"The first 20 ppm of CO2 essentially makes the atmosphere almost opaque at those previously shown wave lengths, so that doubling the concentration to 40 ppm increases the heating effect by only 20 % more. Doubling it again to 80 ppm increases the heating effect by only 7 %.
.... increasing the concentration further diminishes the heating effect, so that by the time we get to the last century’s increase from 280 to 380 ppm, the effect is utterly trivial. It is as though you had blackened a glass window with one coat of paint so that it was 99 % opaque. Adding a second coat increases its opacity by only 1 % more, but it is now completely opaque. Adding a third coat, has no visible effect at all."