Atlantic meridional overturning circulation

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

The Atlantic meridional overturning circulation (AMOC) is the "main current system in the South and North Atlantic Oceans".^[1]: 2238 As such, it is a component of Earth's oceanic circulation system and plays an important role in the climate system. The AMOC includes currents at the surface as well as at great depths in the Atlantic Ocean. These currents are driven by changes in the atmospheric weather as well as by changes in temperature and salinity. They collectively make up one half of the global thermohaline circulation that encompasses the flow of major ocean currents. The other half is the Southern Ocean overturning circulation.^[2]

Topographic map of the Nordic Seas and subpolar basins with surface currents (solid curves) and deep currents (dashed curves) that form a portion of the Atlantic meridional overturning circulation. Colors of curves indicate approximate temperatures.

The AMOC is characterized by a northward flow of warmer, fresher water in the upper layers of the Atlantic, and a southward flow of colder, saltier and deeper waters. These limbs are linked by regions of overturning in the Nordic Seas and the Southern Ocean. Overturning sites are associated with the intense exchange of heat, dissolved oxygen, carbon and other nutrients. They are very important for the ocean's ecosystems and for its functioning as a carbon sink.^[3][4] Thus, if the strength of the AMOC changes, multiple elements of the climate system would be affected.^[1]: 2238

Climate change has the potential to weaken the AMOC through increases in ocean heat content and elevated freshwater flows from the melting ice sheets. Studies using oceanographic reconstructions suggest that the AMOC is now already weaker than it was before the Industrial Revolution.^[5][6] However, there is debate over the relative contributions of different factors. It is unclear how much of it is due to either climate change or due to the circulation's natural variability over hundreds or thousands of years.^[7][8] Climate models predict that the AMOC will weaken further over the 21st century.^[9]: 19 This would affect average temperature over Scandinavia and Great Britain because these regions are warmed by the North Atlantic drift.^[10] Weakening of the AMOC would also accelerate sea level rise around North America and reduce primary production in the North Atlantic.^[11]

Severe weakening of the AMOC may lead to an outright collapse of the circulation, which would not be easily reversible and thus constitute one of the tipping points in the climate system.^[12] A collapse would substantially lower the average temperature and amount of rain and snowfall in Europe.^[13][14] It would also potentially raise the frequency of extreme weather events and have other severe effects.^[15][16] Gold-standard Earth system models indicate that a collapse is unlikely, and would only become plausible if high levels of warming are sustained well after the year 2100.^[17][18][19] Some paleoceanographic research seems to support this idea.^[20][21] However, some researchers fear that the complex models are too stable,^[22] and lower-complexity projections pointing to an earlier collapse are more accurate.^[23][24] One of those projections suggests that AMOC collapse could happen around 2057,^[25] but many scientists are skeptical of the claim.^[26] Some research also suggests that the Southern Ocean overturning circulation may be more prone to collapse.^[27][15]

Overall structure

 

AMOC in relation to the global thermohaline circulation (animation)

The Atlantic meridional overturning circulation (AMOC) is the main current system in the Atlantic Ocean,^[1]: 2238 and it is also part of the global thermohaline circulation, which connects the world's oceans with a single "conveyor belt" of continuous water exchange.^[28] Normally, (relatively) warm and fresh water stays on the surface, while colder, denser and more saline water stays in the ocean depths, in what is known as ocean stratification.^[29] However, deep water eventually gains heat and/or loses salinity in an exchange with the mixed ocean layer, and so it becomes less dense and rises towards the surface. Differences in temperature and salinity exist not only between ocean layers, but also between parts of the World Ocean, and together, they drive the thermohaline circulation.^[28] In particular, the Pacific Ocean is less saline than the others, because it receives large quantities of fresh rainfall.^[30] Its surface water lacks the salinity to sink lower than several hundred meters - meaning that deep ocean water must come in from elsewhere.^[28]

On the other hand, ocean water in the North Atlantic is more saline, partly because the extensive evaporation on the surface concentrates salt within the remaining water, and the sea ice near the Arctic Circle expels salt as it freezes during the winter.^[31] Even more importantly, evaporated moisture in the Atlantic swiftly carried away by atmospheric circulation before it could rain back down. Instead, trade winds move this moisture across Central America, and the westerlies transport it over Africa and Eurasia.^[30] This water is precipitated either over land or in the Pacific Ocean while major mountain ranges, such as the Tibetan Plateau and the Rocky Mountains, prevent any equivalent moisture transport back to the Atlantic.^[32]

