A large portion of Yellowstone National Park is said to be a super-volcanic crater visible from space, more than 40 miles in diameter - last erupted 600,000 years ago and is supposed to be due for another mega-blast that could swaddle the planet in ash.... forget global warming, we'll be talking major ice age.... if that one goes off it will leave most other enviro-horror scenarios in the dust.... literally. Hope it waits at least a few hundred or thousand years more, until humanity has better ways of coping....
All the more reason for mankind to get off the planet.