ref name="RealClimate2024" /> the accuracy of those results is questionable.<ref name="SMC2024" /> There is a consensus the AMOC keeps northern and western Europe warmer than it would be otherwise,<ref name="GTPR2023" /> with the difference of {{convert|4|C-change|F-change}} and {{convert|10|C-change|F-change}} depending on the area.<ref name="ArmstrongMcKay2022" /> For instance, studies of the [[Florida Current]] suggest the Gulf Stream was around 10% weaker from around 1200 to 1850 due to increased surface salinity, and this likely contributed to the conditions known as [[Little Ice Age]].<ref>{{cite journal |last1=Lund |first1=DC |last2=Lynch-Stieglitz |first2=J |author-link2=Jean Lynch-Stieglitz |last3=Curry |first3=WB |title=Gulf Stream density structure and transport during the past millennium |journal=Nature |volume=444 |issue=7119 |pages=601–4 |date=November 2006 |pmid=17136090 |doi=10.1038/nature05277 |bibcode=2006Natur.444..601L |s2cid=4431695 |url=https://authors.library.caltech.edu/56282/5/Fig4.pdf |archive-date=23 January 2023 |access-date=12 April 2022 |archive-url=https://web.archive.org/web/20230123111159/https://authors.library.caltech.edu/56282/5/Fig4.pdf |url-status=live }}</ref>

The AMOC makes the Atlantic Ocean into a more-effective [[carbon sink]] in two major ways. Firstly, the upwelling that takes place supplies large quantities of nutrients to the surface waters, supporting the growth of [[phytoplankton]] and therefore increasing [[marine primary production]] and the overall amount of photosynthesis in the surface waters. Secondly, upwelled water has low concentrations of dissolved carbon because the water is typically 1,000 years old and has not been exposed to anthropogenic {{CO2}} increases in the atmosphere. This water absorbs larger quantities of carbon than the more-saturated surface waters and is prevented from releasing carbon back into the atmosphere when it is downwelled.<ref>{{Cite journal |last1=DeVries |first1=Tim |last2=Primeau |first2=François |date=1 Dec 2011 |title=Dynamically and observationally constrained estimates of water-mass distributions and ages in the global ocean |journal=Journal of Physical Oceanography |volume=41 |issue=12 |pages=2381–2401 |doi=10.1175/JPO-D-10-05011.1 |bibcode=2011JPO....41.2381D |s2cid=42020235 |doi-access=free }}</ref> While [[Southern Ocean]] is by far the strongest ocean carbon sink,<ref name="Long2021">{{cite journal |last1=Long |first1=Matthew C. |last2=Stephens |first2=Britton B. |last3=McKain |first3=Kathryn |last4=Sweeney |first4=Colm |last5=Keeling |first5=Ralph F. |last6=Kort |first6=Eric A. |last7=Morgan |first7=Eric J. |last8=Bent |first8=Jonathan D. |last9=Chandra |first9=Naveen |last10=Chevallier |first10=Frederic |last11=Commane |first11=Róisín |last12=Daube |first12=Bruce C. |last13=Krummel |first13=Paul B. |last14=Loh |first14=Zoë |last15=Luijkx |first15=Ingrid T. |last16=Munro |first16=David |last17=Patra |first17=Prabir |last18=Peters |first18=Wouter |last19=Ramonet |first19=Michel |last20=Rödenbeck |first20=Christian |last21=Stavert |first21=Ann |last22=Tans |first22=Pieter |last23=Wofsy |first23=Steven C. |date=2 December 2021 |title=Strong Southern Ocean carbon uptake evident in airborne observations |journal=Science |volume=374 |issue=6572 |pages=1275–1280 |doi=10.1126/science.abi4355 |pmid=34855495 |bibcode=2021Sci...374.1275L |s2cid=244841359 |url=https://research.rug.nl/en/publications/0601766f-9b82-4c61-a48a-c9773dcfc0a4 }}</ref> The North Atlantic is the largest single carbon sink in the northern hemisphere.<ref>{{Cite journal |last1=Gruber |first1=Nicolas |last2=Keeling |first2=Charles D. |last3=Bates |first3=Nicholas R. |date=20 December 2002 |title=Interannual variability in the North Atlantic Ocean carbon sink |journal=Science |volume=298 |issue=5602 |pages=2374–2378 |doi=10.1126/science.1077077  |pmid=12493911 |bibcode=2002Sci...298.2374G |s2cid=6469504 }}</ref>

=== Abrupt changes during the Late Pleistocene ===
{{See also|Abrupt climate change}}
[[File:Schannwell 2024 Heinrich events.png|thumb|upright=0.7|A reconstruction of how Heinrich events would have likely proceeded, with the Laurentide ice sheet first growing to an unsustainable position, where the base of its periphery becomes too warm, and then rapidly losing ice until it is reduced to sustainable size<ref name="Schannwell2024" />]]

Because the Atlantic meridional overturning circulation (AMOC) is dependent on a series of interactions between layers of ocean water of varying temperature and salinity, it is not static but experiences small, cyclical changes<ref name="Srokosz2015" /><ref name="Latif2022" /> and larger, long-term shifts in response to external forcings.<ref>{{Cite journal |last1=dos Santos |first1=Raquel A. Lopes |display-authors=etal |date=15 Nov 2001 |title=Glacial–interglacial variability in Atlantic meridional overturning circulation and thermocline adjustments in the tropical North Atlantic |journal=Earth and Planetary Science Letters |volume=300 |issue=3–4 |pages=407–414 |doi=10.1016/j.epsl.2010.10.030 }}</ref> Many of those shifts occurred during the [[Late Pleistocene]] (126,000 to 11,700 years ago), which was the final [[geological epoch]] before the current [[Holocene]].<ref name="PesterLS2022">{{cite web |last1=Pester |first1=Patrick |last2=Zimmermann |first2=Kim Ann |date=28 February 2022 |title=Pleistocene epoch: The last ice age |url=https://www.livescience.com/40311-pleistocene-epoch.html |publisher=[[LiveScience]] |access-date=19 May 2024 |archive-date=13 April 2020 |archive-url=https://web.archive.org/web/20200413213808/https://www.livescience.com/40311-pleistocene-epoch.html |url-status=live }}</ref> It also includes the [[Last Glacial Period]], which is colloquially known as the "last ice age".<ref name="Schmidt2011">{{cite web |last1=Schmidt |first1=Matthew W. |last2=Hertzberg |first2=Jennifer E. |date=28 February 2022 |title=Abrupt Climate Change During the Last Ice Age |url=https://www.nature.com/scitable/knowledge/library/abrupt-climate-change-during-the-last-ice-24288097/ |publisher=[[Nature (magazine)|Nature Education Knowledge]] |access-date=19 May 2024 |archive-date=19 May 2024 |archive-url=https://web.archive.org/web/20240519141220/https://www.nature.com/scitable/knowledge/library/abrupt-climate-change-during-the-last-ice-24288097/ |url-status=live }}</ref> Twenty-five abrupt temperature oscillations between the hemispheres occurred during this period; these oscillations are known as [[Dansgaard–Oeschger event]]s (D-O events) after [[Willi Dansgaard]] and [[Hans Oeschger]], who discovered them by analyzing Greenland ice cores in the 1980s.<ref>{{Cite journal|last1=JOHNSEN|first1=S. J.|last2=DANSGAARD|first2=W.|last3=CLAUSEN|first3=H. B.|last4=LANGWAY|first4=C. C.|date=February 1972|title=Oxygen Isotope Profiles through the Antarctic and Greenland Ice Sheets|url=http://dx.doi.org/10.1038/235429a0|journal=Nature|volume=235|issue=5339|pages=429–434|doi=10.1038/235429a0|bibcode=1972Natur.235..429J|s2cid=4210144|issn=0028-0836}}</ref><ref>{{Cite journal|last1=Stauffer|first1=B.|last2=Hofer|first2=H.|last3=Oeschger|first3=H.|last4=Schwander|first4=J.|last5=Siegenthaler|first5=U.|date=1984|title=Atmospheric CO2 Concentration During the Last Glaciation|journal=Annals of Glaciology|volume=5|pages=160–164|doi=10.3189/1984aog5-1-160-164|bibcode=1984AnGla...5..160S|issn=0260-3055|doi-access=free}}</ref>

D-O events are best known for the rapid warming of between 8 °C (15 °F) and 15 °C (27 °F) that occurred in Greenland over several decades.<ref name="Schmidt2011" /> Warming also occurred over the entire North Atlantic region but equivalent cooling over the [[Southern Ocean]] also occurred during these events. This is consistent with the strengthened AMOC transporting more heat from one hemisphere to another.<ref name="Oka2021">{{Cite journal |last1=Oka |first1=Akira |last2=Abe-Ouchi |first2=Ayako |last3=Sherriff-Tadano |first3=Sam |last4=Yokoyama |first4=Yusuke |last5=Kawamura |first5=Kenji |last6=Hasumi |first6=Hiroyasu |date=20 August 2021 |title=Glacial mode shift of the Atlantic meridional overturning circulation by warming over the Southern Ocean |journal=Communications Earth & Environment |volume=2 |issue=1 |page=169 |doi=10.1038/s43247-021-00226-3 |bibcode=2021ComEE...2..169O |doi-access=free }}</ref> The warming of the northern hemisphere would have caused ice-sheet melting and many D-O events appear to have been ended by [[Heinrich event]]s, in which massive streams of icebergs broke off from the then-present [[Laurentide ice sheet]]. As the icebergs melted in the ocean, the ocean water would have become fresher, weakening the circulation and stopping the D-O warming.<ref name="Schannwell2024">{{Cite journal|last1=Schannwell |first1=Clemens |last2=Mikolajewicz |first2=Uwe |last3=Kapsch |first3=Marie-Luise |last4=Ziemen |first4=Florian |date=5 April 2024 |title=A mechanism for reconciling the synchronisation of Heinrich events and Dansgaard-Oeschger cycles |journal=Nature Communications|language=en |volume=15 |issue=1 |page=2961 |doi=10.1038/s41467-024-47141-7 |pmid=38580634 |pmc=10997585 |bibcode=2024NatCo..15.2961S }}</ref>

There is not yet a consensus explanation for why AMOC would have fluctuated so much, and only during this glacial period.<ref name="Dima2018">{{Cite journal |last1=Dima |first1=M. |last2=Lohmann |first2=G. |last3=Knorr |first3=G. |date=21 November 2018 |title=North Atlantic Versus Global Control on Dansgaard-Oeschger Events |journal=Geophysical Research Letters |volume=45 |issue=23 |pages=12,991-12,998 |doi=10.1029/2018GL080035 |bibcode=2018GeoRL..4512991D }}</ref><ref name="Li2018">{{Cite journal|last1=Li|first1=Camille|last2=Born|first2=Andreas |date=10 November 2018 |title=Coupled atmosphere-ice-ocean dynamics in Dansgaard-Oeschger events |journal=Quaternary Science Reviews |volume=203 |pages=1–20 |doi=10.1016/j.quascirev.2018.10.031 |issn=0277-3791 |bibcode=2019QSRv..203....1L |hdl=1956/19927|s2cid=134877256 |hdl-access=free}}</ref> Common hypotheses include cyclical patterns of salinity change in the North Atlantic or a wind-pattern cycle due to the growth and decline of the region's ice sheets, which are large enough to affect wind patterns.<ref name="Schmidt2011" /> As of late 2010s, some research suggests the AMOC is most-sensitive to change during periods of extensive ice sheets and low {{CO2}},<ref>{{cite journal |last1=Sun |first1=Yuchen |last2=Knorr |first2=Gregor |last3=Zhang |first3=Xu |last4=Tarasov |first4=Lev |last5=Barker |first5=Stephen |last6=Werner |first6=Martin |last7=Lohmann |first7=Gerrit |date=21 February 2022 |title=Ice sheet decline and rising atmospheric CO2 control AMOC sensitivity to deglacial meltwater discharge |journal=Global and Planetary Change |volume=210 |page=103755 |doi=10.1016/j.gloplacha.2022.103755 |doi-access=free }}</ref> making the Last Glacial Period a "sweet spot" for such oscillations.<ref name="Li2018" /> It has been suggested the warming of the southern hemisphere would have initiated the pattern as warmer waters spread north through the overall thermohaline circulation.<ref name="Dima2018" /><ref name="Oka2021" /> The [[paleoclimate]] evidence is not currently strong enough to say whether the D-O events started with changes in the AMOC or whether the AMOC changed in response to another trigger.<ref>{{Cite journal |last1=Lynch-Stieglitz |first1=Jean |date=28 October 2016 |title=The Atlantic Meridional Overturning Circulation and Abrupt Climate Change |journal=Annual Review of Marine Science |volume=9 |pages=83–104 |doi=10.1146/annurev-marine-010816-060415  |pmid=27814029 |bibcode=2017ARMS....9...83L }}</ref> For instance, some research suggests changes in sea-ice cover initiated the D-O events because they would have affected water temperature and circulation through [[Ice–albedo feedback]].<ref name="Dima2018" /><ref name="Petersen2013">{{Cite journal |last1=Petersen |first1=S. V. |last2=Schrag |first2=D. P. |last3=Clark |first3=P. U. |date=5 March 2013 |title=A new mechanism for Dansgaard-Oeschger cycles |journal=Paleoceanography and Paleoclimatology |volume=28 |issue=1 |pages=24–30 |doi=10.1029/2012PA002364 |bibcode=2013PalOc..28...24P }}</ref>

[[File:Obase 2021 B-A global.png|thumb|left|The significant warming of the Northern Hemisphere and the equivalent cooling of the South, indicated by [[paleoclimate]] data and replicated in simulations, is consistent with significant AMOC strengthening.<ref name="Obase2021">{{Cite journal |last1=Obase |first1=Takashi |last2=Abe-Ouchi |first2=Ayako |last3=Saito |first3=Fuyuki |date=25 November 2021 |title=Abrupt climate changes in the last two deglaciations simulated with different Northern ice sheet discharge and insolation |journal=[[Scientific Reports]] |language=en |volume=11 |issue=1 |page=22359 |doi=10.1038/s41598-021-01651-2 |pmid=34824287 |pmc=8616927 |bibcode=2021NatSR..1122359O }}</ref>]]

