Greenhouse Effect: The Relative Contributions of Emission Height and Total Absorption

Since the 1970s, results from radiative transfer models unambiguously show that an increase in the carbon dioxide (CO2) concentration leads to an increase of the greenhouse effect. However, this robust result is oftenmisunderstood and often questioned. Acommon argument is that the CO2 greenhouse effect is saturated (i.e., does not increase) as CO2 absorption of an entire atmospheric column, named absorptivity, is saturated. This argument is erroneous first because absorptivity by CO2 is currently not fully saturated and still increases with CO2 concentration and second 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 resulting from an increase of CO2 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%).