April 24, 2022

Nuclear Winter

When I took a broad overview of how destructive nuclear weapons are, one of the areas I looked at was nuclear winter, but I only dealt with it briefly. As such, it was something worth circling back to for a more in-depth look at the science involved.

First, as my opponent here, I'm going to take What the science says: Could humans survive a nuclear war between NATO and Russia? from the prestigious-sounding "Alliance For Science", affiliated with Cornell University, and the papers it cites in hopes of being fair to the other side. Things don't start off well, as they claim that we're closer to nuclear war than any time since the Cuban Missile Crisis, which is clearly nonsense given Able Archer 83 among others. This is followed with the following gem: "Many scientists have investigated this question already. Their work is surprisingly little known, likely because in peacetime no one wants to think the unthinkable. But we are no longer in peacetime and the shadows of multiple mushroom clouds are looming once again over our planet." Clearly, I must have hallucinated the big PR push around nuclear winter back in the mid-80s. Well, I didn't because I wasn't born yet, but everyone else must have.

Things don't get much better. They take an alarmist look at the global nuclear arsenal, and a careful look at the casualties from Hiroshima and Nagasaki, bombs vastly smaller than modern strategic weapons and with rather different damage profiles. Hilariously, their ignorance of the nuclear war literature extends to the point of ignoring fallout because there was relatively little fallout from those two airbursts, although it's well-known that groundbursts produce much more and are likely to be used in a modern nuclear war.

But now we get to the actual science, although before digging in I should point out that there are only a few scientists working on this area. The papers they cite include at least one of Rich Turco, Owen Toon or Alan Robock as an author, sometimes more than one. Turco was lead author on the original 1983 nuclear winter paper in Science and Toon was a co-author, while Robock has also been in the field for decades. The few papers I found elsewhere which do not include one or more of these three tend to indicate notably lower nuclear winter effects.

There are three basic links in the chain of logic behind the nuclear winter models here: how much soot is produced, how high it gets, and what happens in the upper atmosphere.

First, the question of soot production. Environmental Consequences of Nuclear War, by Toon, Robock and Turco gives the best statement of the methodology behind soot production that I've found. Essentially, they take an estimate of fuel loading based on population density, then assume that the burned area scales linearly with warhead yield based on the burned area from Hiroshima. This is a terrible assumption on several levels. First, Hiroshima is not the only case we have for a city facing nuclear attack, and per Effects of Nuclear Weapons p.300, Nagasaki suffered only a quarter as much burned area as Hiroshima thanks to differences in geography despite a similar yield. Taking only the most extreme case for burned area does not seem like a defensible assumption, particularly as Japanese cities at the time were unusually vulnerable to fire. For instance, the worst incendiary attack on a city during WWII was the attack on Tokyo in March 1945, when 1,665 tons of bombs set a fire that ultimately burned an area of 15.8 square miles, as opposed to 4.4 square miles burned at Hiroshima. To put this into perspective, Dresden absorbed 3,900 tons of bombs during the famous firebombing raids, which only burned about 2.5 square miles. Modern cities are probably even less flammable, as fire fatalities per capita have fallen by half since the 1940s.

Nor is the assumption that burned area will scale linearly with yield a particularly good one. I couldn't find it in the source they cite, and it flies in the face of all other scaling relationships around nuclear weapons. Given that most of the burned area will result from fires spreading and not direct ignition, a better assumption is probably to look at the areas where fires will have an easy time spreading due to blast damage, which tends to rip open buildings and spread flammable debris everywhere. per Glasstone p.108, blast radius typically scales with the 1/3rd power of yield, so we can expect damaged area from fire as well as blast to scale with the yield2/3. Direct-ignition radius is more like yield0.4, so for a typical modern strategic nuclear warhead (~400 kT), this will overstate burned area by a factor of 2 for direct-ignition and a factor of 3 for blast radius.

