Nuclear winter theory goes back to the early 1980s when scientists began to use what were then fairly basic climate models. Basically we'd have only one tenth the amount of food we had before the war and they then go on to estimate that could leave up to five billion people around the world eventually dead of starvation. It wouldn't be human extinction but most people would die. That's an existential event that is the end of the world really as we know it by any measure.