Stephen Montzka from the US National Oceanic and Atmospheric Administration and a team of colleagues report that after a long and predicted decline, production of trichlorofluoromethane (CFC-11) - a substance responsible for the second largest destructive impact on the ozone layer - suddenly and mysteriously increased in 2012 and has continued to do so ever since.
"This is the most surprising and unexpected thing that I've observed in 27 years of making these measurements", said Steve Montzka, a research chemist at NOAA and lead author of the paper. "In fact, I was amazed by this".
It's a distressing result for what's widely seen as a global environmental success story, in which nations - alarmed by a growing "ozone hole" - collectively took action to phase out chlorofluorocarbons. The startling resurgence of the chemical, reported in Nature, will likely spark an worldwide investigation to track down the mysterious source. CFC-11 was once commonly used in insulating foams, but it's now banned under the Montreal Protocol and reported production is close to zero.
Zaelke said he was surprised by the findings, not just because the chemical has always been banned, but also because alternatives already exist, making it hard to imagine what the market for CFC-11 today would be. It is thought that about 13,000 tonnes a year has been released since 2013. Emissions of this CFC to the atmosphere reached about 386,000 tons per year at their peak later in the decade. It is only destroyed in the stratosphere, some 9 to 18 miles above the planet's surface, where the resulting chlorine molecules engage in a string of ozone-destroying chemical reactions.
"I hope that somehow the worldwide community can put pressure on South East Asian countries, maybe China, to go and look at whether they can get more information on where the emissions come from".
The chemical is also a powerful greenhouse gas that contributes to global warming.
The Montreal Protocol, signed by more than 200 countries and generally regarded as having a good record of compliance, is created to protect the Earth's ozone layer.
David Fahey, director of NOAA " s Chemical Science Division and co-chair of the United Nations Environment Programme's Ozone Secretariat 's Science Advisory Panel, said ongoing monitoring of the atmosphere will be key to ensuring that the goal of restoring the ozone layer is achieved.
If the study is verified, this would be a clear violation of the Montreal Protocol. "They should tell the industries that's not going to work".
But in the last few years, it looks like someone has started cheating. "That's a tough group of people".
But in 2012 scientists noted that the rate of decline had slowed by 50 per cent, according to the new study. And thanks to the agreement, we've avoided a total ozone layer collapse by mid-century. This, in turn, will delay the ozone layer's recovery, and in the meantime leave it more vulnerable to other threats.
The researchers said that the less rapid decline of CFC-11 could prevent ozone from returning to normal levels, or at least as quickly as hoped.