Thus, the Atlantic surface waters become saltier and therefore denser, eventually sinking (downwelling) to form the North Atlantic Deep Water (NADW).^[33] NADW formation primarily occurs in the Nordic Seas, and it 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.^[34] Labrador Sea Water may play an important role as well; however, increasing evidence suggests that the waters in Labrador and Irminger Seas primarily recirculate through the North Atlantic Gyre, and have little connection with the rest of the AMOC.^[4][35][13]

 

A summary of the path of the thermohaline circulation. Blue paths represent deep-water currents, while red paths represent surface currents

NADW is not the deepest water layer in the Atlantic Ocean: instead, the Antarctic Bottom Water (AABW) is always the densest, deepest ocean layer in any basin deeper than 4,000 m (2.5 mi).^[36] As the upper reaches of this AABW flow upwell, they meld into and reinforce NADW. The formation of NADW is also the beginning of the lower cell of the circulation.^[28][3] The downwelling which forms NADW is balanced out by an equal amount of upwelling. In the western Atlantic, Ekman transport - the increase in ocean layer mixing caused by wind activity - results in particularly strong upwelling in Canary Current and Benguela Current, which are located on the northwest and southwest coast of Africa. Currently, upwelling is substantially stronger around the Canary Current than Benguela Current; an opposite pattern existed until the closure of the Central American Seaway during the late Pliocene.^[37] In the Eastern Atlantic, significant upwelling occurs only during certain months of the year, as this region's deep thermocline means that it is more dependent on the state of sea surface temperature than on wind activity. Further, there is a multi-year upwelling cycle, which occurs in sync with the El Nino/La Nina cycle.^[38]

Meanwhile, NADW moves southwards, and at the southern end of the Atlantic transect, ~80% of it upwells in the Southern Ocean,^[33][39] which connects it with the AMOC's twin, the Southern Ocean overturning circulation.^[40] After upwelling, the water is understood to take one of two pathways. Water surfacing close to Antarctica will likely be cooled by the Antarctic sea ice and sink back into the lower cell of the circulation. Some of this water will rejoin AABW, but the rest of the lower cell flow will eventually reach the depths of the Pacific and Indian oceans.^[28] Meanwhile, water which upwells at the lower, ice-free latitudes moves further northward due to Ekman transport and is committed to the upper cell. This warmer water in the upper cell is responsible for the return flow to the North Atlantic, which mainly occurs around the coastline of Africa and through the Indonesian archipelago. Once this water returns to North Atlantic, it becomes cooler, denser and sinks, feeding back into the North Atlantic Deep Water.^[40][33]

Role in the climate system

 

Heat transfer from the ocean to atmosphere (left) and an increase in Atlantic Ocean heat content (right) observed when the AMOC is strong^[41]

Equatorial areas are the hottest part of the globe, and thermodynamics require this heat to travel polewards. Most of this heat is transported by atmospheric circulation, but warm ocean currents at the surface play an important role as well. Heat transfer from the equator can be either towards the north or the south: the Atlantic Ocean is the only ocean where the heat flow is towards the north.^[42] Much of this heat transfer occurs due to the Gulf Stream, a surface current which carries warm water 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 the thermohaline exchange within the AMOC.^[3] Thus, AMOC alone carries up to 25% of the total heat transport towards the Northern Hemisphere,^[42] and plays a particular importance around the latitudes of Northwest Europe.^[43]

Because atmospheric patterns also play a large role, the idea that Northern Europe would be just as cold as Alaska and Canada without heat transport via the ocean currents (i.e. up to 15–20 °C (27–36 °F) colder) is generally considered false.^[44][45] While one modelling study did suggest that the AMOC collapse could result in Ice Age-like cooling (including sea ice expansion and mass glacier formation) within a century,^[46][47] the accuracy of those results is questionable.^[48] Even so, there is a consensus that the AMOC keeps Northern and Western Europe warmer than it would be otherwise,^[15] with the difference of 4 °C (7.2 °F) and 10 °C (18 °F) depending on the area.^[13] For instance, studies of the Florida Current suggest that the Gulf Stream as a whole was around 10% weaker from around 1200 to 1850 due to increased surface salinity, and this had likely contributed to the conditions known as Little Ice Age.^[49]

