ce samples from Antarctica and Greenland show that atmospheric CO2 levels were so high about 800,000 years ago. During this time, the concentration has fluctuated between 170 and 280 ppm.
In May 2019, the level of carbon dioxide in the atmosphere at the Mauna Loa Observatory in Hawaii exceeded 415 ppm for the first time.
https://scripeps.ucsd.edu/programs/keelingcurve/
Watch a short video ‘The Keeling Curve Hits 415 PPM’: