TL;DR:

Nuclear winter is both plausible and bad, but there’s still a lot of work to be done to determine how bad for various scenarios. How much smoke, what kind of smoke, how high smoke goes, and how long it stays in the atmosphere are some of the main factors in figuring out how long nuclear winter would last and how much temperature change could be expected. It would be good to have more researchers studying this question.

Intro

Many of the people I’ve talked to about nuclear risk fall into two categories.

1. People who think that nuclear winter means literally everyone dies.

Even in the more dire scenarios, it seems likely some portion of humanity would survive given stored food, various shelters, and the geographic spread of humans. Concern here could apply to civilizational collapse with a drastically reduced population and increase the probability that any other GCR could wipe out humans.

and

2. People that think nuclear winter was vastly overblown and isn’t a real problem.

It seems reasonable to be suspicious of arguments made during the cold war, but various newer models show disastrous effects from even relatively small amounts of nuclear weapons (models often look at 100 Hiroshima sized bombs).

Both arguments are either end of disagreements between the scientific and national security crowds during the cold war. These arguments are laid out in a 2008 paper by Rubinson. It seems plausible that this disagreement was harmful to research both through bias and funding limitations.

I intend to lay out what research has shown so far about nuclear winter and talk about the limitations of that research.

Early Nuclear Winter Research

The TTAPS (Turco, Toon, Ackerman, Pollack, and Sagan) papers of 1983 and 1990 investigated how airborne particulates in the form of soot from fires generated by nuclear weapons could affect earth’s climate. In the baseline case, smoke reaches high in the atmosphere, stabilizes, spreads across the northern hemisphere, and is then pushed into southern hemisphere by displacement.

  • The baseline case in the 1983 study was for 5,000 megatons, with 20% of the total explosive yield detonated over cities.
    • This analysis was conducted during the cold war, so concern of a large nuclear exchange between the US and Russia was a factor in deciding how large of a nuclear exchange to model. Modern models use much smaller yields.

In the baseline case (5,000 megatons), temperature drops of about 40 degrees C are expected in northern temperate zones. The upper atmosphere for the baseline case is expected to heat between 30 and 80 degrees C. This is expected to contribute to making soot particles rise and stay in the upper atmosphere for longer, prolonging the cooling effects of earth’s surface.

The 1990 paper summarized the current research as:

  • Nuclear war could generate enough smoke to decrease solar intensity by 50% or more on a hemispheric scale.
  • Sooty smoke from urban fires is the main factor in nuclear winter.
  • Average cooling under smoke clouds could be much as 20 degrees C and interior landmass could cool as much as 40 degrees C.
    • This means subzero temperatures even in summer months.
  • Upper atmosphere could be heated by as much as 100 degrees C.
  • Atmospheric heating would cause smoke to stabilize in the upper atmosphere.
  • Heating would also accelerate spread of smoke between hemispheres.
  • Ozone depletion in the Northern Hemisphere is an additional problem with atmospheric heating. This means harmful UV-B radiation could be at 100-200% above normal in middle latitudes for more than a year after smoke clears.

There are a number of early papers I did not include, because the 1990 TTAPS paper summarizes the work at the time. The paper also summarized much of the uncertainties faced. The unknowns in this area are a pretty long list.

  • How much soot is initially scavenged (rained out) from large plumes of smoke.
  • How absorbing of light is soot released in massive firestorms.
  • Dispersion of smoke over large regions and how that interacts with weather systems.
  • Detailed surface response to long term atmospheric smoke (fog formation, how vegetation responds, etc.).
  • Effects of dust lofted by initial explosions.
  • Duration of ozone depletions.
  • How fire spreads in cities.
  • Air pollutant spread from burning toxic materials.
  • Scope and duration of radioactive fallout including from weapon detonation, damage to nuclear energy facilities, and damage to waste storage sites.
  • Impact of disruptions in social systems.

The TTAPs researchers were aware of many of the uncertainties. Early models were based on asteroid impacts and volcanic eruptions. Dust storms on mars were even considered. It was necessary to use alternatives, because we didn’t and still don’t have good data on how cities burn.

Later Models of Nuclear Winter

Reducing uncertainties has largely been driven by better climate models and better computers to run them on, with accuracy testing by modelling volcanoes and forest fires of which we have more knowledge. Current models show global climatic effects for even relatively small numbers of bombs. Many of the later models focused on modelling the use of about 100 Hiroshima sized bombs. This is a plausible number for a nuclear  war between India and Pakistan.

