The upper atmosphere's ozone layer is our planetary sunblock, filtering harmful ultraviolet sunlight. It's also a case study in unintended consequences and how to address them.
Chlorofluorocarbons, or CFCs, were widely used in refrigerators, spray cans and industrial solvents for most of the 20th century. In 1974, two scientists discovered that sunlight breaks CFCs apart, freeing highly reactive chlorine atoms and destroying atmospheric ozone. Nations mobilized against what became known as the ozone hole and agreed to a 1987 treaty to phase out CFCs.
The Antarctic ozone hole will exist for decades, and Arctic losses may continue. However, say the authors, "On balance, the case of stratospheric ozone is a good example where concerted human effort and wise decision making seem to have enabled us to stay within a planetary boundary."
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