Through molecular diffusion, the concentration of carbon dioxide is nearly constant everywhere [although it varies seasonally].

It has been as high as 4200 ppm during the Lower Carboniferous Period [345-320 million years ago], and as low as 190 ppm during the last Glacial Age [21,000 years ago].

Over the past 360 million years, the concentration has fluctuated widely, but the tendency has been generally downwards.