Ariel: Hello, I’m Ariel Conn with the Future of Life Institute. If you've been listening to our previous podcasts, welcome back. If this is new for you, also welcome, but in any case, please take a moment to follow us, like the podcast, and maybe even share the podcast.

Today, I am excited to present Seth Baum and Robert de Neufville with the Global Catastrophic Risk Institute (GCRI). Seth is the Executive Director and Robert is the Director of Communications, he is also a super forecaster, and they have recently written a report called A Model for the Probability of Nuclear War. This was a really interesting paper that looks at 60 historical incidents that could have escalated to nuclear war and it basically presents a model for how we can determine what the odds are that we could have some type of nuclear war in the future. So, Seth and Robert, thank you so much for joining us today.

Seth: Thanks for having me.

Robert: Thanks, Ariel.

Ariel: Okay, so before we get too far into this, I was hoping that one or both of you could just talk a little bit about what the paper is and what prompted you to do this research, and then we'll go into more specifics about the paper itself.

Seth: Sure, I can talk about that a little bit. So the paper is a broad overview of the probability of nuclear war, and it has three main parts. One is a detailed background on how to think about the probability, explaining differences between the concept of probability versus the concept of frequency and related background in probability theory that's relevant for thinking about nuclear war. Then there is a model that scans across a wide range, maybe the entire range, but at least a very wide range of scenarios that could end up in nuclear war. And then finally, is a data set of historical incidents that at least had some potential to lead to nuclear war, and those incidents are organized in terms of the scenarios that are in the model. The historical incidents give us at least some indication of how likely each of those scenario types are to be.

Ariel: Okay. At the very, very start of the paper, you guys say that nuclear war doesn't get enough scholarly attention, and so I was wondering if you could explain why that's the case and what role this type of risk analysis can play in nuclear weapons policy.

Seth: Sure, I can talk to that. The paper, I believe, specifically says that the probability of nuclear war does not get much scholarly attention. In fact, we put a fair bit of time into trying to find every previous study that we could, and there was really, really little that we were able to find, and maybe we missed a few things, but my guess is that this is just about all that's out there and it's really not very much at all. We can only speculate on why there has not been more research of this type, my best guess is that the people who have studied nuclear war -- and there's a much larger literature on other aspects of nuclear war -- they just do not approach it from a risk perspective as we do, that they are inclined to think about nuclear war from other perspectives and focus on other aspects of it.

So the intersection of people who are both interested in studying nuclear war and tend to think in quantitative risk terms is a relatively small population of scholars, which is why there's been so little research, is at least my best guess.

Robert: Yeah, it's a really interesting question. I think that the tendency has been to think about it strategically, something we have control over, somebody makes a choice to push a button or not, and that makes sense from some perspective. I think there's also a way in which we want to think about it as something unthinkable. There hasn't been a nuclear detonation in a long time and we hope that there will never be another one, but I think that it's important to think about it this way so that we can find the ways that we can mitigate the risk. I think that's something that's been neglected.

Seth: Just one quick clarification, there have been very recent nuclear detonations, but those have all been tests detonations, not detonations in conflict.

Robert: Fair enough. Right, not a use in anger.

Ariel: That actually brings up a question that I have. As you guys point out in the paper, we've had one nuclear war and that was World War II, so we essentially have one data point. How do you address probability with so little actual data?

Seth: I would say “carefully,” and this is why the paper itself is very cautious with respect to quantification. We don't actually include any numbers for the probability of nuclear war in this paper.

The easy thing to do for calculating probabilities is when you have a large data set of that type of event. If you want to calculate the probability of dying in a car crash, for example, there's lots of data on that because it's something that happens with a fairly high frequency. Nuclear war, there's just one data point and it was under circumstances that are very different from what we have right now, World War II. Maybe there would be another world war, but no two world wars are the same. So we have to, instead, look at all the different types of evidence that we can bring in to get some understanding for how nuclear war could occur, which includes evidence about the process of going from calm into periods of tension, or the thought of going to nuclear war all the way to the actual decision to initiate nuclear war. And then also look at a wider set of historical data, which is something we did in this paper, looking at incidents that did not end up as nuclear wars, but pushed at least a little bit in that direction, to see what we can learn about how likely it is for things to go in the direction of nuclear war, which tells us at least something about how likely it is to get there all the way.

Ariel: Robert, I wanted to turn to you on that note, you were the person who did a lot of work figuring out what these 60 historical events were. How did you choose them?

