Catastrophes, whether natural or man-made, that could destroy the human race are often dismissed as alarmist or fanciful, the stuff of science fiction. In fact the risk of such disasters is real, and growing. A collision with an asteroid that might kill a quarter of humanity in 24 hours and the rest soon after; irreversible global warming that might flip, precipitating "snowball earth;" voraciously replicating nanomachines; a catastrophic accident...