Nuclear war is unlikely to cause human extinction

by landfish10 min read7th Nov 202016 comments

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Nuclear WeaponsExistential Risk
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A number of people have claimed that a full-scale nuclear war is likely to cause human extinction. I have investigated this issue in depth and concluded that even a full-scale nuclear exchange is unlikely (<1%) to cause human extinction. 

By a full-scale war, I mean a nuclear exchange between major world powers, such as the US, Russia, and China, using the complete arsenals of each country. The total number of warheads today (14,000) is significantly smaller than during the height of the cold war (70,000). While extinction from nuclear war is unlikely today, it may become more likely if significantly more warheads are deployed or if designs of weapons change significantly.

There are three potential mechanisms of human extinction from nuclear war:

1) Kinetic destruction

2) Radiation

3) Climate alteration

Only 3) is remotely plausible with existing weapons, but let's go through them all.

1) Kinetic destruction

There simply aren't enough nuclear warheads to kill everyone directly with kinetic force, and there likely never will be. There are ~14,000 nuclear weapons in the world, and let’s suppose they have an average yield of something like 1 megaton. This is a conservative guess, the actual average is probably closer to 100 kilotons. With a 1 megaton warhead, you can create a fireball covering 3 km², and a moderate pressure wave that knocks down most residential houses covering 155 km². The former kills nearly everyone and the latter kills a decent percentage of people but not everyone. Let's be conservative and assume the pressure wave kills everyone in its radius. 14,000 * 155 = 2.17 million km². The New York Metro area is 8,683 km². So all the nuclear weapons in the world could destroy about 250 New York Metro areas. This is a lot! But not near enough, even if someone intentionally tried to hit all the populations at once. Total land surface of earth is: 510.1 million km². Urban area, by one estimate, is about 2%, or 10.2 million km.² Since the total possible area destroyed from nuclear weapons is ~2.17 million km² is considerably less than a lower bound on the area of human habitation, 10.2 million km², there should be basically no risk of human extinction from kinetic destruction.

The the circle with the white border indicates the zone of moderate blast damage radius (5 psi): 7.03 km (155 km²) from a 1,000 kiloton warhead, link to nukemap

If you want to check my work there, I was using nuke map

The even more obvious reason why kinetic damage wouldn't lead to human extinction is that nuclear states only threaten one or several countries at a time, and never the population centers of the entire world. Even if NATO countries and Russia and China all went to war at the same time, Africa, South America, and other neutral regions would be spared any kinetic damage. 

2) Radiation

Radiation won't kill everyone because there aren't enough weapons, and radiation from them would be concentrated in some areas and wholly absent from other areas. Even in the worst affected areas, lethal radiation from fallout would drop to survivable levels within weeks.

Here it's worth noting that there is an inherent tradeoff between length of halflife and energy released by radionuclides. The shorter the half life the more energy will be released, and the longer the half life the less energy. The fallout products from modern nuclear weapons are very lethal, but only for days to several weeks. 

From Nuclear War Survival Skills, 1987 edition

Let's try the same calculation we used with kinetic damage, and see if an attack aimed at optimizing fallout for killing everyone could succeed. Using Nukemap again, I'll go with the fallout contour for 100 rads per hour. 400 rads is thought too be enough to kill 50% of people, so 100 rads per hour is likely to kill most all people not in some kind of shelter. We need to switch to using a groundburst detonation rather than an airburst detonation, because groundbursts create far more fallout. A 1mt ground burst would create an area of about 8,000 km²  of >100 rads per hour. Okay, multiple that by 14,000 warheads, and we get 112 million km². That's a lot! It's still less than the  510.1 million km² of earth's land mass, but it's a lot more than the ~10.2 million km² of urban space. Presumably this is enough to cover every human habitation, so in principle, it might be possible to kill everyone with radiation from existing nuclear weapons.

 

The bright red and slightly less bright red indicate fallout contour for 1,000 rads and 100 rads per hour, covering 1,140 km² and 7,080 km² respectively, from a 1,000 kiloton ground burst. Nukemap settings

In practice, it would be almost impossible to kill every human via radiation with the existing nuclear arsenals, even if they were targeted explicitly for this purpose. The first reason is that fallout patterns are very uneven. After a ground burst, fallout is carried by the wind. Some areas will be hit bad and some areas will be hardly affected by fallout. Even if most human population centers were covered, a few areas would almost certainly escape.

