Winter is Coming
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This message comes from Tourism Australia. Australia, a land of contrast, where white sand beaches are almost as plentiful as flat white coffees, and where you can go away for a vacation yet somehow feel right at home. Explore more at Australia.com. Planet Earth, 66 million years ago. The moment just before the end.

The wind tickles a patch of ferns. The shadow of a pine leaf dances lazily over a footprint in the dirt. Somewhere nearby, a lumbering beast stops mid-stride. It's a hulking mass of a creature. Three horns, teeth like shears.

And it swings its head down in a low arc, listening. Then suddenly... A flash of light, thousands of times more blinding than the sun. An asteroid... The size of Mount Everest. ...enters the Earth's atmosphere, moving incredibly fast. Ten times faster than the fastest bullet from a rifle. In the blink of an eye...

This asteroid will crash into Earth's surface on the edge of the ocean with an impact equivalent to 5,000 times the combined destructiveness of the entire nuclear arsenal at the height of the Cold War. It burrows into the Earth 20 miles deep. Everything within 90 miles, every living thing, is instantly vaporized. But this is just the beginning of the end.

A giant fireball. All of the rock and dust and gas. Climbs back up and up and up. All the way into space. And bursts. Bursts through the atmosphere. As the Earth below shakes violently. Magnitude 12 or 13, rippling the entire crust of the Earth. Triggering unimaginably high tsunamis.

The real bummer was all of that material that got ejected into space began to fall back. Millions of tiny specks, each the size of a grain of sand, re-enter the atmosphere and catch fire again. This is like...

A sea of lava descending from the heavens, unleashing hell on Earth. If you want to see it, open up your oven, turn the broiler on, and look at the glow bar.

In this case, the whole sky is glowing. There are no shadows. So you're not going to get away from it. Swaths of life. The culmination of millions of years of evolution. Wilt under the broiler of asteroid debris. Forests catch fire. The air teems with ash and smoke. And then... Comes winter. The soot that stayed in the atmosphere blocked out the sun. Blocked out the sun. The amount of light that would...

get through would be a hundred millionth of the amount of light we normally see. It effectively turned day into night. Temperatures fell below freezing over all the land. No light meant no food. Plants can't photosynthesize. The food chains in the ocean probably collapsed. To say that this was a challenging environment, you know, for life to survive and continue to evolve would be an understatement. It was the end of a world.

And the beginning of another. Over the last few years here at ThruLine, we've been finding ourselves thinking about the end a lot.

What that might look like and feel like. Partly because we live in a world with climate change and nuclear weapons. So we're kind of engineering the asteroid that might destroy us as we speak. And also because a bunch of us have become new parents in that time. Asking ourselves, what future will our kids inherit? What do dinosaurs have to do with any of that?

Well, it turns out that for most of human history, no one had a clue that dinosaurs had walked to Earth before us, or that they disappeared in a sudden flash. But there was a moment in the not-so-distant past when we learned what caused their extinction. And that discovery may have helped save humans from the same fate. That moment is where we'll be taking you in this episode. I'm Randa Abdel-Fattah. And I'm Ramteen Arablui.

You're listening to ThruLine from NPR. Today, we're traveling deep inside the Earth's layers, up above the clouds as the first nuclear weapon is dropped, and all the way to the cosmos. Sometimes we can avoid the worst fate that we can imagine.

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This message comes from Homes.com. What kind of programs does this school have? How are the test scores? These are all things parents ask when they home shop. That's why each listing on Homes.com includes extensive reports on local schools. Homes.com, we've done your homework. Part 1, 45,000 miles per hour. It was 1977.

A geologist named Walter Alvarez was spending a summer in Gubbio, a town in Italy about halfway between Venice and Rome. It sits in a valley alongside a mountain formed by a fault line. And what was once an ancient seafloor became exposed over time. So you can actually see inside the Earth and look at millions of years of the planet's history. So you can think of like a layer cake.

