A Comparison of Climate Forcings Due to Chlorofluorocarbons and Carbon Monoxide

Abstract

The direct radiative forcing of climate by carbon monoxide (CO) is generally considered to be negligible. However, a recent study of localised clear‐sky surface irradiances asserts that the forcing by CO may be comparable to that by CFC‐11. Nevertheless no detailed comparison of CO and CFC climate forcings has yet been made. Thus the present study estimates the radiative impact of the increases in CO, CFC‐11 and CFC‐12 that have occurred since industrialisation. A radiative transfer model is used to reproduce the results of the earlier study. Clouds are then added, and the stratosphere‐adjusted forcing at the tropopause (the “climate forcing”) calculated. Global‐mean anthropogenic climate forcing by CO is determined to be 32% of that by CFC‐11, 12% of that by CFC‐12, and 9% of that by the CFCs combined. Even if the contemporary global‐mean CO concentration is increased by a factor of three, the climate forcing by CO is still only 75% of that due to CFC‐11. Regarding instantaneous clear‐sky forcings, further analysis shows that surface measurements can give a misleading impression of effects at the tropopause. While the indirect effects of CO on climate change are not yet properly quantified, the direct radiative effects appear, as previously thought, to be of only minor significance.