It was over 4000ppm during the Cambrian period 500m years ago, then down to about where it is today in the Carboniferious period, during the 60 million year per

It was over 4000ppm during the Cambrian period 500m years ago, then down to about where it is today in the Carboniferious period, during the 60 million year period where trees evolved to make wood, but nothing had evolved to eat it yet, so dead trees literally just piled up for 60 million years. Then during the Mezozoic it went up into the 2000s again, lots of dino farts or something? Then back under 1000 again and dropping until the last 100 years or so. https://en.wikipedia.org/wiki/Carbon_dioxide_in_Earth%27s_atmosphere#Concentrations_in_the_geologic_past https://en.wikipedia.org/wiki/Carbon_dioxide_in_Earth%27s_at... Of course, there were also mass migrations and extinctions as the environment changed so radically, which seems like something that would be nice to avoid.