AMOC collapse: Atlantic current shutdown is a real danger, suggests simulation

Is there a serious threat of the Atlantic current slowing down Europe as the planet warms? Yes, according to the most detailed computer simulation ever – but the likelihood of this scenario is still very uncertain.

“We’ve shown with our current setup that it’s really possible,” he says. Renee Van Weston at the University of Utrecht in the Netherlands.

At this time, warm water that is extra salty due to evaporation flows northward from the tropics along the surface of the Atlantic Ocean, keeping Europe much warmer than it would otherwise be. As this water cools, it sinks because its high salinity increases its density. It then flows back along the ocean floor into the tropics and southern hemisphere.

This is known as the Atlantic Meridional Overturning Circulation, or AMOC. Studies of past climate have shown that episodes of dramatic cooling around Europe during the past 100,000 years have been associated with a slowing or complete cessation of the overturning current. The so-called tipping pointwhere small changes can change the system to a different state.

The reason for this is thought to be the melting of the ice sheets. If more fresh water enters the North Atlantic, it lowers the salinity and thus the density of the surface water, meaning less of it sinks.

But this modeling has proven difficult. Most shutdown simulations involve adding unrealistically large amounts of fresh water at once. And in some recent simulations with more advanced models, no shutdown has occurred, leading some to question whether this is a possible tipping point at all.

Now van Westen’s team has carried out the most sophisticated simulation to date, requiring a total of six months on the Netherlands’ national supercomputer, called Sunelis. He says it was too expensive.

Unlike previous simulations, the team added fresh water gradually rather than all at once. This created a positive feedback that magnified the effect: as less water sank due to lower salinity, less saline water flowed northward, further reducing salinity.

This eventually shut down the overturning circulation, causing temperatures to rise in the Southern Hemisphere, but fall in Europe. For example, in the model, London cools by an average of 10°C (18°F) and Bergen in Norway by 15°C (27°F). Other results include Local sea levels rise in places like the US East Coast..

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

However, to produce this catastrophe, the researchers had to run the model for 2,500 years. And they had to add a huge amount of freshwater — less than in previous simulations, but still about 80 times more than what was entering the ocean. As the Greenland ice sheet melts.. “So it’s funny and not very realistic,” Van Westen says.

Moreover, no global warming was included in the simulation. The team now plans to run the simulation again to incorporate this.

“This is the most advanced model where such an experiment has been done,” says. Peter Dutleyson at the University of Copenhagen, Denmark, who co-authored a 2023 study that predicted The Atlantic Reversal Current may collapse between 2025 and 2095. Based on how sea surface temperatures are changing.

While the model suggests that it would take a lot of fresh water and many centuries to stop the overturning circulation, there are several reasons to think that climate models underestimate the risk of non-linear changes like the Atlantic tipping point. There are, says Dutleyson.

He says that climate models have to divide the world into large cubes to make the statistics workable, which has a smoothing effect. Models are also graded on how well they simulate the climate of the 20th century, when there was a linear relationship between greenhouse gas emissions and the resulting changes in the future. Can’t be right.

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