Is the Gulf Stream at risk of collapse? Scientists deeply disagree

 

Boel Holm/TT

 

Published 12.30

         

        The map shows the so-called large-scale circulation in the Atlantic, Amoc (Atlantic Meridional                  Overturning Circulation), which includes the Gulf Stream. Photo: Anders Humlebo/TT

Is the Gulf Stream about to collapse and create chaos in the climate on Earth? Several high-profile reports have warned that this is about to happen.

But the question is one of the few climate issues where the scientific community is still deeply in disagreement.

Amoc (Atlantic Meridional Overturning Circulation) is a network of currents in the Atlantic, both at the surface and at depths of several thousand meters. Warm surface water is carried north, cooled in northern latitudes, sinks and returns south along the bottom.

The Gulf Stream is part of the northward surface flow, but is driven primarily by the winds and the Earth's rotation. It will remain even if Amoc weakens significantly.

If the current 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 researchers 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 from each other is already noticeable in the basic question: what is it that is actually being measured?

– The first thing is – which Amoc are we talking about? There are different ways of defining 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 the researchers agree on is how much is still uncertain.

The research is spreading

Fabien Roquet, professor at the Department of Marine Sciences at the University of Gothenburg, agrees that the research situation around 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 significantly smaller changes.

– There is a group of researchers 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 in that case it is 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 the stratification of the world's oceans, i.e. how seawater with different temperatures and salinity 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. But in parts of the North Atlantic we did not see the same development. There, the stratification has changed relatively little, and in some places it has actually decreased.

Too short measurement series

Scientists agree that it is important to learn more about the system, and how and if it is affected by global warming. But reports of a collapse and impending ice age have taken the focus away from how much we actually do not know.

The first continuous measurements began in 2004, from the coast of Florida to northwest Africa. They show how much water is flowing north right there. In 2014, a measurement program was added further north, across the North Atlantic between Canada and Scotland. The measurement series are still too short to be able to reliably distinguish a long-term weakening from natural oscillations, explains Léon Chafik.

– The basic problem is that we have not been measuring for a long time. The 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, there have also been instruments in the passages between Greenland, Iceland, the Faroe Islands and Scotland. There, warm water flows towards Norway and the Arctic, and cold water returns to the depths. They show that the water has become warmer, but not that less water is 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 seeking funding to deploy these instruments. But you have to apply in cycles and there are no guarantees that you will be allowed to continue the measurements. If there is an interruption in the measurement series, it will cost us in the long run.

A hard blow

Recently, the measurements were threatened when the Trump administration announced in May that it would dismantle several ocean observatories within the Ocean Observatories Initiative (OOI) and that instruments and measurement stations would be taken up. A hard blow since the US 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 the instruments that had been taken up would be deployed again.

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 Center.

– It is not just individual research projects that are disappearing, it is 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 opportunity to build the long time series needed would be lost.

Facts

What would happen if Amoc collapsed?

Amoc carries 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 5 to 15 degree colder climate 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 estimates that a collapse is unlikely to occur before 2100.

Heat that is currently transported north would instead remain in the southern hemisphere, which will become 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.