In an effort to stop the widening of a hole in the ozone layer, the international community banned the use of ozone-destroying chemicals through the Montreal Protocol, first signed in 1987.
The treaty has had a sizable impact on preserving the ozone layer. Evidence of its recovery is mounting, although there are still some concerns about the ozone at lower levels of the atmosphere. Of particular note has been the reduction in human-made chlorofluorocarbons (CFCs), one of the primary causes of ozone depletion.
In 2018, however, it became apparent that someone is still using these compounds, and has been for a few years. Researchers detected a mysterious uptick in CFCs, specifically CFC-11, that could hinder the ozone layer's recovery. After they initially narrowed down the source to East Asia, a follow-up study now identifies a single country where most of the CFCs seem to be coming from: China.
'Unexpected and persistent'
CFC-11 was once commonly found in refrigerants, aerosol sprays and Styrofoam. The chemical compound was banned in developed countries by the Montreal Protocol in the mid-1990s, with the rest of the world following suit by 2010.
CFCs last for about 50 years, and some of the compounds still get into the atmosphere today from old appliances and insulation materials made before the '90s. With the ban on their use, however, concentrations of CFC-11 have fallen by 15% since their peak in 1993, according to a 2018 statement from the U.S. National Oceanic and Atmospheric Administration (NOAA).
But that decline has slowed recently. A 2018 study published in the journal Nature found that while the decline of CFC-11 in the atmosphere was consistent from 2002 to 2012, it slowed by 50% after this period. Concurrent with this slowdown in the decline was an increase of CFC-11 emissions between 2014 and 2016.
This CFC spike is both "unexpected and persistent," according to the researchers who uncovered it, with a 25% increase above the average from 2002 to 2012.
Tracing the source
As for where all this new CFC is coming from, researchers engaged in some detective work to find potential suspects. The destruction of buildings with CFC-11 in the insulation or the HVAC was ruled out because it simply didn't fit the data.
The Northern Hemisphere has consistently had slightly higher concentrations of CFC than the Southern Hemisphere, but this has evened out over time thanks to an increase from the Southern Hemisphere. Other gases have not increased over time, so the wind isn't at fault, either. Similarly, changes in atmospheric conditions could account for some of the increase, but only less than half.
This led researchers to conclude the increase is due to an increase in CFC-11 use, whether actual production or as a byproduct, somewhere in East Asia.
Although the 2018 study didn't point fingers beyond that, a follow-up study published in May 2019 did. Led by an international team of scientists, the study found that China accounted for 40% to 60% of the global increase of CFC-11 emissions between 2014 and 2017. Those emissions mostly came from China's northeastern provinces of Shandong and Hebei, according to the study.
"Initially our monitoring stations were set up in remote locations, far from potential sources," explains Ron Prinn, co-author of the new study and professor at the Massachusetts Institute of Technology (MIT), in a statement. "This was because we were interested in collecting air samples that were representative of the background atmosphere, so that we could monitor global changes in concentration and determine their atmospheric lifetimes."
To fix that, researchers set up more monitoring stations closer to industrialized regions. Data from those stations were then used in computer simulations to trace the CFCs back to their sources. These models showed that CFC emissions from eastern China had risen by around 7,000 metric tons per year since 2012, particularly in or around Shandong and Hebei provinces. "We didn’t find evidence of increasing emissions from Japan, the Korean peninsula, or any other country to which our networks are sensitive," adds co-author Luke Western, a post-doctoral researcher at the University of Bristol in the U.K.
This seems to confirm previous reports pointing to China, where some new production and use of CFCs has apparently occurred in defiance of the Montreal Protocol, the researchers concluded.
While China may be responsible for "a substantial fraction" of this CFC comeback, the researchers caution that smaller increases may have also taken place in other countries. "There are large swathes of the world for which we have very little detailed information on the emissions of ozone-depleting substances," says lead author Sunyoung Park from Kyungpook National University in South Korea.
Still, this study represents an important leap forward, its authors say, in scientists' ability to figure out which parts of the world are emitting ozone-depleting chemicals, greenhouse gases and other pollutants, as well as how much they're emitting.
"It is now vital that we find out which industries are responsible for the new emissions," says co-author Ray Weiss, a geochemist at Scripps Institution of Oceanography at the University of California San Diego. "If the emissions are due to the manufacture and use of products such as foams, it is possible that we have only seen part of the total amount of CFC-11 that was produced. The remainder could be locked up in buildings and chillers and will ultimately be released to the atmosphere over the coming decades."
In the meantime, public attention to this problem may be helping at least a little. According to an August 2018 report from the international Environmental Investigation Agency, Chinese authorities have begun to crack down on the illegal production and use of CFC-11.
Editor's note: This article has been updated with new information since it was originally published in May 2018.