A Yellowstone supereruption could set off a global survival crisis, with food shortages and damaged health systems extending the danger long after the initial blasts. Around 631,000 years ago—a geological blink of an eye—catastrophe unfolded in what is now northwestern Wyoming, US. During what is thought to have been a series of immense volcanic eruptions, life within hundreds of kilometers of the blasts was shattered. Ash was dispersed across much of North America, and the effects were felt worldwide. This was a supereruption of the massive volcanic system located in what is now the Yellowstone National Park. Despite the scale, no known human witnessed the cataclysm. As far as we know, early humans had not yet reached the continent. Across oceans, perhaps early humans did wonder at the hazier skies and unusually vivid sunsets. Volcanic particles circulating in the sky cooled their climate and likely affected their weather patterns and lives. Whether they noted these changes—or connected them to any distant event—is unknowable. Today, humanity would certainly notice. We already know that a super-eruption on this scale would be a threat to civilization as we know it. So what exactly would we experience if Yellowstone erupted? We are experts in...