400PPM.
That means that 4/100 of one percent of the atmosphere is CO2.
Or, to put it another way, 99.96% of the atmosphere is NOT CO2.
Water Vapor is 40,000 ppm and Argon is 19,000 ppm. Water is a green house gas, but of course also the stuff of life; as CO2 is. They should be consistent and try to remove H2O from the atmosphere too. That would expose the silliness of this whole exercise.