And then we come to their targeting assumptions, which are, if anything, worse. The only criteria for where weapons are placed are the country in question and how much flammable material is nearby, and they are carefully spaced to keep the burned areas from overlapping. This is obvious nonsense for any serious targeting plan. 100 kT weapons are spaced 15.5 km apart, far enough to spare even many industrial targets if we apply more realistic assumptions about the burned area. A realistic targeting plan would acknowledge that many hardened military targets are close together and would have overlap in their burned areas, and that a lot of nuclear warheads will be targeted at military facilities like missile silos which are in areas with far lower density of flammable materials.

Their inflation of soot production numbers is clearly shown by their own reference 9, which is a serious study of smoke/soot production following an attack on US military assets (excluding missile silos) by 3,030 500 kT warheads. This team estimated that about 21 Tg of soot would be produced after burning an area similar to what Toon, Robock and Turco estimated would be burned after an attack by only 1000 100 kT weapons, which they claim would produce 28 Tg of smoke. They attribute this to redundancy in targeting on the part of the earlier study, instead of their repeatedly taking steps to inflate their soot estimates. They also assume 4,400 warheads from the US and Russia alone, significantly higher than current arsenals. Looking at their soot estimates more broadly, their studies consistently fail to reflect any changes from the world's shrinking nuclear arsenals. A 2007 paper uses 150 Tg as the midpoint of a set of estimates from 1990, despite significant reductions in arsenals between those two dates.

One more issue before we leave this link is that all fuel within the burned area is consumed. This is probably a bad assumption, given that Glasstone mentions that collapsed buildings tend to shield flammable materials inside of them from burning. I don't have a better assumption here, but it's at least worth noting and adding to the pile of worst-case assumptions built into these models.

But what about soot actually getting to high altitudes? After all, if the soot stays in the lower atmosphere, it's going to be rained out fairly quickly. Yes, the people downwind won't have a great time of it for a few days, but we're not looking at months or years of nuclear winter. As best I can tell, the result here is deafening silence. The only factor I can see at any point between stuff burning and things entering the upper atmosphere is a 0.8 in Box 1. Other than that, everything is going into the upper troposphere/stratosphere.

I am extremely skeptical of this assumption, and figured it was worth checking against empirical data from the biggest recent fire, the 2019-2020 Australian bushfires. These burned something like 400 Tg of wood1 which in turn would produce somewhere between 1.3 Tg and 4 Tg of soot based on a 1990 paper from Turco and Toon, depending on how much the fires were like vegetation fires vs urban wood fires. Under the Toon/Robock assumptions, it sounds like we should have at least 1 Tg of soot in the stratosphere, but studies of the fires estimate between 0.4 and 0.9 Tg of aerosols reached the stratosphere, with 2.5% of this being black carbon (essentially another term for soot). This suggests that even being as generous as possible, the actual percentage of soot which reaches the stratosphere is something like 2%, not 80%. The lack of climatic impact of the Kuwait oil fires, which released about 8 Tg of soot in total, also strongly suggests that the relevant assumptions about soot transport into the upper atmosphere need to be examined far more closely. Robock attempts to deal with this by claiming that the Kuwait fires were too spread out and a larger city fire would still show self-lofting effects. The large area covered by the Australian bushfires and the low amount of soot reaching the stratosphere calls this into question.

More evidence of problems in this area comes from a paper by Robock himself, attempting to study the effects of the firebombings in WWII. Besides containing the best sentence ever published in a scientific paper in its plain-language abstract,2 it also fails to find any evidence that there was a significant drop in temperature or solar energy influx in 1945. Robock tries to spin this as positively as he can, but is forced to admit that it doesn't provide evidence for his theory, blaming poor data, particularly around smoke production. I would add in potential issues around smoke transport. More telling, however, is that all of the data points to at most a very limited effect in 1945-1946, with no trace of signal surviving later, despite claims of multi-year soot lifetimes in the papers on nuclear winter.

