Scientist Michael Mann on ‘Low-Probability But Catastrophic’ Climate Scenarios

This week, to accompany our cover story on worst-case climate scenarios, we’re publishing a series of extended interviews with climatologists on the subject — most of them from the “godfather generation” of scientists who first raised the alarm about global warming several decades ago.

Michael Mann is a climatologist at Pennsylvania State University, known primarily for his 1999 “hockey stick graph” of global mean temperatures (which shoots up quite dramatically in the 20th century). He has also been steadily involved in the United Nations’ periodic IPCC state-of-the-planet reports, and with Lee Kump has even adapted those reports into a popular, accessible book summarizing the findings. The book, which was updated to reflect the most recent reports, is called Dire Predictions.

Shortly after this week’s cover story was published, Mann took to Facebook to voice some criticism of it — primarily about its framing, which he described as counterproductively “doomist.” Personally, I don’t think we’re doomed, just facing down a very big challenge. But I own up to the alarmism in the story, which I describe as an effort to survey the worst-case-scenario climate landscape. We have suffered from a terrible failure of imagination when it comes to climate change, I argue, and that is in part because most of us do not understand the real risks and horrors that warming can bring, especially with unabated carbon emissions. For the sake of clarity: I do not believe that the planet will become uninhabitable in 2100. As I write in the story, our complacency will surely be shaken before we get there. But I do believe that it is important to contemplate the possibility that parts of the tropics and equator will become cripplingly hot, for instance, or that our agriculture will suffer huge losses, so that we may be motivated to take action before we get to those eventualities. And I do believe that, absent a significant change in human behavior across the globe, they are plausible eventualities.

Mann also took issue with a few particular points of science. He stressed that the danger of the carbon frozen in the arctic permafrost was not a “game-changing arctic methane time bomb” and, separately, he suggested that the recent upward revision to a particular satellite data set on warming was less significant than I made it out to be. My purpose in raising the permafrost issue was to illustrate how uncertain much of our current modeling can be, not to suggest a sudden methane release would be the major cause of devastating warming: I based none of the warming scenarios described in the piece on a dramatic methane release effect but rather on the high end of the IPCC’s business-as-usual estimate, which gave a roughly 5 percent chance of our hitting eight degrees of warming by 2100. Regarding the data set, I grant that the upward revision may have been less meaningful to the scientists close to the data, who understood it as a revision toward expectations, than it was to journalists covering the development from afar, who focused on the fact of the revision itself.

I have an enormous respect for Mann, and for his perspective on climate change — he has been an invaluable force both as a scientist and as an advocate. That is one reason why I called him up, during my research, to talk to him about what he thought about the low-probability, high-horror possibilities of climate change. Given his criticisms of my story, we’ve decided to run this transcript unedited.

Maybe the way to start is for me to just tell you about what I’m trying to do and what I’m after. My basic perspective as a kind of close but amateur follower of all this stuff is that the business-as-usual forecasts even in the IPCC get us to some pretty scary places. And then there are some reasons to think that those projections may be a little bit conservative, even over the single-century timescale.

It seems to me that there hasn’t been much out there about worst-case scenarios. Maybe that’s in part because scientists have been so anxious that the world — or at least the American public — not impugn their work as speculative or dangerous. And so scientists have felt a need to be a bit restrained in talking about what is possible. But to me it seems like it neglects a lot of really terrifying possibilities and that those possibilities are important to consider because they spur action.

So I’m just hoping to do a piece that walks through what would happen in a world that is four, five, even six degrees warmer: What kinds of threats and challenges we’d face; how likely they are; and what are the things that are quite likely that we should be worrying about quite a lot; and the things that are less likely but so scary that we should be watching out for their arriving and being sure to forestall them when we can. So I’m sort of doing a listening tour, trying to talk to as many people as I can about these things to see what they think in that context. What are things that they think, what are the threats that they think have been underemphasized, what do they think the world would look like if it was four, five, six degrees warmer, and how to talk about that world in a way that’s both sensible and, you know, sort of fair to the risks.
Yeah, I mean, there has been quite a bit made about the so-called scientific reticence — the tendency for scientists in general to actually be conservative in what they state and the conclusions that they state, particularly when it involves the public sphere. Naomi Oreskes, I don’t know if she’s on your list of folks to talk to.

