The Atlantic meridional overturning circulation (AMOC) is the main ocean current system in the Atlantic Ocean. It is part of Earth's ocean circulation system and helps control the planet's climate. The AMOC includes surface and deep ocean currents in the Atlantic that are influenced by changes in weather, temperature, and salinity. These currents make up half of the global thermohaline circulation, with the other half being the Southern Ocean overturning circulation.
The AMOC consists of warm, salty water moving northward in the upper layers of the Atlantic and cold, less salty water flowing southward in the deep ocean. Warm water from the south becomes saltier due to higher evaporation rates in tropical regions. This salty water forms the upper layer of the ocean but becomes denser when it cools, causing it to sink into the deep. This sinking process is a key part of how the AMOC works. The system connects through overturning regions in the Nordic Seas and the Southern Ocean. These areas are important for exchanging heat, oxygen, carbon, and nutrients, which support ocean ecosystems and help the ocean store carbon. Changes in the strength of the AMOC can affect many parts of the climate system.
Climate change may weaken the AMOC by increasing ocean heat and adding more freshwater from melting ice sheets. Studies suggest that by 2015, the AMOC was weaker than it was before the Industrial Revolution. Scientists are still debating how much of this weakening is due to climate change versus natural changes over time. Climate models predict the AMOC will weaken further during the 21st century. This weakening could lower average air temperatures in Scandinavia, Great Britain, and Ireland, as these regions depend on the North Atlantic Current for warmth. It could also speed up sea level rise near North America and reduce the growth of ocean plants in the North Atlantic.
If the AMOC weakens severely, the circulation might collapse. A collapse would be hard to reverse and could be a major tipping point in the climate system. This would cause significant drops in average temperatures and rainfall in Europe, increase extreme weather events, and lead to other serious effects.
Overall structure
The Atlantic meridional overturning circulation (AMOC) is the main current system in the Atlantic Ocean and part of the global thermohaline circulation, which connects all the world's oceans through a continuous exchange of water. Normally, warm, less-salty water stays near the ocean's surface, while deeper water is colder, saltier, and denser. This layering of water is called ocean stratification. Over time, deep water gains heat or loses salt through mixing with the surface layer, becoming less dense and rising. Differences in temperature and salt levels between ocean layers and across the world's oceans drive the thermohaline circulation. The Pacific Ocean has less salt than other oceans because it receives large amounts of fresh rain. Its surface water is not salty enough to sink below several hundred meters, so deep water in the Pacific must come from other regions.
Ocean water in the North Atlantic is saltier than in the Pacific. This happens partly because evaporation on the surface leaves salt behind in the remaining water and partly because sea ice near the Arctic releases salt as it freezes. Also, moisture evaporated from the Atlantic is carried by wind across Central America to the eastern North Pacific, where it falls as rain. Major mountain ranges, such as the Tibetan Plateau, Rocky Mountains, and Andes, block this moisture from returning to the Atlantic.
Because of these processes, Atlantic surface water becomes salty and dense, eventually sinking to form the North Atlantic Deep Water (NADW). NADW forms mainly in the Nordic Seas and involves a mix of different water types, including the Denmark Strait Overflow Water, Iceland-Scotland Overflow Water, and Nordic Seas Overflow Water. Water from the Labrador and Irminger Seas may also play a role, but recent evidence suggests it mostly moves within the North Atlantic Gyre and has little connection to the rest of the AMOC.
The NADW is not the deepest layer in the Atlantic Ocean. The densest and deepest layer is Antarctic Bottom Water (AABW), found in basins deeper than 4,000 meters. As AABW rises, it mixes with and strengthens the NADW. The formation of NADW starts the lower part of the circulation, and the sinking of water is balanced by rising water elsewhere. In the western Atlantic, wind-driven ocean mixing causes strong upwelling in the Canary Current and Benguela Current, located on Africa’s northwest and southwest coasts. As of 2014, upwelling near the Canary Current is stronger than near the Benguela Current, though this pattern was reversed before the Central American Seaway closed during the late Pliocene. In the eastern Atlantic, upwelling occurs only during specific months because this region depends more on surface water temperature than wind activity. Upwelling also follows a cycle linked to El Niño and La Niña events.
