September 16, 1987 | The Montreal Protocol and the Ozone Treaty That Worked

Explore the full date: Today in History – September 16.

On September 16, 1987, diplomats in Montreal adopted a treaty built around an unusual promise: science would keep changing, and the law would change with it. The Montreal Protocol did not end the ozone crisis in a day. It created a practical system for measuring danger, tightening controls and sharing the cost of replacing the chemicals that were damaging Earth’s protective stratospheric ozone layer.

Inside the International Civil Aviation Organization headquarters, delegates were closing a conference that had run from September 14 to 16. Their papers dealt in production figures, consumption baselines and lists of chlorofluorocarbons and halons. The language was technical because the problem was technical. Yet the stakes were planetary: more harmful ultraviolet radiation reaching people, crops and ecosystems if the ozone layer continued to thin.

A warning written in atmospheric chemistry

The ozone layer is not a solid shield. It is a region of the stratosphere where ozone molecules absorb much of the Sun’s most damaging ultraviolet radiation. During the twentieth century, industries found enormous value in stable, nonflammable chemicals such as chlorofluorocarbons, or CFCs. They became common in refrigeration, air-conditioning, foam production, solvents and aerosol propellants.

That stability was the danger. CFCs could persist long enough to reach the stratosphere. There, ultraviolet light broke them apart and released chlorine atoms that could participate in repeated ozone-destroying reactions. In 1974, chemists Mario Molina and F. Sherwood Rowland published the research that helped establish this mechanism. Their work, together with later observations and laboratory studies, transformed an invisible atmospheric process into an urgent policy question.

Governments did not begin from complete certainty. They began with mounting evidence and the possibility of severe, difficult-to-reverse harm. Several countries restricted some aerosol uses of CFCs during the 1970s, but production for other applications continued. Because the atmosphere ignored borders, national rules alone could not solve the problem.

From the Vienna Convention to Montreal

In 1985, governments adopted the Vienna Convention for the Protection of the Ozone Layer. The convention created a framework for research, monitoring and cooperation, but it did not yet impose binding reductions in the chemicals most responsible for depletion. That same year, British Antarctic Survey scientists reported dramatic seasonal ozone losses over Antarctica—the phenomenon soon known as the ozone hole. The discovery gave the negotiations new urgency.

The scientific picture was still developing as diplomats worked. That is one reason the treaty’s design mattered so much. The negotiators did not pretend that a single conference could settle every chemical, timetable and economic consequence. Instead, they built a protocol that could be adjusted as evidence improved.

September 16, 1987: a treaty designed to become stronger

The conference’s Final Act records that the delegates met in Montreal from September 14 through 16 and adopted the Montreal Protocol on September 16. The agreement required participating countries to control the production and consumption of specified ozone-depleting substances. Its initial schedules were cautious by later standards, but the treaty contained the machinery for regular scientific review, amendments and adjustments.

That architecture turned out to be decisive. Parties could add substances, accelerate phaseout schedules and refine compliance rules without discarding the whole agreement. Developed and developing countries accepted common goals with differentiated timetables and support. A financial mechanism later helped eligible countries meet their obligations by funding technology transfer and conversion away from controlled chemicals.

The protocol also addressed trade with non-parties, reporting and national licensing systems. These provisions made participation practical and reduced the incentive for industries simply to move production outside the treaty system. It was environmental diplomacy expressed through measurable inventories and deadlines rather than broad aspiration alone.

What happened next

The Montreal Protocol entered into force on January 1, 1989. Over the following decades, its controls were repeatedly strengthened. CFCs, halons and other ozone-depleting substances were phased out on schedules that varied by substance and national circumstances. The treaty eventually achieved universal ratification, an exceptional status in international environmental law.

The story was not a simple straight line. Replacement chemicals created new problems. Hydrochlorofluorocarbons damaged ozone less than CFCs but still required phaseout. Hydrofluorocarbons did not deplete ozone, yet many were powerful greenhouse gases. The 2016 Kigali Amendment added a global phasedown of HFCs, connecting the treaty’s adaptive structure to climate protection.

