"The CO₂ greenhouse effect is already saturated"

Some denialists argue that the CO₂ in the atmosphere is already having as much greenhouse effect as it can, so adding more won't hurt.  That's sort of true if you only look at a fixed height layer of air close to ground level.   But what matters is the thickness of the blanket.

The effective thickness now is about 5km.  From that height, where the temperature is -18^oC, it radiates heat into space at the same rate that the sun pours energy in.  As you go down through the atmosphere, the air gets warmer at 6.5^oC per km.  So down here the average is about +14.5^oC.  More CO₂ thickens the blanket, raising the altitude of this -18^oC point and increasing the temperature at ground level.

The effect on Venus is dramatic: its atmosphere is 96% CO₂ and the temperature at the surface is 467^oC, hotter even than Mercury.  With no greenhouse effect, it has been estimated the temperature there would be below -10^oC.

Reference: http://www.ccrc.unsw.edu.au/Copenhagen/Copenhagen_Diagnosis_LOW.pdf