Mario Molina

Mario José Molina-Pasquel Henríquez was born in Mexico City on March 19, 1943. He converted a bathroom in the family home into a childhood laboratory, encouraged by an aunt who was a chemist, and studied chemical engineering at UNAM before postgraduate work in Freiburg and a doctorate at Berkeley in 1972. In 1973 he joined F. Sherwood Rowland's group at the University of California, Irvine, as a postdoctoral fellow, and chose from Rowland's list of projects the one that interested nobody: what eventually happens to the chlorofluorocarbons accumulating in the atmosphere.

The answer took three months. CFCs are inert in the troposphere, so nothing removes them; they drift to the stratosphere, where ultraviolet light finally photolyses them and liberates atomic chlorine. Chlorine then attacks ozone catalytically — it destroys an O₃ molecule, is regenerated, and repeats — so a single atom can remove on the order of 100,000 ozone molecules before being scavenged. Molina and Rowland published the calculation in Nature in June 1974 and concluded that continued CFC use would measurably thin the ozone layer that shields the surface from ultraviolet radiation.

The reception was hostile. The aerosol and refrigeration industries disputed the work for a decade, and Rowland found himself effectively blacklisted from industrial consulting. Vindication came from an unexpected direction: in 1985 the British Antarctic Survey reported that the springtime ozone column over Halley Bay had collapsed by more than 40%. Satellites had been recording the hole for years and discarding it as instrument error, because the software had been written to reject readings that extreme. The Montreal Protocol was signed in 1987, and the ozone layer is now measurably recovering — the most successful environmental treaty ever written.

Molina and Rowland shared the 1995 Nobel Prize in Chemistry with Paul Crutzen, making Molina the first Mexican-born scientist to win a Nobel in the sciences. He moved to MIT and later UC San Diego, founded a policy centre in Mexico City bearing his name, advised the Mexican and US governments on climate, and served on President Obama's science council. He died in Mexico City on October 7, 2020 — having spent his final months publishing on aerosol transmission of SARS-CoV-2 and arguing for masks.