It was perceived as one of the greatest environmental threats of the late-20th century. Twenty-five years ago this month, a hole in the ozone layer was detected high in the atmosphere over the frozen wastes of Antarctica; scientists warned it might spread to other parts of the world, leading to dangerous increases in cancer-causing radiation from the Sun.
The Earth's protective layer of ozone shields all life from the damaging effects of ultraviolet (UV) rays, and its gradual depletion by the release of man-made chemicals into the atmosphere threatened a dramatic increase in lethal skin cancers and blinding cataracts – a threat so serious it forced politicians to act.
Just two years after the discovery was publicised in 1985 by a team of three British scientists, the international community had drafted the Montreal Protocol, designed to curb and eventually ban the use and manufacture of ozone-destroying chemicals, such as the chlorofluorocarbons (CFCs) used in products ranging from fridges to aerosol sprays.
The protocol soon led to CFCs being phased out in many countries. Britain ceased production and consumption of CFCs in 1995, followed five years later by other developed nations. By 2009, all UN member states had signed the basic protocol, which was seen as one of the most successful international agreements on the environment.
Now, a quarter of a century after the publication of the key scientific paper documenting the ozone hole, one of the members of the scientific team has said the discovery might not have been made so soon had it not been for a combination of dogged perseverance and good luck.
"My perspective is that luck played its part, as in many other scientific discoveries," said Jonathan Shanklin, who, along with colleagues Joe Farman and Brian Gardiner of the British Antarctic Survey in Cambridge, gathered the key field data.
At that time, in the early 1980s, British science was being squeezed by the Conservative government of Margaret Thatcher. Long-term scientific monitoring programmes were especially threatened. Among them was the one responsible for the annual ozone measurements that had been carried out at the British Antarctic Survey's Halley research station since the late 1950s.
"In the 1980s, the British Antarctic Survey was looking at ways to economise, and the ozone monitoring at Halley was in the frame to be cut. Nothing seemed to be changing and there seemed little reason to keep it going. But it is programmes such as these that provide the crucial evidence for political decisions governing the future of our planet," Dr Shanklin said.
In fact, the measurements at Halley were not originally intended to monitor long-term changes to ozone but to help improve weather forecasting and to verify theories about atmospheric circulation. However, it gradually became obvious that ozone levels in the Antarctic spring – which occurs in October and September – were falling significantly after each southern winter, and were only partly recovering each summer.
There was already scientific speculation, backed up by serious theoretical work, about how the ozone layer might be affected by man-made pollutants such as CFCs in the stratosphere, where the ozone layer is found. Studies into ozone depletion in the 1970s by scientists Paul Crutzen, Mario Molina and Frank Sherwood Rowland led eventually to a Nobel prize in chemistry.
However, in the early 1980s nobody had noticed that the ozone layer above the South Pole was being depleted significantly at the end of each Antarctic winter, when the first rays of sunlight penetrate the darkness of the austral sky to cause ozone-destroying photochemical reactions with the chlorine of CFCs. The 1985 study showed that the lowest values of ozone seen in mid-October had fallen by 40 per cent between 1975 and 1984 – not quite a "hole" but worrying nonetheless.
"As I remember it, there was no real eureka moment in the discovery, more a combination of pieces falling into place," said Dr Shanklin, whose reflections are published in the current issue of Nature. The data was gathered from the ground using relatively simple instruments that looked up through the sky to measure the differences in UV light wavelengths known to be influenced by stratospheric ozone.
"What convinced the team was a graph plotting the minimum 11-day mean, which clearly showed that the spring decline was systematic. Farman crucially developed a chemical theory to explain the observations, linking them to rises in CFCs, and Gardiner carried out the essential quality control on the data," he said.
The study caused consternation, and some disbelief, among scientists in the US who were monitoring the ozone layer by sophisticated satellites. Their initial analysis had shown no such depletion but when they reanalysed the satellite data, they too detected the springtime depletion.
"I don't know what happened behind the scenes with the satellite teams, but I do know that they were overwhelmed by large amounts of data," Dr Shanklin recalled.
More than 20 years after the Montreal Protocol, there are signs that the ozone layer is beginning to recover. It could still take decades for it to return to the state it was in 50 years ago, but had it not been for three scientists who persevered with a seemingly irrelevant, long-term experiment, it would take longer still.