[michaelt]

In the past, things like volcanic eruptions have detectable effects on world temperatures and crop yields.

During the cold war, there was a widespread theory that an all-out nuclear war would produce a similar effect; there are, after all, a great many warheads out there. So it was theorised that even countries that didn't participate in a nuclear war would end up with crop failures and mass starvation. The so-called "nuclear winter" or "nuclear holocaust".

Thankfully this theory has not yet been put to the test.