I just don't see it happening either, at least in our lifetimes. Maybe the eruption of the entire Yellowstone Caldera? Other than that the only thing i can think of would be a Deep Impact-type event.

We have always been lucky here in the States. I know from my parents who lived through it that the Great Depression was a national calamity. But compare things here with the Ukraine during the same time period, where millions literally starved to death. Or China and India, where floods killed millions more and caused widespread famine. Even back then we had the resources to adapt. I think we still do.