It's all about climate, baby. Forget AIDS. Forget ebola. Forget civil wars. In fact, if there were a nuclear war, I'm convinced the greatest loss of life would be in the form of the agricultural fall-out than from multiple nuclear blasts or radiation. Same for the next time the earth is hit by an asteroid.
When Yellowstone last popped its cork about 650,000 or so years ago and most of North America was covered in ash, the earth's average temperature may have only dropped a few degrees, but many scientists believe it wiped out the majority of the earth's plant life and animal species at the time. The domino effect of entire plant and animal species going extinct with a climate jigger would be pretty much beyond our normal comprehension of disaster.