The absorption bands of carbon dioxide are already almost 100% saturated, and have been since before the industrial revolution.
The annual CO2 output from underground coal fires is about the same as from motor vehicles. If you are worried about rising levels of atmospheric CO2, it would be far more cost effective to find better ways of putting out underground coal fires than to try to set a carbon tax high enough to decrease consumption.
Irrelavant, but your point about underground coal fires is well-taken.
"To see why the anthropogenic greenhouse effect does not, however, rely on the direct perturbation of the surface energy budget by greenhouse gas changes, let's consider an idealized limiting case. Suppose that the lowest dozen meters or so of the atmosphere is so full of water vapor or cloud water that it acts like a perfect black body. It is as opaque as it can be to infrared. Now suppose that we double the atmosphere's CO2 content. This doesn't increase the infrared emission to the ground, because the low level air already has so much greenhouse-substance in it that it is radiating like a perfect blackbody, whose emission is determined by its temperature alone. It is radiating as much as it possibly can, for its given temperature. In radiative transfer-speak, its emission is "saturated." Furthermore, since the low layer is opaque to infrared, the CO2-caused change in downward emission aloft does not reach the ground. Does that mean there can be no further global warming in this case? No! What happens is that the increase in CO2 throws the top-of-atmosphere budget out of kilter, forcing the whole troposphere to warm up to bring the planet back into balance. Convection links the whole troposphere, which means the low level air warms up. The warming of the low level air, in turn, increases the flux of energy into the ground by all three of the mechanisms enumerated previously. In particular, the downward infrared flux increases because the air itself has become warmer -- not because it has become more optically thick in the infrared. The increase in downward flux then communicates the warming to the surface."