The Montreal Protocol, the Treaty That Began to Mend the Ozone Hole

In May 1985 three scientists with the British Antarctic Survey, Joseph Farman, Brian Gardiner and Jonathan Shanklin, published a short paper in the journal Nature reporting something that their instruments at Halley Bay had been showing for years and that they had at first assumed must be an equipment fault. Every Antarctic spring, the ozone over the station was thinning dramatically. It was not a small seasonal wobble. A large fraction of the ozone above the continent was disappearing each spring and then returning. Within two years of that paper, the governments of the world had signed a treaty to phase out the chemicals responsible. Within a few more, the treaty had been tightened again and again. It is, by most accounts, the most successful environmental agreement ever made, and the United States under Ronald Reagan helped lead it.

In short: The Montreal Protocol, signed in 1987, phased out the chemicals that were destroying the ozone layer, and scientists now project the layer will recover over the coming decades. It worked because the science became undeniable, substitutes existed, rich countries paid poorer ones to switch, and the treaty restricted trade with holdouts. Climate change is harder in almost every one of those respects, but not in all of them.

Two chemists and a spray can

The story really begins a decade earlier. In 1974 two chemists at the University of California, Irvine, Mario Molina and F. Sherwood Rowland, published a paper arguing that chlorofluorocarbons, the stable, nontoxic, nonflammable chemicals used in refrigerators, air conditioners, spray cans and foam, did not simply vanish after use. They drifted up to the stratosphere, where ultraviolet light broke them apart and released chlorine. Each chlorine atom could destroy a great many ozone molecules in a chain reaction.

The ozone layer matters because it absorbs much of the sun's ultraviolet radiation before it reaches the ground. More ultraviolet radiation means more skin cancer, more cataracts, damage to crops and harm to marine life.

The chemical industry attacked the theory, sometimes harshly. But public concern moved fast. The United States, Canada and a few Nordic countries banned CFCs in most aerosol spray cans in the late 1970s. That was a real early step, but it covered only one use, and global production soon climbed again as other uses grew.

Molina and Rowland, along with the Dutch atmospheric chemist Paul Crutzen, shared the Nobel Prize in Chemistry in 1995 for their work on the chemistry of the ozone layer. By then, the treaty their research had made possible was already working.

From a hole to a treaty in two years

The Antarctic discovery changed the politics. A theory about gradual depletion over decades is easy to put off. A hole over a continent, with satellite images that newspapers could print in vivid colors, is not.

Governments had already agreed, in the Vienna Convention of 1985, on a framework to cooperate on research and protecting the ozone layer. That convention had no binding cuts. It created a structure that could be filled in later, and later came quickly. In September 1987, in Montreal, negotiators agreed on a protocol that required developed countries to cut their consumption of the main CFCs in half by the end of the century, measured against 1986 levels.

The United States played an unusually active role. The State Department's lead negotiator, Richard Benedick, later wrote a detailed account of the talks. Within the Reagan administration there was real division. Some officials doubted the science or worried about cost; at one point a senior official reportedly floated the idea that people could simply protect themselves with hats, sunglasses and sunscreen, an idea that was widely mocked when it became public. But EPA administrator Lee Thomas and the State Department pushed for strong cuts, and in the end Reagan backed them.

The Senate ratified the Montreal Protocol in March 1988 without a single vote against it. It is hard to imagine a modern environmental treaty receiving that kind of vote, and it is worth asking why it happened then.

Why it worked

There are several reasons the ozone fight went better than the climate fight has, and the honest answer to what climate diplomats can learn starts with admitting how lucky the ozone negotiators were.

The industry was small and the substitutes were close. CFCs were made by a limited number of chemical companies, and their uses, while important, were concentrated in a handful of products. In 1988 DuPont, the largest producer, announced it would stop making CFCs, and companies began developing replacement chemicals. Once the biggest producer was on board, the alternatives became a business opportunity rather than a threat.

The harm was concrete and personal. Skin cancer is something people understand. Officials in Washington could see that their own constituents, and their own families, faced a direct risk. The benefits of acting were not decades away and spread across the world; they were measurable in avoided cancers.

The treaty was designed to tighten. The original 1987 cuts were modest, and scientists quickly showed they would not be enough. The protocol included a process for regular scientific assessments and for amendments and adjustments. At meetings in London in 1990 and Copenhagen in 1992, parties accelerated the timeline to a full phaseout of the main ozone-depleting chemicals in developed countries. That was possible because the treaty's structure assumed the first deal would be revised.

The world that did not happen

One way to measure a treaty is to imagine its absence. Atmospheric scientists have done exactly that, building computer models of a world in which CFC production kept growing through the 1990s and beyond. Those studies, sometimes called the world-avoided scenarios, describe a planet where ozone depletion would have spread far beyond Antarctica, with severe thinning over populated regions in the middle latitudes by the middle of this century and ultraviolet levels in summer high enough to burn skin in minutes.

Nobody alive today has to live in that world, which is precisely why it is easy to forget. Successful prevention is invisible. The farmer who never lost a crop to ultraviolet damage, the child who never developed melanoma, the fishery that never collapsed: none of them knows the treaty is the reason. That invisibility is the cruel trick of every good public health and environmental policy, and it is why the people who defend such policies have to keep telling the story of what they prevented.

