‘The riskometer has been going up all the time’: Tim Lenton on tipping points

The following conversation with Tim Lenton, the founding director of the Global Systems Institute at the University of Exeter and lead author of the 2008 paper that formally introduced the idea of tipping points within the Earth’s climate system, is one of several interviews conducted for the March issue of the Bulletin of the Atomic Scientists, which is all about tipping points. You can find the other conversations here, and the rest of the magazine issue here. This interview has been lightly edited and condensed.

Jessica McKenzie: I was wondering if we could start by defining tipping points.

Tim Lenton: I define a tipping point as where you have a situation within some system where amplifying feedback within that system gets so strong that it can overwhelm whatever damping feedback you have in the system, and it creates a self-sustaining or self-propelling change within the system. I would hesitate to do this, but given that you’re the Bulletin of the Atomic Scientists, it would be pretty obvious to take the metaphor of a nuclear explosion as a case where you have runaway feedback. That’s a pretty catastrophic and extreme example, but for a scientist, it’s a classic example of uncontrolled, runaway, amplifying feedback situation. But in general, we’re talking about situations where the amplifying feedback may not be quite so catastrophic or uncontained, but it’s strong enough to support a fundamental change or shift in a system.

McKenzie: My understanding is that you were one of the first people to start to use tipping points in the context of climate science.

Lenton: Yes, with my friend John (Hans Joachim) Schellnhuber, in the early to mid 2000s. Another colleague, Jim Hansen, was also starting to use the language of tipping points. Everybody had probably read Malcolm Gladwell’s book “The Tipping Point,” and you had popular figures using the language quite a lot, so it wasn’t surprising that it found its way into climate discourse.

What John and I did—we didn’t want the language to be used too sloppily, so we went and tried to define what it would mean to have a tipping point in a climate system, and then try to identify the bits of the climate that could be tipped, which we decided to call tipping elements, because we thought of them as elements of the climate system. That was with a bunch of colleagues, also in the UK and the Potsdam Institute in Germany, which John directed for many years. We pulled a workshop together. I think it was either autumn 2004 or 2005, and at the workshop we started an expert elicitation process. We got other expert scientists to help us in our assessment of what the tipping elements were, how they could interact, and things like this. Out of that, I led on writing the paper published in 2008 that first put tipping elements on the map. It took a long time to produce, but it was worth it. It seems to be the one that ultimately set the tone for the field.

McKenzie: Could you tell me how the phrase tipping points maybe works better than what came before, this idea of climate surprises?

Lenton: There was language around abrupt climate change that was used in the scientific circles in the late 90s, 2000s, like climate surprises. If you read the [earlier] Intergovernmental Panel on Climate Change reports, they were using very arcane language. Let me try and get this right: “Large-scale discontinuities in the climate system,” which probably doesn’t mean much to anyone other than a climate specialist.

I’ve always felt that people are readily bamboozled by complexity, or complex systems, but if you have an intuitive way into complexity, it’s okay, because we’re used to living in a world of complexity. The concept of a tipping point can be made intuitive. I often use the example of, when you mess around as a kid, leaning back on your chair and exploring the tipping point where a small nudge can make a big difference to the fate of a system. That’s the tipping point, where you could end up flat on your back, on the floor, or back upright.

I think tipping points has worked partly because people can get the concept, and partly because it’s a faithful description of what can actually happen in systems. They can have this rare time when they’re very, very sensitive to a small nudge, and it’s going to make a big difference. It’s a nice confluence of a metaphor and a truly scientific meaning. And it’s trying to convey this concept of small change making a big difference. Change can be self-propelling. Change can be also very hard to reverse once it’s self-propelling, because you’re fighting against the system’s own momentum. And change, depending on the system, can, from a human perspective, seem quite abrupt. The timescale of change is a quality of a system itself, and some systems are fast systems, and some are slow. But nevertheless, all of that gets kind of nicely wrapped up in one thing that can be readily, easily grasped. And of course, one has to have a little bit of subtlety, like I just did. The speed of change is not always going to be as fast as falling over on your back on the chair.

McKenzie: How has the idea changed over the years?

Lenton: When we put out the first assessment, or synthesis, we were thinking we could identify a couple of systems that could be at risk, at relatively low levels of global warming—meaning one and a half to two degrees above pre-industrial—and some others, maybe another six or seven, that could be at risk at varying levels of more warming. Over time, as we gathered evidence from the real climate, from models, and all the rest of it, I expected some things to go off the map and maybe some other things to come on.

What tended to happen was, the more we learned, more came onto the map of things at risk, than went off the map. And the more we looked at it, assessments tended to, on average, bring the tipping points closer to us. The West Antarctica ice sheet is a classic example, but it’s also true for several others, like the Atlantic overturning circulation, and that happened over the last 15 odd years, as new evidence came in from the collective scientific enterprise trying monitor these systems more carefully, model them better.

