A recent increase in a number of ozone-depleting chlorofluorocarbons (CFCs) poses a climate threat, scientists say.
Five ozone-depleting chlorofluorocarbons significantly increased in the Earth's atmosphere between 2010 and 2020, despite being banned in 2010.
While researchers think they know the source of three of the CFCs, the origin of two remains a mystery.
CFCs are thousands of times more potent greenhouse gases than carbon dioxide.
CFC emissions overall have declined since the 1980s to about 5 per cent of their peak, and the Antarctic ozone hole is still on track for recovery by around 2060.
So these increases came as a surprise to researchers, who published the study today in Nature Geoscience.
"[These five CFCs] are increasing in the atmosphere," study lead author Luke Western from the University of Bristol in the UK and US National Oceanic and Atmospheric Administration said.
"They've been increasing relatively rapidly since 2010."
Under the Montreal Protocol, which was introduced in 1987 in response to the growing hole in the ozone layer, releasing the ozone-depleting gases as aerosols was banned in 2010.
However, CFCs were allowed to continue to be used as feedstocks — ingredients used to form other compounds, or to be produced as by-products of manufacturing.
The researchers think this is the source of three of the CFCs that have increased.
"We think it's likely the increasing emissions of three of those [CFCs] are associated with their use in the productions of other chemicals, in particular ... [hydrofluorocarbons]," Dr Western said.
Hydrofluorocarbons or HFCs are used mostly in refrigeration and air-conditioning, and were intended to be more climate- and ozone-friendly replacements for CFCs. But today's research shows their use is also problematic, according to Dr Western.
"What we're suggesting here is that there may be some cost to ozone and climate when creating these HFCs and HFOs [hydrofluoroolefins], as ozone-depleting CFCs may be released during the process."
The Kigali Amendment, which was ratified in 2016, was an international agreement to reduce the production of HFCs. But it's only happening very gradually, said atmospheric chemist Robyn Schofield from the University of Melbourne.
And HFCs are often being replaced by hydrofluoroolefins, which have their own impacts.
"[HFOs] have a very short atmospheric lifetime, but that doesn't mean that they're not problematic," Dr Schofield said.
"They get washed out [of the atmosphere] and they form [trifluoroacetic acid] and that can be problematic in drinking water."
Warming potential of increased emissions equivalent to small country
Ian Rae from the University of Melbourne's school of chemistry said it was likely that the increase in the three traceable CFCs were from poorly regulated industries.
"Not all countries are as strict in regulating their industries ... countries where the rule of law doesn't rule all that well.
"There's a tendency if you've got an impurity to just let it go. In poorly regulated countries that's what happens — you put it down the drain if you don't want it."
Professor Rae said today's research would likely be addressed at a future Montreal Protocol meeting.
"The Montreal Protocol meeting will pick it up. It will be in their scientific report.
"They will start then asking countries to look more carefully [for the sources] and to report back. How much they will, I don't know."
The overall impact on the Antarctic ozone hole is expected to be fairly negligible, if the emissions continue at their present rate.
However, the bigger issue is the global warming potential of CFCs, Dr Western said.
"CFCs are potent greenhouse gases," he said.
"The emissions of these gases in 2020 were around the same size as the total carbon [dioxide] emissions for a country like Switzerland.
"So the climate impact is quite substantial from these gases and therefore mitigating these emissions would have a large impact on climate — about the same as a small country going carbon neutral."
Dr Schofield said there are viable alternatives to using HFCs that don't produce CFCs as a by-product and are less potent greenhouse gases. However, some applications still pose a challenge.
"It's difficult for space heaters — where refrigerants are used to cool air.
"But we have great replacements, for [other refrigerants]. You can use CO2 as a refrigerant for water heat exchangers, for example."
Discarded appliances and building foams are still the world's largest sources of leaking CFCs.
