The Ozone Layer
The clearest environmental success story in history — a global crisis identified by science, met with international cooperation, and actually solved.
The short version
- The ozone layer — a region of stratosphere 15–35 km above Earth — filters 97–99% of the sun's UV-B radiation; its thinning was confirmed in 1985 after scientists predicted the mechanism in 1974.
- Chlorofluorocarbons (CFCs), synthetic chemicals used in refrigerants and aerosols, were proven to destroy ozone through a catalytic chain reaction: each chlorine atom destroys roughly 100,000 ozone molecules.
- The Montreal Protocol (1987) became the only international treaty in history ratified by every nation on Earth — and it worked. Stratospheric chlorine has peaked and ozone is slowly recovering.
- Without the Montreal Protocol, EPA models project that by 2165 global ozone would have declined 67% and surface UV radiation would have doubled — producing catastrophic increases in cancer and ecosystem collapse.
What it is
The ozone layer is a diffuse band of the Earth's stratosphere, approximately 15 to 35 kilometers above the surface, where ozone molecules (O3) are concentrated enough to absorb approximately 97–99% of the sun's ultraviolet-B (UV-B) and ultraviolet-C (UV-C) radiation. This filtering function is not incidental to life on Earth's surface — it is a precondition for it. UV-B radiation damages DNA, causes skin cancer and cataracts, suppresses immune function, and disrupts photosynthesis in plants and marine phytoplankton, the base of the ocean food chain. The ozone layer is not a static feature but a dynamic equilibrium: UV radiation continuously creates and destroys ozone molecules, with the balance maintaining surface concentrations within a range compatible with complex surface life.
The threat to this equilibrium was identified in a landmark 1974 paper by chemists Sherwood Rowland and Mario Molina, published in Nature. They showed that chlorofluorocarbons — stable synthetic compounds used extensively as refrigerants (Freon), aerosol propellants, and foam-blowing agents — were a previously unrecognized danger. CFCs are inert in the lower atmosphere; they do not break down, react with rain, or dissolve in seawater. This stability allows them to drift upward into the stratosphere over decades, where they are exposed to UV radiation intense enough to liberate chlorine atoms. Each free chlorine atom then catalytically destroys ozone molecules — combining with ozone, releasing a chlorine monoxide molecule that reacts with another oxygen atom to release the chlorine again — repeating the cycle approximately 100,000 times before the chlorine is eventually neutralized. DuPont, the primary U.S. CFC manufacturer, dismissed the Rowland-Molina findings as 'purely speculative' and funded research challenging them. The scientists were right.
Empirical confirmation arrived in 1985, when scientists from the British Antarctic Survey published measurements showing that Antarctic springtime ozone concentrations had declined by more than 40% relative to 1960s baselines — a 'hole' (actually a severe thinning) that appeared each spring as polar stratospheric clouds accelerated ozone-destroying reactions in the extreme cold. The measurements were so anomalous they were initially suspected to be instrument error. NASA satellite data confirmed the findings and showed the hole was large, deep, and expanding. The mechanism was exactly what Rowland and Molina had predicted. By the mid-1980s, there was no serious scientific dispute about the cause. Rowland, Molina, and Dutch atmospheric chemist Paul Crutzen received the Nobel Prize in Chemistry in 1995.
The international response was historically rapid. The Vienna Convention (1985) established a framework for cooperation. The Montreal Protocol on Substances that Deplete the Ozone Layer was negotiated in 1987 and signed by 46 countries, creating the first binding international agreement to phase out a class of industrial chemicals with a global environmental impact. Unlike most international environmental agreements, Montreal had strong enforcement mechanisms: trade restrictions on non-signatories, mandatory technology transfer to developing nations, and a Multilateral Fund to assist compliance. Successive amendments — London (1990), Copenhagen (1992), Montreal (1997), Beijing (1999), and the Kigali Amendment (2016, targeting hydrofluorocarbons, the HFC replacements for CFCs that are potent greenhouse gases) — accelerated timelines and expanded the list of controlled substances. As of 2024, 198 nations have ratified the Montreal Protocol — universal ratification, a distinction it holds alone among international environmental treaties.
Why it matters
The ozone story is the most important case study in successful international environmental cooperation the modern era has produced, and understanding why it succeeded where climate cooperation has not is analytically valuable rather than merely reassuring. The structural differences are significant. CFCs were manufactured by a concentrated global industry — primarily DuPont (U.S.), ICI (U.K.), and Hoechst (Germany) — with a manageable number of actors. Crucially, DuPont had developed HFC-134a, a commercially viable CFC substitute, before the Protocol was finalized; once the company calculated that market position was better served by leading a transition than resisting it, corporate opposition collapsed. This is the precise opposite of the fossil fuel industry's relationship to climate change, where the existing product is the problem and no comparable substitute exists at scale.
The scientific clarity and speed of the ozone case also mattered. From Rowland and Molina's 1974 hypothesis to international treaty in 1987 was thirteen years — fast for a discovery of this consequence. The Antarctic ozone hole provided a visible, dramatic empirical demonstration that accelerated political will. The causal chain from CFC to stratospheric chlorine to ozone destruction was mechanistically unambiguous in a way that the diffuse, probabilistic harms of climate change are not. And crucially, the harm — skin cancer and ecosystem damage from UV radiation — was concrete and personal in ways that politically resonate more readily than temperature anomalies in future decades.
The recovery is real and measurable. The Antarctic ozone hole reached its largest recorded extent in 2006 and has generally trended smaller since. Total global ozone concentration has stabilized and shows early signs of recovery, though the recovery is slow — stratospheric chlorine concentrations peaked around 2000 and are declining, but the ozone layer won't return to 1980 levels over most of the globe until the 2060s–2080s, reflecting the long atmospheric lifetimes of the relevant chemicals. The EPA's 2015 assessment estimated that the Montreal Protocol, by 2165, will have prevented approximately 280 million cases of skin cancer, 45 million cataracts, and 1.5 million skin-cancer deaths among Americans born between 1890 and 2100 — numbers that make it among the most consequential public health interventions in American history, conducted largely invisibly through international environmental law.
The ozone case has specific lessons for the politics of environmental regulation that are often misapplied. It is frequently invoked as proof that global environmental cooperation works and therefore climate change is tractable by the same approach. The differences are more instructive than the similarities. Ozone required approximately 100 countries to phase out one category of industrial chemicals. Climate change requires virtually every economic actor on Earth to fundamentally transform energy systems. Ozone's substitute technologies were available and cost-competitive. Clean energy's substitutes are increasingly cost-competitive but require massive infrastructure transformation. The ozone success was real; it should generate qualified confidence in collective action rather than the assumption that the political conditions will replicate.
Sources & Further Reading
- Stratospheric Sink for Chlorofluoromethanes: Chlorine Atom-Catalysed Destruction of Ozone
- Large losses of total ozone in Antarctica reveal seasonal ClOx/NOx interaction
- The Montreal Protocol on Substances that Deplete the Ozone Layer
- Forty years ago scientists discovered the ozone hole — here's what happened next
- Scientific Assessment of Ozone Depletion: 2022