The above figure of the relative radiative effect of CO2 (prepared courtesy of Joe Eastman who now works at Windlogics) is based on the formula

Radiative Effect  of CO2 = 1.73 (CO2)^0.263

from the paper

Lenton, T.M., 2000: Land and ocean carbon cycle feedback effects on global warming in a simple Earth system model. Tellus. 52B, 1159-1188.

The curve used in the model provides an effective way to illustrate the change of the radiative effect of CO2 as its atmospheric concentration is elevated. While the water vapor overlap is not included, the figure shows the reduction in the increase of its forcing as a function of concentration. This does, not, of course, mean that we should not be concerned with this increase (and as I have written, it is the incompletely know biogeochemical effect of added CO2 which is likely of more concern; e.g. see). However, it does provide a perspective to any…