Funny that they had him and not a geologist who actually studies Yellowstone. I guess he's one of the science-faces in America but there are a dozens of people out there who have more or less spent their careers studying volcanism in Yellowstone who could have interesting insights.
In any case, what was never mentioned here is that Yellowstone isn't a typical volcano but a hotspot. Usually volcanoes are associated with tectonic plate boundaries and are effectively fed through the recycling process of subducting crust. Hotspots, on the other hand, are fed from magmas coming up from much deeper in the Earth (there are some thoughts this this could be coming up from just above the core itself) and their position stable. So while the plates move around you have volcanoes that will form a chain over time. Because of the source of magma and nature of hotspots, they're usually more associated with volcanoes that pour out tons of lava rather than huge explosions. That can be just as bad or worse that a huge explosion depending on how you look at things. There's evidences that there were periods of a couple of millions of years where the Yellowstone Hotspot released enough magma to change the atmosphere enough to cause extinction events. You could still have a huge explosion (or dozens) followed by a large outpouring of lava though.
Hotspots are old and stable features and I think the Yellowstone Hotspot has lavas in western Canada going back about 60-70 million years. They're also pretty regular with their eruptions. That's the sticking point though. When we say something happens regularly the plus or minus on the intervals is thousands of years, especially for something like big hotspot eruption through a continent. We could literally have hundreds of years of major signs before a volcano the size of which they talk about goes off. Of course, then the trick is recognizing those signs for what they are really worth and we really don't have any first hand experience with these things as a conscience species.
We've also known that this could happen for a long time now. Evidence of trouble brewing is why Yellowstone is famous (what do you think causes all those geysers?) Odds are, anything big enough to truly change the face of North America will have a fairly long (in human terms) and dramatic buildup. Of course, the flipside is that geologists might be deluding themselves and these things happen a lot faster than we think. Still, we get fairly decent warnings with "small" volcanoes that we see every few years. If things scale linearly we should have at least a few years if not decades leading to this sort of eruption.*
* Funny thought. If that's the case you'll get all sorts of doubters who can't comprehend anything that happens on timescales slower than a week. Because of that nothing will happen and then the disaster might as well happen immediately. Yay stupid people.
