On the Chlorofluorocarbons Banked in Equipment: Contributions to Emissions and Impacts on the Ozone Layer and the Climate

Chlorofluorocarbon (CFC) banks in equipment such as air conditioning and foams worldwide continue to emit ozone depleting substances after global production stops, so understanding banks and their emissions is critical for evaluating the success of the Montreal Protocol. Recent reports of unexpectedly large emissions of CFC-11 heighten the need to improve the understanding of the size of the banks and quantify their contributions to current releases. Past studies have assessed banks using a bottom-up method that inventories sales and equipment, and a top-down method based on emissions inferred from concentrations along with reported production. Large differences have been reported between these two approaches. Here we develop a new Bayesian probabilistic approach that incorporates a broader range of constraints than either of the earlier approaches individually. We show how the three approaches taken together provide a fuller understanding of key factors and uncertainties, and reconcile the long-standing discrepancies. We apply the Bayesian approach to CFC-11, 12, and 113 to evaluate bank sizes and their consistency with emissions inferred from observations. The implied banks of CFC-11 and CFC-12 are considerably larger than suggested by the top-down approach, and significant banks likely remain present today. Emissions from banks can likely explain much of the emissions of CFC-11 and CFC-12 inferred from global concentration data, including for the recent period since 2000. However, our evaluation of the banks cannot explain the increased emission of CFC-11 reported after 2013, suggesting additional recent unreported production of that gas. Inferred emissions of CFC-113 are subject to large uncertainty, but are much too large to explain from banks, raising questions about unreported sources of this gas as well. Recovery and destruction of the banks beginning in 2000 represents an important opportunity lost to protect the ozone layer and the global climate system. Because significant banks still remain, it is not too late to attain significant improvements in environmental protection if CFC banks are more effectively destroyed in large quantities in the near future.