[radford-neal]

This misses the actual mechanism, which is not as simple as you might think. CO2 at ground level is actually irrelevant. What matters is CO2 at the height in the atmosphere where an infrared photon might escape to space without further interactions with the atmosphere. Increases in CO2 matter only because with more CO2 the height where a photon might escape increases, and the air is colder at this increased height, and hence the amount of energy emitted is (temporarily) less - until the overall temperature of the earth increase so that the temperature at this increased height is back to what it was before.

[fallingfrog]

I don’t think it quite works that way- infrared radiation emitted from the ground can exit directly into outer space.

[radford-neal]

That's true for infrared radiation of a frequency that isn't absorbed by CO2 (or other greenhouse gasses). But a photon at a CO2 absorbtion frequency makes it only a few feet at ground level before being absorbed again.