This is David Sapkoski. He's a historian of science at the University of Illinois Urbana-Champaign and author of the book Catastrophic Thinking. His dad, Jack Sapkoski, was a prominent paleontologist around this time. The layer cake, going from the top being the most recent layers of the history of life all the way down to the earliest ones. Try to picture it.

A swirl of colors, rusty orange, dull brown, chalk white, with fossilized creatures sprinkled throughout. Each layer is a different thickness. And Walter Alvarez was focused on one layer in particular, the K-T boundary. The K-T boundary had been well studied around the world.

It's striking. A very thin layer of clay, much thinner than the others around it, with a distinctive dark, ashy color, kind of like coal. Below the K-T boundary is the Cretaceous layer, marking the age of dinosaurs. It's filled with shards of dinosaur bones and teeth, the outline of their footprints. And above the K-T boundary is the Tertiary layer, marking the age of mammals.

In that one, you won't see many dinosaur fossils. In fact, none. None. None. Given that, paleontologists had already figured out that the dinosaurs went extinct many millions of years ago between the Cretaceous and Tertiary layers. They didn't know how it happened or the amount of time it took, but they assumed natural selection was a really gradual process. The Darwinian view of the world, that over time, the strongest and brightest species went out.

As a geologist, Walter wasn't really interested in dinosaurs. He was just trying to measure how long it had taken these layers of sediment to form. This would be a big breakthrough if he could figure it out. But nothing had worked so far. Until… One day, his father calls up with an idea that has just occurred to him.

We'll get to that idea in a bit. But first, we need to take you on a little detour to learn more about Walter's father, Luis Alvarez. We promise it's going to be worth it. See, Luis Alvarez wasn't just some guy with an idea.

He was a Nobel Prize-winning physicist. The most Walter ever said to me on the record was that he and his father were not close. You know, that's an exact quote. By the way, this is Alec Navala Lee. I am a novelist, critic, and biographer. His upcoming biography is about Louis Alvarez. And he says Walter and Louis weren't close because from the time Walter was born in 1940... His father was occupied elsewhere.

It was the middle of World War II. American scientists were racing to build an atomic bomb before the Germans did. Louis, a young up-and-coming physicist, gets involved and helps the American team figure out the missing link to actually make the bomb detonate. And when the order comes down to use the bomb on Hiroshima, he volunteers to be on the plane. On the observation plane.

following closely behind the Enola Gay as it unleashed, for the first time in history, the power of a nuclear weapon onto a city of 350,000 people. When the bomb goes off, there's a flash of white light that comes from the cockpit. And so at that moment, he knows the bomb has gone off. The entire plane bounced upward when the shockwave from the bomb hits the plane.

So they feel it. They feel that shockwave, even from the air. And he can see the mushroom cloud. But when he looks down, he thinks that they've missed. It's so empty that he thinks that they missed entirely. Peering out the plane window at the devastation wrought by a bomb they'd named Little Boy, the gravity of what just happened begins to dawn on Louis. And then his mind turns to his own little boy, Walter, who was four years old at this point.

Louis pulls out a pen and paper and begins writing him a letter. Dear Walter, This is the first grown-up letter I have ever written to you. It was really for you to read when you were older. During the last few hours, I've been thinking of you and your mother and our little sister Jean. It was tough to take off on this flight not knowing whether I would ever see any of you again.

It's partially an account of what happened that day. But he's also trying to, like, get his thoughts together about what it means to use this bomb. A single plane disguised as a friendly transport can now wipe out a city. He's trying to, in some ways, justify it to himself.

This is kind of like the paragraph that I come back to a lot. What regrets I have about being a party to killing and maiming thousands of Japanese civilians this morning.

are tempered with the hope that this terrible weapon we have created may bring the countries of the world together and prevent further wars. Alfred Nobel thought his invention of high explosives would have that effect by making wars too terrible. But unfortunately, it had just the opposite reaction.

He believes on some level that the atomic bomb is just so destructive that it's going to force the world to approach things differently. Our new destructive force is so many thousands of times worse that it may realize Nobel's dream. The dream of ending all war. Walter did not read it until much later. Over the next few decades, Louis threw himself into work.

