A greenhouse gas that can cause 12,000 times more warming per tonne than carbon dioxide is rising unexpectedly in the atmosphere, despite reports by its major producers, China and India, that they've mostly eliminated emissions of the gas.

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, reports a study led by Kieran Stanley, a visiting research fellow at the University of Bristol.

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 Program in 2017, notes the study, published this week in Nature Communications.

HFC-23 is a byproduct in the production of the refrigerant HCFC-22, which is a greenhouse gas and depletes the ozone layer.

Based on China's and India's reports, scientists had expected to see HFC-23 levels drop 90 per cent between 2015 and 2017 in measurements by the Advanced Global Atmospheric Gases Experiment.

Instead, the difference between reported and measured emissions during that period is equivalent to all of Spain's carbon emissions for one year, the researchers estimate.

"Our study finds that it is very likely that China has not been as successful in reducing HFC-23 emissions as reported," said Stanley in a news release from the University of Bristol.

"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," the study suggests.

HFC-23 produced during the manufacture of HCFC-22 was traditionally released into the atmosphere. 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.

Nevertheless, both countries were reporting reductions, and India passed a regulation in 2016 requiring incineration of HFC-23. The authors of the new study say more regional measurements are needed to verify whether that's actually happening.