Scientists are tracking a sudden increase in a greenhouse gas that is 12,000 times more potent than carbon dioxide, even though its major global producers claim they’ve mostly phased it out.

“Atmospheric gas measurements at five stations around the world show that emissions of HFC-23 or trifluoromethane reached a record high in 2018 of 15,900 tonnes,” CBC reports, citing a new study in the journal Nature Communications. “That’s a lot higher than the 2,400 tonnes of emissions of that gas reported by China and India to the United Nations Environment Programme in 2017.”

HFC-23 is a byproduct of HCFC-22, a refrigerant that both warms the atmosphere and depletes the stratospheric ozone layer.

“Our study finds that it is very likely that China has not been as successful in reducing HFC-23 emissions as reported,” study author Kieran Stanley, a visiting research fellow at the University of Bristol, said in a release. “Alternatively, or additionally, there may be substantial unreported production of HCFC-22 at unknown locations, resulting in unaccounted-for HFC-23 byproduct being vented to the atmosphere.”

“HFC-23 produced during the manufacture of HCFC-22 was traditionally released into the atmosphere,” CBC says. “Under international agreements to protect the ozone layer, the Montreal Protocol and its 2016 Kigali Amendment, HFC-23 is supposed to be destroyed. However, the phaseout is slower for developing countries such as China and India, and isn’t officially yet in effect.”

But even so, “both countries were reporting reductions, and India passed a regulation in 2016 requiring incineration of HFC-23.” Stanley’s study calls for more measurements at the regional level to get a handle on what’s going on.