Annual conferences to re-assess the goals of the treaty make it a living document, which is revised in light of up-to-date scientific data. The Montreal Protocol remains the world’s only successful international environmental treaty with legally binding emissions targets. In the 1980s, geophysicist Joseph Farman confirmed the Rowland-Molina hypothesis when he detected a near-absence of ozone over Antarctica-the “Ozone Hole.” A fierce battle ensued among industry, scientists, environmentalists, and politicians, but in 1987 the U.S signed the Montreal Protocol on Substances that Deplete the Ozone Layer, which ended Freon production.

Greater depletion would lead to crop failures, the collapse of oceanic food systems, and, eventually, the destruction of all life on Earth. A 1 percent decline in the ozone layer’s thickness results in thousands of new cases of skin cancer. Without stratospheric ozone, life as we know it is impossible. The ozone layer absorbs the worst of the sun’s ultraviolet radiation. In 1955, only 10 percent of American cars had air-conditioning. The American car was similarly transformed. And despite millions of Black Americans fleeing the violence of Jim Crow, the South saw greater in-migration than out-migration for the first time-a direct result of AC. Quickly, air-conditioning transitioned from a luxury to a necessity. Cheap materials made boxy, suburban tract housing affordable to most Americans, but the sealed-up, stifling design of these homes required air-conditioning to keep the heat at bay. Architecture could now ignore the local climate. The small, winter resort of Phoenix, Arizona, became a year-round attraction. Non-toxic and non-explosive, Freon was hailed as a “miracle.” It made the modernist skyscraper-with its sealed windows and heat-absorbing materials-possible. Everything changed with the invention of Freon in 1928.

Chemical refrigerants like sulfur dioxide and methyl chloride filled most fridges and coolers, and leaks could kill a child, poison a hospital floor, even blow up a basement. Besides being financially impractical and culturally odd, it was also dangerous. Before World War II, almost no one had air-conditioning at home.

After Cooling by Eric Dean Wilson