The original claim read to me that 100kT was probably pretty close and 1Mt was a big factor of safety (~x10) but whereas the safety factor was actually less than that (~x3). However that’s the advantage of having a safety factor – even if it’s a bit misleading there still is a safety factor in the calculations.
I found the lack of links slightly frustrating here – it would have been nice to see where the OP got the numbers from.
Examining the argument
The argument itself can be summarized as:
Kinetic destruction can’t be big enough
Radiation could theoretically be enough but in practice wouldn’t be
Nuclear winter not sufficient to cause extinction
One assumption in the arguments for 1 & 2 is that the important factor is the average warhead yield and that e.g. a 10Mt warhead doesn’t have an outsized effect. This seems likely and a comment suggests that going over 500kt doesn’t make as much difference as might be thought and that is why warheads are the size that they are.
Arguments 1 & 2 seem very solid. We have done enough tests that our understanding of kinetic destruction is likely to be fairly good so I don’t have much concerns there. Similarly, radiation is well understood and dispersal patterns seem kinda predictable in principle and even if these are wrong the total amount of radiation doesn’t change, just the where it is.
Climate change is less easy to model, especially given that the scenario is a long way out of our actual experience. To trust the conclusion we have to trust the models. I haven’t looked into this – different researchers have come to different conclusions and we hope that the true value is between the worst and best case found (or at least not too much worse than the worst case).
One thing that I found frustrating in the post is that the <1% risk of human extinction from full scale nuclear war is given but the post is not explicit which arguments give such high confidence. The post rightly refers to a number of steps which need to happen to make catastrophic climate change happen:
Given a nuclear exchange how probable are firestorms?
Given firestorms how probable is black carbon to be lifted into the atmosphere?
Given lofting, how probable is it to stay aloft for sufficient time?
Given long term cooling, how probable is human extinction?
But it is not explicit as to which step(s) are most improbable.
The OP points towards an exchange between Robock and Reisner (could a small scale conflict cause a nuclear winter?) where it finds Reisner’s position more compelling. However, finding the position more compelling does not suggest to me that this part of the logic chain can be doing much heavy lifting.
(similarly, I don’t think military planners being aware of the danger of nuclear winter can be doing much heavy lifting to get us up to <1% probability).
So the main part of the logic to conclude high confidence must be that even catastrophic climate change would not cause human extinction.
The OP quotes Luke Oman as saying he believed that it would be ~1 in 10,000 to 1 in 100,000. It also links to him giving his reasoning. This is based mainly on the Toba supervolcano and other similar events failing to wipe out humanity’s ancestors and that the southern hemisphere is likely to be less badly hit than the north in a nuclear exchange. (I would have liked this to have been summarized in the OP as it is probably the strongest argument for non-extinction).
The interview doesn’t go into enough detail for me to assess how strong Luke’s arguments are and this makes it hard for me to get the same level of confidence that Luke has – how close an analogy to full scale nuclear war is Toba likely to be?
The post also suggests food stores and human ingenuity as reasons to think that humanity would avoid extinction.
Given how key this part of the logic chain seems to be I would have liked this section to be more detailed.
Outside the argument
I would have also liked the post to consider uncertainty outside the argument.
A few comments point out other possible mechanisms which might cause additional risk:
Giving <1% probability that full scale nuclear war will cause human extinction implies sufficient confidence that we have thought of all the possible causes of extinction and/or that other possible causes will have similarly low probability of extinction.
This isn’t discussed in the OP which seems like a weakness.
Conclusion
I think that the post (plus the link to the Luke Oman interview) give sufficient evidence – a full scale nuclear exchange is unlikely (<1%) to cause human extinction.
If I wanted to hone in my probability further the post helpfully supplies further reading which is a great feature.
The main subject where I would have liked to see more detail is what I see as the crux of the matter with climate change – given extremely large temperature changes how likely is humanity to go extinct? The arguments in the post itself wouldn’t have been enough for me – it was only reading the Luke Oman interview that convinced me.
Overall I think the post is strong and think it is a good candidate for the 2020 review.
The post claims:
This review aims to assess whether having read the post I can conclude the same.
The review is split into 3 parts:
Epistemic spot check
Examining the argument
Outside the argument
Epistemic spot check
Claim: There are 14,000 nuclear warheads in the world.
