CFC-11 is an ozone-depleting chemical whose phase out agreed upon in the '80s and has been under an worldwide ban since 2010. Production was banned, emissions fell and the hole slowly shrank.
The researchers show that CFC-11 levels, measured at a number of remote monitoring sites around the world, decreased in line with expectations between 2002 and 2011.
The scientists say that the increase is likely a result of new, unreported production of the gas, known as CFC-11, probably in East Asia.
Nature removes 2 percent of the CFC11 out of the air each year, so concentrations of the chemical in the atmosphere are still falling, but at a slower rate because of the new emissions, Montzka said. "Further work is needed to figure out exactly why emissions of CFC-11 are increasing and if something can be done about it soon".
CFC chemicals were used in making foams for furniture and buildings, in aerosols and as refrigerants.
Countries have reported close to zero production of the chemical since 2006 but the study found about 14,300 tons (13,000 metric tons) a year has been released since 2013.
The slowdown in reduction of CFC-11 also has implications for the fight against climate change. This means that the total concentration of ozone-depleting chemicals, overall, is still decreasing in the atmosphere. Even more unexpected was that the rate of decline slowed by 50 percent after 2012. Rather, the evidence "strongly suggests" a new source of emissions, the scientists wrote.
While the new analysis can't definitively explain why emissions of CFC11 are increasing, but Montzka suspects covert production.
Unreported production of CFC-11 outside certain specific carve-out purposes in the treaty would be a "violation of worldwide law", Weller confirmed, though he said that the protocol is "non-punitive" and the remedy would probably involve a negotiation with the offending party or country.
Scientists have detected a 25% uptick in emissions of CFC-11-an ozone-destroying chemical-since 2012.
To put that in perspective, production of CFC-11, marketed under the trade name Freon, peaked at about 430,000 tons per year in the 1980s.
"This is the first time that emissions of one of the three most abundant, long-lived CFCs have increased for a sustained period since production controls took effect in the 1980s", the study concluded.
"The ozone layer remains on track to recovery by mid-century", the UN Environment Programme (UNEP) said in a statement, reacting to the findings.
Montzka said the world's nations are committed to its enforcement. "It is therefore imperative that this finding be discussed at the next Ministerial meeting of Governments given recovery of the ozone layer is dependent on all countries complying with the Montreal Protocol (and its adjustments and amendments) with emissions globally dropping to zero".
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