D-O events are numbered in reverse order; the largest numbers are assigned to the oldest events.<ref name="Dima2018" /> The penultimate event, Dansgaard–Oeschger event 1, occurred some 14,690 years ago and marks the transition from the [[Oldest Dryas]] period to the [[Bølling–Allerød Interstadial]] ({{IPA|da|ˈpøle̝ŋ ˈæləˌʁœðˀ|lang}}), which lasted until 12,890 years [[Before Present]].<ref name="EGL2022">{{cite book |editor-last=Palacios |editor-first=David |editor2-last=Hughes |editor2-first=Philip D. |editor3-last=García-Ruiz |editor3-first=José M. |editor4-last=Andrés |editor4-first=Nuria |title=European Glacial Landscapes: The Last Deglaciation |chapter=The Bølling–Allerød Interstadial |last1=Naughton |first1=Filipa |last2=Sánchez-Goñi |first2=María F. |last3=Landais |first3=Amaelle |last4=Rodrigues |first4=Teresa |last5=Riveiros |first5=Natalia Vazquez |last6=Toucanne |first6=Samuel |pages=45–50|chapter-url=https://www.researchgate.net/publication/363855776 |publisher=Elsevier |year=2022 |doi=10.1016/C2021-0-00331-X |isbn=978-0-323-91899-2 }}</ref><ref name="Rasmussen2006">{{Cite journal |last1=Rasmussen |first1=S. O. |last2=Andersen |first2=K. K. |last3=Svensson |first3=A. M. |last4=Steffensen |first4=J. P. |last5=Vinther |first5=B. M. |last6=Clausen |first6=H. B. |last7=Siggaard-Andersen |first7=M.-L. |last8=Johnsen |first8=S. J. |last9=Larsen |first9=L. B. |last10=Dahl-Jensen |first10=D. |last11=Bigler |first11=M. |date=2006 |title=A new Greenland ice core chronology for the last glacial termination |journal=Journal of Geophysical Research |language=en |volume=111 |issue=D6 |pages=D06102 |doi=10.1029/2005JD006079 |bibcode=2006JGRD..111.6102R |issn=0148-0227 |doi-access=free }}</ref> It was named after the two sites in Denmark with vegetation fossils that could only have survived during a comparatively warm period in the northern hemisphere.<ref name="EGL2022" /> The major warming in the northern hemisphere was offset by southern-hemisphere cooling and little net change in global temperature, which is consistent with changes in the AMOC.<ref name="Obase2021" /><ref name="Shakun2012">{{cite journal |last1=Shakun |first1=Jeremy D. |last2=Clark |first2=Peter U. |last3=He |first3=Feng |last4=Marcott |first4=Shaun A. |last5=Mix |first5=Alan C. |last6=Liu |first6=Zhenyu |last7=Oto-Bliesner |first7=Bette |last8=Schmittner |first8=Andreas |last9=Bard |first9=Edouard |date=4 April 2012 |title=Global warming preceded by increasing carbon dioxide concentrations during the last deglaciation |url=https://www.nature.com/articles/nature10915%3C |journal=[[Nature (journal)|Nature]] |volume=484 |issue=7392 |pages=49–54 |doi=10.1038/nature10915 |pmid=22481357 |bibcode=2012Natur.484...49S |hdl=2027.42/147130 |s2cid=2152480 |access-date=17 January 2023 |hdl-access=free |archive-date=18 January 2023 |archive-url=https://web.archive.org/web/20230118021329/https://www.nature.com/articles/nature10915%3C |url-status=live }}</ref> The onset of the interstadial also caused a period of [[sea level rise]] from ice-sheet collapse that is designated [[Meltwater pulse 1A]].<ref name="Brendryen2020">{{Cite journal |last1=Brendryen |first1=J. |last2=Haflidason |first2=H. |last3=Yokoyama |first3=Y. |last4=Haaga |first4=K. A. |last5=Hannisdal |first5=B. |date=20 April 2020 |title=Eurasian Ice Sheet collapse was a major source of Meltwater Pulse 1A 14,600 years ago |url=https://www.nature.com/articles/s41561-020-0567-4 |journal=[[Nature Geoscience]] |language=en |volume=13 |issue=5 |pages=363–368 |bibcode=2020NatGe..13..363B |doi=10.1038/s41561-020-0567-4 |s2cid=216031874 |access-date=26 December 2023 |hdl-access=free |hdl=11250/2755925}}</ref>

The Bølling and Allerød stages of the interglacial were separated by two centuries of the opposite pattern&nbsp;– northern-hemisphere cooling, southern-hemisphere warming&nbsp;– which is known as the [[Older Dryas]] because the Arctic flower [[Dryas octopetala]] became dominant where forests were able to grow during the interglacial.<ref name="EGL2022" /> The interglacial ended with the onset of the [[Younger Dryas]] (YD) period (12,800–11,700 years ago), when northern-hemisphere temperatures returned to near-glacial levels, possibly within a decade.<ref>{{cite book |last=Wade |first=Nicholas |title=Before the Dawn |location=New York |publisher=Penguin Press |year=2006 |page=123 |isbn=978-1-59420-079-3 }}</ref> This happened due to an abrupt slowing of the AMOC,<ref name="IPCC AR6 WG1 Ch.8" /> which, in a similar manner to [[Heinrich event]]s, was caused by freshening due to ice loss from the Laurentide ice sheet. Unlike true Heinrich events, there was an enormous flow of meltwater through the [[Mackenzie River]] in what is now [[Canada]] rather than a mass iceberg loss.<ref>{{Cite journal |last1=Keigwin |first1=L. D. |last2=Klotsko |first2=S. |last3=Zhao |first3=N. |last4=Reilly |first4=B. |last5=Giosan |first5=L. |last6=Driscoll |first6=N. W. |date=9 July 2018 |title=Deglacial floods in the Beaufort Sea preceded Younger Dryas cooling |journal=Nature Geoscience |volume=11 |issue=8 |pages=599–604 |doi=10.1038/s41561-018-0169-6 |bibcode=2018NatGe..11..599K }}</ref> Major changes in the [[precipitation]] regime, such as the shift of the [[Intertropical Convergence Zone]] to the south, increased rainfall in North America, and the drying of South America and Europe, occurred.<ref name="IPCC AR6 WG1 Ch.8">{{Cite journal |last1=Douville |first1=H. |last2=Raghavan |first2=K. |last3=Renwick |first3=J. |last4=Allan |first4=R. P. |last5=Arias |first5=P. A. |last6=Barlow |first6=M. |last7=Cerezo-Mota |first7=R. |last8=Cherchi |first8=A. |last9=Gan |first9=T.Y. |last10=Gergis |first10=J. |last11=Jiang |first11=D. |last12=Khan |first12=A. |last13=Pokam Mba |first13=W. |last14=Rosenfeld |first14=D. |last15=Tierney |first15=J. |last16=Zolina |first16=O. |year=2021 |editor-last=Masson-Delmotte |editor-first=V. |editor2-last=Zhai |editor2-first=P. |editor3-last=Pirani |editor3-first=A. |editor4-last=Connors |editor4-first=S. L. |editor5-last=Péan |editor5-first=C. |editor6-last=Berger |editor6-first=S. |editor7-last=Caud |editor7-first=N. |editor8-last=Chen |editor8-first=Y. |editor9-last=Goldfarb |editor9-first=L. |title=Chapter 8: Water Cycle Changes |url=https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter08.pdf |journal=Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change |publisher=Cambridge University Press, Cambridge, UK and New York, NY, US |pages=1055–1210 |doi=10.1017/9781009157896.010 |archive-date=29 September 2022 |access-date=19 May 2024 |archive-url=https://web.archive.org/web/20220929084018/https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter08.pdf |url-status=live }}</ref>{{Rp|page=1148}} Global temperatures again barely changed during the Younger Dryas and long-term, post-glacial warming resumed after it ended.<ref name="Shakun2012" />

== Stability and vulnerability ==
{{Further|Multiple equilibria in the Atlantic meridional overturning circulation}}

[[File:Kim 2022 classic Stommel tipping.png|thumb|In the classic Stommel box models, AMOC tipping occurs either because of a large increase in freshwater volumes which makes the circulation impossible (B-tipping), or because of a lower increase which makes it possible for circulation's own variability to push it to collapse (N-tipping). As freshwater input increases, probability of N-tipping increases. If the probability is at 100%, B-tipping occurs <ref name="Kim2022" />]]

The AMOC has not always existed; for much of Earth's history, overturning circulation in the northern hemisphere occurred in the North Pacific. Paleoclimate evidence shows the shift of overturning circulation from the Pacific to the Atlantic occurred 34 million years ago at the [[Eocene–Oligocene extinction event|Eocene-Oligocene transition]], when the Arctic-Atlantic gateway had closed.<ref>{{Cite journal |last1=Hutchinson |first1=David |last2=Coxall |first2=Helen |last3=O'Regan |first3=Matt |last4=Nilsson |first4=Johan |last5=Caballero |first5=Rodrigo |last6=de Boer |first6=Agatha |date=2020-03-23 |title=Arctic closure as a trigger for Atlantic overturning at the Eocene-Oligocene Transition |journal=EGU General Assembly Conference Abstracts |page=7493 |doi=10.5194/egusphere-egu2020-7493 |bibcode=2020EGUGA..22.7493H |s2cid=225974919 |doi-access=free }}</ref> This closure fundamentally changed the thermohaline circulation structure; some researchers have suggested [[climate change]] may eventually reverse this shift and re-establish the Pacific circulation after the AMOC shuts down.<ref name="Molina2021" /><ref name="RealClimate2024" /> Climate change affects the AMOC by making surface water warmer as a consequence of [[Earth's energy imbalance]] and by making surface water less saline due to the addition of large quantities of fresh water from melting ice&nbsp;– mainly from Greenland&nbsp;– and through increasing precipitation over the North Atlantic. Both of these causes would increase the difference between the surface and deep layers, thus making the upwelling and downwelling that drives the circulation more difficult.<ref>{{cite journal |last1=Gierz |first1=Paul |title=Response of Atlantic Overturning to future warming in a coupled atmosphere-ocean-ice sheet model |journal=Geophysical Research Letters |volume=42 |issue=16 |pages=6811–6818 |date=31 August 2015 |doi=10.1002/2015GL065276 |bibcode=2015GeoRL..42.6811G |doi-access=free}}</ref>

In the 1960s, [[Henry Stommel]] did much of the research into the AMOC with what later became known as the Stommel Box model, which introduced the idea of Stommel Bifurcation in which the AMOC could exist either in a strong state like the one throughout recorded history or effectively collapse to a much weaker state and not recover unless the increased warming and/or freshening that caused the collapse is reduced.<ref>{{Cite journal |last1=Stommel |first1=Henry |date=May 1961 |title=Thermohaline convection with two stable regimes of flow |journal=Tellus |volume=13 |issue=2 |pages=224–230 |doi=10.1111/j.2153-3490.1961.tb00079.x |bibcode=1961Tell...13..224S }}</ref> The warming and freshening could directly cause the collapse or weaken the circulation to a state in which its ordinary fluctuations (noise) could push it past the tipping point.<ref name="Kim2022" /> The possibility the AMOC is a bistable system that is either "on" or "off" and could suddenly collapse has been a topic of scientific discussion ever since.<ref>{{Cite journal |doi=10.1029/2011GL047208 |title=Bistability of the Atlantic overturning circulation in a global climate model and links to ocean freshwater transport |year=2011 |last1=Hawkins |first1=E. |last2=Smith |first2=R. S. |last3=Allison |first3=L. C. |last4=Gregory |first4=J. M. |last5=Woollings |first5=T. J. |last6=Pohlmann |first6=H. |last7=De Cuevas |first7=B. |journal=Geophysical Research Letters |volume=38 |issue=10 |pages=n/a |bibcode=2011GeoRL..3810605H | s2cid=970991 |doi-access=free}}</ref><ref>{{Cite journal |last1=Knutti |first1=Reto |last2=Stocker |first2=Thomas F. |date=15 January 2002 |title=Limited predictability of the future thermohaline circulation close to an instability threshold |journal=Journal of Climate |volume=15 |issue=2 |pages=179–186 |doi=10.1175/1520-0442(2002)015<0179:LPOTFT>2.0.CO;2 |bibcode=2002JCli...15..179K |s2cid=7353330 |url=https://boris.unibe.ch/158284/ |archive-date=30 May 2024 |access-date=15 April 2024 |archive-url=https://web.archive.org/web/20240530214755/https://boris.unibe.ch/158284/ |url-status=live }}</ref> In 2004, ''[[The Guardian]]'' published the findings of a report commissioned by Pentagon defense adviser Andrew Marshall that suggests the average annual temperature in Europe would drop by {{convert |6 |F-change |C-change}} between 2010 and 2020 as the result of an abrupt AMOC shutdown.<ref>{{cite news |date=22 Feb 2004 |title=Key findings of the Pentagon |newspaper=The Guardian |url=https://www.theguardian.com/environment/2004/feb/22/usnews.theobserver1 |access-date=2022-10-02 |archive-date=20 March 2023 |archive-url=https://web.archive.org/web/20230320173847/https://www.theguardian.com/environment/2004/feb/22/usnews.theobserver1 |url-status=live }}</ref>

=== Modeling AMOC collapse ===

[[File:Nobre 2023 AMOC 4X BESM.png|thumb|In a typical full-scale climate model, AMOC is greatly weakened for around 500 years but does not truly collapse, even in a test scenario where {{CO2}} concentrations suddenly quadruple.<ref name="Nobre2023" /> There are concerns that this kind of a simulation is too stable <ref name="Valdes2011" />]]

Some of the models developed after Stommel's work suggest the AMOC could have one or more intermediate stable states between full strength and full collapse.<ref>{{Cite journal |last1=Rahmstorf |first1=Stefan |date=12 September 2002 |title=Ocean circulation and climate during the past 120,000 years |journal=Nature |volume=419 |issue=6903 |pages=207–214 |doi=10.1038/nature01090 |pmid=12226675 |bibcode=2002Natur.419..207R |s2cid=3136307 }}</ref> This is more-commonly seen in Earth Models of Intermediate Complexity (EMICs), which focus on certain parts of the climate system like AMOC and disregard others, rather than in the more-comprehensive [[general circulation model]]s (GCMs) that represent the "gold standard" for simulating the entire climate but often have to simplify certain interactions.<ref name="Dijkstra2008">{{Cite journal |last1=Dijkstra |first1=Henk A. |date=28 June 2008 |title=Characterization of the multiple equilibria regime in a global ocean model |journal=Tellus A |volume=59 |issue=5 |pages=695–705 |doi=10.1111/j.1600-0870.2007.00267.x |s2cid=94737971 |doi-access=free }}</ref> GCMs typically show the AMOC has a single equilibrium state and that it is difficult or impossible for it to collapse.<ref name="Drijfhout2010">{{Cite journal |last1=Drijfhout |first1=Sybren S. |last2=Weber |first2=Susanne L. |last3=van der Swaluw |first3=Eric |date=26 October 2010 |title=The stability of the MOC as diagnosed from model projections for pre-industrial, present and future climates |journal=Climate Dynamics |volume=37 |issue=7–8 |pages=1575–1586 |doi=10.1007/s00382-010-0930-z |s2cid=17003970 }}</ref><ref name="Nobre2023">{{cite journal |last1=Nobre |first1=Paulo |last2=Veiga |first2=Sandro F. |last3=Giarolla |first3=Emanuel |last4=Marquez |first4=André L. |last5=da Silva Jr. |first5=Manoel B. |last6=Capistrano |first6=Vinícius B. |last7=Malagutti |first7=Marta |last8=Fernandez |first8=Julio P. R. |last9=Soares |first9=Helena C. |last10=Bottino |first10=Marcus J. |last11=Kubota |first11=Paulo Y. |last12=Figueroa |first12=Silvio N. |last13=Bonatti |first13=José P. |last14=Sampaio |first14=Gilvan |last15=Casagrande |first15=Fernanda |last16=Costa |first16=Mabel C. |last17=Nobre |first17=Carlos A. |date=23 September 2023 |title=AMOC decline and recovery in a warmer climate |journal=Scientific Reports |volume=13 |issue=1 |page=15928 |doi=10.1038/s41598-023-43143-5 |pmid=37741891 |pmc=10517999 |bibcode=2023NatSR..1315928N }}</ref> Researchers have raised concerns this modeled resistance to collapse only occurs because GCM simulations tend to redirect large quantities of freshwater toward the North Pole, where it would no longer affect the circulation, a movement that does not occur in nature.<ref name="Srokosz2015" /><ref name="Liu2017">{{Cite journal |last1=Liu |first1=Wei |last2=Xie |first2=Shang-Ping |last3=Liu |first3=Zhengyu |last4=Zhu |first4=Jiang |date=4 January 2017 |title=Overlooked possibility of a collapsed Atlantic Meridional Overturning Circulation in warming climate |journal=Science Advances |volume=3 |issue=1 |pages=e1601666 |doi=10.1126/sciadv.1601666 |pmid=28070560 |pmc=5217057 |bibcode=2017SciA....3E1666L }}</ref>

[[File:Van Westen 2024 AMOC collapse graph.jpg|thumb|left|In one paper, AMOC collapse only occurs in a full general circulation model after it ran for nearly 2000 years, and freshwater quantities (in Sv) increased to extreme values.<ref name="van Westen2024" /> While the conditions are unrealistic, the model may also be unrealistically stable, and the full implications are not clear without more real-world observations<ref name="SMC2024" />]]

