I recall seeing a long-term (from roughly 18,000 BCE to 3000 CE) visualization of ice-cover, ground cover, and sea level, with numbers on the side showing CO2, human population, and population below current sea level. I recall CO2 less than 200 PPM during the last glaciation, and I think this projection showed a peak around 560 PPM, before gradually declining. Sea levels kept going up though, given the assumptions in this scenario.

But I can't find it again. Anyone know about this?

Wow, that's even better :)