Mr Peter, a year ago we were over the moon that the Montreal Protocol adopted in 1987 seems to be having an impact: a study in your group showed that on average the ozone is increasing again globally – and especially in the moderate northern latitudes. Now, however, an unprecedented ozone hole has been located above the Arctic. What happened?
There had been severe ozone losses over the Arctic before but never what one would call an ozone hole. This turn of events is linked to changing temperature structures in the atmosphere. Every winter in the Antarctic, which is much colder, there are so-called polar stratospheric clouds, upon which the chlorine humans have released into the atmosphere is transferred in an aggressive form, which then destroys the ozone. This can also happen in the Arctic if the stratosphere cools down sufficiently in the wintertime. But the stratosphere doesn’t usually get that cold in the Arctic. Whereas the air masses are self-contained above the isolated Antarctic continent, the Arctic air flows over mountain ranges and is affected by land and sea differences. The resulting air flow disturbances move upwards into the stratosphere, which generally prevents the polar vortex in the north from becoming as stable and cold as in the south. The article in Nature now shows that this north-south difference has become smaller and the situation in the Arctic last winter suddenly became very Antarctic-like.

So an unusually cold stratosphere is to blame. And yet Greenland’s icecap and the Arctic ice are melting dramatically. How does that add up?
Humans are releasing more and more greenhouse gases – especially CO₂ – into the atmosphere that warm the troposphere and cause the ice sheets to melt. However, the same greenhouse gases have a cooling effect on the stratosphere, which leads to more stratospheric clouds upon which chlorine is activated, which in turn ultimately destroys more ozone. So we people are doubly responsible – by emitting greenhouses and releasing chlorine into the atmosphere in the form of CFCs.

So, ultimately, that means the evidence of ozone regeneration that was discovered only a year ago in the mid-latitudes can be cancelled out again by climate warming.
Fortunately, it’s not quite that bad. Of course, such severe destruction of the ozone also has an impact on the mid-latitudes if the air masses with greatly reduced ozone values are carried southwards at the end of the winter. We’re affected one way or another as, through the occurrence of reduced Arctic ozone, the ozone layer has also become somewhat thinner above us. However, if the next few winters return to normal and are not as cold in the stratosphere, the ozone layer will continue to recover as we predicted a year ago. The authors of the new study write that we can’t say when and whether a situation like this will crop up again.

What if greenhouse-gas emissions continue to increase?
If humans don’t reduce greenhouse-gas emissions, these temperature changes in the stratosphere will also persist. This means the stratosphere could also cool down further in the North Pole region. Then the Montreal Protocol will have worked well because it has put a stop to the emission of any more of the gases that damage the ozone. Nonetheless, we would have a problem with regard to the ozone layer in the high latitudes because we keep pumping CO₂ into the atmosphere instead of implementing the Kyoto Protocol and other climate goals.

Did the study come as a surprise to you?
No. The turn of events became apparent last winter and was discussed among scientists. But one figure in the study is bewildering: the graph illustrates the ozone depletion over the Arctic last winter and practically traces the course and ozone depletion in the Antarctic. If you interpret the figure, the mechanism in the Arctic appears to have proceeded exactly like in the Antarctic.

What would it mean if this scenario were to recur every winter?
If the natural variability in the Arctic stratosphere were to become as weak as in the Antarctic, we’d have a winter with a hole in the ozone above the Arctic every year. We didn’t consider this case in our article on the Montreal Protocol. That would mean we’d have to reconsider the books since the large air mass in which the ozone is destroyed in the winter would be imported to northern midlatitudes in the summer. But like I said, such an extreme scenario isn’t compatible with our current knowledge.

Doesn’t that mean we would have to reckon with new requirements for the regeneration of the ozone layer and expect even more extensive changes in the climate conditions?
Scientists have been concerned with this question for a long time. They examine the impact stratospheric changes have on the troposphere underneath. We know from the Antarctic that the hole in the ozone has changed the weather near the ground. Winds have become stronger and temperatures are changing, which may have contributed to the recent large-scale ice losses. There could also be similar changes in the north. But this interplay between chemistry and the climate remains insufficiently described. This requires complex models. We are presently unable to predict the possible consequences, so I’ll have to leave the question unanswered. While the chances are good that the next few winters will be considerably milder in the Arctic stratosphere, as the researchers also stress in the paper we don’t know whether this kind of occurrence could become more frequent.

But that also means we still have a lot of unknowns in our climate models.
Precisely. The dynamic models predict that the climate changes are more likely to disrupt the Arctic stratospheric vortex and not allow the stratosphere to cool down quite so much, which begs the question as to whether this new study is at all compatible with our models: is that just “weather”? A remarkably cold stratospheric winter, just as there was a remarkably hot tropospheric summer in 2003?

But again that just goes to show we’re not fully in the picture when it comes to the processes of climate change…
… that’s right; we’re not. We know that the greenhouse gases cooled the global stratosphere down in the mid-latitudes. About thirteen years ago, I concluded in a publication that if this cooling occurs in the polar regions, we’ll also have an ozone hole in the Arctic. After the models suggested that the stratosphere tends to be disturbed, thus preventing cooling, however, I didn’t pursue it any further. And now we have this new paper. For me, the jury’s still out on whether the Arctic ozone hole is just an exception or whether the alarm bells should be ringing due to the major consistencies with what happened in the Antarctic.

Hole in the ozone

Ozone holes have been appearing in the Antarctic every year from winter to spring since the 1980s. Last winter, however, an ozone hole was observed above the Arctic for the first time. The results of a subsequent study have just been published in the journal Nature. The ozone layer at an altitude of fifteen to forty kilometres, in the stratosphere, protects life on the earth against harmful UV radiation. Chlorofluorocarbons damage the ozone layer and enable more UV radiation to reach the earth, which can cause skin cancer. In 1985 an ozone hole over the South Pole region was detected for the first time. The Montreal Protocol, which was ratified in 1987 and came into force two years later, obligates the signatory states to curb the use of CFCs, such as in sprays, and ultimately dispense with them altogether. It has been repeatedly been refined in recent years on the strength of new findings and meanwhile also contains CFC substitutes and brominated halons, such as those used in fire extinguishers.

Further reading

Manney GL et al.: Unprecedented Arctic ozone loss in 2011, Nature (2011), published online 2 October 2011, DOI: 10.1038/nature10556