Further, the AMOC makes the Atlantic Ocean into a more effective carbon sink in two major ways. Firstly, the upwelling which takes place as part of the system supplies large quantities of nutrients to the surface waters, which supports the growth of phytoplankton and therefore increases marine primary production and the overall amount of photosynthesis in the surface waters. Secondly, upwelled water has low concentrations of dissolved carbon, as the water is typically 1000 years old and has not been sensitive to anthropogenic CO₂ increases in the atmosphere. This water thus absorbs larger quantities of carbon than the more saturated surface waters, and is then prevented from releasing carbon back into the atmosphere when it is downwelled.^[50] While Southern Ocean is by far the strongest ocean carbon sink,^[51] North Atlantic represents the single largest carbon sink in the Northern Hemisphere.^[52]

Stability and vulnerability

Further information: Multiple equilibria in the Atlantic meridional overturning circulation

 

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 ^[21]

Because the Atlantic meriditional overturning circulation is dependent on a series of interactions between layers of ocean water of varying temperature and salinity, it is far from static. Instead, it experiences both smaller, cyclical changes,^[53][7] and larger, long-term shifts in response to external forcings.^[54] This means that climate change can affect the AMOC in two major ways - by making surface waters warmer as an inevitable consequence of Earth's energy imbalance, and by making them less saline due to the addition of large quantities of fresh water from melting ice (mainly from Greenland), and through increasing precipitation over the North Atlantic. Both would increase the difference between the surface and lower layers, and thus make the upwelling and downwelling which drives the circulation more difficult.^[55]

In the 1960s, Henry Stommel had pioneered much of the research into AMOC with what later became known as the Stommel Box model. It introduced the idea of a Stommel Bifurcation, where the AMOC could exist either in a strong state like the one throughout recorded history, or effectively "collapse" to a drastically weaker state, and not recover unless the increased warming and/or freshening which caused the collapse is reduced.^[56] The warming/freshening could cause the collapse directly, or it could simply weaken the circulation to a state where its ordinary fluctuations ("noise") could push it past the point of no return.^[21] The possibility that the AMOC is a bistable system (which is either "on" or "off") and could collapse suddenly has been a topic of scientific discussion ever since.^[57][58] In 2004, The Guardian publicized the findings of a report commissioned by Pentagon defence adviser Andrew Marshall, which suggested that the average annual temperature in Europe would drop by 6 °F (3.3 °C) between 2010 and 2020 as the result of an abrupt AMOC shutdown.^[59]

Modelling AMOC collapse

 

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 increase 4 times.^[60] There are concerns that this kind of a simulation is too stable ^[22]

Some of the models developed after Stommel's work instead suggest that the AMOC could have one or more stable intermediate states, between its full strength and a full collapse.^[61] This is more commonly seen in the so-called 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 models (GCMs), which represent the "gold standard" for simulating the entire climate, but often have to simplify certain interactions.^[62] GCMs typically show that the AMOC has a single equilibrium state, and that it is difficult, if not impossible, for it to collapse.^[63][60]

However, multiple researchers have raised concerns that this modelled resistance to collapse only occurs because the GCM simulations tend to redirect large quantities of freshwater towards the North Pole (where it would no longer affect the circulation), while this movement does not occur in nature.^[53][17]

 

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.^[46] While the conditions are unrealistic, the model may also be unrealistically stable, and the full implications are not clear without more real-world observations^[48]

In 2024, three researchers performed a simulation with one of the CMIP models (the Community Earth System Model), where a classic AMOC collapse had eventually occurred, much like it does in the intermediate-complexity models.^[46] Unlike some other simulations, they did not immediately subject the model to unrealistic meltwater levels, and instead gradually increased the input. However, their simulation had not only run for over 1700 years before the collapse occurred, but they had also eventually reached meltwater levels equivalent to sea level rise of 6 cm (2.4 in)/yr^[48] - about 20 times larger than the 2.9 mm (0.11 in)/yr sea level rise between 1993 to 2017,^[64] 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 taken as a prediction in and of itself, but rather as a high-resolution representation of how currents would start changing before a collapse.^[46] Other scientists agreed that its findings would mainly help with calibrating more realistic studies, particularly once better observational data becomes available.^[48][47]

