In this ClimateGenn episode I speak with researcher, Rene van Weston. We discuss his recent paper looking at how the Atlantic Meridional Overturning Circulation (AMOC) responds in three different carbon emissions scenarios. This work builds on his – and colleagues – previous research that looked at establishing the conditions for the AMOC to collapse.
René et al’s findings shed new light on the risks we are facing from this vital ocean circulation in the Atlantic that connects to the global climate system.
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Transcript:
[Nick Breeze]
René, it’s great to see you again. Thank you very much for taking the time to speak to me. The last time we spoke in 2024, we discussed your research and looked at the near-term risk that the AMOC might abruptly collapse and whether you could detect any sort of early warning signal.
What’s different in this research paper?
[René van Weston]
I think it’s very nice that you also introduced the last paper. And we also had a discussion about, okay, we see this very substantial cooling over Europe, but we also have climate change. And how do they compete?
Will we then see cooling or warming? So this was still a question the public and the media had. And we want to address that, how this looks like under global warming and an AMOC collapse.
So we’re not discussing any about the risk or how likely this is. We are just investigating future scenarios in which we have global warming and an AMOC collapse. Then you can have these very counterintuitive patterns and changes emerging that under global warming, under moderate global warming, there’s about two degrees warming in our climate model, and an AMOC collapse, we can still have substantial cooling over northwestern Europe.
[Nick Breeze]
Okay. And you’ve used very long timescales in this study. Can you talk a little bit about why you’ve done that?
[René van Weston]
So the problem with AMOC tipping from a modelling perspective is that if the AMOC starts to collapse, this takes more than 100 years to reach this new state. So if you take all the climate models that are available, they are simply too short to analyse all the different kind of impacts because the AMOC is still en route to collapse. So therefore you need these very long simulations to get a stable new climate in which the AMOC has stabilised, in which those very strong temperature transient trends are much smaller so that you can look at a new climate.
So that is dominated by its variability and not by its trends. The AMOC has come to a new state, and then it is very easy to look what these new impacts are.
[Nick Breeze]
Okay, so this sort of transition phase is completed and you’re in the new state. And you’ve got… Yes, exactly.
You’ve got three main greenhouse gas scenarios, representative concentration pathways or RCPs. Can you summarise quickly what they are, why you chose these three specifically, and what they broadly told you?
[René van Weston]
Yeah. So the three ones that we do consider is like an AMOC collapse without climate change. So that’s of course not very heuristic, given that we are already at 1.3 degrees warming. Then we have 4.5, and this is a middle-of-the-road scenario. So that is, in our climate model, it is around plus 2 degrees warming. If you take an ensemble of climate models, it’s around 2.7 degrees warming by 2100. And then we have a very high-end scenario that is way beyond the Paris agreements, that is like 4 to 5 degrees warming by 2100. And we chose those because they are somewhat representative for none, somewhere in the middle, and very extreme. But of course, in an ideal world, we would have determined all different kinds of scenarios.
Those were the ones we picked. And we simply looked at how the climate and how the AMOC will evolve under those different kinds of forcing scenarios to see which signal, like cooling or warming, starts to dominate over northwestern Europe.
[Nick Breeze]
I was really interested about the pre-industrial one because you look at it at a glance and think, well, we’re not going there. But it also reminded me that the UK government is investing a lot in geoengineering research. And I interviewed someone a couple of weeks ago who’s talking about a cooling plan for India.
We seem to be moving from a science fiction phase with geoengineering to real research going on. Does your research kind of say in the climate change and AMOC shutdown scenario that you really wouldn’t want to try and go back to a pre-industrial?
[René van Weston]
I mean, it sounds like it could be an absolute disaster. Yes. So this is also sometimes very hard for us now to communicate this with policy makers, because it seems that if you only look at temperature and then only look at Europe, then you see even the AMOC really shuts down.
And the more global warming, the less temperature impacts you have. And this is, of course, a very wrong interpretation and that people need to take this very seriously. In the AMOC tips, there are way more things that are beyond Europe and beyond temperature, like less precipitation, more sea level rise, additional shifts, more winter storms.
So that is very bad news. So people are now saying like, oh, then we should emit more. No, that is a very wrong conclusion.
So this PI under constant pre-industrial forcing conditions, that is only like an ideal test case to see what the pure AMOC effects are. So this is our baseline from which we start. So this is the case where we only have a pure AMOC effects.
And this helps us also to interpret the results on our climate change, because climate change does not only rise temperature, it also changes your atmospheric circulation patterns. It has sea ice effects, which are also in there. So climate change complicates life for us researchers.
And then having like a base reference first, that really helps us to interpret the results in the first place and see whether it gets worse or maybe less worse in that kind of scenario. So that is why we have these different kinds of scenarios. So they are purely for the scientific community and for the public to see like, oh, it can be that bad.
