Hey Lomu,
Sorry, I forgot to hit the reply button last night...
OK, so let's take the thermosphere, for example. Molecules up there are vibrating but, because they are so sparse, there's not much for them with which to collide. Temp at the bottom of the thermosphere, at the interface of the mesosphere, are ~ -100 deg C. At the top, where there's virtually no atmosphere, temperature can be 500+ deg C. The few molecules up there are vibrating like crazy but there's nothing for them to bump into. So the very few molecules of CO2 (theoretically) up there can't kinetically transfer very well at all so they just vibrate in a state of loneliness. They are so far removed from even the bottom layer of the thermosphere and if they make their way down there, they have made the transition from 500+ deg C to -100 deg C.They then, kinetically, have to bump the energy, transmitting through ~40km of sparsely (but increasingly more dense) distributed molecules ranging from temps ~ -100 deg C to 0 deg C at the top of the mesosphere.
Once the energy collides its way there (and keeping in mind that the energy has gone through a temperature range big enough to send H2O through its three phases), it has to transmit through the stratosphere (another 40km and through temp range from 0 deg C back to ~ -40 deg C) before it can interact with the troposphere.At the troposphere, this energy begins to interact with water molecules. H2O is miraculously resistant to heating. These molecules, with their single, polar bonds, take energy in, knock around with other H2O molecules, and just refuse to heat up. A litre of water on the stove takes an incredible amount of energy before it begins phase change. At the top of the troposphere, water molecules are frigid; ~ -40 deg C. At this stage, we're still +10km above the earth's surface and, heading down, temp changes from ~ -40 deg C to whatever the surface is doing at the latitude upon which it arrives. Depending upon latitude, time of day, season, humidity, cloud cover, proximity to the coast, wind speed, etc, etc, the temperature at the surface could range, conservatively, from -20 to 30 deg C.Temperature, as you say, doesn't have the same meaning up there as at the surface. No matter how hot the molecules up there are, (how vigorously they are vibrating), there is no plausible way with which they can kinetically transfer their vibrations down to the troposphere.
So, the energy absorbed by the extra CO2 molecules has to jump through interface hoops of ~ -100, 0, and -40 deg C before it comes into the realm of the troposphere. The troposphere is absolutely dominated by H2O.All this stuff on top of the fact that the earth's surface is hit (wildly unevenly according to day and night, earth's axis, etc, etc) with ~1000W/m/m from the sun, which the hydrological cycle irons out into supremely stable temperatures.
Because there has been no measurable change in the temps at any of these interfaces, the theoretical increase in energy caught by the extra molecules in the thermo, meso, and stratosphere, must be insignificant.
And we know that CO2 is saturated down here at the troposphere due to the pre-existence of 280ppm of CO2 and the extra we've put in. Over and above that, H2O is extremely capable to absorb energy in most of the bands to which CO2 is open to absorbance.
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