After a hiccup in global efforts to reduce ozone-damaging emissions, scientists have found that levels of the major offending gas – trichlorofluoromethane (CFC-11) – have once again started to fall.
The finding, published in two Nature papers, brings welcome news after violations of the global agreement to reduce emissions were discovered and promptly followed up.
“This was a major test for the Montreal Protocol, which it appears to have passed,” says Stephen Montzka, lead author of the first paper, “and this is a great example of how important early warnings from observational systems can be.”
The Montreal Protocol is an international treaty adopted in Canada in 1987 to ban chemicals that were depleting the Earth’s ozone layer by 2010, which has been signed by nearly 200 countries.
It was a major success, producing drastic drops in global emissions over the following couple of decades. But then global monitoring of CFC-11 concentrations, largely over Mauna Loa, Hawaii, revealed the decline had tapered off.
“Since 2013, the concentration decline of CFC-11 slowed unexpectedly owing to increasing emissions, probably from unreported production,” write Montzka and co-authors, “which, if sustained, would delay the recovery of the stratospheric ozone layer.”
Following the discovery in 2018–19 and calls for urgent action by the international community, atmospheric scientists linked most of the new emissions to east China. The Chinese government thus committed to vamp up their inspection measures and reinforce the ban on CFCs.
Renewed analyses of atmospheric data post-2017 indeed linked the renewed drops in emissions to east China. Authors of the second paper report that their emissions plunged by 33% compared to 2014–17, bringing them back to pre-2013 levels.
“The large increase in emissions of CFC-11 from China … have been severely reduced,” says co-author Paul Fraser from Australia’s CSIRO, “and the timeline for the closure of the Antarctic ozone hole, which was to be delayed by these emissions, is now back on track as expected.”
CFCs (chlorofluorocarbons), which have been used for aerosol sprays, refrigeration and plastic foam insulation, deplete the layer of ozone that protects the Earth from harmful ultraviolet rays. The subsequent “hole”, which occurs seasonally, reached 24.7 million square kilometres last year, losing the second highest amount of ozone on record.
Concentrations of CFCs in the Earth’s atmosphere, where they can linger for decades, are measured independently by the National Oceanic and Atmospheric Administration (NOAA), US, and the Advanced Global Atmospheric Gases Experiment (AGAGE).
To pinpoint regional emissions, measurements need to be done relatively close to the suspected source. In China’s case, the recent observations were derived from Gosan, South Korea, and Hateruma, Japan.
Other sources of rogue emissions have been elusive, although monitoring networks eliminated Europe, North America and Australia.
However, evidence suggests renewed efforts to reduce emissions have occurred globally, according to Sunyoung Park, from Kyungpool National University in Korea, and co-authors of the second paper.
But there are still challenges ahead as the two families of less damaging ozone chemicals that replaced the CFCs – hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs) – are still potent greenhouse gases that contribute to climate change and need to be phased out.
On the positive side, the new studies confirm the value of continued observations.
“It highlights the importance of continuing to do high-quality long-term monitoring of ozone depleting chemicals and greenhouse gases in the atmosphere,” says Paul Krummel, also from the CSIRO and a co-author of the second paper.
“This can detect any potential non-compliance with international protocols or treaties, so that action can be taken to mitigate this.”