The Montreal Protocol was designed to protect the stratospheric ozone layer by enabling reductions in the abundance of ozone-depleting substances such as chlorofluorocarbons (CFCs) in the atmosphere1,2,3. The reduction in the atmospheric concentration of trichlorofluoromethane (CFC-11) has made the second-largest contribution to the decline in the total atmospheric concentration of ozone-depleting chlorine since the 1990s1. However, CFC-11 still contributes one-quarter of all chlorine reaching the stratosphere, and a timely recovery of the stratospheric ozone layer depends on a sustained decline in CFC-11 concentrations1. Here we show that the rate of decline of atmospheric CFC-11 concentrations observed at remote measurement sites was constant from 2002 to 2012, and then slowed by about 50 per cent after 2012. The observed slowdown in the decline of CFC-11 concentration was concurrent with a 50 per cent increase in the mean concentration difference observed between the Northern and Southern Hemispheres, and also with the emergence of strong correlations at the Mauna Loa Observatory between concentrations of CFC-11 and other chemicals associated with anthropogenic emissions. A simple model analysis of our findings suggests an increase in CFC-11 emissions of 13 ± 5 gigagrams per year (25 ± 13 per cent) since 2012, despite reported production being close to zero4 since 2006. Our three-dimensional model simulations confirm the increase in CFC-11 emissions, but indicate that this increase may have been as much as 50 per cent smaller as a result of changes in stratospheric processes or dynamics. The increase in emission of CFC-11 appears unrelated to past production; this suggests unreported new production, which is inconsistent with the Montreal Protocol agreement to phase out global CFC production by 2010.
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We thank NOAA station personnel for sample flask collection and on-site instrument operation, maintenance and troubleshooting; and personnel from cooperative institutions involved with flask sampling in Australia (CSIRO), Canada (AES), Ireland (University of Bristol), Israel (Weizmann Institute) and the United States (University of Colorado, Harvard University, University of Wisconsin, and the Scripps Institute for Oceanography). We thank the US National Science Foundation for logistics support at Summit (Greenland) and the South Pole; J. Butler, D. Fahey, S. Reimann, P. Newman and scientists from the Advanced Global Atmospheric Gases Experiment for discussions; S. Davis for MERRA2 reanalysis winds; and P. Novelli for CO data from MLO. The CESM project is supported by the NSF and the Office of Science (BER) of the US Department of Energy. We acknowledge the NOAA Research and Development High Performance Computing Program for computing and storage resources. This work was funded in part by the NOAA Climate Program Office’s AC4 program. The scientific results and conclusions, as well as any views or opinions expressed herein, are those of the authors and do not necessarily reflect the views of NOAA or the Department of Commerce.
Nature thanks J. Laube and the other anonymous reviewer(s) for their contribution to the peer review of this work.