This is really just an opportunity to write a little bit about science, and to advertise other people’s work. There are some who suggest that adding CO2 to the atmosphere can’t lead to warming because CO2 absorption is saturated. This is wrong for a couple of main reasons. One is very simply that it’s simply not true. Eli has a nice post (that partly motivated this post) that illustrates this. There’s also a Realclimate post (by Ray Pierrehumbert) that also shows this. It might be saturated in the middle of the absorption band, but it’s not saturated on the wings.
Another reason it is wrong is that even if it was saturated (which it’s not) adding CO2 would still lead to warming. This is explained in this Realclimate post, by Spencer Weart. Essentially, the presence of greenhouse gases prevents energy from being radiated directly from the surface to space; instead it’s radiated from within the atmosphere. In a simple sense, you can think of there being a layer in the atmosphere where the energy can be radiated directly to space. If you add more greenhouse gases (more CO2), then the layer from which the energy is radiated directly to space will move to a slightly higher altitude. This is because this extra CO2 will trap some of the energy being radiated from the original radiating layer.
However, the temperature of the atmosphere decreases with increasing altitude, and so moving the radiating layer to a higher altitude will reduce the outgoing energy flux. If we were in energy balance before adding the extra CO2, then we’ll now have more energy coming in than going out, and we’ll warm until we’re back in energy balance. Andrew Dessler has a nice video that explains some of the basic physics of the greenhouse effect and what happens as we add more CO2. Again, this post was partly motivated by seeing his video, so I can stop here, and hope that even if I didn’t explain this very clearly, the various posts to which I linked, and Andrew Dessler’s video, will clarify things.