Robert: Well, I wouldn't really say I chose them, I tried to just find every event that was there. There are a few things that we left out because we thought it falls below some threshold of the seriousness of the incident, but in theory you could probably expand it in the scope even a little wider than we did. But to some extent we just looked at what's publicly known. I think the data set is really valuable, I hope it's valuable, but one of the issues with it is it's kind of a convenience sample of the things that we know about, and some areas, some parts of history, are much better reported on than others. For example, we know a lot about the Cuban Missile Crisis in the 1960s, a lot of research has been done on that, there are the times when the US government has been fairly transparent about incidents, but we know less about other periods and other countries as well. We don't have incidents from China's nuclear program, but that doesn't mean there weren't any, it just means it's hard to figure out, and that scenario would be really interesting to do more research on.

Ariel: So, what was the threshold you were looking at to say, "Okay, I think this could have gone nuclear"?

Robert: Yeah, that's a really good question. It's somewhat hard to say. I think that a lot of these things are judgment calls. If you look at the history of incidents, I think a number of them have been blown a little bit out of proportion. As they've been retold, people like to say we came close to nuclear war, and that's not always true. There are other incidents which are genuinely hair-raising and then there are some incidents that seem very minor, that you could say maybe it could have gotten to a nuclear war. But there was some safety incident on an Air Force Base and they didn't follow procedures, and you could maybe tell yourself a story in which that led to a nuclear war, but at some point you make a judgment call and say, well, that doesn't seem like a serious issue.

But it wasn't like we have a really clear, well-defined line. In some ways, we'd like to broaden the data set so that we can include even smaller incidents just because the more incidents, the better as far as understanding, not the more incidents the better as far as being safe.

Ariel: Right. I'd like this question to go to both of you, as you were looking through these historical events, you mentioned that they were already public records so they're not new per se, but were there any that surprised you, and which were one or two that you found the most hair-raising?

Robert: Well, I would say one that surprised me, and this may just be because of my ignorance of certain parts of geopolitical history, but there was an incident with the USS Liberty in the Mediterranean, in which the Israelis mistook it for an Egyptian destroyer and they decided to take it out, essentially, not realizing it was actually an American research vessel, and they did, and what happened was the US scrambled planes to respond. The problem was that most of the planes, or the ordinary planes they would have ordinarily scrambled, were out on some other sorties, some exercise, something like that, and they ended up scrambling planes which had a nuclear payload on them. These planes were recalled pretty quickly. They mentioned this to Washington and the Secretary of Defense got on the line and said, "No, recall those planes," so it didn't get that far necessarily, but I found it a really shocking incident because it was a friendly fire confusion, essentially, and there were a number of cases like that in which nuclear weapons were involved because they happened to be on equipment where they shouldn't have been that was used to respond to some kind of a real or false emergency. That seems like a bigger issue than I would've at first expected, that just the fact that nuclear weapons are lying around somewhere where they could be involved with something.

Ariel: Wow, okay. And Seth?

Seth: Yeah. For me this was a really eye-opening experience. I had some familiarity with the history of incidents involving nuclear weapons, but there turned out to be much more that's gone on over the years than I really had any sense for. Some of it is because I'm not a historian, this is not my specialty, but there were any number of events that it appears that the nuclear weapons were, at least may have been, seriously considered for use in a conflict.

Just to pick one example, in 1954 and 1955 was known as the first Taiwan Straits Crisis, and the second crisis, by the way, in 1958, also included plans for nuclear weapons use. But in the first one there were plans made up by the United States, the Joint Chiefs of Staff allegedly recommended that nuclear weapons be used against China if the conflict intensified and that President Eisenhower was apparently pretty receptive to this idea. In the end, there was a ceasefire negotiated so it didn't come to that, but had that ceasefire not been made, my sense is that ... The historical record is not clear on whether the US would've used nuclear weapons or not, maybe even the US leadership hadn't made any final decisions on this matter, but there any number of these events, especially earlier in the years or decades after World War II when nuclear weapons were still relatively new, in which the use of nuclear weapons in conflict seemed to at least get a serious consideration that I might not have expected.

I'm accustomed to thinking of nuclear weapons as having a fairly substantial taboo attached to them, but I feel like the taboo has perhaps strengthened over the years, such that leadership now is less inclined to give the use of nuclear weapons serious consideration than it was back then. That may be mistaken, but that's the impression that I get and that we may be perhaps more fortunate to have gotten through the first couple decades after World War II without an additional nuclear war. But it might be less likely at this time, though still not entirely impossible by any means.

Ariel: Are you saying that you think the risk is higher now?

Seth: I think the risk is probably higher now. I think I would probably say that the risk is higher now than it was, say, 10 years ago because various relations between nuclear armed states have gotten worse, certainly including between the United States and Russia, but whether the probability of nuclear war is higher now versus in, say, the '50s or the '60s, that's much harder to say. That's a degree of detail that I don't think we can really comment on conclusively based on the research that we have at this point.

