Potency of Greenhouse Molecules

The relative contributions of greenhouse gases to global warming strongly depend on the concentrations of those gases in the atmosphere. The forcings per added molecule were found using a line by line calculation using data obtained from the Hitran database for nearly 100,000 lines of CO_2, CH₄, N₂O and H₂O. The calculation was done for a standard atmosphere temperature profile and took into account the altitudinal dependences of the greenhouse gases. The radiative forcings were computed at the top of the atmosphere and at the tropopause. The forcings at extremely low concentrations <0.1 ppb show a linear dependence on the gas concentration and are of comparable order of magnitude for the 4 gases. Saturation effects are very significant at ambient concentrations. Future forcings due to doubled greenhouse gas concentrations were found and used to determine expressions relating the forcings to concentration change. This is critical for studies of climate sensitivity.