# Summary

Back in June, I published a series of posts on the probability and severity of nuclear war. In light of feedback from Carl Shulman, Kit Harris, MichaelA, David Denkenberger, Topher Brennan, and others, I’ve made several revisions to those posts — especially to the post where I estimate the direct effects of a US-Russia nuclear war as well as post where I explore the severity of the subsequent nuclear winter. These are now reflected in the text, figures, and estimates in the body of those posts.

After making these revisions, my estimate of the number of people that would be killed directly by nuclear detonations during a US-Russia nuclear exchange is about 51 million (90% confidence interval: 30 million — 75 million deaths) — ~43% more than my original estimate of 35 million (90% confidence interval: 23 million — 50 million deaths).

Additionally, my estimate of the amount of smoke that would be lofted into the atmosphere went up from 20 Tg of smoke (90%CI: 7.9 Tg to 39 Tg of smoke) to 30 Tg of smoke (90%CI: 14 Tg to 66 Tg of smoke). Given this, the probability that a US-Russia nuclear exchange would cause a severe nuclear winter — assuming 50 Tg of smoke is the threshold for severe nuclear winter — goes up from just under 1% to about 11%.

The impacts that each individual change had on my results can be seen here, and the original posts can still be found here and here.

# Project Overview

This is one of several posts in Rethink Priorities’ series on nuclear risks. In the first post, I look into which plausible nuclear exchange scenarios should worry us most, ranking them based on their potential to cause harm. In the second post, I explore the make-up and survivability of the US and Russian nuclear arsenals. In the third post, I estimate the number of people that would die as a direct result of a nuclear exchange between NATO states and Russia. In the fourth post, I estimate the severity of the nuclear famine we might expect to result from a NATO-Russia nuclear war. In the fifth post, I get a rough sense of the probability of nuclear war by looking at historical evidence, the views of experts, and predictions made by forecasters. In the sixth and seventh posts, I estimate the direct and indirect effects of nuclear exchanges between (1) India and Pakistan and (2) China and its adversaries. Future work, to be published later in the summer, will explore the contradictory research around nuclear winter, the impact of several nuclear arms control treaties, and the case for and against funding particular organizations working on reducing nuclear risks.

## Revising my estimate of the number of people that would die immediately following a US-Russia nuclear exchange

The changes that had the largest bearing on my estimate of the number of deaths that would be caused by a US-Russia nuclear exchange included:

- Changing the way I estimate the number of nuclear weapons that would be used in a countervalue nuclear exchange in expectation so that I don’t accidentally truncate the tails of the distributions
- Generating a formula that can be directly entered into Guesstimate to estimate the number of deaths caused by a countervalue nuclear exchange rather than using a simplified formula to estimate the parameters for triangular distributions that are then entered into Guesstimate

After making these revisions, my estimate of the number of people that would be killed directly by nuclear detonations during a US-Russia nuclear exchange is about 51 million (90% confidence interval: 30 million — 75 million deaths) — 43% more than my original estimate of 35 million (90% confidence interval: 23 million — 50 million deaths). The impacts that each individual change had on my results can be seen here.

I also added some additional discussion of the probability that a countervalue nuclear exchange would escalate, and sensitivity analysis so that people who disagree with my views on this can see how the results change under more pessimistic assumptions. My sensitivity analysis shows that, if you’re more pessimistic than me about the probability of countervalue targeting and escalation, around 88 million people would be killed in expectation during a US-Russia nuclear exchange

## Revising my estimate of the probability and severity of the nuclear winter that might follow a US-Russia nuclear exchange

The changes that had the largest bearing on my estimate of probability and severity of nuclear winter included:

- Correcting three typos in the formulas in my Guesstimate model (details here)
- Like in my post estimating the direct effects of a US-Russia nuclear war, I change the way I estimate the number of nuclear weapons that would be used in a countervalue nuclear exchange in expectation so that I don’t accidentally truncate the tails of the distributions
- Accounting for the fact that the US and Russia would probably detonate multiple nuclear bombs on large cities in the event of countervalue targeting
- Accounting for the fact that counterforce targeting would likely involve nuclear detonations in and around some very large population centers, despite those cities not being the primary targets of the detonations

After making these revisions and corrections, my estimate of the amount of smoke that would be lofted into the atmosphere went up from 20 Tg of smoke (90%CI: 7.9 Tg to 39 Tg of smoke) to 30 Tg of smoke (90%CI: 14 Tg to 66 Tg of smoke). Given this, the probability that a US-Russia nuclear exchange would cause a severe nuclear winter — assuming 50 Tg of smoke is the threshold for severe nuclear winter — goes up from just under 1% to about 11%. Again, the impacts that each individual change had on my results can be seen here.

