that advances in certain technologies create new threats for the survival of humankind and that the 21st century may be a critical moment in history when humanity's fate is decided. Edited by Nick Bostrom and Milan M. Ćirković, Global Catastrophic Risks was published in 2008, a collection of essays from 26 academics on various global catastrophic and existential risks. Toby Ord's 2020 book The Precipice: Existential Risk and the Future of Humanity argues that preventing existential risks is one of the most important moral issues of our time. The book discusses, quantifies, and compares different existential risks, concluding that the greatest risks are presented by unaligned artificial intelligence and biotechnology. Causes Potential anthropogenic causes of human extinction include global thermonuclear war, deployment of a highly effective biological weapon, an ecological collapse, runaway artificial intelligence, runaway nanotechnology (such as a grey goo scenario), a scientific accident involving a micro black hole or vacuum metastability disaster, overpopulation and increased consumption pose the risk of resource depletion and a concomitant population crash, population decline by choosing to have fewer children, displacement of naturally evolved humans by a new species produced by genetic engineering or technological augmentation. Natural and external extinction risks include high-fatality-rate pandemic, supervolcanic eruption, asteroid impact, nearby supernova or gamma-ray bursts, extreme solar flare, or alien invasion. Without intervention by unexpected forces, the stellar evolution of the Sun is expected to make Earth uninhabitable, then destroy it. Depending on its ultimate fate, the entire universe may eventually become uninhabitable. Probability Natural vs. anthropogenic Experts generally agree that anthropogenic existential risks are (much) more likely than natural risks. A key difference between these risk types is that empirical evidence can place an upper bound on the level of natural risk. Humanity has existed for at least 200,000 years, over which it has been subject to a roughly constant level of natural risk. If the natural risk were sufficiently high, then it would be highly unlikely that humanity would have survived as long as it has. Based on a formalization of this argument, researchers have concluded that we can be confident that natural risk is lower than 1 in 14,000 per year. Another empirical method to study the likelihood of certain natural risks is to investigate the geological record. For example, a comet or asteroid impact event sufficient in scale to cause an impact winter that would cause human extinction before the year