2021 News & Events

Emissions of a banned ozone-depleting gas are back on the decline

12 February 2021
adapted from the story by NOAA Communications

Antarctic ozone concentration
Ozone concentration over Antarctica the week of 14–20 September 2020. Experts define the "ozone hole" as the area in which ozone levels are below 220 Dobson Units (dark blue). Image: NOAA Climate.gov based on TOAST data from the NOAA Environmental Visualization Lab.

A 2018 discovery posed the first real test of the Montreal Protocol. New analyses of global air measurements show that five years after an unexpected spike in emissions of the banned ozone-depleting chemical chlorofluorocarbon CFC-11, emissions dropped sharply between 2018 and 2019.

The abrupt turnaround was detected by both NOAA's global monitoring network and an independent global sampling network, the Advanced Global Atmospheric Gases Experiment (AGAGE), funded in part by NASA. Results showing a global emission decrease in CFC-11, by a team of scientists from the United States, including from NOAA and CIRES, the United Kingdom and Australia, as well as a companion regional analysis by a second international team showing a decline in emissions from eastern Asia, were published in the journal Nature.

The twin papers indicate that efforts to address the first known substantive violation of the international treaty tasked with ensuring stratospheric ozone layer recovery, known as the Montreal Protocol, are working. "This was a major test of the Montreal Protocol, and it appears to have passed," said NOAA GML scientist and CIRES Fellow Stephen Montzka, who led the research team that first documented the problem in 2018. CSL scientists Bob Portmann, Sean Davis, and Eric Ray were co-authors on the research.

Following that initial discovery, members of the treaty called for immediate action. China then announced renewed enforcement and inspections measures. Scientists began looking for results.

"It wasn't long before the NOAA and AGAGE data were indicating a drop in emissions both globally and from eastern China," Montzka said. "This is a great example of how important early warnings from observational systems can be. It's pretty hard to solve a problem you don't know exists.”

CFC-11 is the second-most significant member of a family of chemicals known to deplete Earth's protective ozone layer. Once widely used in the manufacture of aerosol sprays, as blowing agents for foams and packing materials, as solvents, and in refrigeration, its production was banned in 2010, although CFC-11 continues to leak from foam building insulation and appliances manufactured before that year. While the amount of CFC-11 in the atmosphere is only a few hundred parts per trillion, it plays a primary role in the formation of the ozone hole over Antarctica each September, and year-round depletion of the stratospheric ozone layer in the mid-latitudes. Ozone depletion causes thinning of the ozone layer, which shields the planet from potentially harmful ultraviolet radiation that can cause skin cancer and cataracts, suppress immune systems and also damage plants.

Graph depicting the annual changes in the atmospheric concentrations of CFC-11 obtained from analysis of samples from NOAA's monitoring network from 2000 to 2020. This period includes the internationally agreed-upon ban on new CFC-11 production in 2010, the subsequent discovery of new emissions in apparent violation of the ban, and the sharp decline identified in the last two years. Graph: NOAA Global Monitoring Laboratory and Climate.gov

In 2018, a research team led by Montzka with CSL co-authors announced that from 2014 to 2016, emissions of CFC-11 had increased by more than 13,000 tonnes per year (or 14,000 U.S. tons) to about 59,000 tonnes (65,000 U.S. tons) per year, a jump of 25 percent above average emissions from 2002 to 2012. A followup investigation by AGAGE, NOAA and CIRES scientists determined that at least 40 to 60 percent of the global emission increase came from eastern mainland China, primarily from China's Shandong and Hebei provinces.

The new results show that from 2018 to 2019, emissions of CFC-11 decreased globally by 18,000 tonnes per year (or about 20,000 U.S. tons) to 52,000 tonnes per year (57,000 U.S. tons), a decline of 26 percent. Current annual emissions now appear to have returned to pre-2012 levels.

Montzka, S.A., G.S. Dutton, R.W. Portmann, M.P. Chipperfield, S. Davis, W. Feng, A.J. Manning, E. Ray, M. Rigby, B.D. Hall, C. Siso, J.D. Nance, P.B. Krummel, J. Muühle, S. O'Doherty, P.K. Salameh, R.G. Prinn, R.F. Weiss, J.W. Elkins, H. Walter-Terrinoni, and C. Theodoridi, A decline in global CFC-11 emissions during 2018-2019, Nature, doi:10.1038/s41586-021-03260-5, 2021.


The atmospheric concentration of trichlorofluoromethane (CFC-11) has been in decline since the production of ozone-depleting substances was phased out under the Montreal Protocol. Since 2013, the concentration decline of CFC-11 slowed unexpectedly owing to increasing emissions, probably from unreported production, which, if sustained, would delay the recovery of the stratospheric ozone layer. Here we report an accelerated decline in the global mean CFC-11 concentration during 2019 and 2020, derived from atmospheric concentration measurements at remote sites around the world. We find that global CFC-11 emissions decreased by 18 ± 6 gigagrams per year (26 ± 9 per cent; one standard deviation) from 2018 to 2019, to a 2019 value (52 ± 10 gigagrams per year) that is similar to the 2008−2012 mean. The decline in global emissions suggests a substantial decrease in unreported CFC-11 production. If the sharp decline in unexpected global emissions and unreported production is sustained, any associated future ozone depletion is likely to be limited, despite an increase in the CFC-11 bank (the amount of CFC-11 produced, but not yet emitted) by 90 to 725 gigagrams by the beginning of 2020.