It's a really small amount. CO₂ makes up only 0.042481% of the total atmosphere. That's why when they show scary CO₂ graphs they only use ppm charts that represent 0-1000 ppm. If they displayed a full 0-1000000 ppm chart CO₂ would be a straight line throughout history.
And what little effect it has, diminishes with saturation. A concentration increase from 400 to 500 ppm would make hardly any difference at all.