In the movie The Day After Tomorrow, the world’s weather systems are thrown into chaos when an important Atlantic Ocean current abruptly shuts down. Tornadoes rip through Los Angeles, sea levels rise and wipe out most of coastal New York, and large parts of the Northern Hemisphere are plunged into a deep freeze so severe that any exposure to the air kills people almost instantly.
While the movie is probably the best-known example of cli-fi — climate fiction — its premise is loosely based on real science. And it is something scientists are taking very seriously.
The Atlantic Meridional Overturning Circulation (AMOC), is one of several ocean conveyor belts that move water around the globe. It carries warm water from the southern Atlantic northward, where it cools and then sinks. This circulation helps give Europe a milder climate than parts of Canada at the same latitude.
But there are fears that, as in the movie, it could collapse. While the film takes creative licence with the effects such a shutdown would bring, scientists say the real-world consequences would still be severe, including sea level rise and colder weather in Europe. They just would not be as bad as the movie portrays.
Scientists are concerned about the AMOC because of climate change. As we continue pumping greenhouse gases into the atmosphere, global temperatures rise — and so do ocean temperatures. That, in turn, affects how the AMOC works.
Warm water in the Atlantic moves from south to north. As that warmer water travels northward, it cools. Water evaporates, leaving behind salt, which makes the surface water denser and causes it to sink. The colder water then flows back south and the cycle continues.
But sea surface temperatures are rising, and some parts of the Atlantic are becoming less salty, meaning the water does not sink as readily.
Add to that the melting of the Greenland ice sheet — which releases freshwater — and it becomes clear that the AMOC is being disrupted.
But scientists do not agree on by how much.
Wei Liu, an associate professor of climate change at the University of California, Riverside, has written several studies on the AMOC.
He said there is evidence it is already slowing down.
“It has been argued whether the signal could represent long-term decline change, or maybe it’s only part of the variability,” he said.
‘This is real’
There have been dozens of studies looking at the consequences of a slowdown of the AMOC, with varying conclusions.
“Climate models are complicated things,” said Kent Moore, a professor of atmospheric physics at the University of Toronto. “The AMOC is a really complicated process that involves the atmosphere, involves the ocean, involves sea ice, and if you get anything wrong then you may not get a good prediction.”
Some studies suggest there will be a shutdown by the end of the century, while others disagree.
Previous studies estimated that a 30 per cent slowdown could happen by the end of the century, with an error margin of 37 per cent.
More recently, however, one study used a different method to calculate how large a slowdown could be and when it might happen.
It concluded that the AMOC could slow by as much as 50 per cent (±8 percent) by 2100.
“The models that they use, they come up with a much more — not to say better result — but essentially a result where the uncertainty is much lower,” Moore said. “It says this is real. There’s a real possibility of this happening.”
Stefan Rahmstorf, co-head of research in the department on Earth system analysis at the Potsdam Institute for Climate Impact Research, believes this study is the most robust to date.
“I’ve been working on this for 35 years, on this AMOC stability problem. And we always thought the consequences would be very severe,” he said. “Even when it was considered less than 10 per cent likelihood, it was a low-likelihood, high-impact risk. Now the low likelihood is gone.”
Global consequences
As the Atlantic Ocean becomes less salty and warmer, the likelihood of a slowdown increases. There is also the added effect of melting Greenland glaciers. But Rahmstorf said that while many people think Greenland melt is the main driver, that is not actually the case.
“That is a bit of a misconception that is out there. I don’t know exactly why. But the current understanding is that, including the Greenland melt, yes, it will make it a bit worse, but not dramatically.”
While there will not be the kind of deep freeze portrayed in The Day After Tomorrow, temperatures would fall.
“Most people then think [it will get] ice-cold in Europe. That is not entirely wrong, but it is kind of only if you have an AMOC shutdown with not very much global warming,” Rahmstorf said.
But the world is warming, with forecasts suggesting we will surpass the 1.5 C threshold set out in the Paris Agreement.
The AMOC (Atlantic Meridional Overturning Circulation), the ocean current that transfers 100 times the energy output of humans around the Earth. When its circulation slows, we’re in for a wild ride. Watch Wild Wild Weather on CBC Gem.
A recent study suggested that winter temperatures in parts of Europe, including London, could fall as low as -20 C. But summers would still be warm.
Moore, from the University of Toronto, said effects would also be felt here in Canada.
“If the AMOC weakens, the predictions are that sea level will rise along the east coast of Canada, by maybe 25 centimetres,” he said. “It’s not a lot, but the problem is that these things add up…. It’s a real concern.”
He is also worried about the geopolitical fallout, as people may seek to move to warmer climates or away from coasts.
Hurricane Milton grew from a tropical depression to a monstrous Category 5 hurricane in record time due to unprecedented ocean heat that climate scientists link to the planet-warming effects of burning fossil fuels.
Rahmstorf said that if there were a complete AMOC shutdown, cold would not be the only problem. Drying in Europe would become a major issue.
Then there is the instability.
“The contrast between the weather that we get will get much larger, which is kind of bad for agriculture. You might get a very warm spring and all the blossoms are out and suddenly you get a cold air outbreak from Scandinavia and they freeze,” Rahmstorf said. “This is a way of destroying your apple harvest. That happens in Germany actually already.”
The tropical rain belt would shift farther south, and sea levels could rise by about a metre.
The good news is that scientists are forecasting a slowdown, not a complete shutdown, in the near future.
So, if the AMOC slows, will global warming stop? No. It just would not get as warm, the scientists said.
What does this all mean for our future?
“I don’t really worry about myself. I’m not going to experience it, but I’m worried about my grandchildren,” Moore said. “I think all we can do is try to educate people to the fact that these are real risks coming down the pipeline.”