But it is also good to inform it that it can be that bad. And that under climate change, you see that there are some competing effects.
[Nick Breeze]
If we just focus on for a moment, RCP 4.5, which is our current emissions pathway, to what degree do we still see the extended Arctic sea ice? And is it a welcome feedback? Because a lot of people actually lamenting the loss of sea ice.
So does it become a welcome feedback?
[René van Weston]
Yeah. Yeah. So this is also what we are mentioning in our paper is that, first of all, we are only using one climate model, which has too much sea ice in the first place.
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Subscribe FreeSo there are definitely biases, which makes our study very sensitive to the proposed changes is an initial assessment. And we also are communicating this very clearly, we need to have more climate bubbles, doing the same kind of setup to see whether or how valid these results are. But what you see actually, is that the sea ice extends very far to the south in our climate model, because it’s also partly biassed.
So I’m kind of curious to see other climate models to do the same kind of setup and see how far the sea ice goes, whether our model is really exaggerating the sea ice extends, whether it is less or more, that is something that is still out there. And other people need to pick this up other modelling groups.
[Nick Breeze]
Can you just give us an idea when you say really far geographically, what are you sort of indicating?
[René van Weston]
In the case where there was no climate change, it goes as far as South England, which is very spectacular to see also part of the North Sea, the region which is closer to the Netherlands, also part of southwest Scandinavia, and also the British Isles, they are closer to the North Sea. Those are also covered under seasonal very sea ice with climate change, with the multiple climate change scenario. So that is still very far south.
But of course, the question is, is how realistic is this? And what we also show with the high end scenario, if you are adding more and more climate change, then this sea ice edge goes further north. And eventually, it doesn’t reach northwestern Europe at all, if there is too much climate change to really compete with this AMOC induced cooling.
[Nick Breeze]
We’ve already touched on this sort of transition phase and why you’ve used such a long time period. At what point do we start to see tangible effects of the AMOC stopping?
[René van Weston]
So this is also a very fair question. So this study is actually a follow up study already. So we had them preprint released last year, where we first presented these runs.
And then we actually also looked at those transient effects. That paper was unfortunately rejected. And there is now a new version, which is currently under review.
But there we look at some of those transient effects. And there you can see that under 21st century, you can already expect some below average warming if the AMOC starts to dip in those simulations. And if the AMOC weakening is so strong in combination with a given scenario, it can also result in cooling over the 21st century.
So that really depends on which period you’re looking at and which scenario you’re looking at that can already result in some effects over the 21st century. But they are not that substantial compared to the full effects that develop on those very long time scales. And you mainly see those effects when the AMOC is really in a reduced state.
[Nick Breeze]
And there have been some eminent scientists saying the two degrees line will be crossed much earlier. I know there are some other saying, no, it will be more or less what the mean of the model said. But if it is early, that means that we’re going to start seeing the impacts much earlier.
That’s basically what you’re saying.
[René van Weston]
Yeah. So those AMOC impacts that mainly develop when the AMOC really starts to weaken substantially. And it is so in the first, like in the first maybe 10 to 20 years.
So imagine that the AMOC now starts to tip. During this interview, then in the first 10 to 20 years, we don’t see any cooling caused by the AMOC. We will still see this ongoing global warming.
So that is the most dominant signal on the relatively short time scales. But when the AMOC really starts to collapse, it goes, of course, very fast in typical climate time scales. And then we will see those effects start slowly to pop up in our records.
But that is at the moment very hard to distinguish from current climate variability, because the climate variability is so large. Therefore, it is also very hard to detect long term trends. So we need these longer records to really see this.
But if such a scenario would happen, I would expect to see those first kind of indications in about 30 years from now.
[Nick Breeze]
I mean, I know we touched on this a moment ago. But in the worst case scenario, the sort of the top level RCP, it cancels out the cooling. Yeah.
But in that it might look at first glance to be an all right outcome. But the reality is that I mean, you’re in a real different hellscape there.
[René van Weston]
I mean, no, yeah, I completely agree with you, Nick. So we only look at temperatures and then you look like, oh, this could be a good thing for Northwestern Europe. But if you then zoom out and see that the whole world is become completely red, so they have like very high anomalies.
And this, of course, causes more droughts, more heat waves, even more sea level rise and other tipping points that can also tip, for example, the Amazon rainforest, the Greenland ice sheet, the Antarctic ice sheet, especially West Antarctic ice sheet. So like more global warming causes more impacts for society. And this should already motivate to limit our emissions in the first place.
And of course, with more global warming, we have also more risk of those climate tipping points to be triggered.
