385 ppm. But hey, what's a few order of magnitudes amongst friends?
If CO2 made up 2 to 3 ppm in the atmosphere, we would be fucked. Levels are increasing globally at a rate of ~1.5 ppm per year. CO2 behaves roughly according to the following relationship:
ΔF = α ln(C/C0)
Where ΔF is the change in radiative forcing, α is a constant, C is the concentration of CO2 in the atmosphere, and C0 is the initial concentration. This means that any doubling of CO2 concentration should make an equal contribution to the greenhouse effect. In other words, a change from 2 to 4 ppm should give roughly the same forcing as a change from 200 ppm to 400 ppm. We have known this since the 1890's.
Hypercapnia is a red herring.