Updates in these models:

  • In a 5 tg soot injection scenario, 1 tg is rained out and 4 tg rise to the stratosphere(Mills et al, 2008).
  • Several degree cooling over agricultural areas in N America and Eurasia with global average of -1.25 degrees C with cooling still at -0.5 degrees C after ten years(Robock et al, 2007).
  • Higher lofting than previous studies, solar heating causes lofting of black carbon particles to 50-60 km (Mills et al 2014).
  • Extreme heating from black carbon in the middle atmosphere resulting in 20-25% ozone loss in the first 5 years (Mills et al 2014).

Overall, modern climate models show fairly drastic effects from regional nuclear conflicts.

Modeling through real world events

So we haven’t really studied how modern cities burn. 2 cities have been subject of a nuclear attack (Hiroshima and Nagasaki), but precise measurements were not taken and modern cities are made out of different materials. One place where models failed to predict a real world outcome was the Kuwait oil fires.

Unexpected things from the Kuwait oil fires (Hobbs and Radke, 1992):

  • Had only small effects beyond the Persian Gulf.
  • Combustion was fairly efficient, implying soot estimates may be too high.
    • Caveat: Burning of oil in nuclear winter scenarios is likely to be more like burning pools of oil than more concentrated wells.
  • Smoke was never observed above 6 km (well below the stratosphere).
    • To have large global effects, smoke would have to go higher into the atmosphere.

The Kuwait oil fires were less harmful than expected, yet current climate models show worse effects than older ones. There is clearly more work to be done before we arrive at sufficiently accurate models of the climate effects of nuclear war.

Current Model Uncertainties

There will always be some amount of uncertainty, as the number of bombs used, how powerful they are, where they’re dropped and what climate conditions are at the time will vary. These are not the main uncertainties that need to be addressed.

It keeps coming back to how modern cities burn. These papers on nuclear winter all have models that say severity depends on how much soot is generated and how high it goes in the atmosphere. We have various computer models, but models have been generated based off asteroid impacts, volcanoes, Martian dust storms, and forest fires. We don’t have good data from actual burning cities and no one wants to burn a city just to get data. Computer models have improved, but the inputs that we have for these models are limited.

  • What percentage of material is burned when a city catches fire from nuclear explosions?
  • How intense are the fires?
    • Related: How much does it take to generate firestorms?
  • How much soot is generated and what optical properties does it have?

All of these questions have been the subject of some amount of research, but we are still very uncertain in these areas.

Current State of Funding for Nuclear Winter Research

We live in a world where there is not that much research into the topic. The most recent funding for nuclear winter research is a $3 million grant to Alan Robock from the Open Philanthropy Project. It is good the Effective Altruist community has taken notice and invested resources in the problem, but I worry that work is not being conducted by a wide enough range of researchers. It is good that work is being done by Robock as he is one of the foremost experts in the field, but we also need a diverse range of inquiry.

Having a range of inquiry from a wide variety of researchers could help to highlight assumptions being made in various models and aggregate evidence from multiple sources. One of the criticisms of nuclear winter research is that it is politically motivated and done by the same set of researchers. A wide range of researchers could help correct for confirmation bias and political motivation if it is present. More funding should be directed to understanding nuclear winter using multiple research teams.

Sources

  1. Turco, R. P., Toon, O. B., Ackerman, T. P., Pollack, J. B., & Sagan, C. (1984). The climatic effects of nuclear war. Scientific American
  2. Turco, R. P., Toon, B., Ackerman, T. P., Pollack, J. B., & Sagan, C. (1990). Climate and smoke: An appraisal of nuclear winter. Combustion
  3. Hobbs, P. V., & Radke, L. F. (1992). Airborne studies of the smoke from the Kuwait oil fires. Science
  4. Toon, O. B., Turco, R. P., Robock, A., Bardeen, C., Oman, L., & Stenchikov, G. L. (2007). Atmospheric effects and societal consequences of regional scale nuclear conflicts and acts of individual nuclear terrorism. Atmospheric Chemistry and Physics
  5. Robock, A., Oman, L., & Stenchikov, G. L. (2007). Nuclear winter revisited with a modern climate model and current nuclear arsenals: Still catastrophic consequences. Journal of Geophysical Research: Atmospheres
  6. Robock, A., Oman, L., Stenchikov, G. L., Toon, O. B., Bardeen, C., & Turco, R. P. (2007). Climatic consequences of regional nuclear conflicts. Atmospheric Chemistry and Physics
  7. Rubinson, P. H. (2008). Containing science: the US national security state and scientists’ challenge to nuclear weapons during the Cold War
  8. Mills, M. J., Toon, O. B., Lee-Taylor, J., & Robock, A. (2014). Multidecadal global cooling and unprecedented ozone loss following a regional nuclear conflict. Earth’s Future
  9. Frankel, M., Scouras, J., & Ullrich, G. (2015). The Uncertain Consequences of Nuclear Weapons Use. JOHNS HOPKINS UNIV LAUREL MD APPLIED PHYSICS LAB
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