DW: Some parts of the ozone layer are recovering, while other parts are getting worse. This is a bit confusing. What is the current situation?
Markus Rex: That is indeed confusing. And the reason is that these chlorofluorocarbons, or CFCs can destroy ozone efficiently only when the temperatures are really cold. That is typically only the case above the Arctic and the Antarctic, in the polar regions, where the temperatures drop to levels of minus 78 or minus 80 degrees. Then these CFCs can result in the formation of an ozone hole. In the Arctic that happens only once in a while. In some Arctic winters the temperatures are just not low enough to produce large ozone losses and in others they are, and therefore the situation is very different from year to year.
Let's talk about these holes over the poles - what impact do they have then in real terms on the ground?
That is twofold. The thinner the ozone layer is, the more UV-B radiation we get at the ground. And UV-B radiation is harmful, it can cause sunburn, it can also cause skin cancer and therefore we try to avoid large exposure to UV-B radiation. The other effect is that there are strong links between the ozone layer and the climate. Therefore any change in the ozone layer has some consequences for climate at the surface where we live. These linkages are poorly understood and we need to do more research to find out what the links are.
Stopping CFC production has apparently reversed the trend of ozone depletion. How reliable are these measurements and predictions that have led scientists to say this?
In the long term we can safely say that - as long as we all stick to the Montreal Protocol, that international treaty that bans the production of CFCs - the ozone problem will go away by the end of the century and the ozone holes will no longer appear. In the short-term it's much harder to predict because of that link to the temperature. So if the temperatures in the stratosphere - the upper layer of the air - change as part of climate change, then we may even get a worsening situation, particularly in the Arctic, for the next 20 or 30 years.
We've seen countries come together with the Montreal Protocol and sign up to stop CFC production. There's been success there. But in terms of CO2 emissions, why is it so difficult to get countries to agree to cut emissions?
That's much harder to do. We have that nice example of the Montreal Protocol which really banned the production of CFCs and which has a great success at protecting the ozone layer. But it was much easier to do that because we could replace these CFCs by chemicals which have similar properties and the consumer doesn't even notice that the CFCs have been replaced by other things. To reduce the emission of greenhouse gases is much harder. We need to reduce energy consumption, we need to try to produce a larger fraction of our energy from other sources. And that's a much harder and much more costly process.
(Interview: Meggin Leigh)