A COMPARISON OF CLIMATE FORCINGS DUE TO CHLOROFLUOROCARBONS AND CARBON-MONOXIDE

The direct radiative forcing of climate by carbon monoxide (CO) is gen erally considered to be negligible. However, a recent study of localis ed 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 mo del is used to reproduce the results of the earlier study. Clouds are then added, and the stratosphere-adjusted forcing at the tropopause (t he ''climate forcing'') calculated. Global-mean anthropogenic climate forcing by CO is determined to be 32% of that by CFC-11, 12% of that b y CFC-12, and 9% of that by the CFCs combined. Even if the contemporar y 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. Regardi ng instantaneous clear-sky forcings, further analysis shows that surfa ce measurements can give a misleading impression of effects at the tro popause. While the indirect effects of CO on climate change are not ye t properly quantified, the direct radiative effects appear, as previou sly thought, to be of only minor significance.