“The world solved the ozone problem. It can solve climate change”. Such was the headline of an opinion piece in The New York Times in 2019,Footnote 1 when topics other than the COVID-19 pandemic still made headlines. It referred to the fact that the layer of ozone molecules present in the atmosphere was slowly recovering from destruction due to human activities. It was, however, also human action that had enabled the recovery of the ozone layer, and this sparked the outburst of optimism by The New York Times. Ambio had its role in this success story, as it was one of the journals where the scientific discoveries that laid the foundation for political action were discussed.

Ozone (O3) is a natural constituent of the Earth’s atmosphere. It absorbs radiation in the ultraviolet (UV) and infrared (IR) ranges of the electromagnetic spectrum and plays therefore an important role in the regulation of the atmosphere’s radiation and temperature budget. High concentrations of ozone, the so-called ozone layer, are found in the stratosphere, at altitudes of 10 to 50 km in the atmosphere. The ozone layer shields humans, animals, and plants from biologically damaging UV radiation—without the absorptive properties of this molecule, life on Earth would not be possible. Ozone is formed from and reconverted to molecular oxygen via reactions that involve photodissociation. However, the levels of ozone found in the stratosphere can only be explained if additional loss mechanisms are taken into account. These are reaction cycles that involve catalysts such as hydrogen, hydroxyl, and halogen radicals such as bromine and chlorine, and nitrogen oxides (Crutzen 1974).

This understanding was developed gradually in the second half of the last century. The work of chemists Paul Crutzen, Mario Molina, Sherwood Rowland, and colleagues in the 1970s led to the insight that the ozone layer in the stratosphere was being depleted and that the substances responsible for the depletion were emitted by human activities: nitrogen oxides from the use of nitrates as fertilizers, and halogen radicals from chlorofluorocarbons (CFCs) and related compounds, used as propellant gas in aerosol spray cans, and refrigerants in refrigerators and air-conditioners (Molina and Rowland 1974). Crutzen, Molina, and Rowland were awarded the Nobel Prize in Chemistry in 1995 for their work in atmospheric chemistry and specifically ozone formation and decomposition.

Their work eventually led to one of the likely most successful agreements in the history of international environmental politics: The Montreal Protocol. Entered into force in 1989, this international treaty was designed to protect the ozone layer by phasing out the production of ozone-depleting substances, such as CFCs. Political pressure had grown after the detection of the ozone hole over Antarctica in the 1980s (Farman et al. 1985), caused by chlorine atoms produced photochemically in significant amounts in the stratosphere from CFCs that are inert at lower altitudes. The Montreal Protocol indeed led to a halt of the production of ozone-depleting substances, and the recovery of the ozone layer can be observed today. It is a great example for the good that can come out when science, the public, and policy-makers come together.

The scientific discussions that were of eminent importance at the time also happened in part through Ambio: In 1974, Paul Crutzen wrote an article about the possible variations in total ozone due to natural processes and human activities such as superconic aircraft, CFC production, nuclear warfare, and the use of nitrogen fertilizers (Crutzen 1974). Another article, written in 1977 together with Dieter H. Ehhalt, focuses in more detail on the effect of nitrous oxide (N2O) emissions on ozone destruction (Crutzen and Ehhalt 1977). Nitrous oxide emitted from the application of fertilizer, among other processes, breaks down in the stratosphere and represents a source of ozone-depleting nitrogen oxides.

For this Ambio anniversary collection, Dr. Rolf Müller, a colleague of Dieter H. Ehhalt at Forschungszentrum Juelich, describes in his article in more detail the scientific significance of this paper (Müller 2021). Merete Bilde, Professor at Aarhus University and Ambio advisory board member, together with Ole John Nielsen, Professor at University of Copenhagen, and Susan Solomon, Professor at the Massachusetts Institute of Technology in Cambridge, were asked to provide a reflection on the Ambio articles of Paul Crutzen and Dieter H. Ehhalt from the 1970s. Both Bilde and Nielsen, as well as Solomon, herself a renowned scientist pivotal in the understanding of the Antarctic Ozone hole, provide in their reflections a summary of Crutzen and Ehhalts’ articles, and their own view on their significance and legacy for today and the future. Bilde and Nielsen (2021) write that “these two articles represent the beginning of a scientific era, which led to the discovery of the Antarctic ozone hole and political action in the form of the Montreal protocol and its amendments”, and according to Solomon (2021), the articles “remain guideposts today for ongoing international negotiations regarding reducing emissions from fertilizer and limiting nuclear testing”. It is not often in climate sciences that one gets to write about the work of a Nobel laureate and scientist of the format of Paul Crutzen. He is a central figure in the fight for the acceptance of global climate change and has coined the term “Anthropocene”. It is with pride that we can say that Ambio was part of the scientific debate that led to unraveling of the mechanisms of the depletion of the ozone layer. The topic fit well in Ambio at the time and does so now, with its clear link between human activities and their effects on the environment. Paul Crutzen explicitly included policy recommendations in his articles. Here’s to hope that the story of the recovery of the ozone hole will serve as an example for one of the biggest challenges we face today—climate change.