The Impact of Continuing CFC-11 Emissions on the Stratosphere

Chlorofluorocarbons (CFCs) are both powerful ozone depleting substances and greenhouse gases. One of the most abundant CFCs is trichlorofluoromethane (CFC- 11). Its production and consumption has been controlled under the Montreal Protocol, resulting in a rapid decline in emissions starting in the late 1980s. As a result of this emissions decrease and the ~52-year lifetime, tropospheric concentrations of CFC-11 peaked in about 1994, and have since been declining up to the present. However, Montzka et al. [2018] recently showed that CFC-11 emissions have increased over the last few years (2013-2016). This emissions increase cannot be explained by release from existing CFC-11 banks and is most likely associated with new production and consumption from East Asia. It is important to understand and quantify the stratospheric response to potential future CFC-11 emissions increases. In this study, we use the GEOSCCM 3D and GSFC 2D chemistry-climate models to investigate the impact of potential continuing CFC-11 emissions on the stratosphere. We use a scenario in which the average CFC-11 emissions during 2013-2016 (72.5 Gg/yr, as inferred by Montzka et al., 2018 and WMO, 2018) is continued out to 2100. Relative to the WMO (2018) baseline scenario, this resulted in a significant perturbation to the effective equivalent stratospheric chlorine (EESC, +15%), and global and Antarctic spring total ozone (-1% and -10%, respectively). The continuing CFC-11 emissions also resulted in significant delays in the recovery to 1980 levels of ozone and EESC, and had significant impacts on stratospheric temperature, strength of the zonal jets, and the Brewer-Dobson circulation. We also find that the magnitude of the time-integrated ozone response had a strong linear relationship with the cumulative amount of CFC-11 emissions under a wide range of emissions scenarios. This allows for a relatively simple diagnostic that can be used to estimate the ozone loss that would be expected for a given level of CFC-11 emissions. For 2020-2100, global total ozone decreases by 0.3 DU (0.1%) per 1000 Gg of cumulative CFC-11 emissions, and this sensitivity is modestly dependent on future GHG loading under the range of RCP scenarios (RCP2.6-RCP8.5).