The decline in the atmosphere of an ozone-depleting chemical banned by the Montreal Protocol has recently slowed by half, suggesting a serious violation of the 196-nation treaty, researchers revealed Wednesday.
Measurements at remote sites -- including the government-run Mauna Loa Observatory in Hawaii -- of the chemical, known as CFC-11, point to East Asia as the source or renewed production.
"We show that the rate of decline of atmospheric CFC-11 was constant from 2002 to 2012, and then slowed by about 50 percent after 2012," an international team of scientists concluded in a study.
"This evidence strongly suggests increased CFC-11 emissions from eastern Asia after 2012."
The ozone layer in the stratosphere, 10-to-40 kilometres (6-25 miles) above Earth's surface, protects life on the planet from deadly ultraviolet radiation.
The 1987 Montreal Protocol banned industrial aerosols such as chlorofluorocarbons (CFCs) that were chemically dissolving ozone, especially above Antarctica.
At its most depleted, around the turn of the 21st century, the ozone layer had declined by about five percent.
Today, the "hole in the ozone" over the South Pole is showing clear signs of recovery.
"The ozone layer remains on track to recovery by mid-century," the UN Environment Programme (UNEP) said in a statement, reacting to the findings.
But "continued increase in global CFC-11 emissions will put that progress at risk."
The slowdown in reduction of CFC-11 also has implications for the fight against climate change.
"Perhaps even more serious is the role of CFCs as long-lived greenhouse gases," noted Joanna Haigh, a professor at Imperial College London, in commenting on the study.
Two decades ago, CFCs -- more potent by far as greenhouse gases than carbon dioxide or methane -- accounted for around ten percent of human-induced global warming.
- 'Atmospheric detective work' -
Widely used in 1970s and 1980s as propellant in aerosol sprays, as well as in refrigeration and air conditioning systems, CFCs do not exist in nature.
"This is the first time that emissions of one of the three most abundant, long-lived CFCs have increased for a sustained period since production controls took effect in the 1980s," the study concluded.
CFC-11 still contributes about a quarter of all chlorine -- the chemical that triggers the breakdown of ozone -- reaching the stratosphere.
The researchers said that the less rapid decline of CFC-11 could prevent ozone from returning to normal levels, or at least as quickly as hoped.
"A timely recovery of the stratospheric ozone layer depends on a sustained decline of CFC-11 concentrations," the wrote.
Other scientists not involved in the study signalled its importance.
"This is atmospheric detective work at its finest," said Piers Forester, head of the Priestley International Centre for Climate at the University of Leeds.
"The authors pinpoint a new source of CFC-11 to East Asia, breaking Montreal Protocol rules."
The UNEP said that is was "critical that we take stock of this science, identify the causes of these emissions and take necessary action."
A study earlier this year found that the ozone layer is unexpectedly declining in the lower stratosphere -- 10 to 24 kilometres above sea level -- over the planet's populated tropical and mid-latitude regions.
CFCs and other molecules have mainly eroded ozone in the upper stratosphere, and over the poles.
The study, Atmospheric Chemistry and Physics, identified two possible culprits: industrial chemicals not covered by the Montreal Protocol called "very short-lived substances" (VSLSs), or climate change, which would be far more difficult to resolve.