He did wild things like search for hidden chambers in the pyramids and investigate the assassination of JFK. Louis had this kind of superpower for finding things others had missed, all using physics, a tool that had brought humans closer than ever to playing God. So we aren't just passively observing nature, we are actively making things happen. Meanwhile, his relationship with his son grew more distant.

But eventually... Walter did read that letter. Dear Walter. Okay, detour over. Let's get back to Gubbio.

By this point, father and son are on better terms. Walter is feeling stuck in his research, and Louis, hoping to help him out, gives Walter a call. An idea has just occurred to him. He's staring at this gift Walter had given him. A chunk of rock. From Gubbio. Which shows the K-T boundary. Which he'd never seen before.

He thinks it's one of the most exciting things he's ever seen. Why is it so thin? What's up with that color? And why do the dinosaur-era fossils just disappear? For Louis, it was a revelation. It's almost an emotional reaction. Keep in mind, the K-T boundary visible at Gubbio wasn't a secret. Plenty of geologists and paleontologists had seen it. But Louis had an idea using some tools from his universe.

Louis, in his physics work, had developed the ability to measure very trace amounts of rare elements in samples. Rare meaning there's not much of it on Earth. So if you find the same amount of it in a layer in different parts of the world, then we can be pretty sure that they must have gotten formed at the same time. And you can figure out how long that layer took to form.

So then the question was, which rare element to choose? They went with one called iridium, which, besides tiny trace elements at the planet's core... Doesn't naturally occur on Earth. Where can you find it? It tends to be found more in meteorites and asteroids.

What happens is that they find iridium... Like, too much of it. 30 times as much as they are expecting. That's way more than they were expecting. Way more than seemed possible, even. Which makes them think, shoot, we messed up. You know, figure that maybe there'd be contamination in the sample or, you know, some other explanation. They run more tests with samples from different parts of the world...

And after a couple of years. In 1980, concluded the only place it could have come from was a very large asteroid. A very large asteroid. They had stumbled across the answer to a question they hadn't even been asking. What happened to the dinosaurs? Louis couldn't wait to tell the world their theory. And Walter knows better, right? He knows that they're going to be very skeptical.

Soon after, in June 1980, they published a paper with their findings. A very long paper in the journal Science. And to sum up a very long paper, they theorized that an asteroid found its way into the Earth's atmosphere and then, boom, goodbye dinosaurs. Some paleontologists were immediately skeptical. Reminds them of stuff like the flood narrative. Sounds religious. Sounds biblical.

Yeah, to them it sounds like crackpot science. But Louis had witnessed the atomic bomb level a city in an instant. So imagining an asteroid with more than a billion times that power leveling the world wasn't that wild to him. Sure, it might feel apocalyptic in the way that something like Noah's Ark does, but this was different. We can think of...

extinction of the dinosaurs as an apocalypse because it is a revelation to us, not necessarily a scriptural one, but a no less profound revelation of how violent and unpredictable nature can actually be. Even people who have some religious sensibility, I think, would see this new story as undermining human significance.

And even if you had doubts about what came to be known as the Alvarez hypothesis, it was hard to deny that it was a really good story. We can kind of vividly imagine this horrific final moment. Those individual dinosaurs looking up at that bright flash in the sky. A new consciousness is developing which sees the Earth as a single organism and recognizes that an organism at war with itself exists.

Coming up, some scientists begin to connect that bright flash in the sky that wiped out the dinosaurs to a bright flash that could wipe out all of humanity. We are one planet. I am Paul Ekloff from Petaluma, California. You're listening to ThruLine from NPR.

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This message comes from Warby Parker. What makes a great pair of glasses? At Warby Parker, it's all the invisible extras without the extra cost, like free adjustments for life. Find your pair at warbyparker.com or visit one of their hundreds of stores around the country. Part two. It's the end of the world as we know it. Whatever happened to the dinosaurs? That's one of the great mysteries of archaeology.