[Nick Breeze]
Since your last paper, I know it got a lot of media attention. And this issue as well has been getting media attention. I see it more and more in a range of mainstream media outlets.
Given that the emissions are still rising, and there’s a lot of emphasis, a lot of economies are still completely dependent on fossil fuel production and exports and so on. And our economies haven’t decarbonised very fast. We’re on track.
Everyone is sort of almost accepting that we’re on track to hit 2.7 to 3 degrees. I mean, that guarantees the AMOC shuts down and it guarantees that we get these impacts at some point. We don’t know when exactly that’ll be, but it sounds like it might be sooner than later.
But if you were called in now to Brussels or the UK government or wherever, and asked to give a solid briefing, how would you communicate or characterise the risks that you’re finding in all of this?
[René van Weston]
So the paper that was published last week, this doesn’t assess the risk of AMOC tipping in the first place. So then we need to go to a different study, which is currently under review. And then we could actually lay down what the risks are of AMOC tipping on the future climate change.
So that paper is still not published. So therefore it’s a little bit hard to communicate that with at the moment. But what we do know is that there is more risk of AMOC tipping under more global warming.
And we need to limit this as soon as possible. We need to stay far below two degree warming to avoid such a scenario, because adaptation measures against these large shifts we saw in that paper, so that the paper from Vesta and Batze, that only discusses the what-if scenario. And then we see a completely different Europe.
And I’m not an expert on infrastructure or adaptation measures, but I can imagine this is a completely different climate. And it is very hard to take realistic adaptation measures against these kind of large changes. So therefore we’re currently still looking what is the risk, because that is a very important question.
And of course, the question is also will the AMOC tip be on two degree warming, or is this like beyond three degree warming? We need to figure this out. And there are already some studies out there, like for example, the Didlefsen study from 2003, they say like 2057, but there has been some critique on that.
So we need to figure this out to get a really good structure, like this is the risk at the moment, we need to stay below that risk, that I would of course first focus. And if this happens, this is how a future Europe could look like.
[Nick Breeze]
I’m just thinking that I know you’ve got this other paper that you’re publishing, but when you use these RCPs, and we have a broad understanding of what they mean, and you’re here saying, well, you know, our research is telling us that at these different RCPs, we have these different outcomes. And okay, you can verify that with other models and so on. But this is a pretty big signal, because we know it’s like a jigsaw puzzle, once we know what that RCP is, and we know this.
And as you said, at the beginning, it is a kind of trigger that goes way beyond Europe, you talked about the Amazon, you talked about Antarctica, it seems like an incalculable risk. Have you had a policymaker engagement since your last, the last time we spoke back in February 24?
[René van Weston]
So I’ve spoken with a few of them. And I also saw that our study showed up in some policy reports. But there is still a lot of uncertainty around that.
And that is, of course, what scientists want to have crystal clear what the risk is to inform policymakers. But there’s already a very important signal that we are currently already presenting in the scientific world, and also for policymakers that this could be a potential scenario. And there are a lot of indications that we are en route to tipping in the first place.
So this should be already a very clear signal that, hey, we need to limit this. Like, we don’t know for sure what the risks are at the moment of AMOC tipping. But we want to have this risk as low as possible.
And to get this as low as possible, we need to cut our emissions as much as possible. So that is the that is the clear narrative we know. And that doesn’t only hold for the AMOC tipping that only holds also for the other tipping elements and also for more climate chains that also is getting limited by cutting our emissions as low as possible.
[Nick Breeze]
I mean, obviously, it strengthens the case for rapid decarbonisation. But is there still a case that people are making for geoengineering? Because it’s a it’s become a massive topic now.
And there’s a there’s going to be a debate in the UK Parliament later this month. Have you thought anything about that in terms of these risks that we’re facing?
[René van Weston]
So there have been a study from from my colleagues at Utrecht University. And they look, can you save the AMOC with geoengineering as well, so that you cool the climate and then the AMOC recovers again. So that could also be an option that people could explore.
And that really depends, of course, what are the risks? What are the benefits of that? I’m not an expert in that in geoengineering.
But I can imagine that if we know that we cross the AMOC tipping points by some year in the future, and we want, we cannot adapt to that, then geoengineering will become more and more realistic thing, right? So first, we’re moving from from science fiction, to perhaps like a reality. And maybe we don’t want to do this in the first place.
Because also with geoengineering, there are also negative side effects. And maybe those side effects are even worse. So a lot of people are now investigating this further, do we need to use geoengineering in the first place?
Yes or no. And people are curious to find this out.
[Nick Breeze]
Well, look, thank you very much for speaking to me. And I look forward to speaking to you again when your next paper is a fascinating topic. And it’s so visceral in the potential that it can have for society.
Yeah, thank you very much.
[René van Weston]
Thank you so much for the interview, Nick.
