ON THE EVALUATION OF HALOCARBON RADIATIVE FORCING AND GLOBAL WARMING POTENTIALS

Net global warming potentials and instantaneous radiative forcing values that include the cooling from halocarbon-induced ozone destruction have been calculated fbr 14 of the most significant halocarbons. These calculations were performed by incorporating knowledge of direct global warming potentials with an evaluation of the relationship between tropospheric cooling from stratospheric ozone loss and tropospheric halocarbon mixing ratios. The indirect cooling effect is strongly dependent upon the effectiveness of each halocarbon for ozone destruction. Strong net cooling is ascribed to additions of bromocarbon gases, while methyl chloroform and carbon tetrachloride are more nearly climate-neutral, and the CFCs and HCFCs display strong net warming. Consideration of indirect cooling also has important implications for the expected future net halocarbon forcing of the climate system: in the next 20 years, halocarbon radiative forcing is not predicted to decrease as mixing ratios of strongly ozone-depleting gases decline because of faster decreases in radiative cooling than in radiative wanning. Furthermore, continuing production of HFCs as substitutes for CFCs could result in sharply increasing halocarbon radiative heating in the latter part of the 21st century because of the increasing atmospheric burden of these compounds.