Atlantic meridional overturning circulation

The Atlantic meridional overturning circulation (AMOC) is the main ocean current system in the Atlantic Ocean. It is a key part of Earth's ocean circulation and plays a big role in shaping our climate. The AMOC includes currents at the surface and deep down, moving because of changes in weather, temperature, and salinity. These currents make up half of the world's large-scale ocean flow, called the global thermohaline circulation, with the other half happening in the Southern Ocean overturning circulation.

The AMOC works like a giant conveyor belt. Warm, salty water flows north near the surface, while cold, less salty water flows south deep in the ocean. This movement helps move heat, oxygen, carbon, and nutrients around the ocean, which is important for sea life and for storing carbon. When the warm water cools, it becomes heavier and sinks, helping to keep the system moving. This process links areas like the Nordic Seas and the Southern Ocean.

Because of climate change, the AMOC might be getting weaker. More heat in the ocean and extra freshwater from melting ice sheets could slow it down. Some scientists think the AMOC is already weaker than it was before the Industrial Revolution. If it weakens too much, it could even stop working for a while. This would change temperatures and weather patterns, especially in places like Scandinavia, Great Britain, and Ireland, and could also raise sea levels near North America.

Overall structure

The Atlantic meridional overturning circulation (AMOC) is the main current system in the Atlantic Ocean and is part of the world's thermohaline circulation. This global system connects all the oceans like a giant conveyor belt of moving water. Warm, less salty water stays near the surface, while colder, denser, saltier water stays deep below. These differences in temperature and saltiness drive the movement of water around the planet.

The Atlantic Ocean has saltier water than the Pacific Ocean. This is because the Atlantic loses a lot of water through evaporation, which leaves salt behind. Also, when sea ice forms near the Arctic Circle, it pushes salt into the surrounding water. This salty water becomes dense and sinks to form deep water called the North Atlantic Deep Water. This deep water is not the very deepest layer; the densest water is called Antarctic bottom water. As this deep water moves, it eventually rises again in different parts of the ocean, continuing the cycle.

Role in the climate system

The Atlantic Ocean helps move heat around the world. Warm areas near the equator heat up the most, and this heat moves toward the cooler poles. While much of this heat moves through the air, ocean currents also play a big role. In the Atlantic Ocean, warm surface water flows northward, carrying heat to places like northwest Europe. This helps keep those areas warmer than they would be otherwise.

The ocean also helps control how much carbon is in the air. Cold water that rises to the surface brings nutrients that help tiny plants called phytoplankton grow. This process uses carbon from the air. The ocean also takes in carbon from the air when cold, old water rises and absorbs it before sinking back down.

Abrupt changes during the Late Pleistocene

See also: Abrupt climate change

The ocean’s flow isn’t always the same. During a time called the Late Pleistocene, which was the last ice age, the ocean’s flow changed many times. These changes happened along with big shifts in temperature. For example, the northern areas got much warmer very quickly, while the southern areas cooled down. These quick changes happened because the ocean carried more or less heat between the north and south.

These temperature shifts are called Dansgaard–Oeschger events. They happened many times during the ice age. One big change happened about 14,690 years ago, ending a cold period and starting a warmer time. Later, the climate cooled again for a short time called the Younger Dryas. This cooling happened when a lot of fresh water entered the ocean, changing its flow.

Stability and vulnerability

Further information: Multiple equilibria in the Atlantic meridional overturning circulation

The AMOC, the main ocean current system in the Atlantic Ocean, has not always existed. For much of Earth's history, a similar current flowed in the North Pacific. About 34 million years ago, this shifted to the Atlantic when a gateway between the Arctic and Atlantic closed. Today, climate change is affecting the AMOC by warming the surface water and adding fresh water from melting ice, mainly from Greenland. These changes make it harder for the AMOC to keep moving.

In the 1960s, a scientist named Henry Stommel studied the AMOC and suggested it could exist in two states: a strong state, like today, or a much weaker state if warming and fresh water continue. Some models show the AMOC might collapse to a weaker state, while others suggest it is more stable. Recent studies have tried to understand how and when this might happen, but there is still uncertainty. Some scientists think the AMOC might be more likely to collapse than earlier models suggested, especially if warming continues at high levels.

Trends

Until 2024, scientists disagreed about whether a big ocean current in the Atlantic Ocean was slowing down or staying the same. In November 2024, a study tried to solve this problem. The scientists used special computer models to understand the ocean better. The study found that the current has been slowing down since 1950.

Direct measurements of this ocean current have been available since 2004. These measurements help scientists understand how the current is changing. Some earlier smaller measurements suggested parts of the current were weaker, but later data showed recovery. By 2014, there was enough data to show a decline, but scientists debated whether this was due to climate change or natural changes over many years.

Scientists also measure the current by tracking changes in heat movement. In 2017 and 2019, satellite and float data suggested less heat was moving, indicating a fairly stable flow with some changes over many years. The strength of another current near Florida has been stable over the last four decades.

