The whodunit began when scientists caught a whiff of an unexpected climate super-pollutant in the air in 2018, which spurred a global investigation to find its source. A subsequent crackdown on illegal production of the banned chemical has resulted in a return to decreasing emissions of the powerful greenhouse gas, two new studies conclude, effectively closing the case.

Production of CFC-11, which was used primarily to make foam insulation, was slowly phased out before being banned entirely by 2010. The chemical, which destroys atmospheric ozone in addition to warming the planet, was banned under the Montreal Protocol, an international agreement that was finalized in 1987 to help mend the “ozone hole” over Antarctica and now also addresses climate change.  

However, in 2018, researchers detected an “unexpected and persistent increase” in global atmospheric concentrations of CFC-11, which is 7,000 times more effective at warming the planet than carbon dioxide over the near term.  

The study sparked independent on-the-ground investigations that found widespread, illegal CFC-11 production in China. A subsequent scientific study concluded roughly half of the observed increase in atmospheric concentrations resulted from illegal production in Eastern China. The studies and investigations led the Chinese government to a crackdown on the chemical’s production.

Since the Chinese enforcement actions, atmospheric concentrations of CFC-11 have resumed their decline, according to a pair of studies published Wednesday in the journal Nature.

“We raised a red flag,” said Stephen Montzka, a National Oceanic and Atmospheric Administration researcher and the lead author of the initial 2018 study as well as one of the two current studies. “Things weren’t going as they should have and now we have seen big changes from the time we made those announcements to today.”

Montzka said emissions of CFC-11 in 2019, the most current year for which data is available, are at or close to where they were around 2008, before the spike in emissions began.

Despite being banned globally by 2010, some emissions of CFC-11 persist as the chemical slowly leaks back out of the insulation or is released into the atmosphere when buildings or other foam-containing products are demolished.

However, had illegal production in China not occurred in recent years, it’s possible that current emissions, which are calculated from changes in atmospheric concentrations, would be significantly lower. A 2019 technical assessment by the United Nations Environment Programme concluded that current emissions of CFC-11 leaking from foam and other end products would be roughly two-thirds lower than those observed in the current study.

Montzka said it’s hard to say whether CFC-11 emissions are back to where they would have been, but added that illegal production in recent years shouldn’t have a significant, lasting impact.  

“Provided we don’t get any substantial increase again, ultimately we can conclude this pulse didn’t add substantially to… ozone depletion and global warming,” Montzka said.

Avipsa Mahapatra, climate campaign lead with the Environmental Investigation Agency, an environmental organization based in Washington and London, said the findings were a “shot in the arm” for climate and ozone protection.  

“The atmosphere is telling us CFC-11 emissions are now back to a downward trend like they were supposed to be,” said Mahapatra, whose organization published a report, based on undercover visits to Chinese factories that confirmed their illegal production of CFC-11, just months after the initial scientific study. “It’s a landmark example of a global concerted effort that started in 2018 to deal with an epic ozone and climate threat.” 

The current studies also show the effectiveness of international environmental treaties like the Montreal Protocol to reduce emissions.

Despite the good news, Mahapatra said a two-fold challenge remains—ensuring that illegal production of CFC-11 does not recur, and confiscating and destroying any remaining CFC-11 that has not yet been used.  

Montzka said the studies and the impact they had on stopping illegal production of CFC-11 underscore the need for similar monitoring efforts going forward as governments try to reduce emissions to tackle climate change.

“Independent, observation-based assessments allowed us to identify a problem that we didn’t know existed,” Montzka said. “In the future, if we want to control [greenhouse gases], this is an important point to remember.”