While they are not close to absorbing all of the IR that currently escapes into space, they are making the atmosphere more opaque to escaping heat, retaining heat in the atmosphere, as the concentration of these gases increases.
It's a miniscule effect, only changing the temperature by a small percentage of the temperature change caused by water vapor.
So, it is a miniscule effect of CO2 whose concentration is changing only minimally... Are you suggesting that if the level of CO2 doubles, then this miniscule effect will/could double?
The graph itself explains the greenhouse effect of CO2. The absorption effect of CO2 is logarithmic itself. The first 20 ppm absorb most. Increasing the concentration to 40ppm has a much lower effect on the temperature.
http://www.mikechurch.com/joomla/daily-features/global-cooling-for-dummies-pt.-iii.html
