Michael Mann on the Lessons of Climate Change Past

Climatologist Michael Mann discusses his new book on Earth’s climate past, with insights into our climate future.

Renowned Penn climatologist Michael Mann’s latest book, “Our Fragile Moment,” explores the history of climate change and the lessons it can provide into the trajectory of climate change today. The book is Mann’s response to the phenomenon of “climate doomism” which, Mann writes, misrepresents the paleoclimate record to promote climate inaction. In the book, Mann seeks to set the paleoclimate record straight and discusses how human agency remains our greatest tool in preventing the worst impacts of climate change.

Michael Mann is a Presidential Distinguished Professor in the University of Pennsylvania’s Department of Earth and Environmental Science, and director of the Penn Center for Science, Sustainability and the Media. He is also a Faculty Fellow at the Kleinman Center for Energy Policy.

Andy Stone: Welcome to the Energy Policy Now podcast from the Kleinman Center for Energy Policy at the University of Pennsylvania. I’m Andy Stone. Today I’ll be hosting a very special episode of the Energy Policy Now podcast, and one that is particularly timely, given our current moment in which we face increasing urgency to act rapidly to reduce emissions from our energy sector and slow the progress of climate change.

My guest is well known to many of you. Michael Mann is a renowned climatologist who, over the past three decades, has been at the forefront of efforts to communicate the science of climate change to those of us outside of the scientific community and to leaders in government and industry. Michael is the author of six books on science and policy, and a children’s book on climate change. His latest work, titled Our Fragile Moment is being released today, September 26th. In it, Mann dives into Earth’s climate past in search of lessons that may help us to understand the climate trajectory that we are on today. In the book, Mann seeks to address the growing perception that worst-case climate outcomes are foreordained. The book counters with an objective look at what we can and cannot infer about our climate future, based on available science, and what can still be accomplished if we act.

Michael Mann is Presidential Distinguished Professor in the University of Pennsylvania’s Department of Earth and Environmental Science and Director of the Penn Center for Science, Sustainability, and the Media. He is also a Faculty Fellow here at the Kleinman Center. Michael, welcome to the podcast.

Michael Mann: Thanks, it’s great to be with you, Andy.

Stone: So your new book, Our Fragile Moment explores the paleoclimate record, or the history of Earth’s climate. It marks a return to science after your last book which explored climate denialism and misinformation campaigns. Can you tell us what spurred you to return to your focus on science, and in particular to write a book about the science of Earth’s climate past?

Mann: Thanks. There are really two things going on. I just felt that I had written a number of books and had never really written a book that was about what the focus of much of my scientific career has been, which is on the lessons that we can learn from Earth’s past, particularly with respect to climate and the science of paleoclimatology, as you mentioned. But there was another factor here, as well, that really sort of spurred my desire to write about this. It relates to my last book. My last book really focused on the tactics that are being used by polluters to keep us addicted to fossil fuels. Now that denial is no longer credible because we can see the profound impacts of climate change playing out in real time, it just isn’t credible to deny that it’s happening, so polluters and those supporting their agenda of business as usual, use of fossil fuels, have turned to other tactics. Ironically, one of them is doomism, this idea that it’s too late to do anything, because if we truly believe we have no agency, no efficacy in acting, then why bother? So ironically, doomism has been weaponized by bad actors. One of the things that I have repeatedly seen in recent years is the paleoclimate record misrepresented, not in service of an agenda of denial, but in service of an agenda of doomism, and in particular the idea that past climate-related extinction events like the “Great Dying,” which I talk about in the book, the End-Permian extinction, where 90% of all animal species went extinct on Earth, that these events were being used to argue that our own extinction is now ensured. It’s too late to do anything about it.

What you see is a misrepresentation of the factors that played out. For example, this idea that there was a runaway warming event, and it was triggered by the release of methane. By the way, that’s happening today because of the melting permafrost, and therefore we are in for the same thing, and there’s nothing we can do about it. And it’s not true. So I really wanted to diffuse those false arguments for doomism by looking at what the paleoclimate record actually does say about those events. But then I realized that what I really wanted to do was to look at all of the collective lessons that paleoclimate provides us about the climate crisis, going all the way back more than 4 billion years in Earth history.

Stone: It’s so interesting. So you’re looking at the paleoclimate record here, but you’re actually looking at it to set the record straight, to really understand what it really tells us or can or cannot tell us about where we’re going in terms of our climate change at this point.

Mann: That’s exactly right. Absolutely.

Stone: The title of the book is very important. You do go back all the way to the beginning of time, Earth time, and you go over billions of years of history. The book’s title is Our Fragile Moment, and the way I read it is that that refers to a specific moment in geologic time, which is specifically the last 12,000 years, known as the Holocene, as you point out in the book. And this period has been relatively mild in climate terms and very supportive of our human development and the development of society.

