40 Years Of Sounding The Alarm On Nuclear Winter

A white plume of smoke forming a mushroom cloud in the sky
The “Ivy Mike” atmospheric nuclear test in November 1952. Credit: The official Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) Photostream

This week holds anniversaries for two important milestones in nuclear warfare. On November 1, 1952, the United States detonated a massive hydrogen bomb in the Marshall Islands. The new weapon vaporized a whole island, leaving behind a mile-wide crater. That bomb was around 700 times more powerful than the one dropped on Hiroshima seven years prior, and it renewed fears of nuclear annihilation, which would grip the world for generations to come.

Three decades later, on October 30, 1983, millions of Americans flipped open the Sunday paper to find a shadowy, apocalyptic photo with the words: “Would nuclear war be the end of the world?”

Legendary scientist Dr. Carl Sagan, writing for Parade Magazine, introduced the world to “nuclear winter,” the terrifying climate changes that might be brought on by nuclear war. 

Sagan conducted some of the first research on nuclear winter, and he spent years warning politicians, world leaders, and the general public about it. Today, with thousands of nuclear weapons still in existence, the risk of nuclear winter isn’t zero.

Ira talks with another pioneer in nuclear winter research, Dr. Alan Robock, a climate scientist and distinguished professor at Rutgers University, about the science of nuclear winter, how fear of those consequences shaped policies, and what’s happening with the world’s nuclear arsenal now.

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Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow.

This week holds anniversaries for two important milestones in nuclear warfare. November 1952, 71 years ago, the US detonated a terrible new weapon, a massive hydrogen bomb, in the Marshall Islands. It vaporized a whole island, leaving behind a mile-wide crater where the island once was. It was around 700 times more powerful than the bomb dropped on Hiroshima.

Fast forward to almost 40 years ago today, on October 30, 1983, millions of Americans flipped open the Sunday paper to find a shadowy gray, very apocalyptic photo, with the words, “Would nuclear war be the end of the world?” It introduced the world to nuclear winter, the terrifying climate changes that might be brought on by nuclear war.

And it was the iconic scientist, Carl Sagan, that introduced the term. He was an author of the first research on nuclear winter, spent years warning politicians, world leaders, and the general public.

CARL SAGAN: The idea that more nuclear weapons make you safer is an illusion. Beyond a certain point, more nuclear weapons make you less safe.

IRA FLATOW: The threat of nuclear winter has reduced since then. But with thousands of nuclear weapons still in existence, that risk is not zero.

My next guest is also a pioneer in nuclear winter research. Dr. Alan Robock, a climate scientist and distinguished professor at Rutgers University, based in New Brunswick, New Jersey. Welcome back to Science Friday.

ALAN ROBOCK: Thanks very much for having me. By the way, I want to correct one thing.


ALAN ROBOCK: The threat of nuclear winter has not gone down. It’s still with us.

IRA FLATOW: Why is that?

ALAN ROBOCK: Nuclear winter would be caused by smoke from fires started by nuclear weapons dropped on targets. And there are still way too many nuclear weapons in the world. There are enough to produce that much smoke and to produce nuclear winter today, even though the Russian and American arsenals, which make up more than 90% of the world arsenals, have been going down.

IRA FLATOW: Tell us about the details. Give us an idea– for people who did not live through those times like you and I did, talking about nuclear winter– what does the definition mean?

ALAN ROBOCK: If a nuclear war was held and bombs were targeted on cities, they would burn and produce lots of smoke. And the smoke would go up into the atmosphere. And maybe 3% or so of the material burned would turn into smoke, and it would go up to the upper part of the troposphere, which is where we live. But the sun would heat it and loft it into the stratosphere, which is the layer above that.

In the stratosphere, there’s no rain to wash the particles out, and they would last for years. Particles in the lower atmosphere last for a week or so. And so, if you calculate how much smoke there would be, it would last for many years and be blown around the world and absorb sunlight and make it cold and dark at the Earth’s surface. Temperatures could get below freezing even in the summertime in the middle of continents. That’s nuclear winter.

IRA FLATOW: Take us back to 1983. Let’s talk about the social and political atmosphere there. What was happening at that point in nuclear history?