Ariel: Okay. In a little while I'm going to want to come back to current events and ask about that, but before I do that I want to touch first on the model itself, which lists four steps to a potential nuclear war: initiating the event, crisis, nuclear weapon use and full-scale nuclear war. Could you talk about what each of those four steps might be? And then I'm going to have follow-up questions about that next.

Seth: I can say a little bit about that. The model you're describing is a model that was used by our colleague, Martin Hellman, in a paper that he did on the probability of nuclear war, and that was probably the first paper that develops the study of the probability of nuclear war using the sort of methodology that we use in this paper, which is to develop nuclear war scenarios.

So the four steps in this model are four steps to go from a period of calm into a full-scale nuclear war. His paper was looking at the probability of nuclear war based on an event that is similar to the Cuban Missile Crisis, and what's distinctive about the Cuban Missile Crisis is we may have come close to going directly to nuclear war without any other type of conflicts in the first place. So that's where the initiating event and the crisis in this model comes from, it's this idea that there will be some of event that leads to a crisis, and the crisis will go straight to nuclear weapons use which could then scale to a full-scale nuclear war. The value of breaking it into those four steps is then you can look at each step in turn, think through the conditions for each of them to occur and maybe the probability of going from one step to the next, which you can use to evaluate the overall probability of that type of nuclear war. That's for one specific type of nuclear war. Our paper then tries to scan across the full range of different types of nuclear war, different nuclear war scenarios, and put that all into one broader model.

Ariel: Okay. Yeah, your paper talks about 14 scenarios, correct?

Seth: That's correct, yes.

Ariel: Okay, yeah. So I guess I have two questions for you: one, how did you come up with these 14 scenarios, and are there maybe a couple that you think are most worrisome?

Seth: So the first question we can definitely answer, we came up with them through our read of the nuclear war literature and our overall understanding of the risk and then iterating as we put the model together, thinking through what makes the most sense for how to organize the different types of nuclear war scenarios, and through that process, that's how we ended up with this model.

As far as which ones seem to be the most worrisome, I would say a big question is whether we should be more worried about intentional versus accidental, or inadvertent nuclear war. I feel like I still don't actually have a good answer to that question. Basically, should we be more worried about nuclear war that happens when a nuclear armed country decides to go ahead and start that nuclear war versus one where there's some type of accident or error, like a false alarm or the detonation of a nuclear weapon that was not intended to be an act of war? I still feel like I don't have a good sense for that.

Maybe the one thing I do feel is that it seems less likely that we would end up in a nuclear war from a detonation of a nuclear weapon that was not intentionally an act of war just because it feels to me like those events are less likely to happen. This would be nuclear terrorism or the accidental detonation of nuclear weapons, and even if it did happen it's relatively likely that they would be correctly diagnosed as not being an act of war. I'm not certain of this. I can think of some reasons why maybe we should be worried about that type of scenario, but especially looking at the historical data it felt like those historical incidents were a bit more of a stretch, a bit further away from actually ending up in nuclear war.

Robert, I'm actually curious, your reaction to that, if you agree or disagree with that.

Robert: Well, I don't think that non-state actors using a nuclear weapon is the big risk right now. But as far as whether it's more likely that we're going to get into a nuclear war through some kind of human error or a technological mistake, or whether it will be a deliberate act of war, I can think of scary things that have happened on both sides. I mean, the major thing that looms in one's mind when you think about this is the Cuban Missile Crisis, and that's an example of a crisis in which there were a lot of incidents during the course of that crisis where you think, well, this could've gone really badly, this could've gone the other way. So a crisis like that where tensions escalate and each country, or in this case the US and Russia, each thought the other might seriously threaten the homeland, I think are very scary.

On the other hand, there are incidents like the 1995 Norwegian rocket incident, which I find fairly alarming. In that incident, what happened was Norway was launching a scientific research rocket for studying the weather and had informed Russia that they were going to do this, but somehow that message hadn't got passed along to the radar technicians, so the radar technician saw what looked like a submarine launched ballistic missile that could have been used to do an EMP, a burst over Russia which would then maybe take out radar and could be the first move in a full-scale attack. So this is scary because this got passed up the chain and supposedly, President Boris Yeltsin, it was Yeltsin at the time, actually activated the nuclear football in case he needed to authorize a response.

Now, we don't really have a great sense how close anyone came to this, this is a little hyperbole after the fact, but this kind of thing seems like you could get there. And 1995 wasn't a time of big tension between the US and Russia, so this kind of thing is also pretty scary and I don't really know, I think that which risk you would find scarier depends a little bit on the current geopolitical climate. Right now, I might be most worried that the US would launch a bloody-nose attack against North Korea and North Korea would respond with a nuclear weapon, so it depends a little bit. I don't know the answer either, I guess, is my answer.