Now again, the “fragile moment” refers to the fact that climate change threatens to take us away from this nurturing moment. Can you talk about this last 12,000-year period that we’ve been in?

Mann: Yes, absolutely. You’re right, the last 12,000 years is the current, what we call the “interglacial period,” where there isn’t much ice on the face of the Earth, unlike the last Ice Age, which peaked about 20,000 years ago. I would say that probably even a more precise definition of that fragile moment is the time span over which human civilization developed. The first true human civilization, Mesopotamia, developed about 6,000 years ago. Over the last 6,000 years, the temperature of the planet has been relatively stable, and so we have created this massive civilizational infrastructure over this 6,000-year period, during which the global climate has been remarkably stable. That’s the fragile moment because we are, of course, now warming the planet dramatically, and we’re leaving that envelope of stability, and we’re doing it quite rapidly. If we continue on that course, then unfortunately, we will in essence lose that fragile moment. We will lose the resilience that it has provided, and we will suffer the fragility by basically exposing ourselves to a climate that is inhospitable to the infrastructure that we have developed to serve a planetary population of now more than 8 billion people.

Stone: One of the scary prospects here is that we would not come back to this, right? Once we leave it, we don’t come back. Maybe Earth comes back in millions of years, but not in our time. I wonder if you could give us a bit of a level set here. What would our climate look like today, or what would the climate trajectory look like today without anthropogenic climate emissions?

Mann: Yes, Andy, one of the chapters focuses on the Common Era. It’s basically the period of time since the rise of the Roman Empire, so the past 2,000 years, essentially. That sort of gives us a natural baseline because we can look at how climate varied in past centuries before we started warming the planet with fossil fuel burning and carbon pollution. And truly, those last 2,000 years prior to the Industrial Revolution, are really the climate that modern civilization was adapted to.

We can learn all sorts of lessons. We can use that as a baseline to judge how fast sea level is rising, for example, compared to natural levels of sea level rise. We can use it as a baseline to look at potential tipping points. You just alluded to the fact that there are certain irreversible processes that, once we trigger them, there’s no going back. There’s some evidence when we look at the climate of the last 1,000 or 2,000 years that not only is the warming unprecedented over that timeframe, but we see, for example, an anomalous slowdown in the so-called “ocean conveyor.”

If you saw the movie The Day After Tomorrow, the scenario that plays out there in the movie was a caricature of the actual science, but if that conveyor belt circulation that warms parts of the North Atlantic — it actually helps keep the North Atlantic Ocean so productive, biologically speaking. If that ocean circulation pattern collapses, then it will have a number of detrimental impacts on us. It will influence food stocks, fish populations in the North Atlantic, one of the great natural fisheries on the planet. And it would actually cause an even more rapid rise in global sea level, due to oceanography that I won’t get into here.

And so we can actually look at how things are changing, relative to that pre-industrial baseline, and it tells us that, indeed, we are rapidly departing from that baseline that has allowed us and civilization to flourish over the past millennia.

Stone: Well, this North Atlantic Ocean conveyor that you just talked about — I read in the book that that is something that recent science shows the collapse of that conveyor may be a little bit closer than previously had been thought.

Mann: Yes, and that’s one of the themes that emerges in the book. What we see is uncertainty is not a friend. And what I mean by that is uncertainty is sometimes cited by critics who say, “Oh, we’re going to destroy the economy if we move away from fossil fuels. And why should we do this, because there’s so much uncertainty about the science?” The implication is that uncertainty is a reason for inaction. But actually, if we look at the uncertainties and how our knowledge has progressed over the past several decades, as we have alleviated some of that uncertainty, we’re finding that in fact some of the impacts of human-caused warming are proceeding faster than we expected. That is true with ice sheet collapse and sea level rise. It’s true for the conveyor belt, where the climate models predicted that we wouldn’t see that slowing of the conveyor for another century. But we’re already seeing it right now. And probably we’re seeing that right now because Greenland is losing ice earlier than we expected it to, and all that fresh water that’s melting from the surface of the Greenland Ice Sheet is flowing into the North Atlantic, freshening those waters, lightening them — decreasing the density and inhibiting the sinking motion that drives the conveyor.

So all of these things are interconnected. When one system reacts faster, and the change is greater than we predicted, it creates the possibility that other subsystems of the climate are going to do so, as well. The theme here is that uncertainty has not cut in our favor. It has cut against us. As we learn more, as we observe more, we’re seeing that some of the impacts are greater than we expected them to be at this point.

Stone: Let’s look, then, at some of the lessons that specifically climate history might tell us about what’s happening now, where this all may be going. Much of the book focuses on a concept known as “Earth’s equilibrium climate sensitivity,” and I gather that is the degree to which changes in atmospheric carbon dioxide concentrations lead to changes in climate temperature. Tell us about this sensitivity and what the record tells us about how Earth reacts to, again, changing greenhouse gas concentrations in the atmosphere.

