In an effort to stop the widening of the 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 sizable impact on preserving the ozone layer. Evidence of its recovery is mounting, though 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 contributors to ozone depletion.

It appears, however, that someone has been using the compounds for a few years, because there's been an uptick in CFCs, specifically CFC-11, and this could hinder the ozone's recovery.

Who's the mysterious CFC emitter?

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 it still gets into the atmosphere thanks to old appliances and building insulation materials made before the '90s. With the ban on their use, however, concentrations of CFC-11 have fallen by 15 percent since their peak in 1993, according to an NOAA statement.

But that decline has been slowed recently. A 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 down by 50 percent after this period. Concurrent with this slow down in the decline was an increase of CFC-11 emissions between 2014 and 2016.

The increase of CFC is both "unexpected and persistent," according to the researchers who uncovered it, with a 25 percent increase above the average from 2002 to 2012.

A bucket full of empty aerosol cans from the 1990s waiting for proper disposal Aerosols were a common source of CFCs through the 1990s. (Photo: Joseph Sohm/Shutterstock)

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 eliminated as a culprit 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 that the increase is due to an increase in CFC-11 use, whether actual production or as a byproduct, somewhere in East Asia.

Either way, the study isn't pointing fingers; it's instead notifying the world that there's a problem — and it needs to be fixed.

"We're raising a flag to the global community to say, 'This is what's going on, and it is taking us away from timely recovery of the ozone layer,'" NOAA scientist Stephen Montzka and the study's lead author said in the NOAA statement. "Further work is needed to figure out exactly why emissions of CFC-11 are increasing, and if something can be done about it soon."

And acting soon is key. Currently, the damage done to the ozone's recovery is minor, but if left unchecked, it could worsen the hole in the ozone layer. That, in turn, would worsen climate change.