11 August 2021

Ireland is located at a latitude equivalent to parts of Canada and Siberia. These places can see snow all winter and temperatures dip into the negatives. So why is Ireland so much warmer?

A big part of it has to do with an ocean current. Last week, a study warned that this ocean current – the AMOC – is seeing signs that could lead to its collapse.

But what could happen if it does collapse? Why is this happening? And what exactly is the AMOC?

We called a couple of scientists to dive into this oceanography.

Let’s start with the basics – what is AMOC? 

Atlantic Meridional Overturning Circulation (AMOC) – I know, it’s a mouthful – is a large system of ocean currents.

Warm, shallow water moves north from the tropics, and then is returned by a system of colder, deeper currents. Think of it like a conveyer belt that transfers heat across the Atlantic Ocean.

Maynooth University ICARUS Oceanographer Dr Gerard McCarthy describes it as “a river of warm water coming across the Atlantic, kind of cuddling up next to Europe.”

To dig a bit deeper into the science of this, the density of the water affects the depths to which it travels. Dense water will sink if it’s heavier than the water below, while less dense water will rise.

So water from the tropics will sink if its saltier than the rest of the ocean. Currently, the AMOC sees this water travelling near the surface, though this could change. (Keep this in mind when we talk about salinity later on).

Sometimes ‘AMOC’ is used as a synonym for the Gulf Stream. But that’s not exactly right. Maynooth University ICARUS Postdoctoral Researcher Dr Levke Caesar said “it’s part of the AMOC, but it’s more than part of the AMOC.”

The Gulf Stream has a horizontal circulation, a wind drift, as well. It flows beginning from the North part of Florida, along the east coast of the United States, then into the Mid-Atlantic, and finally into the North Atlantic current. Again, bringing heat across the ocean from the south, near the Caribbean and transferring the heat to Europe.

This system is related to the AMOC, but the Gulf Stream is more than just ocean currents. But understanding the concept of the Gulf Stream helps in understanding what AMOC does. Basically, if you can grasp the concept of the Gulf Stream, you already have a basic understanding of the AMOC. 

Okay, so this current transfers heat across the Atlantic. What is happening with it now?

The AMOC is at the weakest it’s been in the last millennium.

Both Dr Caesar and Dr McCarthy determined the stability of AMOC for the past 1,600 years. They analysed everything from sea sediment to coral reefs to figure this out. After a long and relatively stable period, there was an initial weakening, then a more rapid one, and now we are seeing the AMOC in its weakest recorded state.

Even after this finding, the models widely predict more of a slowdown. But there isn’t a point that it is determined to collapse yet. Even the sixth IPCC report reported with “medium confidence” that the AMOC would not collapse before the end of the century.

Rather, it’s more likely that we will see a slow-down. The study released last week looked at this and at how close the AMOC is to a tipping point. It warned that there are early signals to its collapse.

“Once we cross this tipping point, it will be probably close to impossible to stop or reverse it and then it’s really hard to get it back,” Dr Caesar told The Green News.

"We conclude that such [still increasing] greenhouse gas scenarios will cause complete shutdown of the AMOC and SMOC (Southern Ocean overturning circulation), with the latter spurring sea level rise of several meters." https://t.co/7m5I8jFju2

— Barry McMullin (@autofac) July 30, 2021

Do we know why the AMOC might be at its tipping point?

It’s not entirely clear. Ocean systems are complicated.

Part of it has to do with natural variability. But another part is because – you guessed it – of human-caused climate change.

For example, melting ice caps and sea level rise is related to the currents’ weakening. It has to do with the salinity, or the saltiness, of the current. As more freshwater from glaciers melts into the sea, it changes the density of the water, and then depth of the current.

Remember that deeper dive into the science that we talked about earlier on? If the salinity of the ocean changes, it could change the way the warm water travels across the ocean, changing the entire heat transfer of the current.

But Dr McCarthy says this hasn’t started happening yet, as he told The Green News, “We don’t know when it will start happening, So this is kind of a technology crunch point.”

Models can only go so far to predict what will happen in reality.

But why is the AMOC important? And what could happen if we pass the tipping point?

With less warm water transferred across the Atlantic, Ireland and Europe would experience some cooling. But the temperature could be counteracted with global warming. It is isn’t very clear what the net effect would be.

“The whole of the world is warming up. So this is just a relative cooling around it,” Dr McCarthy said.

What is more unpredictable are the potential precipitation changes. In Ireland, we are likely to see drier summers. While it’s unclear what the effects could be in the winter there could perhaps be more intense storms.

Dr McCarthy warned of these potential shifts in precipitation, noting that “rising populations, creaking water infrastructure, drier summers coming out in the future is a serious threat.”

Around the world, there are other “dramatic dangers” of a shifting AMOC. The rainfall and monsoons around the world would likely be affected, especially in areas around the equator. This could lead to catastrophic consequences.

Along with presenting research and warnings, the scientists we talked to urged action.

“Acknowledging that when there’s a danger that it could happen, this might be enough to actually start trying to avoid it,” Dr Caesar urged. “Information is power.”

“It’s about taking the science seriously, and seriously dealing with adaptation and mitigation,” said Dr McCarthy.

“Climate change effects can be avoided. You know, it’s not too late.”

By Sam Starkey