Two other things make extinction by radiation unlikely. Many countries, especially in the southern hemisphere, are unlikely to be affected by fallout much at all. Since most of these countries are likely to be neutral in a conflict, and not near combatant countries, they should be relatively safe from fallout. While fallout might travel hundreds of kms, it still won't reach places separated by greater distances. Fallout that reaches the upper atmosphere will eventually fall back down, but usually after the period of lethal radioactivity.  The other mitigating factor is that in typical nuclear war plans, ground bursts are usually restricted to hardened targets, and air bursts are favored for population and industry centers. This is because air bursts maximize the size of the destructive pressure wave. Air burst detonations result in little lethal fallout reaching the ground, so populations not downwind of military targets would likely be safe from the worst of the radiological effects in a war scenario.

The final protection from extinction by radiation is simply large amounts of mass between people and the radiation source, in other words, fallout shelters. After several weeks, the radionuclides in fallout from ground burst detonations will have decayed to the point where humans can survive outside of shelters. Many fallout shelters exist in the world, and many more could be made easily in a day or two with a shovel, some ground, and some boards.  Even if lethally radioactive fallout from ground bursts covered all population centers, many humans would still survive in shelters.

The risks of extinction from nuclear-weapon-induced-radiation wouldn't be complete without discussing two factors: nuclear power plants and radiological weapons. I'm only going to cover these briefly, but they both don't change the conclusions much.

Nuclear power plants could be targeted by nuclear weapons to create large amounts of fallout with a longer half-life but less energy per unit time. The main concern here is that nuclear power plants and spent fuel sites contain a much greater *mass* of radioactive material than nuclear missiles can carry. The danger comes primarily from spreading the already very radiative spent or unspent nuclear fuel. The risk this poses requires a longer analysis, but the short version is that while nuking a nuclear power plant or stored fuel site would indeed create some pretty long-lived fallout it would still be concentrated in a relatively small area. Fortunately, even a nuclear detonation wouldn't spread the nuclear fuel more than several hundred km at most. Having regions of countries covered in spent nuclear fuel would be awful, but it doesn't much raise the risk of extinction.

Radiological weapons are nuclear weapons designed to maximize the spread of lethal fallout rather than destructive yield. The particular concern from the extinction perspective is that they can be designed to create fallout that continues to emit levels of radiation that can make an area uninhabitable for months to years. These kind of radiological weapons kill more slowly, but they still kill. In principle, radiological weapons could be used to kill everyone on earth. However, in practice, the same constraints that apply to standard nuclear weapons apply to weapons optimized for long-lasting fallout, as well as some additional constraints. 

Radiological weapons wouldn't produce more fallout than standard warheads, they would just produce fallout with different characteristics. As a result the amount of radiological weapons required to cover every part of earth's surface would be massively expensive (likely as expensive as the largest existing nuclear arsenals), and serve no military purpose. Their inefficiency in destruction and death compared to standard nuclear weapons is probably why radiological weapons have never been developed or deployed in large numbers. This makes them an ongoing theoretical concern, but not an existential risk in the immediate future. A concerning development is Russia's claim to have developed a large-yield (100mt) submersible nuclear weapon with the suggestion that it could be used as a radiological weapon, but even if this is true, it's unlikely to be deployed in large numbers.

3) Climate alteration

The bulk of the risk of human extinction from nuclear weapons come from risks of catastrophic climate change, nuclear winter, due to secondary effects from nuclear detonations. However, even in most full-scale nuclear exchange scenarios, the resulting climate effects are unlikely to cause human extinction.

Reasons for this:

a) Under scenarios where a severe nuclear winter occurs as described by Robock et al, some human populations would likely survive.
b) The Robock group’s models are probably overestimating the risk
c) Nuclear war planners are aware of nuclear winter risks and can incorporate these risks into their targeting plans

Before diving into each subject, it’s worth understanding the background of nuclear winter research. In the 1980s a group of atmospheric scientists proposed the hypothesis that a nuclear war would result in massive firestorms in burning cities, which would loft particles high into the atmosphere and cause catastrophic cooling that would last for years. Many found it alarming that such an effect could be possible and go unnoticed for decades while the risk existed. Some scientists also thought the proposed effect was too strong, or unlikely to occur at all. Until a few years ago, if you looked only at peer reviewed literature you would only find papers forecasting severe nuclear winter effects in the event of a nuclear war. Understandably, many people assumed that this was the scientific consensus. Unfortunately, this misrepresented the scientific community’s state of uncertainty about the risks of nuclear war. There have only ever been a small numbers of papers published about this topic (<15 probably), mostly from one group of researchers, despite the topic being one of existential importance.

