Yellowstone Park has recently been struck by swarms of earthquakes--900 in all since December 26--and while seismologists insist that there's no reason for alarm, one has to wonder if their models are sufficiently calibrated to predict the next big one. The USGS places the annual risk of eruption at "1 in 730,000 or 0.00014%" based, perhaps crudely, on the intervals of previous eruptions at the site.
Yellowstone sits on a large and relatively volatile caldera that has erupted massively three times in the last two million years in 600,000 to 800,000 year intervals. The most recent explosion, roughly 600,000 years ago, may have been the "biggest geologic event" on Earth in the last million years, according to one geologist. Ash covered lands as far away as Texas and Missouri in depths up to an inch, and piled hundreds of feet high closer to the eruption. As Bill Bryson pointed out in his book "A Short History of Nearly Everything," another Yellowstone eruption would be ecologically disastrous, wreaking havoc on agriculture throughout North America in addition to damaging Earth's climate. Greenland ice cores from the last "supervolcano" eruption at Toba, in Sumatra, 70,000 years ago, Bryson reports, show evidence of a "volcanic winter" that endured for six years and bringing humans to the brink of extinction.
Here's a paper by Nick Bostrom on existential risks, which includes a passing reference to supervolcanos. In his first footnote, he notes the possibility of ranking the probability of existential risks by using betting markets, but reminds us that it's a slightly problematic tool since "Only a fool would bet on human extinction since there would be no chance of getting paid whether one won or lost."
ADDENDUM: Here's Robin Hanson on volcanos and global cooling: "It would also increase ozone depletion a bit. But these seem minor compared with dire warnings on warming. And people seem to have unfairly lumped this very solid approach with far more speculative approaches, e.g., orbiting shades or iron ocean seeding. Volcano shading is all well-understood physics and chemistry!"