Concentrations of CO2 in the atmosphere were as high as 4,000 parts per million (ppm, on a molar basis) during the Cambrian period about 500 million years ago to as low as 180 ppm during the Quaternary glaciation of the last two million years:
https://en.m.wikipedia.org/wiki/Carbon_dioxide_in_Earth's_at...Now we are at 421 ppm. Still far away from 4000 ppm.