pdq Percentage of CO2 in the air is the issue, not whether there should be CO2.

Whether or not we are responsible for the increase in greenhouse gasses in the air, the increase corresponds to both our emissions and our removal of the biosphere's ability to handle the increased emissions. If you look at the temperatures of Venus, Jupiter and Mars, they are all higher than can be accounted for by direct solar radiation, very likely due to the preponderance of greenhouse gasses in their atmospheres not allowing as much thermal reradiation as the "thinner atmosphere" planets allow.