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important part of the Earth system almost single-handedly. Thomas Midgley was one such person. An obscure American chemist, whose main inventions took place in the 1920s and 30s, this one man had “more impact on the atmosphere than any other single organism in earth history,” as historian John McNeill puts it.1 This was not Midgley’s intention, of course. As a loyal servant of General Motors Corporation, he merely had the task of seeking nonflammable coolants for the company’s Frigidaire division, which was looking to replace the explosive and dangerous methyl chloride gas used in its fridges. Midgley’s solution was Freon, his trade name for a nonflammable synthesis of chlorine, fluorine, and carbon. Today we know it better as chlorofluorocarbon, or CFC.

The ozone-layer story demonstrates the double-edged sword of human technological prowess. On the one hand, modern science allows unremarkable individuals to have remarkable impacts at the planetary level through the deployment of everyday technologies that later turn out to be unintentionally but extremely damaging. On the other, science now equips us with the knowledge to at best identify these damaging impacts before they happen, or at worst to notice when they appear and take steps to counter them. Until recently, history suggests that technological applied science has tended to outstrip environmental science, as some of the other planetary boundary areas show. For example, the Haber-Bosch process to create synthetic nitrogen was invented early in the twentieth century and was producing millions of tonnes of ammonia per year before the downsides of nitrate pollution and oceanic dead zones were spotted. (As John McNeill notes: “Midgley was the Fritz Haber of the atmosphere.”) Similarly, toxic pesticides like DDT were in very widespread use before their negative effects on bird species and biodiversity in general were noted.

Humanity was both lucky and clever when it came to the ozone layer. We were lucky because had Thomas Midgley made a different choice from the periodic table he habitually carried around in his pocket—using bromine rather than chlorine as the basis for his new synthetic Freon refrigerants—almost the entire planetary ozone layer would have been destroyed as early as the 1970s, well before the science of atmospheric chemistry was well enough developed to recognize the problem.2 This is because bromine is far more destructive even than chlorine in the stable parts of the upper atmosphere where the ozone layer forms. But humans were also clever because our understanding of atmospheric chemistry did develop, in leaps and bounds in the 1970s and 1980s, and consequently when the real-world ozone hole did open up over Antarctica, scientists—who had already raised theoretical concerns about the impacts of CFCs—were quick to spot it and rapidly identified the likely cause.

Some of the key figures in the development of ozone-layer science are worth noting. The first person to accurately measure the gradual accumulation of CFCs in the atmosphere was none other than the Gaia hypothesis originator and intellectual father of Earth-system science, James Lovelock. On a research ship to Antarctica in 1971–2, Lovelock took the opportunity to test out his new invention, an electron capture detector for gas chromatography. This exquisitely sensitive instrument allowed for the first time the measurement of trace gases present in the air in only tiny quantities. On board Lovelock discovered that CFC (chemical formula CCL3F) was present at around 50 parts per trillion everywhere he took measurements, from the green coasts of Ireland to the storm-lashed Southern Ocean. This was the first scientific proof that a wholly man-made gas had become ubiquitous throughout the planet’s air. It was an epochal discovery, but far from raising the alarm, in his resulting scientific paper (published in Nature in 1973) Lovelock instead asserted reassuringly that “the presence of these compounds constitutes no conceivable hazard”—a claim he later acknowledged as a “gratuitous blunder.”3