The cooling effects are generally not so saturated. Added CO2 provides many more emitters to radiate thermal energy into space from above the water condensation altitude in the troposphere. This is a cooling effect. CO2 also absorbs a bit of the longer wave infrared radiation from the sun before it reaches the surface. It has a heat capacity which is 27% higher than that of nitrogen molecules, so it carries more heat energy upward in a convection current, which is a cooling effect which does not saturate with added CO2 in the atmosphere. The fact that the CO2 in a given layer of atmosphere at a given altitude also radiates energy to cooler molecules in layers of air above it is also a cooling effect.
The net result is that at the lowest concentrations of CO2 in the atmosphere, the net effect is a warming effect. Additions of further CO2 warm less and less, until finally a concentration is reached where further CO2 causes a slight cooling, which becomes a greater cooling as still more CO2 is added to the atmosphere. The net temperature effect attributed to CO2 in the atmosphere passes through a maximum at some atmospheric concentration which it is extremely important to determine. Indeed, learning the complete nature of the net temperature effect versus atmospheric concentration of CO2 is the scientific task a rational program of climate science would take as one of its central goals to understand.
I usually read the weekly reports of the Science and Environmental Policy Project (SEPP), which has long been headed by the physicist Fred Singer. He has recently retired as Chairman of that organization, but the most recent weekly report of SEPP found here notes that Fred Singer has come to understand the significance of the competition between the warming and the cooling effects of CO2. Here is the relevant excerpt from the 16 December weekly report of SEPP:
Warming and Cooling? S. Fred Singer, our founder and newly elected Chairman Emeritus, is busily working on an interesting question: can carbon dioxide, a greenhouse gas, cause a cooling as well as a warming? The answer is YES, depending on subsidiary conditions. The notion has been checked by several atmospheric physicists. One issue is putting the concept into a format that is easily understandable, without many highly technical equations. The concept has the potential of partially explaining the hiatus in measured atmospheric warming despite increasing carbon dioxide (CO2). If correct, adding more CO2 will produce a cooling, not a warming of the atmosphere. Does it sound counter-intuitive? YES!While my discussions with Fred Singer have been few and not very recent, it was my evaluation that he was one of the brighter and most open-minded of the people who had been a luke-warmer. I am eager to read his argument on this subject of the competing warming and cooling effects of CO2. This is the very question I have been raising since December of 2010 and taking endless abuse for doing so. Until now, I have taken pleasure in seeing the expectations for the warming effects of CO2 consistently dropping among the better scientists considering this issue, but Fred Singer coming around to the realization that added CO2 may actually cause a net cooling is a real milestone.