In 2024, three researchers performed a simulation with one of the Community Earth System Models (CIMP) in which a classic AMOC collapse had occurred, much like it does in intermediate-complexity models.<ref name="van Westen2024">{{Cite journal |last1=van Westen |first1=René M. |last2=Kliphuis |first2=Michael |last3=Dijkstra |first3=Henk A. |date=9 February 2024 |title=Physics-based early warning signal shows that AMOC is on tipping course |journal=Science Advances |volume=10 |issue=6 |pages=eadk1189 |doi=10.1126/sciadv.adk1189 |pmid=38335283 |pmc=10857529 |arxiv=2308.01688 |bibcode=2024SciA...10K1189V }}</ref> Unlike some other simulations, they did not immediately subject the model to unrealistic meltwater levels but gradually increased the input. Their simulation had run for over 1,700 years before the collapse occurred and they had also eventually reached meltwater levels equivalent to a [[sea level rise]] of {{convert |6 |cm |abbr=on}} per year,<ref name="SMC2024" /> about 20 times larger than the {{convert |2.9 |mm |abbr=on}}/year sea level rise between 1993 and 2017,<ref name="WCRP2018">{{cite journal |author=WCRP Global Sea Level Budget Group |year=2018 |title=Global sea-level budget 1993–present |journal=Earth System Science Data |volume=10 |issue=3 |pages=1551–1590 |bibcode=2018ESSD...10.1551W |doi=10.5194/essd-10-1551-2018 |doi-access=free |hdl=20.500.11850/287786 |hdl-access=free }}</ref> and well above any level considered plausible. According to the researchers, those unrealistic conditions were intended to counterbalance the model's unrealistic stability and the model's output should not be regarded as a prediction but rather as a high-resolution representation of the way currents would start changing before a collapse.<ref name="van Westen2024" /> Other scientists agreed this study's findings would mainly help with calibrating more-realistic studies, particularly once better observational data becomes available.<ref name="SMC2024">{{Cite web |date=9 February 2024 |title=expert reaction to modelling study suggesting Atlantic Ocean circulation (AMOC) could be on course to collapse |url=https://www.sciencemediacentre.org/expert-reaction-to-modelling-study-suggesting-atlantic-ocean-circulation-amoc-could-be-on-course-to-collapse/ |access-date=12 April 2024 |website=Science Media Centre |language=en |archive-date=20 April 2024 |archive-url=https://web.archive.org/web/20240420004531/https://www.sciencemediacentre.org/expert-reaction-to-modelling-study-suggesting-atlantic-ocean-circulation-amoc-could-be-on-course-to-collapse/ |url-status=live }}</ref><ref name="RealClimate2024">{{cite web |last1=Rahmstorf |first1=Stefan |date=9 February 2024 |title=New study suggests the Atlantic overturning circulation AMOC "is on tipping course" |url=https://www.realclimate.org/index.php/archives/2024/02/new-study-suggests-the-atlantic-overturning-circulation-amoc-is-on-tipping-course/ |publisher=RealClimate }}</ref>

Some research indicates classic EMIC projections are biased toward AMOC collapse because they subject the circulation toward an unrealistically constant flow of freshwater. In one study, the difference between constant and variable freshwater flux delayed collapse of the circulation in a typical Stommel's Bifurcation EMIC by over 1,000 years. The researchers said this simulation is more consistent with reconstructions of the AMOC's response to [[Meltwater pulse 1A]] 13,500–14,700 years ago and indicates a similarly long delay.<ref name="Kim2022">{{Cite journal |last1=Kim |first1=Soong-Ki |last2=Kim |first2=Hyo-Jeong |last3=Dijkstra |first3=Henk A. |last4=An |first4=Soon-Il |date=11 February 2022 |title=Slow and soft passage through tipping point of the Atlantic Meridional Overturning Circulation in a changing climate |journal=npj Climate and Atmospheric Science |volume=5 |issue=13 |doi=10.1038/s41612-022-00236-8 |s2cid=246705201 |doi-access=free |bibcode=2022npCAS...5...13K }}</ref> In 2022, a paleoceanographic reconstruction found a limited effect from massive freshwater forcing of the final [[Holocene]] deglaciation ~11,700–6,000 years ago, when the sea level rise was around {{cvt|50|m|ft}}. It suggested most models overestimate the effects of freshwater forcing on the AMOC.<ref name="He2022">{{Cite journal |last1=He |first1=Feng |last2=Clark |first2=Peter U. |date=7 April 2022 |title=Freshwater forcing of the Atlantic Meridional Overturning Circulation revisited |url=https://www.nature.com/articles/s41558-022-01328-2 |journal=Nature Climate Change |volume=12 |issue=5 |pages=449–454 |doi=10.1038/s41558-022-01328-2 |bibcode=2022NatCC..12..449H |s2cid=248004571 |archive-date=14 April 2023 |access-date=3 October 2022 |archive-url=https://web.archive.org/web/20230414023824/https://www.nature.com/articles/s41558-022-01328-2 |url-status=live }}</ref> If the AMOC is more dependent on wind strength&nbsp;– which changes relatively little with warming&nbsp;– than is commonly understood, then it would be more resistant to collapse.<ref>{{Cite journal |last1=Hofmann |first1=Matthias |last2=Rahmstorf |first2=Stefan |date=December 8, 2009 |title=On the stability of the Atlantic meridional overturning circulation |journal=Proceedings of the National Academy of Sciences |volume=106 |issue=49 |pages=20584–20589 |doi=10.1073/pnas.0909146106 |pmid=19897722 |pmc=2791639 |doi-access=free }}</ref> According to some researchers, the less-studied [[Southern Ocean overturning circulation]] (SOOC) may be more vulnerable to collapse than the AMOC.<ref name="Liu2022" />

High-quality [[Earth system model]]s indicate a collapse is unlikely and would only become probable if high levels of warming (≥{{convert|4|C-change|F-change}})<ref name="ArmstrongMcKay2022" /> are sustained long after 2100.<ref name="Liu2017" /><ref name="Bakker2016" /><ref name="Sigmond2020" /> Some [[paleoceanography|paleoceanographic]] research seems to support this idea.<ref name="He2022" /><ref name="Kim2022" /> Some researchers fear the complex models are too stable<ref name="Valdes2011" /> and that lower-complexity projections pointing to an earlier collapse are more accurate.<ref name="Lohmann2021" /><ref name="Boers2021" /> One of those projections suggests AMOC collapse could happen around 2057<ref name="Ditlevsen2023" /> but many scientists are skeptical of the projection.<ref name="SMC2023" /> Some research also suggests the Southern Ocean overturning circulation may be more prone to collapse than the AMOC.<ref name="Liu2022">{{cite journal |last1=Liu |first1=Y. |last2=Moore |first2=J. K. |last3=Primeau |first3=F. |last4=Wang |first4=W. L. |date=22 December 2022 |title=Reduced CO2 uptake and growing nutrient sequestration from slowing overturning circulation |journal=Nature Climate Change |volume=13 |pages=83–90 |doi=10.1038/s41558-022-01555-7 |osti=2242376 |s2cid=255028552 }}</ref><ref name="GTPR2023">{{cite report |last1=Lenton |first1=T. M. |last2=Armstrong McKay |first2=D.I. |last3=Loriani |first3=S. |last4=Abrams |first4=J.F.  |last5=Lade |first5=S.J. |last6=Donges |first6=J.F. |last7=Milkoreit |first7=M. |last8=Powell |first8=T. |last9=Smith |first9=S.R. |last10=Zimm |first10=C. |last11=Buxton |first11=J.E. |last12=Daube |first12=Bruce C. |last13=Krummel |first13=Paul B. |last14=Loh |first14=Zoë |last15=Luijkx |first15=Ingrid T. |year=2023 |title=The Global Tipping Points Report 2023 |url=https://global-tipping-points.org/download/4608/ |publisher=University of Exeter }}</ref> In October 2024, 44 climate scientists published an open letter, 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, with devastating impacts especially for Nordic countries. They called on Nordic countries to ensure the implementation of the [[Paris Agreement]] to prevent it.<ref name="Letter">{{cite web |title=Open Letter by Climate Scientists to the Nordic Council of Ministers |url=https://en.vedur.is/media/ads_in_header/AMOC-letter_Final.pdf |access-date=31 October 2024 |archive-date=30 October 2024 |archive-url=https://web.archive.org/web/20241030191940/https://en.vedur.is/media/ads_in_header/AMOC-letter_Final.pdf |url-status=live }}</ref><ref name="Livescience">{{cite web |last1=Pare |first1=Sascha |title=Key Atlantic current could collapse soon, 'impacting the entire world for centuries to come,' leading climate scientists war |url=https://www.livescience.com/planet-earth/rivers-oceans/key-atlantic-current-could-collapse-soon-impacting-the-entire-world-for-centuries-to-come-leading-climate-scientists-warn |access-date=31 October 2024 |website=Live Science|date=22 October 2024 }}</ref>

== Trends ==
Until 2024 there was a disagreement between observations showing a slowdown of the circulation and climate models showing a stable circulation. In November 2024, ''[[Nature Geoscience]]'' published a study which tried to solve the problem. The scientists used "Earth system and eddy-permitting coupled ocean–sea-ice models". Then observations and models corresponded to each other much better. The study found a slowdown of 0.46 sverdrups per decade since 1950.<ref name=Nature>{{cite journal |last1=M. Pontes |first1=Gabriel |last2=Menviel |first2=Laurie |title=Weakening of the Atlantic Meridional Overturning Circulation driven by subarctic freshening since the mid-twentieth century |journal=Nature Geoscience |date=18 November 2024 |volume=17 |issue=12 |pages=1291–1298 |doi=10.1038/s41561-024-01568-1 |url=https://www.nature.com/articles/s41561-024-01568-1 |access-date=20 November 2024 |archive-date=18 November 2024 |archive-url=https://web.archive.org/web/20241118193521/https://www.nature.com/articles/s41561-024-01568-1 |url-status=live }}</ref>
===Observations===
[[File:95307main fig4m.jpg|thumb|1992–2002 altimeter data from NASA Pathfinder indicated a slowing (red) in the subpolar gyre region. This was used as a proxy for the AMOC before the initiation of RAPID, and before subsequent research demonstrated the subpolar gyre often behaves separately from the larger circulation <ref name="Lozier2019" />]]

[[File:Smeed 2018 RAPID trendlines.jpg|thumb|RAPID tracks both the AMOC itself (third line from the top, labelled MOC) as well as its separate components (three lower lines) and the AMOC flow combined with the subpolar gyre and/or the western boundary current flow (upper two lines) AMOC flow during 2004–2008 appears stronger than afterwards<ref name="Smeed2018" />]]

Direct observations of the strength of the AMOC have been available since 2004 from [[Rapid Climate Change-Meridional Overturning Circulation and Heatflux Array|RAPID]], an ''in situ'' mooring array at 26°N in the Atlantic.<ref>{{cite journal |last=Schiermeier |first=Quirin |year=2007 |title=Climate change: A sea change |journal=[[Nature (journal)|Nature]] |volume=439 |issue=7074 |pages=256–60 |bibcode=2006Natur.439..256S |doi=10.1038/439256a |pmid=16421539 |s2cid=4431161|doi-access=free }}</ref><ref name="Smeed2018" /> Observational data needs to be collected for a prolonged period to be of use. Thus, some researchers have attempted to make predictions from smaller-scale observations; for instance, in May 2005, submarine-based research from [[Peter Wadhams]] indicated downwelling in the [[Greenland Sea]]&nbsp;– a small part of the AMOC system&nbsp;– which was measured using giant water columns nicknamed chimneys, transferring water downwards was at less than a quarter of its normal strength.<ref>{{cite news |author=Leake, Jonathan |date=8 May 2005 |title=Britain faces big chill as ocean current slows |newspaper=The Sunday Times |url=http://www.timesonline.co.uk/article/0,,2087-1602579,00.html |url-status=dead |archive-url=https://web.archive.org/web/20060112155518/http://www.timesonline.co.uk/article/0%2C%2C2087-1602579%2C00.html |archive-date=12 January 2006 }}</ref><ref name="RealClimate2005">{{cite web |last=Schmidt |first=Gavin |date=26 May 2005 |title=Gulf Stream slowdown? |url=https://www.realclimate.org/index.php/archives/2005/05/gulf-stream-slowdown/ |publisher=[[RealClimate]] }}</ref> In 2000, other researchers focused on trends in the [[North Atlantic Gyre]] (NAG), which is also known as the Northern Subpolar Gyre (SPG).<ref>{{cite web |date=16 April 2004 |title=Satellites record weakening North Atlantic Current |url=https://www.sciencedaily.com/releases/2004/04/040416010914.htm |publisher=[[ScienceDaily]] |access-date=12 April 2024 |archive-date=18 June 2024 |archive-url=https://web.archive.org/web/20240618044636/https://www.sciencedaily.com/releases/2004/04/040416010914.htm |url-status=live }}</ref> Measurements taken in 2004 found a 30% decline in the NAG relative to the measurement in 1992; some interpreted this measurement as a sign of AMOC collapse.<ref>{{cite web |last=Pearce |first=Fred |date=30 November 2005 |title=Failing ocean current raises fears of mini ice age |url=https://www.newscientist.com/article.ns?id=dn8398 |publisher=[[New Scientist]] |access-date=12 April 2022 |archive-date=20 September 2008 |archive-url=https://web.archive.org/web/20080920022806/https://www.newscientist.com/article.ns?id=dn8398 |url-status=live }}</ref> RAPID data have since shown this to be a statistical anomaly,<ref name="Schiermeier2007">{{cite journal |last=Schiermeier |first=Quirin |year=2007 |title=Ocean circulation noisy, not stalling |journal=Nature |volume=448 |issue=7156 |pages=844–5 |bibcode=2007Natur.448..844S |doi=10.1038/448844b |pmid=17713489 |doi-access=free}}</ref> and observations from 2007 and 2008 have shown a recovery of the NAG.<ref>{{cite journal |last1=Våge |first1=Kjetil |last2=Pickart |first2=Robert S. |last3=Thierry |first3=Virginie |last4=Reverdin |first4=Gilles |last5=Lee |first5=Craig M. |last6=Petrie |first6=Brian |last7=Agnew |first7=Tom A. |last8=Wong |first8=Amy |last9=Ribergaard |first9=Mads H. |year=2009 |title=Surprising return of deep convection to the subpolar North Atlantic Ocean in winter 2007–2008 |url=https://archimer.ifremer.fr/doc/00000/6415/ |journal=Nature Geoscience |volume=2 |issue=1 |pages=67–72 |bibcode=2009NatGe...2...67V |doi=10.1038/ngeo382 |hdl=1912/2840 |hdl-access=free |archive-date=8 May 2021 |access-date=12 April 2022 |archive-url=https://web.archive.org/web/20210508154128/https://archimer.ifremer.fr/doc/00000/6415/ |url-status=live }}</ref> It is now known the NAG is largely separate from the rest of the AMOC and could collapse independently of it.<ref name="ArmstrongMcKay2022" /><ref name="ArmstrongMcKayExplainer" /><ref name="GTPR2023" />

By 2014, there was enough processed RAPID data up until the end of 2012; these data appeared to show a decline in circulation which was 10 times greater than that which was predicted by the most-advanced models of the time. Scientific debate about whether it indicated a strong impact of climate change or a large interdecadal variability of the circulation began.<ref name="Srokosz2015">{{Cite journal |last1=Srokosz |first1=M. A. |last2=Bryden |first2=H. L. |date=19 June 2015 |title=Observing the Atlantic Meridional Overturning Circulation yields a decade of inevitable surprises |journal=Science |volume=348 |issue=6241 |pages=3737–3743 |doi=10.1126/science.1255575 |pmid=26089521 |s2cid=22060669 |doi-access=free }}</ref><ref>{{Cite journal |last1=Roberts |first1=C. D. |last2=Jackson |first2=L. |last3=McNeall |first3=D. |date=31 March 2014 |title=Is the 2004–2012 reduction of the Atlantic meridional overturning circulation significant? |journal=Geophysical Research Letters |volume=41 |issue=9 |pages=3204–3210 |bibcode=2014GeoRL..41.3204R |doi=10.1002/2014GL059473 |s2cid=129713110 |doi-access=free }}</ref> Data up until 2017 showed the decline in 2008 and 2009 was anomalously large but the circulation after 2008 was weaker than it was in 2004–2008.<ref name="Smeed2018">{{Cite journal |last1=Smeed |first1=D.A. |display-authors=etal |date=29 January 2018 |title=The North Atlantic Ocean Is in a State of Reduced Overturning |journal=Geophysical Research Letters |volume=45 |issue=3 |pages=1527–1533 |bibcode=2018GeoRL..45.1527S |doi=10.1002/2017GL076350 |s2cid=52088897 |doi-access=free}}</ref>

