In this century a number of events could extinguish humanity. The probability of these events may be very low, but the expected value of preventing them could be high, as it represents the value of all future human lives. We review the challenges to studying human extinction risks and, by way of example, estimate the cost effectiveness of preventing extinction-level asteroid impacts.

In this article, I discuss a subset of catastrophic events—those that could extinguish humanity.¹ It is only in the last century, with the invention of nuclear weapons, that some of these events can be both caused and prevented by human action. While extinction events may be very improbable, their consequences are so grave that it could be cost effective to prevent them.