I’m very glad Robock, Toon, and others have spent much of their careers studying nuclear winter effects, and their models are useful in estimating potential climate change caused by nuclear war. However, I’ve become less convinced over time the Robock model is largely correct. See section B below for why I’ve changed my mind. However, I’m quite uncertain about the probability of strong cooling effects from nuclear war, and am still quite concerned about the potential for severe cooling, even if the risk of extinction from such events is small.

A:  Under scenarios where a severe nuclear winter occurs as described by Robock et al, some human populations would likely survive.

The latest and most detailed model of potential cooling effects from a fullscale nuclear exchange comes from, Robock et al.,  “Nuclear winter revisited with a modern climate model and current nuclear arsenals: Still catastrophic consequences” found here.

The effects from this model are severe. In the 150Tg case, after a year, summer temperatures in the Northern hemisphere are 10-30 degrees C cooler. The effects are less severe at the equator (5 degrees C), but basically all places in the world are affected. The most likely outcome is that most people starve to death. Many would freeze too, but starvation is likely the greatest risk. Even in this model, it appears that in equatorial regions, some farming would still be possible, enough for some populations to survive. After a 10-15 years, agriculture in most of the world would be possible at reduced capacity.

Surface air temperature changes for the 150 Tg case averaged for June, July, and August of the year of smoke injection and the next year. Robock et al., 2007


Carl Shulman asked one of the authors of this paper, Luke Oman, his probability that the 150Tg nuclear winter scenario discussed in the paper would result in human extinction, the answer he gave was “in the range of 1 in 10,000 to 1 in 100,000.” This strikes me as quite plausible, though one expert opinion is no substitute for a deep analysis. The Q&A with Oman contains his reasoning for this assessment.

Two different analyses are required to calculate the chances of human extinction from nuclear winter. The first is the analysis of the climate change that could result from a nuclear war, and the second is the adaptive capacity of human groups to these climate changes. I have not seen an in depth analysis of the latter, but I believe such an assessment would be worthwhile.  

My own guess is that humans are capable of surviving far more severe climate shifts than those projected in nuclear winter scenarios. Humans are more robust than most any other mammal to drastic changes in temperature, as evidenced by our global range, even in pre-historic times. While a loss of most agriculture would likely kill most people on earth, modern technology would enable some populations to survive. Great stores of food currently exist in the world, and it is l likely that some of these would be seized and protected by small groups, providing enough food to last for years. While even such populations with food stores wouldn’t have enough to survive for 10-15 years, such food stores would give groups time to adapt to new food sources. The organization ALLFED has explored a number of alternative food sources that could keep populations alive in the event of a nuclear war or other large solar disruption, and I expect great necessity to drive the discovery of even more in the event of such a disaster.

B: The Robock group’s models are probably overestimating the risk

The nuclear winter model at its simplest: Nuclear detonations → Fires in cities → Firestorms in cities → Lofted black carbon into the upper atmosphere → black carbon persists in upper atmosphere, reflecting sunlight and causes massive cooling

Each step is required in order for the effect to occur. If nuclear war causes massive fires in cities but does not lead to firestorms that loft particles, then no long term cooling is going to occur. Some of these steps are easier to model than others. Based on my reading of the literature, the greatest uncertainties involve the dynamics of cities burning after a nuclear attack, and whether the conditions would produce firestorms sufficient to loft large numbers of particles high enough in the atmosphere to persist for years.

We’re finally beginning to see some healthy debate about some of these questions in the scientific literature. Alan Robock’s group published a paper in 2007 that found significant cooling effects even from a relatively limited regional war. A group from Los Alamos, Reisner et al, published a paper in 2018 that reexamined some of the assumptions that went into Robock et al’s model, and concluded that global cooling was unlikely in such a scenario. Robock et al. responded, and Reisner et al responded to the response. Both authors bring up good points, but I find Reisner’s position more compelling. This back and forth is worth reading for those who want to investigate deeper. Unfortunately Reisner’s group has not published an analysis on potential cooling effects from a modern full scale nuclear exchange, rather than a limited regional exchange. Even so, it’s not hard to extrapolate that Reisner’s model would result in far less cooling than Robock’s model in the equivalent situation.

