First, IIRC, recent research has not been kind to the nuclear winter x-risk hypothesis. Depopulating most of North America would be bad, but not literally the end of the world. If only some people in Madagascar survive, then they can in principle build the next technological civilization over the next 1000 years or so.
Throwaway for limited OPSEC
You're right here, though maybe with a few interesting caveats that others might find interesting from the (very niche) field.
Classic nuclear winter (everyone dies on snowball earth) was fairly quickly ruled out, and the worst case scenarios of present day teams most concerned on the issue seem unlikely. For example, the 150 Tg (a Tg being a million tonnes of soot in the stratosphere, where it persists) requires 4,400 unique (non overlapping) detonations over the most dense cities in the list, all of which make a firestorm. That's more than the total strategic arsenals available, some of which will be destroyed, fired at targets not in cities or held back, targeting is heavily duplicated in nuclear planning to ensure kills and not every urban detonation will cause a firestorm.
Add in losing agricultural inputs, access to mechanization in fields and mass logistics should the war also seriously disrupt global industry and civilizational complexity, and you have the conditions for a lot of mortality (that's actually true even without the winter, it just makes it much worse). This isn't certain, but it's really risky, and deaths in non target countries could be in the billions.
Like you say, it's hard to go from that to human extinction, and I don't personally think it's too plausible myself, but we have never run the experiment of putting our society in a situation where 80% are likely to die (absolute worst case following big rearmament, I would guess). Catastrophes can spiral, people could take risky actions as a result that contain x risks, maybe we cannot recover, it's full of unknown unknowns to quote the man who actually did the most for disarmament arguably in living memory.
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Throwaway for limited OPSEC
You're right here, though maybe with a few interesting caveats that others might find interesting from the (very niche) field.
Classic nuclear winter (everyone dies on snowball earth) was fairly quickly ruled out, and the worst case scenarios of present day teams most concerned on the issue seem unlikely. For example, the 150 Tg (a Tg being a million tonnes of soot in the stratosphere, where it persists) requires 4,400 unique (non overlapping) detonations over the most dense cities in the list, all of which make a firestorm. That's more than the total strategic arsenals available, some of which will be destroyed, fired at targets not in cities or held back, targeting is heavily duplicated in nuclear planning to ensure kills and not every urban detonation will cause a firestorm.
However, nuclear winter is unfortunately still possible, or at least the National Academy of Sciences is concerned enough not to rule it out at all and more research is being funded: https://nap.nationalacademies.org/catalog/27515/potential-environmental-effects-of-nuclear-war. Models which exclude the possibility of stratospheric injection don't include latent heating (a huge deal) - https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2022JD036667 and you only need ~400 firestorms injecting soot to take out something like 30-50% of global crop yields via 47 Tg, depending on how well you adapt agriculture https://www.researchgate.net/publication/395439565_Strategic_crop_relocation_could_substantially_mitigate_nuclear_winter_yield_losses .
Add in losing agricultural inputs, access to mechanization in fields and mass logistics should the war also seriously disrupt global industry and civilizational complexity, and you have the conditions for a lot of mortality (that's actually true even without the winter, it just makes it much worse). This isn't certain, but it's really risky, and deaths in non target countries could be in the billions.
Like you say, it's hard to go from that to human extinction, and I don't personally think it's too plausible myself, but we have never run the experiment of putting our society in a situation where 80% are likely to die (absolute worst case following big rearmament, I would guess). Catastrophes can spiral, people could take risky actions as a result that contain x risks, maybe we cannot recover, it's full of unknown unknowns to quote the man who actually did the most for disarmament arguably in living memory.
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