ALAN ROBOCK: There was still an arms race. Everybody was worried about a nuclear war between the US and the Soviet Union. And there were massive demonstrations to try and ban the bomb, get rid of nuclear weapons. And this was in democracies, in the United States and in England in particular.

At the same time, a group of scientists in California calculated how much smoke there would be and what would be the effects if there was a nuclear war. And they published a paper in 1983. The Parade Magazine article you mentioned was written by Carl and came out before the actual journal article in the journal Science. And it said there would be nuclear winter if there was a nuclear war.

That was pretty shocking to people. Nobody had looked at these indirect effects of nuclear war. Everybody knew how horrific the direct effects would be– blast, fire, radioactivity– but nobody had looked at these indirect effects before.

IRA FLATOW: Yeah. And that Parade Magazine, I think that was the most popular magazine in the country because everybody got it bundled with their Sunday newspaper. That really did catch attention, right? People really paid attention to that.

ALAN ROBOCK: Absolutely. I was at the University of Maryland then. And there was a two-day workshop at the Shoreham Hotel, in Washington D.C., about this, where Carl came. But there was also a satellite connection to Moscow, and there were some Russian scientists, Alexandrov and Stenchikov, who had done the same calculation.

In fact, their paper was published a week before the TTAPS paper, the Nuclear Winter paper, but they got the same result. If you put that much smoke in the atmosphere, it will get cold and dark at the Earth’s surface. And so they were talking about it from Russia. And Americans were talking about it. And that also got a lot of publicity.

IRA FLATOW: And if I recall at that time, that’s important that you got similar results and it could not be considered propaganda by one country or the other.

ALAN ROBOCK: Absolutely. I was part of the Soviet-American scientific exchange, and we met Russians and worked with Russians. And we scientists have a lot in common. So it was collaborative and it was useful to work with multiple people.

So a couple of years later, Mikhail Gorbachev and Ronald Reagan met in Iceland to discuss what to do about the nuclear arms race. And they were both quoted as saying that they listened to the scientists from both sides and accepted the science because it was not propaganda. Ronald Reagan said a great many reputable scientists are telling us that such a war would end up in no victory for anyone because we would wipe out the Earth as we know it. So he called me reputable.


IRA FLATOW: But how refreshing it is to hear that politicians listen to scientists.

ALAN ROBOCK: Absolutely. And Gorbachev said models made by Russian and American scientists showed that a nuclear war would result in a nuclear winter that would be extremely destructive to all life on Earth.

So I think we played a role in helping convince them to stop the arms race and start reducing the arms race. They realized that we had way too many weapons already and building more was going to make us less safe, not safer.

IRA FLATOW: You must still get some satisfaction on that contribution.

ALAN ROBOCK: I do. I mean, once you’ve saved the world from nuclear weapons, what do you do with the rest of your life?


IRA FLATOW: Well, one thing you did is, last year, your study in Nature Food looked at the famine that could come again from nuclear winter. How many people today would that affect?

ALAN ROBOCK: So the number of weapons started going down in the 1980s, and it kept going down. But it didn’t stop going down. There’s still about 10,000 nuclear weapons in the world. And for the last decade, they really haven’t gone down at all.

So people said, OK, so there’s weapons, there’s smoke. How would it affect food? And so we calculated how much smoke there would be from different scenarios of a war between India and Pakistan or the US and Russia and how much it would affect the temperature, the precipitation, the amount of sunlight. And we calculated, with a crop model, how the major crops would grow in every country around the world.

And what we found is a war between India and Pakistan could produce famine in the next year. It could kill 1 to 2 billion people.


ALAN ROBOCK: A war between the US and Russia could kill more than 5 billion people– most of the people in the world– at places far removed from the war, where there were no bombs dropped. This is the indirect effect.

IRA FLATOW: So it wouldn’t even take an all out war between Russia and the US to kill a billion or so people?

ALAN ROBOCK: That’s right. And so that paper got a lot of attention because of those dramatic results. Nobody had actually calculated directly what the effects would be on food. We used data from the Food and Agriculture Organization, FAO, for every country– what do they grow? What did they eat? How much did they import and export?

