> We know it was close to 7000 ppm during the Cambrian explosion 500 million years ago. We know it was in the thousands of ppm for most of life's existence on this planet.

I'm really unclear on the point here. A rapid transition to the climate of the Cambrian period would annihilate most civilization - The globe was entirely free of ice at the end of the Cambrian and temperatures were 7 degrees celsius higher than today.

While it's plausible for humans to adapt to significantly colder temperatures, it's difficult for Human's to adapt to higher temperatures in a short period. There are limits to what A/C systems can handle, and I'd doubt that people would enjoy required bathing times per day for temperature reasons.