On the other hand, some research indicates that the classic EMIC projections are biased towards collapse because they subject the circulation towards 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 1000 years. The researchers suggested that this simulation is more consistent with the reconstructions of AMOC response to Meltwater pulse 1A (13,500-14,700 years ago), which indicate a similarly long delay.^[21] Moreover, a paleoceanographic reconstruction from 2022 found only a limited impact from massive freshwater forcing of the final Holocene deglaciation ~11,700–6,000 years ago, when the sea level rise amounted to around 50 m (160 ft). It suggested that most models overestimate the impact of freshwater forcing on AMOC.^[20] Further, if AMOC is more dependent on wind strength (which changes relatively little with warming) than is commonly understood, then it would be more resistant to collapse.^[65] And according to some researchers, the less-studied Southern Ocean overturning circulation may be more vulnerable than the AMOC.^[27]

Trends

Observations

 

1992-2002 altimeter data from NASA Pathfinder indicated a slowing (red) in the subpolar gyre region. This was used as a proxy for AMOC before the initiation of RAPID, and before subsequent research demonstrated that the subpolar gyre often behaves separately from the larger circulation ^[4]

 

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^[66]

Direct observations of the strength of the AMOC have been available only since 2004 from RAPID, an in situ mooring array at 26°N in the Atlantic.^[67][66] Further, observational data needs to be collected for a prolonged period of time 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 that downwelling in the Greenland Sea (a small part of the larger AMOC system) reduced to less than a quarter of its normal strength, as measured by a number of giant water columns (nicknamed "chimneys") transferring water downwards.^[68][69] Other researchers in 2000s have focused on trends in the North Atlantic Gyre (also known as the northern subpolar gyre, or SPG).^[70] When measurements taken in 2004 found a 30% decline in SPG relative to the previous measurement in 1992, some have intepreted it as a sign of AMOC collapse.^[71] However, RAPID data have soon shown this to be a statistical anomaly,^[72] and observations from 2007/2008 have shown a recovery of the SPG.^[73] Further, it is now known that the North Atlantic Gyre is largely separate from the rest of the AMOC, and could collapse independently of it.^[13][74][15]

By 2014, there was enough processed RAPID data up until the end of 2012, and it appeared to show a decline in circulation which was 10 times greater than what was predicted by the most advanced models of the time. Scientific debate began over whether it indicated a strong impact of climate change, or simply large interdecadal variability of the circulation.^[53][75] Data up until 2017 had shown that the decline in 2008-2009 was anomalously large, but the circulation after 2008 was nevertheless weaker than it was in 2004-2008.^[66]

Another way to observe the AMOC is through tracking changes in heat transport only, since they would necessarily be correlated with the overall current flows. In 2017 and 2019, estimates derived from heat observations made by NASA's CERES satellites and international Argo floats suggested 15-20% less overall heat transport than implied by the RAPID, and indicated a fairly stable flow, with only a limited indication of decadal variability.^[76][77]

Reconstructions

Recent past

 

A 2021 comparison of the post-2004 RAPID observations with the 1980-2004 reconstructed AMOC trend had indicated no real change across 30 years ^[78]

Climate reconstructions allow research to piece together hints about the past state of the AMOC, though these techniques are necessarily less reliable than the direct observations. In February 2021, RAPID data was combined with the reconstructed trends from 25 years before RAPID. Doing so had shown no evidence of an overall AMOC decline over the past 30 years.^[78] Likewise, a Science Advances study published in 2020 found no significant change in the AMOC circulation relative to 1990s, even though substantial changes have occurred across the North Atlantic Ocean over the same period.^[79] A March 2022 review article concluded that while there may be a long-term weakening of the AMOC caused by global warming, it remains difficult to detect when analyzing its evolution since 1980 (including both direct, as that time frame presents both periods of weakening and strengthening, and the magnitude of either change is uncertain (in range between 5% and 25%). The review concluded with a call for more sensitive and longer-term research.^[80]

20th century

 

A 120-year trend of sea surface temperature differences from the mean warming trend - a proxy for AMOC state - shows no net change until around 1980.^[7]

Some reconstructions reach deeper into the past, attempting to compare the current state of the AMOC with that from a century or so earlier. For instance, a 2010 statistical analysis found an ongoing weakening of the AMOC since the late 1930s, with an abrupt shift of a North Atlantic overturning cell around 1970.^[81] In 2015, a different statistical analysis interpeted a specific cold pattern in certain years of temperature records as a sign of AMOC weakening. It concluded that the AMOC has weakened by 15–20% in 200 years, and that the circulation was slowing throughout most of the 20th century. Between 1975 and the 1995, the circulation would have been weaker than at any time over the past millennium. While this analysis had also shown a limited recovery after 1990, the authors cautioned that a decline is likely to reoccur in the future.^[5]

