On the Cause of the Relative Greenhouse Strength of Gases such as the Halocarbons

This note examines some of the factors important in determining the large radiative impact, relative to carbon dioxide, of increased concentrations of gases in the optically thin limit (such as the halocarbons at their present day concentrations). A narrow-band radiative transfer model is used to show that an absorber with the same integrated band strength as CFC-12, but with almost the same spectral variation of tropopause net flux change as occurs for small variations in carbon dioxide concentration, is 400 times more effective than carbon dioxide, on a molecule-per-molecule basis; this can be compared with the relative strength of 20 000 for CFC-12. This illustrates that the dominant reason for the relative strength of such gases is not their position in the 8–13 μm window. It is not possible to unambiguously separate the possible reasons (spectral position, preexisting amounts and spectroscopic strength) for the variations in relative strength, as they are all related.