Scientific assessments now report that ozone recovery is underway, although the timetable varies by region and depends on continued compliance. The result is not evidence that the original danger was exaggerated. It is evidence that coordinated action changed the trajectory.

Why the Montreal Protocol matters

The protocol is often described as the world’s most successful environmental agreement. Its importance lies not only in the substances it controlled, but in the relationship it built between science and policy. Researchers measured the atmosphere; assessment panels translated evidence; governments revised rules; industry developed substitutes; finance helped make compliance possible.

That model did not eliminate conflict or uncertainty. It gave institutions a way to act before every detail was known and to correct course afterward. On September 16, 1987, the delegates in Montreal did not sign the final page of the ozone story. They created a system capable of continuing it.

Why cooperation held

Environmental agreements often struggle when the costs of action arrive immediately but the benefits are dispersed across borders and decades. The Montreal system addressed that problem through institutions. Scientific assessment panels regularly reviewed atmospheric evidence. Technical and economic panels examined substitutes and feasibility. Meetings of the parties converted those findings into negotiated schedules.

The Multilateral Fund, established in 1991, helped developing countries pay agreed incremental costs of compliance. That support was not charity added at the margins; it was part of the bargain that made global participation possible. Countries also reported production and consumption data, creating a shared basis for measuring progress and identifying non-compliance.

A recovery measured over generations

Ozone chemistry moves on long timescales. Many controlled substances remain in the atmosphere for decades, so recovery cannot be judged from one season. The World Meteorological Organization and UNEP periodically assess trends, model future concentrations and distinguish the Antarctic ozone hole from broader changes at mid-latitudes and in the Arctic.

Those assessments project a return toward 1980 ozone values during the middle or later decades of this century, with regional differences. The dates are projections, not guarantees. Unexpected emissions, illegal production, climate interactions and failures of compliance can alter them. Continued observation is therefore part of success, not evidence that the treaty is unfinished.

The protocol’s legacy is a disciplined form of optimism: a planetary environmental threat can be reduced when scientific warnings are translated into adjustable rules, financial support and verifiable action.

How chlorine can destroy ozone repeatedly

The chemistry made small concentrations consequential. Ultraviolet radiation could break a chlorine atom from a CFC molecule high in the atmosphere. That chlorine could react with ozone, then be regenerated through subsequent reactions and attack again. Because the atom acted catalytically, one release could participate in many cycles before being removed from the active chain.

Polar stratospheric clouds helped create especially destructive conditions over Antarctica. Reactions on the cloud particles converted chlorine reservoirs into forms that became highly active when sunlight returned in the Antarctic spring. The result was not a literal hole but an extreme seasonal thinning of ozone across a vast region.

Adjustment without starting over

The protocol distinguished between amendments and adjustments. Parties could tighten control measures for substances already covered through a procedure more flexible than negotiating an entirely new treaty. New chemicals and obligations could be added through amendments. This legal engineering allowed the regime to respond as evidence strengthened and substitute technologies became available.

The London, Copenhagen, Montreal and Beijing changes progressively expanded and accelerated controls. The sequence shows why the 1987 text should not be judged only by its first targets. Its durability came from accepting that the initial agreement was incomplete and providing a legitimate process for making it more ambitious.

From ozone protection to climate benefits

Many ozone-depleting substances are also powerful greenhouse gases. Phasing them out therefore avoided warming as well as ultraviolet exposure. The transition was complicated because some replacements, especially HFCs, protected ozone but contributed strongly to climate change.

The Kigali Amendment of 2016 addressed that second-generation problem by establishing an HFC phasedown. It did not change the protocol into a general climate treaty. It used an existing, functioning system to control a family of chemicals already connected to the industries the ozone regime regulated.

This capacity to confront unintended consequences is another reason September 16 matters. The agreement succeeded not by predicting every future problem in 1987, but by remaining capable of incorporating problems its authors could not yet fully see.

Continue exploring

• Explore another September 16 story: the Wall Street bombing of 1920.
• Browse more discoveries in Science & Space.
• Read how LIGO made the first direct detection of gravitational waves.
• Follow Mae Jemison’s launch aboard STS-47.
• Explore Luna 2’s impact on the Moon.