Stratospheric ozone: depletion and recovery

Contrast this week directly with last week: stratospheric ozone ("good ozone," high in the atmosphere) absorbs harmful ultraviolet radiation and protects life at the surface, while ground-level ozone ("bad ozone," from Week 22) is a harmful pollutant -- the identical molecule, but its location entirely determines whether it helps or harms. Chlorofluorocarbons (CFCs), once common in refrigerants and aerosol sprays, were found in the 1970s and 1980s to drift into the stratosphere and catalytically destroy ozone molecules -- a single chlorine atom can break apart tens of thousands of ozone molecules before it is finally removed from the stratosphere, since a catalyst is not consumed by the reaction it speeds up.

This discovery, along with the observed Antarctic "ozone hole," led to the Montreal Protocol of 1987, an international treaty phasing out CFCs and other ozone-depleting substances. It has since been ratified by every country in the United Nations -- a genuinely rare feat -- and atmospheric scientists widely point to it as one of the most successful environmental policies ever enacted: the ozone layer is on a documented recovery path, with major assessments projecting a return to pre-1980 levels by roughly the 2060s to 2070s. Whatever one's views on other environmental policy questions, this case is worth knowing as a clear example of science, public awareness, and international agreement leading to a measured result.