From The Inconvenient Skeptic:
|Northern Hemisphere (NH) Outgoing Longwave Radiation (OLR) as a function of NH temperature|
The direct dependence is a 4th order one, but even a simple linear regression gives an R^2 of 0.96. Every 1 °C change in temperature results in a 2.2 W/m^2 increase in NH OLR. There is simply no physical method by which CO2 is capable of overcoming this barrier. There never has been and there never will be. This is why global warming by means of CO2 level is impossible and always will be impossible.