CO2 levels may be something of a red herring.
No model based purely on CO2 levels has been able to show or predict future global temperatures, it would be nice if it were that simple.
CO2 levels varied greatly long before man entered the picture.
What caused that?
Many articles on the subject start with the premise that CO2 levels are the primary influence on global temperature. This has proven to be a false but popular theory. CO2 sometimes lags and sometimes leads temperature change.
http://wattsupwiththat.com/2012/04/1...n-et-al-paper/