Which brings us neatly to the last question, involving how long anything which does reach the stratosphere will last. A 2019 paper from Robock and Toon suggests that the e-folding life will be something like 3.5 years, while a paper published the same year and including both men as authors has smoke from the 2017 Canadian wildfires persisting in the stratosphere for a mere 8 months, which they themselves noting that this is 40% shorter than their model predicted. They attempt to salvage the thesis here, even suggesting that organic smoke will contribute more than expected, but this looks to me like reporting results different from what they actually got. They attempt to salvage this by claiming that the smoke will reach higher, but at this point, I simply don't trust their models without a through validation on known events, and Kuwait is only mentioned in one paper, where they claim it doesn't count.

A few other aspects bear mentioning. First is the role of latitude. Papers repeatedly identify subtropical fires as particularly damaging, apparently due to some discontinuity in the effects of smoke at 30° latitude which greatly increases smoke persistence. This seems dubious, given that Kuwait falls just within this zone, as does most of Australia, where the wildfires have yet to show this kind of effect. Second, all of the nuclear winter papers are short on validation data, usually pointing only to modeling of volcanic aerosols, which they themselves usually admit are very different from the soot they're modeling. There is generally no discussion of validation against more relevant data.

A few academic papers also call the Toon/Robock conclusion into question, most notably this one from a team at Los Alamos National Laboratory. They model one of the lower-end scenarios, an exchange of 100 15 kT warheads in a hypothetical war between India and Pakistan, which Toon and Robock model as producing 5 Tg of soot and a significant nuclear winter. This team used mathematical models of both the blast and the resulting fire, and even in a model specifically intended to overestimate soot production got only 3.7 Tg of soot, of which only about 25% ever reached above 12 km in altitude and persisted in the long term, with more typical simulations seeing only around 6% of the soot reaching that altitude. The other three-quarters stayed in the lower atmosphere, where it was rapidly removed by weather. This was then fed into the same climate models that were used by Robock and Toon, and the results were generally similar to earlier studies of a 1 Tg scenario, which showed some effect but nowhere near the impacts Robock predicted from the scale of the conflict. It's worth noting that these models appear to have significantly overestimated soot lifetime in the stratosphere, as shown by the data from the Canadian fire.

Robock argued back against the paper, claiming that the area it looked at was not densely populated enough, lowering the production of soot and preventing a firestorm from forming. The Los Alamos team responded, running simulations with higher fuel density that showed a strongly nonlinear relationship between fuel density and soot production, with a factor of 4 increase in fuel density doubling soot and a factor of 72 increasing soot production by a factor of only 6, as oxygen starvation limited the ability of the fire to burn. In both of these cases, the percentage of soot reaching an altitude where it could persist in the stratosphere was around 6%, and the authors clearly emphasize that their earlier work is a reasonably upper bound on soot production.

So what to make of all of this? While I can't claim a conclusive debunking of the papers behind nuclear winter, it's obvious that there are a lot of problems with the papers involved. Most of the field is the work of a tiny handful of scientists, two of whom were involved from its beginnings as an appendage of the anti-nuclear movement, while the last has also been a prominent advocate against nuclear weapons. And in the parts of their analysis that don't require a PhD in atmospheric science to understand or a supercomputer simulation to check, we find assumptions that, applying a megaton or two of charity, bespeak a total unfamiliarity with nuclear effects and targeting, which is hard to square with the decades they have spent in the field.3 A tabulation of the errors in their flagship paper is revealing:

Error FactorCause
1.5Overestimating the number of warheads
2Targeting for flammability rather than efficacy
2Flammability for Hiroshima rather than normal city
1-2Flammability for 1940s city instead of 2020s city
2-3Linear burn area scaling
4-1380% soot in the stratosphere vs 6%-20%

