In 1974, scientists Mario Molina and F. Sherry Rowland published a paper theorising that CFCs could destroy ozone in Earth's stratosphere. Until then CFCs were thought to be harmless, but Molina and Rowland suggested that assumption was wrong. Their findings were attacked by industry, who insisted their products were safe. Among scientists, their research was contested. Projections indicated that ozone depletion would be minor – between 2-4% – and many thought it would happen on a timescale of centuries.
The use of CFCs continued unabated and by the 1970s they were ubiquitous worldwide, used as coolants in refrigerators and air conditioners, in aerosol spray cans and as industrial cleaning agents.
A mere decade later, in 1985, the British Antarctic Survey confirmed a hole in the ozone layer and suggested a link to CFCs – vindicating the work of Molina and Rowland, who were eventually awarded the 1995 Noble Prize in chemistry. Even worse, the depletion was happening much quicker than had been anticipated. "It was really quite shocking," says Shanklin, now an emeritus fellow at the British Antarctic Survey.
From then on, scientists raced to figure out how and why this was happening. A chemical mystery
In 1986, as the Antarctic winter neared its end, Susan Solomon, a researcher with the US government National Oceanic and Atmospheric Administration, led a team of scientists to McMurdo Base in search of answers. At the time, scientists were debating three possible theories, one of which Solomon had proposed: that the answer might lie in surface chemistry involving chlorine on polar stratospheric clouds, which occur at high latitudes and only form during very low temperatures in polar winter.
"It was a great mystery," says Solomon, now professor of atmospheric chemistry and climate science at MIT. Her research explained how and why the ozone hole occurs in Antarctica. "All the data pointed towards the combination of the increase of chlorine from the human use of CFCs and the presence of polar stratospheric clouds as being the trigger for what happened."
Satellite monitoring confirmed ozone depletion extended over a vast region – 7.7 million square miles (20 million sq km).
The serious threat posed by ozone depletion – rises in skin cancer and cataracts in humans, harm to plant growth, agricultural crops and animals and reproductive problems in fish, crabs, frogs and phytoplankton, the basis of the marine food chain – spurred international action and collaboration. But considering how grave a threat the ozone hole was deemed to be, why do we not often hear about it anymore?
"It's not the same cause for alarm that it once was," says Laura Revell, associate professor of environmental physics at the University of Canterbury, New Zealand. This is largely due to the unprecedented international steps that governments took to tackle the problem.
Thinking ozone depletion would be small and far into the future, international policymakers initially took a cautious approach to ozone protection. In 1977, a global action plan was adopted, calling for monitoring of ozone and solar radiation, research on ozone depletion’s effect on human health, ecosystems and the climate and a cost-benefit assessment of control measures. A few months before the discovery of the ozone hole by the British scientists, this led to the 1985 Vienna Convention, calling for further research. But it didn't include legally binding controls for CFC reduction, disappointing many.
After the ozone hole discovery, heavy investment in scientific research, marshalling of economic resources and coordinated international political action helped to turn things around.
In 1987, the Montreal Protocol was adopted to protect the ozone layer by phasing out the chemicals which deplete it. To support compliance, the treaty recognised "common but differentiated responsibilities", staggering phase-out schedules for developed and developing countries and establishing a multilateral fund to provide financial and technical assistance to help developing countries meet their obligations. During the 1990s and early-2000s, the production and consumption of CFCs was brought to a halt. By 2009, 98% of the chemicals agreed to in the treaty had been phased out.Six amendments — which the treaty allows when scientific evidence shows further action is needed — have led to ever-tightening restrictions on substances introduced to replace CFCs, such as hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs). While good for the ozone layer, these replacements turned out to be bad for the climate. The global warming potential of the most commonly used HCFC, for example, is almost 2,000 times stronger than carbon dioxide.
The treaty's climate benefits have been a positive side effect. In 2010, emissions reductions due to the Montreal Protocol were between 9.7 to 12.5 gigatons of CO2 equivalent, approximately five to six times more than the target of the Kyoto Protocol, an international treaty adopted in 1997 that aimed to reduce greenhouse gas emissions. The 2016 adoption of the Kigali Amendment, which will limit the use of HFCs, will help avoid up to 0.5 C of global warming by 2100.
"You could argue [the Montreal Protocol] is a much more successful bit of climate protection legislation than any of the other [climate] agreements we've had to date," says Revell. Since its adoption, the Montreal Protocol has been signed by every country on Earth – to date the only treaty to be universally ratified. It's widely considered a triumph of international environmental cooperation. According to some models, the Montreal Protocol and its amendments have helped prevent up to two million cases of skin cancer yearly and avoided millions of cataract cases worldwide.
Had the world not banned CFCs, we would now find ourselves nearing massive ozone depletion. "By 2050, it's pretty well-established we would have had ozone hole-like conditions over the whole planet, and the planet would have become uninhabitable," says Solomon.
Solomon credits three factors for the swift action on the problem: the clear and present danger the ozone hole posed to human health made it personal to people, vivid satellite imagery made it perceptible and there were practical solutions to it – ozone-depleting substances could be replaced fairly quickly and easily.