By the early 1980s, Louis and Walter Alvarez's dinosaur hypothesis was starting to get attention in local papers, on TV and radio. "Alvarez says that an asteroid crashed into the Earth and wiped out the dinosaurs." But there were still some things Louis and Walter Alvarez didn't know. Potential flaws in their hypothesis. For one, where was the crater that this massive asteroid supposedly left behind?

And was the environment really inhospitable enough to kill off all the dinosaurs, even the ones way outside the impact zone, on different continents, in different hemispheres? They needed more information, more evidence, and more scientists across disciplines were getting involved. So we knew about the Alvarez discovery before it was published. Enter Brian Toon.

At the time, I lived in the San Francisco Bay Area and worked at NASA Ames. We're working on the Earth's atmosphere and climate. Brian had grown up in the shadow of the Cold War. And by the time he was at NASA in the early 80s... There were 70,000 nuclear weapons on the planet. Most of those weapons were part of the Soviet or U.S. arsenals, and they were pointed at each other.

70,000 nuclear weapons. So when he came across the Alvarez hypothesis about a single asteroid strike wiping out the dinosaurs, his mind immediately went to that. Because this asteroid strike, which had the estimated energy of tens of millions of nuclear weapons, would have produced a massive fireball. A fireball which would likely have risen hundreds of kilometers above the surface of the Earth.

Then Brian called up a friend of his. Richard Turco. Who worked for a company called R&D Associates, which did research for the Pentagon and Department of Energy and had already done modeling of nuclear explosions. So anyway, Brian asked his friend if, hypothetically...

A nuclear weapon went off. Is there a way to calculate how much dust it would kick up? He said, "Well, yeah, I know how much dust would be raised by a nuclear weapon." But what if all of them went off? How much dust would be raised then? Let me go think about that. So he went out. Ran the numbers. Tens of thousands of weapons go off. And concluded. They're going to put enough dust into the upper atmosphere to cause a big climate problem. And then there were the fires.

A nuclear blast emits enough light to ignite flammable materials in cities and forests, pumping even more smoke into the atmosphere.

And so we heard about their idea of smoke and we thought, "Oh my gosh, we didn't think of the smoke." So we put the fires into the model in urban areas. Computer models were a relatively new thing at this time. Simulations of reality that were helping scientists like Brian test out theories in a virtual world. And so we discovered that, you know, there might be a prolonged period

where there'd be so much smoke in the atmosphere that it would cause temperatures below freezing over the lands. A long winter.

In October 1981, Brian took the model he and some colleagues had made to a conference organized exclusively to discuss and test Louis and Walter Alvarez's hypothesis. But for Brian, this wasn't just about dinosaurs or the past anymore. It could be the key to humanity's future, to avoiding our own potential end.

This was a big idea, and they wanted one of the most well-known scientific thinkers of the time on the case. "Should we call up Carl?" Carl Sagan. "The cosmos is also within us. We're made of star stuff." Yes, that Carl Sagan. "The journey for each of us begins here. We're going to explore the cosmos in a ship of the imagination." Carl Sagan was in people's living rooms talking about the wonders of the universe.

"Drawn by the music of cosmic harmonies, it can take us anywhere in space and time." Carl was an astronomer turned bona fide celebrity, the so-called showman of science by Time magazine.

His television show, A Personal Voyage, was the highest-rated PBS program at the time, reaching over 500 million viewers worldwide. He had an almost like kind of Mr. Rogers-like demeanor, you know, to him. Calm, almost gentle. Contemplating the stars. Organized collections of 10 billion, billion, billion atoms. Billions and billions, you know, that was sort of his catchphrase. He was completely authentic.

This is Ann Druyan. She co-wrote and co-created Cosmos with Carl. And from the moment they'd met… I heard this that was so thrilling. And it was the laugh of a person who had no fear. Her life would never be the same. There was no facade. There was no baloney. There was no presentation of self to impress. Carl Sagan was magnetic.

