The official US government number seems to be 412 parts per million CO2 from all causes. We can call that 0.04%.

Atmospheric water vapor varies but is usually said to be 4-5% at most. Lets go with 4%.

That sugests that H2O is 1000 times more abundant than CO2 and probably 1000 times more important as a “greenhouse gas.”

Most scientists question the accuracy of this type of data (such as temperature) that dates from before 1850. Before 1780 it is worthless. People who claim to know the atmospheric CO2 level 2 million years ago surely lie about other stuff too.