Until the early 1970s, it could be said that, like politics, all chemistry was local. That changed in dramatic fashion with a series of discoveries concerning the global effects of a family of chemicals called chlorofluorocarbons, or CFCs. These compounds had played a key role in the midcentury chemical revolution, allowing such innovations as safe refrigeration, cheap aerosol deodorants, and widespread air conditioning. First commercialized by ­DuPont in the early 1930s under the trade name Freon, CFCs appeared to be the perfect industrial chemical: nontoxic, nonflam­mable, and odorless. But in 1973, a pair of chemists at the University of California, Irvine--Sherwood Rowland and his postdoctoral fellow Mario Molina--began to explore the fate of the CFC gases that were being emitted into the atmosphere. Molina began the investigation of CFCs in October of that year, and by Christmas, the researchers had their answer: the CFCs were breaking down in the atmospheric ozone layer, which begins 15 kilometers above the earth, ends roughly 30 kilometers later, and absorbs much of the sun's deadly ultraviolet radiation.

The researchers found that the CFCs wafted up through the lower atmosphere intact, too stable to react with the swirling brew of chemicals around them. But once they reached the mid-­stratosphere, above most of the protective layer of ozone, the intense solar radiation broke the CFC molecules apart, releasing chlorine. Two simple reactions gave Rowland and Molina concern: Cl + O₃ = ClO + O₂, and ClO + O = Cl + O₂. That is, chlorine (Cl) reacted with ozone (O₃), generating chlorine monoxide (ClO), which in turn reacted with an oxygen atom to release another chlorine; the net result was that the chlorine was destroying ozone without depleting itself. "When we found the chain reactions" occurring in the ozone layer, remembered Rowland this fall, the fate of CFCs "suddenly went from a scientific curiosity to an environmental worry."