So she’s written on this, specifically with regard to climate change and how there has been a tendency for scientists to often understate, sort of, the potential risks and the timeframe on which they may unfold because of this sort of combination of innate conservatism among scientists and also sort of this assault on science by climate-change deniers.

I think the intent of that assault has been to sort of cow scientists into retreating from the public discourse and frankly intimidating scientists into being very guarded and very conservative about their public statements. I think that’s been the intent, or one of the intents, of the fossil-fuel-industry-funded attack on climate science. This is something I’ve written about in a book about my own experiences, The Hockey Stick and the Climate Wars, and more recently this book with Tom Toles, The Madhouse Effect. Which is sort of more up to date. And we talk quite a bit about that. So I think your observation is correct. There has been a tendency to understate risk.

And the other point that you make is a very important one, which is that when it comes to societal decision-making, it’s critical to not just consider the most likely impacts, but those sort of low-probability but catastrophic sort of cost scenarios. What we call the so-called tails of the probability distribution. The things that may not be, we can’t conclude that they’re likely, but we can’t rule them out, and if they were to happen, they would have such a catastrophic impact that it makes sense to take them into account. And sort of do any cost-benefit analysis of the importance of acting to avert ongoing warming and climate change. So one scenario, and something I’ve done a fair amount of work on: you have a combination of sea-level rise and potentially stronger tropical storms and hurricanes. The flood risk for New York City may be such that we literally have to abandon New York City and many of our largest coastal cities and many of our largest naval bases like the one in Norfolk, Virginia. All would be very vulnerable to even modest amounts of additional sea-level rise. If you look at the last IPCC report, which is sort of the state of scientific consensus, the report concluded that the sort of upper end of the range of likely sea-level rise was somewhere in the neighborhood of a meter. Over three feet of sea-level rise by the end of the century. But the science that has been done since then, just over the last couple of years, now suggests that we probably have to double that. The most likely sea-level rise by the end of this century is probably somewhere in the neighborhood of six feet, two meters, and that has to do with science that’s been done since the last IPCC report, work by climate scientists to put into the models some of the processes that were not really resolved by the models previously, and that’s a recurring theme.

Uncertainty, of course, can cut both ways, but in many respects, the progress in the science over the last decade or so has been such that we tend to see more risk. We tend to see the probability distribution shifting in the direction of greater impact, larger magnitude impacts. And that’s true with sea-level rise, it’s true with tropical storms and hurricanes, it is true with drought, where we are seeing a pretty dramatic increase in both the extent of drought in middle latitudes. In California, of course, the worst drought on record. The paleo-climate scientists tell us it’s probably the worst drought in at least 1,200 years. There is science that’s emerging, some of that is science that we’ve done, that has isolated sort of subtle mechanisms by which climate change can influence extreme events like droughts and like floods and like heat waves, and again these are things that, there are processes involved that are pretty subtle, and not well resolved by the global models that have typically been used for climate-change projections, and, again, by the nature of the science, is that these sorts of impacts are more likely than the projections would have had us believe just five years ago or ten years ago. So there is this recurring theme of the science moving in the direction of the impacts being larger than we expected and part of that is a function of the reticence of scientists and the tendency to sort of be very conservative.

A good example is the last IPCC report — actually, sorry, well, two IPCC reports ago, the fourth assessment report, and this got quite a bit of attention at the time, this was back in 2006 where the report gave an upper sort of end range of sea-level rise that was just on the order of a foot, a little over a foot over the next century and there was sort of an almost like an asterisk next to that conclusion, it was almost buried in the footnotes if you will, was the fact that they didn’t include the contribution from melting ice sheets, from the Greenland and Antarctic ice sheets, because they couldn’t estimate that contribution. And therein lies the rub, because it’s the major driver and it can only weigh in in one direction, so there’s sort of an asymmetric neglect of processes that almost invariably imply worse changes than what are being projected. So, yeah, your thesis, I would agree with your basic thesis here. A worst-case scenario — and when you asked about business as usual, and I assume by business as usual you mean if we don’t really, if the world adopts a Trump-like approach to climate change over the next century where we literally don’t act at all, we continue to escalate our burning of fossil fuels, we don’t move away from fossil fuels towards renewable energy. In that sort of scenario, then, by the end of the century, you’re probably talking about a four to five degree Celsius, seven to nine degree Fahrenheit warmer globe. The warming is even greater than that where people live because we live on continents rather than in the oceans, and the land warms up faster than the oceans so when people quote a global average temperature that’s actually misleading. Most of us will experience more warming than the global average because the global average is held down by the oceans, that don’t warm as much. Doesn’t help us, those of us that live on land. So more than the seven to nine degree Fahrenheit warming of the globe …

