The rate of decline of trichlorofluoromethane (CFC-11) in the atmosphere has slowed by approximately 50 per cent since 2012, according to a study in Nature. The authors suggest that this may be due to emissions from new sources of production.

The Montreal Protocol was designed to protect the ozone layer by reducing the abundance of ozone-depleting substances, such as chlorofluorocarbons (CFCs), in the atmosphere. The reduction in levels of CFC-11 has made the second-largest contribution to the decline in the total atmospheric concentration of ozone-depleting chlorine since the 1990s.

Stephen Montzka and colleagues 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. Based on simulations, the authors suggest that there has been an increase in CFC-11 emissions since 2012. The increase appears unrelated to past production, and the authors suggest that this may be due to new production, which has not been reported to the United Nations Environment Programme’s Ozone Secretariat. This would be inconsistent with the agreed phase-out of CFC production by 2010 in the Montreal Protocol.