Michael E. Mann has served on the front lines of climate science and, as a consequence, the wars over climate policy, for more than a decade. Probably still best known as one of the creators of the hockey stick graph, which shows an abrupt rise in the planet’s average temperature since the 1900s, he is also a dedicated science communicator who has previously published five books aimed at the general public, including a book for children.

Now a presidential distinguished professor and the director of the Penn Center for Science, Sustainability and the Media at the University of Pennsylvania, Mann has just published his sixth book, “Our Fragile Moment: How Lessons from Earth’s Past Can Help Us Survive the Climate Crisis. Yale Climate Connections discussed the book with Mann over Zoom. 

This interview has been edited for brevity and clarity. Where helpful, further details have been provided in parentheses. 

A graph shows average global temperatures from the past 1,000 years, sharply curving up in the recent past similar to the end of a hockey stick
The hockey stick. (Image credit: IPCC)

Yale Climate Connections: Your new book seems rather different from your previous books. You step further back; you paint a bigger picture of a much longer history. Why this book now? Why offer the public a deep dive into Earth’s climatic history at this critical juncture in the effort to address human-caused climate change? 

Michael E. Mann: You’re right. I wrote “Dire Predictions” (2008, second edition in 2015) as a layperson’s guide to the IPCC reports. “The Hockey Stick and the Climate Wars” (2012) was my own story of the battles I’ve fought because of the graph of global warming I had created. With cartoonist Tom Toles, I’ve also written about climate denial (“The Madhouse Effect,” 2016). In “The New Climate War” (2021), I wrote about the politics of climate policy. I’ve even authored a children’s book (“The Tantrum That Saved the World,” with artist Megan Herbert, 2018). 

So I’ve done all that. What I hadn’t written about was the topic that was my bread and butter as a climate scientist. I cut my teeth on paleoclimate. What are the lessons we can learn from the deep past? 

There are a lot of people who think that the science supports this view, but it doesn’t. 

Michael E. Mann

And that came together with this other thread — the observation that the primary obstacle to action on climate change no longer seems to be denial; it’s the idea that we lack agency, doomism: “It’s too late to prevent catastrophic, runaway warming and the extinction of all life.” 

There are a lot of people who think that the science supports this view, but it doesn’t. 

So it was those two things that came together. I felt it was time for me to get back to the science. And I felt like I was in a position now to take this on in a big-picture way.

YCC: So this is less an explicit call to political action — although it is that, too — than a “Keep calm, carry on”?

Mann: Yeah, it’s using an exploration of Earth’s climate history to motivate what I think is the right framing for the problem that we face today: the pairing of urgency and agency. That it’s not too late. And the paleoclimate record actually reinforces both of those points.

A rainbow book cover that reads Our fragile Moment how lessons from Earth's past can help us survive the climate crisis.

YCC: Let’s dig into the details. One of the points you make in your book over and over again is that many factors, on many different time scales, interact with each other to produce the climate. Can you give us some examples of how these factors interacted in different times to produce very different climates? 

Mann: On the very long time scales, there are two factors that ensured a certain amount of resilience in the climate system — homeostasis — over billions of years: the brightening of the sun and Earth’s carbon cycle. [When] you warm up the planet, there’s more cycling of water, which scrubs more carbon out of the atmosphere and sends it off in rivers to the oceans, where it gets buried. This natural restoring mechanism kept the climate in stasis. 

But there are exceptions. And ‘Snowball Earth’ is my favorite. In that case, it was the development of a new photosynthetic mechanism by a bacteria that produced oxygen. The sudden rise in oxygen scavenged the methane from the atmosphere, dramatically lowering the greenhouse effect and plunging us into the destabilizing effects involved in glaciation and into a snowball state. 

So on the longest time scales, there’s this competition between factors that, most of the time, instill resilience in the climate system. But there are exceptions, and Snowball Earth is a reminder that if you hit the system hard enough it can spin out of control. 

YCC: Asteroids would be another exception. 

Mann: Absolutely! Asteroids are an acute example. You’re hitting the system very hard — literally. That’s what chapter four is all about. And in that case, yet again it was a scenario of cooling. It was a sudden cooling due to the KPG impact that killed off all the non-avian dinosaurs. 

One of the points I drive home there is that the dinosaurs lacked the ability to see the asteroid coming. We don’t. Interestingly, the book was already underway when the movie “Don’t Look Up” came out, which presents that analogy directly. 

An illustration of Earth from outer space showing the planet covered in ice
An artist’s depiction of Snowball Earth. (Image credit: Oleg Kuznetsov / CC BY-NC 4.0)

YCC: You bring in the concept of hysteresis to explain the sometimes puzzling patterns of climate change. What is hysteresis? 

Mann: Hysteresis is this very important phenomenon in complex nonlinear systems. In a linear system, if you push it a little, it moves a little. In a nonlinear system, however, if you push it a little, it moves a little. But sometimes a small push can pass some threshold that leads to a tipping point, a dramatic transition into a new state. And once you’re in that new state, you can’t get back to the old one. 

I use a number of examples in the book. Daisyworld is one, the simple example that James Lovelock gave us as a proof of concept for the Gaia hypothesis. 

A more salient example today is the Greenland ice sheet and sea level rise. The last time Greenland didn’t have an ice sheet was in the Pliocene, 2.5 million years ago. And at the time, CO2 levels were about what they are today. Looking at that you might think, “We’re in trouble; we’re going to lose the Greenland ice sheet. It’s a goner.” 

That would be true if the system behaved linearly. But it matters where you came from. If you’re coming from a warm state and gradually cool, you’re going to get a different result than if you’re coming from a cool state and gradually warm. The Greenland ice sheets won’t form at the same point in those two scenarios. 