We assume that trade would stop. Like we hoarded toilet paper during the pandemic, people would keep their food. So a few countries that don’t have many people, that are big exporters now, would have enough food– places like Argentina or Australia.

But if you think refugees are a problem now, there’d be flotillas of hungry people, arriving in their shores. So it’s not a decision just to go live there and wait and wait. So yes, it’s still a problem. And we’re still doing more work to try and use many more climate models, more crop models, and to see how robust our results are.

IRA FLATOW: And the Nuclear Club, the number of countries that have nuclear weapons, has increased.

ALAN ROBOCK: There are nine countries with nuclear weapons. And that hasn’t changed in quite a while. So there’s the five permanent members of the UN Security Council– just by chance– the US, Russia, England, France, and China. There’s India and Pakistan. And there’s North Korea and there’s Israel– although Israel doesn’t even admit that they have nuclear weapons.

IRA FLATOW: So between all of them, how many weapons do you think exist?

ALAN ROBOCK: There are about 12,000 altogether. And some of them in the US and Russian arsenals are not deployed. They’re on shelves. They’re sort of waiting to be dismantled. The US and Russia have a treaty, called the New START Treaty, which limits each country to 1,550 strategic weapons. Those are on bombers, missiles, and submarines. But each bomber counts as one. So maybe each country has about 2,000 that could be used.

But the other countries have a couple hundred. England, France, China, India, Pakistan, Israel, and North Korea might have 40 or so. So people claim that, what are our weapons for? Oh, we’re not going to attack somebody. They’re there for defense. They’re for deterrence. And so I asked them, how many weapons do you need to put onto your enemy’s capital to deter them from attacking you?

And when I ask audiences that, the answer is often one because it would be so horrible. So maybe they need two if the first one doesn’t work. So these countries decided that a couple of hundred are more than enough. But the US and Russia still have thousands. And so I don’t understand why we have so many.

IRA FLATOW: Why did you feel it was time– in recent history– like last year– to bring up the topic of nuclear winter again? Was there something going on that you detected?

ALAN ROBOCK: About 15 years ago, Brian Toon and Rich Turco approached me at a conference– the American Geophysical Union meeting– and said somebody asked us, what would happen if India and Pakistan had a nuclear war? Because these are two new nuclear nations. Each one had about 100 weapons at the time. And they calculated how much smoke would come.

And it wouldn’t be as much as from the US and Russia, but it would be more than 5 million tons of smoke. And they asked me to calculate the climate response. I had a student, Luke Oman, who was using our model for looking at volcanic eruptions, which put different sulfur gases up in the stratosphere and not smoke. And so we put it into our model. And we found temperatures would cool– it wouldn’t be nuclear winter, but it would be the greatest climate change in recorded human history. Temperatures would be colder than the Little Ice Age, about 3 degrees Fahrenheit colder than average.

And then we took our climate model– we said, in the 1980s, the climate models, the computers were very primitive– let’s go back and see if it really was nuclear winter. Because people criticized the models. And we put the smoke in from US and Russia. And sure enough, temperatures got below freezing in the summertime in Iowa, in Ukraine, places which are breadbaskets.

IRA FLATOW: It’s interesting that you talk about that now. Because I think that people of a certain age– my age, your age– we remember the existential threat posed by nuclear weapons. We remember people talking about nuclear winter. But today, with the Russians rattling their nuclear sabers again in Ukraine, I don’t get the sense that people understand just how deadly a threat nuclear weapons pose– if not from the military side, but from the nuclear winter side. Do you feel that also?

ALAN ROBOCK: No. In 2014, the International Campaign to Abolish Nuclear Weapons, which is an organization of anti-nuclear organizations, along with Alexander Kmentt, Austrian ambassador, organized three international conferences on the humanitarian impact of nuclear weapons. The direct effects– they had Hibakusha, people that had been there for the bombings of Hiroshima and Nagasaki– but also these indirect effects of starvation. And more than that, there was one in Norway, one in Mexico, one in Austria. More than 100 nations sent ambassadors– sent delegates to it.