As NADW moves southward, about 80% of it rises in the Southern Ocean, connecting to the Southern Ocean overturning circulation. After rising, the water takes one of two paths. Water near Antarctica may be cooled by sea ice and sink again, rejoining AABW or flowing to the Pacific and Indian Oceans. Water rising at lower latitudes moves northward due to wind-driven mixing and becomes part of the upper circulation. Warm water in the upper circulation returns to the North Atlantic, mainly near Africa and through the Indonesian archipelago. Once this water reaches the North Atlantic, it cools and becomes denser, sinking again to feed back into the NADW.
Role in the climate system
Equatorial areas are the hottest parts of the globe. Due to thermodynamics, this heat moves toward the poles. Most of this heat is transported by atmospheric circulation, but warm surface ocean currents also play an important role. Heat from the equator moves either northward or southward. The Atlantic Ocean is the only ocean in which heat flows northward. 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 is driven by winds, its northern-most segment, the North Atlantic Current, receives much of its heat from thermohaline exchange in the AMOC. Thus, the AMOC carries up to 25% of the total heat toward the northern hemisphere and plays an important role in the climate around northwest Europe.
Atmospheric patterns also play a large role in heat transfer. The idea that the climate in northern Europe would be as cold as that in northern North America without ocean current heat transport (up to 15–20 °C [27–36 °F] colder) is generally considered incorrect. While one modeling study suggested that a collapse of the AMOC could result in Ice Age-like cooling, including sea-ice expansion and glacier formation within a century, the accuracy of those results is questionable. There is a consensus that the AMOC keeps northern and western Europe warmer than it would be otherwise, with differences of 4 °C (7.2 °F) and 10 °C (18 °F) depending on the area. For example, studies of the Florida Current suggest the Gulf Stream was about 10% weaker from around 1200 to 1850 due to increased surface salinity, and this likely contributed to the conditions known as the Little Ice Age.
The AMOC makes the Atlantic Ocean a more-effective carbon sink in two major ways. First, upwelling supplies large quantities of nutrients to surface waters, supporting the growth of phytoplankton and increasing marine primary production and photosynthesis in surface waters. Second, upwelled water has low concentrations of dissolved carbon because it is typically 1,000 years old and has not been exposed to increased atmospheric carbon dioxide. This water absorbs more carbon than surface waters and is prevented from releasing carbon back into the atmosphere when it is downwelled. While the Southern Ocean is the strongest ocean carbon sink, the North Atlantic is the largest single carbon sink in the northern hemisphere.
The Atlantic meridional overturning circulation (AMOC) depends on interactions between layers of ocean water with different temperatures and salinities. It is not static but experiences small, cyclical changes and larger, long-term shifts in response to external factors. Many of these shifts occurred during the Late Pleistocene (126,000 to 11,700 years ago), the final geological epoch before the current Holocene. This period also includes the Last Glacial Period, known colloquially as the "last ice age." Twenty-five abrupt temperature oscillations between the hemispheres occurred during this time; these are called Dansgaard–Oeschger events (D-O events), named after Willi Dansgaard and Hans Oeschger, who discovered them by analyzing Greenland ice cores in the 1980s.
D-O events are best known for the rapid warming of 8 °C (15 °F) to 15 °C (27 °F) in Greenland over several decades. Warming also occurred across the North Atlantic, but equivalent cooling occurred in the Southern Ocean during these events. This is consistent with the strengthened AMOC transporting more heat between hemispheres. The warming of the northern hemisphere likely caused ice-sheet melting, and many D-O events appear to have ended due to Heinrich events, in which massive streams of icebergs broke off from the Laurentide ice sheet. As the icebergs melted, ocean water became fresher, weakening the circulation and stopping the D-O warming.
