‘Metaphors can grow legs’: David Armstrong McKay on tipping points

The following conversation with David Armstrong McKay, a lecturer and researcher at the University of Sussex and the lead author of a 2022 paper that reassessed the risk of all the tipping elements in the Earth 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: Could you start by defining tipping points?

David Armstrong McKay: The definition that we’ve been using is when change in a system becomes self-sustaining once the system has been pushed beyond a particular threshold, and that triggers a kind of state shift, so the system will completely change state in a way which is often abrupt or irreversible, but not necessarily.

McKenzie: And where did the term come from, what’s its history?

McKay: It’s an interesting combination of ideas that sort of merged together over time. The phrase itself, ‘tipping point,’ was popularized, but not coined, by Malcolm Gladwell in his book. That was in 2000, and for a few years it was used kind of informally. It was starting to be picked up by climate scientists around 2004-2005, but it didn’t enter the scientific literature for a few years. It was only really with the 2008 paper that my colleague Tim Lenton led that it became scientifically formalized. In the years before, it was just people using it as a phrase, not necessarily citing Malcolm Gladwell’s book, but just picking up on the fact that it was becoming quite a common term. There was kind of a tipping point around the use of the phrase ‘tipping point’ with that book. The actual phrase itself goes back further to, I think it was paper in the 1950s to do with racial segregation. The term had a darker history before it was popularized by Malcolm Gladwell.

McKenzie: These ideas that are now associated with climate tipping points, those are not necessarily new; “climate surprises” became tipping points. Could you say more about the history of this idea that the climate system is unstable, and that there are different phases or states?

McKay: Some of that comes from geology. It used to be thought that pretty much everything had been formed in the flood, and then a lot of 19th and 20th century geologists said “no, things happen with gradual layers. Everything happens gradually. It’s this slow, uniform process.”

Sometime in the 1960s, there started to be more fine-grained data and evidence coming from geology and the Earth sciences more broadly, particularly from ice cores out of Greenland and sediment cores from the ocean, that started to show that there were these abrupt shifts in the past, particularly around the last glacial period, the last peak Ice Age. The end of the last glacial period was surprisingly fast. They were expecting, based on this uniformitarian, old-school view, that it should have been a really gradual up and down, of the glacial and interglacial periods. And actually, it turns out to be quite a fast process.

McKenzie: And when you say fast—what is fast?

McKay: Geologically fast. But sometimes actually fast. If you look at some of the Greenland cores, there were some time periods where you could get 10-degrees of warming over a few decades, or something like that, which is actually really fast. But on the global scale, we’re talking more like a few 100 years to 1,000 years, which, to a geologist, is fast.

McKenzie: Ten degrees of warming over a couple decades sounds pretty fast to me! How have the ideas around tipping points changed over time?

McKay: A lot of the shift has to do with how it’s been defined and refining the definition. A lot of the stuff about climate surprises were to do with either abrupt shifts or irreversible shifts, and they’re still kind of somewhat mixed together. Even in the last IPCC report, they have this table which has abrupt shifts, irreversible shifts, and tipping points, all within the same category. Originally, particularly if you’re using the Gladwellian definition, that emphasizes the abruptness. But when you’re looking at something like an ice sheet, that can have a self-sustaining-change type tipping point. But on a human time scale, that’s not abrupt to us. It’s abrupt to the ice sheet. If the Greenland ice sheet collapses within 1,000 years, potentially, that is abrupt for an ice sheet, but it’s not abrupt for us.

Over time a large part of the community has been focusing more on this self-sustaining change dynamic, which often leads to abrupt shifts, but not necessarily abrupt. Not necessarily abrupt from a human perspective, at least. So that’s the definition that we’ve been using. But it’s not universal. There are people who still prefer to use the kind of abrupt focus definition, because it taps into the imagery that most people have when they think of a tipping point. There are differences in approach there, but I think over time, mostly it’s that self-sustaining definition that people have picked up.

McKenzie: Can you talk about why the term is a bit controversial or divisive?

