In the early 1980s, a small team of British scientists stood on the ice at a remote research station in Antarctica and could not believe their own instruments. Every spring, over the South Pole, the ozone in the upper atmosphere seemed to be collapsing — thinning so dramatically that they assumed their ageing equipment must be broken. It was not. They had found a hole in the sky, and the story of how it was made, discovered, and then healed is one of the rare occasions when humanity spotted a planetary disaster and actually managed to head it off.
Ozone is a form of oxygen that, high in the stratosphere, forms a thin layer shielding the surface of the Earth from the Sun's most damaging ultraviolet radiation — the wavelengths that cause skin cancer, cataracts and harm to crops and plankton. Life on land depends on it. And in the twentieth century, without anyone intending it, we began to destroy it.
The trouble with a miracle gas
The culprits were chlorofluorocarbons, or CFCs — synthetic chemicals invented in the 1920s and 1930s and hailed as miracles of modern industry. They were cheap, stable, non-toxic and non-flammable, ideal as the coolant in refrigerators and air conditioners, the propellant in aerosol spray cans, and the blowing agent in foams. Their very stability was the problem. Because CFCs are so inert, they do not break down in the lower atmosphere; instead they drift slowly upward over years until they reach the stratosphere, where the Sun's ultraviolet light finally shatters them, releasing chlorine atoms. In 1974 two chemists, Mario Molina and Sherwood Rowland, published a paper warning that a single chlorine atom, acting as a catalyst, could destroy many thousands of ozone molecules before being removed — meaning that these humble spray-can gases could, over time, eat away the ozone layer.
The theory was controversial, and the chemical industry resisted it, but the warning proved prophetic in a way and place no one had predicted. In 1985, scientists of the British Antarctic Survey — Joe Farman, Brian Gardiner and Jonathan Shanklin — published measurements showing that springtime ozone over Antarctica had fallen catastrophically, by around a third or more compared with earlier decades. The "ozone hole," as it was quickly named, was far larger and more sudden than the models had forecast. The extreme cold of the polar winter, it turned out, created special icy clouds on which the chlorine chemistry ran with terrifying efficiency once the spring sunlight returned.
The evidence the satellites flagged
The most chastening detail is that satellites had been recording the very same collapse for years, and the readings had very nearly been dismissed. NASA's instruments had measured the plunging ozone values, but the numbers were so extreme that the computers had been programmed to flag them as probable instrument errors for later review, on the assumption that no natural reading could be that low — the raw data itself was archived, not discarded. It took a handful of scientists on the ground, with old-fashioned instruments and stubborn attention, to notice what the machines had queried. Only when the British result was published did NASA go back, pull the flagged data out of storage, and confirm the hole in dramatic satellite images.
What happened next is why the ozone story is told as a triumph rather than a tragedy. The evidence was alarming and the science, once the hole was undeniable, was hard to dispute. In 1987 the nations of the world negotiated the Montreal Protocol, an international treaty to phase out the production of CFCs and related ozone-destroying chemicals. It was later strengthened repeatedly, eventually earned ratification by every country on Earth, and is often called the most successful environmental treaty ever signed. Industry, under pressure and with alternatives available, largely complied, and the manufacture of the worst offenders was wound down.
Because CFCs linger so long, the ozone layer heals only slowly, over generations, and the Antarctic hole still opens each spring. But the amount of ozone-destroying chlorine in the atmosphere has passed its peak and is falling, and scientists now expect the layer to recover to its mid-twentieth-century state around the middle of this century. In 1995 Molina, Rowland and the atmospheric chemist Paul Crutzen shared the Nobel Prize in Chemistry for working out the chemistry that had raised the alarm.
The lesson has hung over environmental science ever since, both as encouragement and as warning. It is possible for the world to recognise an invisible, slow-moving, global threat and to act together in time to fix it. But it nearly did not happen — and it only happened because a few people on the Antarctic ice trusted a reading that everyone else, including the satellites, had assumed was a mistake.
Quiz nuggets
- The ozone layer shields Earth from harmful ultraviolet radiation; it is depleted by chlorofluorocarbons (CFCs) used in fridges, aerosols and foams.
- In 1974 Mario Molina and Sherwood Rowland warned that chlorine from CFCs could destroy ozone.
- The Antarctic ozone hole was reported in 1985 by the British Antarctic Survey team of Joe Farman, Brian Gardiner and Jonathan Shanklin.
- Satellite data had recorded the depletion earlier, but automated software flagged the readings as probable instrument errors for review, and the raw data was archived rather than discarded.
- The 1987 Montreal Protocol phased out CFCs and is regarded as the most successful environmental treaty; Molina, Rowland and Paul Crutzen won the 1995 Nobel Prize in Chemistry.