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Brian Lehrer: It's The Brian Lehrer Show on WNYC. Good morning again, everyone. We'll take a break now from discussing the Democrats' campaign transition. We will come back to it for a call-in and any more developments for our last segment this morning. Now, we continue our WNYC Centennial series, 100 Years of 100 Things. We're up to Thing 5, which is very relevant to the presidential campaign.

It's 100 years of global warming at a time when we've got a pro-warming party and an anti-warming party, or another way to put it, a "do something" versus a "no climate policy" party. Judging from the Republican Convention and speeches and the written party platform, they are doubling down on a "no climate policy" approach as a centerpiece of their populism. For this 100-year segment, I've got the temperature data here with a very revealing curve of what exactly has changed globally and locally in our temperature patterns since 1924.

With us to help explain the very big picture is Michael Mann, professor of Earth and Environmental Science at the University of Pennsylvania with a secondary appointment in the Annenberg School for Communication, a very interesting combination. As such, he is also director of the Penn Center for Science, Sustainability, and the Media. Michael Mann is the author of several books, including his latest, perfect for the series, Our Fragile Moment: How Lessons from Earth's Past Can Help Us Survive the Climate Crisis. Professor Mann, it's great to have you for this. Welcome to WNYC.

Professor Michael Mann: Thanks, Brian. It's great to be with you.

Brian Lehrer: Considering what's in your book, we might have to retitle this segment. Instead of 100 Years of 100 Things, it might have to be 100 million years or four and a half billion years. How far back do you go in tracing the warming and cooling periods of the Earth's history?

Professor Mann: Well, more than four billion years and life has been around on this planet for the better part of that four billion years, at least 3.8 billion years ago. The purpose of this book was really to take the numerous lessons that Earth's history has to offer us when it comes to the obvious crisis we face today, not slow, long-term natural climate change, but very rapid human-caused warming.

Brian Lehrer: Listeners, be patient. We'll get to 1924 and that temperature crawl to the present, but with this opportunity with Professor Mann, let's do some of this prehistory. Can I start with what you call the gradual brightening of the sun over eons? What period of time roughly was that and how the brightening of what you call the faint young sun affected the Earth?

Professor Mann: This was a puzzle that the great Carl Sagan thought about quite a bit. He, in fact, formulated an explanation to this conundrum, which is the fact that the Earth was teeming with life nearly four billion years ago, had liquid oceans with microbes, but the sun was about 30% dimmer than it is today. If you do the standard calculations, Earth should have been a frozen planet, and yet it wasn't. It was a habitable planet with liquid oceans and with life.

What Sagan realized was that the way out of that paradox was that the greenhouse effect was actually much stronger than it is today. Carbon dioxide, methane, other gases have this warming influence on the lower atmosphere and the surface. It's the increase in those gases from human activity today that poses such a great threat. Billions of years ago, it turns out that very high levels of those greenhouse gases actually kept Earth habitable at a time when it should have been frozen.

Brian Lehrer: One of the things we worry about with climate change today is mass extinction events that many species might literally go extinct because of the changes we're bringing. You wrote about the greatest known extinction event in the history of the Earth 250 million years ago. What happened 250 million years ago?

Professor Mann: We called it the Great Dying, but it wasn't so great if you were a life form at that time. 90% of all species perished during this event. 96% of all ocean biota perished during this event. We have, over time, figured out the puzzle of what exactly happened and what appears to have caused it. Now, some years ago, there was a hypothesis that it was due to a massive warming event from the release of methane frozen in the sea bed.

Methane is very potent greenhouse gas. There are large amounts of it that are frozen at the ocean bottom or along the continental shelves of the Arctic Ocean. At that time, the ocean warmed up. There was this idea that, well, maybe that released a whole bunch of methane. Methane is an extremely potent greenhouse gas, so that caused this very abrupt warming nearly 9-degree Fahrenheit warming of the planet.

That was an example that people would point to for the possibility that we are threatening our own extinction today through the warming of the planet and the methane that is supposedly being released from the melting permafrost. Well, it turns out that's not actually what happened. It was actually just a period of intense volcanic activity that tapped into rich carbon reservoirs within the solid earth and pumped very large amounts of carbon dioxide into the atmosphere. The same greenhouse gas that we are producing from fossil fuel burning today.

