The Montreal Protocol—a 1987 international agreement to end production of ozone-destroying chemicals like freon—seems miraculous compared to the long struggle to achieve meaningful action on climate change. Even more astonishing is that the agreement has worked. Those chemicals (known as CFCs) take a long time to flush out of the atmosphere, but monitoring has shown that the flushing is proceeding largely according to plan.

That keeps the hole in the ozone layer on track to shrink over the coming decades. However, a new study shows that someone has been cheating in the last few years.

A group of researchers led by Stephen Montzka of the US National Oceanic and Atmospheric Administration had been tracking the progress of CFCs and noticed something off with CFC-11. This chemical has been used as a refrigerant, solvent, and propellant for aerosol spray cans, as well as in the production of styrofoam. As with the other CFCs, nations agreed to end production of CFC-11 entirely. While there may still be some older machines leaking CFC-11, these sources should gradually disappear over time, allowing the decline of its atmospheric concentration to accelerate.

Hiding the decline?

Instead of an accelerating decline, CFC-11 showed a steady drop of 2.1 parts-per-trillion each year between 2002 and 2012. Since then, its decline has actually slowed. Between 2015 and 2017, CFC-11 dropped at only 1.0 part-per-trillion per year.

There are a few possible explanations to sort through. The most important one is natural variations in the transport of emitted CFCs into the stratosphere, which depends on weather patterns. But some of them can be eliminated quickly. A sudden uptick in the demolition of old buildings with CFC-11 refrigerants in their HVAC systems doesn’t seem to plausibly fit the data, for example.

Careful analysis of the data and some modeling can help us choose among the remaining explanations. First off, the concentration of these gases has always been a little higher in the Northern Hemisphere than the Southern Hemisphere, because most of the sources are in the north. Over the last few years, the difference between the two hemispheres has increased a bit. Similar gases haven’t done that, which points to increased emissions from the Northern Hemisphere rather than just a change in the winds.

Second, measurements from atop Mauna Loa in Hawaii show correlations between CFC-11 concentrations and a few other gases known to come from industrial emissions. That means CFC-11 isn’t the only human pollutant seeing an uptick over the same time span.

Finally, the researchers used some models to find out what kinds of emissions would fit the pattern of measurements around the world. Modeling weather patterns since 2000 shows that natural variability in atmospheric circulation could explain some portion of the changing trend—but less than half. The measurements can only really be explained by an increase in emissions from Eastern Asia.

A new source

At the height of use in the 1980s, humans released 350,000 tons of CFC-11 each year—a number that dropped to 54,000 tons per year in the early 2000s. An additional 6,500 to 13,000 tons released each year in Eastern Asia would be enough to change the declining trend in just the way we’ve observed. An increase that large seems to require renewed production of CFC-11—violating the Montreal Protocol.

“This is the first time that emissions of one of the three most abundant, long-lived CFCs have increased for a sustained period since production controls took effect in the late 1980s,” the researchers write. “A delay in ozone recovery [...] is anticipated, with an overall importance depending on the trajectory of CFC-11 emissions and concentrations in the future.”

Seeing as nations are required to track CFC production and report accurate numbers to the United Nations group that oversees the Montreal agreement, this is going to be a contentious conclusion. The researchers chose their words carefully, and the network of measurements isn’t complete enough to point the finger at a specific nation. Still, the list of suspects is short, and some nation needs to find and snuff out the illicit industrial activity within its borders in order to hold up its end of the Montreal Protocol.

Nature, 2018. DOI: 10.1038/s41586-018-0106-2 (About DOIs).