The Ozone Hole and the Montreal Protocol

Thomas Midgley Jr. of Frigidaire synthesized the first chlorofluorocarbons in 1928, marketed as Freon — non-toxic, non-flammable, stable refrigerants that replaced dangerous ammonia and propane in refrigerators and, from the 1940s, powered aerosol sprays and air conditioning. The chemicals' very stability, celebrated at the time, was the property that would let them survive for decades in the atmosphere. Production grew into a multi-billion-dollar global industry.

Location: Ohio, United States

In a paper in Nature (28 June 1974), Mario Molina and F. Sherwood Rowland of UC Irvine calculated that CFCs, released at then-current rates, would survive transport to the stratosphere where ultraviolet light would split off chlorine atoms, each of which could catalytically destroy tens of thousands of ozone molecules — thinning the layer that shields life from ultraviolet radiation. The chemical industry disputed the finding for years, and the science was initially controversial, but the paper set the research agenda. The United States banned CFCs in aerosol sprays in 1978, the first national restriction.

Joe Farman, Brian Gardiner, and Jonathan Shanklin of the British Antarctic Survey published in Nature (16 May 1985) ground-based measurements showing springtime ozone over Antarctica's Halley Bay had collapsed by more than a third since the 1970s — a hole far deeper than theory had predicted, later shown to be amplified by polar stratospheric clouds. NASA satellite data had recorded the same collapse but had rejected the readings as instrument error until the British paper triggered reanalysis. The discovery transformed ozone depletion from a theoretical risk into an observed planetary emergency.

After the Vienna Convention (1985) established a framework for research and cooperation, 46 countries signed the Montreal Protocol on Substances that Deplete the Ozone Layer on 16 September 1987, committing developed countries to phase out production of five major CFCs and halons on a fixed schedule, with developing countries given longer timelines. The treaty's design — binding schedules, trade sanctions against non-parties, and later amendments adding more chemicals — proved adaptable as the science hardened. It entered into force on 1 January 1989.

Location: Montreal, Canada

At the London meeting of the parties in June 1990, the protocol was strengthened to phase out CFCs entirely by 2000 for developed countries and established the Multilateral Fund, the first financial mechanism to pay developing countries for the incremental costs of treaty compliance. The fund — replenished by industrialized countries — financed the closure of CFC production lines and technology transfer across the developing world, and became the model for later climate finance debates. Industry, which had lobbied against controls through the 1980s, largely pivoted to producing substitutes as HCFCs and HFCs.

Location: London, United Kingdom

The 1995 Nobel Prize in Chemistry was awarded to Paul Crutzen, Mario Molina, and F. Sherwood Rowland for their work on atmospheric ozone chemistry, the first Nobel for environmental science, validating the science that industry and skeptics had contested for two decades. Ratification of the protocol continued to spread, and in 2009 it became the first treaty in history to achieve universal ratification — all 196 parties — a standing no other environmental agreement has matched. By the 2000s, global CFC production had fallen from over a million tonnes annually to near zero for controlled uses.

The parties to the Montreal Protocol adopted the Kigali Amendment, extending the treaty to hydrofluorocarbons (HFCs) — the chemicals industry had adopted as CFC substitutes that harm the climate rather than the ozone layer but are potent greenhouse gases. The amendment schedules an 80-percent-plus reduction in HFC production and consumption by the late 2040s and is projected to avoid up to half a degree Celsius of warming by 2100. The United States ratified it in 2022, and it entered into force in January 2019 for early parties.

Location: Kigali, Rwanda

A study led by Stephen Montzka of NOAA (Nature, May 2018) found that the atmospheric decline of CFC-11 had slowed unexpectedly after 2012, indicating some party was illegally producing and releasing the banned chemical at tens of thousands of tonnes per year — the first documented violation of the protocol. Investigative reporting and atmospheric analyses traced the production to factories in eastern China, and Chinese enforcement raids shut down the illegal operations; follow-up studies confirmed the emissions had ceased by 2019. The episode tested the treaty's verification system, which relies on atmospheric monitoring rather than on-the-ground inspection.

The 2022 Scientific Assessment of Ozone Depletion, published in January 2023 by the World Meteorological Organization and UN Environment Programme, reported that the ozone layer is on track to recover to 1980 levels by approximately 2066 over Antarctica, 2045 over the Arctic, and 2040 for the rest of the world, with CFC and HCFC concentrations continuing to decline. The assessment confirmed the protocol has avoided tens of millions of skin cancer cases by limiting ultraviolet exposure. The Montreal Protocol is widely cited as the model case of science, diplomacy, and industry working at planetary scale — and a point of comparison in the slower politics of climate change.