What Happens If Atlantic Ocean Currents Cease To Churn?

17:34 minutes

an illustrated map of the world showing parts of the northern hemisphere and the atlantic ocean. current lines indicating the north atlantic current is highlighted red at the bottom coming upwards along north america towards denmark and the UK before turning orange and yellow.
Topographic map of the Nordic Seas and subpolar basins with schematic circulation of surface currents (solid curves) and deep currents (dashed curves) that form a portion of the Atlantic meridional overturning circulation. Colors of curves indicate approximate temperatures. Credit: R. Curry, Woods Hole Oceanographic Institution/Science/USGCRP/Wikimedia/CC BY 3.0

Early last month, the Intergovernmental Panel on Climate Change released its latest report. It was a grim document, concluding that global warming had already set in motion irreversible levels of sea level rise, along with other changes that are threatening lives and health around the globe.

The report focused in part on climate tipping points, or phenomena that, if they occur, could lead to a long term re-setting of our global climate and cascades of dangerous changes. Included among tipping points like the loss of the Amazon rainforest and melting of the permafrost, was the potential shutdown of the Atlantic meridional overturning circulation—the AMOC, for short.

That circulation, a set of currents that includes the Gulf Stream, ferries cold water from the poles toward the equator, and distributes heat from the equator to northern latitudes. And it’s powered by two things that are both changing as the climate warms: the temperature of ocean water, and the varying concentrations of salt in that water. 

Climate models that use data from thousands of years ago can help us predict what might happen if the AMOC shuts down. Because the currents are a huge source of heat redistribution globally, a shutdown could have a complex array of consequences, from rainfall disruptions in the southern hemisphere, to even greater sea level rise on North America’s east coast. And if it shuts down completely, it may not come back on again in any of our lifetimes.

Unfortunately, researchers have been finding evidence that the circulation is, in fact weakening, including a study published in the journal Nature Climate Change in early August. Ira talks to Levke Caesar, a researcher at Maynooth University’s ICARUS Climate Research Center. While not affiliated with the latest research, her work has helped map the ongoing pattern of weakening in the AMOC

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Segment Guests

Levke Caesar

Levke Caesar is a postdoctoral researcher in the ICARUS Climate Research Centre at Maynooth University in Maynooth, Ireland.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow.


The current depends upon a delicate balance of salt and fresh water. No one is taking into account how much fresh water has been dumped into the ocean because of melting polar ice. I think we’ve hit a critical desalinization point. I think we’re on the verge of a major climate shift.


IRA FLATOW: When Dennis Quaid, back in the 2004 film, The Day After Tomorrow, gave the bad news about melting glaciers stopping the flow of warm ocean currents, like the Gulf Stream, not a lot of people had heard about such a possibility. And while the movie may have overdramatized the effects, a UN report released last month, though less dramatic, was no less cautionary. The IPCC report focused on the current weakening of this crucial ocean circulation, and the potentially irreversible changes resulting from a shutdown of what is formerly known As the Atlantic Meridional Overturning Circulation, AMOC, for short.

Joining me now is Dr. Levke Caesar, a postdoctoral researcher at the Icarus Climate Research Center at Maynooth University in Ireland. Her research has helped identify the current weakening of this crucial circulation, and some of the historic trends that might help us understand what could happen next. Welcome to Science Friday.

DR. LEVKE CAESAR: Thank you. Yeah, it’s great to talk to you.

IRA FLATOW: Please introduce us to this system of currents in the Atlantic Ocean, this so-called conveyor belt for heat.

DR. LEVKE CAESAR: Yes, so, the system is called AMOC, which is short for Atlantic Meridional Overturning Circulation. And basically, it’s a large system of different ocean currents that connect the Southern Ocean with the North Atlantic. So, the simple picture is that we have warm and salty water that is flowing near or just below the surface from the South Atlantic, through the tropics, towards the subpolar North Atlantic. And there, in the North, the water releases heat to the way colder atmosphere. And as cold water is denser than warm water, this leads to sinking off the water masses to deeper ocean layers. And they, in the deeper ocean, this water has formed the southward return flow of the conveyor belt, where water is flowing from the North Atlantic back to the South Atlantic.

This flow of warm surface waters in and of cold, deep water out of the North Atlantic, basically leads to a redistribution of heat. And we’re talking about a lot of heat energy here. Basically, that the maximum northward heat transport by this ocean circulation system sums up to about 1.3 petawatts. Peta, that is basically a one with 15 zeros. And to give you an idea, 1.3 petawatts, that’s basically or approximately the energy produced by a million medium sized nuclear power stations.

IRA FLATOW: So that’s a lot of heat that’s being released. And where does it go, and what effect does it have?

DR. LEVKE CAESAR: Yes, it is. While it’s released over the North Atlantic, and due to the prevailing wind systems with mainly westerly winds, a lot of this heat is actually transported towards the European continent. And is one of the reasons why when we look at average winter temperatures for cities like Stockholm or Dublin, they are about 10 degrees warmer than cities in Canada, like Montreal for example, or Quebec, even though the latter are at the same latitude.

