Efforts to curb emissions of a powerful greenhouse gas commonly produced as a by-product of refrigerant manufacture might be falling short, and it seems eastern China is a major culprit.

The hydrofluorocarbon gas, HFC-23, is around 14,700 times as powerful as carbon dioxide at warming the globe and has long been the subject of national and international climate-change mitigation efforts. Those efforts gained new traction nearly a decade ago when China and India — the world’s largest producers of the chemical — agreed to dial down its emissions. New research¹, however, confirms that emissions continued to rise in subsequent years, and an analysis of data from atmospheric-monitoring stations suggests that factories in eastern China are responsible for nearly half of the total.

The chemists policing Earth’s atmosphere for rogue pollution

The rogue emissions are one of several air-pollution sources under discussion at the latest meeting of the Montreal Protocol, held in Nairobi, Kenya, this week. Signed in 1987, the Montreal Protocol is generally considered the most effective international environmental treaty in history, having halted the destruction of the ozone layer while also slowing down global warming. But scientists have often played a role, scanning the atmosphere for chemicals, such as ozone-depleting chlorofluorocarbons (CFCs), that governments have agreed to phase out.

“Science has been instrumental in evaluating compliance under the treaty,” says Megan Lickley, a climate scientist at Georgetown University in Washington DC.

Finding the source

HFC-23, which also has some specialty industrial uses, has a controversial history. In 2007, scientists raised the alarm about HFC-23 transactions stemming from a United Nations programme that allowed wealthy countries to purchase carbon credits from lower-income nations. Destroying the chemical was an easy source of carbon credits, and it was also enormously profitable for factories in lower-income countries. The fear was that some factories increased their HFC-23 production for the sole purpose of selling carbon credits.

China feels the heat over rogue CFC emissions

Major HFC-23 producers eventually agreed to halt emissions of the chemical unilaterally. In 2020, however, atmospheric scientists reported evidence² of an increase in HFC-23 emissions that ran counter to an expected decline of 87% from 2014 to 2017. A second team followed up this August with a more detailed analysis¹ of atmospheric samples collected on an island off the southern tip of South Korea, downwind of China. They confirmed the increase, and pinpointed eastern China as the source of nearly half of the unexpected HFC-23 found in the atmosphere from 2015 to 2019, contradicting a 99% reduction in emissions reported by China.

Overall, the analysis suggests that HFC-23 emissions in China nearly doubled, from some 5,000 tonnes in 2008, to around 9,500 tonnes in 2019, although those emissions were not covered under the Montreal Protocol at the time. (In 2016, the protocol was amended such that governments agreed to destroy HFC-23 ‘to the extent practicable’, but only from 2020 onwards.)

The monitoring stations that scientists use to track global trends in trace gases such as HFC-23 are too sparse and remote to pinpoint where all the remaining emissions of the chemical are coming from.

A spokesperson for the Chinese embassy in Washington DC declined to answer questions regarding the HFC-23 emissions trends. Nature was unable to reach China’s Ministry of Ecology and Environment for comment.

Considering the evidence, “it may be that China has got a problem”, says Durwood Zaelke, the president of the Institute for Governance & Sustainable Development, an advocacy group based in Washington DC. But Zaelke says that scientists have raised similar evidence in the past, and governments have ultimately taken action under the treaty. “History suggests that this will be taken care of.”

Smaller fish

The HFC-23 case has parallels to that of trichlorofluoromethane, or CFC-11, an ozone-destroying chemical that was used in spray-foam insulation before being banned in 2010. A team led by scientists at the US National Oceanic and Atmospheric Administration (NOAA) revealed a mysterious spike in emissions in 2018, and a year later those emissions were tracked back to factories in northeast China. Scientists later confirmed that CFC-11 emissions declined sharply in 2019 and 2020, which limited the chemical’s effect on stratospheric ozone.

‘This shouldn’t be happening’: levels of banned CFCs rising

“It was clear that there was a big response, and emissions dropped,” says Steve Montzka, an atmospheric chemist at NOAA in Boulder, Colorado, who discovered the problem.

Countries will have their first opportunity to take action on the recently published evidence of ongoing HFC-23 emissions as the meeting comes to a close this week. Discussions have also arisen regarding new evidence of ongoing emissions of several CFCs, including some that are probably produced as building blocks — or feedstocks — for other chemicals, and are thus exempt from controls under the Montreal Protocol.

Lickley says that the exemption was originally created under the assumption that it would not lead to huge emissions, because most of the feedstocks would be consumed in the synthesis of new chemicals rather than be released into the environment. But the latest evidence on CFCs suggests that some of these substances are already beginning to accumulate in the atmosphere. “This underscores the need for the parties to the Montreal Protocol to tighten controls on feedstocks,” she says.

For Montzka, the current focus on these lesser, residual emissions of HFCs and CFCs can be taken as a sign of broader success. Action taken under the Montreal Protocol has already stabilized the ozone layer by phasing out the bulk of CFCs, and the 2016 amendment curbing HFC emissions could on its own avert as much as 0.5 °C of global warming by 2100, according to a 2022 scientific assessment by the World Meteorological Organization and the United Nations Environment Programme. “We’ve gotten all of the big fish in the pond, and now there are a bunch of little fish swimming around,” he says. Catching the small ones, he adds, “will require some more work”.