Atlantic meridional overturning circulation: Difference between revisions

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] [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] [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]],. andThose 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]] }}</ref>

The AMOC is composed of a northward flow of warm, less-more saline water in the Atlantic's upper layers and a southward, return flow of cold, salty, deep water. TheseWarm 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}}

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 }}</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}}</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 }}</ref> Major mountain ranges such as the [[Tibetan Plateau]] and the [[Rocky Mountains]] 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 |page=2602 |doi=10.1038/s41467-024-46856-x |pmid=38521775 |pmc=10960852 |bibcode=2024NatCo..15.2602Y }}</ref>

The NADW is not the deepest water layer in the Atlantic Ocean; the [[Antarctic Bottombottom Waterwater]] (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 NinoNiño]]/La NinaNiña 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>

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}}</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>

Because the Atlantic meriditional 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]] }}</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]] }}</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>

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 }}</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 }}</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 }}</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-albedoIce–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 }}</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_The_Bolling-Allerod_Interstadial |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 }}</ref> The onset of the interstadial also caused a period of [[sea- level rise]] from ice-sheet collapse that is designated [[Meltwater Pulsepulse 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– northern-hemisphere cooling, southern-hemisphere warming&nbsp;–which– 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 |pages=599–604 |doi=10.1038/s41561-018-0169-6 }}</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 }}</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" />

[[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 fof 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– mainly from Greenland&nbsp;–and– 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>

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>

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 to 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-paper-warning-of-a-collapse-of-the-atlantic-meridional-overturning-circulation/ |access-date=12 April 2024 |website=Science Media Centre |language=en}}</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 }}</ref> If the AMOC is more dependent on wind strength&nbsp;–which– which changes relatively little with warming&nbsp;–than– 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" />

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>

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}}</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– as that time frame presents both periods of weakening and strengthening&nbsp;–and– 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}}</ref>

[[File:Latif_2022_AMOC_120y_seriesLatif 2022 AMOC 120y series.png|thumb|A 120-year trend of sea surface temperature ''differences'' from the mean warming trend&nbsp;–a– a proxy for AMOC state&nbsp;–shows– 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}}</ref> In 2015, a different statistical analysis interpreted a cold pattern in some years of temperature records as a sign of AMOC weakening. It concludednthe 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 }} {{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 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 |newspaper=[[The Guardian]] |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]]}}</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/ }}</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 }}</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 (North Atlantic)|''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}}</ref> Later research found atmospheric changes, such as an increase in low cloud cover<ref name="CB2020" /> and a strengthening of the [[North Atlantic Oscillationoscillation]] (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 }}</ref>

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– 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>

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 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"; one expert said the projection had "feet of clay".<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}}</ref>

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>

{{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]. 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] [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}}</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}}</ref>

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}}</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 |page=4235 |doi=10.1038/s41467-018-06683-3 |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 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'two decades’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" }}</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– including the AMOC itself&nbsp;–could– 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>

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 |doi=10.1007/s00382-024-07185-3 }}</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}}</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}}</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}}</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}}</ref> In 2002, research compared AMOC shutdown to [[Dansgaard-OeschgerDansgaard–Oeschger event]]s&nbsp;–abrupt– abrupt temperature shifts that occurred during the [[lastLast glacialGlacial periodPeriod]]. 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 |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 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 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– 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}}</ref>

In 2024, one modeling study predicted more-severe cooling in Europe of between {{cvt|10|C-change|F-change}} and {{cvt|30|C-change|F-change}} within a century on land, and up to {{cvt|18|F-change|C-change}} at sea. 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. Scandinavia and parts of Britain would eventually become cold enough to support ice sheets.<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" />

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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 }}</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}}</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 [[savannahsavanna]] 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" />

* [[Loop Current]]