IRA FLATOW: And so, as the globe continues to warm, and especially as the ice in the Arctic continues to melt, the idea here is that we might lose the difference in salinity that drives the circulation. And that’s why researchers are worried about it getting weaker, or even stopping entirely.

DR. LEVKE CAESAR: Yes, one of the main drivers of this circulation system is this sinking of water masses in the subpolar North Atlantic. And for the sinking to happen, the surface waters have to be denser than the waters below. And helping in that is a high salinity value, because more saline water is denser than fresh water. And, for the water being cold, would also help, because colder water is denser than warm water. And, as you said, under global warming, we are seeing so much freshwater input into the subpolar North Atlantic, because of the melting of the Greenland ice sheet, because of the melting of the Arctic sea ice, also because we see enhanced precipitation over the North Atlantic under global warming. And all this fresh water is basically diluting the surface waters. And they are therefore weakening the sinking of water masses that are driving the overturning circulation.

IRA FLATOW: So we’re seeing then, actual evidence of the circulation weakening? Dr. Caesar, how do you assess an entire ocean climate system? Where do you get your data? How do you collect it?

DR. LEVKE CAESAR: I mean, you have this flow of water, but its exact location, its exact width, the depth, they vary over time. So if you really want to get the full picture, you basically need to cover the whole width of the Atlantic. And that’s a few thousand kilometers. And we actually have gone a good way in doing that. And in 2004, such an area spanning the whole width of the Atlantic was installed, approximately at the height of Florida, that is 26 degrees north. And this is now, with all its instruments and monitoring system, is giving us a lot of information about how the AMOC evolved from 2004 onward.

For any information about the overturning circulation before 2004, we have to rely on so-called proxy data. That is climate variables that are directly affected by the overturning circulation, and can therefore be an indicator for its strength by the changes that we see in those climate variables. And for the overturning circulation, there are basically a variety of proxy data. So for example, one thing is, that we look at the surface temperatures in the subpolar North Atlantic, because that is a region where the AMOC transports most of its heat into. And when we see it cooling there, this is an indicator of the overturning circulation slowing down.

And other proxies that we can add– and we actually did this– is for example, that we looked at the grain size found in Ocean sediments. Because when you look at the right place, then these can give you an idea of the strength of the bottom current of the overturning circulation, just by the fact that a faster current can actually carry larger grain sizes with it. And what we see is that for the main part of this time period of the last 1,600 years, the overturning circulation was fairly stable. But it did start to slow down, probably at the end of the 19th century, already by a little bit, but mainly since the mid part of the 20th century. Since then we’ve seen approximately a decline between 10% and 20% of the overturning circulation. And this is likely going to continue in the future, as this is what climate models predict.

IRA FLATOW: That’s very interesting. What about the possibility of the circulation stopping entirely? Is this a real risk, as the IPCC climate change report has assessed in recent years? If the current stops, cold water stays at the poles, warm water stays at the equator, is that a real possibility?

DR. LEVKE CAESAR: It is. Basically, and we have several lines of evidence why we think it is a real possibility. On the one hand side, we know from paleoclimate data, that this has very likely happened already in the past. When we look at temperature records for example, from the Greenland ice cores, we see that there are pronounced spikes in the temperature, where they were way below the previous value. And think this is linked to a shutdown of the overturning circulation in this time period.

We also see this in climate models, that when we drive these models with a lot of warming, with a lot of fresh water input into the North Atlantic, that the overturning circulation can shut down in these models. So, while there is this evidence that this can happen, we are not that close to actually really estimating when this will happen, in terms of how much global warming is still OK for the overturning circulation to keep going, and where is the critical value, or threshold, when the overturning circulation might actually shutdown.

We are, and this is what the IPCC at least states, is that it’s unlikely to happen when we stay below 2 degrees of warming. It is not impossible to happen below 2 degrees of global warming. But the risk definitely increases when we see more warming. We’re really not that close to really giving you the exact number here.

IRA FLATOW: Yeah, I get it. But what are the consequences if this happens? What happens to people in Europe that are depending on this warm water to give them a milder climate?

DR. LEVKE CAESAR: This is really a huge question, and it’s a vast question in the sense it really depends on how much of a slowdown do we see, and also how much global warming did happen until that moment. Because, in some aspects, global warming and the sort of overturning circulation work in different directions. Because if the overturning circulation shuts down, then we would expect a cooling in the North Atlantic region. And if the two effects happen at about the same time, then they superimpose. And it really kind of depends on how strong each factor is.

But what we do know is that even just a weaker overturning circulation has strong impacts on both natural and human systems. So there are studies showing that a weaker circulation will lead to a decrease in marine productivity in the North Atlantic, basically because the ecosystem is just accustomed to the overturning circulation being there and running. So that will definitely affect all life in the North Atlantic. There are studies showing that because of the changes in the sea surface temperature patterns that will happen when the overturning circulation continues to slow down, that this will lead to more severe winter storms in Europe, especially the Northwestern part.

