The Atlantic Meridional Overturning Circulation (AMOC) is a network of currents in the Atlantic Ocean, both at the surface and at depths of several thousand metres. Warm surface water is carried north, cooled at northern latitudes, sinks and returns south along the bottom.
The Gulf Stream is part of the northward surface flow, but is driven primarily by winds and the Earth's rotation. It would remain even if AMOC weakened significantly.
If the system were to weaken significantly or collapse, it would have major consequences for the climate, not least in Sweden and the other Nordic countries, where, according to climate models, it would become significantly colder, especially in winter.
"If AMOC weakens, the climate will be affected globally, not just in northern Europe. I think all scientists agree on that," says Léon Chafik, oceanographer and climate researcher at Stockholm University.
But the question is whether it is happening, when it will happen and at what rate it will happen. It is very difficult to predict.
How far apart the researchers are is already evident in the basic question: what is actually being measured?
The first thing is - which AMOC are we talking about? There are different ways to define it mathematically. Then there is the geographical question. Some talk about AMOC at higher latitudes, some at lower latitudes. And there is not just one. There are AMOCs, in the plural, because the circulation behaves differently in different places.
What researchers agree on is how much is still uncertain.
Research is spreading
Fabien Roquet, professor at the Department of Marine Sciences at the University of Gothenburg, agrees that the research situation surrounding AMOC is uncertain.
It is difficult to observe the circulation as a whole and we lack long, robust time series. It is also difficult to represent in models. Some show a sharp weakening, others much smaller changes.
There is a group of scientists who strongly emphasize the risk of a collapse. It should be taken seriously, but the overall scientific assessment is that the probability is very low and that if it does happen, it will be several decades in the future. At the same time, there are other major consequences of climate change that are already underway.
Fabien Roquet has studied ocean stratification, that is, how seawater with different temperatures and salinities settles into different layers. A common hypothesis is that increased stratification in the North Atlantic weakens the AMOC circulation, but his results question that picture.
Stratification is increasing rapidly in some areas, especially in the tropical Pacific. However, in parts of the North Atlantic we did not see the same development. There, stratification has changed relatively little, and in some places it has actually decreased.
For short measurement series
Scientists agree that it is important to learn more about the system, and how and whether it is affected by global warming. But reports of a collapse and an impending ice age have taken the focus off how much we actually don't know.
The first continuous measurements began in 2004, from the coast of Florida to north-west Africa. They show how much water is flowing northward there. In 2014, a measurement programme was added further north, across the North Atlantic between Canada and Scotland. The measurement series are still too short to reliably distinguish a long-term weakening from natural fluctuations, explains Léon Chafik.
The basic problem is that we haven't been measuring for a long time. Natural variations can be 30 to 50 years long, or longer. This makes it difficult to say for sure whether what we are seeing is natural variation or the beginning of a long-term weakening.
Since the mid-1990s, instruments have also been installed in the passages between Greenland, Iceland, the Faroe Islands and Scotland. There, warm water flows in towards Norway and the Arctic, and cold water flows back out into the depths. They show that the water has become warmer, but not that there is less water flowing.
Perhaps the system is more resilient than previously thought. But it could also be that what we are seeing right now is part of a much longer-term weakening.
Léon Chafik is one of several researchers who, in a report to the Nordic ministers, has tried to raise the AMOC issue at ministerial and EU level. The idea is to get long-term funding in place to keep the measurements going.
The problem has been that researchers are looking for funding to deploy these instruments. But you have to apply in cycles and there are no guarantees that you will be able to continue the measurements. If there is an interruption in the measurement series, it will cost us in the long run.
Hard blow
The measurements were recently threatened when the Trump administration announced in May that it would dismantle several ocean observatories within the Ocean Observatories Initiative (OOI) and remove instruments and measurement stations. A severe blow, since the United States is responsible for around half of the world's ocean monitoring.
The protests became massive and in June the research agency NSF announced that it was pausing the plans and that instruments that had been taken up would be put back out.
The episode shows how vulnerable the global infrastructure for ocean observations is and how quickly long measurement series can be at risk if funding or political decisions change, explains Albert Norström, associate professor at the Stockholm Resilience Centre.
It's not just individual research projects that are disappearing, it's large parts of the basic infrastructure used to monitor how the ocean changes over time. If measurements were to be discontinued now, future data and the entire possibility of building the long time series needed would be lost.
AMOC brings heat northwards in the Atlantic, from the tropics and subtropics. The heat is released into the atmosphere in the north and reaches Europe with the westerly winds.
A model study published in Science Advances in 2024 gives a climate 5 to 15 degrees colder in parts of Northern Europe in the event of a complete collapse. The figure applies to a single climate model under highly simplified assumptions, and other studies give less cooling. The UN's climate panel assesses that a collapse is unlikely to occur before 2100.
Heat that is currently transported north would instead remain in the Southern Hemisphere, which is becoming warmer.
A collapse would also affect weather patterns globally, including by shifting the tropical rain belt that millions of people depend on to grow their food.