There is a political lesson in that, too. Montreal had the advantage of a dramatic discovery that made the danger real before the worst harm arrived. Climate change has had no single equivalent moment. Instead it arrives as a steady accumulation of hotter summers, heavier floods and bigger fires, each one arguable on its own.

The money and the stick

Two features of Montreal stand out for anyone who cares about how rich and poor countries work together.

The first is the money. Developing countries were given more time to phase out the chemicals, and at the 1990 London meeting the parties created a Multilateral Fund to pay the extra costs those countries faced in switching to substitutes. Developed countries pay into the fund. It has helped finance thousands of projects across the developing world, from converting factories to training refrigeration technicians. It is a direct transfer from countries that created the problem to countries that needed help solving it, and it is a big part of why nearly every country joined. The Montreal Protocol became the first treaty in the history of the United Nations to be ratified by every member state.

The second is the stick. The protocol restricted trade in controlled substances with countries that had not joined. That made staying outside costly: a country that refused to join could find itself cut off from supplies of the chemicals its industries depended on and from markets for products made with them. Unlike the Paris Agreement, which relies on transparency and peer pressure, Montreal had a concrete economic consequence for holdouts.

It is tempting to say climate diplomacy should simply copy this. The strongest counterpoint is that the scale is incomparably different. CFCs were a useful class of chemicals; fossil fuels are the foundation of almost every economy on Earth, powering electricity, transportation, industry and food production. A trade restriction on countries that do not cut carbon would mean restricting trade in steel, cement, fertilizer and nearly everything else. That is not impossible, and the European Union is experimenting with border carbon charges, but it is a much larger political lift than banning imports of refrigerants.

Kigali, and the climate bonus

Montreal has also turned out to be one of the most important climate treaties in the world, partly by accident. CFCs and many of the substitutes that replaced them are powerful greenhouse gases. By phasing them out, the protocol avoided a substantial amount of warming that would otherwise have occurred.

But some replacement chemicals, especially hydrofluorocarbons, or HFCs, created a new problem. HFCs do not deplete ozone, but many of them trap heat far more effectively than carbon dioxide. As air conditioning spread across hot, fast-growing countries, HFC use was set to soar.

In 2016, in Kigali, Rwanda, the parties agreed to amend the Montreal Protocol to phase down HFCs. The Kigali Amendment used the same structure that had worked before: schedules for developed and developing countries, support from the Multilateral Fund, and the expectation of tightening over time.

In the United States, Congress acted in a way that should be better known. The American Innovation and Manufacturing Act, passed in December 2020 as part of a large spending package and signed by President Trump, directed the EPA to phase down HFC production and consumption by 85 percent over about fifteen years. The bill had bipartisan sponsors, and much of the American chemical and air-conditioning industry supported it, partly because domestic companies had invested in the next generation of refrigerants and wanted a market for them. In 2022 the Senate gave its consent to the Kigali Amendment with bipartisan support.

That is a climate policy that passed with Republican and Democratic votes, backed by industry, signed by a president who had pulled out of Paris. You can find other bills that crossed party lines on the common ground page.

What climate diplomats can borrow

The ozone layer is healing. The most recent international scientific assessment, released in 2023 by the UN Environment Programme, the World Meteorological Organization and other agencies, concluded that the ozone layer is on track to recover to its 1980 levels around the middle of this century over most of the world, and later over Antarctica. That is a slow recovery, measured in decades, because the chemicals already released linger in the atmosphere for a long time. But it is recovery, and it was caused by policy.

Here is what I think climate policy can borrow from Montreal, and what it cannot.

It can borrow the habit of designing for revision. Montreal's first targets were too weak, and everyone moved past them quickly. Paris uses a similar ratchet, and the lesson of Montreal is that the ratchet works only if each turn actually tightens.

It can borrow the idea of paying for the transition in poorer countries in a dedicated, predictable way. The Multilateral Fund was not charity; it was the price of getting everyone to the table. Climate finance has been far less reliable, and the difference shows.

It can borrow the strategy of working sector by sector where a deal is possible, as Kigali did with refrigerants and as negotiators are now attempting with methane, shipping and aviation.

What it cannot easily borrow is the luck. The ozone problem had a small industry, ready substitutes, a vivid image and a direct health threat. Climate change has a vast industry, substitutes that are getting cheaper but require building enormous amounts of new infrastructure, harms that are vivid only after the fact, and costs that fall heavily on people far away and not yet born. That makes the job harder. It does not make it impossible.

What you can do

The ozone story is a reminder that international agreements live or die on domestic politics, which gives every reader a role.

• Learn who voted for the last bipartisan climate wins. The common ground page lists bills both parties passed together, which is a better map of where the next deal might come from than any speech.
• Weigh in on refrigerant and methane rules. The EPA writes the rules that put the HFC phasedown into practice. The public comment page shows federal rules open for comment, grouped by issue, with how to write a comment.
• Ask your senators about climate finance. Support for international funds like Montreal's runs through appropriations. You can find your senators and representative by address.
• Follow the global data. The climate dashboard tracks carbon dioxide at Mauna Loa and the global temperature record, the numbers the next treaty will have to bend.

The scientists at Halley Bay almost threw out their own data because it looked too strange to be true. They checked it, published it and changed the world. The next breakthrough may look just as strange at first. Our job is to make sure the politics can move as quickly as the science.