The AMOC is also measured by tracking changes in heat transport that would be correlated with overall current flows. In 2017 and 2019, estimates derived from heat observations made by [[NASA]]'s [[Clouds and the Earth's Radiant Energy System|CERES]] satellites and international [[Argo (oceanography)|Argo floats]] suggested 15–20% less heat transport was occurring than was implied by the RAPID, and indicated a fairly stable flow with a limited indication of decadal variability.<ref>{{Cite journal |last1=Trenberth |first1=Kevin E. |last2=Fasullo |first2=John T. |date=8 February 2017 |title=Atlantic meridional heat transports computed from balancing Earth's energy locally |journal=Geophysical Research Letters |volume=44 |issue=4 |pages=1919–1927 |doi=10.1002/2016GL072475 |doi-access=free |bibcode=2017GeoRL..44.1919T }}</ref><ref>{{Cite journal |last1=Trenberth |first1=Kevin E. |last2=Zhang |first2=Yongxin |last3=Fasullo |first3=John T. |last4=Cheng |first4=Lijing |date=15 July 2019 |title=Observation-Based Estimates of Global and Basin Ocean Meridional Heat Transport Time Series |journal=Journal of Climate |volume=32 |issue=14 |pages=4567–4583 |doi=10.1175/JCLI-D-18-0872.1  |bibcode=2019JCli...32.4567T }}</ref>

The strength of [[Florida Current]] has been measured as stable over the last four decades after correction for changes in [[Earth's magnetic field]].<ref name="u785">{{cite journal | last1=Volkov | first1=Denis L. | last2=Smith | first2=Ryan H. | last3=Garcia | first3=Rigoberto F. | last4=Smeed | first4=David A. | last5=Moat | first5=Ben I. | last6=Johns | first6=William E. | last7=Baringer | first7=Molly O. | title=Florida Current transport observations reveal four decades of steady state | journal=Nature Communications | volume=15 | issue=1 | date=5 September 2024 | issn=2041-1723 | pmid=39237510 | pmc=11377814 | doi=10.1038/s41467-024-51879-5 | page=7780| bibcode=2024NatCo..15.7780V }}</ref>

=== Reconstructions ===
==== Recent past ====
[[File:Worthington 2021 RAPID vs reconstructions.png|thumb|A 2021 comparison of the post-2004 RAPID observations with the 1980–2004 reconstructed AMOC trend had indicated no real change across 30 years <ref name="Worthington2021" />]]

Climate reconstructions allow research to assemble hints about the past state of the AMOC, though these techniques are necessarily less reliable than direct observations. In February 2021, RAPID data was combined with reconstructed trends from data that were recorded 25 years before RAPID. This study showed no evidence of an overall decline in the AMOC over the past 30 years.<ref name="Worthington2021">{{Cite journal |last1=Worthington |first1=Emma L. |last2=Moat |first2=Ben I. |last3=Smeed |first3=David A. |last4=Mecking |first4=Jennifer V. |last5=Marsh |first5=Robert |last6=McCarthy |first6=Gerard |date=15 February 2021 |title=A 30-year reconstruction of the Atlantic meridional overturning circulation shows no decline |url=https://os.copernicus.org/articles/17/285/2021/ |journal=Ocean Science |volume=17 |issue=1 |pages=285–299 |bibcode=2021OcSci..17..285W |doi=10.5194/os-17-285-2021 |doi-access=free |archive-date=31 October 2023 |access-date=3 October 2022 |archive-url=https://web.archive.org/web/20231031160718/https://os.copernicus.org/articles/17/285/2021/ |url-status=live }}</ref> A ''[[Science Advances]]'' study published in 2020 found no significant change in the AMOC circulation compared to that in the 1990s, although substantial changes have occurred across the North Atlantic in the same period.<ref>{{Cite journal |last1=Fu |first1=Yao |last2=Li |first2=Feili |last3=Karstensen |first3=Johannes |last4=Wang |first4=Chunzai |date=27 November 2020 |title=A stable Atlantic Meridional Overturning Circulation in a changing North Atlantic Ocean since the 1990s |journal=Science Advances |volume=6 |issue=48 |bibcode=2020SciA....6.7836F |doi=10.1126/sciadv.abc7836 |pmc=7695472 |pmid=33246958}}</ref> A March 2022 review article concluded while [[global warming]] may cause a long-term weakening of the AMOC, it remains difficult to detect when analyzing changes since 1980, including both direct&nbsp;– as that time frame presents both periods of weakening and strengthening&nbsp;– and the magnitude of either change is uncertain, ranging between 5% and 25%. The review concluded with a call for more-sensitive and longer-term research.<ref>{{Cite journal |last1=Jackson |first1=Laura C. |last2=Biastoch |first2=Arne |last3=Buckley |first3=Martha W. |last4=Desbruyères |first4=Damien G. |last5=Frajka-Williams |first5=Eleanor |last6=Moat |first6=Ben |last7=Robson |first7=Jon |date=1 March 2022 |title=The evolution of the North Atlantic Meridional Overturning Circulation since 1980 |url=https://www.nature.com/articles/s43017-022-00263-2 |journal=Nature Reviews Earth & Environment |volume=3 |issue=4 |pages=241–254 |bibcode=2022NRvEE...3..241J |doi=10.1038/s43017-022-00263-2 |s2cid=247160367 |archive-date=14 April 2023 |access-date=3 October 2022 |archive-url=https://web.archive.org/web/20230414005837/https://www.nature.com/articles/s43017-022-00263-2 |url-status=live }}</ref>

==== 20th century ====

[[File:Latif 2022 AMOC 120y series.png|thumb|A 120-year trend of sea surface temperature ''differences'' from the mean warming trend&nbsp;– a proxy for AMOC state&nbsp;– shows no net change until around 1980.<ref name="Latif2022" />]]

Some reconstructions have attempted to compare the current state of the AMOC with that from a century or so earlier. For instance, a 2010 statistical analysis found a weakening of the AMOC has been continuing since the late 1930s with an abrupt shift of a North-Atlantic overturning cell around 1970.<ref>{{cite journal |author=Mihai Dima |author2=Gerrit Lohmann |year=2010 |title=Evidence for Two Distinct Modes of Large-Scale Ocean Circulation Changes over the Last Century |url=https://epic.awi.de/id/eprint/20790/1/Dim2009a.pdf |journal=Journal of Climate |volume=23 |issue=1 |pages=5–16 |bibcode=2010JCli...23....5D |doi=10.1175/2009JCLI2867.1 |archive-date=23 January 2023 |access-date=12 April 2022 |archive-url=https://web.archive.org/web/20230123111159/https://epic.awi.de/id/eprint/20790/1/Dim2009a.pdf |url-status=live }}</ref> In 2015, a different statistical analysis interpreted a cold pattern in some years of temperature records as a sign of AMOC weakening. It concluded the AMOC has weakened by 15–20% in 200 years and that the circulation slowed during most of the 20th century. Between 1975 and 1995, the circulation was weaker than at any time over the past millennium. This analysis had also shown a limited recovery after 1990 but the authors cautioned another decline is likely to occur in the future.<ref name="Rahmstorf2015">{{cite journal |last1=Rahmstorf |first1=Stefan |last2=Box |first2=Jason E. |last3=Feulner |first3=Georg |last4=Mann |first4=Michael E. |last5=Robinson |first5=Alexander |last6=Rutherford |first6=Scott |last7=Schaffernicht |first7=Erik J. |year=2015 |title=Exceptional twentieth-century slowdown in Atlantic Ocean overturning circulation |url=https://eprints.ucm.es/32657/1/robinson10postprint.pdf |journal=[[Nature Climate Change]] |volume=5 |issue=5 |pages=475–480 |bibcode=2015NatCC...5..475R |doi=10.1038/nclimate2554 |issn=1758-678X |archive-date=9 September 2016 |access-date=12 April 2022 |archive-url=https://web.archive.org/web/20160909080138/http://eprints.ucm.es/32657/1/robinson10postprint.pdf |url-status=live }} {{Closed access}} [http://wedocs.unep.org/bitstream/handle/20.500.11822/17802/Exceptional_twentiethcentury_slowdown_in_Atla.pdf PDF in UNEP Document Repository] {{Webarchive|url=https://web.archive.org/web/20190712144829/http://wedocs.unep.org/bitstream/handle/20.500.11822/17802/Exceptional_twentiethcentury_slowdown_in_Atla.pdf |date=12 July 2019 }}</ref>

In 2018, another reconstruction suggested a weakening of around 15% has occurred since the mid-twentieth century.<ref>{{Cite journal |last1=Caesar |first1=L. |last2=Rahmsdorf |first2=S. |last3=Robinson |first3=A. |last4=Feulner |first4=G. |last5=Saba |first5=V. |date=11 April 2018 |title=Observed fingerprint of a weakening Atlantic Ocean overturning circulation |journal=Nature |volume=556 |issue=7700 |pages=191–196 |bibcode=2018Natur.556..191C |doi=10.1038/s41586-018-0006-5 |pmid=29643485 |s2cid=4781781 |url=http://mural.maynoothuniversity.ie/13174/1/CL-geography_observed.pdf |archive-date=21 March 2024 |access-date=15 April 2024 |archive-url=https://web.archive.org/web/20240321182346/https://mural.maynoothuniversity.ie/13174/1/CL-geography_observed.pdf |url-status=live }}</ref> A 2021 reconstruction used over a century of ocean-temperature-and-salinity data, which appeared to show significant changes in eight independent AMOC indices that could indicate "an almost complete loss of stability". This reconstruction was forced to omit all data from 35 years before 1900 and after 1980 to maintain consistent records of all eight indicators.<ref name="Boers2021">{{cite journal |last1=Boers |first1=Niklas |title=Observation-based early-warning signals for a collapse of the Atlantic Meridional Overturning Circulation |journal=Nature Climate Change |date=August 2021 |volume=11 |issue=8 |pages=680–688 |doi=10.1038/s41558-021-01097-4 |bibcode=2021NatCC..11..680B |s2cid=236930519 |url=https://publications.pik-potsdam.de/pubman/item/item_25859_3/component/file_25954/25859oa.pdf }}</ref> These findings were challenged by 2022 research that used data recorded between 1900 and 2019, and found no change in the AMOC between 1900 and 1980, and a single-[[sverdrup]] reduction in AMOC strength did not occur until 1980, a variation that remains within range of natural variability.<ref name="Latif2022" />

Sediment analyses shows a weakening of the AMOC by 20% from the middle of the 20th century.<ref>{{cite web |last1=Menviel |first1=Laurie |title=Meltwater from Greenland and the Arctic is weakening ocean circulation, speeding up warming down south |url=https://www.unsw.edu.au/newsroom/news/2024/11/meltwater-greenland-arctic-weakening-ocean-circulation |website=UNSW Sydney |access-date=29 November 2024 |archive-date=30 November 2024 |archive-url=https://web.archive.org/web/20241130162334/https://www.unsw.edu.au/newsroom/news/2024/11/meltwater-greenland-arctic-weakening-ocean-circulation |url-status=live }}</ref>

==== Millennial scale ====

[[File:Michel 2022 AMOC reconstruction vs PMIP3.png|thumb|Model simulations of the Atlantic Multidecadal Variability over the past millennium (green) largely match a reconstruction based on coral and marine sediment evidence (blue) until the late 20th century. The sharp divergence could be caused by increasing "memory" of past atmospheric changes in the AMOC. This could precede its destabilization.<ref name="Michel2022" />]]

According to a 2018 study, in the last 150 years, the AMOC has demonstrated exceptional weakness when compared to the previous 1,500 years and indicated a discrepancy in the modeled timing of AMOC decline after the [[Little Ice Age]].<ref name="Thorn2018">{{Cite journal |last1=Thornalley |first1=David JR |display-authors=etal |date=11 April 2018 |title=Anomalously weak Labrador Sea convection and Atlantic overturning during the past 150 years |journal=Nature |volume=556 |issue=7700 |pages=227–230 |bibcode=2018Natur.556..227T |doi=10.1038/s41586-018-0007-4 |pmid=29643484 |s2cid=4771341 |url=https://centaur.reading.ac.uk/76608/1/Combined_accepted_text_figs_extended.pdf |archive-date=21 March 2024 |access-date=15 April 2024 |archive-url=https://web.archive.org/web/20240321182452/https://centaur.reading.ac.uk/76608/1/Combined_accepted_text_figs_extended.pdf |url-status=live }}</ref> A 2017 review concluded there is strong evidence for past changes in the strength and structure of the AMOC during abrupt climate events, such as the [[Younger Dryas]] and many of the [[Heinrich event]]s.<ref>{{cite journal|title=The Atlantic Meridional Overturning Circulation and Abrupt Climate Change|year=2017|journal=Annual Review of Marine Science|author=Jean Lynch-Stieglitz|volume=9 |pages=83–104 |author-link=Jean Lynch-Stieglitz|doi=10.1146/annurev-marine-010816-060415|pmid=27814029 |bibcode=2017ARMS....9...83L}}</ref> In 2022, another millennial-scale reconstruction found the Atlantic multidecadal variability strongly displayed increasing "memory", meaning it is now less likely to return to the mean state and instead would proceed in the direction of past variation. Because this pattern is likely connected to the AMOC, it could indicate a "quiet" loss of stability that is not seen in most models.<ref name="Michel2022">{{Cite journal |last1=Michel |first1=Simon L. L. |last2=Swingedouw |first2=Didier |last3=Ortega |first3=Pablo |last4=Gastineau |first4=Guillaume |last5=Mignot |first5=Juliette |last6=McCarthy |first6=Gerard |last7=Khodri |first7=Myriam |date=2 September 2022 |title=Early warning signal for a tipping point suggested by a millennial Atlantic Multidecadal Variability reconstruction |journal=Nature Communications |volume=13 |issue=1 |page=5176 |doi=10.1038/s41467-022-32704-3 |pmid=36056010 |pmc=9440003 |bibcode=2022NatCo..13.5176M}}</ref>

In February 2021, a major study in ''[[Nature Geoscience]]'' reported the preceding millennium saw an unprecedented weakening of the AMOC, an indication the change was caused by human actions.<ref name="Caesar2021" /><ref name="Harvey">{{cite news |last=Harvey |first=Fiona |author-link=Fiona Harvey |date=26 Feb 2021 |title=Atlantic Ocean circulation at weakest in a millennium, say scientists |url=https://www.theguardian.com/environment/2021/feb/25/atlantic-ocean-circulation-at-weakest-in-a-millennium-say-scientists |access-date=2021-02-27 |newspaper=[[The Guardian]] |archive-date=22 November 2023 |archive-url=https://web.archive.org/web/20231122093529/https://www.theguardian.com/environment/2021/feb/25/atlantic-ocean-circulation-at-weakest-in-a-millennium-say-scientists |url-status=live }}</ref> The study's co-author said the AMOC had already slowed by about 15% and effects now being seen; according to them: "In 20 to 30 years it is likely to weaken further, and that will inevitably influence our weather, so we would see an increase in storms and heatwaves in Europe, and sea level rises on the east coast of the US."<ref name="Harvey" /> In February 2022, ''Nature Geoscience'' published a "Matters Arising" commentary article co-authored by 17 scientists that disputed those findings and said the long-term AMOC trend remains uncertain.<ref name="Kilbourne2022" /> The journal also published a response from the authors of 2021 study, who defended their findings.<ref>{{Cite journal |last1=Caesar |first1=L. |last2=McCarthy |first2=G.D. |last3=Thornalley |first3=D. J. R. |last4=Cahill |first4=N. |last5=Rahmstorf |first5=S. |date=17 February 2022 |title=Reply to: Atlantic circulation change still uncertain |journal=Nature Geoscience |volume=15 |issue=3 |pages=168–170 |bibcode=2022NatGe..15..168C |doi=10.1038/s41561-022-00897-3 |s2cid=246901654 |url=https://discovery.ucl.ac.uk/id/eprint/10144398/ |archive-date=4 December 2024 |access-date=15 April 2024 |archive-url=https://web.archive.org/web/20241204070955/https://discovery.ucl.ac.uk/id/eprint/10144398/ |url-status=live }}</ref>

