Only thing I can think of at the moment, geologically-speaking and within a short time span could be a Super-volcano eruption that engulfs the atmosphere in smo

Only thing I can think of at the moment, geologically-speaking and within a short time span could be a Super-volcano eruption that engulfs the atmosphere in smoke, causing another ice-age. Another method of danger could be a meteor impact, though it's time span is unknown until we detect it, and not sure if that fits your geological category.