How much CO2 is enough?

So, a while back I admitted that there were some questions I had about the whole globule warning thing, and vowed to do some research to rectumify the situation.

In the comments, Mike W made a comment that mirrored something that I had already sort of intuitively guessed at. Essentially, that if the sun creates heat by warming the surface, at some point you will reach an equilibrium state where there isn't enough energy left to heat anything else, so adding more CO2 is essentially irrelevant.

You see, a great deal of the chest-clutching, hair-fire, screaming-in-the-streets catastrophizing about global warming seems to be centered around the idea that all this industrial pollution from capitalist factories and irresponsible SUV owners is CREATING the greenhouse efffect.

'Cept dat, the greenhouse effect was already there. It HAS to be, or life on this planet ceases to exist. There needs to be a certain amount of CO2 "blanket" to prevent radiant heat from escaping too quickly, thus moderating temp extremes, and enabling the kind of warm, humid environment on which so much of life as we know it depends.

So, the question becomes not IF there is a greenhouse effect, but, can we have too much of a good thing?

If the sun creates radiant heat through the absorption of UV and Visible light by "dark" surfaces on the Earth, followed by the "reflection" of this energy as infra-red, it would seem to me to be intuitively obvious that the RATE at which this energy/light strikes the Earth would be the determining factor in how much heat is generated.

Because, as stated by Mike W, if only X amount of energy is created, then only X amount is available to "excite" CO2 molecule to a higher energy state (called "heating" in layman's terms). So, while I can certainly see how you could ultimately have too LITTLE CO2, and thus be unable to retain sufficient heat, I begin to question if you can really have too much? Once you reach a certain threshold amount, or a "point of equilibrium," there will simply not be enough energy left to charge up any more CO2 molecules!

Air is heated by the "blackbody radiation" from the ground. This energy is then transfered via convection to other molecules in the atmosphere: H2O and CO2 being the most popular. As these molecules transfer their heat energy to the other molecules around them, they lose energy by exactly the same amount (conservation of energy), thus cooling down. Since one molecule can transfer energy to several others, and so on in a geometric progression, each successive "bump" transfers less and less energy, until the medium again reaches its thermal equilibrium state (the Zeroth law of thermodynamics). At some point, all of this radiant energy will have convected/dissapated through the atmosphere. The farther away from the stove you get, the less heat you feel, right?

Once the threshold (equilibrium) point has been reached, double or tripling the concentration of CO2 will have no effect. There simply isn't enough energy left to heat the rest of the molecules past their equilibrium state.

The only problem I could see is that as the CO2 heats, it expands, thus taking up more volume, and so the heating could potentially happen closer to the surface, thus increasing the "felt effects." Yet, I just can't get past the idea that radiant heat, behaving as a wave, will propogate through the atmospheric medium at a fairly fixed rate.

Again, I'm still reading stuff, so please point out if my logic here is flawed. But until I'm convinced otherwise, I'm going to lean towards the rate of heating by the sun as a far greater determinent of global temp than the concentration of "greenhouse gases" in the atmosphere.