There is no consensus on why the AMOC fluctuated so much, and only during this glacial period. Common hypotheses include cyclical patterns of salinity change in the North Atlantic or wind-pattern cycles caused by the growth and decline of ice sheets, which affect wind patterns. Research from the late 2010s suggests the AMOC is most sensitive to change during periods of extensive ice sheets and low carbon dioxide levels, making the Last Glacial Period a "sweet spot" for such oscillations. Some research suggests warming in the southern hemisphere initiated the pattern as warmer waters spread north through thermohaline circulation. Paleoclimate evidence is not strong enough to determine whether D-O events started with changes in the AMOC or if the AMOC changed in response to another trigger. For example, some research suggests changes in sea-ice cover initiated the D-O events because they would have affected water temperature and circulation through ice–albedo feedback.
D-O events are numbered in reverse order, with the largest numbers assigned to the oldest events. The penultimate event, D-O event 1, occurred about 14,690 years ago and marks the transition from the Oldest Dryas period to the Bølling–Allerød Interstadial, which lasted until 12,890 years before present. It was named after two sites in Denmark with vegetation fossils that could only have survived during a warm period in the northern hemisphere. The major warming in the northern hemisphere was offset by cooling in the southern hemisphere, with little net change in global temperature, consistent with changes in the AMOC. The onset of the interstadial caused a period of sea level rise from ice-sheet collapse, designated Meltwater pulse 1A.
The Bølling and Allerød stages of the interglacial were separated by two centuries of the opposite pattern—northern-hemisphere cooling and southern-hemisphere warming—known as the Older Dryas because the Arctic flower Dryas octopetala became dominant where forests grew during the interglacial. 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. This happened due to an abrupt slowing of the AMOC, caused by freshening from ice loss in the Laurentide ice sheet. Unlike true Heinrich events, an enormous flow of meltwater occurred through the Mackenzie River in present-day Canada rather than a mass iceberg loss. Major changes in precipitation patterns, such as the shift of the Intertropical Convergence Zone to the south, increased rainfall in North America and caused drying in South America and Europe. Global temperatures barely changed during the Younger Dryas, and long-term post-glacial warming resumed after it ended.
Stability and vulnerability
The AMOC did not exist for most of Earth's history. In the past, ocean currents in the northern hemisphere were mainly in the North Pacific. Evidence from ancient climates shows that ocean currents shifted from the Pacific to the Atlantic about 34 million years ago, during a time called the Eocene-Oligocene transition. This change happened when a water passage between the Arctic and Atlantic Ocean closed. This closure changed how ocean currents moved in the deep sea. Some scientists think climate change might one day reverse this shift and restart the Pacific circulation if the AMOC stops working. Climate change affects the AMOC by warming surface water due to Earth’s energy imbalance and by reducing the saltiness of surface water. This happens because melting ice, especially from Greenland, adds fresh water to the ocean, and because more rain falls over the North Atlantic. These changes increase the difference between surface and deep water layers, making it harder for the ocean currents that drive the AMOC to function.
In the 1960s, a scientist named Henry Stommel studied the AMOC and created a model called the Stommel Box model. This model introduced the idea that the AMOC can exist in two states: a strong state, like the one seen in recorded history, or a weak state where it nearly stops working. If the AMOC weakens due to warming or fresh water, it might not recover unless these changes are reduced. Warming and fresh water could directly cause the AMOC to collapse or weaken to a point where natural changes in the system might push it past a critical point. Scientists have long discussed whether the AMOC is a system that can be either "on" or "off" and might suddenly collapse.
Some models developed after Stommel’s work suggest the AMOC could have one or more stable states between full strength and full collapse. These are more common in models called Earth Models of Intermediate Complexity (EMICs), which focus on parts of the climate system like the AMOC and ignore other factors. More detailed models, called general circulation models (GCMs), usually show the AMOC has only one stable state and is unlikely to collapse. Researchers have pointed out that GCMs may avoid collapse because they move large amounts of fresh water to the North Pole, where it doesn’t affect the AMOC. This movement does not happen in real life.