Ariel: Okay. You guys brought up a whole bunch of things that I had planned to ask about, which is good. I mean, one of my questions had been are you more worried about intentional or accidental nuclear war, and I guess the short answer is, you don't know? Is that fair to say?

Seth: Yeah, that's pretty fair to say. The short answer is, at least at this time, they both seem very much worth worrying about.

As far as which one we should be more worried about, this is actually a very important detail to try to resolve for policy purposes because this speaks directly to how we should manage our nuclear weapons. For example, if we are especially worried about accidental or inadvertent nuclear war, then we should keep nuclear weapons on a relatively low launch posture. They should not be on hair-trigger alert because when things are on a high-alert status, it takes relatively little for the nuclear weapons to be launched and makes it easier for a mistake to lead to a launch. Versus if we are more worried about intentional nuclear war, then there may be some value to having them on a high-alert status in order to have a more effective deterrence in order to convince the other side to not launch their nuclear weapons. So this is an important matter to try resolving, but at this point, based on the research that we have so far, it remains, I think, somewhat ambiguous.

Ariel: I do want to follow up with that. Everything I've read, there doesn't seem to be any benefit really to having things like our intercontinental ballistic missiles on hair-trigger alert, which are the ones that are on hair-trigger alert is my understanding, because submarines and the bombers still have the capability to strike back. Do you disagree with that?

Seth: I can't say for sure whether or not I do disagree with that because it's not something that I have looked at closely enough, so I would hesitate to comment on that matter. My general understanding is that hair-trigger alert is used as a means to enhance deterrence in order to make it less likely that either side would use their nuclear weapons in the first place, but regarding the specifics of it, that's not something that I've personally looked at closely enough to really be able to comment on.

Robert: I think Seth's right that it's a question that needs more research in a lot of ways and that we shouldn't answer it in the context of... We didn't figure out the answer to that in this paper. I will say, I would personally sleep better if they weren't on hair-trigger alert. My suspicion is that the big risk is not that one side launches some kind of decapitating first strike, I don't think that's really a very high risk, so I'm not as concerned as someone else might be about how well we need to deter that, how quickly we need to be able to respond. Whereas, I am very concerned about the possibility of an accident because... I mean, readings these incidents will make you concerned about it, I think. Some of them are really frightening. So that's my intuition, but, as Seth says, I don't think we really know. There’s more, at least in terms of this model, there’s more studying we need to do.

Seth: If I may, to one of your earlier questions regarding motivations for doing this research in the first place, I feel like to try giving more rigorous answers to some of these very basic nuclear weapons policy questions, like “should nuclear weapons be on hair-trigger alert, is that safer or more dangerous,” we can talk a little bit about what the trade-offs might be, but we don't really have much to say about how that trade-off actually would be resolved. This is where I think that it's important for the international security community to be trying harder to analyze the risks in these structured and, perhaps, even quantitative terms so that we can try to answer these questions more rigorously than just, this is my intuition, this is your intuition. That's really, I think, one of the main values for doing this type of research is to be able to answer these important policy questions with more confidence and also perhaps, more consensus across different points of view than we would otherwise be able to have.

Ariel: Right. I had wanted to continue with some of the risk questions, but while we're on the points that you're making, Seth, what do you see moving forward with this paper? I mean, it was a bummer to read the paper and not get what the probabilities of nuclear war actually are, just a model for how we can get there, how do you see either you, or other organizations, or researchers, moving forward to start calculating what the probability could actually be?

Seth: The paper does not give us final answers for what the probability would be, but it definitely makes some important steps in that direction. Additional steps that can be taken would include things like exploring the historical incidence data set more carefully to check to see if there may be important incidents that have been missed, to see for each of the incidents how close do we really think that that came to nuclear war? And this is something that the literature on these incidents actually diverges on. There are some people who look at these incidents and see them as being really close calls, other people look at them and see them as being evidence that the system works as it should, that, sure, there were some alarms but the alarms were handled the way that they should be handled and that the tools are in place to make sure that those don't end in nuclear war. So exactly how close these various incidents got is one important way forward towards quantifying the probability.

Another one is to come up with some sense for what the actual population of historical incidences relative to the data set that we have, we are presumably missing some number of historical incidents, some of them might be smaller and less important, but there might be some big ones that maybe they happened and we don't know about it because they are only in literatures in other languages, we only did research in English, or because all of the evidence about them is classified government records by whichever governments were involved in the incident, and so we need to-

Ariel: Actually, I do actually want to interrupt with a question real quick there, and my apologies for not having read this closer, I know there were incidents involving the US, Russia, and I think you guys had some about Israel. Were there incidents mentioning China or any of the European countries that have nuclear weapons?