If you think of a big riskometer, the riskometer has been going up all the time, the more we learned. Some things maybe went down, but overall, it definitely went up.

McKenzie: When you’re talking about tipping elements, or tipping points within the climate systems, how do you differentiate between the temperature tipping point, versus the moment that self-amplifying feedback kicks in?

Lenton: People always want certainty if they can get it. But in a complex system like the climate, you might have what we’d call an irreducible uncertainty. So even if there is a particular tipping point for a system—firstly, it’s hard to know what that is. Even if we knew what it was, there’s a level of internal variability in the climate. It fluctuates, and it’s got a stochastic or random quality to it. So you’re never going to be able to perfectly predict when you get across the tipping point. It’s deeper than saying you can’t really predict the weather, but this does bear some relation to that.

So, how do we try and get a handle on where the thresholds are? There are several lines of attack. You look at Earth history. Between the last two ice ages, the interglacial period, got in some places warmer than we are now, and that triggered the loss of a large chunk of the Greenland ice sheet, and in other interglacials, the loss of the West Antarctic Ice Sheet. So that gives us a historical anchor—some constraints—on those tipping points.

We also have models capturing what you might call the key reinforcing feedbacks of, let’s say, the loss of the Greenland ice sheet. And you run those models towards equilibrium at different levels of warming, and you see at some point, bam!—you get the tipping point, and Greenland can no longer be sustained. Of course, the subtlety is different models might give different answers to that tipping point. So then people like me try to synthesize that information and see, well, this model says here, this one says here: Here’s the range.

This is also true for our state of the art climate models. There’s quite a lot of tipping points going on in those models, even though they don’t capture everything, but they’ll often differ over what level of warming [results in tipping]. So you try to capture the range of that, and with enough information, it’s like having a distribution of estimates, of where the tipping point might be.

McKenzie: And then of course these tipping elements interact with each other, correct?

Lenton: Spot on Jessica. I just talked as if they were acting independently. But they are also contingent. They’re causally coupled, so the probabilities, or whatever you want to think of them as, are not independent. Meaning, if A has happened, maybe the probability of B has gone up. That’s certainly true for the melting of the Greenland ice sheet, making it more likely that the Atlantic overturning circulation will be tipped. But the converse is if the Atlantic overturning circulation tipped first, and Greenland hadn’t tipped, it would make tipping Greenland harder, or less likely, because it’s really a local tipping point of temperature, if you like, for Greenland, not a global one. It’s a beautiful, complex system, as usual.

The bad news is that the direction of those interactions tends to indicate that tipping one thing makes tipping another more likely, which then leads into a discourse about whether you could have a tipping cascade, which is a bit like a chain reaction, for the nuclear physicist reading, in the worst-case scenario. That would be a really extreme case, where tipping one thing made tipping another inevitable. We don’t think that that’s often the case. Thank whoever.

McKenzie: Is that what’s been called the hot house Earth scenario or runaway global warming?

Lenton: Yes, it is, although even though I was involved in that work, I think it is more realistic to talk about a “wet house Earth” or “water world,” where we would see large-scale breakdowns of ice sheets. Weirdly, or interestingly, if you destabilize the ice sheet on, say, Greenland, sea level rise will be biggest at the other pole, at Antarctica, and raising the sea levels is one way of destabilizing an ice sheet that floats on the ocean, in the case of West Antarctica.

The prospects for some kind of bad cascade, I see more as committing to a water world or redrawing coastlines. But if you tip the Atlantic overturning circulation, we know from Earth history, you tip the monsoons in West Africa and in India off, and that would be a catastrophe. So those are the genuine cases of connected or cascading tipping that are actually much more pertinent than runaway warming.

The real question we should ask is whether we can rule runaway warming out—because it’s a really bad scenario. It’s been known for a long time, in the broadest sense, that you can create a runaway greenhouse effect, and that’s a really interesting problem to do with why Venus is uninhabitable today.

But luckily, we’re a fair way away from triggering that tipping point. That’s basically where the Earth can’t keep itself cool by shedding long wave radiation from the surface to space, because the atmosphere is just too opaque, so full of water vapor, it won’t let the heat out. Then we’re screwed, full stop.

McKenzie: It’s like a wet bulb temperature for Earth…

Lenton: If the whole atmosphere is saturated and heat can’t escape the surface as fast as it’s coming in, then you’re in runaway [warming]. That’s quite hard to do, thankfully; usually you have some places where you have healthy circulation, dry columns of descending air in the atmosphere, and you’d have heat frantically trying to escape through those windows in the atmosphere.

But let’s turn away from that, because that’s a bigger-picture concern for any living things far in the future, hopefully.

McKenzie: Climate tipping points are now widely discussed within science and mass media. What has it been like watching the term take on a life of its own? And have there been places where you’re like, “oh, that’s not quite right, I wish that wasn’t adopted in that way?”