When we look at the Great Dying, it wasn't actually an example of runaway, methane-driven warming, something that we wouldn't be able to control today if it were happening. It was a result of the input of carbon dioxide into the atmosphere. We can do something about that today. We need to stop the activities, particularly fossil fuel burning that is elevating carbon dioxide concentrations in the atmosphere. The difference between now and then is that we have agency. We can actually do something about the problem. We know what the cause is. It's us. We know that we still have an opportunity to avert the worst impacts of climate change if we act concertedly and dramatically and rapid.

Brian Lehrer: We will get to the policy options ahead of us for the next 100 years. Continuing with the long arc of history that you trace in your book, we get much closer to the present instead of 250 million years ago. Only 10,000 years ago, really recent, when a series of volcanic eruptions created conditions that you write might be the best historical example of rapid global warming triggered by a massive release of carbon, so what happened 10,000 years ago?

Professor Mann: Oh, well, I think what you're describing is the PETM, which is an acronym that stands for the Paleocene-Eocene thermal maximum, the PETM. That was actually about 56 million years ago.

Brian Lehrer: Oh, okay.

Professor Mann: That was still pretty far back. 10,000 years ago, we were coming out of the last ice age. That was a climate change driven by the changes in Earth's orbit relative to the sun, the changes, the distribution of sunlight, particularly at the Arctic, which controls the buildup of ice. The more ice that you have in the Arctic, the colder the planet because ice reflects away sunlight. It's a cooling effect.

The less ice you have, the more you warm the planet. Those astronomical factors, the relationship between our orbit and the sun about 12,000 years ago was such that it favored warming of the Arctic, melting of ice, and the Great Thaw as we came out of the last ice age. There's a lot we can learn from that event as well. It turns out it allows us to assess a quantity that we call the climate sensitivity.

Climate sensitivity, fancy term for how much warming do you get when you double the concentration of carbon dioxide in the atmosphere. It's a basic metric of the impact of fossil fuel burning on the planet today. You can actually back out an estimate of that quantity from past natural changes, including the last glacial maximum when we had extensive ice sheets covering where you are in New York City.

Brian Lehrer: As we head toward the present, here's a question. Given everything you've just been describing about the first several billion years of the history of the Earth, given all the wide swings in temperature that you document in your book and we're just describing some of-- and that we're not man-made, how do we know we're actually causing this?

Professor Mann: It's a great question. There are a couple of things here. We can look at the natural factors that might be implicated, say volcanoes or small but measurable fluctuations in how bright the sun is. We know that those factors play out over decades and centuries. We can take models of the Earth's system, climate models, and subject those to those natural factors. When you do that, what you find is that the planet shouldn't have warmed over the last century.

In fact, it should have cooled just a bit over the past several decades because there was a decrease in the frequency of cooling, volcanic eruptions, solar output as best we can estimate it. It was flat or even slightly declining. The natural factors over the last half-century should have caused us to cool. They can't explain the warming. The only thing that can explain the warming is the increase in the concentration of greenhouse gases and, in particular, carbon dioxide, which we know is coming from fossil fuel burning.

We can actually look at the isotopes of the carbon that make up the carbon dioxide in the atmosphere. Those isotopes give us a fingerprint. They tell us where it's coming from. Could it be a natural source of carbon dioxide or is it fossil fuel burning? It turns out that we can very clearly see the fingerprint of fossil fuel burning in the buildup of carbon dioxide that is causing the rapid warming of the planet today.

Brian Lehrer: As we move into the period that this series actually is officially concerned with the last 100 years, I'm curious, is there a year that you mark as the beginning of the man-made global warming era, a year when a certain kind of industrial technology first came into use, or anything like that?

Professor Mann: I would point to the year 1982.

Brian Lehrer: What?

Professor Mann: Actually, that is well after scientists were first speculating that we were warming the planet by increasing the concentration of greenhouse gases. The reason I mention 1982, it's the year of an internal report that was published by ExxonMobil, the world's largest publicly traded fossil fuel company. In that 1982 report, Exxon's own scientists made an estimate of how much the carbon dioxide level would increase in the atmosphere and how much warming would result from that if we continued to burn fossil fuels. This was ExxonMobil's own scientists. Their prediction was spot-on. We are exactly where ExxonMobil's scientists predicted we would be if we continued to burn fossil fuels at historic rates.

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Brian Lehrer: Go ahead.

Professor Mann: Just one last bit. They used the term "catastrophic" to describe the consequences. This isn't Al Gore. This isn't the Intergovernmental Panel on Climate Change. This is ExxonMobil's own scientists who warned of catastrophic consequences, but they buried that report. Instead, ExxonMobil and other fossil fuel companies doubled down in a campaign to deny the reality of climate change.

