In 1977, the new job for then-24-year-old Jonathan Shanklin at the British Antarctic Survey (BAS) held little glamor. The meteorologist was tasked with wading through years of backlog of hand-scrawled data from a BAS instrument at a research station in Antarctica, meticulously checking and correcting measurements of damaging UV light reaching Earth to gauge how much ozone, our planet’s natural sunscreen, hovers above the continent.

What began as a routine, even tedious, assignment soon unearthed a precipitous drop in springtime ozone levels over Antarctica since the late 1970s, culminating in the 1985 revelation of significant thinning of our planet’s delicate protective shield. Subsequent satellite monitoring by NASA showed this attenuation was not just localized over the British research stations but spanned most of Antarctica, with pockets extending beyond the continent. 

Shanklin and his team linked the so-called “ozone hole” to the breakdown of human-made chlorofluorocarbons (CFCs) prevalent in spray cans, refrigerators and air conditioners. The findings confirmed the theory that these components, mainly chlorine and bromine, are emitted into the air over the tropics and then dispersed globally by winds. 

Antarctica's large, ocean-surrounded landmass makes the ozone layer above it particularly vulnerable to destruction by CFCs. The continent’s geography creates exceptionally cold winters that foster the formation of clouds in the ozone layer above it. When springtime sunlight returns, these clouds enable chemical reactions that transform chlorine and bromine into potent ozone-destroying agents. 

The discovery of this phenomenon drew the world’s attention to the impact of human activity on the global environment, jolting it into an unprecedented recovery effort. This response was underpinned by a landmark international treaty known as the Montreal Protocol on Substances That Deplete the Ozone Layer, or Montreal Protocol for short. Since enacted on January 1, 1989, the treaty has been used to phase out nearly 100 dangerous gases, making it one of the most successful environmental treaties ever negotiated and implemented. 

“The Montreal Protocol was founded on principles of strong scientific evidence and international cooperation,” says Luke Western, a research fellow at the University of Bristol who quantifies emissions of ozone-depleting substances at both global and local levels. “Scientists, industry and policymakers continue to work closely to ensure that these principles are upheld.”

But because CFCs linger for hundreds of years in the layer of Earth’s atmosphere where most of the ozone is, the Antarctic ozone layer isn’t expected to fully recover until the mid-2060s. 

“Despite a ban on the production of CFCs, emissions to the atmosphere continue to this day as they leak out of old refrigerators, air conditioning units and insulation foams,” says Western. “The total amount of these gases in the atmosphere continues to decline but it will take several decades before they reach levels we saw in 1980.”

On March 22, 1985—roughly a month before the publication of Shanklin and his team’s findings—representatives from 28 countries, including most CFC producers such as the United States, the European Union and Japan, signed the Vienna Convention. This agreement, while lacking legally binding targets for reducing ozone-depleting substances, established principles for global cooperation and phase-out schedules later implemented in the Montreal Protocol.

On September 16, 1987, when the Montreal Protocol was adopted, it was signed by 46 countries. It has since been signed by all 197 member states of the United Nations and is the first UN treaty to have achieved universal ratification. 

“People only had to look at a picture to physically see the atmospheric chemistry,” Pawan Bhartia, a NASA scientist who presented a satellite image at a 1985 meeting that revealed for the first time the ozone hole’s sheer size and magnitude, said in a 2012 interview with the space agency. “It didn’t take much persuasion to convince the policymakers to take action.”

The treaty’s success stemmed from strong leadership and novel negotiating techniques, including small, informal discussions that fostered open dialogue and trust. This was particularly key in facilitating agreement on complex issues like the future Multilateral Fund, which provided financial and technical assistance to developing countries to help them adopt ozone-friendly technologies and meet phase-out targets. This framework, where essentially richer countries have assisted poorer countries, “has led to an efficient and successful phase out of ozone-depleting substances,” says Western.

The treaty's brilliance, however, was its built-in mechanism for regular review, allowing it to adapt to new scientific findings. This enabled the implementation of stricter regulations and addition of ozone-depleting substances as evidence emerged. In 2016, for instance, the Kigali Amendment was adopted to phase down hydrofluorocarbons, or HFCs, which are potent planet-warming gases often used as replacements for banned ozone-depleting substances in refrigerators and air-conditioners. 

“It is humbling to think that industry across the world has had to change because of what seemed to be a small discovery over an obscure part of Antarctica that most people had never heard of,” Shanklin said in an interview with the UK Research and Innovation, the UK’s lead funder of science and research, including BAS’s work. 

The first statistically significant evidence of Antarctic ozone hole recovery appeared in the mid-2010s. In 2016, three decades after the world banned CFCs, scientists announced that the ozone hole had shrunk by about 4 million square kilometers, a crucial indication of recovery. 

Direct satellite observations of the ozone hole two years later corroborated this finding, showing declining levels of damaging chlorine due to the international CFC ban.

Yet, while human-caused ozone depletion may be under control, Western notes that unusual weather patterns and volcanic eruptions have in the past caused unexpected but temporary reductions in ozone concentrations. 

For instance, the eruption of the Hunga Tonga-Hunga Ha'apai submarine volcano in January 2022 was notably unusual, because it injected an unprecedented amount of water vapor directly into the stratosphere. Scientists have linked this influx to the earlier formation and prolonged existence of stratospheric clouds during the Antarctic winter, leading to a rapid decline in ozone concentrations and the early onset of the Antarctic ozone hole in 2023.

“Wildfires and volcanic eruptions can deplete the ozone layer,” says Western. Yet, he adds, “their impact is difficult to predict.”

Then, in 2000, unusually cold conditions over Antarctica amplified the formation of polar stratospheric clouds. This, coupled with the persistent presence of ozone-depleting substances, resulted in the largest ozone hole on record, spanning approximately 11 million square miles (28.3 million square kilometers), roughly three times the size of the United States.

In 2009, scientists at NASA modeled what the world would have been like without the Montreal Protocol and continued CFC use. Their simulation showed the ozone hole would have essentially covered the globe by midcentury, and a clear summer noon in New York would have caused sunburn in just 10 minutes.

That’s the world we collectively avoided.