What is the Atlantic Meridional Overturning Circulation, and why are scientists concerned that it is weakening?
In the movie the day after tomorrowthe world’s weather conditions are thrown into chaos when an important current in the Atlantic Ocean is suddenly shut down. Hurricanes pass through Los Angeles, sea levels rise and erode much of New York’s coastline, and large parts of the Northern Hemisphere are submerged in ice so dense that any exposure to the air kills people almost instantly.
Although the movie is probably the best-known example of cli-fi – climate fiction – its premise is loosely based on real science. And it’s something scientists take seriously.
The Atlantic Meridional Overturning Circulation (AMOC), is one of several ocean belts that move water around the world. It carries warm water from the southern Atlantic to the north, where it cools and sinks. This circulation helps give Europe a cooler climate than parts of Canada in the same area.
But there is a fear that, like in the movie, it might fall. While the film takes creative license with the consequences such a shutdown could bring, scientists say the real-world consequences will still be dire, including rising sea levels and cold weather in Europe. They just couldn’t be as bad as the movie shows.
Scientists are concerned about the AMOC because of climate change. As we continue to pump greenhouse gases into the atmosphere, global temperatures rise – and so do ocean temperatures. That, in turn, affects how the AMOC works.
The warm waters of the Atlantic move from south to north. As that warm water moves north, it cools. The water evaporates, leaving behind salt, which makes the surface water denser and causes it to sink. The cold water then moves back south and the cycle continues.
But ocean temperatures are rising, and parts of the Atlantic are becoming less saline, meaning the water is shallower.
Add to that the melting of the Greenland ice sheet – which is releasing fresh water – and it’s clear that the AMOC is being disrupted.
But scientists disagree about how much.
Wei Liu, aassociate professor of climate change at the University of California, Riverside, has written several studies on the AMOC.
He said there is evidence that it is already slowing down.
“There has been some debate as to whether the signal could represent a long-term decline, or whether it is part of diversification,” he said.
‘This is true’
There have been a number of studies looking at the effects of decreasing AMOC, with different conclusions.
“Climate models are complex things,” said Kent Moore, a professor of atmospheric physics at the University of Toronto. “The AMOC is a really complex process that involves the atmosphere, involves the ocean, involves sea ice, and if you get anything wrong you might not get a good forecast.”
Some studies suggest there will be a shutdown by the end of the century, while others disagree.
Previous studies have estimated that a 30 percent decline could occur by the end of the century, with a margin of error of 37 percent.
Recently, however, some research has used a different method to calculate how big the decline could be and when it might happen.
It concluded that the AMOC could slow by 50 percent (± 8 percent) by 2100.
“The models they use, they come up with a lot more – not to say a better result – but a result where the uncertainty is much lower,” Moore said. “Say this is true. It’s really possible for this to happen.”
Stefan Rahmstorf, head of research at the Earth system analysis department at the Potsdam Institute for Climate Impact Research, believes the study is the most robust to date.
“I have worked on this for 35 years, on this problem of AMOC stability. And we always thought that the consequences would be very severe,” he said. “Even when it was thought that the probability was less than 10 percent, it was a low risk, with a high impact. Now the low probability is gone.”
Global results
As the Atlantic Ocean becomes less saline and warmer, the likelihood of a slowdown increases. There is also an additional effect of the melting of the Greenland ice sheets. But Rahmstorf said that while many people think Greenland melting is the biggest driver, it isn’t.
“That’s a misconception out there. I don’t really know why. But the current understanding is, including Greenland, yes, it will make it worse, but not significantly.”
While there won’t be any kind of deep snow exposed to it the day after tomorrowtemperatures would drop.
“Many people then think [it will get] freezing cold in Europe. That’s totally wrong, but it’s kind of only if you have an AMOC shutdown without global warming,” Rahmstorf said.
But the world is warming, with forecasts suggesting we will exceed the 1.5 C limit set in the Paris Agreement.
AMOC (Atlantic Meridional Overturning Circulation), an ocean current that transmits 100 times the power of human emissions around the world. When its transmission slows down, we are in for an extraordinary journey. Watch Wild Wild Weather on CBC Gem.
Recent research has suggested that winter temperatures in parts of Europe, including London, could drop as low as 20 C. But summers will still be warm.
Moore, from the University of Toronto, said the results will be felt here in Canada.
“If the AMOC weakens, the predictions are that sea levels will rise along the east coast of Canada, perhaps by 25 centimeters,” he said. “It’s not much, but the problem is that these things add up…. It’s really worrying.”
He is also concerned about the geopolitical fallout, as people may want to move to warmer areas or further away from the coast.
Hurricane Milton grew from a tropical depression to a major Category 5 storm in record time due to unprecedented ocean temperatures that climate scientists link to the planet’s warming effects of fossil fuel burning.
Rahmstorf said that if there was a complete shutdown of the AMOC, the cold would not be the only problem. Drought in Europe can be a big problem.
Then there is instability.
“The difference between the weather we get will be very big, which is kind of bad for agriculture. You can get a very warm spring and all the flowers come out and suddenly you get a blast of cold air from Scandinavia and they freeze,” said Rahmstorf. “This is a way to destroy your apple crop. That’s already happening in Germany.”
The tropical rain belt would move south, and sea levels would rise by about a meter.
The good news is that scientists predict a slowdown, not a complete shutdown, in the near future.
So, if the AMOC slows down, will global warming stop? No. It wouldn’t have warmed that much, scientists say.
What does all this mean for our future?
“I’m not really worried about myself. I’m not going to deal with it, but I’m worried about my grandchildren,” Moore said. “I think what we can do is try to educate people that these are real dangers that are coming in the future.”
