Scientists have detected a massive and mysterious rise in a banned ozone-eating chemical which they believe is originating from around China, Mongolia and the Korean peninsula.
They have warned that unless the production of chlorofluorocarbons (CFCs) is identified and stopped, the recovery of the ozone layer will be jeopardised.
CFCs were in wide use as refrigerants and propellants in aerosols and refrigerators until the 1980s, when emergency action by the UN saw a number of climate change treaties ratified by all member states – the first treaties to achieve this.
Despite all countries having ratified the Montreal Protocol, which seeks to reduce CFCs in the atmosphere, rogue polluters have seen emissions rise by 25% since 2012.
The emissions were detected by Dr Stephen Montzka, a researcher at the US National Oceanic and Atmospheric Administration (NOAA) in Colorado, and international colleagues who monitor the atmosphere.
"I have been doing this for 27 years and this is the most surprising thing I've ever seen. I was just shocked by it," he said.
The Earth's ozone layer is a region of the stratosphere which absorbs up to 99% of the sun's ultraviolent radiation, protecting life near the surface of the planet.
In the 20th century scientists discovered that CFC emissions were causing enormous holes in this layer over the poles – driving global warming and sea level rises.
In 2001, scientists at NASA said they located the largest ozone hole ever recorded, covering 11.5 million square miles over Antarctica.
"Although levels of these chemicals have been declining since the 1990s due to the Montreal Protocol, they will remain in the atmosphere for years affecting ozone levels well into the century," explained Dr Susan Stagan, a NASA scientist.
Dr Stagan said that the hole over the Antarctic was partly seasonal, growing and shrinking based on other variables – including whether the pole was tilted towards or away from the sun.
However the role of emissions in producing it were widely accepted scientific fact.
It is not clear who is behind the emissions, even if they could be roughly located to East Asia, according to an article published in the journal Nature.
The rise means that someone is either making the banned compound or they are the result of byproducts which have not been reported as required, Dr Montzka said.
Dr Ross Salawitch, an atmospheric scientist at the University of Maryland, called it "rogue production" and said if it continued "the recovery of the ozone layer would be threatened."
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Erik Solheim, head of UN Environment, said: "If these emissions continue unabated, they have the potential to slow down the recovery of the ozone layer.
"It's therefore critical that we identify the precise causes of these emissions and take the necessary action."
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