The concrete global treaty to protect our planet's atmosphere, known as the Montreal Protocol, is beginning to show cracks.

A few years after a mysterious spike in a known ozone-destroying chemical, another of the protocol's banned substances is also on the rise.

Despite global promises to dial down the use of hydrofluorocarbon (HFC) - a 'super' greenhouse gas with a global warming potential up to 3,000 times that of CO2 - reported reductions do not match the reality.

Someone, somewhere, appears to be fudging the numbers.

"When we saw the reports of enormous emissions reductions from India and China, we were excited to take a close look at the atmospheric data," says atmospheric chemist Matt Rigby from the University of Bristol.

"This potent greenhouse gas has been growing rapidly in the atmosphere for decades now, and these reports suggested that the rise should have almost completely stopped in the space of two or three years."

Except it didn't. In 2018, HFC concentrations actually set an all-time record, despite the fact scientists were expecting a 90 percent drop in emissions between 2015 and 2017.

Using atmospheric data from 2015 through 2018, Rigby and his colleagues show that when reported emissions were at their lowest in 17 years, actual emissions hit their highest at any point in history.

More specifically, it's the hydroflurocarbon HFC-23 - also known as fluoroform - that has hit new highs. It's a bad one, too, with one tonne of the gas having the same greenhouse potential as nearly 12,000 tons of carbon dioxide.

The exact source of this greenhouse gas is still unknown, but given the magnitude of the mismatch and the fact that China and India are two of the largest emitters, the authors suspect these two have not been successful in reducing HFC-23 like they claim.

HFCs were originally introduced as a substitute for ozone-destroying chemicals in refrigeration and air-conditioning products. But while their use is a good thing for ozone, their capacity to act as a potent greenhouse gas has since become all too clear. That's why, in 2016, an amendment was added to reduce the climate impact of HFCs.

"To be compliant with the Kigali Amendment to the Montreal Protocol, countries who have ratified the agreement are required to destroy HFC-23 as far as possible," says atmospheric scientist Kieran Stanley from the Goethe University Frankfurt.

"Although China and India are not yet bound by the amendment, their reported abatement would have put them on course to be consistent with Kigali."

Some numbers are clearly slipping through the cracks, and officials might not even know. Last year, a New York Times report found that violations of CFC-11 emissions were probably coming from factories in China that were ignoring the rules to save money.

But there is another explanation. The authors say they cannot exclude the possibility that China has indeed met its HFC reduction commitment of 98 percent.

In this case, however, there would have to be either a 780 percent increase from developed countries, a 690 percent increase from India, or we haven't accounted for 4,250 percent of the reported global production.

"The magnitude of the CO2-equivalent emissions shows just how potent this greenhouse gas is," says Rigby.

"We now hope to work with other international groups to better quantify India and China's individual emissions using regional, rather than global, data and models."

The study was published in Nature Communications