Mann: Yes, it’s a concept that as scientists we use as a sort of shorthand for what is our best estimate of the warming effect of increasing carbon dioxide levels? And to be very specific and technical, the equilibrium climate sensitivity asks this question: If you double the concentration of carbon dioxide in the atmosphere, how much warming do you get when the system finally equilibrates to that new CO2 level?

For example, pre-industrial levels of CO2 back in the 1600s or 1700s were about 280 parts per million CO2 in the atmosphere. So a doubling would be about 560 parts per million. We’re at about 420 right now, so we’re on our way to doubling, and if we fail, for example, to substantially reduce carbon emissions, then later this century, we will reach that doubling. And when Earth warms up fully in response to that doubling, the equilibrium climate sensitivity is: How much warming will we get? The best estimate is that that’s probably about 3 degrees, between 5 and 6 degrees Fahrenheit warming of the planet, if you double CO2 concentrations. Obviously that would have huge detrimental impacts because right now, we’re talking about trying to limit warming below 1-1/2 Celsius, 3 Fahrenheit, where we’ll start to see some far worse climate impacts. And here we’re talking about more than 3 degrees Celsius, more than 5 degrees Fahrenheit warming. That tells us how bad it gets, how much warming and all of the impacts that are associated follow. How much warming do you get when you increase CO2 that way?

We can look to the past, because we’ve got a short instrumental record. It only goes back about 150 years. There are all sorts of competing factors. There are increasing greenhouse gasses, but there’s sulfur pollution from smokestacks that has cooled some regions. There are natural factors like volcanic eruptions and small, but measurable changes in the brightness of the sun. All of those things are competing with each other over this short period of time. So from the modern record alone, it’s really hard to pin down that quantity, to really estimate what the warming effect is of a doubling of carbon dioxide concentrations.

So we turn to other past episodes — the last glacial maximum 20,000 years ago; the early Cretaceous period, when CO2 levels were very high, and dinosaurs roamed the planet and made it all the way up into the Polar Regions. So we can use some of these past events, and the data is fuzzier. We have to turn to paleoclimate data that have their own uncertainties, but we can use these other past natural climate changes as independent data points, to try to pin down that quantity of equilibrium climate sensitivity. Collectively, they tell us that we probably have it about right, that the warming effect is probably about 3 degrees Celsius, or 5 to 6 degrees Fahrenheit warming of the planet, if you double CO2 levels. That means we don’t want to double CO2 levels. We want to avoid that scenario if we possibly can, and that’s going to take quite a bit of action. We’re on a trajectory right now where, in the absence of any meaningful progress, we could well warm the planet that much over the next century.

Stone: You say, and this was in the book, that some of the more fundamental scientific uncertainties — and this is a quote from the book — “are unlikely to be definitively resolved on the timeframe that we might like, namely the next few years during which we have to make critical decisions about climate policy.” As you said, we must fall back once again on the evergreen principle that uncertainty is not our friend. This is a reason for more, not less urgent action. And I want to call this out. This seems to be a critical point that you’re making in the book. And it’s 180 degrees counter to what we’ve been told over the last 30 or 40 years.

Mann: Right.

Stone: “We don’t know what’s happening with climate change, so don’t worry about it. Take it easy. Don’t risk the economy,” as it went, “by doing too much.” You’re saying just the opposite. Uncertainty is, in fact, a cause for action. Do you think this message will be heard today?

Mann: Well, I’ve certainly been trying to convey it. Other climate scientists and climate communicators I know have been really emphasizing this point in recent years in the climate discourse. But there is a concerted effort by some bad actors to misrepresent what uncertainty means. So it’s not entirely in good faith. Some of the people making that argument, I think they know better. I think they know that there’s some real uncertainties that could come back to bite us. Even Exxon-Mobil, back in the early 1980s, their own scientists used the term “catastrophic” in an internal report to describe the potential consequences of on-going fossil fuel burning and the warming that would result. That’s not Al Gore. It’s not the U.N. Intergovernmental Panel on Climate Change. That was Exxon-Mobil.

So in an honest moment, even the fossil fuel companies will concede the argument that uncertainty is a reason for inaction really is fallacious. It’s not made in good faith, and indeed, as we’re saying here, the uncertainties are not breaking in our favor. They’re breaking against us. One of the things I would emphasize in that regard is just the onslaught of unprecedented extreme weather events that we’ve seen, certainly this past summer, but other recent summers. Here in Philadelphia and elsewhere around the planet, we are seeing a greater increase in these very persistent, extreme summer weather events than the models predicted. And that’s probably because the models are not resolving some of the atmospheric physics that govern the way the jet stream changes when you warm up the planet. Some of our own research has been focused on that question.

