Indirect climate forcing from ozone depleting substances

Abstract. Many halocarbons are powerful greenhouse gases and also influence climate indirectly through depletion of stratospheric ozone which opposes their direct greenhouse effect. Changes in effective radiative forcing (ERF) from historical ozone depletion have been diagnosed from model experiments with perturbed halocarbons run under the sixth Coupled Model Intercomparison Project. This is more negative than the offline stratospheric-temperature-adjusted radiative forcing (SARF). Including effects of ozone depletion on the methane lifetime makes the historical net ERF of ozone depleting substances consistent with zero. The Integrated Ozone Depletion (IOD) metric has been used to apportion this ERF between the halocarbon species and thereby derive indirect 100-year Global Warming Potentials (GWP100s) for a suite of halocarbons. The indirect GWP100 for CFC-11 is enough to make the net GWP100 likely negative, whereas the indirect contribution for CFC-12 is smaller due to a combination of longer stratospheric lifetime and fewer chlorine atoms. use of the online ERF, rather than the offline SARF, allows the model physics to account for changes in stratospheric temperature (as well as tropospheric temperature, water vapour and clouds) rather than estimating stratosphere temperature changes using fixed dynamical heating. This online calculation of radiative forcing rather than offline leads to approximately double the indirect GWPs compared to World Meteorological Organization assessments. This formalism can be used with other estimates of ozone ERF, as the indirect GWPs scale linearly with this quantity.