C: Nuclear war planners are aware of nuclear winter risks and can incorporate these risks into their targeting plans

A very simple way to reduce risks from nuclear winter is to refrain from targeting cities with nuclear weapons. The proposed mechanism behind nuclear winter results from cities burning, not ground bursts on military targets. I’ve spoken with some of the officials in the US defense establishment responsible for nuclear war planning, and they’re well aware of the potential risks from nuclear winter. Of course, being aware of the risks does not guarantee they will have reasoned about the risks well, or have engaged in good risk management practices. However, the fact that this risk is well publicized makes it more likely that nuclear war planners will take steps to minimize blowback risk from climate effects.

It’s hard to know to what extent this has been done. Nuclear war plans are classified, and as far as we know current US nuclear war plans do target cities under some circumstances but not under others. However, the defense establishment has access to classified information and models that we civilians do not have, in addition to all the public material. I’m confident that nuclear war planners have thought deeply about the risks of climate change from nuclear war, even though I don’t know their conclusions or bureaucratic constraints. All else being equal, the knowledge of these risks makes military planners less likely to accidentally cause human extinction.

Conclusion

This post discussed the three plausible mechanisms of human extinction caused by nuclear weapons. The fact that one of these mechanisms, nuclear winter, wasn’t characterized until the 1980s, is a good reminder of the possibility of unknown unknowns. While nuclear tests provided information about the effects of these weapons, the test environments were significantly different than war environments. Large model uncertainties remain. Given that the greatest existential threat from nuclear war appears to be from climate impacts, it would be great to see more researchers study the climate effects from nuclear war and the resilience capacity of different human groups.

There appear to be several interventions possible for reducing existential risk from nuclear war. At the policy level, a commitment from the largest nuclear powers to refrain from targeting the majority of cities would reduce risk of accidental omnicide. Improving the maximum resilience capacity of human populations best positioned to survive a nuclear winter would also make humanity less vulnerable to nuclear winter, and could also protect against other existential threats.

Further reading

Toby Ord conducts a quantitative estimate of extinction risk from nuclear war in:
The Precipice: Existential Risk and the Future of Humanity

Nuclear War as a Global Catastrophic Risk

Nuclear winter and human extinction: Q&A with Luke Oman (by Carl Shulman)

http://www.overcomingbias.com/2012/11/nuclear-winter-and-human-extinction-qa-with-luke-oman.html

Nuclear Winter Responses to Nuclear War Between the United States and Russia in the Whole Atmosphere Community Climate Model Version 4 and the Goddard Institute for Space Studies ModelE

https://www.semanticscholar.org/paper/Nuclear-Winter-Responses-to-Nuclear-War-Between-the-Coupe-Bardeen/560033106c2d599bcace3ce4cb6c67d5b713ec50

Climate Impact of a Regional Nuclear Weapons Exchange: An Improved Assessment Based On Detailed Source Calculations

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2017JD027331?fbclid=IwAR0SlQ_naiKY5k27PL0XlY-3jsocG3lomUXGf3J1g8GunDV8DPNd7birz1w

Comment on “Climate Impact of a Regional Nuclear Weapon Exchange: An Improved Assessment Based on Detailed Source Calculations” by Reisner et al.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2019JD030777

Reply to Comment by Robock et al. on “Climate Impact of a Regional Nuclear Weapon Exchange: An Improved Assessment Based on Detailed Source Calculations”

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2019JD031281 

Comparing Economic and Crop Models: The Case of Climatic and Agricultural Impacts of Nuclear War

https://www.gtap.agecon.purdue.edu/resources/download/9185.pdf

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I agree that nuclear war - and even nuclear winter - would be very unlikely to directly cause human extinction. My loose impression is that other EAs who have looked into this agree as well.

However, I'm not sure if it's good to publicize work on existential risk from nuclear war under this headline, and with this scope. Here is why:

