” CO2 is now at low level and it has been 5 times higher in the past”
Now here’s the kicker: so is the O2 levels —— no one knows why it was so high 60MYA, but a dino alive then would die quickly of O2 starvation in today’s atmosphere.
Above 8000 ppm of CO2, neurological effects are a concern for humans. Submariners have a bit of experience dealing with elevated levels. Young and fit individuals suffer the least effects—that’s true of many other stress causation.
https://www.nap.edu/read/11170/chapter/5