McKay: This goes back to when it was first being suggested and discussed, in the early 2000s. There was this worry that the phrase undersells the uncertainty around these thresholds. It suggests that it could be possible to identify with high precision a particular number, a particular warming level, that if we go beyond that, we know that tipping will definitely happen, when actually, if you look at it, and if you look our 2022 paper, there are these really big ranges or potential thresholds. Even some of the better-defined ones, like the Greenland ice sheet, the range is between 0.8 and three degrees of warming, with collapse becoming more likely as you go up through that range. But we really don’t know exactly when it will happen.

So tipping points are sometimes implied to be far more definite than they are. You can say, well, actually, there’s a lot of uncertainty. But then people ask, if there’s that much uncertainty, is it useful?

I would argue it’s still useful, because it does have that extra sense of irreversible lock-in of some of these changes that are being made. But it does complicate the picture a lot. Sometimes people do portray it as far more definite than they should.

I think the other side of it is that people will look at that threshold and think, the only thing we should be doing is throwing absolutely everything at trying to stop it, which could encourage something like an emergency intervention, geoengineering, that sort of thing. Or if we’re close to going over it, or we go over it, then the flip side is fatalism, people think there’s nothing we can do. All we can do is adapt, and maybe we can’t even adapt, because the changes will be so big, especially if people assume that it’s a global climate tipping point, rather than tipping points in smaller parts of the climate system. Those are the main issues.

McKenzie: Maybe it’s because I’ve been reading so much about it, but it seems so obvious that it’s something within each individual system, and not a single, specific temperature.

McKay: It’s sometimes frustrating. There are not very many papers that suggest there’s one global number at which point there’s runaway global warming, but that’s how it’s often interpreted. And to be fair, there are papers that kind of suggest that, and there are people who have done public science communication who have implied that, and that doesn’t help. That kind of muddies the whole concept to a point where it’s not usable.

McKenzie: What is the connection between tipping points and the runaway greenhouse effect, or the hothouse Earth scenario. Is there any scientific credence to that scenario at all, right now?

McKay: There is a link, but it is a hypothesis. It’s still speculative. And the issue I have with it is that sometimes it is presented as kind of a definite, proven thing, or at least there’s lots of evidence for it, whereas it still remains a hypothesis that is being tested and doesn’t have a huge amount of evidence. The 2018 paper that proposed it suggested that there are various feedbacks and tipping points that could basically, once you got to, they speculated two degrees of warming, that would cause more warming, which would trigger more warming, and so on. And that would cause us to drift towards four degrees of warming over the course of 1,000 years, or something like that. The implication was that there was some kind of emergent global tipping point coming out of some of these smaller scale tipping points and feedbacks and they would kind of add up to a global tipping point.

At the moment, we don’t actually have evidence supporting that. It’s something that needs to be explored and tested. But as it stands, there are other hypotheses for why, for example, the Holocene was so stable. That is part of the argument they make in that paper, saying the Holocene is stable because it was in one of these stable states, and that we can be knocked out of that stable state into a hotter one. There are alternative hypotheses for why the Holocene was stable that don’t need that stable state. And we don’t actually know if there’s a stable state at four degrees of warming that we could be knocked into. The idea needs a lot more evidence and discussion before they’re presented as definite things that should guide global policy.

McKenzie: If this was the only thing someone were to read about tipping points, what would you want their takeaway to be?

McKay: The thing I always want people to take away is to not just to appreciate the urgency, because there is urgency with tipping points even though these things are highly uncertain. They might lock in these big future changes, big shifts, for generations. But we always need to bear in mind that it’s never too late. Even if we do pass one of these tipping points, or several tipping points, that doesn’t mean that we give up, because we can still prevent further tipping points. It doesn’t kick off this runaway warming scenario, as far as we know. Each one that is triggered will lock in some level of damage, and that’s bad. We should avoid that, but we can still prevent further ones, and that’s what I would want people to come away with, so they don’t come away with this sense that there is a tipping point, and if we pass it, then it means all hope is lost. That’s not what we’re trying to say here.

McKenzie: It’s definitely a tricky communications line to walk.

McKay: It’s useful, but it’s very easy to misinterpret. That’s kind of the issue with it. It’s a powerful metaphor, but metaphors can grow legs and run in different directions to what you want.