Stratospheric Impacts of Continuing CFC-11 Emissions Simulated in a Chemistry-Climate Model

Abstract

Trichlorofluoromethane (CFC-11, CFCl3) is a major anthropogenic ozone-depleting substance and greenhouse gas, and its production and consumption are controlled under the Montreal Protocol. However, recent studies show that CFC-11 emissions increased during 2014–2017 relative to 2008–2012. In this study, we use a chemistry-climate model to investigate the stratospheric impacts of potential CFC-11 emissions continuing into the future. As a sensitivity test, we use a high CFC-11 scenario in which the inferred 2013–2016 average emissions of 72.5 Gg/yr is sustained to year 2100. This increases equivalent effective stratospheric chlorine by 15% in 2100, relative to the WMO (2018) baseline scenario in which future emissions decay with a bank release rate of 6.4%/year. Consistent with recent studies, the resulting ozone response has a linear dependence on the accumulated CFC-11 emissions, yielding global and Antarctic spring total ozone sensitivity per 1,000 Gg of −0.37 and −3.9 DU, respectively, averaged over 2017–2100. The deepened ozone hole reduces UV heating, causing a colder Antarctic lower stratosphere in spring/early summer. Through thermal wind balance, this accelerates the circumpolar jet which in turn alters planetary and gravity wave propagation through the Southern Hemisphere stratosphere, and modifies the Brewer-Dobson circulation. Age of air in the high scenario is slightly younger than the baseline in the lower stratosphere globally during 2090–2099, with a maximum change of −0.1 years. Coupled atmosphere-ocean model simulations show that the resulting greenhouse gas impact of CFC-11 is small and not statistically significant throughout the troposphere and stratosphere.