Nuclear winter and human extinction: Q&A with Luke Oman

In Reasons and Persons, philosopher Derek Parfit wrote:

I believe that if we destroy mankind, as we now can, this outcome will be much worse than most people think. Compare three outcomes:

1. Peace

2. A nuclear war that kills 99% of the world’s existing population.

3. A nuclear war that kills 100%

2 would be worse than 1, and 3 would be worse than 2. Which is the greater of these two differences? Most people believe that the greater difference is between 1 and 2. I believe that the difference between 2 and 3 is very much greater… If we do not destroy mankind, these thousand years may be only a tiny fraction of the whole of civilized human history.

The ethical questions raised by the example have been much discussed, but almost nothing has been written on the empirical question: given nuclear war, how likely is scenario 3?

The most obvious path from nuclear war to human extinction is nuclear winter: past posts on Overcoming Bias have bemoaned neglect of nuclear winter, and high-lighted recent research. Particularly important is a 2007 paper by Alan Robock, Luke Oman, and Georgiy Stenchikov:  “Nuclear winter revisited with a modern climate model and current nuclear arsenals: Still catastrophic consequences.” Their model shows severe falls in temperature and insolation that would devastate agriculture and humanity’s food supply, with the potential for billions of deaths from famine in addition to the direct damage.

So I asked Luke Oman for his estimate of the risk that nuclear winter would cause human extinction, in addition to its other terrible effects. He gave the following estimate:

The probability I would estimate for the global human population of zero resulting from the 150 Tg of black carbon scenario in our 2007 paper would be in the range of 1 in 10,000 to 1 in 100,000.

I tried to base this estimate on the closest rapid climate change impact analog that I know of, the Toba supervolcanic eruption approximately 70,000 years ago.  There is some suggestion that around the time of Toba there was a population bottleneck in which the global population was severely reduced.  Climate anomalies could be similar in magnitude and duration.  Biggest population impacts would likely be Northern Hemisphere interior continental regions with relatively smaller impacts possible over Southern Hemisphere island nations like New Zealand.

Luke also graciously gave a short Q & A to clarify his reasoning:

Q1: What food sources would you expect to sustain surviving human populations with severe nuclear winter? The months of existing grain stocks? Slaughtering livestock herds? Intensive fishing? Electric greenhouse agriculture? Simply less-effective agriculture?

A: My thought was that food sources would be mainly fishing as well as less-effective agriculture, assuming little or no access to fertilizer or fuel.

Q2: If nuclear arsenals become much larger in the future, e.g. 100x as large, damage would presumably scale sublinearly (only so many cities to ignite). Could the detonation of millions of nuclear weapons make a material difference to your estimate?

A: Yes it would make a difference but as you state I would definitely think it would scale sublinearly. The largest thing that I would think, more so than the number above a certain point, would be how much the Southern Hemisphere is involved. In the 2007 paper scenario it is assuming largely NH mid-high latitude injection so there is likely large difference in black carbon aerosol amounts in the respective hemispheres. This is one of the largest differences between the 150 Tg of BC scenario and that of Toba, which was a tropical eruption and presumably spread much more evenly over both hemispheres.

Q3: Am I right in thinking that the estimate is based on the reasoning that many Toba-level events must have taken place in the last tens of millions of years, but did not wipe out our prehuman ancestors (even if perhaps eliminating some other lineages of hominids, or bringing human ancestor populations near minimal sustainable size), so the probability per event must be low (plus our access to modern technology)?

A: Yes that was my thinking.

Q4: 1 in 10,000 to 1 in 100,000 is quite a low probability, although one that could be justified if we were sure that similar events had happened many times. However, it is also low enough for model uncertainty to matter. In particular, how much probability mass can we place in nuclear winter being less or more dangerous than a Toba-level eruption? Should we assign a 1-10% probability in it being materially worse than Toba in terms of human extinction risk? In other words, how fat are the tails of the distribution for nuclear winter climate models?

A: Yes there is definitely plenty of model uncertainty when dealing with these kinds of scenarios. This question sort of goes back to my answer to number 2 in that the impacts would likely be different in the respective hemispheres, with the Northern Hemisphere more likely to be Toba-like in climate impacts. My thought for the extinction question was to treat the Southern Hemisphere as the rate limiting step. So, in the scenario we assumed, the NH climate impacts might have a 20-30% chance of being materially worse but the SH maybe around 1-5% chance of being worse.

Also, I was thinking of something in the range of 1,000-5,000 as the Minimum Viable Population (MVP) but if it is on the high end it could lower my estimated probability somewhat, but probably not significantly.  Probably one of the biggest uncertainties on my end is my climate change estimate for Toba.  Papers after ours suggest a smaller climate impact due to different aerosol size assumptions than we used.  So if indeed there was a population bottleneck around Toba and the climate anomalies were significantly smaller than we assumed, this would likely significantly raise extinction probabilities.

