Radiative forcing of climate change by CFC-11 and possible CFC replacements

The infrared absorption cross sections of CFC-11 and 16 other halogenated compounds have been measured. These spectra were used in detailed line-by-line calculations to derive radiative forcing values. The radiative forcing values for 14 of these gases have not, to our knowledge, been previously reported in the literature. The accuracy of a computationally inexpensive narrowband scheme, which included the effect of clouds and stratospheric adjustment, was investigated. Global warming potentials are presented where atmospheric lifetimes are available. In light of the substantial disagreement in values for the forcing due to CFC-11 reported in the literature and its use as a standard to which other halogenated gases are often compared, we examined the sensitivity of CFC-11 forcing to a number of assumptions. We find that the uncertainties in the calculated value of the radiative forcing caused by neglect of the temperature and pressure dependence of the IR absorption spectrum are much smaller than those resulting from uncertainties in the absolute absorption cross sections or the vertical profile of CFC-11. Our best estimate is 0.285 W m⁻² ppbv⁻¹, which is 30% higher than the value adopted by the Intergovernmental Panel on Climate Change and is believed to be accurate to within about 10%. For the other gases represented here the lack of detailed knowledge of the likely vertical and horizontal distribution probably represents the most significant uncertainty in evaluating their radiative forcing.