I also added a bit more on the controversy behind the foundational nuclear winter research, which is quite controversial (for example, see: Singer, 1985; Seitz, 2011; Robock, 2011; Coupe et al., 2019; Reisner et al., 2019; Pausata et al., 2016; Reisner et al., 2018).^{[1]} I hope to write more about this controversy in the future, but for the purposes of my estimations, I’ve assumed that the nuclear winter research comes to the right conclusion. However, if one discounted the expected harm caused by US-Russia nuclear war for the fact that the nuclear winter hypothesis is somewhat suspect, the expected harm could shrink substantially.

Thanks again to those who offered feedback, and also to Jaime Sevilla, Ozzie Gooen, Max Daniel, and Marinella Capriati for feedback and support implementing the revisions.

# Credits

This essay is a project of Rethink Priorities. It was written by Luisa Rodriguez. Thanks to Carl Shulman, Kit Harris, MichaelA, David Denkenberger, Topher Brennan for their feedback on the original posts, and to Jaime Sevilla, Ozzie Gooen, Max Daniel, Marinella Capriati, and Peter Hurford for feedback and support implementing the revisions. Thanks also to Matt Gentzel, Carl Shulman, and Seth Baum for providing guidance and feedback on the larger project.

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# Bibliography

Coupe, J., Bardeen, C. G., Robock, A., & Toon, O. B. (2019). 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. *Journal of Geophysical Research: Atmospheres,* 124(15), 8522-8543, https://doi.org/10.1029/2019JD030509.

Pausata, F. S. R., J. Lindvall, A. M. L. Ekman, and G. Svensson. (2016). Climate effects of a hypothetical regional nuclear war: Sensitivity to emission duration and particle composition, *Earth’s Future,* 4 (11), 498–511, https://doi.org/10.1002/2016EF000415

Reisner, J., Koo, E., Hunke, E., Dubey, M. (n.d.). Reply to Comment by Robock et al. on “Climate impact of a regional nuclear weapons exchange: An improved assessment based on detailed source calculations.” *Journal of Geophysical Research: Atmospheres.* Advance online publication, https://doi.org/10.1029/2019JD031281.

Reisner, J., D’Angelo, G., Koo, E., Even, W., Hecht, M., Hunke, E., et al. (2018). Climate impact of a regional nuclear weapons exchange: An improved assessment based on detailed source calculations. *Journal of Geophysical Research: Atmospheres,* 123(5), 2752–2772, https://doi.org/10.1002/2017JD027331.

Robock, A. (2011). Nuclear winter is a real and present danger. _Nature, _ 475, 37, https://doi.org/10.1038/473275a.

Singer, F. (1985). On a "Nuclear Winter." *Science*, 227(4685), 356, https://doi.org/10.1126/science.227.4685.356.

Seitz, R. (2011). Nuclear winter was and is debatable. *Nature,* 475, 37, https://doi.org/10.1038/473275a.

# Notes

Also see the summary of the nuclear winter controversy in Wikipedia’s article on nuclear winter. ↩︎

This post represents the culmination of research into the severity of the risks of nuclear war. I think the series as a whole was very helpful in figuring out how much the EA movement should prioritize nuclear risk and whether nuclear risk represented a true existential risk. Moreover, I think this post in particular was a great example of how there can be initial errors in analysis and how these errors can be thoughtfully corrected.

Disclaimer: I am co-CEO at Rethink Priorities and supervised some of this work, but I am writing this review in a personal capacity as a personal reflection. I did not share this review with anyone at RP, so it's quite possible other people might disagree and I may be wrong here, so do not take this as an official RP position.

Thank you for updating your research. I understand that only a handful of scientists are working on the nuclear winter problem. It seems like this is an area where effective altruism and yourself can make a major difference. I do have a few questions about nuclear winter since you mentioned looking into that subject in greater detail for future publications.

If cities burn without creating a firestorm to lift black carbon into the stratosphere then would a nuclear winter persist for years or would it quickly rain out?

Smoke is the result of incomplete combustion. A firestorm generates enormous temperatures due to the blast furnace effect heating fuels to temperatures between 1,400F and 2,000F. Carbon, like in diamonds, burns at 1,292F to create CO2. A smokeless incinerator is able to burn plastics without releasing black smoke and only release CO2 and water vapor. Do the sources you reference already account for the combustion of pure black carbon in a high temperature firestorm?

A few pyrocumulonimbus clouds have been studied. They appear to be mostly water vapor. If a firestorm releases a large amount of water vapor that condenses into ice as it rises then would the black carbon act as condensation nuclei? Would an ice coating change the color and stop the self-heating and rising necessary to reach the stratosphere? If an ice coated black carbon particle does reach the stratosphere then how does that impact the longevity of a nuclear winter?

If the sources are ambiguous then is this something that smaller scale table top experiments or additional observations can factually determine?