Or would it just spread everywhere?
Or would it just spread everywhere?
The sun shines on the ground in a wide band of the spectrum and heats it up. The hot ground re-radiates mainly in the infrared. The result is equilibrium. CO2 reflects infrared more than the broad spectrum of solar radiation. Thus the ground with CO2 above it still gets heated as much by the sun but the infrared it radiates is partly now reflected back on itself. Thus the equilibrium is altered and the ground gets hotter.
So, even if the CO2 didn't spread everywhere, it would still be causing heating locally. The heat would eventually spread everywhere.
Any gas released into Earth's atmosphere would diffuse quite quickly.
What I was trying to say is, because any gas will diffuse from an area of high concentration to one with a lower concentration, there is no place in Earth's atmosphere one can release gas without it diffusing throughout it. The best you can do is slow the process down.