[Spirochete]

Concentrations reached 600 parts per million at the time of the dinosaurs and they did quite well.

Much higher than 600. Samples from Late Cretaceous amber bubbles indicate CO2 was around 3,500 ppm. The oceans didn't acidify; fossils show luxuriant coral reefs from that time as well as huge chalk deposits. The climate was not hot, but mild from pole to pole.

[Vigilanteman]

Carbon dioxide only makes up less than one-half of one percent of the Earth’s atmosphere while nitrogen makes up 78 percent of the atmosphere. Oxygen, argon, water vapor and other trace gases make up the rest.

Isn't 500 ppm only 5/100th of 1%? One-half of 1% would be 5000 ppm, wouldn't it?