The classic worry is https://en.wikipedia.org/wiki/Nuclear_winter https://en.wikipedia.org/wiki/Nuclear_winter: the soot thrown up by the ensuing urban fire sto

The classic worry is https://en.wikipedia.org/wiki/Nuclear_winter https://en.wikipedia.org/wiki/Nuclear_winter: the soot thrown up by the ensuing urban fire storms causing temperatures, rainfall to plummet, and hence food production. On reading the scenarios the odds of total human extinction are lower than I recollected, but 80% of world pop dying of starvation dwarfs the direct death toll (usually reckoned in the hundreds of millions)