The hole we actually fixed

September 16 is the International Day for the Preservation of the Ozone Layer. The date is not a metaphor. It is the anniversary of the Montreal Protocol, signed in 1987 — a treaty that did something environmental agreements are not famous for: it worked.

Ozone (O₃) in the stratosphere absorbs ultraviolet light that would otherwise reach the ground. In 1974, Mario Molina and F. Sherwood Rowland argued that chlorofluorocarbons — stable, useful chemicals in spray cans, fridges, and foam — could drift up, break apart in UV, and catalyze the destruction of ozone. Each chlorine atom could kill many ozone molecules before it left the system. In 1985, a British Antarctic Survey paper reported a seasonal crater in the ozone over Antarctica: the "ozone hole." Satellite data had been discarding the extreme values as errors. They were not errors.

The political response was unusually fast. The Montreal Protocol opened for signature on 16 September 1987. It committed countries to phase out the worst ozone-depleting substances. Amendments tightened the list. The ozone hole did not vanish overnight — CFCs persist for decades — but production collapsed, and the hole has been shrinking. Assessments from the World Meteorological Organization and UN Environment Programme now put Antarctic ozone on track to recover around mid-century if the controls hold.

This is not a story that "science solved it and everyone clapped." There were industry fights, developing-country phase-in schedules, and a later amendment (Kigali, 2016) aimed at HFCs, the replacements that spare ozone but warm the climate. The hole still appears every southern spring. Recovery is a slope, not a switch.

It is still one of the few large environmental problems where the mechanism was identified, the chemical was named, the treaty was signed, and the sky measurably improved. That is rare enough to be worth a Monday note.

There is no ozone-molecule tee in the shop this week. That is on purpose. The story is the chemistry and the treaty, not a flask. If you want that molecule on a shirt, Friday's vote has an O₃ option.