And in certain areas, even more pronounced than that.
Twice that much in the Arctic, where we’re already seeing some of the worst impacts. So the loss of sea ice and the ramifications and all this stuff sort of circles back, again getting at this issue of processes that haven’t really been fully incorporated or represented in the main climate model projections, because when you lose Arctic sea ice, when you warm up the Arctic that fast, you lose Arctic sea ice faster than you expected. Greenland melts faster than you expected. The Greenland ice is of course going, Greenland is losing ice quite a bit faster. The climate models projected that Greenland and the West Antarctic ice sheet probably wouldn’t lose ice mass until the middle of this century. Just ten years ago, that would’ve been sort of the state-of-the-art projection, and what we’ve learned is that first of all, the satellite measurements tell us it’s already happening, so clearly that’s not correct, they’re already losing ice; and now we’re beginning to understand the processes by which that happens and we’re beginning to incorporate those into models and the sea-level-rise projections are increasing, the projection of melting is increasing, and when you melt all that Greenland ice and that water flows into the North Atlantic, not only does it contribute to sea-level rise, it freshens the North Atlantic and it potentially shuts down the so-called conveyor-belt ocean circulation. The Day After Tomorrow scenario — of course the movie is a caricature of the science but there is an underlying grain of truth to that, and we published an article just a couple of years ago, my colleague [Stefan Rahmstorf] and others showing that this already appears to be happening.

Shutdown of conveyor-belt ocean-circulation pattern — that could, you know, it’s not going to lead to about anything that’s portrayed in the movie The Day After Tomorrow, but it would mean potential decrease in the productivity in the North Atlantic, which we rely upon for fish, seafood and fishing, worst drought, far more extreme heat waves, longer duration, more intense heat waves.

A good fraction of the tropics are now too warm essentially for human habitation and there are studies that show that once you warm temperatures by even a few degrees by what they are currently in the tropics, basically you can’t work outside. Productivity plummets, agriculture plummets, basically these regions become unlivable so you’ve got a smaller amount of surface area on the Earth to support a growing global population. You see a tendency for worse drought and decreased freshwater availability over much of the continents of the world. Again, as I mentioned, food, productivity …

That seems like to me a really scary and also quite underappreciated by the broader public, the effect on food.
No, absolutely. Food, water, land, you know? The basic resources that we rely upon. All of them are adversely impacted by climate change and with a growing global population. So you’ve got more competition over fewer resources among a growing global population. It’s a recipe for a conflict nightmare. And this is why when you talk to national-security experts, many of them will tell you that climate change may be the greatest security threat we face in the years ahead, it’s what they call a threat multiplier. It heightens existing tensions, it heightens conflict, especially when you’re talking about more competition for fewer resources. And interestingly enough, and this is not actually that widely appreciated, this is sort of a dystopian scenario that Hollywood imagined decades ago. The movie Soylent Green, with Charlton Heston — if you watch the very beginning of the movie, it’s briefly mentioned, but the underlying cause of that dystopian future is global warming, is climate change. So there was — it was sort of an oddly prescient, some of these early-’70s dystopian novels and films and Soylent Green foresaw exactly the sort of future that we’re talking about where climate change leads to decreased resources. In that case, it was about food for a growing global population, and it’s a dystopian future. A worst-case scenario — a worst-case future does not look that different from the dystopian visions that Hollywood has already provided us.


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On Sept 19, 2023 ahead of the Climate Ambition Summit in New York City, climate activists gathered for a rally and civil disobedience outside Bank of America Tower in Midtown Manhattan as part of the March to End Fossil Fuels wave of actions resulting in multiple arrests. Activists demand Bank of America to “Defund Climate Chaos and Defend Human Rights” Photo: Erik McGregor (CC BY-NC 2.0 Deed)

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