Why scientists are concerned about the AMOC, and how it could impact the East Coast

Do you think back the apocalyptical motion-picture show, The daytime After Tomorrow?

The 2004 shoot depicted catastrophic climate impacts and the planet being thrown into another ice age. While the film was ultimately Hollywood fiction, the root cause of that climate shift was based on science.

Specifically it was triggered by the shutdown of the Atlantic Meridional Overturning Circulation, or AMOC for short.

And now scientists are increasingly concerned about the AMOC slowdown, its potential tipping point and the global consequences.

The ocean currents that could change the Atlantic coast

So what is the AMOC?

In short, it’s a part of a massive system of ocean currents which are acting as a global conveyor belt, carrying warm surface water into the North Atlantic, where it sinks into the deep ocean and then returns southward. Eventually that water overturns back to the surface, completing the cycle.

The massive amount of heat, water and carbon it transports around the planet, makes the AMOC one of the world’s most important ocean systems, regulating global weather and sustaining marine ecosystems.

While an AMOC shutdown wouldn’t lead to the apocalyptic deep freeze depicted in the Day After Tomorrow, the global impacts would be catastrophic and far reaching.

And now there’s mounting evidence that thanks to climate change, the AMOC is already slowing and the tipping point towards a full shutdown is approaching. 

A recent University of Miami study looked at data from 4 locations in the Atlantic, including the Scotian Shelf off the coast of Nova Scotia.

What researchers found was that the AMOC has already slowed by 10 to 20 percent over the past two decades.

Professor Stefan Rahmstorf from Potsdam Institute for Climate Impact Research in Germany, has been researching the AMOC for 35 years. He says the AMOC slowing has been long predicted by climate models.

“Now with global warming, we are warming up the surface ocean. So the surface ocean becomes less dense than the deep waters that have already been there from previous decades of colder climate. And that means you can't get that deep mixing anymore and you can't get the renewal of deep water.”

The study also estimates that the AMOC system could slow by 51% by the year 2100, under a mid-range greenhouse gas emissions scenario, with a margin of error of plus or minus eight percentage points. 

While scientists have long predicted a weakening of the AMOC, climate models have offered different predictions on the timing, leading to uncertainty.

Rahmstorf says this latest study was able to significantly narrow that uncertainty by comparing the climate models against real-time observational data.

“What they found is that unfortunately the more pessimistic models with the greater weakening of the circulation by 2100 are the more realistic ones that match the observational data. And they end up at a strength that is only half as much as what we presently have. And that, according to another study that I co-authored last year, is probably past the tipping point where it's kind of past the point of no return.”

“Once you're past the tipping point, you can't stop it anymore because it continues by self amplifying feedback, even if we stopped emissions already. After that tipping point, we practically can't do anything against the full shutdown. Even if this only slowly plays out, it reaches the full shutdown early in the next century.”

A further slowdown and eventual shutdown would bring abrupt impacts to global agriculture, food production, climate and weather patterns, local ecosystems and a further rising of sea levels.

Models have predicted sea-levels will continue to rise along the North Atlantic shores, anywhere from 1 foot to 1 metre when factoring in an AMOC slowdown and shutdown.

Even on the low end of the scale, that would have massive impacts to our coastline, according to Douglas Wallace, who is a professor of Oceanography at Dalhousie University.

“The combination of spring tides, king tides with storm surges, we know how much damage they can do in the province, just from the last 30 to 40 years of incidents like that. I think with this AMOC weakening, the risk is that it will become even more dramatic because you've got the factor that, the weakening of the AMOC will basically… less water will be going to Europe, more of it will be staying on our shores, so that causes sea level rise.”

In addition to rising sea levels, Wallace says a further AMOC slowdown would have other huge implications for Atlantic Canada, as the Labrador current weakens and the Scotian Shelf becomes warmer. 

“Certainly more intense winter storms, especially the Nor’easters there. I think we can suspect that they would become more intense for sure. And then I think there's other other factors. It does look like AMOC slowdown is associated with a kind of a movement of the Gulf Stream. So that seems to imply that waters off Nova Scotia will be warmer, which in a way sounds good, but it carries implications for storms, of course, but also for fish, because warmer water often means lower oxygen, and that's not great for organisms that live in the sea. So I think there's a variety of impacts, which are all potentially really substantial for us here in Atlantic Canada, certainly Nova Scotia.”

Both Wallace and Rahmstorf say more data collection and research on the AMOC is needed so we can be better informed and prepared for what's coming. 

However in the end, Rahmstorf says we could bypass that need and just cut to the heart of the issue.

“And for me, the main response to this is actually reducing that risk. You know, I don't want more research money. I want this real danger for my children and the generations after to be minimized. And the way to do that is just really stick to the Paris climate agreement and get out of fossil fuels as fast as possible.”

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Meteorologist

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