Brian Lehrer: That's when some of the information really came to light, but the warming period, the industrial era started well before that. A lot of people say 1850 or so, huh?

Professor Mann: Yes. In fact, we published an article some years ago, suggesting that it probably started even a little bit earlier. You probably have to go back to maybe 1780. We were already engaged in activities, mostly agriculture, a little bit of fossil fuel burning, but mostly agriculture in the late 1700s, early 1800s. We were already warming the planet even then. If you really want a true pre-industrial baseline with which to measure the warming that's taken place, you probably have to go back to the mid-1700s.

Brian Lehrer: Now, I have a temperature chart, listeners, from the National Weather Service for Central Park with average temperatures for each month going all the way back to 1869. To start with average temperature for the whole years, in those first few decades, the 1870s and 1880s, the average annual temperature at Central Park hovers in the low 50s from 49 to 53 degrees roughly, depending on the year. By the time we get to 1924, 100 years ago, the range is 52 to 54 degrees, just a few degrees higher. We would never have an annual average in the 40s again. Was there global warming already noticeable from industrial pollution in 1924?

Professor Mann: No question. The warming was underway, arguably even since the beginning of the 1800s or the late 1700s. You can very clearly see that in some of the long historical records that we have, including records from New York. Now, if you're a skeptic, you might say, "Well, I don't trust those thermometers. They've been doctored." Climate scientists are often accused of doing these sorts of things in today's political climate, but you could even throw out all those thermometer records. There are botanical records or what we call phenological records.

Botanists, the gardeners who kept records about the dates at which various plants bloom or the dates at which insects hatch or birds arrive, and you can very clearly see the warming in those records as well. One last thing about New York City like my city here in Philadelphia. It's a large urban center. There's been even a little bit of extra warming here from what we call the urban heat island effect. Those of us who live in the East Coast cities, New York City, Philadelphia, have experienced even more warming than the rest of the country because of that extra contribution of urban heat island effects.

Brian Lehrer: Listers, I'm going to let you in on a little behind-the-scenes thing that we're watching. We're told that Vice President Harris might be giving her first speech since President Biden ended his campaign and endorsed her around 11:30. That would be a few minutes from now. Only we will take it if it happens during the show, so here in our WNYC Centennial series, 100 Years of 100 Things, with 100 years of global warming today with University of Pennsylvania climate scientist Michael Mann.

I'm going to jump ahead on these temperature charts, so we make sure we get some of this in here. We cited that one period of global warming in Central Park based on overall average temperatures up until 1924. Then going up through the decades, the average annual in Central Park in the 1950s through the 1980s ranges from 53 degrees to 57 degrees, warmer than the lower 50s that we saw in the 1880s.

Then jumping to recent years, since 2010, it's taken another little leap to where the annual average temperature in Central Park is more like 56, 57 degrees, pushing into the upper 50s in some years compared to the lower 50s when they first started measuring. That curve seems like an indicator. I also noticed on that Central Park temperature chart, the seasonal extremes have gotten warmer. The National Weather Service chart goes month by month since 1869.

Looking at January, we see the average monthly temperatures in those early decades would range from the high 20s to the mid-30s, depending on the year. By the 1990s, we have no more average January temperatures in the 20s. That doesn't happen anymore. Some years, the average is peaking into the low 40s. From 2010 to the present, average January temperatures in the 40s are quite common, even reaching a January average of 50 degrees in 2015. Do these gradually warmer Decembers and Januaries over the last 100 years tell us something specifically important about the nature of the warming that we're seeing?

Professor Mann: Certainly, they do. The warming is ubiquitous. We see it during all the seasons. It might sound nice actually to some people, the idea of more mild winters. If you don't enjoy all that winter has to offer, you might say, "Well, hey, that's an improvement." Be careful what you wish for because while winters have become milder, of course, the flip side of that is that the summers have become far more extreme.

Brian Lehrer: Let me do those numbers and keep it moving ahead since we might get interrupted. Similarly, in July, the average July temperatures in Central Park would be in the mid-70s in the early days. When we say "average," that's the temperature at each time of day all averaged together for the month, so it includes nightly lows as well as the daytime highs. In 1924, that average for July in Central Park was 74 degrees. In recent years, sometimes that average surpasses 80. That's, again, the average temperature, including nighttime lows. Last year, it was 79 degrees. Also important, right?

