Asteroids are assumed by impact specialists to exceed a “global catastrophe threshold” if they are more than about 1.5 km across. A collision by such an object could alter the Earth’s climate, kill billions of people, and cause civilization to collapse. Only a small fraction of the casualties would be directly due to the impact. The blast wave, heat, falling debris, shaking, and tsunami would kill millions. The vast majority of deaths, however, would be slow and indirect: starvation, exposure, disease, or violence related to societal disruption. The concept of a catastrophe threshold comes from “nuclear winter” studies. The impact threat, at its core, is a climate-change threat. There is geological evidence for such catastrophes in the ancient past, sometimes so severe that they have led to mass extinctions. Dinosaurs and many other organisms appear to have been wiped out 65 million years ago by a 10-km-wide asteroid. About 12,900 years ago, woolly mammoths and many other great beasts went extinct, and the Native American Clovis culture suddenly disappeared at the same time. Some scientists have speculated that a comet exploded over the Canadian ice sheet, causing a climate change so abrupt and severe that these humans and animals could not adapt. However, spontaneous global changes of this speed and magnitude occur much more often than impacts. Scientists are just beginning to see the patterns of past climate catastrophes and are working to unravel the reasons. What are the odds of a global catastrophe in our lifetimes, or those of our children? The probability of such an event happening in the near future may be small, but it is not zero. Humans have the technology to prevent some, but not all potential catastrophes. We must accept the very real possibility that the world as we know it could come to an end in our own lifetimes.