"It's the most surprising and unexpected observation I've made in my 27 years" of measurements, said study lead author Stephen Montzka, a research chemist at the National Oceanic and Atmospheric Administration.
The scientists say that the increase is likely a result of new, unreported production of the gas, known as CFC-11, probably in East Asia.
Emissions of a banned, ozone-depleting chemical are on the rise, a group of scientists reported Wednesday, suggesting someone may be secretly manufacturing the pollutant in violation of an global accord.
That speculation is due to increased CFC-11 emissions, a big issue that could delay ozone restoration efforts and contribute to a warming planet.
"This evidence strongly suggests increased CFC-11 emissions from eastern Asia after 2012". The chemical, called CFC11, was used for making foam, degreasing stains and for refrigeration.
The finding that the destruction of ozone was creating a large "hole" over the Antarctic led to the signing of the Montreal Protocol in 1987. "Further work is needed to figure out exactly why emissions of CFC-11 are increasing and if something can be done about it soon".
However, a study recently published in Nature reveals that CFC-11 production may be happening somewhere in the world despite the Montreal Protocol.
Because CFC-11 still accounts for one-quarter of all chlorine present in today's stratosphere, expectations for the ozone hole to heal by mid-century depend on an accelerating decline of CFC-11 in the atmosphere as its emissions diminish- which should happen with no new CFC-11 production.
"Emissions today are about the same as it was almost 20 years ago", he said. But the apparent increase in emissions of CFC-11 has slowed the rate of decrease by about 22 percent, the scientists found.
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. In 2012, however, the rate of decline suddenly reduced by about 50% - indicating that new source of production had started up. After considering a number of possible causes, Montzka and his colleagues concluded that CFC emissions must have increased after 2012.
The USA ceased production in 1996 and other countries agreed to phase out CFC production by 2010.
However, if no action is taken on the new source of emissions, it could be highly significant.
Zaelke said he was surprised by the findings, not just because the chemical has always been banned, but also because alternatives already exist, making it hard to imagine what the market for CFC-11 today would be. CFC-11 concentrations have declined by 15 percent from peak levels measured in 1993 as a result.
If the source of these new emissions can be identified and controlled soon, the damage to the ozone layer should be minor, Montzka said.
The Montreal Protocol, signed by more than 200 countries and generally regarded as having a good record of compliance, is created to protect the Earth's ozone layer.
Overall, it is important to underscore that the ozone layer is slowly recovering and ozone-depleting substances are still declining.
Keith Weller, a spokesman for the United Nations Environment Program, which administers the Montreal Protocol, said the findings will have to be verified by the scientific panel to the Protocol, and then would be put before the treaty's member countries.
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