  • You only discuss whether nuclear war would somewhat directly cause human extinction - i.e. by either immediately killing everyone, or causing everyone to starve within, say, the next 20 years. However, you don't discuss whether nuclear war could cause a trajectory change of human civilization that make it more vulnerable to future existential risk. For example, if nuclear war would cause an irrecoverable loss of post-industrial levels of technology that would arguably constitute an existential catastrophe itself (by basically removing the chance of close-to-optimal futures) and also make humanity more vulnerable to natural extinction risk (e.g. they can no longer do asteroid deflection). FWIW, I think the example I just gave is fairly unlikely as well; my point here just is that your post doesn't tell us anything about such considerations. It would be entirely consistent with all evidence you present to think that nuclear war is a major indirect existential risk (in the sense just discussed).
  • For this reason, I in particular disagree "that the greatest existential threat from nuclear war appears to be from climate impacts" (as you say in the conclusion). I think that in fact the possibly greatest existential threat from nuclear war is a negative trajectory change precipitated by 'the collapse of civilization', though we don't really know how likely that is or whether this would in fact be negative on extremely long timescales.
  • (Note I'm not intending for this to just be a special case of the true but somewhat vacuous general claim that, for all we know, literally any event could cause a negative or positive trajectory change. The point is that the unprecedented damage caused by large-scale nuclear war seems unusually likely to cause a trajectory change in either direction.)
  • [Less important:] I'm somewhat less optimistic than you about point 3C, i.e. nuclear war planners being aware of nuclear winter. I agree they are aware of the risk. However, I'm not sure if they have incentives to care. They might not care if they view "large-scale nuclear war that causes all our major cities to be destroyed" or "nuclear war that leads to a total defeat by an adversary" as essentially the worst possible outcomes, which seems at least plausible to me. Certainly I think they won't care about the risk as much as a typical longtermist - from an impartial perspective, even a, say, 1% risk of nuclear winter would be very concerning, whereas it could plausibly be a minor consideration when planning nuclear war from a more parochial perspective. Perhaps even more importantly, even if they did care as much as a longtermist, it's not clear to me if the strategic dynamics allow them to adjust their policies. For example, a nuclear war planner may well think that only a 'countervalue' strategy of targeting adversaries' population centers has a sufficient deterrence effect.

So overall I think our epistemic situation is: We know that one type of existential risk from nuclear war is very small, but we don't really have a good idea for how large total existential risk from nuclear war is. It's of course fine, and often a good idea for tractability or presentation reasons, to focus on only one aspect of a problem. But given this epistemic situation, I think the cost of spreading a message that can easily be rounded off to "nuclear war isn't that dangerous [from a longtermist perspective]" are high, particularly since perceptions that nuclear war would be extremely bad may be partly causally responsible for the fact that we haven't yet seen one.

Note I'm not claiming that this post by itself has large negative consequences. No nuclear power is going to chance their policies because of an EA Forum post. But I'd be concerned if there was a growing body of EA work with a messaging like this. For future public work I'd feel better if the summary was more like "nuclear war wouldn't kill every last human within a few decades, but is still extremely concerning from both a long-termist and present-generation perspective" + some constructive implications (e.g. perhaps focus more on how to make post-collapse recovery more likely or to go well).

I think I gave the impression that I'm making  a more expansive claim than I actually mean to make, and will edit the post to clarify this.  The main reason I wanted to write this post is that a lot of people, including a number in the EA community, start with the conception that a nuclear war is relatively likely to kill everyone, either for nebulous reason or because of nuclear winter specifically. I know most people who've examined it know this is wrong, but I wanted that information to be laid out pretty clearly, so someone could get a summary of this argument. I think that's just the beginning in assessing existential risk from nuclear war, and I really wouldn't want people to read my post and walk away thinking "nuclear war is nothing to worry about from a longtermist perspective." 

I agree that "We know that one type of existential risk from nuclear war is very small, but we don't really have a good idea for how large total existential risk from nuclear war". I'm planning to follow this post with a discussion of existential risks from compounding risks like nuclear war, climate change, biotech accidents, bioweapons, & others.

It feels like I disagree with you on the likelihood that a collapse induced by nuclear war would lead to permanent loss of humanity's potential / eventual extinction. I currently think humans would retain the most significant basic survival technologies following a collapse and then reacquire lost technological capacities relatively quickly. (I discussed this investigation here though not in depth). I'm planning too write this up as part of my compounding risks post or as a separate one.

Agreed that it's very hard to know the sign on a huge history-altering event, whether it's a nuclear war or covid.

The main reason I wanted to write this post is that a lot of people, including a number in the EA community, start with the conception that a nuclear war is relatively likely to kill everyone, either for nebulous reason or because of nuclear winter specifically.

This agrees with my impression, and I do think it's valuable to correct this misconception. (Sorry, I think it would have been better and clearer if I had said this in my first comment.) This is why I favor work with somewhat changed messaging/emphasis over no work.

It feels like I disagree with you on the likelihood that a collapse induced by nuclear war would lead to permanent loss of humanity's potential / eventual extinction.

