What is the ozone layer, and did we fix the hole?

Ozone is a molecule of three oxygen atoms, concentrated in a layer of the stratosphere roughly 15 to 35 kilometers up. It absorbs the majority of the sun's ultraviolet-B and essentially all UV-C radiation, which would otherwise reach the surface at levels causing severe damage to DNA, sharply increasing skin cancer and cataracts and harming crops and marine plankton. Ozone is continuously created and destroyed by sunlight in a natural balance, and the layer's thickness is measured in Dobson units.

In the 1970s chemists identified that chlorofluorocarbons, then used widely in refrigeration, aerosols, and foam, drift into the stratosphere where UV breaks them apart, releasing chlorine atoms that catalytically destroy ozone, with a single chlorine atom capable of destroying many thousands of ozone molecules. The Antarctic ozone hole was documented in 1985, forming each southern spring because polar stratospheric clouds provide surfaces that accelerate the chemistry. The 1987 Montreal Protocol phased out the responsible compounds and was ratified universally, which is unique among environmental agreements.

The recovery is real and measurable. Scientific assessments by the World Meteorological Organization and UN Environment Programme have found the ozone layer on track to return to 1980 levels over the middle decades of this century, with the Antarctic hole recovering more slowly and later. This is the strongest available example of an identified global atmospheric problem being addressed successfully through coordinated policy. Two footnotes: the substitutes adopted, HFCs, turned out to be potent greenhouse gases and are themselves being phased down under the Kigali Amendment, and ozone in the stratosphere is protective while ozone at ground level is a harmful pollutant, which is why the same molecule appears in both good and bad news.