Nice catch! It doesn't... the ice core measurements do show that it was waaay lower 800,000 yrs ago at 185ppm. To go beyond that they use other methods that are

Nice catch! It doesn't... the ice core measurements do show that it was waaay lower 800,000 yrs ago at 185ppm. To go beyond that they use other methods that are less accurate, but still good enough to at least get in the ballpark. Its admittedly a huge ballpark... 4M yrs is the conservative estimate while 25M yrs is the upper limit of what makes sense. Here is my source: https://psmag.com/when-s-the-last-time-our-co2-levels-were-this-high-128b19503511#.wntfjaxei https://psmag.com/when-s-the-last-time-our-co2-levels-were-t...