Seth: Yeah, I think there were probably incidents involving all of the nuclear armed countries, certainly involving China. For example, China had a war with the Soviet Union over their border some years ago and there was at least some talk of nuclear weapons involved in that. Also, the one I mentioned earlier, the Taiwan Straits Crises, those involved China. Then there were multiple incidents between India and Pakistan, especially regarding the situation in Kashmir. With France, I believe we included one incident in which a French nuclear bomber got a faulty signal to take off in combat and then it was eventually recalled before it got too far. There might've been something with the UK also. Robert, do you recall if there were any with the UK?

Robert: Yes, there was, during the Falklands war, apparently, they left with nuclear depth charges. It's actually not really, honestly clear to me why you would use a nuclear depth charge, but there's not any evidence they ever intended to use them but they sent out nuclear armed ships, essentially, to deal with a crisis in the Falklands.

There's also, I think, an incident in South Africa as well when South Africa was briefly a nuclear state.

Ariel: Okay. Thanks. It's not at all disturbing.

Robert: It's very disturbing. I will say, I think that China is the one we know the least about. Some of the incidents that Seth mentioned with China, the danger or the nuclear armed power that might have used nuclear weapons was the United States. So there is the Soviet-China incident, but we don't really know a lot about the Chinese program and Chinese incidents. I think some of that is because it's not reported in English and to some extent it's also that it's classified and the Chinese are not as open about what's going on.

Seth: Yeah, the Chinese are definitely much, much less transparent than the United States, as are the Russians. I mean, the United States might be the most transparent out of all of the nuclear armed countries.

I remember some years ago when I was spending time at the United Nations I got the impression that the Russians and the Chinese were actually not quite sure what to make of the Americans’ transparency, that they found it hard to believe that the US government was not just putting out loads of propaganda and misinformation that it didn't make sense to them that we just actually put out a lot of honest data about government activities here, and that's just the standard and that you can actually trust this information, this data. So yeah, we may be significantly underestimating the number of incidents involving China and perhaps Russia and other countries because their governments are less transparent.

Ariel: Okay. That definitely addresses a question that I had, and my apologies for interrupting you earlier.

Seth: No, that's fine. But this is one aspect of the research that still remains to be done that would help us figure out what the probabilities might be. It would be a mistake to just calculate them based on the data set as it currently stands, because this is likely to be only a portion of the actual historical incidents that may have ended in nuclear war.

So these are the sorts of details and nuances that were, unfortunately, beyond the scope of the project that we were able to do, but it would be important work for us or other research groups to do to take us closer to having good probability estimates.

Ariel: Okay. I want to ask a few questions that, again, are probably going to be you guys guessing as opposed to having good, hard information, and I also wanted to touch a little bit on some current events. So first, one of the things that I hear a lot is that if a nuclear war is going to happen, it's much more likely to happen between India and Pakistan than, say, the US and Russia or US and ... I don't know about US and North Korea at this point, but I'm curious what your take on that is, do you feel that India and Pakistan are actually the greatest risk or do you think that's up in the air?

Robert: I mean, it's a really tough question. I would say that India and Pakistan is one of the scariest situations for sure. I don't think they have actually come that close, but it's not that difficult to imagine a scenario in which they would. I mean, these are nuclear powers that occasionally shoot at each other across the line of control, so I do think that's very scary.

But I also think, and this is an intuition, this isn't a conclusion that we have from the paper, but I also think that the danger of something happening between the United States and Russia is probably underestimated, because we're not in the Cold War anymore, relations aren't necessarily good, it's not clear what relations are, but people will say things like, "Well, neither side wants a war." Obviously neither side wants a war, but I think there's a danger of the kind of inadvertent escalation, miscalculation, and that hasn't really gone away. So that's something I think is probably not given enough attention. I'm also concerned about the situation in North Korea. I think that that is now an issue which we have to take somewhat seriously.

Seth: I think the last five years or so have been a really good learning opportunity for all of us on these matters. I remember having conversations with people about this, maybe five years ago, and they thought the thought of a nuclear war between the United States and Russia was just ridiculous, that that's antiquated Cold War talk, that the world has changed. And they were right and their characterization of the world as it was at that moment, but I was always uncomfortable with that because the world could change again. And sure enough, in the last five years, the world has changed very significantly that I think most people would agree makes the probability of nuclear war between the United States and Russia substantially higher than it was five years ago, especially starting with the Ukraine crisis.

There's also just a lot of basic volatility in the international system that I think is maybe underappreciated, that we might like to think of it as being more deterministic, more logical than it actually is. The classic example is that World War I maybe almost didn't happen, that it only happened because a very specific sequence of events happened that led to the assassination of Archduke Ferdinand and had that gone a little bit differently, he wouldn't have been assassinated and World War I wouldn't have happened and the world we live in now would be very different than what it is. Or, to take a more recent example, it's entirely possible that had the 2016 FBI director not made an unusual decision regarding the disclosure of information regarding one candidate's emails a couple weeks before the election, the outcome of the 2016 US election might've gone different and international politics would look quite different than it is right now. Who knows what will happen next year or the year after that.

