CAMBRIDGE, MASSACHUSETTS - The reign of the dinosaurs was ended 66 million years ago by an asteroid that crashed into what is now Chicxulub, Mexico. Although this lump of rock and metal was not particularly large – probably about ten kilometers across – it struck the Earth at more than 60,000 kilometers per hour, generating an explosion billions of times greater than that of the atomic bomb dropped on Hiroshima, and killing all life within 1,000 kilometers.

More ominously, the explosion sent a massive cloud of dust and ash into the upper atmosphere, blocking the sun for years. This prevented photosynthesis and led to sharply reduced temperatures, which is why scientists reckon that this atmospheric dust and sulfate aerosols ultimately killed the dinosaurs and many other species. If a similar asteroid or comet were to crash into Earth today, it would cause another mass-extinction event, wiping out most species and human civilization as we know it. This distant possibility is an example of a natural existential risk: an event not caused by humans that leads to the extinction or near-extinction of our species. But there are also anthropogenic, human-created, existential risks. As the University of Oxford philosopher Toby Ord argues in his thought-provoking new book, ‘The Precipice: Existential Risk and the Future of Humanity,’ these risks should most concern us now and in the coming century.

Risk and Reward

Ord recognizes that science and technology are humankind’s most potent tools for solving problems and achieving prosperity, but reminds that dangers are associated with such power, particularly when in the wrong hands or wielded without concern for long-term and unintended consequences.

More to the point, Ord argues anthropogenic existential risk has reached an alarmingly high level because we have developed tools capable of destroying