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Slowdown or possible shutdown of the thermohaline circulation
The slowdown or shutdown of the thermohaline circulation is a hypothesized effect of climate change on a major ocean circulation. The Gulf Stream is part of this circulation, and is part of the reason why northern Europe is warmer than it would normally be; Edinburgh has the same latitude as Moscow. The Thermohaline Circulation influences the climate all over the world. The impacts of the decline and potential shutdown of the AMOC could include losses in agricultural output, ecosystem changes, and the triggering of other climate tipping points. Other likely impacts of AMOC decline include reduced precipitation in mid-latitudes, changing patterns of strong precipitation in the tropics and Europe, and strengthening storms that follow the North Atlantic track. Finally, a decline would also be accompanied by strong sea level rise along the eastern North American coast.

AMOC stability
Atlantic overturning is not a static feature of global circulation, but rather a sensitive function of temperature and salinity distributions as well as atmospheric forcings. Paleoceanographic reconstructions of AMOC vigour and configuration have revealed significant variations over geologic time complementing variation observed on shorter scales.

Reconstructions of a "shutdown" or "Heinrich" mode of the North Atlantic have fuelled concerns about a future collapse of the overturning circulation due to global climate change. The physics of a shutdown would be underpinned by the Stommel Bifurcation, where increased freshwater forcing or warmer surface waters would lead to a sudden reduction in overturning from which the forcing must be substantially reduced before restart is possible. In 2022, a study suggested that the strongly increasing "memory" of the past multidecadal variations in the system's circulation could act as an early warning indicator of a tipping point.

An AMOC shutdown would be fuelled by two positive feedbacks, the accumulation of both freshwater and heat in areas of downwelling. AMOC exports freshwater from the North Atlantic, and a reduction in overturning would freshen waters and inhibit downwelling. Similar to its export of freshwater, AMOC also partitions heat in the deep-ocean in a global warming regime – it is possible that a weakened AMOC would lead to increasing global temperatures and further stratification and slowdown. However, this effect would be tempered by a concomitant reduction in warm water transport to the North Atlantic under a weakened AMOC, a negative feedback on the system. 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 metres. It suggested that most models overestimate the impact of freshwater forcing on AMOC.

To complicate the issue of positive and negative feedbacks on temperature and salinity, the wind-driven component of AMOC is still not fully constrained. A relatively larger role of atmospheric forcing would lead to less dependency on the thermohaline factors listed above, and would render AMOC less vulnerable to temperature and salinity changes under global warming.

Multiple equilibria versus single equilibrium
As well as paleoceanographic reconstruction, the mechanism and likelihood of collapse has been investigated using climate models. Earth Models of Intermediate Complexity (EMICs) have historically predicted a modern AMOC to have multiple equilibria, characterised as warm, cold and shutdown modes. This is in contrast to more comprehensive models, which bias towards a stable AMOC characterised by a single equilibrium. However, doubt is cast upon this stability by a modelled northward freshwater flux which is at odds with observations. An unphysical northward flux in models acts as a negative feedback on overturning and falsely biases towards stability. On the other hand, it was also suggested that the stationary freshwater forcing used in the classic EMICs is too simplistic, and a 2022 study which modified a Stommel's Bifurcation EMIC to use more realistic transient freshwater flux found that this change delayed tipping behavior in the model by over 1000 years. The study suggested that this simulation is more consistent with the reconstructions of AMOC response to Meltwater pulse 1A, when a similarly long delay was observed.

AMOC weakening mechanism

Studies have linked AMOC reduction to the Artic sea-ice loss. Multiple models that were run using quadrupled carbon dioxide concentrations indicate a time-evolving AMOC structure that is weakened by the concurrent decrease in Artic sea-ice volume.

When the atmospheric carbon dioxide concentration increases abruptly, the North Atlantic Ocean surface warms and results in melting of Artic sea-ice. This consequently results in surface water freshening and salinity decrease. The combined effect of elevated surface water temperature and lower salinity causes changes in North Atlantic surface water densities which affects the advective oceanic processes to reduce AMOC strength. However, which of the previous mentioned effects have a more dominating effect on density is model dependent.

A 2023 research paper predicts the AMOC a sequential declining mechanism under warming climatic conditions as follows:


 * 1) Increased carbon dioxide concentrations causing a summer/fall pulse of warm air/water.
 * 2) Southward advection of Artic sea ice melt due to the warming.
 * 3) A reduction in North Atlantic water salinity and density causing a weakening of AMOC.
 * 4) Displacement of the climatological density southward due to the resulting fresh surface water pool.
 * 5) Displacement causing the veering off of the Gulf Stream reducing heat and salt transport from lower latitudes attributing to further diminish AMOC.

AMOC - Possible recovery

Multiple studies share light on possible AMOC strength recovery either due to ocean only processes such as destabilization of ocean stratification or through atmospheric interactions with ocean surface. The pace of recovery postulated varies in different study depending on the mechanism used to explain the recovery.

A study that uses models that combine ocean-atmospheric processes postulates the recovery mechanism as a result of weakening of vertical stratification due to wind driven Ekman pumping that brings denser, saltier waters to the surface combining with atmospheric radiative heating effects. The models in the study however show that the resulting AMOC will have characteristics different from the original such as stratification and thickness. The domino effects that it can have on the surrounding ecology and environment can be extremely damaging.

Impacts of a slowdown
Don Chambers from the University of South Florida College of Marine Science mentioned: "The major effect of a slowing AMOC is expected to be cooler winters and summers around the North Atlantic, and small regional increases in sea level on the North American coast." James Hansen and Makiko Sato stated: "AMOC slowdown that causes cooling ~1 °C and perhaps affects weather patterns is very different from an AMOC shutdown that cools the North Atlantic several degrees Celsius; the latter would have dramatic effects on storms and be irreversible on the century time scale."

