Montreal Protocol (1987): The Treaty That Saved the Ozone Layer

The Montreal Protocol is the international treaty, signed on 16 September 1987 in Montreal, Canada, that set out to protect the Earth’s ozone layer by phasing out chlorofluorocarbons (CFCs) and other ozone-depleting chemicals. Strengthened many times since, it has eliminated around 99 percent of these substances and was the first United Nations treaty ratified by every country in the world. It is widely regarded as the most successful global environmental agreement ever made.

Background: a threat to the ozone layer

The ozone layer, a band of ozone gas in the stratosphere roughly 15 to 35 kilometres above the ground, absorbs most of the Sun’s harmful ultraviolet-B radiation. Without it, rates of skin cancer and cataracts would rise and crops and marine life would suffer.

In 1974 the chemists Mario Molina and F. Sherwood Rowland warned that CFCs, then used widely in aerosol sprays, refrigerators, air conditioners and foam, would drift into the stratosphere and release chlorine atoms that destroy ozone. Industry disputed the theory, but the United States, Canada, Norway and Sweden banned most CFC aerosol sprays in the late 1970s. Global CFC use nevertheless kept growing as other uses expanded.

The UN Environment Programme (UNEP), led by the Egyptian scientist Mostafa Tolba, began work on an international agreement. The result was the Vienna Convention for the Protection of the Ozone Layer, adopted on 22 March 1985. It committed countries to cooperate on research but contained no binding cuts.

The ozone hole raises the stakes

Two months after Vienna, in May 1985, scientists of the British Antarctic Survey reported a dramatic seasonal loss of ozone over Antarctica. The discovery of the ozone hole was far worse than any computer model had predicted, and NASA satellite images soon made it visible to the public. Although the precise cause was still being studied, the discovery gave negotiators a powerful sense of urgency.

Negotiating the treaty

Formal talks on a protocol began in December 1986. Positions differed sharply. The United States, Canada and the Nordic countries pressed for strong cuts. The European Community, home to major CFC producers, at first favoured only a cap on production capacity. Japan and the Soviet Union were also cautious, and many developing countries feared that controls would deny them affordable refrigeration.

American diplomat Richard Benedick, the chief US negotiator, later described the talks in his book Ozone Diplomacy. A key factor was a shift in industry: in 1986 the Alliance for Responsible CFC Policy, an American industry group, accepted that some limits were justified, which made agreement easier.

On 16 September 1987 delegates in Montreal signed the Montreal Protocol on Substances that Deplete the Ozone Layer. It required developed countries to freeze consumption of five main CFCs at 1986 levels and then cut it by 50 percent by mid-1999, and to freeze halons. Developing countries received a ten-year grace period. The treaty entered into force on 1 January 1989.

Strengthening the treaty

The Montreal Protocol was designed to be adjusted as science improved, and this proved crucial. In 1988 international scientists confirmed that CFCs were the main cause of the Antarctic ozone hole, and the chemical company DuPont, the largest producer, announced that it would phase out CFC production.

Parties then tightened the rules at a series of meetings:

• London, 1990: CFCs, halons and carbon tetrachloride to be phased out completely in developed countries by 2000. The parties also created the Multilateral Fund to pay the extra costs of developing countries’ transition, which helped bring India and China on board.
• Copenhagen, 1992: The phase-out of CFCs in developed countries was brought forward to the end of 1995, and hydrochlorofluorocarbons (HCFCs) and methyl bromide were added.
• Montreal, 1997 and Beijing, 1999: Further controls on methyl bromide, HCFC production and trade, and a licensing system to combat smuggling.
• 2007: Parties agreed to accelerate the phase-out of HCFCs.

Global production of CFCs fell steeply in the 1990s and ended in developing countries in 2010. Mario Molina, Sherwood Rowland and the Dutch atmospheric chemist Paul Crutzen shared the 1995 Nobel Prize in Chemistry for their work on ozone chemistry.

Key people and institutions

• Mostafa Tolba: executive director of UNEP from 1975 to 1992, who chaired difficult sessions and pushed delegations towards compromise.
• Richard Benedick: the chief US negotiator, whose account of the talks remains a standard history of the treaty.
• Mario Molina and F. Sherwood Rowland: the chemists whose 1974 theory first linked CFCs to ozone loss and who campaigned for controls.
• Joseph Farman, Brian Gardiner and Jonathan Shanklin: the British Antarctic Survey scientists who revealed the ozone hole.
• Susan Solomon: the American atmospheric chemist who proposed and helped confirm, through Antarctic expeditions in 1986 and 1987, the role of polar stratospheric clouds in ozone destruction.