Q5: There are widespread popular claims that nuclear winter would create a significant chance of human extinction. Could you name other climate scientists who would estimate higher probability than yourself?

A: I haven’t really read any accounts where there was a probability placed on human extinction. I certainly could be offbase with my estimate, it is not something I have done before. I don’t know offhand anyone that would estimate higher but I am sure there might be people who would. [I asked two colleagues] who did respond back to me, saying in general terms “very close to 0″ and “very low probability.”

I guess a big question that I don’t really know or have a good feel for is how big is the discontinuity between something impacting 99% of the population and that of 100%. In my thinking about this problem I thought it might be quite large but maybe it is not. It also probably makes a big difference if the forcing is a pulse that goes away after several years or a constant forcing.

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  • dmytryl

    If not for extra 70 million deaths, a point can be made that 100% is morally preferable to 99%

    The biosphere will recover, there’s at least a hundred million years left for other attempts, we look like an early attempt, and the fact of nuclear war is evidence against humans being the species you’d want to be trying. (Depending to how cynical we are with regards to human ability to learn from own mistakes). Some species with less dexterous hands, that have to evolve greater intelligence before runaway technological progress, may do better.

    If you are to count people that will never be born as a loss, you ought to count the species that will never emerge as a loss too.

    • http://profile.yahoo.com/J7722QFLAZAWSLNH4OAWDPOB7M Rxxx

      But they might not have anything remotely like human “values”

      • dmytryl

        And we may not have anything remotely like their “values”… anyhow my point is that not everyone sees “mankind” as having moral value beyond plurality of “human”. Aesthetic value, maybe.

        If you count unborn, you ought to go rally against abortions and all contraceptives including abstinence, tbh. Counting unborn sort of dilutes the point regarding nuclear war killing you and everyone you know.

      • http://www.facebook.com/people/Jorge-Emilio-Emrys-Landivar/37403083 Jorge Emilio Emrys Landivar

        “If you count unborn, you ought to go rally against abortions and all contraceptives including abstinence, tbh.”
        Not, really.  You are using your biases to try to blur lines that to others are distinct.

    • http://www.facebook.com/people/Jorge-Emilio-Emrys-Landivar/37403083 Jorge Emilio Emrys Landivar


      If you are to count people that will never be born as a loss, you ought to count the species that will never emerge as a loss too.
      “Why?

  • http://profile.yahoo.com/J7722QFLAZAWSLNH4OAWDPOB7M Rxxx

    Interesting article, though the fact that Luke Oman gave a 1 in 10,000 probability on a question where model uncertainty is so obviously a problem does not exactly inspire confidence in him. The list of forseeable “gotchas” which could change this result includes: 

    - interactions between radiation and modern pathogens

    - effects of reduced genetic diversity of modern crops, especially staple crops like wheat and rice, and of crops which have been engineered to produce no seeds

    - interactions between elevated CO2 levels and nuclear winter-causing soot

    - the fact that particles from Toba and soot from nuclear war are not the same substance, and may have subtly different effects

    - the significantly reduced number of hunter-gatherer tribes today, combined with uncertainty about how modern humans could adapt meaning that there are fewer “viable” humans today than 70,000 years ago

    - anthropic bias/survivorship bias 

    - the fact that there would be billions of decaying human bodies after a nuclear war, as opposed to very few in any given time or place after Toba. These bodies might cause very serious disease problems

    - uncertainty about the existence of a Cobalt-based “doosday device”

    - ideological wildcards, such as a popular human extinction movement in one key country which still has intact technology. Such a movement could be precipitated by the nuclear war, and use satellite and aviation technology to root out the last survivors elsewhere in the world. Small indigenous tribes which are hard to spot might be killed off by natural processes

    Based on reading this article, I would estimate perhaps a 1 in 100 chance that nuclear war would lead relatively directly to human extinction, taking into account model uncertainty.

  • http://juridicalcoherence.blogspot.com/ srdiamond

    This is ridiculous. Any response I make to Shulman is deleted 

    HANSON: AT LEAST ANNOUNCE TO THE READERS THAT YOU ARE PRACTICING DELETION TO PROTECT PARTICULAR POSTERS.

    Otherwise, you’re just acting like a common thug.

    • http://overcomingbias.com RobinHanson

      No one is deleting your comments, and we have no comments waiting for approval in our queue at the moment. The system shows four comments by you made in the last hour available for all to see.

      • VV

         I can’t see them.

    • John Maxwell IV

      It’s always possible that your comments are being deleted not ’cause they express disagreement, but because they express disagreement of a sort that doesn’t seem especially intelligent or insightful.  Your example of a deleted comment seems like it might have been of this sort…  If extinction is something we want to avoid, it’s useful to know how probable it is given nuclear winter, so we know how much to prioritize prevention of nuclear winter vs other concerns.  This seems pretty basic.

