Relative Potency of Greenhouse Molecules

Abstract: The forcings due to changing concentrations of Earth's five most important, naturally occurring greenhouse gases, H$_2$O, CO$_2$, O$_3$, N$_2$O and CH$_4$ as well as CF$_4$ and SF$_6$ were evaluated for the case of a cloud-free atmosphere. The calculation used over 1.5 million lines having strengths as low as $10^{-27}$ cm. For a hypothetical, optically thin atmosphere, where there is negligible saturation of the absorption bands, or interference of one type of greenhouse gas with others, the per-molecule forcings are of order $10^{-22}$ Watts for H$_2$O, CO$_2$, O$_3$, N$_2$O and CH$_4$ and of order $10^{-21}$ Watts for CF$_4$ and SF$_6$. For current atmospheric concentrations, the per-molecule forcings of the abundant greenhouse gases H$_2$O and CO$_2$ are suppressed by four orders of magnitude. The forcings of the less abundant greenhouse gases, O$_3$, N$_2$O and CH$_4$, are also suppressed, but much less so. For CF$_4$ and SF$_6$, the suppression is less than an order of magnitude because the concentrations of these gases is very low. For current concentrations, the per-molecule forcings are two to four orders of magnitude greater for O$_3$, N$_2$O, CH$_4$, CF$_4$ and SF$_6$ than those of H$_2$O or CO$_2$. Doubling the current concentrations of CO$_2$, N$_2$O or CH$_4$ increases the forcings by a few per cent. A concentration increase of either CF$_4$ or SF$_6$ by a factor of 100 yields a forcing nearly an order of magnitude smaller than that obtained by doubling CO$_2$. Important insight was obtained using a harmonic oscillator model to estimate the power radiated per molecule. Unlike the most intense bands of the 5 naturally occurring greenhouse gases, the frequency-integrated cross sections of CF$_4$ and SF$_6$ were found to noticeably depend on temperature.