Bans on certain ozone-depleting gases have been in effect since the late 1980s, but scientists recently identified four new, man-made gases that could be harmful to this protective layer.
The kicker is they don't know where they're coming from.
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A team of experts identified one hydrochlorofluorocarbons and three chlorofluorocarbons from air samples collected in Tasmania and ice samples in Greeland. Analysis of the samples showed these gas emissions into the atmosphere began in the 1960s, according to the study published in Nature Geoscience.
Though combined emissions are said to be small compared to peak emissions of ozone-depleting gases in the 1980s, the study authors said they are "clearly contrary to the intentions behind the Montreal Protocol, and raise questions about the sources of these gases."
Due to their relatively low levels now, lead author Johannes Laube of the University of East Anglia in England told Reuters they are "not yet a threat to the ozone layer," but they still merit investigation due to their harmful potential.
"It does set off an alarm bell because we thought production of all the CFCs had been shut down," Paul Newman, chief atmospheric scientist at the NASA Goddard Space Flight Center and co-chairman on the Montreal Protocol's assessment panel, said, according to the Wall Street Journal.
The Montreal Protocol was established in 1987 as a treaty, ratified by 197 countries in the United Nations, to limit production of CFCs. A total ban was enacted in 2010.
“The identification of these four new gases is very worrying as they will contribute to the destruction of the ozone layer," Laube said. "We don’t know where the new gases are being emitted from and this should be investigated. Possible sources include feedstock chemicals for insecticide production and solvents for cleaning electronic components."
"The concentrations found in this study are tiny. Nevertheless, this paper reminds us we need to be vigilant and continually monitor the atmosphere for even small amounts of these gases creeping up, either through accidental or unplanned emissions," Piers Forster, from the University of Leeds, told BBC. "Of the four species identified, CFC-113a seems the most worrying as there is a very small but growing emission source somewhere, maybe from agricultural insecticides. We should find it and take it out of production."
CFCs are the main cause of the ozone hole over Antarctica, but last year, NASA reported that the Antarctic ozone hole was the smallest it had been in 20 years.