In 2024, three scientists used a model called the Community Earth System Model (CIMP) to simulate the AMOC. Their simulation showed a classic collapse of the AMOC, similar to what happens in simpler models. Unlike other simulations, they gradually increased the amount of meltwater in the model over more than 1,700 years. Eventually, the model reached meltwater levels equal to a sea level rise of 6 centimeters (2.4 inches) per year. This is much higher than the 2.9 millimeters (0.11 inches) per year rise observed between 1993 and 2017. The researchers said these extreme conditions were used to test the model’s stability, not to predict the future. Other scientists agreed that this study helps improve understanding of how the AMOC might change before a collapse, especially as more data becomes available.
Some research suggests simpler models overestimate the AMOC’s likelihood of collapsing because they assume a constant flow of fresh water. In one study, using a variable flow of fresh water instead of a constant one delayed the AMOC’s collapse by over 1,000 years in a typical model. This result matched evidence from ancient climates, such as the Meltwater Pulse 1A event 13,500–14,700 years ago. A 2022 study found that massive fresh water input during the end of the last ice age had a limited effect on the AMOC, suggesting models may overestimate the impact of fresh water. If the AMOC depends more on wind strength, which changes little with warming, it might be more stable than models suggest. Some scientists think the Southern Ocean’s circulation might be more vulnerable to collapse than the AMOC.
High-quality models suggest the AMOC is unlikely to collapse unless warming continues for many years after 2100. Some ancient climate studies support this idea. However, some scientists worry that more detailed models are too stable and that simpler models predicting an earlier collapse might be more accurate. One model suggests the AMOC could collapse around 2065, but many scientists are unsure about this prediction. In October 2024, 44 climate scientists published a letter stating that recent studies suggest the risk of AMOC collapse has been underestimated and that it could happen in the next few decades, with serious effects for Nordic countries. They urged Nordic nations to follow the Paris Agreement to reduce this risk.
Trends
Before 2024, scientists had different ideas about the Atlantic Meridional Overturning Circulation (AMOC). Observations showed the AMOC was slowing down, but computer models suggested it stayed the same. In November 2024, a study published in Nature Geoscience helped resolve this disagreement. The scientists used special computer models that study ocean and sea ice. After this study, the observations and models matched better. The study found the AMOC has slowed by 0.46 sverdrups each decade since 1950.
Direct measurements of the AMOC’s strength began in 2004 with RAPID, a system of underwater sensors at 26°N in the Atlantic. To understand long-term trends, scientists need data collected over many years. Some researchers used smaller-scale observations to predict AMOC changes. For example, in 2005, research by Peter Wadhams found that water sinking in the Greenland Sea was weaker than usual. In 2000, other scientists studied the North Atlantic Gyre (NAG), a part of the AMOC. Measurements in 2004 showed a 30% drop in the NAG compared to 1992, but later RAPID data showed this was a statistical anomaly, and the NAG recovered by 2008. Scientists now know the NAG operates separately from the rest of the AMOC and could weaken on its own.
By 2014, RAPID data up to 2012 showed a decline in the AMOC 10 times larger than predicted by models. Scientists debated whether this was due to climate change or natural changes in the circulation. Data up to 2017 showed the biggest drop in 2008–2009, but after that, the AMOC was weaker than it was between 2004–2008.
The AMOC is also measured by tracking heat transport, which relates to ocean currents. In 2017 and 2019, satellite and float data suggested 15–20% less heat was being moved than RAPID data indicated, showing a stable flow with limited changes over decades.
Measurements of the Florida Current, a part of the AMOC, have remained stable for 40 years after adjusting for changes in Earth’s magnetic field.