So I think we can maybe make some generalizations about which conflicts seem more likely or less likely, especially at the moment, but we should be really cautious about what we think it's going to be overall over 5, 10, 20, 30 year periods just because things really can change substantially in ways that may be hard to see in advance.

Robert: Yeah, for me, one of the lessons of World War I is not so much that it might not have happened, I think it probably would have anyway -- although Seth is right, things can be very contingent -- but it's more that nobody really wanted World War I. I mean, at the time people thought it wouldn’t happen because it was sort of bad for everyone and no one thought, "Well, this is in our interest to pursue it," but wars can happen that way where countries end up thinking, for one reason or another, they need to go, they need to do one thing or another that leads to war when in fact everyone would prefer to have gotten together and avoided it. It's suboptimal equilibrium. So that's one thing.

The other thing is that, as Seth says, things change. I'm not that concerned about what's going on in the week that we're recording this, but we had this week the Russian ambassador saying he would shoot down US missiles aimed at Syria and the United States' president responding on Twitter, that they better get ready for his smart missiles. This is, I suspect, won't escalate to a nuclear war. I'm not losing that much asleep about it. But this is the kind of thing that you would like to see a lot less of, this is the kind of thing that's worrying and maybe you wouldn't have anticipated this 10 years ago.

Seth: When you say you're not losing much sleep on this, you're speaking as someone who has, as I understand, it very recently, actually, literally lost sleep over the threat of nuclear war, correct?

Robert: That's true. I was woken up early in the morning by an alert saying a ballistic missile was coming to my state, and that was very upsetting.

Ariel: Yes. So we should clarify, Robert lives in Hawaii.

Robert: I live in Hawaii. And because I take the risk of nuclear war seriously, I might've been more upset than some people, although I think that a large percentage of the population of Hawaii thought to themselves, "Maybe I'm going to die this morning. In fact, maybe, my family's going to die and my neighbors and the people at the coffee shop, and our cats and the guests who are visiting us," and it really brought home the danger, not that it should be obvious that nuclear war is unthinkable but when you actually face the idea ... I also had relatively recently read Hiroshima, John Hersey's account of, really, most of the aftermath of the bombing of Hiroshima, and it was easy to put myself in that and say, "Well, maybe I will be suffering from burns or looking for clean water," and of course, obviously, again, none of us deserve it. We may be responsible for US policy in some way because the United States is a democracy, but my friends, my family, my cat, none of us want any part of this. We don't want to get involved in a war with North Korea. So this really, I'd say, it really hit home.

Ariel: Well, I'm sorry you had to go through that.

Robert: Thank you.

Ariel: I hope you don't have to deal with it again. I hope none of us have to deal with that.

I do want to touch on what you've both been talking about, though, in terms of trying to determine the probability of a nuclear war over the short term where we're all saying, "Oh, it probably won't happen in the next week," but in the next hundred years it could. How do you look at the distinction in time in terms of figuring out the probability of whether something like this could happen?

Seth: That's a good technical question. Arguably, we shouldn't be talking about the probability of nuclear war as one thing. If anything, we should talk about the rate, or the frequency of it, that we might expect. If we're going to talk about the probability of something, that something should be a fairly specific distinct event. For example, an example we use in the paper, what's the probability of a given team, say, the Cleveland Indians, winning the World Series? It's good to say what's the probability of them winning the World Series in, say, 2018, but to say what's the probability of them winning the World Series overall, well, if you wait long enough, even the Cleveland Indians will probably eventually win the World Series as long as they continue to play them. When we wrote the paper we actually looked it up, and it said that they have about a 17% chance of winning the 2018 World Series even though they haven't won a World Series since like 1948. Poor Cleveland- sorry, I'm from Pittsburgh so I get to gloat a little bit.

But yeah, we should distinguish between saying what is the probability of any nuclear war happening this week or this year, versus how often we might expect nuclear wars to occur or what the total probability of any nuclear war happening over a century or whatever time period it might be.

Robert: Yeah. I think that over the course of the century, I mean, as I say, I'm probably not losing that much sleep on any given week, but over the course of a century if there's a probability of something really catastrophic, you have to do everything you can to try to mitigate that risk.

