Tukuk-OSOzone (O₃) is oxygen’s short-lived cousin. High in the stratosphere, a thin concentration a few parts per million absorbs the Sun’s harmful UV-B — enough to protect land life, and the amount in the entire layer is so little that if compressed to tropospheric density it would be a few millimetres thick. Fragile, essential, invisible: the layer forms where UV splits O₂ and freed atoms attach to more O₂, continuously made and destroyed.
In 1974 two chemists proposed that a family of human-made gases — chlorofluorocarbons, inert, cheap, non-toxic, perfect refrigerants — were destroying that layer: CFCs drift up intact, UV shreds them over the pole, and each released chlorine atom catalytically destroys thousands of ozone molecules before it is removed. The 1985 discovery of the Antarctic ozone hole proved them right faster than anyone expected. By the late 1980s, measurements showed the hole deepening each spring and ozone thinning globally.
The response is the template for doing planetary problems well: the 1987 Montreal Protocol phased CFCs out with binding dates, differentiated duties for rich and poor economies, and became the first treaty ratified by every country on Earth. Production of the worst ozone killers is essentially zero now, and monitored recovery is real — models and observations project the Antarctic hole closing around mid-century, decades after the chemistry predicted a fix would even be attempted. One caveat for honesty: the gases that replaced CFCs (HFCs) are harmless to ozone but potent greenhouse gases, which is why the 2016 Kigali Amendment extended the same machinery to them.