=== Possible indirect signs ===

[[File:16-008-NASA-2015RecordWarmGlobalYearSince1880-20160120.png|thumb|The cold blob visible on NASA's global mean temperatures for 2015, the warmest year on record up to 2015 since 1880. Colors indicate temperature evolution ([[NASA]]/[[NOAA]]; 20 January 2016).<ref name="NASA-20160120">{{cite web |last1=Brown |first1=Dwayne |last2=Cabbage |first2=Michael |last3=McCarthy |first3=Leslie |last4=Norton |first4=Karen |title=NASA, NOAA Analyses Reveal Record-Shattering Global Warm Temperatures in 2015 |url=http://www.nasa.gov/press-release/nasa-noaa-analyses-reveal-record-shattering-global-warm-temperatures-in-2015 |date=20 January 2016 |work=[[NASA]] |access-date=21 January 2016 |archive-date=20 January 2016 |archive-url=https://web.archive.org/web/20160120183259/http://www.nasa.gov/press-release/nasa-noaa-analyses-reveal-record-shattering-global-warm-temperatures-in-2015 |url-status=live }}</ref>]]

Some researchers have interpreted a range of recently observed climatic changes and trends as being connected to a decline in the AMOC; for instance, a large area of the [[North Atlantic Gyre]]<ref name="Allan2019" /> near Greenland has cooled by {{convert|0.39|C-change|F-change}} between 1900 and 2020, in contrast to substantial ocean warming elsewhere.<ref name="Fan2023" /> This cooling is normally seasonal; it is most-pronounced in February, when cooling reaches {{convert|0.9|C-change|F-change}} at the area's epicenter but it still experiences warming relative to pre-industrial levels during warm months, particularly in August.<ref name="Allan2019">{{Cite journal |last1=Allan |first1=David |last2=Allan |first2=Richard P. |date=5 December 2019 |title=Seasonal Changes in the North Atlantic Cold Anomaly: The Influence of Cold Surface Waters From Coastal Greenland and Warming Trends Associated With Variations in Subarctic Sea Ice Cover |journal=Journal of Geophysical Research: Oceans |language=en |volume=124 |issue=12 |pages=9040–9052 |doi=10.1029/2019JC015379 |bibcode=2019JGRC..124.9040A |url=https://centaur.reading.ac.uk/87865/1/Allan20JGR.pdf |archive-date=19 November 2023 |access-date=15 April 2024 |archive-url=https://web.archive.org/web/20231119073749/http://centaur.reading.ac.uk/87865/1/Allan20JGR.pdf |url-status=live }}</ref> Between 2014 and 2016, waters in the area stayed cool for 19 months before warming,<ref name="Shi2023">{{Cite journal |last1=Shi |first1=Jian |last2=Wang |first2=Jiaqi |last3=Ren |first3=Zixuan |last4=Tang |first4=Cong |last5=Huang |first5=Fei |date=3 May 2023 |title=Cold blobs in the subpolar North Atlantic: seasonality, spatial pattern, and driving mechanisms |journal=Ocean Dynamics |volume=73 |issue=5 |pages=267–278 |doi=10.1007/s10236-023-01553-z |bibcode=2023OcDyn..73..267S }}</ref> and media described this phenomenon as the ''[[cold blob]]''.<ref name=WP2015Sep />

The cold-blob pattern occurs because sufficiently fresh, cool water avoids sinking into deeper layers. This freshening was immediately described as evidence of a slowing of the AMOC slowdown.<ref name=WP2015Sep>{{cite news|url=https://www.washingtonpost.com/news/energy-environment/wp/2015/09/30/everything-you-need-to-know-about-the-cold-blob-in-the-north-atlantic-ocean/|title=Everything you need to know about the surprisingly cold 'blob' in the North Atlantic ocean|date=September 30, 2015|newspaper=The Washington Post|first=Chris|last=Mooney|archive-date=1 February 2023|access-date=12 April 2022|archive-url=https://web.archive.org/web/20230201134633/https://www.washingtonpost.com/news/energy-environment/wp/2015/09/30/everything-you-need-to-know-about-the-cold-blob-in-the-north-atlantic-ocean/|url-status=live}}</ref> Later research found atmospheric changes, such as an increase in low cloud cover<ref name="CB2020" /> and a strengthening of the [[North Atlantic oscillation]] (NAO) have also played a major role in this local cooling.<ref name="Fan2023">{{Cite journal |last1=Fan |first1=Yifei |last2=Liu |first2=Wei |last3=Zhang |first3=Pengfei |last4=Chen |first4=Ru |last5=Li |first5=Laifang |date=12 June 2023 |title=North Atlantic Oscillation contributes to the subpolar North Atlantic cooling in the past century |journal=Climate Dynamics |volume=61 |issue=11–12 |pages=5199–5215 |doi=10.1007/s00382-023-06847-y |bibcode=2023ClDy...61.5199F }}</ref> The overall importance of the NAO in the phenomenon is disputed<ref name="Shi2023" /> but cold-blob trends alone cannot be used to analyze the strength of the AMOC.<ref name="CB2020">{{cite web |last1=McSweeney |first1=Robert |date=29 June 2020 |title=Scientists shed light on human causes of North Atlantic's 'cold blob' |url=https://www.carbonbrief.org/scientists-shed-light-on-human-causes-of-north-atlantics-cold-blob/ |publisher=Carbon Brief |access-date=13 April 2024 |archive-date=29 March 2024 |archive-url=https://web.archive.org/web/20240329091620/https://www.carbonbrief.org/scientists-shed-light-on-human-causes-of-north-atlantics-cold-blob/ |url-status=live }}</ref>

Another possible early indication of a slowing of the AMOC is the relative reduction in the North Atlantic's potential to act as a carbon sink. Between 2004 and 2014, the amount of carbon sequestered in the North Atlantic declined by 20% relative to 1994–2004, which the researchers considered evidence of AMOC slowing. This decline was offset by a comparable increase in the South Atlantic, which is considered part of the Southern Ocean.<ref>{{Cite journal |last1=Müller |first1=Jens Daniel |last2=Gruber |first2=N. |last3=Carter |first3=B. |last4=Feely |first4=R. |last5=Ishii |first5=M. |last6=Lange |first6=N. |last7=Lauvset |first7=S. K. |last8=Murata |first8=A. |last9=Olsen |first9=A. |last10=Pérez |first10=F. F. |last11=Sabine |first11=C. |last12=Tanhua |first12=T. |last13=Wanninkhof |first13=R. |last14=Zhu |first14=D. |date=10 August 2023 |title=Decadal Trends in the Oceanic Storage of Anthropogenic Carbon From 1994 to 2014 |journal=AGU Advances |volume=4 |issue=4 |page=e2023AV000875 |doi=10.1029/2023AV000875 |bibcode=2023AGUA....400875M |hdl=10261/333982 |hdl-access=free }}</ref> While the total amount of carbon absorption by all carbon sinks is generally projected to increase throughout the 21st century, a continuing decline in the North Atlantic sink would have important implications.<ref>{{Cite book |last1=Canadell |first1=J.G. |last2=Monteiro |first2=P.M.S. |last3=Costa |first3=M.H. |last4=Cotrim da Cunha |first4=L. |last5=Cox |first5=P.M. |last6=Eliseev |first6=A.V. |last7=Henson |first7=S. |last8=Ishii |first8=M. |last9=Jaccard |first9=S. |last10=Koven |first10=C. |last11=Lohila |first11=A. |title=Climate Change 2021 – the Physical Science Basis |chapter=Global Carbon and Other Biogeochemical Cycles and Feedbacks |editor-last=Masson-Delmotte |editor-first=V. |editor2-last=Zhai |editor2-first=P. |editor3-last=Piran |editor3-first=A. |editor4-last=Connors |editor4-first=S.L. |editor5-last=Péan |editor5-first=C. |editor6-last=Berger |editor6-first=S. |editor7-last=Caud |editor7-first=N. |editor8-last=Chen |editor8-first=Y. |editor9-last=Goldfarb |editor9-first=L. |chapter-url=https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter05.pdf |year=2021 |volume=2021 |pages=673–816 |doi=10.1017/9781009157896.007 |bibcode=2021AGUFM.U13B..05K |isbn=9781009157896 |archive-date=6 April 2023 |access-date=23 May 2022 |archive-url=https://web.archive.org/web/20230406183849/https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter05.pdf |url-status=live }}</ref> Other processes that were attributed in some studies to AMOC slowing include increasing salinity in the South Atlantic,<ref>{{Cite journal |last1=Zhu |first1=Chenyu |last2=Liu |first2=Zhengyu |date=14 September 2020 |title=Weakening Atlantic overturning circulation causes South Atlantic salinity pile-up |journal=Nature Climate Change |volume=10 |issue=11 |pages=998–1003 |doi=10.1038/s41558-020-0897-7|bibcode=2020NatCC..10..998Z |s2cid=221674578 }}</ref> rapid [[ocean deoxygenation|deoxygenation]] in the [[Gulf of St. Lawrence]],<ref>{{cite journal |last1=Claret |first1=Mariona |last2=Galbraith |first2=Eric D. |last3=Palter |first3=Jaime B. |last4=Bianchi |first4=Daniele |last5=Fennel |first5=Katja |last6=Gilbert |first6=Denis |last7=Dunne |first7=John P. |title=Rapid coastal deoxygenation due to ocean circulation shift in the northwest Atlantic |journal=Nature Climate Change |date=17 September 2018 |volume=8 |issue=10 |pages=868–872 |doi=10.1038/s41558-018-0263-1|pmid=30416585 |pmc=6218011 |bibcode=2018NatCC...8..868C }}</ref><ref>{{Cite web |date=17 September 2018 |title=Large-scale shift causing lower-oxygen water to invade Canada's Gulf of St. Lawrence |url=https://phys.org/news/2018-09-large-scale-shift-lower-oxygen-invade-canada.html |access-date=13 April 2024 |website=Phys.org |language=en |archive-date=5 March 2024 |archive-url=https://web.archive.org/web/20240305021341/https://phys.org/news/2018-09-large-scale-shift-lower-oxygen-invade-canada.html |url-status=live }}</ref> and an approximately 10% decline in phytoplankton productivity across the North Atlantic over the past 200 years.<ref>{{cite journal |last1=Osman |first1=Matthew B. |last2=Das |first2=Sarah B. |last3=Trusel |first3=Luke D. |last4=Evans |first4=Matthew J. |last5=Fischer |first5=Hubertus |last6=Grieman |first6=Mackenzie M. |last7=Kipfstuhl |first7=Sepp |last8=McConnell |first8=Joseph R. |last9=Saltzman |first9=Eric S. |title=Industrial-era decline in subarctic Atlantic productivity |journal=Nature |date=6 May 2019 |volume=569 |issue=7757 |pages=551–555 |doi=10.1038/s41586-019-1181-8|pmid=31061499 |bibcode=2019Natur.569..551O |s2cid=146118196 }}</ref>

== Projections ==
=== Individual models ===
[[File:Bellomo 2021 CMIP AMOC differences.png|thumb|Climate models are often calibrated by comparing their simulations after {{CO2}} concentrations have been suddenly quadrupled. Under those conditions, older fifth-generation climate models (top) simulate substantially smaller declines in AMOC strength than the sixth generation (bottom) <ref name="Bellomo2021" />]]

Historically, [[Coupled Model Intercomparison Project|CMIP]] models, the gold standard in climate science, show the AMOC is very stable; although it may weaken, it will always recover rather than permanently collapse&nbsp;– for example, in a 2014 idealized experiment in which {{CO2}} concentrations abruptly double from 1990 levels and do not change afterward, the circulation declines by around 25% but does not collapse, although it recovers by only 6% over the next 1,000 years.<ref>{{Cite journal |last1=Zhu |first1=Jiang |last2=Liu |first2=Zhengyu |last3=Zhang |first3=Jiaxu |last4=Liu |first4=Wei |date=14 May 2014 |title=AMOC response to global warming: dependence on the background climate and response timescale |journal=Climate Dynamics |volume=44 |issue=11–12 |pages=3449–3468 |doi=10.1007/s00382-014-2165-x }}</ref> In 2020, research estimated if warming stabilizes at {{convert|1.5|C-change|F-change}}, {{convert|2|C-change|F-change}} or {{convert|3|C-change|F-change}} by 2100; in all three cases, the AMOC declines for an additional 5–10 years after the temperature rise ceases but does not approach collapse, and partially recovers after about 150 years.<ref name="Sigmond2020">{{Cite journal |last1=Sigmond |first1=Michael |last2=Fyfe |first2=John C. |last3=Saenko |first3=Oleg A. |last4=Swart |first4=Neil C. |date=1 June 2020 |title=Ongoing AMOC and related sea-level and temperature changes after achieving the Paris targets |journal=Nature Climate Change |volume=10 |issue=7 |pages=672–677 |doi=10.1038/s41558-020-0786-0|bibcode=2020NatCC..10..672S |s2cid=219175812 }}</ref>

Many researchers have said collapse is only avoided due to biases that persist across the large-scale models.<ref name="Drijfhout2010" /><ref name="Valdes2011">{{Cite journal |last=Valdes |first=Paul |date=2011 |title=Built for stability |journal=Nature Geoscience |language=en |volume=4 |issue=7 |pages=414–416 |doi=10.1038/ngeo1200 |bibcode=2011NatGe...4..414V |issn=1752-0908}}</ref> While models have improved over time, the sixth and {{as of|2020|alt=as of 2020}}<ref>{{cite web |last1=Hausfather |first1=Zeke |date=2 December 2019 |title=CMIP6: the next generation of climate models explained |url=https://www.carbonbrief.org/cmip6-the-next-generation-of-climate-models-explained/ |publisher=Carbon Brief }}</ref> current generation CMIP6, retains some inaccuracies. On average, those models simulate much greater AMOC weakening in response to greenhouse warming than the previous generation;<ref name="Bellomo2021">{{Cite journal |last1=Bellomo |first1=Katinka |last2=Angeloni |first2=Michela |last3=Corti |first3=Susanna |last4=von Hardenberg |first4=Jost |date=16 June 2021 |title=Future climate change shaped by inter-model differences in Atlantic meridional overturning circulation response |journal=Nature Communications |volume=12 |issue=1 |page=3659 |doi=10.1038/s41467-021-24015-w |pmid=34135324 |bibcode=2021NatCo..12.3659B |pmc=8209213 }}</ref> when four CMIP6 models simulated the AMOC under the [[Shared Socioeconomic Pathways|SSP3-7]] scenario in which {{CO2}} levels more than double from 2015 values by 2100 from around 400 [[parts per million]] (ppm) to over 850 ppm,<ref name="IPCC_2021_WGI_SPM">IPCC, 2021: [https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_SPM.pdf Summary for Policymakers] {{Webarchive|url=https://web.archive.org/web/20210811205522/https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_SPM.pdf |date=11 August 2021 }}. In: [https://www.ipcc.ch/report/ar6/wg1/ Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change] {{Webarchive|url=https://web.archive.org/web/20230526182346/https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter09.pdf |date=26 May 2023 }} [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, New York, US, pp. 3−32, doi:10.1017/9781009157896.001.</ref>{{rp|14}} they found it declined by over 50% by 2100.<ref name="Hassan2022">{{Cite journal |last1=Hassan |first1=Taufiq |last2=Allen |first2=Robert J. |display-authors=etal  |date=27 June 2022 |title=Air quality improvements are projected to weaken the Atlantic meridional overturning circulation through radiative forcing effects |journal=Communications Earth & Environment|volume=3 |issue=3 |page=149 |doi=10.1038/s43247-022-00476-9|bibcode=2022ComEE...3..149H |s2cid=250077615 |doi-access=free }}</ref> The CMIP6 models are not yet capable of simulating [[North Atlantic Deep Water]] (NADW) without errors in its depth, area or both, reducing confidence in their projections.<ref name="Heuze2021">{{Cite journal |last1=Heuze |first1=Celine |date=13 January 2021 |title=Antarctic Bottom Water and North Atlantic Deep Water in CMIP6 models |journal=Ocean Science |volume=17 |issue=1 |pages=59–90 |doi=10.5194/os-17-59-2021 |doi-access=free|bibcode=2021OcSci..17...59H }}</ref>

