How bad are we talking about? We're a long way from the 1980's when the Soviet arsenal was some 45,000 warheads and the USAs was similar magnitude. There was a paper in Nature late last year * that noted that a nuclear exchange between the Russia and the USA, even with the far reduced arsenals under New START (1,550 warheads each), would still lead to global famine that would kill some 5 billion people.

But what if only Russia fired their missiles, 90% of them failed, and the remaining 10% exploded in their launcher? Most of those launch tubes are deep underground to protect them from first-strike by the USA, and would not release the same kind of soot as if they exploded in the air.

Even an exchange between India and Pakistan would result in almost 30 million of dead **, but again, that's using math where nuclear weapons (one-hundred 15Kt warheads) are detonated on or above ground.

I'm not remotely suggesting we try out and find out, I like not starving to death, tymv. The math, however, is interesting in a morbid curiosity sort of way.

* https://www.nature.com/articles/s43016-022-00573-0 ** https://thebulletin.org/2022/10/nowhere-to-hide-how-a-nuclea...