Even using the most conservative numbers here, an all-out exchange between the US and Russia would produce a nuclear winter that would at most resemble the one that Robock and Toon predict for a regional nuclear conflict, although it would likely end much sooner given empirical data about stratospheric soot lifetimes. Some of the errors are long-running, most notably assumptions about the amount of soot that will persist in the atmosphere, while others seem to have crept in more recently, contributing to a strange stability of their soot estimates in the face of cuts to the nuclear arsenal. All of this suggests that their work is driven more by an anti-nuclear agenda than the highest standards of science. While a large nuclear war would undoubtedly have some climatic impact, all available data suggests it would be dwarfed by the direct (and very bad) impacts of the nuclear war itself.

1 Based on reported CO2 emissions of 715 Tg and a wood-to-CO2 ratio of 1 to 1.8.

2 "We discovered that [the bombing of Hiroshima and Nagasaki] was actually the culmination of a genocidal U.S. bombing campaign."

3 A source I expect many of my readers will have looked at is Luisa Rodriguez's writeup on nuclear winter for Effective Altruism organization Rethink Priorities. Her conclusion is that a US-Russia nuclear war is unlikely to be an existential risk, as she believes Robock and Toon, who form the basis of most of her analysis, have overestimated the soot from an actual war. It's obvious that I agree that they have done so, and also think they have exaggerated the climatic consequences.


  1. April 24, 2022Lambert said...

    A discontinuity at the horse latitudes sounds plausible. Soot on one side will pass up through the tropical Hadley cell and on the other the temperate Ferrel cell. Proponents of geoengineering recommend releasing particulates into the polar regions so maybe the dynamics of the stratosphere above the Ferrel cell (it spins the other way from the Hadley and polar cells) make it uniquely bad at holding soot.

  2. April 24, 2022ike said...

    or a supercomputer simulation to check,

    As a man does computational fluid dynamics on a supercomputer professionally, I feel like a laugh. How many computational cells are there in this model that they are basing these grandiose claims on?

  3. April 24, 2022bean said...


    There are details in the paper. I have no idea if it's a lot by your standards or not. Just that I'm not messing around with atmospheric soot simulations.


    Not saying there isn't a discontinuity. Just that we do have data on soot from the horse latitudes, and it doesn't support things being hugely different.

  4. April 24, 2022Emilio said...

    My question is: how many Russians nuclear weapons would really work?

  5. April 24, 2022ike said...


    It is a shame you don't remember some of the numbers. I tried ctrl+F for a few minutes before I needed to throw up and found nothing.

    10 million elements is a good ballpark number for a small model of a single engine component. 50 to 500 million is probably more reasonable if you were doing something more complicated.

    As you may have guessed, there is a great deal of bad blood between working fluid scientists and 'academic atmospheric scientists'. "Nobody believes a CFD model without validation unless it proves G***** W******", is a lession that gets drilled into every new hire.

    Re: horse latitudes

    One thing to remember is that they are a giant column of DESENDING air.


    I feel like 2x is reasonable for 'Hiroshima vs Japanese city', but that there should be '5-20x Japanese vs average city' line. I remember reading lots of period accounts of how well they burned.

    The 'Targeting' line could stand to be 3-10x (I read the line in the paper where they talk about their methodology for targeting. Somehow they have less than zero understanding of what a general would want to hit.)

    Should probably add a line for 'interception, duds, & destroyed before launch' 1.2-3x?

  6. April 24, 2022bean said...


    A quick look at one of their abstracts gives me 2.14 million elements, which based on what you say makes me suspicious of this. (I did some CFD work in college, but it was a long time ago and I didn't get very far.)

    As for the table, I'm trying to be conservative. The blast areas are large enough I don't think you'd get quite the ratio that Tokyo/Dresden suggests. Applying more deflators is easy enough to do.


    We genuinely don't know. There's going to be some duds, but if they've been taking care of their warheads, it's probably 90% or more. Of course, that assumes that someone hasn't been taking that money and using it for his mistress. In that case, Russia is currently doing 40% with its missiles in Ukraine.