Climate reconstructions, which use indirect evidence to study the past, help scientists understand the AMOC’s history. In 2021, RAPID data was combined with older records to show no overall decline in the AMOC over the past 30 years. A 2020 study found no major changes in the AMOC compared to the 1990s, though other parts of the North Atlantic showed changes. A 2022 review noted that while global warming might weaken the AMOC long-term, changes since 1980 are hard to detect because the AMOC has both weakened and strengthened at different times.
Some reconstructions compare the AMOC’s current state to earlier periods. A 2010 study found the AMOC has been weakening since the late 1930s, with a sudden change around 1970. A 2015 study linked cooler temperatures in some years to a 15–20% AMOC weakening over 200 years, with the circulation slowing most of the 20th century. Between 1975 and 1995, the AMOC was weaker than any time in the past 1,000 years. A 2018 study suggested a 15% weakening since the mid-20th century. A 2021 reconstruction using ocean temperature and salinity data showed possible instability in the AMOC, but it excluded data from 35 years before 1900 and after 1980 to keep records consistent. A 2022 study challenged these findings, saying no major changes occurred between 1900 and 1980, and a small weakening in the AMOC began in 1980, which fits natural variability.
Sediment analysis showed the AMOC weakened by 20% since the mid-20th century. A 2018 study found the AMOC has been unusually weak in the last 150 years compared to the past 1,500 years. A 2017 review showed evidence of past AMOC changes during sudden climate events like the Younger Dryas. A 2022 study found the AMOC’s long-term patterns now show less chance of returning to normal, which might indicate a slow loss of stability not seen in most models.
In 2021, a major study in Nature Geoscience said the AMOC weakened significantly over the past millennium, likely due to human activity. The study’s authors warned the AMOC could weaken further in 20–30 years, leading to more storms, heatwaves in Europe, and rising sea levels on the U.S. East Coast. A 2022 article in Nature Geoscience questioned these findings, saying the AMOC’s long-term trend is still unclear. The original study’s authors defended their results.
Some scientists link recent climate changes to the AMOC’s weakening. For example, a large area near Greenland has cooled by 0.39°C (0.70°F) since 1900, unlike other parts of the ocean that have warmed. This cooling is usually seasonal but has lasted longer in some periods, like 2014–2016, when the area stayed cool for 19 months. Scientists call this the "cold blob." This happens when fresh, cool water stays on the surface instead of sinking, which is linked to a slower AMOC. Later research found changes in weather patterns, like more low clouds and stronger winds, also contributed to this cooling.
Projections
Historically, CMIP models, the most trusted method in climate science, show the AMOC is very stable. Even if it weakens, it usually recovers instead of collapsing completely. For example, in a 2014 test where CO₂ levels doubled from 1990 levels and stayed the same, the AMOC declined by about 25% but did not collapse. It recovered slightly, by 6%, over the next 1,000 years. In 2020, research estimated that if warming stops at 1.5°C, 2°C, or 3°C by 2100, the AMOC would weaken for about 5–10 years after warming stops but would not collapse. It would partially recover after about 150 years. Many scientists believe that models avoid predicting collapse because they have errors that affect large-scale simulations.
Although models have improved over time, the current generation of CMIP6 models still has some inaccuracies. On average, these models show greater AMOC weakening in response to greenhouse gas warming than earlier models. For example, four CMIP6 models simulated the AMOC under the SSP3-7 scenario, where CO₂ levels more than double from 2015 values by 2100. They found the AMOC declined by over 50% by 2100. These models still have trouble accurately simulating the North Atlantic Deep Water (NADW), which reduces confidence in their predictions.