Lenton: There’s always meaning slippage.

If I step back a bit, I was hoping we were wrong 15-20 years ago. We were raising a flag because we were pretty sure this was a genuine risk and it was real, but you would have wanted to be wrong in the sense that we’d overestimated the risk. It would still have been a good service to provide. I feel there was a fundamental problem then and still now, to not see climate change as a risk management problem. We were really trying to shift people’s thinking.

I was told 15, 20 years ago, by many people, especially colleagues in climate science, to basically shut up. That this was gratuitous alarmism, catastrophism, and all the rest of it. But the sad thing is, reality intervened and started to show everybody that the climate is changing and faster and more convulsively than we thought. There’s little pleasure in being right when being right means we’re all facing bigger and more dangerous risks than we thought we were.

Of course, there’s still debate and discussion. Right now, I would observe, there’s a mixed discourse where inevitably there are people saying publicly, “well, this isn’t a very helpful framing of the problem.” And there are other people over-egging the pudding and maybe misusing the terminology. This is just what happens in this space of complex issues. We’re all trying to shift our way of thinking and get our brains around a complex situation. It’s deeply political, whether we like it or not. I, of course, would wish all the discourse to be as clear as it could be and as faithful to science as it can be. But I’m not an idiot. I know the world we live in, so I think the job of scientists like me is just to try to be very clear.

This is about a risk assessment approach. It’s not about a classic scientific approach, about what’s the most likely thing to happen. This is not that. This is a risk assessment. This is asking, what are some of the worst things that could happen, and what can we do to control those risks?

At least it’s on the collective radar now. Hopefully we’re getting some shift at the deepest level in people’s worldviews about the nature of the problem we’re facing. We’re 400 years into the scientific revolution of being told that the world is like a clockwork machine and outputs are proportional to inputs. But we’ve all lived through a couple of decades of experience where we realize the world is not a clockwork machine, and outputs are definitely not proportionate to inputs, and things can go very nonlinear. We’re all in a bit of mental recalibration. The dominant cultural worldview is being repeatedly challenged by reality. That’s my philosophical take on it, I suppose.

McKenzie: I understand that you’ve turned your attention to positive tipping points. Can you tell me a little bit about why you wanted to do that?

Lenton: If you’re one of the world experts on the risks, and you can see how risky the risks look, you do a certain amount of work, and you’re like: “Okay, I know enough about the problem to know how bad it is. I’m not going to gain a lot, and the world’s not going to gain a lot, if I just continue to diagnose the problem. Can I find any credible grounds of hope that there is a way for us, humans, collectively, and our technology, and society, to change fast enough in a direction that would limit these existential risks?” That’s where I got on a scientific journey, trying to see if I could find the evidence, as well as the mechanisms, for self-propelling change towards zero greenhouse gas emissions, and to stop the net destruction of nature. It took a bit of work, it took empirical examples at country scale in major sectors of the economy starting to tip for me to convince myself that this was credible.

I’m just trying to bring what skill set or toolkit I might have to help here as a public good. It’s obvious that we’re not acting fast enough on climate change. There is some action in the right direction. It just needs to go about an order of magnitude faster. And if you need to achieve a radical acceleration of action, you’ve got to look at strong amplifying feedbacks within systems and ask yourself, what can activate them and make them stronger? And that led me on a journey, because I began to see the role that social activists and social movements had played in tipping technology changes, like electric vehicles in Norway, or the global solar panel revolution, or wind power offshore Denmark.

One of my distant relatives was a famous suffragette, so I have this kind of pride in her, and in that history of a very small group of people who completely changed social norms for everybody, and a bunch of connections started to click into place for me. So I ended up writing a book about positive tipping points, which is in production.

I mean, what are scientists? Apart from being motivated by the beauty of nature and wanting to understand it, you’re kind of a professional problem solver. So you see this enormous problem, climate change, which you’re busy diagnosing and telling everybody about, but part of your instinct is to try and solve problems. I think that’s how I remain sane at the same time—not just shut up shop basically and go into denialism or despair.

McKenzie: If this is the one thing someone is reading about tipping points, what would you want them to take away from this conversation?

Lenton: It would be that we’ve all got some agency to be part of triggering positive tipping points that can accelerate us out of the existential trouble that’s otherwise going to be caused by the bad tipping points in the climate and the Earth system.

It’s a different problem than escalating nuclear war or whatever. That’s for sure. But in some ways, possibly in a more empowering way—I mean, we could all protest rightly against nuclear escalation, but here there are actually more ways in which we have agency to change the outcome. No one’s denying it isn’t a big and messy problem, but at the same time, think of all the other benefits. That’s the other thing that never gets stressed. Cleaner air, cleaner water, and better mental health and happier kids. There’s everything to gain.