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for refrigeration. CFCs quickly entered the market under the trade name Freon® and were successfully used all over the world.

      In the economic boom of post-WWII, millions of people, especially Americans and Europeans, were suddenly able to afford cars and television sets, kitchen appliances, larger homes and long-distance travel. New supermarkets featured an abundance of foods as agriculture became more mechanized and productive. Chemical factories created new products that were designed to make life easier and more pleasant and that list included Freon. In the 1950s and 1960s, CFC use sharply increased because owning a refrigerator was now taken for granted. However, it wasn’t long before discarded refrigerators dumped in landfills began leaking CFCs into the atmosphere. And because CFCs were odorless and colorless, no one noticed what was happening.

      Unwittingly, Thomas Midgley’s invention was causing immense damage to one of the systems most vital to protect life on Earth—a system that had taken hundreds of millions of years to evolve and that had made it possible for humanity to develop in the first place: the ozone layer. At a height of between six and thirty miles up in the Earth’s atmosphere, the ozone layer intercepts most of the sun’s harmful ultraviolet rays. Without the ozone layer, life on Earth, at least on land, would hardly be possible for higher diurnal life forms.

The environmental historian John McNeill later wrote that Midgley “had more impact on the atmosphere than any other single organism in Earth’s history.”² In the 1960s, no one knew about the dangers posed by chemicals mixing together, high above Earth’s then three billion inhabitants. CFCs, nitrous-oxide, also known as “laughing gas,” another ozone-depleting substance used in farming, and the burning of fossil fuels like coal and petroleum, were causing tremendous damage. Not even Paul Crutzen knew.

In 1958, our Dutch engineer applied for the position of computer specialist at the University of Stockholm’s Meteorological Institute. He got the job because interviewers at the Institute believed that he would be a fast learner. This was the young man’s entry into the world of scientific research, the career of his dreams. Not only was he a fast learner in the field of computer science, he also began to attend lectures in mathematics, statistics and meteorology. In 1963 he graduated and began a career as a scientist. Without deliberately planning it, he found himself in one of the hotspots of global environmental research. Among the young professors at Stockholm was Bert Bolin, who went on to co-found the Intergovernmental Panel on Climate Change (IPCC), which he then chaired, from 1988 to 1997.³

      Crutzen picked an area of research that was relatively new: the chemical processes that take place in Earth’s upper atmosphere. At first, he didn’t realize how significant the gaps in then-current knowledge would prove to be. He was interested in the natural processes and how the protective layer of ozone in the atmosphere, constantly renews itself.

Then, his impact on Earth’s future really began: he was one of the very first scientists to ask whether there are chemical processes that harm the ozone layer. Until then, this idea had seemed quite improbable. “The general feeling at the time was that ‘nature is so big and mankind so small,’” says Crutzen today, in retrospect. “Nobody had thought that man-made substances could have a huge effect on stratospheric ozone.”⁴ Crutzen’s initial research into how nitrous oxide, naturally released by soils, might damage the ozone layer, led him to a quite different discovery—that human activity was a threat to the ozone layer.⁵

Ever-increasing quantities of CFC molecules released into the atmosphere from landfills were not the only threat to the ozone layer. At around the same time, aviation engineers were developing large, high-tech nitrous oxide turbines. The United States, France, Great Britain and the Soviet Union had plans to build fleets of Supersonic Transport Aircraft (SSTs) to make it possible for civilians and the military to travel at supersonic speeds. Together with the American chemist Harold Johnston, Crutzen recognized the harm caused by nitrous oxide being released into the stratosphere so he used cool scientific logic to counter the dream that humans should be able to travel like gods to anywhere on Earth in a few hours. At the beginning of the 1970s he did some meticulous calculations to show that the additional nitrous oxide that would be emitted into the atmosphere from a fleet of 500 high-flying SSTs could cause “serious decreases in the total atmospheric ozone layer and changes in the vertical distributions of ozone, at least in certain regions.”⁶

      The work of Paul Crutzen and his colleagues sent a new message to the world: humanity had become so powerful and dominant through science, technology and modern lifestyles that we could harm Earth’s protective ozone layer.

      Inspired by these warnings, other scientists began to look for additional chemicals that might be changing the ozone layer. Mario Molina and Sherwood Rowland made a discovery in 1974: they established that CFCs are particularly effective in destroying ozone molecules. When Crutzen heard about this groundbreaking work, he immediately contributed his research and calculations showing that the hypotheses of his American colleagues were correct and that in the foreseeable future, forty percent of the world’s ozone layer might be depleted if the use of CFCs went unchecked. The consequences would be devastating; incidents of skin cancer and genetic mutation would multiply and some regions of the Earth might well become unfit for human life.

      There were many objections to the researchers’ theories, especially from the chemical industry that feared for its profits from the sale of CFCs and artificial fertilizers. The “hole” in the ozone layer could be a natural occurrence, critics argued. The harmfulness of CFCs was not proven. It would cause enormous damage to the economy to ban them, since there were no alternatives.

But in the mid-1980s, polar researchers led by Joe Farman returned from Antarctica with data showing that above a continent not populated by humans, the ozone layer was shrinking, especially during the southern hemisphere’s spring and that hazardous ultraviolet rays were reaching the Earth’s surface without hindrance. In 1985 Farman and his team published their results,⁷ revealing why the thinning of the ozone layer was over Antarctica, of all places, and not over industrial areas: CFCs adhere particularly well to ice crystals.

      The Antarctic explorers’ findings shocked public opinion even more than Rachel Carson’s Silent Spring. Before this, millions of people had associated their refrigerators and freezers with the idea of cold beer and instant pizzas as well as other convenience foods. They sprayed underarm deodorant in the mornings to smell nice at work in the office where, on hot days, they were grateful for air conditioning. Now, all of a sudden, these symbols of modern prosperity were seen in a completely different light. What had been thoughtlessly emptied into the atmosphere was suddenly coming back to haunt, in the form of a “hole” in the ozone layer. The individual actions of millions were a major hazard that threatened human life on Earth, and risked damaging the conditions needed for any terrestrial life.

      No matter how aggressively the chemical industry opposed demands made by environmentalists and scientists like Crutzen, Molina and Rowland to ban CFCs, the case was won in 1987. The United Nations drew up the Montreal Protocol, the single most effective international environmental treaty to date, which called for a gradual phase-out of harmful CFCs, used primarily as coolants. In 1997, the Kyoto Protocol was ratified, which aimed to drastically cut carbon dioxide emissions and nitrous oxide (N₂O) emissions.

Since then, a slow but constant regeneration of the ozone layer over Antarctica has taken place. In 2012 and 2013, scientists at NASA and the Alfred Wegener Institute in Germany reported that the Antarctic ozone “hole” had become noticeably smaller, for the first time.⁸ There is now a chance that the Antarctic ozone “hole” will disappear by 2050–—so long as climate change and the extreme durability of CFC molecules

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