Assessment: True
Claim: Average warhead yield <1 Mt, probably closer to 100kt
Assessment: Probably true, possibly misleading. Values I found were:
US
W78 warhead: 335-350kt
W87 warhead: 300 or 475 kt
Russia
R-36 missile: 550-750 kt
R29 missile: 100 or 500kt
The original claim read to me that 100kT was probably pretty close and 1Mt was a big factor of safety (~x10) but whereas the safety factor was actually less than that (~x3). However that’s the advantage of having a safety factor – even if it’s a bit misleading there still is a safety factor in the calculations.
I found the lack of links slightly frustrating here – it would have been nice to see where the OP got the numbers from.
Examining the argument
The argument itself can be summarized as:
Kinetic destruction can’t be big enough
Radiation could theoretically be enough but in practice wouldn’t be
Nuclear winter not sufficient to cause extinction
One assumption in the arguments for 1 & 2 is that the important factor is the average warhead yield and that e.g. a 10Mt warhead doesn’t have an outsized effect. This seems likely and a comment suggests that going over 500kt doesn’t make as much difference as might be thought and that is why warheads are the size that they are.
Arguments 1 & 2 seem very solid. We have done enough tests that our understanding of kinetic destruction is likely to be fairly good so I don’t have much concerns there. Similarly, radiation is well understood and dispersal patterns seem kinda predictable in principle and even if these are wrong the total amount of radiation doesn’t change, just the where it is.
Climate change is less easy to model, especially given that the scenario is a long way out of our actual experience. To trust the conclusion we have to trust the models. I haven’t looked into this – different researchers have come to different conclusions and we hope that the true value is between the worst and best case found (or at least not too much worse than the worst case).
One thing that I found frustrating in the post is that the <1% risk of human extinction from full scale nuclear war is given but the post is not explicit which arguments give such high confidence. The post rightly refers to a number of steps which need to happen to make catastrophic climate change happen:
Given a nuclear exchange how probable are firestorms?
Given firestorms how probable is black carbon to be lifted into the atmosphere?
Given lofting, how probable is it to stay aloft for sufficient time?
Given long term cooling, how probable is human extinction?
But it is not explicit as to which step(s) are most improbable.
The OP points towards an exchange between Robock and Reisner (could a small scale conflict cause a nuclear winter?) where it finds Reisner’s position more compelling. However, finding the position more compelling does not suggest to me that this part of the logic chain can be doing much heavy lifting.
(similarly, I don’t think military planners being aware of the danger of nuclear winter can be doing much heavy lifting to get us up to <1% probability).
So the main part of the logic to conclude high confidence must be that even catastrophic climate change would not cause human extinction.
The OP quotes Luke Oman as saying he believed that it would be ~1 in 10,000 to 1 in 100,000. It also links to him giving his reasoning. This is based mainly on the Toba supervolcano and other similar events failing to wipe out humanity’s ancestors and that the southern hemisphere is likely to be less badly hit than the north in a nuclear exchange. (I would have liked this to have been summarized in the OP as it is probably the strongest argument for non-extinction).
The interview doesn’t go into enough detail for me to assess how strong Luke’s arguments are and this makes it hard for me to get the same level of confidence that Luke has – how close an analogy to full scale nuclear war is Toba likely to be?
The post also suggests food stores and human ingenuity as reasons to think that humanity would avoid extinction.
Given how key this part of the logic chain seems to be I would have liked this section to be more detailed.
Outside the argument
I would have also liked the post to consider uncertainty outside the argument.
A few comments point out other possible mechanisms which might cause additional risk:
Triggering supervolcano
New technologies
Inability to restart civilisation
Giving <1% probability that full scale nuclear war will cause human extinction implies sufficient confidence that we have thought of all the possible causes of extinction and/or that other possible causes will have similarly low probability of extinction.
This isn’t discussed in the OP which seems like a weakness.
Conclusion
I think that the post (plus the link to the Luke Oman interview) give sufficient evidence – a full scale nuclear exchange is unlikely (<1%) to cause human extinction.
If I wanted to hone in my probability further the post helpfully supplies further reading which is a great feature.
The main subject where I would have liked to see more detail is what I see as the crux of the matter with climate change – given extremely large temperature changes how likely is humanity to go extinct? The arguments in the post itself wouldn’t have been enough for me – it was only reading the Luke Oman interview that convinced me.
Overall I think the post is strong and think it is a good candidate for the 2020 review.