...during periods of mass extinction.
What he's describing is the very scenario we are trying to avoid -- the way the world looks after the tipping point tips.
http://en.wikipedia.org/wiki/Global_warming#Pre-human_global_warmingAs far as his "three times the CO2" argument, ask him this rhetorical question:
"Just how long ago was that?"
Beause it's rather ridiculous to reach back to the times of a supercontinent (no land mass at the poles for glaciers to sit on) to find a "precedent." We know more accurately how CO2 has affected things recently than it did that far back when entire biospheres were completely different. Heck, those were back in the days of the supercontinent.
What's with this guy's obsession with 23 feet of sea level rise? 13 feet would submerge
most coastal cities on the planet. The 3 feet expected for this century, if it doesn't turn out to be more (estimates are always going up) will do plenty of damage in his lifetime.
And there are plenty of other reasons to worry, like ocean acidification killing off the food supply for a huge chunk of the world's population, more and bigger forest fires, drought, and severe weather. (Not to mention spontaneous clathrate release from the ocean floor like what is thought to have caused two of the largest extinction level events, but if he doesn't believe in melting permafrost methane despite the fact that you can go up there and see/smell it bubbling out, then in his fantasy-land he'll think the clathrates were put there by the great pollyanna in the sky for us to burn as fuel.)
(EDIT: the main point is it's not temperature, perse, but rapid change in temperature, that stress the biosphere. If you can get that one point through his thick skull chalk it up as a win and move on.)