For a long time, it was thought that the collapse of the Atlantic Ocean current AMOC would not happen this century. However, new research from, among others, the KNMI shows that in many climate models, the AMOC reaches a tipping point before 2100 and will then collapse. The likelihood of this is particularly high if CO2 emissions continue to rise, but even with significant reductions in emissions in line with the Paris Climate Agreement, the risk remains around 25 percent. The collapse of the AMOC would interrupt the heat supply to the northern ocean, leading to summer droughts and extremely cold winter extremes in Northwestern Europe.

This conveyor belt in the ocean provides a relatively mild climate in Europe and influences weather patterns worldwide.

The AMOC transports warm and salty surface water from the tropics northward. In winter, this water cools significantly, sinks around Greenland (see image 1), and flows back southward in the deep sea. This conveyor belt in the ocean provides a relatively mild climate in Europe and influences weather patterns worldwide.

How Warming Can Bring the AMOC to a Standstill 

Due to global warming, the surface seawater is also getting warmer. At the same time, increased precipitation is making the water in the North Atlantic less salty. This makes the surface water lighter and it sinks less quickly into the depths. This leads to a weakening of the AMOC, resulting in less warm, salty water flowing northward. Once set in motion by global warming, this process reinforces itself: the weaker the AMOC, the less salty water is transported north, the lighter the surface water, the less water sinks, and the weaker the driving force of the AMOC.

IPCC Models 

According to the latest reports from the UN climate panel IPCC, the chance of the AMOC completely collapsing this century is small. This new study examined what happens after 2100, and it shows that in many of the IPCC model simulations, the AMOC significantly decreases in strength this century, before completely collapsing not long after. This entire process can take 50 to 100 years.

It is becoming increasingly clear that the likelihood of this is not as small as previously thought.

For a long time, this tipping point was seen as something with a small chance but significant consequences. This remains true: particularly Northwest Europe will face severe consequences if the AMOC indeed collapses, with dry summers, extremely low winter temperatures, and significantly lower agricultural yields. However, it is becoming increasingly clear that the chance of this is not as small as previously thought.

When the Water No Longer Mixes 

This study examined how the strength of the AMOC changes in the IPCC model simulations up to the year 2300.

In this study, the changes in AMOC strength in the IPCC model simulations up to the year 2300 were examined. In all nine simulations with high emissions that were run for such a long time, the AMOC comes to a standstill. This also occurs in future projections with medium and low emissions.

The collapse of the AMOC after 2100 is preceded in the models by the cessation of vertical mixing of water in the North Atlantic, halfway through this century. This seems to be the tipping point, after which the collapse of the AMOC becomes inevitable as a result of a self-reinforcing process. The heat transport by the AMOC then decreases to less than 20 percent of the current amount, in some models almost to zero, according to the study.

KNMI researcher Sybren Drijfhout states that “recent observations in the areas where the water sinks deeply in the northern Atlantic have already shown a decrease in the past five to ten years. It may be a temporary dip, but it corresponds with the predictions of the models.”

Chance of Collapse 

As a measure of AMOC strength, the researchers used the amount of water moving northward at 26 degrees north latitude. All future projections where this water transport has dropped below 10 Sv (1 Sv = 1 million m3/s) by 2100 show a collapsed AMOC in the following centuries.

Based on this, the researchers conclude that in at least 70 percent of the model simulations with high CO2 emissions (the so-called SSP5-8.5 emissions scenario), the AMOC will collapse. For the moderate emissions scenario (SSP2-4.5), this is 37 percent, and for the low emissions scenario (SSP1-2.6), it is 25 percent.

According to co-author Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research, their study “perhaps underestimates the risk: these standard models generally do not take into account the extra freshwater released by the melting of ice in Greenland, which would likely further disrupt the system.”