I think the upper end of Halstead’s <1%-3.5% x-risk estimate is implausible for a few reasons:
1. As his paper notes and his climate x-risk writeup further discusses, extreme change would probably happen gradually instead of abruptly.
2. As his paper also notes, there’s a case that issues with priors and multiple lines of evidence imply the tails of equilibrium climate sensitivity are much less fat than those used by Weitzman. As I understand it, ECS > 10 would imply paleoclimate estimates are highly misleading and estimates based on the instrumental record are highly misleading and climate models are highly misleading. I don’t know how this sort of reasoning relates to Earth system feedbacks, but I guess the thresholds for them to become relevant would be less likely to be crossed.
3. Even if some of it were abrupt, a 10 degree rise would probably not be an existential disaster in the strict sense, though it would be horrible. (On the other hand, maybe a less than 10 degree rise would still have some risk of causing an existential disaster through some indirect effect on the stability of civilization.)
4. All estimates of the chance that a particular development will cause an existential disaster have to account for the possibility that some other development will have caused an existential disaster by that time and the possibility that some other development will have made humanity mostly immune to existential disasters.
I think the upper end of Halstead’s <1%-3.5% x-risk estimate is implausible for a few reasons:
1. As his paper notes and his climate x-risk writeup further discusses, extreme change would probably happen gradually instead of abruptly.
2. As his paper also notes, there’s a case that issues with priors and multiple lines of evidence imply the tails of equilibrium climate sensitivity are much less fat than those used by Weitzman. As I understand it, ECS > 10 would imply paleoclimate estimates are highly misleading and estimates based on the instrumental record are highly misleading and climate models are highly misleading. I don’t know how this sort of reasoning relates to Earth system feedbacks, but I guess the thresholds for them to become relevant would be less likely to be crossed.
3. Even if some of it were abrupt, a 10 degree rise would probably not be an existential disaster in the strict sense, though it would be horrible. (On the other hand, maybe a less than 10 degree rise would still have some risk of causing an existential disaster through some indirect effect on the stability of civilization.)
4. All estimates of the chance that a particular development will cause an existential disaster have to account for the possibility that some other development will have caused an existential disaster by that time and the possibility that some other development will have made humanity mostly immune to existential disasters.