Let me elevate a comment in a subtread to the top level: nuclear winter is bunk, real effects would be on the scale of historical volcanic eruptions: https://news.ycombinator.com/item?id=8768412
BUT, if you want a disaster of that scale, nature will be happy to provide! Just have a big meteorite hit an ocean; hitting a big and deep body of water is critical, because otherwise "too much" of the heat radiates back into space. That's a big reason the vast majority of fielded nuclear weapons never got very big, there effects doesn't scale well.
There's some good science fiction that covers this and how to respond, email me at my contact address if you want a recommendation that is necessarily a spoiler.
BUT, if you want a disaster of that scale, nature will be happy to provide! Just have a big meteorite hit an ocean; hitting a big and deep body of water is critical, because otherwise "too much" of the heat radiates back into space. That's a big reason the vast majority of fielded nuclear weapons never got very big, there effects doesn't scale well.
There's some good science fiction that covers this and how to respond, email me at my contact address if you want a recommendation that is necessarily a spoiler.