Jörg Mäder and Thomas Peter from the Institute for Atmospheric and Climate Science of ETH Zurich are genuinely surprised by the media response to their latest publication put up for discussion in Atmospheric Chemistry and Physics. Chlorofluorocarbons (CFCs), which make a considerable contribution to damaging the ozone layer, are scarcely still an issue in the climate debate today, even though they are also greenhouse gases in addition to having an ozone-destroying effect. ETH Zurich Professor Thomas Peter explains in this interview why it was comparatively easy, in the context of the ozone debate, to push through regulations that were accepted by almost all countries, and why this is so difficult in the case of CO₂ reduction.

Prof. Peter, you are surprised that your study is arousing interest in the media today. Why?
We were well aware of the significance of the study, but we did not expect that it would also interest the general public so much. Another reason was that it had been rejected by the scientific journal “Nature” after almost getting through the “review process”. However, it occasionally happens that even valuable studies fail to survive an expert appraisal behind closed doors. That is why we submitted now to an open access journal and exposed our study to public discussion. The first expert review is positive.

The suspicion that the Montreal Protocol was effective is not new, but was never clearly proved.
Up to now it has been clearly apparent that global atmospheric chlorine concentrations, which are the main factor responsible for the chemical degradation of ozone, are decreasing. The same is true for bromine, which also attacks ozone and which was used for a long time in fire extinguishers. Demonstrating this improvement in the ozone itself is difficult, however, because the thickness of the ozone layer measured locally at a single location is subject to large natural fluctuations on which a slow anthropogenic decrease of the ozone layer is superimposed. The problem is: how can we prove it? And how can we prove that this process has been halted and even reversed by political action?

But you have obviously found a good way to provide this evidence. The first commentary on your study stresses the method’s clarity and simplicity.
Making ozone measurements from a station on the ground is inherently very simple, but using them to deduce changes of 5 to 10 percent over several decades is a challenge. The problem is a statistical one, for which we need long-term measurements. This is why Jörg Mäder used worldwide data from all the measuring stations that have measured the ozone layer thickness practically every day for decades. He analysed the size of the variations and trends, and which factors contribute to temperature variations as a measure of climate change, for example, or to increases in chlorine as a measure of chemical effects. Based on this data, we obtained a picture of the measuring stations at which the ozone layer thickness had increased, and those where it had decreased. The number of measuring stations with a significant increase was larger by far.

And how did you reach your conclusion?
Having analysed the data statistically, we have concluded that the measured ozone increase can only be explained in conjunction with the Montreal Protocol. We did this by comparing what the measurements at the stations would have needed to look like if the concentrations of chlorine and bromine in the stratosphere had continued to rise in a linear manner, in contrast to the reduction in chlorine and bromine actually observed after the Montreal Protocol: the comparison between these two scenarios clearly shows that the ozone increase at most of the stations in recent years can be explained only through the effectiveness of the Montreal Protocol.

Why does the ban not become visible in the stratospheric concentrations of chlorine and bromine until about six years later?
Firstly, the typical exchange time between the troposphere and the stratosphere is two years, which causes an initial delay before the pollutants enter the stratosphere. An additional factor is that the original version of the Montreal Protocol provided for only very limited restrictions on CFC production, and was improved only by subsequent agreements at a later date. This delayed the effect.

A regulation achieved a successful result. Why was that so easy in the case of CFCs, and why is it so difficult when it comes to reducing climate-influencing greenhouse gases?
In the end the CFC problem was a walkover compared to the problem of CO₂. The latter is linked to the backbone of our industrialised society – the energy industry – and has totally different orders of magnitude. Nevertheless, the fact that it was possible to initiate a worldwide environmental treaty so successfully at that time, and one which it has lasted until today, remains interesting. There are some fascinating observations on this from the psychological viewpoint. During a student project at the Environmental Sciences Department we carried out a case study on this topic in which we compared press reports after Montreal and after Kyoto. The statistical assessment revealed, among other things, that the idea of “health” was quoted much more often in the ozone articles as an argument for the Montreal Protocol than in the debate surrounding the Kyoto Protocol on Climate Protection. Presumably the risk of becoming ill with skin cancer due to a reduced ozone layer was a strong incentive motivating almost all countries to sign the Montreal Protocol. There was also no discussion about “winners” and “losers” in the situation at that time, because everyone felt threatened by an illness. However, no-one identifies with the people who are caught up in a mud-slide, or who live in flood areas or whose roof falls on their heads during a hurricane – all events that could become more frequent as a result of climate change.

We can now be pleased about the beneficial effect of the CFC regulations, but how will climate change affect the ozone layer?
The climate affects the ozone layer, but, conversely, CFCs and ozone are themselves greenhouse gases. Thus a complex relationship exists between chemistry, the atmosphere and climate. We know that greenhouse gases like CO₂ increase the temperature at ground level and in the troposphere, whereas there are also greenhouse gases that cool the stratosphere. The latter slows down chemical reactions and causes ozone to be degraded more slowly. Thus initially there is a positive effect that supports ozone recovery. Therefore presumably our ozone layer will be even thicker at the end of the century than in the last thousand years. But the question is whether that is a good thing. If too much UV radiation is filtered out, it can cause vitamin D deficiency in humans, but that will occur in the second half of the century at the earliest.

What happens in the polar regions?
In the northern polar regions we expect the recovery of the ozone layer to progress approximately as in the mid-latitudes. In contrast, under the special conditions prevailing at the south pole, the ozone hole will not close until later, perhaps around 2070 or 2080.

About the CFC problems:

The ozone layer in the stratosphere at a height of 15 to 40 kilometres protects life on Earth from harmful UV radiation. Chlorofluorocarbons damage the ozone layer, allowing more UV radiation to reach the Earth. This can cause skin cancer in humans. The Ozone Hole over the south polar region was first described in 1985. The Montreal Protocol was ratified in 1987 and came into force in 1989. It obliged countries to reduce the use of CFCs, e.g. in spray-cans, and finally to abandon them altogether. It has been improved several times in the past few years on the basis of new knowledge, and now also includes CFC substitutes and bromine-containing halons of the kind used in fire extinguishers.