I think, honestly, some terrible things are going to happen in 21st century. I don't know what they are, but that's just how life is. I don't know which things they are. Maybe it will involve a nuclear war of some kind. But you can also differentiate among types of nuclear war. If one nuclear bomb is used in anger in the 21st century, that's terrible, but wouldn't be all that surprising or mean the destruction of the human race. But then there are the kinds nuclear wars that could potentially trigger a nuclear winter by kicking so much soot up into the atmosphere and blocking out the sun, and might actually threaten not just the people who were killed in the initial bombing, but the entire human race. That is something we need to look at, in some sense, even more seriously, even though the chance of that is probably a fair amount smaller than the chance of one nuclear weapon being used. Not that one nuclear weapon being used wouldn't be an incredibly catastrophic event as well, but I think with that kind of risk you really need to be very careful to try to minimize it as much possible.

Ariel: Real quick, I got to do a podcast with Brian Toon and Alan Robock a little while ago on nuclear winter, so we'll link to that in the transcript for anyone who wants to learn about nuclear winter, and you brought up a point that I was also curious about, and that is: what is the likelihood, do you guys think, of just one nuclear weapon being used and limited retaliation? Do you think that is actually possible or do you think if a nuclear weapon is used, it's more likely to completely escalate into full-scale nuclear war?

Robert: I personally do think that's possible because I think a number of the scenarios that would involve using a nuclear weapon or not between the United States and Russia, or even the United States and China, so I think that some scenarios involve a few nuclear weapons. If it were an incident with North Korea, you might worry that it would spread to Russia or China, but you can also see a scenario in which North Korea uses one or two nuclear weapons. Even with India and Pakistan, they don't necessarily, I wouldn't think they would necessarily, use all -- what do they have each, like a hundred or so nuclear weapons -- I wouldn't necessarily assume they would use them all. So there are scenarios in which just one or a few nuclear weapons would be used. I suspect those are the most likely scenarios, but it's really hard to know. We don't know the answer to that question.

Seth: There are even scenarios between the United States and Russia that involve one or just a small number of nuclear weapons, and the Russian military has the concept of the de-escalatory nuclear strike, which is the idea that if there is a major conflict that is emerging and might not be going in a favorable way for Russia, especially since their conventional military is not as strong as ours, that they may use a single nuclear weapon, basically, to demonstrate their seriousness on the matter in hopes of persuading us to back down. Now, whether or not we would actually back down or escalate it into an all-out nuclear war, I don't think that's something that we can really know in advance, but it's at least plausible. It's certainly plausible that that's what would happen and presumably, Russia considers this plausible which is why they talk about it in the first place. Not to just point fingers at Russia, this is essentially the same thing the NATO had in the earlier point in the Cold War when the Soviet Union had the larger conventional military and our plan was to use nuclear weapons in a limited basis in order to prevent the Soviet Union from conquering Western Europe with their military, so it is possible.

I think this is one of the biggest points of uncertainty for the overall risk, is if there is an initial use of nuclear weapons, how likely is it that additional nuclear weapons are used and how many and in what ways? I feel like despite having studied this a modest amount, I don't really have a good answer to that question. This is something that may be hard to figure out in general because it could ultimately depend on things like the personalities involved in that particular conflict, who the political and military leadership are and what they think of all of this. That's something that's pretty hard for us as outside analysts to characterize. But I think, both possibilities, either no escalation or lots of escalation, are possible as is everything in between.

Ariel: All right, so we've gone through most of the questions that I had about this paper now, thank you very much for answering those. You guys have also published a working paper this month called A Model for the Impacts of Nuclear War, but I was hoping you could maybe give us a quick summary of what is covered in that paper and why we should read it.

Seth: Risk overall is commonly quantified as the probability of some type of event multiplied by the severity of the impacts. So our first paper was on the probability side, this one's on the impact side, and it scans across the full range of different types of impacts that nuclear war could have looking at the five major impacts of nuclear weapons detonation, which is thermal radiation, blast, ionizing radiation, electromagnetic pulse and then finally, human perceptions, the ways that the detonation affects how people think and in turn, how we act. We, in this paper, built out a pretty detailed model that looks at all of the different details, or at least a lot of the various details, of what each of those five effects of nuclear weapons detonations would have and what that means in human terms.

Ariel: Were there any major or interesting findings from that that you want to share?

Seth: Well, the first thing that really struck me was, "Wow, there are a lot of ways of being killed by nuclear weapons." Most of the time when we think about nuclear detonations and how you can get killed by them, you think about, all right, there's the initial explosion and whether it's the blast itself or the buildings falling on you, or the fire, it might be the fire, or maybe it's a really high dose of radiation that you can get if you’re close enough to the detonation, that's probably how you can die. In our world of talking about global catastrophic risks, we also will think about the risk of nuclear winter and in particular, the effect that that can have on global agriculture. But there's a lot of other things that can happen too, especially related to the effect on physical infrastructure, or I should say civil infrastructure, roads, telecommunications, the overall economy when cities are destroyed in the war, those take out potentially major nodes in the global economy that can have any number of secondary effects, among other things.