Scientific assessment panels, reporting regularly to the parties, became a core part of the treaty. Their reports, and those of a technology panel that judged which substitutes were ready, allowed governments to tighten deadlines with confidence.

How the treaty works

The protocol lists controlled substances in annexes and gives each a timetable for reduction. Countries report their production, imports and exports each year to the Ozone Secretariat, based in Nairobi. Developing countries, known as Article 5 parties after the article that defines them, follow later deadlines. A non-compliance procedure encourages countries that fall behind to return to their targets rather than punishing them, and parties may not trade controlled substances with states outside the treaty.

The Kigali Amendment

Many CFC substitutes, especially hydrofluorocarbons (HFCs), do not destroy ozone but are extremely powerful greenhouse gases. On 15 October 2016 the parties agreed the Kigali Amendment in Rwanda, which requires a gradual phase-down of HFCs. It entered into force on 1 January 2019. Studies cited by UNEP estimate that a full phase-down could avoid up to about 0.4 degrees Celsius of global warming by 2100.

The Kigali Amendment made explicit something scientists had long noted: the Montreal Protocol is also an important climate treaty. Because CFCs are potent greenhouse gases, a widely cited 2007 study led by Guus Velders estimated that the protocol had avoided emissions equal to about 135 billion tonnes of carbon dioxide between 1990 and 2010, several times the target of the Kyoto Protocol.

Evidence of recovery

Because CFCs remain in the atmosphere for decades, the ozone layer heals slowly. Concentrations of ozone-depleting chlorine and bromine in the stratosphere peaked around the late 1990s and have been falling since. The Antarctic ozone hole still forms each southern spring, and its size varies with weather, but scientists have measured signs of healing since about 2000.

The 2022 Scientific Assessment of Ozone Depletion by the World Meteorological Organization and UNEP projected that, if current policies continue, the ozone layer should return to its 1980 values around 2066 over Antarctica, around 2045 over the Arctic and around 2040 for the rest of the world.

The treaty has faced setbacks. In 2018 scientists reported an unexpected rise in emissions of CFC-11, which was traced largely to illegal production in eastern China. After enforcement action, emissions fell again by 2019 and 2020. Researchers have also studied the possible effects of rocket launches, wildfire smoke and proposals for solar geoengineering on the ozone layer.

Why it succeeded

Historians and diplomats point to several reasons for the treaty’s success:

• Clear science: a well-understood mechanism and a visible ozone hole.
• Available substitutes: industry could develop alternatives, so the costs of change were manageable.
• Flexible design: regular scientific assessments allowed controls to be tightened quickly.
• Fairness: a grace period and the Multilateral Fund, which has provided several billion US dollars, helped developing countries take part.
• Trade measures: limits on trade with non-parties encouraged countries to join.

Former UN Secretary-General Kofi Annan described the protocol as perhaps the single most successful international agreement to date, a view repeated by many later commentators. Its success influenced later climate agreements, including the Paris Agreement, although cutting carbon dioxide has proved far harder than replacing CFCs.

By the numbers

Timeline

• 1974, June: Molina and Rowland publish their warning about CFCs in Nature.
• 1985, 22 March: The Vienna Convention for the Protection of the Ozone Layer is adopted.
• 1985, 16 May: British scientists report the Antarctic ozone hole.
• 1987, 16 September: The Montreal Protocol is signed.
• 1989, 1 January: The protocol enters into force.
• 1990, June: The London Amendment sets a full CFC phase-out and creates the Multilateral Fund.
• 1992, November: The Copenhagen Amendment accelerates the phase-out and adds HCFCs.
• 2009, September: The protocol becomes the first UN treaty ratified by every country.
• 2016, 15 October: The Kigali Amendment on HFCs is agreed.
• 2023, January: The WMO/UNEP assessment confirms the ozone layer is on track to recover.

Why it matters

The Montreal Protocol showed that nations could act together to solve a global environmental problem, guided by science and with support for poorer countries. It prevented a large rise in skin cancer and other damage from ultraviolet radiation, and it slowed climate change as a side effect. Its record remains the main example cited when people ask whether international environmental cooperation can work.