I'm not sure we disagree. My current best guess is that most plausible kinds of civilizational collapse wouldn't be an existential risk, including collapse caused by nuclear war. (For basically the reasons you mention.) However, I feel way less confident about this than about the claim that nuclear war wouldn't immediately kill everyone. In any case, my point was not that I in fact think this is likely, but just that it's sufficiently non-obvious that it would be costly if people walked away with the impression that it's definitely not a problem.

I'm planning to follow this post with a discussion of existential risks from compounding risks like nuclear war, climate change, biotech accidents, bioweapons, & others.

This sounds like a very valuable topic, and I'm excited to see more work on it. 

FWIW, my guess is that you're already planning to do this, but I think it could be valuable to carefully consider information hazards before publishing on this [both because of messaging issues similar to the one we discussed here and potentially on the substance, e.g. unclear if it'd be good to describe in detail "here is how this combination of different hazards could kill everyone"]. So I think e.g. asking a bunch of people what they think prior to publication could be good. (I'd be happy to review a post prior to publication, though I'm not sure if I'm particularly qualified.)

FWIW, my guess is that you're already planning to do this, but I think it could be valuable to carefully consider information hazards before publishing on this [both because of messaging issues similar to the one we discussed here and potentially on the substance, e.g. unclear if it'd be good to describe in detail "here is how this combination of different hazards could kill everyone"]. So I think e.g. asking a bunch of people what they think prior to publication could be good. (I'd be happy to review a post prior to publication, though I'm not sure if I'm particularly qualified.)

Yes, I was planning to get review prior to publishing this. In general when it comes to risks from biotechnology, I'm trying to follow the principles we developed here: https://www.lesswrong.com/posts/ygFc4caQ6Nws62dSW/bioinfohazards I'd be excited to see, or help workshop, better guidance for navigating information hazards in this space in the future. 

I think I gave the impression that I'm making  a more expansive claim than I actually mean to make, and will edit the post to clarify this.

Have you made these edits yet, or is this still on the to-do list? Having just read the post, I strongly agree with Max's assessment, and still think readers could very easily round this post's claims off to "Nuclear war is very unlikely to be a big deal for longtermists". The key changes that I'd see as valuable would be: 

  • changing the title (maybe to something like "Nuclear war is unlikely to directly cause human extinction")
  • explicitly saying something in the introductory part about how the possibilities of nuclear war causing indirect extinction or other existential catastrophes/trajectory changes are beyond the scope of this post
  • (There may of course also be other changes that would accomplish similar results)

I also do think that this post contains quite valuable info. And I'd agree that there are some people, including in the EA community, who seem much too confident that nuclear war would directly cause extinction (though, like Max, I'm not aware of anyone who meets that description and has looked into the topic much). 

So if this post had had roughly those tweaks / when you make roughly those tweaks, I'd think it'd be quite valuable. (Unfortunately, in its present form, I worry that the post might create more confusion than it resolves.)

I'd also be excited to see the sort of future work you describe on compounding risks and recovery from collapse! I think those topics are plausibly important and sorely under-explored. 

I agree it's very unlikely that a nuclear war discharging current arsenals could directly cause human extinction. But the conditional probability of extinction given all-out nuclear war can go much higher if the problem gets worse. Some aspects of this:

-at the peak of the Cold War arsenals there were over 70,000  nuclear weapons, not 14,000
-this Brookings estimate puts spending building the US nuclear arsenal at several trillion current dollars, with lower marginal costs per weapon, e.g. $20M per weapon and $50-100M all-in for for ICBMs
-economic growth since then means the world could already afford far larger arsenals in a renewed arms race
-current US military expenditure is over $700B annually,  about 1/30th of GDP; at the peak of the Cold War in the 50s and 60s it was about 1/10th; Soviet expenditure was proportionally higher
-so with 1950s proportional military expenditures, half going to nukes, the US and China could each produce 20,000+ ICBMs, each of which could be fitted with MIRVs and several warheads, building up to millions of warheads over a decade or so; the numbers could be higher for cheaper delivery systems
-economies of scale and improvements in technology would likely bring down the per warhead cost
-if AI and robotics greatly increase economic growth the above numbers could be increased by orders of magnitude
-radiation effects could be intentionally greatly increased with alternative warhead composition
-all-out discharge of strategic nuclear arsenals is also much more likely to be accompanied by simultaneous deployment of other WMD, including pandemic bioweapons (which the Soviets pursued as a strategic weapon for such circumstances)and drone swarms (which might kill survivors in bunkers); the combined effects of future versions of all of these WMD at once may synergistically cause extinction 

This may be in the Brookings estimate, which I haven't read yet, but I wonder how much cost disease + reduction in nuclear force has affected the cost per warhead / missile. My understanding is that many military weapon systems get much more expensive over time for reasons I don't well understand.