In 2018, another reconstruction suggested a weakening of around 15% since the mid-twentieth century.^[82] A 2021 reconstructon drew on over a century of ocean temperature and salinity data, which appeared to show significant changes in eight independent AMOC indices, to the point they could indicate "an almost complete loss of stability". However, this reconstruction was forced to omit all data from 35 years before 1900 and after 1980 to maintain consistent records of all eight indicators.^[24] All these findings were challenged by 2022 research, which used nearly 120 years of data between 1900 and 2019 and found no change between 1900 and 1980, with a single-sverdrup reduction in AMOC strength not emerging until 1980 – a variation which remains within range of natural variability.^[7]

Millennial-scale

 

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 ^[83]

2018 research suggested that the last 150 years of AMOC demonstrated exceptional weakness when compared to the previous 1500 years, and it indicated a discrepancy in the modeled timing of AMOC decline after the Little Ice Age.^[84] A 2017 review concluded that 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 events.^[85] In 2022, another millennial-scale reconstruction suggested that the Atlantic multidecadal variability displayed strongly increasing "memory", meaning that it is now less likely to return to the mean state, and instead would procede in the direction of past variation. Since this pattern is likely connected to AMOC, this could indicate a "quiet" loss of stability not seen in most models.^[83]

In February 2021, a major study in Nature Geoscience had reported that the preceding millennium had seen an unprecedented weakening of the AMOC, an indication that the change was caused by human actions.^[6][86] Its co-author said that AMOC had already slowed by about 15%, with impacts now being seen: "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."^[86] In February 2022, Nature Geoscience published a "Matters Arising" commentary article co-authored by 17 scientists, which disputed those findings and argued that the long-term AMOC trend remains uncertain.^[8] The journal had also published a response from the authors of 2021 study to "Matters Arising" article, where they defended their findings.^[87]

Possible indirect signs

 

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).^[88]

Some researchers have interpreted a range of recently observed climatic changes and trends as being connected to the AMOC slowdown. For instance, a major area of the North Atlantic Gyre^[89] near Greenland has cooled by 0.39 °C (0.70 °F) between 1900 and 2020, in contrast to substantial ocean warming elsewhere.^[90] This cooling is normally seasonal - it is most pronounced in February, when the cooling reaches 0.9 °C (1.6 °F) at the area's epicenter, but it still experiences warming relative to the preindustrial during the warmer months, particularly in August.^[89] However, between 2014 and 2016, the waters in the area stayed cool for 19 months before finally experiencing warming,^[91] and this phenomenon was colloquially described as the "Cold blob" in the media.^[92]

Physically, the cold blob pattern occurs because even cool waters avoid sinking into the deeper layers if they are fresh enough. This freshening was immediately described as the evidence of AMOC slowdown.^[92] Later research found that atmospheric changes, such as an increase in low cloud cover^[93] and a strengthening of the North Atlantic Oscillation (NAO) have also played a major role.^[90] While the overall importance of NAO in the phenomenon is disputed,^[91] it is clear that cold blob trends cannot be used to analyse AMOC strength on its own.^[93]

Another possible early indication of the AMOC slowdown 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 slowdown. This decline was offset by the comparable increase in the South Atlantic (considered part of the Southern Ocean).^[94] While the total amount of carbon absorbed by all carbon sinks is generally projected to increase throughout the century, a decline in the North Atlantic sink would have important implications if it were to continue.^[95] Other processes which were attributed in some studies to AMOC slowdown include increasing salinity in the South Atlantic,^[96] "rapid" deoxygenation in the Gulf of St. Lawrence,^[97][98] and a ~10% decline in phytoplankton productivity across the North Atlantic over the past 200 years.^[99]

Projections

Individual models

 

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) ^[100]

Historically, CMIP models (the gold standard in climate science) suggest that the AMOC is very stable, in that while it may weaken, it'll always recover in time, rather than collapse outright. However, many researchers have suggested this was only due to the biases persistent across these models, such as an insufficient analysis of the impacts on circulation caused by Greenland ice sheet meltwater intrusion.^[63][22]

Some scientists experimented with bias correction to address it. In 2017, an experiment with Community Climate System Model simulated an idealized scenario where CO₂ concentrations abruptly double from 1990 levels and do not change afterwards. The AMOC collapsed after 300 years when bias correction was applied to the model.^[17] In an earlier CO₂ doubling experiment performed without bias correction, the circulation declined by ~25% instead of collapsing, but then it only recovered by 6% over 1,000 years.^[101]

