Since the 1970's, results from radiative transfer models unambiguously show that an increase in the CO 2 concentration leads to an increase of the greenhouse effect. However, this robust result is often misunderstood and often questioned. A common argument is that the CO 2 greenhouse effect is saturated (i.e. does not increase) as CO 2 absorption of an entire atmospheric column, named absorptivity, is saturated. This argument is erroneous firstly because absorptivity by CO 2 is currently not fully saturated and still increases with CO 2 concentration, and secondly because a change in emission height explains why the greenhouse effect may increase even if the absorptivity is saturated. However, these explanations are only qualitative. In this article, we first propose a way of quantifying the effects of both the emission height and absorptivity and we illustrate which one of the two dominates for a suite of simple idealized atmospheres. Then, using a line by line model and a representative standard atmospheric profile, we show that the increase of the greenhouse effect due to an increase of CO 2 from its current value is primarily due (about 90%) to the change in emission height. For an increase of water vapor, the change in absorptivity plays a more important role (about 40%) but the change in emission height still has the largest contribution (about 60%).