Warheads could be altered to increase the duration of radiation effects from fallout, but this would would also reduce their yield, and would represent a pretty large change in strategy. We've gone 70 years without such weapons, which the recent Russian submersible system as a possible exception. It seems unlikely such a shift in strategy will occur in the next 70 years, but like 3% unlikely rather than really unlikely.

It's a good point that risks of extinction could get significantly worse if different/more nuclear weapons were built & deployed, and combined with other WMDs. And the existence of 70k+ weapons in the cold war presents a decent outside view argument that we might see that many in the future. I'll edit the post to clarify that I mean present and not future risks from nuclear war.

Thanks for the post. The updated nuclear winter modeling is Coupe 2019 - it has similar results to the 2007 work. The new work was funded by Open Phil. I agree that extinction is quite unlikely, but I think there are a few routes to get there. Many people mention the lower climate impacts in places like New Zealand, but they do not consider the possibility of refugees overwhelming New Zealand. You may very well be right that some food storage will be protected, but I don't think it's guaranteed that people could or would forcibly repel the desperate people trying to get food. Furthermore, if there is an eventual collapse of anthropological civilization (cooperation outside tribes), we may have to go back to hunting and gathering, and there is evidence that that transition may not go well (The Secret of Our Success book). And the current hunter gatherers generally don't have much food storage, so they would likely die out.

But then I agree with Max Daniel that even if extinction is unlikely, I think worse trajectory changes have significant probability, which is supported by the poll of GCR researchers here.

As Robock points out, even without firestorms (simultaneous burning of a large area), some smoke can go into the upper troposphere and be lofted into the stratosphere. Indeed, this has been demonstrated for wildfires (which are not firestorms).

There are four different spellings of 'Reisner' (which is correct) in this paragraph:

Alan Robock’s group published a paper in 2007 that found significant cooling effects even from a relatively limited regional war. A group from Los Alamos, Reisner et al, published a paper in 2018 that reexamined some of the assumptions that went into Robock et al’s model, and concluded that global cooling was unlikely in such a scenario. Robock et al. responded, and Riesner et al responded to the response. Both authors bring up good points, but I find Rieser’s position more compelling. This back and forth is worth reading for those who want to investigate deeper. Unfortunately Reiser’s group has not published an analysis on potential cooling effects from a modern full scale nuclear exchange, rather than a limited regional exchange. Even so, it’s not hard to extrapolate that Reiser’s model would result in far less cooling than Robock’s model in the equivalent situation.

Thanks, fixed!

Two different analyses are required to calculate the chances of human extinction from nuclear winter. The first is the analysis of the climate change that could result from a nuclear war, and the second is the adaptive capacity of human groups to these climate changes. I have not seen an in depth analysis of the former, but I believe such an assessment would be worthwhile.  

Do you mean "I have not seen an in depth analysis of the latter"? I.e. humans' adaptive capacity?

Thanks, fixed!

This all seems very plausible to me, I've always been sceptical of the idea that nuclear war (or climate change, or a pandemic) could kill all humans on the planet. There's a lot of us, we're very widely distributed and we're very adaptable, we'd probably just end up with a new world order dominated by Australia, New Zealand, Africa and Latin America. It would be great to have more research on nuclear winter, especially since it would overlap with climate modelling and potentially with geoengineering projects designed to deliberately cool the planet, so understanding this better would be great for the future.

Nuclear war would still be very bad, and  I think we should probably be concerned with non-existential risks, since after a near miss it would take a very long time for things to return to where they are today.

The idea that we should lobby the military to be selective in its nuclear targeting is interesting, I'm not sure how tractable that is but the pessimist in me does suspect that militaries are the only branch of government incentivised to think long term.

It seems to me that this post fills an important void. Several commenters have said that they fear that letting people know that nuclear war wouldn't mean the end of the world would encourage them to take nuclear war less seriously.

First, let me say that I've spent forty years working to try to reduce the danger of nuclear war and (as I've gotten older) to eliminate nuclear weapons. I've received outside funding during only six of those years. Not asking for sympathy, I just want you to realize that I am quite serious about this stuff.