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 that 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, where greenhouse gas emissions increase continuously. When the two scenarios were extended past 2100, 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 an outright collapse.^[18] In 2020, a different team of researchers simulated RCP 4.5 and RCP 8.5 between 2005 and 2250 in a Community Earth System Model integrated with an advanced ocean physics module. Due to the module, the circulation was subjected to 4-10 times more freshwater when compared to the standard run. It simulated very similar results for RCP 4.5 as the 2016 study, while under RCP 8.5, the circulation declines by two-thirds soon after 2100, but does not collapse past that level.^[102]

 

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 and/or aerosol cooling would have a marginal impact by comparison ^[103]

The sixth generation of climate models, CMIP6, became available around 2020.^[104] On average, these models simulate much greater AMOC weakening in response to greenhouse warming than the previous generation.^[100] In 2022, when four CMIP6 models simulated AMOC under the SSP3-7 scenario, the circulation declined by over 50% by 2100.^[103] SSP3-7 is a high warming scenario where CO₂ levels more than double from 2015 values by 2100 (from ~400 to over 850 ppm) - yet, it involves lower emissions than the RCP 8.5 scenario simulated in 2016 and 2020 studies.^[105]: 14 The study also found that different air pollution mitigation policies could have an impact of around 10%. Aggressive mitigation of particulates and ground-level ozone without action on greenhouse gases would exacerbate AMOC decline to ~63%, because sulfate pollution also causes cooling through global dimming. Aggressively reducing atmospheric methane emissions alone would have an opposite impact, and a simultaneous reduction of both would cancel out their effects.^[103]

Another study published in 2020 analyzed how the AMOC would be impacted by temperature stabilizing at 1.5 °C (2.7 °F), 2 °C (3.6 °F) (the two Paris Agreement goals, both well below the warming under of RCP 4.5) or 3 °C (5.4 °F) by 2100 (slightly above the expected warming by 2100 under RCP 4.5). In all three cases, the AMOC declines for an additional 5–10 years after the temperature rise ceases, but it does not come close to collapse, and experiences some recovery after about 150 years.^[19]

In 2023, a statistical analysis of output from multiple intermediate-complexity models suggested that AMOC collapse would most likely happen around 2057, with the 95% confidence range between 2025 and 2095.^[25] This study had received a lot of attention, but also a lot of criticism, since the intermediate-complexity models are considered less reliable in general, and may confuse a major slowdown of the circulation with its complete collapse. Further, the study relied on proxy temperature data from the Northern Subpolar Gyre region, which other scientists do not consider representative of the entire circulation, believing it is potentially subject to a separate tipping point instead. Some scientists have still described this research as "worrisome" and noted that it can provide a "valuable contribution" once better observational data is available, but there was widespread agreement amongst experts that the paper's proxy record was "insufficient", with one saying the projection had "feet of clay". Some went as far as to say the study used old observational data from 5 ship surveys which "has long been discredited" by the lack of major weakening seen in direct observations since 2004, "including in the reference they cite for it".^[26]

Major review studies

 

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^[106][13]

Large review papers and reports are capable of evaluating model output together with direct observations and historical reconstructions in order to make expert judgements beyond what models alone can show. Around 2001, the IPCC Third Assessment Report projected high confidence that the thermohaline circulation would tend to weaken rather than stop, and that the warming effects would outweigh the cooling, even over Europe.^[107] 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 confidence level.^[108]

In 2021, the IPCC Sixth Assessment Report again assessed that the AMOC is very likely to decline within the 21st century, and expressed high confidence that changes to it would be reversible within centuries if the warming was reversed.^[9]: 19 Unlike the Fifth Assessment Report, it had only expressed medium confidence rather than high confidence in AMOC avoiding a collapse before the end of the century. This reduction in confidence was likely influenced by several review studies drawing attention to the circulation stability bias within general circulation models,^[109][110] as well as simplified ocean modelling studies suggesting that the AMOC may be more vulnerable to abrupt change than what the larger-scale models suggest.^[23]

