31 years on from the international treaty which banned CFCs, Reading atmospheric scientist Michaela Hegglin reflects on what’s been achieved and whether we’ve really solved the problem of ozone layer depletion.

False-colour view of the total ozone over the Antarctic pole.
False-colour view of total ozone over the Antarctic pole. The purple and blue indicates where there is the least ozone, and the yellows and reds are where there is more ozone. Image credit: NASA Ozone Watch

Today is International Day for the Preservation of the Ozone Layer. Ever heard of it? Some of you may know that a crucial treaty got signed on that very day in 1987, but not that the United Nations would have marked this historical event by giving it the status of an International Day. There is a pretty good reason for it.

The big deal

The Montreal Protocol, whose 31st birthday is celebrated on September 16, is an international treaty established in 1987 to protect the ozone layer from human-made ozone depleting substances.

Hailed to be the most effective international environmental agreement to date, the protocol addressed one of the most pressing environmental issues of the 20th century1.

Ozone is an unusual type of oxygen molecule, with three oxygen atoms per molecule instead of the two atoms that make up normal oxygen. The ozone layer resides in the stratosphere – the upper layer of the atmosphere. It and protects Earth from ultra-violet (UV) radiation, and its development enabled life to crawl out of the oceans and take over the land millions of years ago2. By the same token, stratospheric ozone depletion leads to increases in UV which can cause skin cancer, eye cataracts, and damage to the immune system, as well as affecting health and growth of animals and plants.

A brief history of the ozone layer

English scientist James Lovelock was the first to measure – with a home-made device – the abundance of human-made chlorofluorocarbons (CFCs) in the atmosphere.  This finding triggered Mario Molina and Sherwood Rowland’s hypothesis4 in the early 1970s that CFCs could only be destroyed in the stratosphere, where they release chlorine atoms. They predicted that these chlorine atoms might react with ozone and pose a threat to the ozone layer.

When Joe Farman and colleagues at the British Antarctic Survey5 discovered the Antarctic ozone hole in 1985, it proved Molina and Rowland’s hypothesis to be not only correct, but also far more threatening than anyone had imagined. It spurred research activities to understand why such severe ozone depletion was found over Antarctica alone, and led to political action under the Montreal Protocol in 1987.

The realization that more severe ozone depletion would spread further across the globe if we were to continue releasing CFCs into the atmosphere, along with technological advances that made replacement of CFCs possible, helped governments to tighten the regulations on CFCs through several Amendments to the Montreal Protocol.

Is the Ozone layer recovering?

 Almost 50 years after Molina and Rowland’s hypothesis, research on the stratospheric ozone layer continues, but now the focus is on whether the ozone layer is on its way to recovery6,7,8. First signs of a healing are indeed detected in the upper stratosphere6, for the midlatitude regions of both hemispheres7, and also in the Antarctic8.

However, research shows that ozone-depleting substances are not the only factor affecting the natural balance of the ozone layer. Detection of how well the ozone layer is recovering is obscured by air pollution, which increases levels of ozone near to the Earth’s surface8.

Also, climate change alters temperatures and transport of air in the stratosphere with consequences for the ozone distribution8,9. Indeed, recent observations have shown disconcerting decreases in ozone in the lower region of the stratosphere10 that may have been a result of changing ozone transport11.

Hope for the future

More observations and research are needed to understand the current evolution of the ozone layer. But in today’s world, in which we mainly hear negative news stories concerning the state of the environment, it may be good to reflect on the Montreal Protocol and its success on the 16th September. After all, it prevented a major threat to people and the living world around us.

This international success story should give us some hope that if we really wanted to, we could also change the current course of climate change.

Michaela Hegglin is Associate Professor in Atmospheric Chemistry at the University of Reading’s Department of Meteorology. Her research is concerned with the the upper troposphere/lower stratosphere; a region of particular importance in the climate system yet which is difficult to access by current measurement systems. More recently her research has been focusing on  questions around air pollution transport and air quality, and impacts on human and ecosystem health.


1 Annan, K. A. We the Peoples: The Role of the United Nations in the Twenty-First Century (UN, 2000).

2R. P. Wayne, Chemistry of Atmosphere (Oxford Univ. Press, Oxford, ed. 3, 2000), p. 775.

3Lovelock, J. E.; Maggs, R. J.; Wade, R. J. “Halogenated Hydrocarbons in and over the Atlantic”. Nature 241, no. 5386 (1973): 194. doi:10.1038/241194a0.

4Molina, Mario J., and F. Sherwood Rowland. “Stratospheric sink for chlorofluoromethanes: chlorine atom-catalysed destruction of ozone.” Nature 249.5460 (1974): 810.

5Farman, Joseph C., Brian G. Gardiner, and Jonathan D. Shanklin. “Large losses of total ozone in Antarctica reveal seasonal ClOx/NOx interaction.” Nature 315, no. 6016 (1985): 207.

6World Meteorological Organization (WMO), Scientific Assessment of Ozone Depletion: 2014, World Meteorological Organization, Global Ozone Research and Monitoring Project –Report No. 55, 416 pp., Geneva, Switzerland, 2014. Available at https://www.esrl.noaa.gov/csd/assessments/ozone/2014/

7Solomon, Susan, Diane J. Ivy, Doug Kinnison, Michael J. Mills, Ryan R. Neely, and Anja Schmidt. “Emergence of healing in the Antarctic ozone layer.” Science (2016): aae0061.

8Shepherd, T. G., D. Plummer, J. Scinocca, M. I. Hegglin, C. Reader, V. Fioletov, E. Remsberg, T. von Clarmann, H. J. Wang, Reconciliation of halogen-induced ozone loss with the total-column ozone record, Nature Geoscience, 7 (6), 443–449, doi:10.1038/NGEO2155, 2014.

9Hegglin, M. I., and T. G. Shepherd, Large climate-induced changes in UV index and stratosphere-to-troposphere ozone flux, Nature Geoscience 2, 687-691, 2009.

10Ball W., et al., Evidence for a continuous decline in lower stratospheric ozone offsetting ozone layer recovery, Atmos. Chem. Phys., 18, 1379-1394, https://doi.org/10.5194/acp-18-1379-2018, 2018.

 11 Wargan, K., Orbe, C., Pawson, S., Ziemke, J. R., Oman, L. D., Olsen, M. A., et al. (2018). Recent decline in extratropical lower stratospheric ozone attributed to circulation changes. Geophysical Research Letters, 45, 5166–5176. https://doi.org/10.1029/2018GL077406