It's just a really wide array of effects, and that's one thing that I'm happy for with this paper is that for, perhaps, the first time, it really tries to lay out all of these effects in one place and in a model form that can be used for a much more complete accounting of the total impact of nuclear war.

Ariel: Wow. Okay. Robert, was there anything you wanted to add there?

Robert: Well, I agree with Seth, it's astounding what the range, the sheer panoply of bad things that could happen, but I think that once you get into a situation where cities are being destroyed by nuclear weapons, or really anything being destroyed by nuclear weapons, it can unpredictable really fast. You don't know the effect on the global system. A lot of times, I think, when you talk about catastrophic risk, you're not simply talking about the impact of the initial event, but the long-term consequences it could have -- starting more wars, ongoing famines, a shock to the economic system that can cause political problems, so these are things that we need to look at more. I mean, it would be the same with any kind of thing we would call a catastrophic risk. If there were a pandemic disease, the main concern might not be the pandemic disease would wipe out everyone, but that the aftermath would cause so many problems that it would be difficult to recover from. I think that would be the same issue if there were a lot of nuclear weapons used.

Seth: Just to follow up on that, some important points here, one is that the secondary effects are more opaque. They're less clear. It's hard to know in advance what would happen. But then the second is the question of how much we should study them. A lot of people look at the secondary effect and say, "Oh, it's too hard to study. It's too unclear. Let's focus our attention on these other things that are easier to study." And maybe there's something to be said for that where if there's really just no way of knowing what might happen, then we should at least focus on the part that we are able to understand. I'm not convinced that that's true, maybe it is, but I think it's worth more effort than there has been to try to understand the secondary effects, see what we can say about them. I think there are a number of things that we can say about them. The various systems are not completely unknown, they're the systems that we live in now and we can say at least a few intelligent things about what might happen to those after a nuclear war or after other types of events.

Ariel: Okay. My final question for both of you then is, as we're talking about all these horrible things that could destroy humanity or at the very least, just kill and horribly maim way too many people, was there anything in your research that gave you hope?

Seth: That's a good question. I feel like one thing that gave me some hope is that, when I was working on the probability paper, it seemed that at least some of the events and historical incidents that I had been worried about might not have actually come as close to nuclear war as I previously thought they had. Also, a lot of the incidents were earlier within, say, the '40s, '50s, '60s, and less within the recent decades. That gave me some hope that maybe things are moving in the right direction.

But the other is that as you lay out all the different elements of both the probability and the impacts and see it in full how it all works, that really often points to opportunities that may be out there to reduce the risk and hopefully, some of those opportunities can be taken.

Robert: Yeah, I'd agree with that. I'd say there were certainly things in the list of historical incidents that I found really frightening, but I also thought that in a large number of incidents, the system, more or less, worked the way it should have, they caught the error of whatever kind it was and fixed it quickly. It's still alarming, I still would like there not to be incidents, and you can imagine that some of those could've not been fixed, but they were not all as bad as I had imagined at first. So that's one thing.

I think the other thing is, and I think Seth you were sort of indicating this, there's something we can do, we can think about how to reduce the risk, and we're not the only ones doing this kind of work. I think that people are starting to take efforts to reduce the risk of really major catastrophes more seriously now, and that kind of work does give me hope.

Ariel: Excellent. I'm going to end on something that ... It was just an interesting comment that I heard recently, and that was: Of all the existential risks that humanity faces, nuclear weapons actually seem the most hopeful because there's something that we can so clearly do something about. If we just had no nuclear weapons, nuclear weapons wouldn't be a risk, and I thought that was an interesting way to look at it.

Seth: I can actually comment on that idea. I would add that you would need not just to not have any nuclear weapons, but also not have the capability to make new nuclear weapons. There is some concern that if there aren't any nuclear weapons, then in a crisis there may be a rush to build some in order to give that side the advantage. So in order to really eliminate the probability of nuclear war, you would need to eliminate both the weapons themselves and the capacity to create them, and you would probably also want to have some monitoring measures so that the various countries had confidence that the other sides weren't cheating. I apologize for being a bit of a killjoy on that one.

Robert: I'm afraid you can't totally reduce the risk of any catastrophe, but there are ways we can mitigate the risk of nuclear war and other major risks too. There's work that can be done to reduce the risk.

Ariel: Okay, let’s end on that note. Thank you both very much!

Seth: Yeah. Thanks for having us.

Robert: Thanks, Ariel.

Ariel: If you’d like to read the papers discussed in this podcast or if you want to learn more about the threat of nuclear weapons and what you can do about it, please visit futureoflife.org and find this podcast on the homepage, where we’ll be sharing links in the introduction.