That’s just the United States. Modeling by meteorologists has found that the aerosols released could spread globally if the eruption occurred during the summer. Over the short term, as the toxic cloud blocked sunlight, global average temperatures could plunge significantly — and not return to normal for several years. Rainfall would decline sharply. That might be enough to trigger a die-off of tropical rain forests. Farming could collapse, beginning with the Midwest. It would be, as a group of researchers wrote in a 2015 report on extreme geohazards for the European Science Foundation, “the greatest catastrophe since the dawn of civilization.”

Supervolcanoes like Yellowstone represent what are known as existential risks — ultra-catastrophes that could lead to global devastation, even human extinction. They can be natural, like supereruptions or a major asteroid impact of the scale that helped kill off the dinosaurs, or they can be human-made, like nuclear war or an engineered virus. They are, by definition, worse than the worst things humanity has ever experienced. What they are not, however, is common — and that presents a major psychological and political challenge.

Though asteroids get the press and the Michael Bay movies, existential risk experts largely agree that supervolcanoes — of which there are 20 scattered around the planet — are the natural threat that poses the highest probability of human extinction. But that’s not the same thing as high. The probability of a supereruption at Yellowstone in any given year is 1 in 730,000.

But extremely unlikely isn’t the same thing as impossible, even though it’s human nature to conflate the two. What sets existential risks apart from everyday dangers isn’t likelihood but consequence.

Let’s say, as scientists have modeled, that a supereruption might kill 10 percent of the global population. Even if such eruptions occur roughly every 714,000 years — the low end of the frequency range — the death toll of that catastrophe equates to the expected loss of over 1,000 people annually, averaged out between now and when that supervolcano finally blows. If they occur roughly every 45,000 years — the high end of the range — that annual expected death toll jumps to some 17,000.