Recently, a group of forward-looking thinkers compiled a list of catastrophes that could kill off 10 per cent or more of the human population within five years. This Gizmodo video explains how it could actually happen.
The UK-based Global Priorities Project released its report back in April, and it defined global catastrophic risks as "events or processes that would lead to the deaths of approximately a tenth of the world's population, or have a comparable impact." So a catastrophe like this wouldn't necessarily result in total human extinction, but it would make a pretty serious dent in world population.
The likeliest risks include nuclear war and pandemics (both natural and deliberately engineered), followed by disasters stemming from runaway climate change, geoengineering gone amok, and disruptions posed by artificial intelligence. The report also includes low-probability -- but high impact -- events, like asteroid impacts and supervolcanic eruptions. The authors conclude their report by proposing a number of solution, ranging from the proliferation of medicines through to internationally binding treaties.
It's a grim assessment, but at least it will allow us to prepare for such eventualities -- even if they're extremely unlikely to happen.