The increase in emission of CFC-11 appears unrelated to past production; this suggests unreported new production, which is inconsistent with the Montreal Protocol agreement to phase out global CFC production by 2010.
However, results from the new analysis of NOAA atmospheric measurements show that from 2014 to 2016, emissions of CFC-11 increased by more than 14,000 tons per year to about 65,000 tons per year, or 25 percent above average emissions during 2002 to 2012.
Officially, production of CFC-11 is supposed to be at or near zero - at least, that is what countries have been telling the United Nations body that monitors and enforces the Montreal Protocol. The startling resurgence of the chemical, reported in Nature, will likely spark an global investigation to track down the mysterious source.
High in the atmosphere, ozone shields Earth from ultraviolet rays that cause skin cancer, crop damage and other problems.
Nearly no CFC-11 has been been produced since 2006 - or so we thought.
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.
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Zaelke said he was surprised by the findings, not just because the chemical has always been banned but also because alternatives exist, making it hard to imagine what the market for CFC-11 today would be.
They considered a range of alternative explanations for the growth, such as a change in atmospheric patterns that gradually remove CFC gases in the stratosphere, an increase in the rate of demolition of buildings containing old residues of CFC-11, or accidental production.
Trichlorofluoromethane, or CFC-11, is the second-most abundant ozone-depleting gas in the atmosphere and a member of the family of chemicals most responsible for the giant hole in the ozone layer that forms over Antarctica each September. The source of the new emissions has been tracked to east Asia, but finding a more precise location requires further investigation.
CFC-11 still contributes about a quarter of all chlorine - the chemical that triggers the breakdown of ozone - reaching the stratosphere.
But if the emissions of CFC-11 continue, recovery could be delayed by about a decade, said Stephen A. Montzka, the lead author of a report detailing the findings published Wednesday in the journal Nature. "It is critical that we take stock of this science, identify the causes of these emissions and take necessary action", he said. Scientists say there's more of it - not less - going into the atmosphere and they don't know where it is coming from. If not remedied soon, however, substantial delays in ozone layer recovery could be expected. But the apparent increase in emissions of CFC-11 has slowed the rate of decrease by about 22 percent, the scientists found.