Climate reconstructions help scientists understand the past state of the ocean current. In February 2021, a study combining recent data with older records found no overall decline in the current over the past 30 years. Other studies have found changes in the North Atlantic but no clear long-term decline since the 1990s. Some reconstructions suggest the current has weakened since the late 1930s, with an especially weak period between 1975 and 1995, followed by a limited recovery.

Some researchers have linked recent climate changes to a possible decline in the ocean current. For example, a large area of the North Atlantic near Greenland has cooled, which some think might be connected to the current slowing. However, other factors like changes in cloud cover also play a role. There have also been observations of changes in carbon absorption and other ocean properties that some link to the current’s possible slowdown, though these findings are debated.

Main article: Atlantic meridional overturning circulation

Main articles: Ocean circulation , Thermohaline circulation

Further information: Climate change and the Atlantic Ocean

Projections

Historically, scientists use special computer programs to study how Earth’s climate might change. These programs, called CMIP models, show that a big ocean current in the Atlantic, called the AMOC, stays strong even when the planet gets warmer. For example, when scientists tested what would happen if the amount of a certain gas in the air suddenly doubled, the AMOC got weaker but did not completely stop. Even after many years, it only recovered a little.

More recent studies suggest that the AMOC might weaken more than we thought. Some newer computer programs show that by the year 2100, this current could lose more than half of its strength. However, these programs also have some problems, so scientists are still learning.

To get better results, some scientists adjust their computer programs. In one test, the AMOC completely stopped after 300 years. Other studies show that by the year 2100, the AMOC might weaken by about 18% to 37%, depending on how much the Earth warms. If warming continues, it could weaken even more in the following years.

In 2023, a study suggested the AMOC might collapse around the year 2065, but many scientists think this might not be accurate because the methods used were not very reliable.

New long-duration CMIP6 projections

In 2025, scientists looked far into the future using advanced computer programs. They found that under a scenario where the Earth warms a lot, the deep part of the AMOC could almost stop by the year 2100. This would change the way water moves in the North Atlantic.

Big reviews of many studies help scientists understand what might happen to the AMOC. Around 2001, scientists thought the AMOC would get weaker but not stop. By 2014, they still thought a sudden stop was very unlikely.

In 2021, a major report said the AMOC is very likely to get weaker this century, but it probably won’t collapse before 2100. However, scientists were less sure about this than before.

In 2022, a study looked at many possible climate changes and said the AMOC could collapse if the Earth gets very warm. If this happens, it could change weather patterns and affect ecosystems and human activities.

A report in 2025 suggested that melting ice and warming waters might be slowing the AMOC, which could make Northern Europe much colder. Another study published in February 2025 found that, across many models, the AMOC is strong enough to handle large changes and is unlikely to collapse this century.

Effects of AMOC slowdown

As of 2024, scientists are unsure if the main ocean current in the Atlantic Ocean, called the AMOC, has slowed down. But they agree that if the Earth keeps changing climate, this slowdown is very likely to happen. If the AMOC weakens, it could change how much rain falls in many places, especially in middle parts of the world and in Europe. It might also make storms stronger along the North Atlantic coast.

A weaker AMOC could also make sea levels rise more quickly along the U.S. East Coast. This happens because the ocean water near the coast gets warmer and expands. This effect has been linked to times when the AMOC slowed down temporarily in the past.

Some scientists think a slower AMOC might make Europe a little cooler, about 1 °C (1.8 °F). Other places, like Siberia, might feel different effects. For example, research shows that when the AMOC was weaker in the past, winters in Siberia were milder. Some studies suggest that a slower AMOC could actually reduce the economic costs of climate change a little, because Europe makes up a big part of the world's economy. However, most scientists believe that a slower AMOC would trap more heat in the oceans, making global warming worse overall.

One study from 2021 suggested that a slowdown of the AMOC could connect other important parts of the Earth's climate system, like the ice sheets in Greenland and West Antarctica, and the Amazon rainforest. While the AMOC slowdown alone might not cause these systems to change suddenly, it could lower the temperature thresholds at which these systems might change dramatically. This could lead to a series of big climate changes over many centuries.

Effects of an AMOC shutdown

A complete stop of the AMOC, the main ocean current in the Atlantic, would be very hard to reverse and might take thousands of years. It would cause much cooler temperatures in Europe, especially in places like Britain, Ireland, France, and the Nordic countries. Some research suggests that temperatures in Europe could drop by as much as 10 °C (18 °F) if this happens.

Such a change could also affect farming in Great Britain, making it harder to grow food because of colder temperatures and less rain. In addition, severe weather might become more common, with stronger winter storms and more snow. Scientists are still studying how these changes might influence other parts of the world, like the Amazon rainforest and weather patterns in the Pacific Ocean.