And he was busy. He was on TV. He and Ann, his future wife, were falling in love. Oh, and he was also Brian Toon's PhD advisor. You gave him an idea. He would respond to that idea. And you'd often come back with some related idea that you would have never thought of. A lot of them were not right. But he just threw ideas out right and left. He had this gift of

for not just kind of simplifying complex science, but making science feel personal and important to an everyday person, right? Why should we care about what happened 14 billion years ago or even hundreds of millions of years ago? It was thinking about why science gives us insight into the big questions that matter about existence.

And at that time, there was no bigger question than whether nuclear war between the U.S. and the Soviet Union could end our existence altogether. So when Carl got Brian's call, he eagerly agreed to start working alongside Brian and the other scientists to answer that very question. I'm pretty sure it was named Nuclear Winter in my living room. Nuclear Winter. Those two words summed up everything the model showed—

But not everyone was so excited about their research. The head of NASA Ames was so mad, it looked like steam was coming out of his ears. His face was bright red. He said, I'm going to fire you and everybody in your building if you keep working on this. Why was it seen as being so threatening? He was afraid Ronald Reagan would shut down NASA Ames to stop it.

President Ronald Reagan wasn't a fan of any science that called for more government regulation. And he believed that the more nuclear weapons the U.S. had, the less likely the Soviets would be to attack. But Carl talked to the head of NASA Ames and convinced him to let this research continue. For the scientists working on this, this wasn't about politics. It was about following where the science took them.

And some of them, like Brian Toon, thought: "We thought if science people came up and said nuclear weapons are going to kill most of the people on the planet, that the major political figures and politicians, the Department of Defense would say, 'Oh my gosh, this is an important thing. We should do something about it.' We were just as naive as Louis Alvarez, who thought nuclear weapons would end wars."

But Carl knew better. He didn't think a single scientific paper was going to be enough to get people's attention. Carl's pretty much a loose cannon. Here now reporting from Washington, Ted Koppel. November 20th, 1983. Ted Koppel, the host of Nightline. The most watched news program at the time. Was hosting a roundtable all about nuclear weapons. We are joined here in Washington by a live audience and a distinguished panel of guests.

Carl Sagan was one of the guests, along with some of the country's most powerful people. Former Secretary of State Henry Kissinger. William F. Buckley Jr. Former Secretary of Defense Robert McNamara. Along with Holocaust survivor Elie Wiesel. And former National Security Advisor Brent Snowcroft. So it's like these people are talking and, you know, many, many millions of people stayed up and stayed tuned for this event.

Most Americans tuning into this roundtable would have just seen a made-for-TV movie called The Day After, which debuted that night to an audience of 100 million viewers in the U.S. Roger, copy. This is not an exercise. Roger, understand. Major Reinhardt, we have a massive attack against the U.S. at this time. Over 300 missiles inbound now. And it's billed as an authentic depiction of the aftermath of a nuclear war. ♪♪

And this is one of the first times that anybody's tried to actually realistically reproduce what a nuclear war would look like. Hello? Is anybody there? And a text appears on screen at the very end saying, the catastrophic events you've just witnessed are, in all likelihood, less severe than the destruction that would actually occur in the event of a full nuclear strike against the United States.

And then, as soon as the day after credits end, Carl and the other roundtable guests appear, ready to discuss the film everyone just saw. Henry Kissinger, not a fan. I think that this film presents a very simple-minded notion

Neither was William F. Buckley Jr. The guy who wrote it says, I would like to see people starting to question the value of defending this country with a nuclear arsenal. But Sagan... Sagan turns to the camera and says... It's my unhappy duty...

to point out that the reality is much worse than what has been portrayed in this movie and this new emerging reality has significant policy implications.

The nuclear winter that will follow even a small nuclear war involves a pall of dust and smoke, which would reduce the temperatures, not just in the northern mid-latitudes, but pretty much globally, to sub-freezing temperatures for months. There is a real possibility of the extinction of the human species from such a war. It was a mic drop moment. Imagine a room awash in gasoline.

