Stephen Montzka and colleagues at the US National Oceanic and Atmospheric Administration (NOAA) looked at levels of CFC-11 in the atmosphere using measurements taken at the Mauna Loa Observatory in Hawaii. "I was astounded by it really". The startling resurgence of the chemical, reported in Nature, will likely spark an worldwide investigation to track down the mysterious source.
However, a study recently published in Nature reveals that CFC-11 production may be happening somewhere in the world despite the Montreal Protocol.
"Somebody's cheating", said Durwood Zaelke, founder of the Institute for Governance and Sustainable Development and an expert on the Montreal Protocol, in a comment on the new research.
But for now, the scientists don't know exactly who, or where, that person would be.
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.
Atmospheric levels of a key ozone-depleting chlorofluorocarbon (CFC) are not going down as fast as they should be, researchers in the USA have discovered.
As a result of the controls, CFC11 concentrations have declined by 15% from peak levels measured in 1993.
The researchers have calculated that an additional 6,500 to 13,000 tons emitted each year in Eastern Asia would be enough to account for the trend. Rather, the evidence "strongly suggests" a new source of emissions, the scientists wrote.
As ScienceAlert reported, a large reservoir of CFC-11 is contained in foam insulation in buildings, chillers, and appliances manufactured decades ago, but none of the factors could justify the sudden spike over past few years. The chemical stays in the air for about 50 years.
The ozone layer in the stratosphere, 10 to 40 kilometres above Earth's surface, protects life on the planet from deadly ultraviolet radiation.
The protocol was a huge success, slowly shrinking the giant hole that forms over Antarctica each September.
"In the end, we concluded that it's most likely that someone may be producing the CFC-11 that's escaping to the atmosphere".
The researchers warn that continued production of the gas could delay recovery of the ozone layer. Together, this analysis suggested the emissions are coming from east Asia.
In response to these new reports, the United Nations Environment Programme released a statement, highlighting the efficacy of the Montreal Protocol with science at its very core.
The USA ceased production in 1996 and other countries agreed to phase out CFC production by 2010.
But Zaelke thought the finding could promote tougher action. "That's a tough group of people".
At its most depleted, around the turn of the 21st century, the ozone layer had declined by about five percent.
Although Montzka and his colleagues could not pinpoint the exact location of the new emissions, some of their observations and models offer clues as to where they might be coming from.