[[File:Hassan 2022 AMOC aerosols.png|thumb|left|If {{CO2}} concentrations were to double by 2100 from their 2015 values, then the AMOC strength would decline by over 50%. Reductions in methane warming or sulfate aerosol cooling, or both, would have an effect of around 10% by comparison <ref name="Hassan2022" />]]

To address these problems, some scientists experimented with bias correction. In another idealized {{CO2}} doubling experiment, the AMOC collapsed after 300 years when bias correction was applied to the model.<ref name="Liu2017" /> One 2016 experiment combined projections from eight then-state-of-the-art CMIP5 climate models with the improved Greenland ice-sheet melt estimates. It found by 2090–2100, the AMOC would weaken by around 18% (3%–34%) under the intermediate [[Representative Concentration Pathway]] 4.5, and by 37% (15%–65%) under the very high Representative Concentration Pathway 8.5, in which [[greenhouse gas emissions]] increase continuously. When the two scenarios were extended past 2100, the AMOC stabilized under RCP 4.5 but continued to decline under RCP 8.5, leading to an average decline of 74% by 2290–2300 and a 44% likelihood of a complete collapse.<ref name="Bakker2016">{{Cite journal |last1=Bakker |first1=P |last2=Schmittner |first2=A |last3=Lenaerts |first3=JT |last4=Abe-Ouchi |first4=A |last5=Bi |first5=D |last6=van den Broeke |first6=MR |last7=Chan |first7=WL |last8=Hu |first8=A |last9=Beadling |first9=RL |last10=Marsland |first10=SJ |last11=Mernild |first11=SH |last12=Saenko |first12=OA |last13=Swingedouw |first13=D |last14=Sullivan |first14=A |last15=Yin |first15=J |date=11 November 2016 |title=Fate of the Atlantic Meridional Overturning Circulation: Strong decline under continued warming and Greenland melting |journal=Geophysical Research Letters |volume=43 |issue=23 |pages=12,252–12,260 |doi=10.1002/2016GL070457|bibcode=2016GeoRL..4312252B |hdl=10150/622754 |s2cid=133069692 |hdl-access=free }}</ref>

In 2020, another team of researchers simulated RCP 4.5 and RCP 8.5 between 2005 and 2250 in a [[Community Earth System Model]] that was integrated with an advanced ocean physics module. Due to the module, the AMOC was subjected to four-to-ten times more freshwater when compared to the standard run. It simulated for RCP 4.5 very similar results to those of the 2016 study while below RCP 8.5, the circulation declines by two-thirds soon after 2100 but does not collapse past that level.<ref>{{Cite journal |last1=Sadai |first1=Shaina |last2=Condron |first2=Alan |last3=DeConto |first3=Robert |last4=Pollard |first4=David |date=23 September 2020 |title=Future climate response to Antarctic Ice Sheet melt caused by anthropogenic warming |journal=Science Advances |volume=6 |issue=39 |doi=10.1126/sciadv.aaz1169|pmid=32967838 |pmc=7531873 |bibcode=2020SciA....6.1169S }}</ref>

In 2023, a statistical analysis of output from multiple intermediate-complexity models suggested an AMOC collapse would most likely happen around 2057 with 95% confidence of a collapse between 2025 and 2095.<ref name="Ditlevsen2023">{{Cite journal |last1=Ditlevsen |first1=Peter |last2=Ditlevsen |first2=Susanne |date=2023-07-25 |title=Warning of a forthcoming collapse of the Atlantic meridional overturning circulation |journal=Nature Communications |language=en |volume=14 |issue=1 |pages=4254 |doi=10.1038/s41467-023-39810-w |pmid=37491344 |issn=2041-1723|pmc=10368695 |arxiv=2304.09160 |bibcode=2023NatCo..14.4254D }}</ref> This study received a lot of attention and criticism because intermediate-complexity models are considered less reliable in general and may confuse a major slowing of the circulation with its complete collapse. The study relied on [[proxy (climate)|proxy]] temperature data from the Northern Subpolar Gyre region, which other scientists do not consider representative of the entire circulation, believing it may be subject to a separate tipping point. Some scientists have described this research as "worrisome" and noted it can provide a "valuable contribution" once better observational data is available but there was widespread agreement among experts the paper's proxy record was "insufficient".<ref name="SMC2023" /> Some experts said the study used old observational data from five ship surveys that "has long been discredited" by the lack of major weakening seen in direct observations since 2004, "including in the reference they cite for it".<ref name="SMC2023">{{Cite web |date=25 July 2023 |title=expert reaction to paper warning of a collapse of the Atlantic meridional overturning circulation |url=https://www.sciencemediacentre.org/expert-reaction-to-paper-warning-of-a-collapse-of-the-atlantic-meridional-overturning-circulation/ |access-date=11 August 2023 |website=Science Media Centre |language=en |archive-date=12 October 2023 |archive-url=https://web.archive.org/web/20231012194242/http://www.sciencemediacentre.org/expert-reaction-to-paper-warning-of-a-collapse-of-the-atlantic-meridional-overturning-circulation/ |url-status=live }}</ref>

In November 2024, ''[[Nature Geoscience]]'' published a study which tried to solve the problem of the differences between observation showing a slowdown of the circulation and the climate models which showing a stable circulation. The scientists used some new methods and models. Then, the observations and the models corresponded to each other much better. The study found a slowdown of 0.46 sverdrups per decade since 1950. In the future, 2 °C temperature rise will weaken AMOC by 33% in comparison to a state without climate change, which could be reached over the coming decade. The result will be big changes in climate and ecosystems. It also found that AMOC weakening raise temperature and salinity in the [[South Atlantic Ocean]] through the propagation of [[Kelvin wave|Kelvin]] and [[Rossby wave|Rossby waves]].<ref name=Nature/>

=== Major review studies ===

[[File:Tipping_points_2022_list.jpeg|thumb|AMOC is considered to be one of the several major parts of the climate system which could pass tipping point around a certain level of warming and eventually transition to a different state as a result. The graphic shows the levels of warming where this tipping is most likely to occur for a given element<ref>{{Cite web |title=Tipping Elements – big risks in the Earth System |url=https://www.pik-potsdam.de/en/output/infodesk/tipping-elements |publisher=[[Potsdam Institute for Climate Impact Research]] |access-date=31 January 2024 |archive-date=31 January 2024 |archive-url=https://web.archive.org/web/20240131155942/https://www.pik-potsdam.de/en/output/infodesk/tipping-elements |url-status=live }}</ref><ref name="ArmstrongMcKay2022" />]]

Large review papers and reports are capable of evaluating model output, direct observations and historical reconstructions to make expert judgements beyond what models alone can show. Around 2001, the [[IPCC Third Assessment Report]] projected high confidence the AMOC thermohaline circulation would weaken rather than stop and that warming effects would outweigh cooling, even over Europe.<ref name=TAR_WG1_Ch9>{{Cite book |year=2001 |author =(( IPCC TAR WG1)) |author-link=IPCC |chapter=9.3.4.3 Thermohaline circulation changes |chapter-url =http://www.grida.no/climate/ipcc_tar/wg1/357.htm |title=Climate Change 2001: The Scientific Basis |series=Contribution of Working Group I to the [[IPCC Third Assessment Report|Third Assessment Report]] of the Intergovernmental Panel on Climate Change |editor1=Houghton, J.T. |editor2=Ding, Y. |editor3=Griggs, D.J. |editor4=Noguer, M. |editor5=van der Linden, P.J. |editor6=Dai, X. |editor7    =Maskell, K. |editor8=Johnson, C.A. |publisher=Cambridge University Press |url=https://archive.org/details/climatechange2000000unse |isbn=978-0-521-80767-8 |df=dmy-all |url-access =registration}} (pb: {{ISBNT|0-521-01495-6}})</ref> When the [[IPCC Fifth Assessment Report]] was published in 2014, a rapid transition of the AMOC was considered "very unlikely" and this assessment was offered at a high level of confidence.<ref>{{cite web|title=IPCC AR5 WG1|url=http://www.ipcc.ch/pdf/assessment-report/ar5/wg1/WG1AR5_ALL_FINAL.pdf |website=IPCC|page=Table 12.4|url-status=dead|archive-url=https://web.archive.org/web/20150824065154/http://www.ipcc.ch/pdf/assessment-report/ar5/wg1/WG1AR5_ALL_FINAL.pdf |archive-date=24 August 2015}}</ref>

In 2021, the [[IPCC Sixth Assessment Report]] again said the AMOC is "very likely" to decline within the 21st century and that there was a "high confidence" changes to it would be reversible within centuries if warming was reversed.<ref name="SROCC_SPM_20190925"/>{{rp|19}} Unlike the Fifth Assessment Report, it had only "medium confidence" rather than "high confidence" in the AMOC avoiding a collapse before the end of the 21st century. This reduction in confidence was likely influenced by several review studies that draw attention to the circulation stability bias within [[general circulation model]]s,<ref>{{Cite journal |last1=Mecking |first1=J.V. |last2=Drijfhout |first2=S.S. |last3=Jackson |first3=L.C. |last4=Andrews |first4=M.B. |date=2017-01-01 |title=The effect of model bias on Atlantic freshwater transport and implications for AMOC bi-stability |journal=Tellus A: Dynamic Meteorology and Oceanography |volume=69 |issue=1 |pages=1299910 |doi=10.1080/16000870.2017.1299910 |bibcode=2017TellA..6999910M |s2cid=133294706|doi-access=free }}</ref><ref>{{Cite journal |last1=Weijer |first1=W. |last2=Cheng |first2=W. |last3=Drijfhout |first3=S. S. |last4=Fedorov |first4=A. V. |last5=Hu |first5=A. |last6=Jackson |first6=L. C. |last7=Liu |first7=W. |last8=McDonagh |first8=E. L. |last9=Mecking |first9=J. V. |last10=Zhang |first10=J. |date=2019 |title=Stability of the Atlantic Meridional Overturning Circulation: A Review and Synthesis |journal=Journal of Geophysical Research: Oceans |language=en |volume=124 |issue=8 |pages=5336–5375 |doi=10.1029/2019JC015083 |bibcode=2019JGRC..124.5336W |issn=2169-9275 |s2cid=199807871|doi-access=free }}</ref> and simplified ocean-modelling studies suggesting the AMOC may be more vulnerable to abrupt change than larger-scale models suggest.<ref name="Lohmann2021">{{Cite journal |last1=Lohmann |first1=Johannes |last2=Ditlevsen |first2=Peter D. |date=2021-03-02 |title=Risk of tipping the overturning circulation due to increasing rates of ice melt |journal=Proceedings of the National Academy of Sciences |language=en |volume=118 |issue=9 |pages=e2017989118 |doi=10.1073/pnas.2017989118 |issn=0027-8424 |pmc=7936283 |pmid=33619095|bibcode=2021PNAS..11817989L |doi-access=free }}</ref>

The synthesis report of the IPCC Sixth Assessment Report summarized the scientific consensus as follows: "The Atlantic Meridional Overturning Circulation is very likely to weaken over the 21st century for all considered scenarios (high confidence), however an abrupt collapse is not expected before 2100 (medium confidence). If such a low probability event were to occur, it would very likely cause abrupt shifts in regional weather patterns and water cycle, such as a southward shift in the tropical rain belt, and large impacts on ecosystems and human activities."<ref>{{cite book |url=https://www.ipcc.ch/report/ar6/syr/downloads/report/IPCC_AR6_SYR_FullVolume.pdf |title=Climate Change 2023: Synthesis Report. Contribution of Working Groups I, II and III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change |date=2023 |publisher=United Nations |location=Geneva, Switzerland |pages=18, 78 |access-date=21 November 2024 |archive-date=25 July 2023 |archive-url=https://web.archive.org/web/20230725180253/https://www.ipcc.ch/report/ar6/syr/downloads/report/IPCC_AR6_SYR_FullVolume.pdf |url-status=live }}</ref> 

In 2022, an extensive assessment of all potential [[tipping points in the climate system|climate tipping points]] identified 16 plausible climate tipping points, including a collapse of the AMOC. It said a collapse would most likely be triggered by {{convert|4|C-change|F-change}} of global warming but that there is enough uncertainty to suggest it could be triggered at warming levels of between {{convert|1.4|C-change|F-change}} and {{convert|8|C-change|F-change}}. The assessment estimates once AMOC collapse is triggered, it would occur between 15 and 300 years, and most likely at around 50 years.<ref name="ArmstrongMcKay2022">{{Cite journal |last1=Armstrong McKay |first1=David |last2=Abrams |first2=Jesse |last3=Winkelmann |first3=Ricarda |last4=Sakschewski |first4=Boris |last5=Loriani |first5=Sina |last6=Fetzer |first6=Ingo |last7=Cornell |first7=Sarah |last8=Rockström |first8=Johan |last9=Staal |first9=Arie |last10=Lenton |first10=Timothy |date=9 September 2022 |title=Exceeding 1.5°C global warming could trigger multiple climate tipping points |url=https://www.science.org/doi/10.1126/science.abn7950 |journal=Science |language=en |volume=377 |issue=6611 |pages=eabn7950 |doi=10.1126/science.abn7950 |pmid=36074831 |hdl=10871/131584 |s2cid=252161375 |issn=0036-8075 |hdl-access=free |archive-date=14 November 2022 |access-date=3 October 2022 |archive-url=https://web.archive.org/web/20221114143835/https://www.science.org/doi/10.1126/science.abn7950 |url-status=live }}</ref><ref name="ArmstrongMcKayExplainer">{{Cite web |last=Armstrong McKay |first=David |date=9 September 2022 |title=Exceeding 1.5°C global warming could trigger multiple climate tipping points – paper explainer |url=https://climatetippingpoints.info/2022/09/09/climate-tipping-points-reassessment-explainer/ |access-date=2 October 2022 |website=climatetippingpoints.info |language=en |archive-date=18 July 2023 |archive-url=https://web.archive.org/web/20230718085705/https://climatetippingpoints.info/2022/09/09/climate-tipping-points-reassessment-explainer/ |url-status=live }}</ref> The assessment also treated the collapse of the [[North Atlantic Current|Northern Subpolar Gyre]] as a separate tipping point that could tip at between {{convert|1.1|C-change|F-change}} degrees and {{convert|3.8|C-change|F-change}}, although this is only simulated by a fraction of climate models. The most likely tipping point for the collapse of Northern Subpolar Gyre is {{convert|1.8|C-change|F-change}} and once triggered, the collapse of the gyre would occur between 5 and 50 years, and most likely at 10 years. The loss of this convection is estimated to lower the global temperature by {{convert|0.5|C-change|F-change}} while the average temperature in Europe would decrease by around {{convert|3|C-change|F-change}}. There would also be substantial effects on regional precipitation levels.<ref name="ArmstrongMcKay2022" /><ref name="ArmstrongMcKayExplainer" />

The "State of the cryosphere" report, dedicates significant space to AMOC, saying it may be enroute to collapse because of ice melt and water warming. Impacts will include cooling of Northern Europe faster than 3°C per decade, "with no realistic means of adaptation". At the same time, the [[Antarctic Circumpolar Current |Antarctic Circumpolar Current (ACC)]] is also slowing down and the [[Weddell Sea Bottom Water]] is losing volume, what can impact global ocean circulation and climate.<ref>{{cite book |title=State of the Cryosphere 2024 Lost Ice, Global Damage |date=November 2024 |publisher=International Cryosphere Climate Initiative |pages=x (in the beginning), 8, 48, 52 |url=https://articles.unesco.org/sites/default/files/medias/fichiers/2024/11/State%20of%20the%20Cryosphere%20Report%202024.pdf |access-date=20 November 2024 |archive-date=19 November 2024 |archive-url=https://web.archive.org/web/20241119235250/https://articles.unesco.org/sites/default/files/medias/fichiers/2024/11/State%20of%20the%20Cryosphere%20Report%202024.pdf |url-status=live }}</ref> [[UNESCO]] mentions that the report in the first time "notes a growing scientific consensus that melting Greenland and Antarctic ice sheets, among other factors, may be slowing important ocean currents at both poles, with potentially dire consequences for a much colder northern Europe and greater sea-level rise along the U.S. East Coast."<ref>{{cite news |title=State of the Cryosphere Report 2024 Lost Ice, Global Damage |url=https://www.unesco.org/en/articles/state-cryosphere-report-2024 |website=UNESCO |access-date=20 November 2024 |archive-date=21 November 2024 |archive-url=https://web.archive.org/web/20241121185035/https://www.unesco.org/en/articles/state-cryosphere-report-2024 |url-status=live }}</ref>

