A full-scale thermonuclear exchange between major powers would not kill humanity through blast and radiation alone. The existential threat is nuclear winter — the injection of soot and particulates into the stratosphere blocking sunlight for years, collapsing agriculture globally, and triggering famine at a scale no civilisation could withstand.
Models suggest that even a "limited" regional nuclear exchange between India and Pakistan — a fraction of global arsenals — could reduce global temperatures by 1.5°C for a decade and kill more people through crop failure than through direct detonation. A full exchange would likely reduce global average temperatures by 8–10°C.
Beyond immediate casualties, nuclear conflict risks destroying the accumulated institutional knowledge that makes modern medicine, engineering, and science possible — the people who know how things work, and the facilities they work in.