Is the Montreal Protocol really working?

By Michaela Hegglin

The Montreal Protocol, which celebrated its 30th birthday last year, is an international treaty established in 1987 to protect the ozone layer from human-made ozone depleting substances. The Montreal Protocol has been hailed as the most effective international environmental agreement to date, and addressed one of the most pressing environmental issues of the 20th century. But … is the Montreal Protocol really working?

Montreal Protocol history in a nutshell
It was the English scientist James Lovelock who was the first to measure the abundance of chlorofluorocarbons (CFCs) with a homemade gas-chromatograph and to realise that these human-made substances were found ubiquitously in both the northern and southern hemispheres (Lovelock et al., 1973). The finding triggered Mario Molina and Sherwood Rowland’s hypothesis in the early 1970s (Molina and Rowland, 1974) that CFCs could only be destroyed in the stratosphere where they release chlorine atoms, which then would be able to destroy ozone catalytically and pose a threat to the ozone layer. The discovery of the Antarctic ozone hole in 1985 by Joe Farman and colleagues at the British Antarctic Survey (Farman et al., 1985) proved their hypothesis to be not only correct, but also far more threatening than had been imagined even by them. 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 advancements that made replacement of CFCs possible, helped governments to tighten the regulations on CFCs through several Amendments to the Montreal Protocol. 

Ozone layer research today
Almost 50 years after Molina and Rowland’s hypothesis, research on the stratospheric ozone layer is still ongoing, but now focuses on the question of whether the Montreal Protocol and associated Amendments is working and whether the ozone layer is beginning to recover. In particular, researchers now know about the confounding effects that climate change and tropospheric pollution can have on attempts to detect ozone recovery. In the WMO Scientific Ozone Assessment Report 2014, the key statements in the Summary for Policy Makers on this topic point out that indications of ozone recovery since 2000 are found in global total column observations (although not yet attributable to the decline in ozone depleting substances, ODS), and that ozone increases have been found in the upper stratosphere, half of which were attributable to ODS decline (with the other half attributed to climate-change and its effects on stratospheric temperatures) (WMO, 2014). Since the last assessment, studies by Shepherd et al. (2014) on the total column ozone evolution at mid-latitudes and Solomon et al. (2016) over Antarctica attributed ozone recovery to declining ODS concentrations with the help of complex model simulations that help distinguish ODS-related changes from those induced by climate parameters and other natural factors such as volcanic aerosol.   

A disconcerting finding …
More recently, however, a study published by Ball et al. (2018) found on the basis of observations alone that ozone in the lower stratosphere is in fact not recovering but in continuous decline (see Figure 1).  The study applied a more refined statistical method than usually used in the research field of stratospheric ozone, with which the authors were better able to take into account natural variations in ozone. The paper received much publicity, since its findings imply that the Montreal Protocol is not working as expected. On the other hand, some colleagues in the field were quick to denounce the paper and its conclusions. Is it time to worry?

Figure 1: Ozone changes as derived from different stratospheric ozone data records (taken from Ball et al., 2018).

While these findings are indeed disconcerting, the changes in the lower stratosphere do not seem to have had a discernible effect on total column ozone (at least not yet) (Weber et al., 2018). The changes are also (at least partially) compensated by increases in tropospheric ozone (Ball et al., 2018; Shepherd et al., 2014). Were the decline in lower stratospheric ozone to continue, however, the consequences could become more serious. In fact, scientists were expecting ozone decline in the lower stratosphere as a consequence of climate change due to a strengthening of the stratospheric circulation (Hegglin and Shepherd, 2009). These changes would lead to a substantial increase in harmful radiation reaching Earth’s surface in the tropics, where UV levels are low to begin with and where most people live.

The way ahead
What is ultimately needed to answer the question of whether the Montreal Protocol is working is to attribute the causes of the observed changes. Are they indeed the result of non-compliance with the Montreal Protocol’s regulations, or just the result of natural variability? Or are they instead due to climate change? If the latter were the case, the Montreal Protocol would be working but concern for the protection of the ozone layer would have to shift towards regulating climate change. More evaluations of the currently available ozone data record are needed, to confirm (or refute) the results of Ball et al. These evaluations should in particular take into account an aging fleet of ozone instruments flying in space, since they may well show signs of degradation potentially affecting the measurements. The finding also highlights that a renewed commitment to measure vertically resolved ozone in the stratosphere and ozone depleting substances in the troposphere is required to be able to satisfy future needs of monitoring the ozone layer. Both these are essential to see whether trends are to continue and to help attribute the changes to either increasing greenhouse gases or ozone depleting substances.


Ball, W. T., Alsing, J., Mortlock, D. J., Staehelin, J., Haigh, J. D., Peter, T., Tummon, F., Stübi, R., Stenke, A., Anderson, J., Bourassa, A., Davis, S. M., Degenstein, D., Frith, S., Froidevaux, L., Roth, C., Sofieva, V., Wang, R., Wild, J., Yu, P., Ziemke, J. R., and Rozanov, E. V., 2018. Evidence for a continuous decline in lower stratospheric ozone offsetting ozone layer recovery. Atmos. Chem. Phys., 18, 1379-1394,

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

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

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

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

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

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

Weber, M., Coldewey-Egbers, M., Fioletov, V. E., Frith, S. M., Wild, J. D., Burrows, J. P., Long, C. S., and Loyola, D., 2018. Total ozone trends from 1979 to 2016 derived from five merged observational datasets – the emergence into ozone recovery. Atmos. Chem. Phys., 18, 2097-2117,

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