And there are two implacable enemies in that room. One of them has 9,000 matches. The other has 7,000 matches. Each of them is concerned about who's ahead, who's stronger. Well, that's the kind of situation we are actually in. Soon after that roundtable, Carl, Brian Toon and the other scientists published their scientific paper on nuclear winter.

But the message was already out there, and Carl and Anne made sure people outside of the scientific community kept talking about it. That same year, they wrote an article together explaining the theory in one of the most popular publications of the time, Parade magazine. Because it was inserted in the Sunday papers across the nation. Which most people would have gotten delivered to their front doorstep.

And that morning when they opened up the paper, they would have seen a photo of planet Earth alight with nuclear flashes and the words, Would nuclear war be the end of the world? A special report by Carl Sagan. He was proud of the fact that the public would be getting this information at the same time as the scientific elite did.

By the end of 1983, tens of millions of Americans had heard of nuclear winter. And as far as the average American was concerned, the messenger and man behind it all was Carl Sagan. People are used to having a shaman. It goes back 100,000 generations, you know, since we've been telling stories to each other at night around the campfire. Coming up...

Carl Sagan sets out to tell the story of nuclear winter to anyone who'll listen and finds himself in a battle for the future of the human race. My name's Amanda. I'm calling from Portland, Maine, and you're listening to Through One.

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Part 3. The Pale Blue Dot. I remember it vividly. Carl would have a look on his face like he had something really great to tell me.

And he said, I've been told that Gorbachev wants to know more about nuclear winter. Mikhail Gorbachev, the leader of the Soviet Union, America's enemy in the Cold War. And not long after, Karl, Anne, and their young daughter found themselves on a plane to Moscow. There was a reception the day before. And Gorbachev was there, and...

And so was our daughter, who was wearing the most adorable outfit, which made people confuse her with the Azerbaijani junior skating champion. The following day, Karl made his way to the Kremlin to meet with the Soviet Central Committee and Gorbachev. A few hours went by.

And then... He came back from this meeting. Anne could instantly tell it went well. He was completely energized. He started off talking about how the room looked, who was there, but mostly... Mostly, he was really talking about just how interested they were and how sober, how sober they were at this information. You know, the possibility that there could be a nuclear winter was...

A waking nightmare. It was 1986 when Carl got that invitation to meet with Gorbachev. He'd spent the years since the parade article on a campaign to warn the world of the existential threat of nuclear war. Going everywhere. Spreading the gospel of nuclear winter. We didn't have anybody to do PR. We didn't have anybody financing this. It was personal.

He hosted a TV special. For months, there would be a dark, cold, and deadly nuclear winter, no matter in what season the war might occur. Gave lectures around the country. Anything that involves the future is fundamentally threatened by the danger of nuclear war. Testified before Congress. One response by a senior practitioner was the following. He said, look...

He even met with the Pope at the Vatican. Meanwhile, a symphony of media coverage continued to surround the idea.

I just felt eager to be on the cutting edge of understanding. What's the world like? Why is it changing? This is Andrew Revkin. He's a longtime science journalist who got his start back in the 80s covering nuclear winter. There were so many things happening in that period between the United States and the Soviet Union. There was this arms race. There was the economics. These competing views of humanity.

of what the world economy should be. There was a technology race. Computers were advancing rapidly. And then each country had these client states that were actually killing each other. But for Carl, this competitive approach missed the bigger picture. This was about all of our futures. So people on the other side of the Iron Curtain needed to hear about nuclear winter too.

For that, he needed the help of scientists inside the Soviet Union. To try to beat the drama of disarmament and that nuclear winter demonstrated that nuclear war was unwinnable. And they did, because they really, you know, anyone who knows anything about science realizes that it's a global enterprise.

Eventually, nuclear winter theory began to gain traction in the Soviet Union. The way it felt to me at the time when I was reporting this, it seemed pretty clear that the Soviets saw an advantage in playing up the specter of nuclear winter as a counterweight to Ronald Reagan's Star Wars. Many of you seriously believe that a nuclear freeze would further the cause of peace.

