As the planet warms, is there a serious risk that the Atlantic Current that warms Europe will slow down and stop? Yes, according to the most detailed computer simulation ever performed. The likelihood of this scenario remains highly uncertain.

“We have demonstrated that it is indeed possible with our current setup,” he says. René van Westen At Utrecht University in the Netherlands.

Now, warm water, made more salty by evaporation, flows north from the tropics along the surface of the Atlantic Ocean, keeping Europe much warmer than it would otherwise be. When this water cools, it sinks because it becomes more salty and denser. It then returns to the tropics and flows along the ocean floor into the southern hemisphere.

This is known as the Atlantic Meridional Overturning Circulation (AMOC). Studies of past climate suggest that the dramatic cooling episodes that have occurred around Europe over the past 100,000 years or so have been associated with so-called tipping points, when reverse currents slow down or stop completely, and small changes in may convert one system to another. state.

The cause is thought to be melting ice sheets. The influx of large amounts of fresh water into the North Atlantic reduces salinity, which in turn reduces surface water density and reduces the amount of water that sinks.

However, this has proven difficult to model. Most shutdown simulations require adding unrealistically large amounts of fresh water at once. Some also question whether this is a potential tipping point, since recent simulations using more advanced models have not shown any shutdowns.

Now, van Westen's team has run the most sophisticated simulation to date, which took a total of six months to run on the Dutch state-run supercomputer Sunellius. It was very expensive, he says.

Unlike previous simulations, the team added fresh water gradually rather than all at once. This created a positive feedback that amplified the effect. The decrease in salinity reduced the amount of water sinking, which reduced the amount of brine flowing north, further reducing salinity.

This eventually broke the overturning circulation, causing temperatures to rise in the Southern Hemisphere but plummet in Europe. For example, in this model, London would be 10°C (18°F) cooler on average, and Bergen, Norway would be 15°C (27°F) cooler on average. Other impacts include localized sea level rise in areas such as the East Coast of the United States.

Additionally, some of the changes seen in the model before the collapse are consistent with changes seen in the real Atlantic Ocean in recent decades.

But to cause this collapse, the researchers had to run the model for 2,500 years. And they needed to add huge amounts of fresh water. Although less than previous simulations, it is still about 80 times the amount that is currently flowing into the ocean from the melting Greenland ice sheet. “So it's absurd and not very realistic,” Van Westen said.

Furthermore, this simulation did not include global warming. The team now plans to rerun the simulation with that in mind.

“This is the most cutting-edge model in which such experiments have been performed,” he says. Peter Ditlefsen He is a co-author of a 2023 study predicting that the Atlantic overturning current could break up between 2025 and 2095, based on changes in sea surface temperatures.

The model suggests it will take large amounts of fresh water and centuries to stop the circulation from reversing, but why do we think climate models are underestimating the risk of nonlinear changes like the Atlantic tipping point? There are several, Ditlefsen said.

Climate models need to divide the world into large cubes to make their calculations workable, he says, and this has a smoothing effect. Additionally, the model has been calibrated based on how well it simulates the 20th century climate, although there was a linear relationship between greenhouse gas emissions and the resulting changes. may not be applicable in the future.

“We should expect the model to be less sensitive than the real world,” Ditlevsen says.