The Montreal Protocol to protect the ozone layer is humanity's outstanding environmental success. Since it was passed in 1987, the concentration of most of the gasses threatening our atmospheric shield has fallen, and the ozone layer has started to recover. However, the decline in a key ozone-depleting gas has slowed, suggesting there is some illegal source. At current rates, this will hinder the recovery of the Earth's sunscreen, causing thousands of extra cases of skin cancer. If these secret emissions increase further, they could pose a threat to us all.
Trichlorofluoromethane (CFC-11) was one of the gasses phased out under the Montreal Protocol to protect the ozone layer. Nevertheless, a quarter of the ozone-destroying chlorine in the stratosphere comes from this one industrial gas. The concentration of CFC-11 in the atmosphere fell steadily from 2002 to 2012 at a rate of 2.1 parts per trillion per year, but a team from the National Oceanic and Atmospheric Administration report in Nature that since then it has been declining only half as rapidly.
CFC-11 has an atmospheric lifetime of around 43-67 years, so it should be decreasing faster than this. Some stocks manufactured before the ban have been reaching the atmosphere as buildings containing CFC-11 products are demolished, but this shouldn't be accelerating. There is a widening gap between concentrations in the Northern and Southern Hemisphere, a sure sign of increased emissions north of the equator. Most other ozone destroyers are continuing to fall in line with expectations.
After accounting for changes in atmospheric circulation that exaggerated the problem, the authors estimate that at least 7,000 tonnes (7,700 tons) of fresh CFC-11 is being produced each year, despite official production being almost zero.
The paper doesn't try to identify the sources, but there are hints they lie in East Asia.
A decade after chemists first warned of the dangers that chlorine-containing gasses could pose to the ozone layer, which protects the Earth from ultraviolet radiation intense enough to kill most living things if it reached the surface, the world decided to act.
The chemicals in question were phased out. Production of CFC-11 – which before the ban had been used as a refrigerant, propellant in spray cans, and for blowing foam – was to cease by 2010. Falling concentrations of CFC-11 in the atmosphere accounted for much of the progress made in reversing the decline in ozone levels over Antarctica. Without this restoration, the ecosystems of the Southern Ocean, the base of much of the marine food web, would have been at risk of collapse.
Besides its ozone-depleting capacity, CFC-11 is the fifth most important cause of global warming, although far behind carbon dioxide and methane. Since replacements are available for all its known uses, its revival is presumably to save costs.