In 2022, an extensive assessment of all potential climate tipping points identified 16 plausible climate tipping points, including a collapse of the AMOC. It suggested that a collapse would most likely be triggered by 4 °C (7.2 °F) of global warming, but that there's enough uncertainty to suggest it could be triggered at warming levels as low as 1.4 °C (2.5 °F), or as high as 8 °C (14 °F). Likewise, it estimates that once AMOC collapse is triggered, it would most likely take place over 50 years, but the entire range is between 15 and 300 years.^[13][74] That assessment also treated the collapse of the Northern Subpolar Gyre as a potential separate tipping point, which could occur at between 1.1 °C (2.0 °F) degrees and 3.8 °C (6.8 °F) (although this is only simulated by a fraction of climate models). The most likely figure is 1.8 °C (3.2 °F), and once triggered, the collapse of the gyre would most likely take 10 years from start to end, with a range between 5 and 50 years. The loss of this convection is estimated to lower the global temperature by 0.5 °C (0.90 °F), while the average temperature in Europe decreases by around 3 °C (5.4 °F). There are also substantial impacts on regional precipitation.^[13][74]

Impacts of AMOC slowdown

 

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) ^[111]

While there is not yet consensus on whether there has already been a consistent slowdown in AMOC circulation, there is little doubt that it would occur in the future under continued climate change.^[35] According to the IPCC, the most likely impacts of future AMOC decline are the reduced precipitation in mid-latitudes, changing patterns of strong precipitation in the tropics and Europe, and strengthening storms that follow the North Atlantic track. Similarly, decline would also be accompanied by an acceleration of sea level rise along the U.S. East Coast:^[35] with at least one such event already connected to a temporary AMOC downturn.^[112] This effect would be caused by the increased warming and thus thermal expansion of coastal waters, since they would transfer less of their heat towards Europe. It is one of the reasons why sea level rise in that area is estimated to be 3–4 higher than the global average.^{[113][114][115]}

Multiple scientists believe that a partial slowdown would result in limited cooling in Europe,^[116] perhaps around 1 °C (1.8 °F).^[117][111] While some research suggests that the reduction in oceanic heat uptake would be the dominant effect from AMOC slowdown and result in increased warming,^[118] this view is in the minority.^[13] A 2021 assessment suggested that while climate tipping points in general would likely increase the social cost of carbon (common measure of economic impacts of climate change) by about 25% (potentially more than doubling it), AMOC slowdown is likely to reduce it by about −1.4%. According to their logic, cooling caused by the slowdown would counteract the effects of warming in Europe, which represents a larger fraction of the global GDP than the regions negatively impacted by the slowdown.^[119] However, the study was unable to estimate nonmarket damages, and many scientists consider its numbers a severe underestimate, or at most a lower bound on economic impacts.^[120][121]

 

A proposed tipping cascade where the AMOC would mediate a connection between the other tipping elements.

On the other hand, a 2021 study suggested that other well-known tipping points, such as the Greenland ice sheet, West Antarctic Ice Sheet and the Amazon rainforest would all be connected to AMOC. According to this study, changes to AMOC are unlikely to trigger tipping elsewhere on their own. However, AMOC slowdown would provide a connection between these elements, and reduce the global warming threshold beyond which any of those four elements (including the AMOC itself) could be expected to tip, as opposed to thresholds established from studying those elements in isolation. This connection could potentially cause a cascade of tipping across multi-century timescales.^[122]

In 2020, a study evaluated the effects of projected AMOC weakening in the 21st century under the Representative Concentration Pathway 8.5, which portrays a future of continually increasing emissions. In this scenario, a weakened AMOC would also slow down Arctic sea ice decline and delay the emergence of an ice-free Arctic by around 6 years, as well as preventing over 50% of sea ice loss on the edges of Labrador Sea, Greenland Sea, Barents Sea, and Sea of Okhotsk in the years 2061–2080. It also found a southward displacement of Intertropical Convergence Zone, with the associated rainfall increases to the north of it over the tropical Atlantic Ocean and decreases to the south, but cautioned that those trends would be dwarved by the far larger changes in precipitation associated with RCP 8.5. Finally, it found that this slowdown would further deepen Icelandic Low and Aleutian Low due to the displacement of westerly jets.^[123] 2022 research suggested that 20th century winter weather extremes in Siberia were milder when the AMOC was weaker.^[41]

Impacts of an AMOC shutdown

See also: Abrupt climate change

Cooling

 

Modelled 21st century warming under the "intermediate" global warming scenario (top). The potential collapse of the subpolar gyre in this scenario (middle). The collapse of the entire Atlantic Meriditional Overturning Circulation (bottom).