      • Carl Shulman

        John,

        No it isn’t, all four bloggers are accounted for. Trike Apps is on the case, looking for the cause of the commenting problems (there have apparently been some other problems with comments not appearing involving the Disqus system).

      • http://juridicalcoherence.blogspot.com/ srdiamond

         That’s a really shameful remark, Maxwell 4. Obviously, I think many of your remarks are stupid. The point is that civilized behavior requires notification of deletions. It seems pretty much everyone but you agrees with that

        While it’s not true apparently that any deletions have occurred, the possibility has at least smoked you out as a person ready to defends arbitrary announced arbitrary measures and stands ready to justify them as defense of common sense.

        On the topic in question, you in fact didn’t even understand the poster’s concern–making you rather unqualified to understand my criticism of it and decidedly pompous trying to evaluate it. He was not determining the probability of nuclear winter causing extinction, as his thesis was that a nonextinction nuclear winter is worse than outright extinction! So, your proposed use is not his point. Nor is it a sensible point–unless someone can show how we can and should calibrate our policies so that if nuclear winter occurs, it will produce the “better” result of outright extinction!

      • http://juridicalcoherence.blogspot.com/ srdiamond

        While it’s not true apparently that any deletions have occurred, the possibility has at least smoked you out as a person ready to defends arbitrary announced arbitrary measures and stands ready to justify them as defense of common sense.

        Should be: a person who stands ready to defend unnoticed deletions, justifying them as defense of common sense.

        But–My apologies to Robin for jumping to conclusions.

      • http://juridicalcoherence.blogspot.com/ srdiamond

        He was not determining the probability of nuclear winter causing extinction,

        Meant to say: He isn’t prioritizing on the probability that nuclear winter will cause extinction, as his thesis is that a nonextinction nuclear winter is worse than outright extinction!

        I continue to be amazed at how the author’s very point is missed in the service of understanding the essay as  contribution against existential risk.

        (Sorry for the number of corrections. Annoyance makes for carelessness.)

      • http://www.facebook.com/profile.php?id=599840205 Christian Kleineidam

        “The point is that civilized behavior requires notification of deletions.”

        That’s not how the modern web works. Running a blog or a forum means that you have to delete a lot of stuff. Most of that stuff happens to be commercial spam. As a result nearly nobody has a blanket policy of making every deletion public. 

        Are there valid reason for making decisions about deleting personal attacks public? Yes, but there are also reason against it. Claiming that no civilised place has such policis is wrong.

      • http://juridicalcoherence.blogspot.com/ srdiamond

        Are there valid reason for making decisions about deleting personal attacks public? Yes, but there are also reason against it. Claiming that no civilised place has such policis is wrong.

        This comment is so irrelevant and blind to context that I’d expect to see it on Less Wrong.

  • http://juridicalcoherence.blogspot.com/ srdiamond

    I’ve had 4 responses to Carl Shulman deleted soon after appearing in the last couple of weeks. The latest was:

    Why would or should anyone care about this probability estimate, which neither expresses an integrating theory or makes any difference for practical effort? Perhaps it has value as an intellectual exercise, or more probably–in Hansonian terms–to signal knowledge and expertise without actually contributing to useful knowledge?

    I think this question appears interesting only to those who have a misunderstanding of probability, who think that probability is an actual property of events or an objective relationship between hypothesis and evidence. (Believing in fantasies like Kolmogorov simplicity as the univocal measure of parsimony does lead to this error.) Such concepts only appear to work in degenerate cases, like in games of chance. 

    Seems like a gigantic distraction for aspiring intellectuals. The complete reverse of “saving the world.”

    The evaporations were all to postings or Comments by Carl Shulman.

    • Robert Wiblin

      How would you go about saving the world without coming up with probability estimates for possible outcomes?

      • http://juridicalcoherence.blogspot.com/ srdiamond

        My complaint isn’t with probability estimates but with estimates of probabilities so miniscule relative to context that they make no practical contribution to deliberation. I suppose you could find a hypothetical context where the degree of tininess of the probability of total extinction compared to near-total extinction is a relevant consideration. But no such actual practical context was presented, and I doubt any actually exists.

        What’s the probability that the next time you step out into the street, a vehicle will strike you dead? For you, it’s an “existential question.” But is it worth answering?

      • Robert Wiblin

        Wouldn’t it be useful to know the probability in this case is low rather than high? I still don’t understand the complaint.

      • http://juridicalcoherence.blogspot.com/ srdiamond

        Wouldn’t it be useful to know the probability in this case is low rather than high? I still don’t understand the complaint.

