This story originally appeared on Yale Environment 360 and is part of the Climate Desk collaboration.

When the Montreal Protocol marked its 30th anniversary in 2017, it seemed like an unalloyed triumph for environmental common sense. By banding together to address a planetary emergency, the 197 signatory nations had officially ended production and use of chemicals responsible for depleting the ozone layer in the upper atmosphere, an essential shield against the sun’s ultraviolet radiation. It was a “milestone for all people and our planet,” according to António Guterres, secretary-general of the United Nations. “The Earth’s Ozone Hole Is Shrinking,” one celebratory headline announced. “Without the Ozone Treaty,” another advised, “You’d Get Sunburned in 5 Minutes.”

But an unexpected recent spike in emissions of chlorofluorocarbons, the major ozone-depleting chemicals, now suggests it’s far too soon to close the file on ozone depletion. A new study published this week in Nature pins down the source of 7,000 metric tons a year of new CFC-11 (trichlorofluoromethane) emissions to the provinces of Shandong and Hebei on the northeastern coast of China. That’s an area half the size of Texas, with a population of about 170 million people, including the city of Beijing. The bulk of these emissions are believed to come from small factories that are using CFC-11, in violation of the Montreal Protocol, to manufacture foam insulation used in refrigerators and buildings.

The new study accounts for roughly half the emissions reported last year in Nature, by some of the same authors. Based on data from the Mauna Loa Observatory in Hawaii, the previous study reported new emissions of about 13,000 tons a year of CFC-11 from somewhere in eastern Asia starting in 2012.That was two years after the 2010 date for ending all CFC production under the terms of the Montreal Protocol. It was “the first time that emissions of one of the most abundant CFCs have increased for a sustained period since production controls took effect in the late 1980s,” according to that study.

Protecting the ozone layer has thus come to look increasingly like an endless search for new and unsuspected threats, as well as for old threats unexpectedly revived. It’s a search conducted primarily by a patchy network of monitoring stations managed in a multinational collaboration, on islands and mountaintops around the world. These stations measure trace concentrations of about 50 ozone-depleting chemicals and greenhouse gases in parts per trillion in the upper atmosphere. The protocol of sampling many times each day for decades on end is difficult and expensive, requiring a long-term commitment from the country where each observatory is located. Hence only 15 observatories now participate in the Advanced Global Atmospheric Gases Experiment (AGAGE), leaving huge geographic gaps in coverage.

The new study is based on reports from monitoring stations in Gosan, South Korea, and in Hateruma, in southern Japan. Because of the gaps in the AGAGE network, however, it’s unlikely that scientists will ever be able to detect the source of the rest of the reported 13,000 tons of new emissions. And because the monitoring stations measure only emissions, not production, that 7,000-ton figure probably represents only a small fraction of the total CFC-11 production, even within Shandong and Hebei provinces, according to Sunyoung Park, one of the study’s authors and an atmospheric chemist at South Korea’s Kyungpook National University. The remainder is now banked in the foam insulation or other products it was used to manufacture and will slowly leak out over the course of the century.

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These reported violations of the Montreal Protocol come at an especially perilous moment. The so-called Kigali Amendment to the protocol went into effect in January. It begins an 80-percent phase-down by midcentury in the production and use of hydrofluorocarbons, the chemicals that replaced CFCs in air conditioning, refrigeration, and foam insulation starting in the 1990s. HFCs are much less harmful than CFCs for the ozone layer. But they have turned out to be thousands of times more powerful than carbon dioxide as a cause of global warming, with the potential to add half a degree Celsius of warming by the end of the century. The danger now, according to some observers, is that the shift away from HFCs will push some manufacturers to fall back on an illegal trade in CFCs.

The growth in CFC-11 emissions in northeast China from 2008 to 2017, showing a significant spike between 2014 to 2017. RIGBY ET AL, NATURE 2019

The new emissions aren’t large enough so far to be catastrophic. A separate study last year in the journal Geophysical Research Letters found that the level of ozone-depleting chemicals in the atmosphere over Antarctica continues to decline at a rate of about 0.8 percent a year. But that could change as the new emissions make their way into the stratosphere and begin to break down, a process that typically takes about five years, according to Susan Strahan, an atmospheric scientist with the Universities Space Research Association at NASA’s Goddard Space Flight Center. The reported emissions, if continued, will slow the rate at which these chemicals decrease and could delay recovery of the ozone layer by a decade or more, until the end of the century, according to Stephen Montzka, of the US National Oceanographic and Atmospheric Administration, an author on both Nature studies. On the other hand, Montzka says, “If they go away quickly, the influence should be small.”