Jeff Masters has a nice summary of the science on the global devastation of even a limited regional nuclear war. He starts with the accounts popularized by Carl Sagan and others in the 1980s, which really did move the needle on global public and political consciousness on the issue. There was also a 2008 paper about the global consequences of a relatively small India-Pakistan exchange. Since then the science has been updated several times, including using the latest climate models. The results are always bad, with even the limited regional war disrupting global agriculture for up to a decade and killing 2 billion people. It would just be cold, in the summer, where food is normally grown, for ten years. Human beings would starve on an unimaginable scale. By contrast, a huge volcanic eruption like the one in Indonesia in 1815 could cause a similar effect, but it would last only a few years. (Nonetheless, we should have a plan for that one, no? Something that happens every few hundred years is common and you have to have a plan for it!) This should probably be the #1 political issue no matter what else is going on. Where are the courageous leaders today?