We also see effects for the United States. Mainly an increase in the regional sea level around the Atlantic, so that is the east part of the United States. That’s actually a simple physical mechanism to understand. Normally, the northward surface flow of the overturning circulation leads to a deflection of water masses to the right, away from the US east coast. That’s basically just due to the Earth’s rotation, that is the Coriolis force, which moves objects, such as currents that are flowing northward in the northern hemisphere to the right. And as this current slows down, this effect will weaken, and then more water can pile up at the US east coast, which leads to an enhanced sea level rise. And we do believe that we already see a little bit of that right now, due to the already weaker overturning circulation. But that’s just in the range of a few centimeters at the moment.

IRA FLATOW: You know, there was that movie from 2004, The Day After Tomorrow, that dramatizes this phenomenon. In the Hollywood version, the current stops so fast that you have a wall of winter descending in a matter of days. That’s not a correct image that you’re predicting, is it?

DR. LEVKE CAESAR: No, luckily not. Well, for a movie, for a Hollywood blockbuster to work, everything has to happen fast. But, when we as the scientists, talk about an abrupt climate change, that will still take a few decades before the overturning circulation has weakened so much that we would basically say it’s now in a collapsed, or a shut down state. I said, OK luckily, and this maybe does not seem that fast. But of course, just a few decades, or if such a huge change in the climate system happens within a few decades, that actually is still fast for even us humans, but of course also for other ecosystems. Because the question is, will we be able to adapt within the few decades. And I’m a little bit doubtful of that actually.

IRA FLATOW: This is Science Friday from WNYC Studios. Talking with Dr. Levke Caesar about the weakening of the circulating current in the Atlantic Ocean. I would imagine once you reach that tipping point, then you flip. There’s no flipping back any time real soon.

DR. LEVKE CAESAR: That’s true. Again, it depends on the timescales we’re looking at. It is possible. We also see that some paleoclimate data, for the overturning circulation to recover. But that will take much longer than it takes for to shut down. So we’re then talking about hundreds or even thousands of years. And I guess from our human perspective, we can say it’s kind of irreversible at that moment.

There are, I guess, ideas of people thinking, maybe we can just pour a lot of salt into the North Atlantic. Or even kind of try to drag the water masses ourselves, but you have to keep in mind, the system is huge. The overturning circulation transports about 20 million cubic meters of water per second. That’s about 100 times the size of the Amazon River, which is the world’s largest river. I’d say it’s numbers of water masses that you can’t really picture in your head. And it will be extremely difficult for us humans to really interfere with the system on a local scale, and trying to keep it running. So what has to be done to keep it running, that is something that the climate system basically has to provide for. The density differences that drive the system have to remain in place for it to keep going. So, general climate protection basically, trying to stop global warming would be our best chance.

IRA FLATOW: Yeah. You said we can’t know yet how soon a shutdown would be. We have gaps in our knowledge. What are the gaps in the data? What would you like to know? What more research and where should that be focused?

DR. LEVKE CAESAR: So basically, we always start off with what we do know, and we do know that it can potentially collapse. And we do know what the driver is, which is this fresh water input into the North Atlantic, or at least, simplified, we can say that this is the case. And now what we’re missing, is the link of how much fresh water will lead to a collapse of, or shutdown of the overturning circulation. We can look at climate models, but those really give you just a certainty of range. So, a vast range of numbers of how much fresh water could disrupt the system. And we have to specify that a little bit better.

And then, even more importantly, we have to know what is the link between how much global warming will lead to how much fresh water input. And I’d say we probably have to look at climate models there a little bit more. Just because it’s probably difficult to just look at palaeodata to try to estimate this range, because the states that the climate was in when the overturning circulation likely collapsed before was so different to what it is now. So it’s not perfectly comparable.

And while we do that, I think we should also take in mind that even though we, right now think that, at least for the next few decades, it’s unlikely for the overturning circulation to shut down, there is still a low risk. And while it’s just a low risk, it is a low risk, high impact scenario, in the sense that if it really does shut down, that would be a lot of disruptions all over the globe, especially in the North Atlantic region. So, while we try to estimate this better, we should probably try to just prevent it from happening anyway.

IRA FLATOW: Does this keep you awake at night?

DR. LEVKE CAESAR: If I think about it, yeah. But honestly, if I would think about it too much, then I would probably hardly ever sleep at all. I know that there is a huge problem, and I know we have to do something to stop it. But I also think that I’m trying to do something, in the sense that I am a climate researcher. But I can’t think about it every time I’m awake because then that would drive me crazy. So basically, I just hope that humanity, as a whole, gets themselves together and tries to stop global warming. But, it actually does worry me a lot. So sometimes I just try to push it from my mind.

IRA FLATOW: Yeah. I think we all do at times. I want to thank you very much, Dr. Caesar, for taking time to be with us today.

DR. LEVKE CAESAR: Yes, you’re welcome. That was a really interesting talk. Thank you.

IRA FLATOW: You’re welcome. Dr. Levke Caesar is a postdoctoral researcher at the Icarus Climate Research Center at Maynooth University in Ireland.

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