To fix these issues, scientists have tried adjusting models for errors. In one test, applying bias correction to a model caused the AMOC to collapse after 300 years. In 2016, a study combined results from eight CMIP5 models with better Greenland ice melt estimates. It found that under the RCP 4.5 scenario, the AMOC would weaken by about 18% by 2100. Under the RCP 8.5 scenario, the AMOC would weaken by 37%. If these scenarios continued past 2100, the AMOC stabilized under RCP 4.5 but continued to decline under RCP 8.5, leading to a 74% drop by 2290–2300 and a 44% chance of complete collapse. In 2020, another study used a model with advanced ocean physics. It found similar results to the 2016 study under RCP 4.5 but showed the AMOC declined by two-thirds under RCP 8.5 after 2100, though it did not collapse completely.
In 2023, a study using data from several models suggested the AMOC might collapse around 2065, with a 95% chance of collapse between 2037 and 2109. However, this study faced criticism because the models used are less reliable and might confuse a major slowdown with a complete collapse. The study used temperature data from the Northern Subpolar Gyre, which some scientists say does not represent the whole AMOC. Experts called the research "worrisome" but noted it could be useful once better data is available. They agreed the data used was not strong enough to support the findings.
A 2025 study extended CMIP6 simulations beyond 2100 and tracked the deep northern overturning cell linked to NADW. Under the high-emissions scenario SSP5-8.5, all nine models showed the AMOC weakened from about 14–26 Sv to 1–6 Sv by 2100. This was accompanied by a shift in the depth of maximum overturning and a breakdown of deep winter convection in subpolar regions. These changes were linked to feedbacks in the models. The models also showed a shallow, wind-driven overturning cell remained active after the deep cell weakened.
Large review papers and reports can evaluate model results, direct observations, and historical data to provide expert opinions. In 2001, the IPCC Third Assessment Report said the AMOC would weaken but not stop, with warming effects outweighing cooling. In 2014, the IPCC Fifth Assessment Report said a rapid AMOC transition was "very unlikely." In 2021, the IPCC Sixth Assessment Report said the AMOC is "very likely" to weaken in the 21st century and that changes would be reversible if warming stops. However, confidence in avoiding collapse before 2100 was reduced to "medium" due to model biases and simpler studies suggesting the AMOC may be more vulnerable than larger models suggest.
The IPCC Sixth Assessment Report summarized that the AMOC is very likely to weaken by the end of the century but that an abrupt collapse is not expected before 2100. If such an event occurred, it would cause sudden changes in weather patterns, rainfall, and harm ecosystems and human activities. In 2022, a study identified 16 potential climate tipping points, including AMOC collapse. It estimated collapse would most likely happen at 4°C of warming but could occur between 1.4°C and 8°C. Once triggered, collapse would take 15–300 years, most likely around 50 years. The collapse of the Northern Subpolar Gyre was also considered a separate tipping point, likely to occur at 1.8°C and take 5–50 years. This collapse could lower global temperatures by 0.5°C and European temperatures by 3°C, with major effects on regional rainfall.
A report on the cryosphere (ice and snow) said the AMOC may be approaching collapse due to ice melt and warming. This could cause Northern Europe to cool rapidly, faster than 3°C per decade, with no realistic way to adapt. At the same time, other regions might experience different changes.
Effects of AMOC slowdown
In 2024, scientists are not sure if the AMOC has been slowing down consistently. However, most agree that if climate change continues, the AMOC will likely slow down in the future. According to the IPCC, a weaker AMOC could lead to less rain in mid-latitude regions, more intense rain in tropical and European areas, and stronger storms that move along the North Atlantic. In 2020, research showed that a weaker AMOC might slow the loss of Arctic sea ice and cause atmospheric changes similar to those during the Younger Dryas, such as the Intertropical Convergence Zone shifting southward. These changes would be much more extreme under high-emissions scenarios.
A weaker AMOC would increase sea level rise along the U.S. East Coast. At least one event linked to temporary AMOC slowdowns has already been observed. This would happen because coastal waters would warm more, expand, and transfer less heat toward Europe. This is why sea level rise along the U.S. East Coast is expected to be three to four times higher than the global average.