So indeed we fall back on sort of the evergreen precautionary principle, which is in the presence of uncertainty, when the stakes are great, you want to weigh in on the side of precaution. Another way of saying that is there is no Planet B, and if we make this planet unlivable, there’s no other planet to go to. And so it really does underscore the fragility of this moment and the importance of acting before it’s too late to preserve this moment.

Stone: Michael, getting back to the fundamental question here that we discussed at the beginning of our conversation. That is, what do we learn from the climate record about where we’re going in the future? I think the takeaway here is, as you say in the book, “The collective evidence supports neither fatalism nor complacency. We’re not on a worst-case path, and we’re not definitely on a best-case path.” What, in a nutshell, are we looking at at this point?

Mann: We’re not going to avoid dangerous and damaging climate change because it’s here. Look at what happened in Maui. Look at Canada this summer. Look at the fact that Philadelphia had the worst air quality in the world for several days this summer because of the Canadian wildfire smoke that made it down here. Look at the people who lost their lives in flash floods just outside of Philadelphia, in Bucks County. So we’re not going to avoid damage and death. It’s already here, so it comes down to how bad we are willing to let it get. If we can keep warming below around 3 degrees Fahrenheit, the science tells us we can avert some of the worst and more irreversible impacts, climate impacts. And there’s still time to do that. The obstacles are not physical. They’re not technological. They’re entirely political at this point.

If we go beyond that, if we warm the planet 3 degrees Celsius, 5 to 6 degrees Fahrenheit, let alone more than that, then we do start to enter into very dangerous territory, where the paleoclimate record tells us that there really isn’t a precedent for the changes that will be in store for us if we do that, and where questions about the stability of our global civilization — where even that starts to come into question, once we warm the planet more than that amount.

So the bottom line, the message of the book is we are in this very fragile moment. There is still an opportunity, a shrinking window of opportunity, to preserve that fragile moment, but if we go beyond it, then we do start to venture into unprecedented territory and again, the viability of human civilization in that scenario is in question.

Stone: And the other message that I think goes along with that is this message of agency. Unlike prior episodes of climate change, this one is caused by us, which means we also have a very direct possibility to do something about it. That is the agency, and that is really the message of the book that I also took away here. Don’t forget that agency. Don’t dip into fatalism, into doomism and give up on that power. We have to act.

Mann: Yes, absolutely. There’s a chapter, chapter 4 on this so-called K/Pg. We used to call it the KT extinction event. It’s the asteroid that struck the planet and killed off the dinosaurs 66 million years ago. It was a major extinction event, and there are a whole lot of pop culture references in that chapter.

Stone: That recent Leo DiCaprio film, Don’t Look Up, right?

Mann: Well, that, but also in the 1980s, the band The Police on their album Synchronicity — I was in high school at the time. There was a song, “Walking in Your Footsteps.” If you listen carefully to the lyrics, what they were saying was with the rising threat of a global thermonuclear war, and the great scientist Carl Sagan — I talk quite a bit about him in the book — had alerted us to the potential dangers of a nuclear winter. If there was a global thermonuclear war, we would blanket out the sun; we would dramatically cool off the planet. And it turns out, in the 1980s, just as we were beginning to understand that, we also made the discovery back in 1980, Alvarez and Alvarez, that what killed off the dinosaurs was an asteroid strike that created essentially the equivalent of a nuclear winter.

So the song “Walking in Your Footsteps” by The Police, “Hey, mighty brontosaurus, don’t you have a lesson for us?” And it’s really about exactly what you’re asking. The dinosaurs couldn’t see it coming. They had no agency. They couldn’t do anything about it. We don’t have that excuse. We see the metaphorical asteroid, and you’re right, there’s a “don’t look up” sort of metaphor there because that film with Leo DiCaprio was a metaphor for us failing to recognize the coming asteroid or comet of the climate crisis.

And so that is the key lesson. The dinosaurs had no agency. They didn’t have the ability to control their fate. We do. We don’t have that excuse.

Stone: Michael, thank you very much for talking.

Mann: Thank you. It has been a great conversation.

Stone: Today’s guest has been Michael Mann, Director of the Penn Center for Science, Sustainability, and the Media, and author of the new book Our Fragile Moment.


Michael Mann

Presidential Distinguished Professor
Michael E. Mann is the Presidential Distinguished Professor in the Department of Earth and Environmental Science, with a secondary appointment in the Annenberg School for Communication. He is a faculty fellow with the Kleinman Center for Energy Policy.

Andy Stone

Energy Policy Now Host and Producer
Andy Stone is producer and host of Energy Policy Now, the Kleinman Center’s podcast series. He previously worked in business planning with PJM Interconnection and was a senior energy reporter at Forbes Magazine.