What that means for us is that we probably have a little margin of safety. That’s because we already have a Greenland ice sheet, it’s reflecting sunlight. It’s helping keep the climate cool. If you have an ice sheet, the system resists change. But eventually, you reach a point where it’s so warm that the ice sheet collapses. 

So that’s hysteresis. You’ve got a little margin for error. But if you warm the planet past two degrees Celsius, I would say all bets are off for Greenland. One and a half degrees Celsius, maybe we have a decent shot at keeping it. 

YCC: One of the surprising takeaways from your book concerns methane hydrates. But I’d like to ask my question in a way that acknowledges your affection for popular culture: How did you learn to stop worrying and love the methane bomb?

Mann: I love the reference, which connects with the Strangelove ocean I discuss in the book, a climate model Wally Broecker developed and named in a nod to the film, “Dr. Strangelove: Or How I Learned to Stop Worrying and Love the Bomb.”

YouTube video

So methane bombs. One of the premises that’s very prevalent among climate doomers, those who say “It’s too late,” is that methane escaping from the permafrost will warm Earth beyond the level that humans can tolerate and lead to our extinction. 

There are prominent players out there who make this claim, and they point to two episodes in the paleoclimate record as putative evidence.

The first is the end-Permian extinction, which was the greatest in Earth’s history, in which 90% of all organisms went extinct, including 96% of marine organisms. This was due, they say, to a huge burst of methane gas. 

But the evidence does not actually support that. It looks very much like it was driven by the volcanic outgassing of CO2. Methane does not appear to have been a major player. There was no runaway warming during the end-Permian extinction, the climate warmed as much as we would expect it to, given the standard estimate of the climate’s sensitivity to CO2 and the amount of CO2 released. 

The second episode is the PETM — Paleocene Eocene Thermal Maximum — about 10 million years after the dinosaurs perished. At that time, Earth’s temperature increased fairly rapidly on a geological scale, which is to say tens of thousands of years. Today we’re releasing CO2 and are warming the planet on the scale of tens of years. 

Again, the doomers point to the PETM, a period of sudden warming, as an example of a runaway warming scenario driven again by catastrophic releases of methane.

‘The truth is bad enough.’

Michael E. Mann, quoting the late Stephen Schneider.

So I went through the history of the science on this period, and there were some credible scientific papers early on making the argument that methane feedbacks might have played a prominent role. But we have a lot more data now. And it turns out that methane was only a small part of it. It was mostly — get ready for the punchline — the release of CO2. And it wasn’t runaway warming; it was steady warming — as now.

As long as we continue to steadily warm the planet, we will be in trouble. As my great friend and mentor, the late Stephen Schneider, used to say: “The truth is bad enough.”

We don’t need to invoke runaway methane warming to explain the past or to motivate urgent action today. 

YCC: Your book is coming out after a very dramatic summer. It seems that 2023-24 could be another dramatic year like 1998, with other factors like an El Nino and an eruption amplifying the steadily increasing effects of rising CO2 levels. Right now, climate deniers are trying to explain away this year’s dramatic rise as entirely due to Tonga, the “natural” volcano in the South Pacific. 

Mann: Exactly!

YCC: Then when this current spike in temperatures ends, we can predict that …

Mann: They’ll say there’s been a period of cooling!

YCC: Exactly! Would it be effective to preempt them? Are we getting better at dealing with these people? 

Mann: I’m glad you asked about that. Because it’s been really fascinating. You’re seeing two different groups trying to hijack the discussion. The deniers want it to be Tonga. “Natural.” “Volcanoes rule.” 

The geoengineering advocates want it to be the aerosols from ships. “You see, we can cool the climate back down the same way.”

YCC: Just to be clear, the point about the aerosols refers to the lower-sulfur diesel fuel now burned by the big container ships? 

Mann: Yes. There were some changes in policy in 2020, in particular. 

But neither of those hypotheses stand up to the slightest bit of scrutiny. And yet they were getting so much play. 

There’s been this weird nexus between geoengineering and doomism. The doomers say there’s no way we can solve this problem; the planet’s going to warm too much. So we have to turn to some other intervention. And lo and behold, [the geoengineers are ready] to shoot particles into the stratosphere. 

To me it was fascinating. You saw these two constituencies promoting alternative realities. By contrast, the reasonable climate voices — like my friends Gavin Schmidt and Zeke Hausfather and Katharine Hayhoe — were saying, “No, it’s global warming and the transition to El Nino.”

Going from a La Nina to an El Nino gives us a tenth of a degree of warming. That explains the difference between 2022 and 2023. But why are 2022 and 2023 both among the warmest years on record? It’s global warming. 

So, yeah, it’s been a challenge this summer, with all of that noise coming from both sides, to keep the discussion focused on “the truth is bad enough.” 

The warming is right in line with what the models predicted. James Hansen and others did an amazing job with the limited computers and models they had. Nonetheless, they were bang on. Hansen got it right. The warming of the planet is more or less exactly what was predicted, given the continued burning of fossil fuels. 

Some of the impacts are exceeding what was predicted, but we’re not getting runaway warming. 

YCC: Which leads to the take-away message you deliver on the second-to-last page of your book: “There’s no point beyond which we shouldn’t keep trying to limit warming. Every fraction of a degree matters to the level of suffering climate disruption will rain down on us.”

Mann: I’m not the first person to say that. But, yes, it’s the point I stress at the end. Because it’s true.

Michael Svoboda, Ph.D., is the Yale Climate Connections books editor. He is a professor in the University Writing Program at The George Washington University in Washington, D.C., where he has taught since...