        That’s probably because I expressed it confusedly by describing the issue as the probability being relatively miniscule

        But you’re saying that it’s useful to know the probability of stepping off the street and being struck dead? (Actually, to make the analogy, I should say the comparative probabilities of getting maimed versus getting killed.) I think computing it is a waste of time. Would you act differently–better–if you know?

        Similarly, what are we going to do based knowing that some remnant of humanity will almost certainly survive nuclear winter–versus thinking that total extinction is improbable but not so overwhelmingly so?

        Perhaps you’re of the “more is better” school. If it has potential use, it’s useful. (Kind of the way Katja’s family thought about all their barely “useful” possessions cluttering her house.) I’d advise reading Gut Reactions: The Intelligence of the Unconscious as an antidote. 

      • Paul Christiano

        I see a number of possible risks on which our actions might have an effect. For example, there are existing efforts to mitigate the risk of nuclear war, or decrease the size of nuclear arsenals, to which we can contribute.  Personally, my main altruistic concern is reducing existential risk in particular, so the value of those activities depends on the extent to which nuclear war constitutes an existential risk. This information seems obviously relevant. 

        I don’t really understand where you are coming from. Do you think few enough people would care about existential risk that it’s not worth talking about? Do you think that people who seem to care are deluded in some way? Do you think that there is nothing we can do to reduce the risk of nuclear winter (despite presumably knowing full well that many people spend time doing just that)?

      • http://juridicalcoherence.blogspot.com/ srdiamond

        For example, there are existing efforts to mitigate the risk of nuclear war, or decrease the size of nuclear arsenals, to which we can contribute.

        As I pointed out responding to Maxwell 4, it isn’t enough to point out that an analysis is somehow connected with the subject of existential risk. You have to show some relation between the particulars of the analysis and some different courses of action. Carl Schulman’s argument has no practical bearing on whether we should take measures to reduce the chances of nuclear war. (Nobody has tried to argue otherwise.)

        These replies, yours and Maxwell 4′s, seem to reinforce that this is all about signaling far-mode concerns while doing nothing about them except irrelevant analysis that mentions the appropriate high-status terms.

      • VV

         @f26939f398e5b2e21ea353b06370c426:disqus  It seems to me that people involved with the SI, tend to disproportionately worry about “Pascal’s mugging” risks that have very low probability but absolutely catastrophic outcomes.

        Anything less than human extinction is not catastrophic enough to worry, apparently.

        Maybe it’s due to the total utilitarian ethics that’s popular among these people: If everybody dies the aggregate utility goes to zero, while if a small but viable population remains, re-population can occur and the total utility can grow once again.

        Or maybe it’s just that thinking of yourself as the saviour of all mankind strokes your ego more than being the saviour of most of mankind.

    • http://juridicalcoherence.blogspot.com/ srdiamond

      Personally, my main altruistic concern is reducing existential risk in particular, so the value of those activities depends on the extent to which nuclear war constitutes an existential risk. This information seems obviously relevant.

      But the argument doesn’t concern the risks of nuclear war but the relative risks of outright extinction versus destruction of what we value in humanity without killing every singe human. I would think both of those risks would be termed existential.

    • Carl Shulman

      Trike found that some of your comments have been triggering Disqus spam filters. If you systematically use more spam-associated language and post features when replying to my posts and comments, that may explain the pattern.

      • http://juridicalcoherence.blogspot.com/ srdiamond

        Come on! That’s an incredibly lame response. You have an example of one of the filtered items (that somehow got through when embedded in another post!) Was it spam? What “spam-related” terms should I have avoided.

        Do you really intend to let Discus “filter” comments to this blog? If you’re really serious about it, you should so announce. I don’t think it’s something readers reasonably expect: to be filtered permanently–not merely delayed–because of “spam-related terms”

        If you have no control over arbitrary acts by Discus, get rid of it! It’s only principled. (And Discus hasn’t, to my perception, improved this blog in the least.)

      • Carl Shulman

        “What “spam-related” terms should I have avoided.” 

        I have no idea. I’m telling you what seems to be the best causal explanation of what happened to your comments. That doesn’t mean I endorse spam filter false positives, nor that I will be deciding for Robin how to fix the problem. I thought you would want to know what happened rather than be left in the dark, so I told you. That’s all.

      • http://juridicalcoherence.blogspot.com/ srdiamond

        Sorry for my tone, in that case. I had thought that this being now called a “group blog,” you shared responsibility for its policies. I accept your correction of my premise. Thank you for your efforts.

  • http://www.southernmanblog.com/ Southern Man

    All of the aboveground nuclear testing in the 50s and 60s was equivalent to a modest-scale nuclear war, with no measurable effect on the global environment.

    • Carl Shulman

      The claimed effects rely on burning cities throwing up material into the upper atmosphere. Nuclear test sites were selected not to create such enormous fires.

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