Atlantic meridional overturning circulation

[1]: 2238 Climate change may weaken the AMOC through increases in ocean heat content and elevated flows of freshwater from melting ice sheets.

[6][7] There is debate over the relative contributions of different factors and it is unclear how much of this weakening is due to climate change or the circulation's natural variability over millennia.

[10]: 19  This weakening would reduce average air temperatures over Scandinavia, Great Britain, and Ireland, because these regions are warmed by the North Atlantic Current.

[19] 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.

[28] 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.

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[clarification needed] and through the Indonesian archipelago.

[14] For instance, studies of the Florida Current suggest the Gulf Stream was around 10% weaker from around 1200 to 1850 due to increased surface salinity, and this likely contributed to the conditions known as Little Ice Age.

[43] 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[45][8] and larger, long-term shifts in response to external forcings.

[58] 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.

Paleoclimate evidence shows the shift of overturning circulation from the Pacific to the Atlantic occurred 34 million years ago at the Eocene-Oligocene transition, when the Arctic-Atlantic gateway had closed.

[70][71] In 2004, The Guardian published the findings of a report commissioned by Pentagon defense adviser Andrew Marshall that suggests the average annual temperature in Europe would drop by 6 °F (3.3 °C) between 2010 and 2020 as the result of an abrupt AMOC shutdown.

[65] 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 50 m (160 ft).

[82][16] In October 2024, 44 climate scientists published an open letter, claiming that according to scientific studies in the past few years, the risk of AMOC collapse has been greatly underestimated, it can occur in the next few decades, with devastating impacts especially for Nordic countries.

[111] 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.

[7][114] 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.

"[114] 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.

[115] 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[117] near Greenland has cooled by 0.39 °C (0.70 °F) between 1900 and 2020, in contrast to substantial ocean warming elsewhere.

[120] Later research found atmospheric changes, such as an increase in low cloud cover[121] and a strengthening of the North Atlantic oscillation (NAO) have also played a major role in this local cooling.

[87] 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 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".

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.

[14][100] In October 2024, 44 climate scientists published an open letter to the Nordic Council of Ministers, claiming that according to scientific studies in the past few years, the risk of AMOC collapse has been greatly underestimated, that it can occur in the next few decades, and that some changes are already happening.

At the same time, the Antarctic Circumpolar Current (ACC) is also slowing down and the Weddell Sea Bottom Water is losing volume, what can impact global ocean circulation and climate.

[141] UNESCO mentions that the report in the first time "notes a growing scientific consensus that melting Greenland and Antarctic ice sheets, among other factors, may be slowing important ocean currents at both poles, with potentially dire consequences for a much colder northern Europe and greater sea-level rise along the U.S. East Coast.

[145] 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.

[152][153] According to some research, the dominant effect on an AMOC slowdown would be a reduction in oceanic heat uptake, leading to increased global warming,[154] but this is a minority opinion.

[39] A 2015 study led by James Hansen found a shutdown or substantial slowing of the AMOC will intensify severe weather because it increases baroclinicity and accelerates northeasterly winds up to 10–20% throughout the mid-latitude troposphere.

[164] 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[166] to an increase in ENSO strength,[67] 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.[167][168][169] A 2021 study used a simplified modeling approach to evaluate the effects of an AMOC collapse on the Amazon rainforest, and its hypothesized dieback and transition to a savanna state in some climate-change scenarios.

[37][38][39] A 2005 paper said severe disruption of the AMOC would collapse North Atlantic plankton counts to less than half of their normal biomass due to increased stratification and the large decline in nutrient exchange among ocean layers.

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.
AMOC in relation to the global thermohaline circulation (animation)
A summary of the path of the thermohaline circulation. Blue paths represent deep-water currents, while red paths represent surface currents
Heat transfer from the ocean to atmosphere (left) and an increase in Atlantic Ocean heat content (right) observed when the AMOC is strong [ 32 ]
A reconstruction of how Heinrich events would have likely proceeded, with the Laurentide ice sheet first growing to an unsustainable position, where the base of its periphery becomes too warm, and then rapidly losing ice until it is reduced to sustainable size [ 44 ]
The significant warming of the Northern Hemisphere and the equivalent cooling of the South, indicated by paleoclimate data and replicated in simulations, is consistent with significant AMOC strengthening. [ 57 ]
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 [ 65 ]
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 CO 2 concentrations suddenly quadruple. [ 73 ] There are concerns that this kind of a simulation is too stable [ 74 ]
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. [ 37 ] While the conditions are unrealistic, the model may also be unrealistically stable, and the full implications are not clear without more real-world observations [ 39 ]
1992–2002 altimeter data from NASA Pathfinder indicated a slowing (red) in the subpolar gyre region. This was used as a proxy for the AMOC before the initiation of RAPID, and before subsequent research demonstrated the subpolar gyre often behaves separately from the larger circulation [ 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 [ 92 ]
A 2021 comparison of the post-2004 RAPID observations with the 1980–2004 reconstructed AMOC trend had indicated no real change across 30 years [ 105 ]
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. [ 8 ]
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. [ 111 ]
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). [ 116 ]
Climate models are often calibrated by comparing their simulations after CO 2 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) [ 128 ]
If CO 2 concentrations were to double by 2100 from their 2015 values, then the AMOC strength would decline by over 50%. Reductions in methane warming or sulfate aerosol cooling, or both, would have an effect of around 10% by comparison [ 132 ]
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 [ 135 ] [ 14 ]
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) [ 143 ]
A proposed tipping cascade where the AMOC would mediate a connection between the other tipping elements.
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).