  7. April 24, 2022ike said...

    I did some CFD work in college, but it was a long time ago and I didn’t get very far.

    Wise choice - the stuff makes you crazy.

    Yeah, that is pretty light weight for a model - especially if they are trying to do something complicated with it. The more I think about it, the more I am sure that their model is going to be crazy sensitive to tiny changes in system parameters. Change the turbulence-intensity of the wind coming from the NE of the model from 6.5% to 7%, and you result changes by 80% - that sort of thing.


    You have put your finger on why the USA is such a strong supporter of the Test Ban. It is an annoyance to her, but it is very hard on China and especially Russia.

  8. April 25, 2022Emilio said...

    Well, we must hope that the Russian Belles are out in force...

    Thanks to both.

  9. April 26, 2022John Schilling said...

    Typo: Direct-ignition area scales as yield^0.8, the radius only scales as yield^0.4

  10. April 26, 2022bean said...

    Oops. I think I must have misread your email. Fixed now.

  11. April 28, 2022Eric Rall said...

    Clearly, I must have hallucinated the big PR push around nuclear winter back in the mid-80s. Well, I didn’t because I wasn’t born yet, but everyone else must have.

    I was pretty young in the mid-to-late 80s, but old enough to remember hallucinating a ton of PR around the dire threat of nuclear winter. I also remember the original Civilization game's manual in the early 90s apologizing nuclear war causing global warming rather than the expected nuclear winter due to engine limitations, namely that they reused the mechanism for industrial pollution and global warming to stand in for fallout and nuclear winter instead of building a whole additional mechanic for it.

  12. April 28, 2022Echo said...

    Given the latest "we must go nuclear to show Russia how tough we are" WSJ article, this whole series is well timed.
    If you write a follow-up on fallout shelter planning, I'll definitely print it out for future reference.

    Haha who am I kidding, I live within eyesight of the Puget Sound naval bases; Bangor's going to get enough ground bursts to make the entire region glow no matter which way the wind's blowing.

  13. April 28, 2022bean said...

    There's a book called "Nuclear War Survival Skills" that the author kindly put in the public domain, and which I believe is the standard for this stuff.

    As for you specifically, yeah, that doesn't sound like a good situation to be in. If it was the old days, the Soviets might spare the People's Republic of Seattle, but it is not.

  14. April 28, 2022Lambert said...

    I should try making one of those expedient electroscope dosimeters that Kearney describes someday. A UV LED might be ionising enough to test it out.

    @ike what's the CFD field like? I've been meaning to find a specialism lately.

  15. April 28, 2022ike said...


    Do you enjoy shouting at a computer at 3AM because your model keeps diverging and you don't understand why (even with 10 years experience)?

    That said, if you enjoy CAD, you will do fine.

    Another quote, "In the teststand, everyone believes the results except the guy that ran the test. With CFD only the guy who ran it believes it."

  16. August 12, 2023Vasco said...

    Hi there,

    I think crossposting this post to EA Forum would be great. You can just copy-paste the text, and the links will not be broken. You may have to have the footnotes manually, though. I am also happy to crosspost it myself, if you would prefer that for some reason.

  17. August 12, 2023bean said...

    I at least called this to their attention when I entered the contest last year, although I may have gotten distracted talking about things like submarine availability. If you want to crosspost it, feel free.

  18. August 13, 2023Vasco said...

    Done (https://forum.effectivealtruism.org/posts/Ysq53coRwgSWHHz2x/nuclear-winter-scepticism)!

  19. April 15, 2024Tim said...

    Loved your article. I would like to interview you on the subject of nuclear winter for my nuclear war YouTube channel as I believe that Sagan, et al were grossly miscalculating its effects.

  20. April 16, 2024bean said...

    I'd be happy to do that. Email me at battleshipbean at gmail, and we can figure out how to do it.

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