the mean molecular weight of air is ~28.97, CO2 is 44 thus CO2 is heavier than air and allot heavier than water which is 18. relative concentration of CO2 actually drops with height. true, it is "close" to linear but a linear relative to a highly exponential drop of air density. absolute density on the other hand has an even higher exponential decrease over elevation. your assumption that CO2 has the same concentration is not quite valid. and water vapor actually reaches bit further up and it's concentration attenuation is not as pronounced. basic physics on a macroscopic scale.
reality is exactly as you portrait it. turbulence, currents, uneven distributions, but for some reason you actually think CO2 is fixed in time and space and does not move in the atmosphere, it doesn't get absorbed by cold water, precipitation, plants - most notably algae, released from oceans and microorganisms. no. it is just as dynamic.
lets make a scientific mistake and analyze the hardest hitting CO2 versus the thinnest humidity and lets assume, that you are right and CO2 can actually reach as far up as water.
the lowest concentration you mentioned on the surface is 0.2% (two tenth of a percent). i assume this is mass based.
the highest measurement of CO2 i could find is 650ppm (parts per million) - i assume this is mole fraction i don't want to nerf it by using mass fraction. 650 CO2 molecules have a total atomic mass of 28600u (650*44). 1 000 000 particles of air weigh 28970000u. so the mass based ratio against DRY (another buff) air is 28600u*100/28970000u. and that is 0.098%.
keep in mind that the CO2 loves to dissolve in cold liquid water and de-gas in hot water. so i put together the measurements inside
a chimney against water in a place where even CO2 is the thinnest. and you still have at least 2x lowest possible concentration of water at the surface.
the global values are somewhere around 0.403% water and 0.039% CO2 (both are volume based)
couple that, with the radiation bands (they are not thin spikes as in your image) of vapor, reflectivity of ice and droplets, absorption of droplets, so on and so on. not to mention that size of the droplets/crystals is a major factor. it is quite clear that water is by far the main transmission of this machine, and the engine is the Sun (which has a weather of it's own)http://en.wikipedia.org/wiki/File:Solar_Spectrum.png
now, if CO2 can be a "trigger" or not, that remains to be seen. so far i did not find any valid argument.