In February 2025, a study published in ''[[Nature (journal)|Nature]]'' concluded that the AMOC is resilient to extreme greenhouse gas and North Atlantic freshwater forcings across 34 climate models, suggesting that an AMOC collapse is unlikely in the 21st century.<ref name=Nature_20250226>{{cite journal |last1=Baker |first1=J.A. |last2=Bell |first2=M.J. |last3=Jackson |first3=L.C. |last4=Vallis |first4=G.K. |last5=Watson |first5=A.J. |last6=Wood |first6=R.A. |title=Continued Atlantic overturning circulation even under climate extremes |journal=Nature |doi=10.1038/s41586-024-08544-0 |volume=638 |pages=987-994 |date=26 February 2025|hdl=10871/139390 |hdl-access=free }}</ref>

== Effects of AMOC slowdown ==

[[File:Tzedakis 2018 AMOC slowdown LIG.png|thumb|AMOC was weaker than now during the last interglacial period, and this had been connected to the cooling of North Atlantic Ocean temperatures and the reduction in precipitation over Europe and Africa (blue) <ref name="Tzedakis2018" />]]

{{As of|2024}}, there is no consensus on whether a consistent slowing of the AMOC circulation has occurred but there is little doubt it will occur in the event of continued climate change.<ref name="AR6_WG1_Chapter92">Fox-Kemper, B., H.T. Hewitt, C. Xiao, G. Aðalgeirsdóttir, S.S. Drijfhout, T.L. Edwards, N.R. Golledge, M. Hemer, R.E. Kopp, G.  Krinner, A. Mix, D. Notz, S. Nowicki, I.S. Nurhati, L. Ruiz, J.-B. Sallée, A.B.A. Slangen, and Y. Yu, 2021: [https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter09.pdf Chapter 9: Ocean, Cryosphere and Sea Level Change] {{Webarchive|url=https://web.archive.org/web/20221024162651/https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter09.pdf |date=24 October 2022 }}. In [https://www.ipcc.ch/report/ar6/wg1/ Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change] {{Webarchive|url=https://web.archive.org/web/20230526182346/https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter09.pdf |date=26 May 2023 }} [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. 1211–1362, {{doi|10.1017/9781009157896.011}}</ref> According to the IPCC, the most-likely effects of future AMOC decline are reduced precipitation in mid-latitudes, changing patterns of strong precipitation in the tropics and Europe, and strengthening storms that follow the North Atlantic track.<ref name="AR6_WG1_Chapter92" /> In 2020, research found a weakened AMOC would slow [[Arctic sea ice decline|the decline in Arctic sea ice]].<ref name="Liu2020" /> and result in atmospheric trends similar to those that likely occurred during the [[Younger Dryas]],<ref name="IPCC AR6 WG1 Ch.8" /> such as a southward displacement of [[Intertropical Convergence Zone]]. Changes in precipitation under high-emissions scenarios would be far larger.<ref name="Liu2020">{{Cite journal |last1=Liu |first1=Wei |last2=Fedorov |first2=Alexey V. |last3=Xie |first3=Shang-Ping |last4=Hu |first4=Shineng |date=26 June 2020 |title=Climate impacts of a weakened Atlantic Meridional Overturning Circulation in a warming climate |journal=Science Advances |volume=6 |issue=26 |pages=eaaz4876 |doi=10.1126/sciadv.aaz4876 |pmid=32637596 |pmc=7319730 |bibcode=2020SciA....6.4876L }}</ref>

A decline in the AMOC would be accompanied by an acceleration of sea level rise along the [[U.S. East Coast]];<ref name="AR6_WG1_Chapter92" /> at least one such event has been connected to a temporary slowing of the AMOC.<ref>{{cite news|url=http://www.usclivar.org/research-highlights/extreme-sea-level-rise-event-linked-amoc-downturn|archive-url=https://web.archive.org/web/20150518093925/http://www.usclivar.org/research-highlights/extreme-sea-level-rise-event-linked-amoc-downturn|url-status=dead|archive-date=18 May 2015|title=Extreme sea level rise event linked to AMOC downturn|date=25 March 2015|publisher=CLIVAR|first1=Jianjun |last1=Yin  |first2=Stephen |last2=Griffies |name-list-style=amp }}</ref> This effect would be caused by increased warming and thermal expansion of coastal waters, which would transfer less of their heat toward Europe; it is one of the reasons sea level rise along the U.S. East Coast is estimated to be three-to-four times higher than the global average.<ref>{{cite news|url=https://www.washingtonpost.com/news/energy-environment/wp/2016/02/01/why-the-u-s-east-coast-could-be-a-major-hotspot-for-sea-level-rise/|title=Why the U.S. East Coast could be a major 'hotspot' for rising seas|first=Chris|last=Mooney|date=February 1, 2016|newspaper=The Washington Post|archive-date=31 March 2020|access-date=12 April 2022|archive-url=https://web.archive.org/web/20200331080306/https://www.washingtonpost.com/news/energy-environment/wp/2016/02/01/why-the-u-s-east-coast-could-be-a-major-hotspot-for-sea-level-rise/|url-status=live}}</ref><ref>{{cite journal| last1=Karmalkar | first1=Ambarish V. | last2=Horton | first2=Radley M. |date=23 September 2021 | title=Drivers of exceptional coastal warming in the northeastern United States |journal = [[Nature (journal)|Nature Climate Change]]| volume = 11 | issue=10 |pages=854–860 | doi=10.1038/s41558-021-01159-7 | bibcode=2021NatCC..11..854K | s2cid=237611075 }}</ref><ref>{{cite web |url=https://news.climate.columbia.edu/2021/09/23/the-u-s-northeast-coast-is-a-global-warming-hot-spot-says-study/ |title=Why the U.S. Northeast Coast Is a Global Warming Hot Spot |last1=Krajick |first1=Kevin |publisher=[[Columbia Climate School]] |date=23 September 2021 |access-date=2023-03-23 |archive-date=27 September 2023 |archive-url=https://web.archive.org/web/20230927015159/https://news.climate.columbia.edu/2021/09/23/the-u-s-northeast-coast-is-a-global-warming-hot-spot-says-study/ |url-status=live }}</ref>

[[File:Wunderling 2021 tipping cascade.png|thumb|left|A proposed tipping cascade where the AMOC would mediate a connection between the other tipping elements.]]

Some scientists believe a partial slowing of the AMOC would result in limited cooling of around {{convert|1|C-change|F-change}} in Europe.<ref name="PhysorgJan2016">{{cite news|url=http://phys.org/news/2016-01-greenland-ice-sheet-affect-global.html|title=Melting Greenland ice sheet may affect global ocean circulation, future climate|date=January 22, 2016|author=University of South Florida|work=Phys.org|archive-date=19 August 2023|access-date=12 April 2022|archive-url=https://web.archive.org/web/20230819184753/https://phys.org/news/2016-01-greenland-ice-sheet-affect-global.html|url-status=live}}</ref><ref>{{cite web|url=http://csas.ei.columbia.edu/2015/09/21/predictions-implicit-in-ice-melt-paper-and-global-implications/|archive-url=https://web.archive.org/web/20150923214610/http://csas.ei.columbia.edu/2015/09/21/predictions-implicit-in-ice-melt-paper-and-global-implications/|url-status=dead|archive-date=2015-09-23|first1=James |last1=Hansen |first2=Makiko |last2=Sato |year=2015|title=Predictions Implicit in "Ice Melt" Paper and Global Implications}}</ref><ref name="Tzedakis2018">{{cite journal |last1=Tzedakis |first1=P. C. |last2=Drysdale |first2=R. N. |last3=Margari |first3=V. |last4=Skinner |first4=L. C. |last5=Menviel |first5=L. |last6=Rhodes |first6=R. H. |last7=Taschetto |first7=A. S. |last8=Hodell |first8=D. A. |last9=Crowhurst |first9=S. J. |last10=Hellstrom |first10=J. C. |last11=Fallick |first11=A. E. |last12=Grimalt |first12=J. O. |last13=McManus |first13=J. F. |last14=Martrat |first14=B. |last15=Mokeddem |first15=Z. |last16=Parrenin |first16=F. |last17=Regattieri |first17=E. |last18=Roe |first18=K. |last19=Zanchetta |first19=G. |date=12 October 2018 |title=Enhanced climate instability in the North Atlantic and southern Europe during the Last Interglacial |journal=Nature Communications |volume=9 |issue=1 |page=4235 |doi=10.1038/s41467-018-06683-3 |pmid=30315157 |pmc=6185935 |bibcode=2018NatCo...9.4235T |hdl=11343/220077 |hdl-access=free }}</ref> Other regions would be differently affected; according to 2022 research, 20th-century winter-weather extremes in [[Siberia]] were milder when the AMOC was weakened.<ref name="Wang2022">{{Cite journal |last1=Wang |first1=Huan |last2=Zuo |first2=Zhiyan |last3=Qiao |first3=Liang |last4=Zhang |first4=Kaiwen |last5=Sun |first5=Cheng |last6=Xiao |first6=Dong |last7=Lin |first7=Zouxing |last8=Bu |first8=Lulei |last9=Zhang |first9=Ruonan |date=4 November 2022 |title=Frequency of the winter temperature extremes over Siberia dominated by the Atlantic Meridional Overturning Circulation |journal=npj Climate and Atmospheric Science |volume=5 |issue=1 |page=84 |doi=10.1038/s41612-022-00307-w |bibcode=2022npCAS...5...84W |doi-access=free }}</ref> According to one assessment, a slowing of the AMOC is one of the few climate tipping points that are likely to reduce the [[social cost of carbon]], a common measure of [[economic impacts of climate change]], by −1.4% rather than increasing it, because Europe represents a larger fraction of global [[GDP]] than the regions that will be negatively affected by the slowing.<ref>{{Cite journal |last1=Dietz |first1=Simon |last2=Rising |first2=James |last3=Stoerk |first3=Thomas |last4=Wagner |first4=Gernot |date=24 August 2021 |title=Economic impacts of tipping points in the climate system |journal=[[Proceedings of the National Academy of Sciences]] |volume=118 |issue=34 |pages=e2103081118|doi=10.1073/pnas.2103081118 |pmid=34400500 |pmc=8403967 |bibcode=2021PNAS..11803081D |doi-access=free }}</ref> This study's methods have been said to have underestimating climate impacts in general.<ref>{{Cite journal |last1=Keen |first1=Steve |last2=Lenton |first2=Timothy M. |last3=Garrett |first3=Timothy J. |last4=Rae |first4=James W. B. |last5=Hanley |first5=Brian P. |last6=Grasselli |first6=Matheus |date=19 May 2022 |title=Estimates of economic and environmental damages from tipping points cannot be reconciled with the scientific literature |journal=Proceedings of the National Academy of Sciences |volume=119 |issue=21 |pages=e2117308119 |doi=10.1073/pnas.2117308119 |doi-access=free |pmid=35588449 |pmc=9173761 |bibcode=2022PNAS..11917308K }}</ref><ref>{{Cite journal |last1=Dietz |first1=Simon |last2=Rising |first2=James |last3=Stoerk |first3=Thomas |last4=Wagner |first4=Gernot |date=19 May 2022 |title=Reply to Keen et al.: Dietz et al. modeling of climate tipping points is informative even if estimates are a probable lower bound |journal=Proceedings of the National Academy of Sciences |volume=119 |issue=21 |pages= e2201191119 |doi=10.1073/pnas.2201191119 |doi-access=free |pmid=35588452 |pmc=9173815 |bibcode=2022PNAS..11901191D }}</ref> According to some research, the dominant effect on an AMOC slowdown would be a reduction in oceanic heat uptake, leading to increased global warming,<ref name="Chen2018">{{Cite journal |last1=Chen |first1=Xianyao |last2=Tung |first2=Ka-Kit |date=18 July 2018 |title=Global surface warming enhanced by weak Atlantic overturning circulation |journal=Nature |volume=559 |issue=7714 |pages=387–391 |doi=10.1038/s41586-018-0320-y |pmid=30022132 |bibcode=2018Natur.559..387C |s2cid=49865284 }}</ref> but this is a minority opinion.<ref name="ArmstrongMcKay2022" /><ref name="CB2018">{{cite web |last1=McSweeney |first1=Robert |date=18 July 2018 |title=Slowdown of Atlantic conveyor belt could trigger 'two decades' of rapid global warming |url=https://www.carbonbrief.org/slowdown-atlantic-conveyor-belt-could-trigger-two-decades-rapid-global-warming/ |publisher=[[Carbon Brief]] |quote="However, writing on the website RealClimate, climate scientists Prof Michael Mann, from Penn State, and Prof Stefan Rahmstorf, from the Potsdam Institute for Climate Impact Research, say that a weaker AMOC bringing warming runs counter to existing research" |access-date=19 May 2024 |archive-date=19 May 2024 |archive-url=https://web.archive.org/web/20240519143340/https://www.carbonbrief.org/slowdown-atlantic-conveyor-belt-could-trigger-two-decades-rapid-global-warming/ |url-status=live }}</ref>

A 2021 study said other well-known tipping points, such as the Greenland ice sheet, the [[West Antarctic Ice Sheet]] and the [[Amazon rainforest]] would all be connected to the AMOC. According to this study, changes to the AMOC alone are unlikely to trigger tipping elsewhere but an AMOC slowdown would provide a connection between these elements and reduce the global-warming threshold beyond which any of those four elements&nbsp;– including the AMOC itself&nbsp;– could be expected to tip, rather than the thresholds that have been established from studying those elements in isolation. This connection could cause a cascade of tipping over several centuries.<ref>{{Cite journal |last1=Wunderling |first1=Nico |last2=Donges |first2=Jonathan F. |last3=Kurths |first3=Jürgen |last4=Winkelmann |first4=Ricarda |author4-link=Ricarda Winkelmann |date=3 June 2021 |title=Interacting tipping elements increase risk of climate domino effects under global warming |url=https://esd.copernicus.org/articles/12/601/2021/ |url-status=live |journal=Earth System Dynamics |volume=12 |issue=2 |pages=601–619 |bibcode=2021ESD....12..601W |doi=10.5194/esd-12-601-2021 |issn=2190-4979 |archive-url=https://web.archive.org/web/20210604054226/https://esd.copernicus.org/articles/12/601/2021/ |archive-date=4 June 2021 |access-date=4 June 2021 |s2cid=236247596|doi-access=free }}</ref>

== Effects of an AMOC shutdown ==
=== Cooling ===
[[File:Sgubin2017 spg amoc collapse.jpg|thumb|upright=0.7|Modelled 21st century warming under the "intermediate" global warming scenario (top). The potential [[North Atlantic Current#Climate change|collapse of the subpolar gyre]] in this scenario (middle). The collapse of the entire Atlantic Meriditional Overturning Circulation (bottom).]]