But a freeze now would make us less, not more secure, and would raise, not reduce, the risks of war. Around the same time nuclear winter theory was bubbling up, President Ronald Reagan had announced a new idea of his own. The Strategic Defense Initiative, better known as Star Wars. Let me share with you a vision of the future which offers hope. Reagan proposed a high-tech space shield that would shoot down enemy nukes before they could strike the U.S.,

In other words, Reagan was saying we could actually win a nuclear war, with the asterisk that the technology hadn't been developed yet. Still, that introduced a scary possibility. If destruction was no longer mutually assured, what would stop an invulnerable country from launching a nuclear attack? And a lot of peace activists, including many scientists,

Carl Sagan was a vocal opponent of Star Wars. It is ruinously expensive. It abrogates a large number of treaties that the United States has solemnly signed.

And in addition, it is likely to bring about nuclear war itself if the Soviets were to believe, as they say, that it is part of an American plan for a first strike. Soviet leader Mikhail Gorbachev condemned the U.S. Star Wars missile defense program, warning that it will increase the risk of accidental war.

Carl Sagan found himself in the middle of a delicate political chess match, which is how he ended up at the Kremlin explaining nuclear winter theory to Gorbachev. By the way, he and Anne were also invited to the White House three times to dine with Reagan. And we said no each time very politely, but no thank you. Because this was when his administration was committing unspeakable atrocities in Central America.

Ronald Reagan was the person who ripped Jimmy Carter's solar panels off the roof of the White House. His contempt for science and reality even, you know, it was just the headline always about him.

We just didn't want to be complicit. Anne also acknowledges that while Carl and other scientists working on nuclear winter theory never let politics impact the science itself, when it came to telling the story, they did have a political agenda. The entire human species was living in a hostage drama. And so, yes, we had definite political feelings. We're parents.

We have children. Is that political or is that biological? I don't know. I think it's biological. It's survival. And although they never met with him, it's clear Reagan was aware of nuclear winter. During one press conference in 1985, he mentioned a volcanic eruption from the 1800s, which had cooled the earth and produced famines.

Then Reagan asked, quote, If one volcano can do that, what are we talking about with the whole nuclear exchange, the nuclear winter that scientists have been talking about? Since the original nuclear winter idea was proposed, several groups have tried to study the problem, and each has come up with a different computer model for the global effects of nuclear war. I learned something about science through that period.

New science often leads to more questions, and that was absolutely the case for nuclear winter. I think that some people were worried in the scientific community about overstating the case. Nuclear winter essentially exists in data and models, and models are highly imperfect simulations of how the world works.

And this is, I think, part of the reason why some folks did resent Carl Sagan. I think they thought he was a showboat a little bit, you know? He jumped the gun. Scientists are human beings, and they realize they have the capacity to influence world events. The scientists who were involved with Sagan, they all kind of were riding this wave. And journalists like me were riding the wave, too. By the mid-1980s, a few scientists had begun rebranding the theory. Nuclear fall.

They argued it wouldn't lead to full extinction of the human race. But it would still be really bad. Anybody who needs to know the weather report after a nuclear war in order to be deterred is already crazy.

But one scientist, Fred Singer, disagreed and said it could actually bring on a nuclear summer. What they've forgotten is that the smoke cloud also keeps in the heat. Singer was a prominent climate change skeptic, and it was later discovered that throughout his career, he had been paid by oil and tobacco companies who had hired a handful of scientists to quell concerns about the impact these companies were having on the environment.

Some historians call them merchants of doubt. Professional purveyors of uncertainty. People who have set out deliberately to confuse the public about science. Who would just frame the questions around overstatement and uncertainties being the real story on any issue. And these voices got a lot of airtime. We in the media...

We're addicted to conflict and debate, and so it's easy to build stories that kind of mischaracterize the uncertainty by making it a us-and-them fight. At the same time, David Sapkoski says there was some truth to the claim that Carl Sagan and other scientists sounding the alarm about nuclear winter were making things sound as catastrophic as possible.