Partly because of that seriousness, I think it is essential to foreground reality in all arguments against nuclear weapons. In fact, I think it is the key to elimination.

I've spent some time studying attitudes toward nuclear war and I think landfish is exactly right that people often have unrealistic ideas about it. Mostly they exaggerate its consequences. I believe these exaggerations are far more harmful to attempts to reduce or eliminate nuclear weapons than any realistic explanation of the reality might be.

Let me explain.

For the most part, ordinary people, when they don't use the exact words "nuclear war" refer to nuclear war as "the end of the world" or "apocalypse" or "Armageddon." These terms are colorful, exaggerated, and unrealistic. They are, in some ways, fascinating anthropological evidence. Two of these terms are religious terms (apocalypse and Armageddon). Why would it make sense to refer to a twenty-first century military phenomenon (nuclear war), in religious or mythical terms?

Why would millions of people use an apocalyptic story dating back thousands of years to refer to nuclear war? The short answer is that I think in some way apocalypse is a narrative that they are familiar with. They take a strange and uncomfortable phenomenon, like nuclear war, and they make it familiar and less threatening by covering it with a story they know.

But there is an important point about apocalypse stories -- they are all in the hands of God. If nuclear war is really the apocalypse then it is out of our control. Only God controls the apocalypse. Only God determines where the Last Battle at the End of Days will be fought (a hill called Armageddon).

Exaggerating nuclear war to mean the end of the world makes it horrible, no doubt, but it also makes it (perhaps comfortingly) beyond our control. It gives us a pass not to worry about the problem.

At least that is the way it seems to me.

I think facing the realities of nuclear war is essential.

the amount of radiological weapons radiological weapons required to cover every part of earth's surface would be massively expensive (likely as expensive as the largest existing nuclear arsenals), and serve no military purpose.

Do you mean that radiological weapons serve no military purpose, or that the amount required to cover every part of the Earth's surface  would serve no military purpose (relative to a more moderate amount of them)?

If the former, that claim surprises me, given that: 

  • a large part of the military purpose of nuclear weapons in general is deterrence
  • it sounds like radiological weapons would in some cases cause more deaths per kiloton than standard nuclear weapons
    • So other nations may in some cases be more deterred by a country with some radiological weapons than a country with only standard nuclear weapons

If the latter, then it seems like your analysis is consistent with the view that there may still end up being a quite large amount of radiological weapons? If so, that seems potentially important, as it could perhaps affect indirect extinction risks or other existential risks from nuclear war (e.g., by perhaps increasing the expected number of deaths from a nuclear exchange, and changing where and how those deaths occur).

(Note: I don't really know much about radiological weapons, have no particular reason to believe governments are likely to develop them, and would indeed be surprised if they developed enough to cover the Earth's surface.)

many people assumed that this was the scientific consensus. Unfortunately, this misrepresented the scientific community’s state of uncertainty about the risks of nuclear war. There have only ever been a small numbers of papers published about this topic (<15 probably), mostly from one group of researchers, despite the topic being one of existential importance.

...
We’re finally beginning to see some healthy debate about some of these questions in the scientific literature. Alan Robock’s group published a paper in 2007 that found significant cooling effects even from a relatively limited regional war. A group from Los Alamos, Reisner et al, published a paper in 2018 that reexamined some of the assumptions that went into Robock et al’s model, and concluded that global cooling was unlikely in such a scenario. Robock et al. responded, and Riesner et al responded to the response. Both authors bring up good points, but I find Rieser’s position more compelling. This back and forth is worth reading for those who want to investigate deeper.

I've always found it a bit weird that so few researchers have work on such an important question. It's good to hear the more researchers are now engaging with the nuclear winter modeling. Besides genuine scientific disagreements about the modeling, I wasn't surprised to find that Wikipedia also notes there are some doubts about the emotional and political bias of the researchers involved:

As MIT meteorologist Kerry Emanuel similarly wrote a review in Nature that the winter concept is "notorious for its lack of scientific integrity" due to the unrealistic estimates selected for the quantity of fuel likely to burn, the imprecise global circulation models used, and ends by stating that the evidence of other models, point to substantial scavenging of the smoke by rain.[179] Emanuel also made an "interesting point" about questioning proponent's objectivity when it came to strong emotional or political issues that they hold.[11]

I think that funding another group of climate modellers to conduct nuclear winter simulations independently of the Robock group would provide a valuable second perspective on this. Alternatively, an adversarial collaboration between the Robock group and some nuclear winter opponents could also produce valuable results.