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to 100 DU or even less. Such severe depletion was not observed in the Arctic. Judged by these criteria, there was no Arctic ozone hole in 2011 — only the most extreme episode of ozone loss seen in the Arctic so far. However, the evolution of HNO3, HCl and ClO species was strikingly Antarctic-like and different from what has been observed in other Arctic winters. On the basis of these considerations, Manney et al. conclude, reasonably enough, that there was an Arctic ozone hole in 2011. Was the ozone loss seen in 2011 a truly exceptional event, or should we expect repeated episodes of extensive Arctic ozone depletion as a result of climate change? Although increasing concentrations of carbon dioxide in the atmosphere warm the troposphere, they cool the overlying stratosphere, and observations have shown that the global stratosphere has cooled significantly during the past 30 years. Furthermore, there is evidence to suggest that the coldest Arctic winters are becoming colder — although there is no statistically significant trend because Arctic winter temperatures are highly variable. Comprehensive models of the climate system do not produce consistent predictions for the evolution of Arctic temperatures and ozone loss in the twenty-first century; the refinement of such predictions will undoubtedly be an important topic of research in the immediate future. Another consideration is that the abundance of ozone-destroying chlorineand bromine-containing species in the atmosphere is decreasing following the adoption of the Montreal Protocol, which bans substances that deplete the ozone layer, such that the impact of cold winters on stratospheric ozone will lessen steadily in the future. All things considered, a repeat of this year’s episode is possible, but not likely. Nonetheless, the behaviour of Arctic ozone in 2011 is a nice demonstration that our basic understanding of stratospheric chemistry is robust, and that extensive ozone loss will occur wherever the right conditions are present. ■