I’m not sure about the naturally occuring part. CO2 concentration has risen dramatically in the last 50 years, by an amount known to be consistent with human emmissions. I’m not so sure that that is bad thing. CO2 is definitely “green” in that it encourages plant growth. Linden (iirc) thinks that the amount of CO2 in the atmosphere is way beyond the saturation level for atmospheric warming. An increase in 50% in CO2 is likely to increase the CO2 absorption component of global temperature less than 1%.
Nonetheless, there is no demonstrable correlation between atmospheric CO2 concentrations and global air temperatures, as demonstrated most recently by the past 17-18 year history of flat temperature readings in the face of rising CO2 levels, and in the past, by higher temperatures during periods of lower CO2 levels (e.g. - the Medieval Warm Period).
Skepticism is also warranted with regard to the use of temperature records by AGW promoters as a means of documenting atmospheric conditions in the past - largely because of "cherry-picking" of such samples (most notoriously by the University of East Anglia's scientists in 2009).
Additionally, there is evidence that many (indeed, over 90%) of American temperature stations no longer meet minimum standards (as established by NOAA) for locational accuracy, and tend to be located in urban heat islands and near heat-generating airports.
Other cherry-picked data include core and tree ring samples; in one case I saw, climate scientists used only three out of several hundred samples to make their case.
Bottom line is: it's not science, it's politics.