The cause is persistent emissions of so-called feedstock chemicals, which are still permitted as raw materials in industry. These ozone-depleting substances have so far been excluded from international agreements because, according to the current study, their emissions and use have been significantly underestimated.
Although ozone-depleting chemicals such as carbon tetrachloride (CCl₄) or certain chlorofluorocarbons (CFCs) are no longer used in refrigerators and foams, they continue to serve as feedstocks in industrial processes for the production of modern refrigerants and plastics. Until now, these so-called feedstock chemicals have flown under the radar of international agreements because the quantities produced and leakage rates were significantly underestimated.
Researchers have now used global measurements to show that during the production and processing of these substances, approximately three to four percent escapes into the atmosphere through leaks. Furthermore, their use has increased significantly in recent decades.
In a study published in Nature Communications, they have now calculated that, as a result, the ozone layer is likely to recover about seven years later than previously assumed – unless emissions are reduced. “These substances are not only ozone-depleting but also highly harmful to the climate. Lower emissions would thus benefit both the ozone layer and the climate,” says Stefan Reimann, an atmospheric scientist at Empa and lead author of the study.
Researchers at the University of Bristol used atmospheric transport models to track these chemicals as they move through the atmosphere. By comparing their simulations with long‑term measurements from the Advanced Global Atmospheric Gases Experiment (AGAGE), a network Bristol has been involved in since 1978, they were able to derive global emissions estimates.
Professor Matt Rigby, an atmospheric scientist in the School of Chemistry at the University of Bristol said: “The measurements show that emissions linked to fluorochemical production are substantially higher than expected. This suggests much greater leakage of ozone‑depleting and climate‑warming gases than the controls of the Montreal Protocol assumed, with clear implications for climate change and ozone layer recovery."
Stefan Reimann, a fellow atmospheric scientist and lead author of the study said: “These substances are not only ozone-depleting but also highly harmful to the climate. Lower emissions would thus benefit both the ozone layer and the climate.”
When the Montreal Protocol was negotiated in the 1980s and later strengthened, it led to a global ban on ozone-depleting substances in everyday products. Feedstock chemicals, however, were exempt from this ban. At the time, industry assumed that only about 0.5 percent of the quantities produced would escape into the atmosphere and that the use of these substances would decline in the long term.
However, the use of feedstock chemicals has risen as they feature in refrigerant substitutes following the ban on CFCs, as well as rapidly growing use in the polymer industry being used to help create lithium-ion batteries for electric cars.
Based on these developments, the international research team calculated various future scenarios. They compared, for example, the originally assumed, very low emission rates with the values measured today from the use of feedstock chemicals. The established benchmark from 1980, when global ozone depletion was first observed, serves as a reference. Until now, it was assumed that this original state of the ozone layer would be reached again around the year 2066.
However, the new calculations show that if feedstock emissions remain at current levels, this timeline will shift by about seven years. The stratospheric ozone layer would therefore not fully recover until around 2073. The margin of uncertainty for this estimate ranges from six to eleven years.
The feedstock chemicals released not only damage the ozone layer but also act as powerful greenhouse gases. If nothing changes, these additional climate-damaging emissions will reach around 300 million metric tons of CO₂ equivalents per year by mid-century – comparable to the current annual CO₂ emissions of a country like England or France. Reducing these emissions would therefore have a dual benefit.
Paper:
'Continuing industrial emissions are delaying the recovery of the stratospheric ozone layer' by S. Reimann et al, in Nature Communications