How do we know what CO2 level were billions of years ago? Or just millions? Temperature recordings from before WW2 are patchy at best and very limited in geographical coverage. Recordings from the 19th century are unreliable and a real rarity and anything before the Napoleonic wars is pure guesswork. We dont even know how many children rulers of the Middle Ages had and if they were real at all sometimes. Our knowledge about the past just some hundred years back is very iffy. But we know what the CO2 levels from 100 million years ago, a million times farther back are. Bullshit.
Last July, the concentration of CO2 in the atmosphere was 408 ppm. In the last 400 million years, a CO2 concentration 300 ppm was considered high. Therefore the high concentration of CO2 we are experiencing now was never reached in the last half billion year. This increase will for sure have an impact on how plants will be growing in the next decades.