There are plenty of factors affecting our climate—but after a wholesale ban more than a decade ago, chlorofluorocarbons (CFCs) shouldn’t be one of them. The Montreal Protocol—an international environmental treaty finalized in 1987—banned the ozone-depleting chemicals, with a full-on phase-out by 2010, with a goal of healing the ozone completely by 2060.

Unfortunately, new readings and research published this week in Nature show that levels of some CFCs have actually risen over the last decade.

What Are Chlorofluorocarbons?

Chlorofluorocarbons are chemicals that contain chlorine, fluorine, and carbon. In the past, CFCs could be found in refrigeration systems, air conditioning, aerosol sprays, and more.

Once these chemicals reach the upper atmosphere, they undergo reactions that harm the stratosphere—aka lead to holes in the ozone layer—and can remain present for hundreds of years.

It seems that levels of CFC-113a, CFC-114a, CFC-115, CFC-13, and CFC-112a have risen rapidly between 2010 and 2020, a reality that atmospheric chemists find mysterious.

Where Are These Chemicals Coming From?

While three of the increasing CFCs may be being released accidentally as a byproduct of manufacturing, the cause of the rise of two (CFC-13 and CFC-112a) are unknown.

“We don’t know of any chemical process where this [CFC-13 will show up as a by-product,” atmospheric scientist Martin Vollmer told Nature. Though bizarre, the detection is ultimately a good thing: Once the source of the banned CFC is traced, the emissions can be stopped.