Absolutely. They were fear-mongering. They're presenting us with some of the more pessimistic models because they want us to wake up. I call it prophecy, not fear-mongering. If something will happen that will destroy absolutely everything you care about, even if that's a small possibility, especially human-made possibility, we've engineered for ourselves

What you have to do is sound the alarm. As many people as possible should have the ability to do something to avert it. This goes back to Plato and Aristotle and what's called the noble lie, where you're consciously not speaking the entire truth in service of the better outcome for society. Because you know it would be inconveniently complicated or difficult

Louis Alvarez, who had helped kickstart all of this with that dinosaur extinction theory, had some doubts about nuclear winter, though he made sure never to share them publicly.

It was a means to achieving that long-awaited peace he'd hoped nuclear weapons would bring. He wrote in his memoir, "...the most encouraging feature of the nuclear winter scenario is that no one has been able to disprove it. It has had a very salutary effect on the thinking of military planners on both sides of the world." "...in Moscow, the hammer and sickle is lowered for the last time, and an era comes to an end."

Gorbachev said it best today, just moments before he resigned his post as the last president of the Soviet Union. "If you have to go, you have to go. It's that time," he said. Both Gorbachev and Reagan wrote that the reason that they controlled nuclear weapons was because of this discovery of nuclear winter.

And while there were economic and political realities that pushed the Soviet Union towards collapse... I do think that it had a role in the, you know, quote-unquote fall of communism. Louis Alvarez died in 1988. He didn't see the end of the Cold War in 1991 or the discovery of the asteroid crater a year before that, which put to rest most doubts about the dinosaur extinction theory.

Soon after the Cold War ended, Anne remembers sitting in the audience of a talk given by the Soviet cosmonaut Alexei Leonov. Who was the very first human ever to walk in space. He had also been in the room at the Kremlin several years earlier when Carl Sagan explained nuclear winter to Gorbachev. Alexei Leonov, he began his talk by saying words to the effect of, there's someone in our universe

audience who we owe more to than any other. And that's Carl Sagan. Because when Carl Sagan came to debrief us on the nuclear winter, after it was over and Dr. Sagan left the room, Gorbachev said to us, well, it's over, isn't it?

Carl Sagan died in 1996, with some questions still hanging over exactly how nuclear winter theory would play out in reality. While some more recent models suggest it could be better than he predicted, others show it might actually be even worse. The Earth is a very small stage.

in a vast cosmic arena. When someone tells you, that's Earth. On it, as Carl brilliantly said, everyone you ever loved, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives. That's it. That's the reality of our true circumstances in the universe. Think of the rivers of blood spilled by all those generals and emperors

so that in glory and triumph they could become the momentary masters of a fraction of a dot. You see how nonsensical it is to divide that planet up and to be willing to destroy it. To me, it underscores our responsibility to deal more kindly with one another and to preserve and cherish the Pale Blue Dot, the only home we've ever known.

That's it for this week's show. I'm Randabdil Fattah. And I'm Ramteen Arablui. And you've been listening to ThruLine from NPR. This episode was produced by me. And me. And. Lawrence Wu. Julie Kane. Anya Steinberg. Casey Miner. Christina Kim. Devin Katayama. Sarah Wyman. Irene Noguchi. Voice over work in this episode was done by Casey Hermans.

And thank you to NPR's Indicator podcast co-host and reporter, Waylon Wong, for letting her husband, Alec Naval Ali, use her home studio to record with us for this episode. Thank you to Jonnet Oaks, Keandre Starling, Johannes Dergi, Tony Cavan, Nadia Lansi, Edith Chapin, and Colin Campbell. Fact-checking for this episode was done by Kevin Vogel. This episode was mixed by Robert Rodriguez.

Music for this episode was composed by Ramtin and his band, Drop Electric, which includes... And finally, if you have an idea or like something you heard on the show, write us at throughline at npr.org. And if you don't already, please follow us on Apple, Spotify, and the NPR app. That way, you'll never miss an episode. Thanks for listening.

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