And they were really concerned. They were really afraid of it. They got it. And then they went to the United Nations. And in 2017, in the United Nations General Assembly, they passed a treaty to prohibit nuclear weapons. And that treaty came into force after 50 countries ratified it. And now, half the countries in the world have signed it– 69 have ratified it.

So this is the will of the rest of the world, that we want the nuclear nations to get rid of them. We’re afraid. So what you said might be true in the United States, when people are concerned about other things and it hasn’t gotten to the top of their list of concerns. But the rest of the world is really afraid about it.

And ICAN, in 2017, got the Nobel Peace Prize for warning the world about these humanitarian impacts and for getting this treaty passed.

IRA FLATOW: This is Science Friday, from WNYC Studios.

In case you’re just joining us, I’m talking with Dr. Alan Robock about the threat of nuclear winter.

And speaking of prizes, you and other pioneers in nuclear winter work, including Carl Sagan, received the Future of Life Award last year. And basically, that’s an award for people who save the world. That must have great meaning to you because, as you said before, your research helped save humanity.

ALAN ROBOCK: Well, I really haven’t saved the world yet because we still have all these nuclear weapons.

In the 1980s, after I did my work, I published papers on nuclear winter– I was up for promotion. And I couldn’t get much funding for it, but I couldn’t not work on it. It was too important. So I kept working on it. Eventually, I did get promoted.

And so, yeah, it’s really rewarding to do something that’s– and I met my wife because I was at a conference– I was giving a talk about nuclear winter. I met Carl Sagan. I met Fidel Castro, who invited me down to Cuba to talk about this. And Cuba was one of the first countries to sign the Treaty on the Prohibition of Nuclear Weapons. And so it’s all connected. So nuclear winter has been good for me.


IRA FLATOW: You’ve had a wonderful life nuclear winter-wise, so to speak.


One last question before we go. We’re currently in another crisis that could change the world as we know it. And I’m talking about climate change. Are there any lessons learned from warning against nuclear winter that we could use today about climate change?

ALAN ROBOCK: Well, I’ve been working on climate change my whole career. I’m a climate scientist. The world is gradually warming. We know what’s causing it. And we know that the reason we haven’t solved the problem is because the fossil fuel companies are so powerful and rich that they stand in the way of switching the world to green energy.

The same is a problem with nuclear weapons. The military-industrial complex is making trillions of dollars building new weapons. When Obama got the New START Treaty signed, to get the Senate to agree to ratify it, he promised a modernization of our nuclear arsenal. And so now we’re spending over $1 trillion to build more missiles that are going to be buried in the ground out in the West that are never going to be used, more submarines, more bombers and weapons.

And so companies like Lockheed, Raytheon, and Boeing are making lots of money doing this. And they have lobbyists that are ensuring that people stay afraid and convince them that we need more weapons. And so the opposition is sort of similar. And it’s very frustrating that, as a lowly college professor, I don’t have the money to go out and lobby. But if people would go out and protest, like happened in the 1980s, we could change policy. But people have to get excited about it.

I’m glad you’re airing this interview so people can learn about this. But that’s the problem. That’s the similarities I see.

IRA FLATOW: Is there any hopeful note you can end on here?

ALAN ROBOCK: When Putin threatened to use nuclear weapons, that got people’s attention. The Oppenheimer movie got people’s attention. But when I was in Cuba, when I gave my talk and it was shown on nationwide television the next day– and when I walked into a hotel to see if I was on TV, there was a Julia Roberts movie on. And I said, could you change the channel? There, I was giving a lecture.

Which taught me that intellectually is not the way to do it. You have to touch people’s feelings. You need to have a movie with a beautiful scientist from St. Petersburg and a handsome scientist from Oak Ridge. Meanwhile, there’s a skirmish on the Kashmiri border.

So Brian Toon and I have written a book, which we want to get published, about dead dinosaurs and nuclear winter. We’re trying to get a movie made. We’re trying to get people’s attention that way, rather than just publishing journal articles– even in places like Scientific American.

IRA FLATOW: Well, we’ll have you back when that’s all done, OK, Alan.

ALAN ROBOCK: That would be great. Thank you.

IRA FLATOW: Dr. Alan Robock, climate scientist and distinguished professor at Rutgers University, based in New Brunswick, New Jersey.

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