I can't really remember how this particular topic got started, but I started telling him about a very interesting recent Nova episode I saw a couple days ago. As luck would have it, there's an online transcript of that very episode, as well as part two (which I didn't see, and I read the transcript during the conversation). You should go read them. The beginning of the first episode should serve as a decent teaser:
1500 years ago something extreme happened to the world's climate-something that must have terrified those who witnessed it.
The sun began to go dark.
Rain poured red, as if tinted by blood.
Clouds of dust enveloped the earth.
Cold gripped the land for two years.
Then came drought,
Whole cities were wiped out - civilisations crumbled.
There is evidence of a catastrophe-a catastrophe whose consequences affected the entire world-and may have changed the course of human history.