Professor Mann: Yes, absolutely. If you look at, for example, the number of 100-degree days, 90-degree days, or 100-degree days, we see very substantial, more than a doubling or, in some cases, a tripling of those numbers. Even what might seem like a modest warming of a couple of degrees Fahrenheit leads to a dramatic increase in the frequency of these very extreme heat events. That's what we're seeing. I'll point out one other thing.

A couple of winters ago, I lived here in Philadelphia. Now, Philadelphia and New York City for the first time in history had no measurable snow that winter. That's something we haven't seen before. We are literally seeing a fundamental change in what winter looks like, in what winter feels like. We're losing part of our heritage in a sense because of the warming of the winters. Obviously, the warming of the summers is deeply problematic because of the extreme heat and the heat stress, which can be deadly that is associated with it.

Brian Lehrer: All of that so far was for New York. Now, we get to global temperatures since 1924. For this, I looked at the federal government's climate.gov website, which gives us some nice graphics and some very concise bullet points, which I will read. They say, "Earth's temperature has risen by an average of 0.1," so 1/10 of a degree Fahrenheit per decade since 1850.

Do the math. That's about 2 degrees Fahrenheit in total. It continues the rate of warming since 1982. There's that number that you cited, Professor. I mean that year that you cited. The rate of warming since 1982 is more than three times as fast as the rate before. It's going up about 3/10 of a degree per decade rather than 1/10 of a degree. Then it tells us 2023 was the warmest year since global records began in 1850 by a wide margin.

It was 2.1 degrees Fahrenheit above the 20th-century average of 57 degrees. It was 2.4 degrees Fahrenheit above the pre-industrial average. That's considered 1850 to 1900. Finally, this climate.gov site tells us the 10 warmest years in historical record. The 10 warmest years in the historical record have all occurred in the past decade, 2014 to 2023. That seals the deal, doesn't it?

Professor Mann: It does. Actually, a number of years ago, we published an article that estimated the likelihood of these record warm streaks where 9 of the 10 warmest years were in the last decade or, as you said, the 10 warmest years now have been in the last decade. You can calculate the likelihood that that would happen by chance alone because people say, "Oh, well, maybe we're just unlucky. We've had a number of warm years. It's an unlucky roll of the climate dice." You can calculate, in the absence of human-caused warming, how likely something like that would be. We're talking 1 in 50,000-year events. Something that is extremely unlikely to have been observed in the absence of human-caused warming. Once again, we can clearly see the influence of human activity in the warming that we've seen.

Brian Lehrer: Where might we go in the next 100 years? I see in your book that you have aggressive policy scenarios, keeping today's policy scenarios, and what you call no-policy scenarios. I argued at the top that we have a climate policy party and a climate no-policy party in this election year. Do you, with all you study this, have a good grasp of how much short-term pain, economic pain mostly, which less wealthy people bear the brunt of, is required to stave off catastrophic warming in the next 100 years?

Professor Mann: Well, I would even flip the script there. The consequences of the extreme weather events, the wildfires and heat waves, and damaging storms that we're experiencing, the cost of climate damage way outweighs the cost of taking action, i.e., of moving away from fossil fuels towards clean energy infrastructure. In fact, you could argue, the latter is an investment. It's an investment in clean energy probably provides far more jobs than the largely automated fossil fuel industry.

From a cost-benefit standpoint, there's no question here. Obviously, the most sensible thing to do is to decarbonize our infrastructure as quickly as possible. A little bit of good news here, as bad as things might seem, we can prevent them from getting worse. The science of the last decade or so tells us that when we bring our carbon emissions to zero, the planet literally stops warming up.

We used to think that there would be a delay of decades that the planet would still continue to warm up because of the sluggishness of the oceans, for example, as they slowly heat up in response to the pollution. We have all the carbon pollution we've already added, but it turns out because of the ocean and its ability to absorb carbon dioxide. If you work it all out, when our carbon emissions go to zero, the planet stops warming up. There is an immediate and direct consequence of our efforts to decarbonize our world.

That's why it's so important to do it quickly. If we do it dramatically and quickly, we can prevent the planet from warming more than a catastrophic 1.5 Celsius or roughly 3-degree Fahrenheit. That danger mark, we're coming close to it, but we can avoid crossing it if we take dramatic action. The most important thing people can do, of course, right now is vote in this upcoming election because it will determine whether or not we take a leadership role in combating the greatest threat we face on this planet, the climate crisis.

