He is getting it from scientific observations that 97% of the 0.04% of the CO2 (that makes up 100% of the CO2 in the atmosphere) comes from natural processes such as volcanic eruptions, plant and animal decomposition, etc.

The other 3% comes from human activities, such as breathing, industrial activities, etc.

It's independently verifiable fact... if you don't believe him, feel free to DYOR.