A full AMOC shutdown will be largely irreversible,^[35] with a recovery likely lasting thousands of years.^[124] It is expected to trigger substantial cooling in Europe,^[125][12] particularly in the British Isles, France and the Nordic countries.^[126][127] 2002 research compared AMOC shutdown to Dansgaard-Oeschger events - abrupt temperature shifts which occurred during the last glacial period. That paper indicated local cooling of up to 8 °C (14 °F) in Europe.^[128] In 2022, a major review of tipping points had concluded that AMOC collapse would lower global temperatures by around 0.5 °C (0.90 °F), while regional temperatures in Europe would go down by between 4 °C (7.2 °F) and 10 °C (18 °F).^[13][74]

In 2020, a study had assessed the impact of an AMOC collapse on farming and food production in Great Britain.^[129] It found an average temperature drop of 3.4 °C (6.1 °F) (after the impact of warming was subtracted from collapse-induced cooling.) Moreover, AMOC collapse would lower rainfall during the growing season by around <123mm, which would in turn reduce the land area suitable for arable farming from the 32% to 7%. The net value of British farming would decline by around £346 million per year, or over 10%.^[14]

In 2024, one modelling study suggested even more severe cooling in Europe - between 10 °C (18 °F) and 30 °C (54 °F) within a century for land and up to 18 °F (10 °C) on sea. This would result in sea ice reaching into the territorial waters of the British Isles and Denmark during winter, while the Antarctic sea ice would diminish instead. Scandinavia and parts of Britain would become cold enough to eventually support ice sheets.^{[46][47][130]} However, these findings do not include the counteracting warming from climate change itself, and the modelling approach used by the paper is controversial.^[48]

A 2015 study led by James Hansen indicated that the shutdown or substantial slowdown of the AMOC will generally intensify severe weather, as it increases baroclinicity and accelerates northeasterly winds up to 10–20% throughout the midlatitude troposphere. This could boost winter and near-winter cyclonic "superstorms", associated with near-hurricane-force winds and intense snowfall.^[16] This paper had also been controversial.^[131]

Other

 

 

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.^[132] 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

In 2017, a study evaluated the effects of a shutdown on El Niño–Southern Oscillation (ENSO), but found no overall impact, with divergent atmospheric processes cancelling each other out.^[133] 2021 research suggested that an AMOC slowdown or collapse could nevertheless increase the strength of El Niño–Southern Oscillation and thus amplify climate extremes, especially if it causes another overturning circulation to develop in the Pacific Ocean.^[134]

In contrast, a 2022 study showed that an AMOC collapse is likely to accelerate the Pacific trade winds and Walker circulation, while weakening Indian and South Atlantic subtropical highs.^[135] The next study from the same team showed that the result of those altered atmospheric patterns is a ~30% reduction in ENSO variability and a ~95% reduction in the frequency of extreme El Niño events. Unlike today, central El Niño events become more frequent than the currently dominant eastern Pacific El Niño events.^[136] At the same time, this would essentially make a La Nina state dominant across the globe, likely leading to more frequent extreme rainfall over eastern Australia and worse droughts and bushfire seasons over southwestern United States.^[137]

A 2021 study used a simplified modelling approach to evaluate the impact of a shutdown on the Amazon rainforest and its hypothesized dieback and transition to a savannah state in some climate change scenarios. It suggested that a shutdown would enhance rainfall over the southern Amazon due to the shift of an Intertropical Convergence Zone and thus would help to counter the dieback and potentially stabilize at least the southern part of the rainforest.^[138] 2024 research suggested that the seasonal cycle of the Amazon could reverse ouright - dry seasons would become wet, and wet seasons dry.^[46][47][48]

A 2005 paper suggested that a severe AMOC "disruption" would collapse North Atlantic plankton counts to less than half of their normal biomass due to the increased stratification and the severe drop in nutrient exchange amongst the ocean layers.^[11] 2015 research simulated global ocean changes under AMOC slowdown and collapse scenarios and found that it would greatly decrease dissolved oxygen content in the North Atlantic, even as it would slightly increase globally due to greater increases across the other oceans.^[139]

See also

•  Climate change portal
•  Earth sciences portal
•  Oceans portal

• 8.2-kiloyear event
• Climate security
• Loop Current
• Pacific decadal oscillation
• Paleosalinity
• West Greenland Current

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External links

• Project THOR University of Hamburg project to study the thermohaline circulation
• "An Abrupt Climate Change Scenario and Its Implications for United States National Security" (2003 study)
• (1999 study)
• Why is there a thermohaline circulation in the Atlantic but not the Pacific? (2005 technical report)
• Original article at the Encyclopedia of Earth