Some scientists think a partial slowdown of the AMOC might cause Europe to cool by about 1°C (1.8°F). Other regions would experience different effects. Research from 2022 found that Siberian winter weather extremes were milder when the AMOC was weaker. One study suggested that a slower AMOC could lower the social cost of carbon by −1.4%, because Europe contributes more to the global economy than regions harmed by the slowdown. However, this study may have underestimated climate impacts overall. Some research argues that a slower AMOC would reduce the ocean’s ability to absorb heat, increasing global warming, though this view is less common.
A 2021 study linked other major climate tipping points, such as the Greenland ice sheet, the West Antarctic Ice Sheet, and the Amazon rainforest, to the AMOC. The study said changes to the AMOC alone are unlikely to trigger tipping points elsewhere. However, a slower AMOC could connect these systems, lowering the global-warming threshold at which tipping points might occur. This connection could lead to a chain of tipping events over several centuries.
Effects of an AMOC shutdown
A complete collapse of the AMOC cannot be reversed easily. Recovery would take thousands of years. A shutdown of the AMOC is expected to cause a lot of cooling in Europe, especially in Britain and Ireland, France, and the Nordic countries. In 2002, research compared an AMOC shutdown to Dansgaard–Oeschger events—sudden temperature changes that happened during the Last Glacial Period. According to that study, Europe could cool by up to 8 °C (14 °F). In 2022, a major review of tipping points concluded that an AMOC collapse would lower global temperatures by about 0.5 °C (0.90 °F), while temperatures in Europe would fall by between 4 °C (7.2 °F) and 10 °C (18 °F).
A 2020 study examined the effects of an AMOC collapse on farming and food production in Great Britain. It found that, after accounting for warming, the average temperature in Great Britain would drop by 3.4 °C (6.1 °F). The growing season would also see about 123 mm (4.8 in) less rainfall, reducing the area of land suitable for growing crops from 32% to 7%. The total value of British farming would decrease by about £346 million per year—over 10% of its value in 2020.
In 2024, a study modeled the effects of an AMOC collapse on a pre-industrial world. It predicted average sea surface temperatures in northwest Europe would fall by 10 °C (18 °F), and average February temperatures on land in northern and western Europe could drop between 10 °C (18 °F) and 30 °C (54 °F) within a century. This change could bring sea ice into the territorial waters of the British Isles and Denmark during winter, while Antarctic sea ice would decrease. These findings do not include the warming caused by climate change, and the study’s methods have been debated.
A 2015 study led by James Hansen found that a shutdown or slowdown of the AMOC could worsen severe weather. This happens because it increases baroclinicity and speeds up northeasterly winds by up to 10–20% in the mid-latitude troposphere. This could lead to more frequent and stronger winter and near-winter cyclonic storms with hurricane-force winds and heavy snowfall. This study has also been debated.
Several studies have looked at how an AMOC collapse might affect the El Niño–Southern Oscillation (ENSO). Results vary, ranging from no major impact to stronger ENSO activity, or a shift toward dominant La Niña conditions. This could reduce El Niño extremes by about 95%, increase extreme rainfall in eastern Australia, and worsen droughts and wildfire seasons in the southwestern U.S.
A 2021 study used a simplified model to examine how an AMOC collapse might affect the Amazon rainforest and its potential shift to a savanna in some climate scenarios. It found that an AMOC collapse could increase rainfall in the southern Amazon due to the movement of the Intertropical Convergence Zone, helping to prevent the rainforest from shrinking and possibly stabilizing the southern region. A 2024 study suggested the Amazon’s seasonal cycle could reverse, with dry seasons becoming wet and wet seasons becoming dry.
A 2005 paper stated that severe disruption of the AMOC would reduce North Atlantic plankton levels to less than half their normal amount due to increased ocean layer separation and reduced nutrient exchange. A 2015 study simulated global ocean changes under AMOC slowdown and collapse scenarios, finding that these events would greatly lower dissolved oxygen in the North Atlantic. However, dissolved oxygen would slightly increase globally due to greater increases in other ocean regions.