1) Climate change takes many years. Let me remind you of a recent one called the end of Little Ice Age. Before that, there was Medieval Warm Period. The one cur

1) Climate change takes many years. Let me remind you of a recent one called the end of Little Ice Age. Before that, there was Medieval Warm Period. The one currently happening is unprecedented but likely not impossible to adapt to, nor fix on century lengths. It will be more painful the longer we let it run though. 2) Big nuclear exchanges? Nuclear exchanges as projected are expected to be local, we don't even have enough nukes to raze all major cities. 3) Economic depression has happened multiple times so far. With various interesting results, from World War II to Computer Era. :) The closest thing I can think of is "Rock Falls, Everyone Dies". Generally a rock moving at km/s from out of Solar System. Last time this happened dinosaurs got extinct. The other one is a nearby gamma ray emitting supernova. This could sterilize most of the planet.