A complete collapse of the AMOC will be largely irreversible<ref name="AR6_WG1_Chapter92" /> and recovery would likely take thousands of years.<ref>{{Cite journal |last1=Curtis |first1=Paul Edwin |last2=Fedorov |first2=Alexey V. |date=6 April 2024 |title=Collapse and slow recovery of the Atlantic Meridional Overturning Circulation (AMOC) under abrupt greenhouse gas forcing |journal=Climate Dynamics |volume=62 |issue=7 |page=5949 |doi=10.1007/s00382-024-07185-3 |bibcode=2024ClDy...62.5949C }}</ref> A shutting down of the AMOC is expected to trigger substantial cooling in Europe,<ref>{{cite news |url=https://www.sciencedaily.com/releases/2004/12/041219153611.htm |title=Shutdown Of Circulation Pattern Could Be Disastrous, Researchers Say |date=20 December 2004 |work=ScienceDaily |author=University Of Illinois at Urbana-Champaign |archive-date=13 January 2005 |access-date=12 April 2022 |archive-url=https://web.archive.org/web/20050113152254/http://www.sciencedaily.com/releases/2004/12/041219153611.htm |url-status=live }}</ref><ref name="CarbonBrief">{{Cite web |date=2020-02-10 |title=Explainer: Nine 'tipping points' that could be triggered by climate change |url=https://www.carbonbrief.org/explainer-nine-tipping-points-that-could-be-triggered-by-climate-change |access-date=2021-09-04 |website=Carbon Brief |language=en |archive-date=11 February 2020 |archive-url=https://web.archive.org/web/20200211090247/https://www.carbonbrief.org/explainer-nine-tipping-points-that-could-be-triggered-by-climate-change |url-status=live }}</ref> particularly in Britain and Ireland, France and the [[Nordic countries]].<ref>{{Cite web|url=https://www.weatheronline.co.uk/reports/wxfacts/North-Atlantic-Drift-Gulf-Stream.htm|title=Weather Facts: North Atlantic Drift (Gulf Stream) &#124; weatheronline.co.uk|website=www.weatheronline.co.uk|access-date=12 April 2023|archive-date=1 September 2023|archive-url=https://web.archive.org/web/20230901093719/https://www.weatheronline.co.uk/reports/wxfacts/North-Atlantic-Drift-Gulf-Stream.htm/|url-status=live}}</ref><ref>{{Cite web|url=https://oceancurrents.rsmas.miami.edu/atlantic/north-atlantic-drift.html|title=The North Atlantic Drift Current|website=oceancurrents.rsmas.miami.edu|access-date=12 April 2023|archive-date=4 September 2011|archive-url=https://web.archive.org/web/20110904074050/http://oceancurrents.rsmas.miami.edu/atlantic/north-atlantic-drift.html|url-status=live}}</ref> In 2002, research compared AMOC shutdown to [[Dansgaard–Oeschger event]]s&nbsp;– abrupt temperature shifts that occurred during the [[Last Glacial Period]]. According to that paper, local cooling of up to {{cvt|8|C-change|F-change}} would occur in Europe.<ref>{{cite journal |last1=Vellinga |first1=M. |last2=Wood |first2=R.A. |title=Global climatic impacts of a collapse of the Atlantic thermohaline circulation |journal=Climatic Change |volume=54 |issue=3 |pages=251–267 |year=2002 |doi=10.1023/A:1016168827653 |bibcode=2002ClCh...54..251V |s2cid=153075940 |url=http://www.ocean.washington.edu/people/faculty/luanne/classes/pcc586/papers/vellingawood_thc2002.pdf |url-status=dead |archive-url=https://web.archive.org/web/20060906202415/http://www.ocean.washington.edu/people/faculty/luanne/classes/pcc586/papers/vellingawood_thc2002.pdf |archive-date=6 September 2006 }}</ref> In 2022, a major review of tipping points concluded an AMOC collapse would lower global temperatures by around {{cvt|0.5|C-change|F-change}} while regional temperatures in Europe would fall by between {{cvt|4|C-change|F-change}} and {{cvt|10|C-change|F-change}}.<ref name="ArmstrongMcKay2022" /><ref name="ArmstrongMcKayExplainer" />

A 2020 study assessed the effects of an AMOC collapse on farming and food production in Great Britain.<ref>{{cite journal |last1=Ritchie |first1=Paul D. L. |last2=Smith |first2=Greg S. |last3=Davis |first3=Katrina J. |last4=Fezzi |first4=Carlo |last5=Halleck-Vega |first5=Solmaria |last6=Harper |first6=Anna B. |last7=Boulton |first7=Chris A. |last8=Binner |first8=Amy R. |last9=Day |first9=Brett H. |last10=Gallego-Sala |first10=Angela V. |last11=Mecking |first11=Jennifer V. |last12=Sitch |first12=Stephen A. |last13=Lenton |first13=Timothy M. |last14=Bateman |first14=Ian J. |title=Shifts in national land use and food production in Great Britain after a climate tipping point |journal=Nature Food |date=13 January 2020 |volume=1 |pages=76–83 |doi=10.1038/s43016-019-0011-3 |hdl=10871/39731 |s2cid=214269716 |hdl-access=free }}</ref> It found within Great Britain an average temperature drop of {{cvt|3.4|C-change|F-change}} after the effect of warming was subtracted from collapse-induced cooling. A collapse of the AMOC would also lower rainfall during the growing season by around {{cvt|123|mm}}, which would in turn reduce the area of land suitable for arable farming from 32% to 7%. The net value of British farming would decline by around £346&nbsp;million per year&nbsp;– over 10% of its value in 2020.<ref name="Phys2020">{{Cite web |date=13 January 2020 |title=Atlantic circulation collapse could cut British crop farming |url=https://phys.org/news/2020-01-atlantic-circulation-collapse-british-crop.html |access-date=3 October 2022 |website=[[Phys.org]] |language=en |archive-date=29 April 2023 |archive-url=https://web.archive.org/web/20230429164259/https://phys.org/news/2020-01-atlantic-circulation-collapse-british-crop.html |url-status=live }}</ref>

In 2024, one study that modeled the effect of an AMOC collapse on a pre-industrial world, predicted a more severe cooling in Europe. It predicted the average sea surface temperatures in northwest Europe  falling {{cvt|10|C-change|F-change}} and the average February temperatures on land falling between {{cvt|10|C-change|F-change}} and {{cvt|30|C-change|F-change}} within a century in northern and western Europe. This change would result in sea ice reaching into the territorial waters of the British Isles and Denmark during winter while Antarctic sea ice would diminish.<ref name="van Westen2024" /><ref name="RealClimate2024" /><ref>{{Cite news |last=Watts |first=Jonathan |date=2024-02-09 |title=Atlantic Ocean circulation nearing 'devastating' tipping point, study finds |url=https://www.theguardian.com/environment/2024/feb/09/atlantic-ocean-circulation-nearing-devastating-tipping-point-study-finds |access-date=2024-02-10 |work=The Guardian |language=en-GB |issn=0261-3077}}</ref> These findings do not include the counteracting warming from climate change, and the modeling approach used by the paper is controversial.<ref name="SMC2024" />

A 2015 study led by [[James Hansen]] found a shutdown or substantial slowing of the AMOC will intensify severe weather because it increases [[baroclinicity]] and accelerates northeasterly winds up to 10–20% throughout the mid-latitude [[troposphere]]. This could boost winter and near-winter cyclonic "superstorms" that are associated with near-hurricane-force winds and intense snowfall.<ref name="Hansen2015">{{Cite journal |last1=Hansen |first1=J. |last2=Sato |first2=M. |last3=Hearty |first3=P. |last4=Ruedy |first4=R. |last5=Kelley |first5=M. |last6=Masson-Delmotte |first6=V. |last7=Russell |first7=G. |last8=Tselioudis |first8=G. |last9=Cao |first9=J. |last10=Rignot |first10=E. |last11=Velicogna |first11=I. |last12=Tormey |first12=B. |last13=Donovan |first13=B. |last14=Kandiano |first14=E. |last15=von Schuckmann |first15=K. |last16=Kharecha |first16=P. |last17=Legrande |first17=A. N. |last18=Bauer |first18=M. |last19=Lo |first19=K. |display-authors=5 |date=2015-07-23 |title=Ice melt, sea level rise and superstorms: evidence from paleoclimate data, climate modeling, and modern observations that 2&nbsp;°C global warming is highly dangerous |url=https://acp.copernicus.org/preprints/15/20059/2015/acpd-15-20059-2015.pdf |journal=Atmospheric Chemistry and Physics Discussions |volume=15 |issue=14 |pages=20059–20179 |bibcode=2015ACPD...1520059H |doi=10.5194/acpd-15-20059-2015 |doi-access=free |archive-date=15 March 2023 |access-date=15 May 2022 |archive-url=https://web.archive.org/web/20230315105631/https://acp.copernicus.org/preprints/15/20059/2015/acpd-15-20059-2015.pdf |url-status=live }}</ref> This paper has also been controversial.<ref>{{cite news |date=23 July 2015 |title=James Hansen's controversial sea level rise paper has now been published online |newspaper=The Washington Post |url=https://www.washingtonpost.com/news/energy-environment/wp/2015/07/23/controversial-sea-level-rise-paper-is-now-published-online/ |archive-date=26 November 2019 |access-date=13 April 2024 |archive-url=https://web.archive.org/web/20191126005216/https://www.washingtonpost.com/news/energy-environment/wp/2015/07/23/controversial-sea-level-rise-paper-is-now-published-online/ |url-status=live }}</ref>
=== Other ===
{{multiple image
| total_width = 300
| width1 =
| image1 = NOAA_Nino.jpg
| alt1 = Impacts of El Niño on climate
| caption1 =
| width2 =
| image2 = NOAA_Nina.jpg
| alt2 = Impacts of La Niña on climate
| caption2 =
| footer_background = <!-- footer background as a 'hex triplet' web color prefixed by # e.g. #FFFFFF -->
| footer_align = center
| footer = Changes to temperature and precipitation during El Niño (left) and La Niña (right). The top two maps are for Northern hemisphere winter, the bottom two for summer.<ref name="Wald 2021">{{cite book |last1=Wald |first1=Lucien |title=Fundamentals of solar radiation |date=2021 |publisher=CRC Press |location=Boca Raton |isbn=978-0-367-72588-4 |chapter=Definitions of time: from year to second}}</ref> While the El Niño–Southern Oscillation occurs due to the processes in the Pacific Ocean, a connection between the Pacific and the Atlantic means that changes in AMOC can conceivably affect it}}

Several studies have investigated the effect of a collapse of the AMOC on the [[El Niño–Southern Oscillation]] (ENSO); results have ranged from no overall impact<ref>{{cite journal |last1=Williamson |first1=Mark S. |last2=Collins |first2=Mat |last3=Drijfhout |first3=Sybren S. |last4=Kahana |first4=Ron |last5=Mecking |first5=Jennifer V. |last6=Lenton |first6=Timothy M. |title=Effect of AMOC collapse on ENSO in a high resolution general circulation model |journal=Climate Dynamics |date=17 June 2017 |volume=50 |issue=7–8 |pages=2537–2552 |doi=10.1007/s00382-017-3756-0 |s2cid=55707315 |doi-access=free |hdl=10871/28079 |hdl-access=free }}</ref> to an increase in ENSO strength,<ref name="Molina2021">{{cite journal |last1=Molina |first1=Maria J. |last2=Hu |first2=Aixue |last3=Meehl |first3=Gerald A. |title=Response of Global SSTs and ENSO to the Atlantic and Pacific Meridional Overturning Circulations |journal=Journal of Climate |date=22 November 2021 |volume=35 |issue=1 |pages=49–72 |doi=10.1175/JCLI-D-21-0172.1 |osti=1845078 |s2cid=244228477 |doi-access=free }}</ref> and a shift to a dominant La Niña conditions with an about 95% reduction in El Niño extremes but more-frequent extreme rainfall in eastern Australia, and intensified droughts and wildfire seasons in the southwestern U.S.<ref>{{cite journal |last1=Orihuela-Pinto |first1=Bryam |last2=England |first2=Matthew H. |last3=Taschetto |first3=Andréa S. |title=Interbasin and interhemispheric impacts of a collapsed Atlantic Overturning Circulation |journal=Nature Climate Change |date=6 June 2022 |volume=12 |issue=6 |pages=558–565 |doi=10.1038/s41558-022-01380-y |bibcode=2022NatCC..12..558O |s2cid=249401296 }}</ref><ref>{{cite journal |last1=Orihuela-Pinto |first1=Bryam |last2=Santoso |first2=Agus |last3=England |first3=Matthew H. |last4=Taschetto |first4=Andréa S. |title=Reduced ENSO Variability due to a Collapsed Atlantic Meridional Overturning Circulation |journal=Journal of Climate |date=19 July 2022 |volume=35 |issue=16 |pages=5307–5320 |doi=10.1175/JCLI-D-21-0293.1 |bibcode=2022JCli...35.5307O |s2cid=250720455 |hdl=1959.4/unsworks_80930 |hdl-access=free }}</ref><ref>{{Cite web |date=2022-06-06 |title=A huge Atlantic ocean current is slowing down. If it collapses, La Niña could become the norm for Australia |url=https://theconversation.com/a-huge-atlantic-ocean-current-is-slowing-down-if-it-collapses-la-nina-could-become-the-norm-for-australia-184254 |access-date=2022-10-03 |website=[[The Conversation (website)|The Conversation]] |language=en |archive-date=24 November 2023 |archive-url=https://web.archive.org/web/20231124084313/http://theconversation.com/a-huge-atlantic-ocean-current-is-slowing-down-if-it-collapses-la-nina-could-become-the-norm-for-australia-184254 |url-status=live }}</ref>

A 2021 study used a simplified modeling approach to evaluate the effects of an AMOC collapse on the [[Amazon rainforest]], and its hypothesized dieback and transition to a [[savanna]] state in some climate-change scenarios. This study found an AMOC collapse would increase rainfall in the southern Amazon due to the shift of an [[Intertropical Convergence Zone]], and this would help to counter the dieback and potentially stabilize the southern part of the rainforest.<ref>{{cite journal |last1=Ciemer |first1=Catrin |last2=Winkelmann |first2=Ricarda |last3=Kurths |first3=Jürgen |last4=Boers |first4=Niklas |title=Impact of an AMOC weakening on the stability of the southern Amazon rainforest |journal=The European Physical Journal Special Topics |date=28 June 2021 |volume=230 |issue=14–15 |pages=3065–3073 |doi=10.1140/epjs/s11734-021-00186-x |bibcode=2021EPJST.230.3065C |s2cid=237865150 |doi-access=free }}</ref> A 2024 study found the seasonal cycle of the Amazon could reverse with dry seasons becoming wet and ''vice versa''.<ref name="van Westen2024" /><ref name="RealClimate2024" /><ref name="SMC2024" />

A 2005 paper said severe disruption of the AMOC would collapse North Atlantic plankton counts to less than half of their normal biomass due to increased [[Ocean stratification|stratification]] and the large decline in nutrient exchange among ocean layers.<ref name="Schmittner2005">{{cite journal |last1=Schmittner |first1=Andreas |title=Decline of the marine ecosystem caused by a reduction in the Atlantic overturning circulation |journal=Nature |date=31 March 2005 |volume=434 |issue=7033 |pages=628–633 |doi=10.1038/nature03476 |pmid=15800620 |bibcode=2005Natur.434..628S |s2cid=2751408 }}</ref> A 2015 study simulated global ocean changes under AMOC slowing and collapse scenarios, and found these events would greatly decrease [[dissolved oxygen]] content in the North Atlantic, although dissolved oxygen would slightly increase globally due to greater increases across other oceans.<ref>{{cite journal |last1=Yamamoto |first1=A. |last2=Abe-Ouchi |first2=A. |last3=Shigemitsu |first3=M. |last4=Oka |first4=A. |last5=Takahashi |first5=K. |last6=Ohgaito |first6=R. |last7=Yamanaka |first7=Y. |title=Global deep ocean oxygenation by enhanced ventilation in the Southern Ocean under long-term global warming |journal=Global Biogeochemical Cycles |date=5 October 2015 |volume=29 |issue=10 |pages=1801–1815 |doi=10.1002/2015GB005181 |bibcode=2015GBioC..29.1801Y |s2cid=129242813 |doi-access=free }}</ref>

== See also ==
{{Portal|Climate change|Earth sciences|Oceans}}
* [[8.2-kiloyear event]]
* [[Climate security]]
* [[Loop Current]]
* [[North Atlantic Deep Water]]
* [[Pacific decadal oscillation]]
* [[Paleosalinity]]
* [[Sverdrup balance]]
* [[West Greenland Current]]

== References ==
{{Reflist|30em}}

{{Climate change}}
{{physical oceanography}}

[[Category:Climate change and the environment]]
[[Category:Effects of climate change]]
[[Category:Oceanography]]
[[Category:Natural hazards]]
[[Category:Future problems]]
[[Category:Currents of the Atlantic Ocean]]