The ozone layer is critical to life, limiting the amount of harmful ultraviolet solar radiation that reaches the planet’s surface. CFC-11 is also a potent greenhouse gas, with roughly 4,750 times the heat-trapping potential of carbon dioxide.

The new report underscores the need for enforcement of international environmental agreements even when the hazards are clear and profound. And it is a reminder that China’s intensifying environmental challenges have global consequences.

“This is a huge problem,” a US State Department official said on Wednesday. The official said the department plans to review the report but has not yet concluded that China is the source of the new emissions.

“If it’s a problem in another country, we’re also going to be suffering,” the official said.

Any production and use of CFC-11 is a violation of the Montreal Protocol, a 1987 agreement that phased out such chemicals in favour of ones that do not damage the atmosphere. The global accord was reached after scientists revealed the existence of an expanding hole in the ozone layer over Antarctica.

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Last year’s report did not identify the source of the new emissions beyond saying it is most likely they came from eastern Asia. But authors of the new report identify the provinces of Shandong and Hebei in eastern China as the likely source for at least 40 per cent of the emissions.

The researchers based their conclusion on air samples from monitoring stations in South Korea and Japan. Those sampling stations, which feature instruments that can tease out the molecular components of the air, showed periodic spikes in CFC-11.

The researchers combined that data with weather forecasts and observations of wind patterns and ran a series of computer models to pinpoint the most likely origin of the emissions. The results pointed to the two Chinese provinces.

“When the wind is blowing in a straight line from that source to the measuring station, you see a spike,” said lead author Matt Rigby, an atmospheric scientist at the University of Bristol.

“We hope to work with Chinese colleagues in the future to see if similar signals are visible in their data,” Rigby said.

The Montreal Protocol is often held up as a model of global cooperation in protecting the environment, and for what nations can do collectively to combat climate change.

The parties to the protocol were alarmed by last year’s report and called for “urgent action” to investigate the sources of emissions and enforce the international agreement.

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“We cannot relax our vigilance for a second. We cannot let this go unaddressed,” Tina Birmpili, head of the UN Environment’s Ozone Secretariat, said last summer.

The protocol has largely worked as intended, and the authors of the new report reiterate that central message: The ozone layer continues to recover despite the evidence of new emissions.

The problem is that the recovery may be happening more slowly than expected. Levels of CFC-11 in the atmosphere have not declined as rapidly as computer models predicted they would.

There might have been relatively benign explanations. A great deal of CFC-11 is already “banked” in the built world, in the form of insulation that gradually leaks the gas into the atmosphere.

Rigby and his colleagues looked at the possibility that new construction, and the replacement of building stock and destruction of older infrastructure, could have released the chemical into the air.

But the numbers did not add up. The most likely explanation is new production and use of the chemical, he said.

Rigby said that the new emissions of CFC-11 have the global warming equivalent of all the CO2 emitted annually by human activity in the city of London.

Stephen Montzka, a National Oceanic and Atmospheric Administration scientist and co-author of Wednesday’s study, as well as the 2018 report, said the latest findings leave important unanswered questions. While researchers traced roughly half of the troubling CFC-11 emissions to the two provinces in China, details about the source for other emissions remain elusive.

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“The guessing game is now: Where is the other half coming from?” Montzka said, noting that the study detailed emissions only in a limited region of China, which encompasses about a third of its population.

The spike in emissions of CFC-11 raises other questions about why anyone would still manufacture it.

The short answer is that CFC-11 is favoured by some companies because it is cheaper and more effective than climate-friendly alternatives. That is the conclusion of an investigation published last year by the Environmental Investigation Agency, a Washington-based environmental watchdog group.

The researchers talked with representatives of 18 companies in 10 Chinese provinces who acknowledged their use of the banned chemical. Six of those companies were in Shandong and Hebei.

After last year’s disclosure of new emissions, Chinese authorities cracked down on illegal use of CFCs, seizing 29 tonnes of chemicals and closing some rogue factories, according to Avipsa Mahapatra, who works for the watchdog group.

But she said the Nature paper indicates that this seized material represents only a fraction of what’s being produced and used. She said her investigators talked with companies that hid their use of CFC-11, for example by displaying legal chemicals that could be shown to government inspectors.

“We are just beginning to understand the scale of the problem,” she said.