Increasing CO2 in the atmosphere decreases its effect as a greenhouse gas.
The graph below shows the behavior of greenhouse effect of CO2 as CO2 concentration increases. Calculations were performed using the ModTran program.
The baseline for the chart below, “0” represents the atmosphere with all other gases present, including water vapor, except CO2.
The results were obtained by applying ModTran to the method described in the page Compute Delta T
Doubling CO2 increases the greenhouse effect by 2 watts/m^2 out of 240 watts/m^2; i.e., less than 1%.