The Montreal Protocol: How the World Healed Its Sky

When scientists in the 1970s first sounded the alarm that humanity was eroding the stratospheric ozone layer, their warning cut through political boundaries and industrial interests. The ozone layer—our planet’s thin, ultraviolet-absorbing shield—was being chemically destroyed by human-made chlorofluorocarbons (CFCs) and halons used in aerosols, refrigerators, and air conditioners.

By the early 1980s, satellite data and field measurements over Antarctica revealed an alarming ozone hole, confirming the worst fears of atmospheric chemists. In one of the most remarkable acts of global cooperation, the world’s nations responded not with denial, but with action.

A Timeline of Discovery and Response

1974 — The Chemistry of Destruction
Mario Molina and F. Sherwood Rowland published their seminal paper explaining how CFCs rise into the stratosphere, where UV radiation breaks them apart, releasing chlorine atoms that catalyze ozone destruction. A single chlorine atom could destroy tens of thousands of ozone molecules before being deactivated.

1985 — The Antarctic Ozone Hole
British Antarctic Survey scientists reported a dramatic seasonal depletion of ozone over Antarctica. The discovery shocked policymakers and accelerated diplomatic talks already underway.

1987 — The Montreal Protocol Signed
Negotiated under the United Nations Environment Programme (UNEP), the Montreal Protocol on Substances that Deplete the Ozone Layer was adopted in September 1987. It required industrialized nations to phase out key ozone-depleting substances (ODS) and provided financial and technical assistance to developing countries to follow suit.

How the Montreal Protocol Works

The Protocol is both simple and flexible, two qualities that explain its enduring success.

Science-based targets: ODSs are listed in annexes, and control measures are updated as new evidence emerges.
Universal participation: All 198 UN member states are parties—making it the first treaty in history to achieve universal ratification.
Differentiated responsibility: Wealthier nations phase out ODSs first and support developing nations via the Multilateral Fund, which finances technology transitions.
Built-in revision mechanism: Periodic meetings (the Meetings of the Parties) allow for adjustments based on the latest science.

The Results: A Healing Atmosphere

Since 1989, production of the most harmful CFCs and halons has dropped by over 99%. Atmospheric concentrations of chlorine and bromine have begun to decline, and the ozone layer shows measurable signs of recovery.
Models predict that—if compliance continues—the Antarctic ozone hole will close by around 2060.

The Montreal Protocol has also delivered a climate dividend: many ODSs are potent greenhouse gases. By preventing their release, the treaty has avoided the equivalent of hundreds of billions of tonnes of CO₂ emissions, making it one of the most effective climate policies ever enacted.

The Kigali Amendment: Expanding the Legacy

In 2016, nations adopted the Kigali Amendment, targeting hydrofluorocarbons (HFCs)—chemicals developed to replace CFCs but later found to be powerful greenhouse gases.
While HFCs do not destroy ozone, their phase-down under the same legal framework reinforces the Protocol’s status as a living, evolving treaty.

Lessons for Today’s Climate Challenge

The Montreal Protocol is more than a treaty—it’s a case study in what global environmental governance can achieve when:

Science is trusted and acted upon.
Costs are shared fairly between nations.
Policies include mechanisms for learning and revision.

In a world now wrestling with the scale and urgency of climate change, the Montreal Protocol stands as proof that multilateral action can not only halt environmental harm but reverse it.

Key Takeaways

Enacted: 1987 Entered into force: 1989
Parties: 198 (universal)
ODS reduction: >99%
Estimated ozone recovery: mid-21st century
Climate co-benefit: Avoided warming ≈ 0.5 °C by 2050

References

Molina, M. J. & Rowland, F. S. (1974). Stratospheric sink for chlorofluoromethanes: chlorine atom–catalysed destruction of ozone. Nature, 249, 810–812.
United Nations Environment Programme (UNEP). (1987). The Montreal Protocol on Substances that Deplete the Ozone Layer.
World Meteorological Organization (2022). Scientific Assessment of Ozone Depletion: 2022.
Velders, G. J. M. et al. (2007). The importance of the Montreal Protocol in protecting climate. PNAS, 104(12), 4814–4819.

“The Montreal Protocol is perhaps the single most successful international agreement to date.”
— Kofi Annan, Former Secretary-General of the United Nations