A 2005 paper suggested that a severe AMOC slowdown would collapse North Atlantic plankton counts to less than half of their pre-disruption biomass due to the increased stratification and the severe drop in nutrient exchange amongst the ocean layers. In 2019, a study suggested that the observed ~10% decline in the phytoplankton productivity in the North Atlantic may provide evidence for this hypothesis.

Downturn of the Atlantic meridional overturning circulation has been tied to extreme regional sea level rise. A 2015 paper 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. In 2018, AMOC slowdown was also tied to increasing coastal deoxygenation. In 2020, it was linked to increasing salinity in the South Atlantic.

A study published in 2016 found further evidence for a considerable impact of a slowdown on sea level rise around the U.S. East Coast. The study confirms earlier research findings which identified the region as a hotspot for rising seas, with a potential to divert 3–4 times in the rate of rise, compared to the global average. The researchers attribute the possible increase to an ocean circulation mechanism called deep water formation, which is reduced due to AMOC slow down, leading to more warmer water pockets below the surface. Additionally, the study noted, "Our results suggest that higher carbon emission rates also contribute to increased [sea level rise] in this region compared to the global average." In 2021, another paper had also suggested that the slowdown had played a role in the northeastern coast of the United States ending up as one of the fastest-warming regions of North America.

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. In 2021, a conceptual network model was developed, connecting the AMOC, Greenland ice sheet, West Antarctic Ice Sheet and the Amazon rainforest (all well-known climate tipping points) through a set of simplified equations. It suggested that while changes to AMOC are unlikely to trigger tipping behaviour in those other elements of the climate system on their own, any other climate element transitioning towards tipping would also affect the others through a connection mediated by the AMOC slowdown, potentially initiating a tipping cascade across multi-century timescales. Consequently, AMOC slowdown would 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.

A 2021 assessment of the economic impact of climate tipping points found that while tipping points in general would likely increase the social cost of carbon by about 25%, with a 10% chance of tipping points more than doubling it, AMOC slowdown is likely to do the opposite and reduce the social cost of carbon by about −1.4%, since it would act to counteract the effects of warming in Europe, which is more developed and thus represents a larger fraction of the global GDP than the regions which would be impacted negatively by the slowdown. The following year, this finding, and the broader findings of the study, were severely criticized by a group of scientists including Steve Keen and Timothy Lenton, who considered those findings to be a severe underestimate. The authors have responded to this criticism by noting that their paper should be treated as the starting point in economic assessment of tipping points rather than the final word, and since most of the literature included in their meta-analysis lacks the ability to estimate nonmarket climate damages, their numbers are likely to be underestimates.

Impacts of a shutdown
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 for a long time. 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 Fahrenheit between 2010 and 2020 as the result of an abrupt AMOC shutdown.

In general, a shutdown of the thermohaline circulation (THC) caused by global warming would trigger cooling in the North Atlantic, Europe, and North America. This would particularly affect areas such as the British Isles, France and the Nordic countries, which are warmed by the North Atlantic drift. Major consequences, apart from regional cooling, could also include an increase in major floods and storms, a collapse of plankton stocks, warming or rainfall changes in the tropics or Alaska and Antarctica, more frequent and intense El Niño events due to associated shutdowns of the Kuroshio, Leeuwin, and East Australian Currents that are connected to the same thermohaline circulation as the Gulf Stream, or an oceanic anoxic event — oxygen below surface levels of the stagnant oceans becomes completely depleted – a probable cause of past mass extinction events.

In 2002, a study had suggested that an AMOC shutdown may be able to trigger the type of abrupt massive temperature shifts which occurred during the last glacial period: a series of Dansgaard-Oeschger events – rapid climate fluctuations – may be attributed to freshwater forcing at high latitude interrupting the THC. 2002 model runs in which the THC is forced to shut down do show cooling – locally up to 8 °C (14 °F). 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.

A 2015 study led by James Hansen found that the shutdown or substantial slowdown of the AMOC, besides possibly contributing to extreme end-Eemian events, will cause a more general increase of severe weather. Additional surface cooling from ice melt increases surface and lower tropospheric temperature gradients, and causes in model simulations a large increase of mid-latitude eddy energy throughout the midlatitude troposphere. This in turn leads to an increase of baroclinicity produced by stronger temperature gradients, which provides energy for more severe weather events. This includes winter and near-winter cyclonic storms colloquially known as "superstorms", which generate near-hurricane-force winds and often large amounts of snowfall. These results imply that strong cooling in the North Atlantic from AMOC shutdown potentially increases seasonal mean wind speed of the northeasterlies by as much as 10–20% relative to preindustrial conditions. Because wind power dissipation is proportional to the cube of wind speed, this translates into an increase of storm power dissipation by a factor ~1.4–2,. However, the simulated changes refer to seasonal mean winds averaged over large grid-boxes, not individual storms.

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. In 2021, a study using a Community Earth System Model suggested that an AMOC slowdown could nevertheless increase the strength of El Niño–Southern Oscillation and thus amplify climate extremes, especially if another Meridional Overturning Circulation develops in the Pacific Ocean in response to AMOC slowdown. 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. 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, El Niño events become more frequent in the central rather than eastern Pacific El Niño events. 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.

In 2020, a study had assessed the impact of an AMOC collapse on farming and food production in Great Britain. It estimated that AMOC collapse would reverse the impact of global warming in Great Britain and cause an average temperature drop of 3.4 °C. Moreover, it 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%.

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.