Brian Lehrer: For people who saw the movie Don't Look Up, which is that satirical warning about ignoring climate change even without mentioning climate change, Leonardo DiCaprio who played the scientist who was being ignored while seeing the path of an asteroid or comet hurtling toward the Earth representing global warming, DiCaprio cited you as a model for how he approached the role. Did he ever contact you or get more specific about why he name-checked you in describing it?

Professor Mann: Yes, it was funny. It caused some concern, I have to say, on the part of my family, my wife. Because if you've seen the movie, he does engage in some dubious behavior, a little bit of philandering there. Hopefully, it wasn't meant to be--

Brian Lehrer: It wasn't that part of your life.

[laughter]

Professor Mann: Exactly. I've gotten to know Leo pretty well over the last decade and a half because, of course, he's been a leading spokesperson in the world of entertainment, in the film world when it comes to the climate crisis. He has been outspoken. He's consulted with climate scientists. I've talked with him a number of times. I imagine that because I'm one of the climate scientists, he knows best. He probably drew upon maybe some of my mannerisms.

I'll give you one last funny story here. I actually watched the film before it came out. Netflix sent us a code so we could watch it at home on our big-screen television. I'd been invited to the premiere, but they ended up canceling a whole bunch of things there because of COVID. We decided to just watch it at home. We're no more than five minutes into the film. My daughter says, "Dad, he's you."

She could actually see the mannerisms that I did not see that apparently, according to Leo, were, I guess, reflective of some aspects of my personality. It was amusing, but it was a powerful film. I think it got across a very important message that we should treat this like a meteor that's going to strike the planet. Again, the difference here unlike the meteor, the asteroid that killed the dinosaurs, we have agency. We can do something about it, but we have to act now.

Brian Lehrer: I guess there aren't many scientists in the world who get to actually have a real-life answer to the question. Who would you like to play you in the movies?

Professor Mann: [laughs]

Brian Lehrer: I know you got to go in a minute. I think you have a stat in the book that I'm curious about. It's a stat of a maximum temperature increase from what our current consumption of fossil fuels would produce and it was 5 degrees Fahrenheit. Did I see that right? Because if we keep consuming gas and oil and electricity and meat and everything at the current rates and keep producing methane and carbon as fast as we've been, why would it max out at all?

Professor Mann: Yes, so that's an interesting point. The fact is that we are already transitioning away from fossil fuels towards clean energy. The age of fossil fuels is coming to an end. There's nothing that's going to stop that. Just like the age of whale oil came to an end in the 1900s when fossil fuels became widely available. The same thing is happening here. Renewable energy is increasingly going to be cheaper. It's cleaner. There are so many reasons why we're going to move in that direction. Our carbon emissions will come down.

Here's the problem. If it just left to the vagaries of the marketplace without regulation or without incentives to move in that direction, it'll happen too slowly. It'll happen too slowly to avoid, say, warming of about 5 degrees Fahrenheit as you mentioned. That would be truly catastrophic, a truly catastrophic level of warming. We haven't seen anything yet when it comes to the consequences we would see at that level of warming. That's business as usual. That's what it would give us.

The good news is we are making progress. If we can accelerate that progress, then it's time to hold warming below 3 degrees Fahrenheit, and that's a level of warming-- Look, the warming that's already happening is causing displacement. It's leading to catastrophic events and huge economic toll that it's taking. We're already encountering dangerous impacts of the warming that's happened.

If we can keep the warming below 3 degrees Fahrenheit, scientists who study adaptation and come to the conclusion that that's a level of warming that we can cope with, we will have to adapt to it. We will have to institute all sorts of measures to instill greater resilience. The planet will remain within our adaptive capacity if we can keep warming below 3 degrees Fahrenheit. If we let it get much beyond that, then it really starts to challenge the stability of our infrastructure. That's what makes this, to quote the title of my book, such a fragile moment.

Brian Lehrer: Listeners, that's today's episode of our WNYC Centennial series, 100 Years of 100 Things. This has been Thing 5, 100 years of global warming. On Wednesday, we'll do Thing 6 related, 100 years of air conditioning. For today, we thank Michael Mann, professor of Earth and Environmental Science at the University of Pennsylvania and director of the Penn Center for Science, Sustainability, and the Media, and author of the book Our Fragile Moment: How Lessons from the Earth's Past Can Help Us Survive the Climate Crisis. Thank you so much for joining us for this.

Professor Mann: Thank you, Brian. It was a real pleasure talking with you.

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