The Montreal Protocol
The treaty that actually worked
Description
In May 1985, three British Antarctic Survey scientists Joseph Farman, Brian Gardiner and Jonathan Shanklin published a short paper in Nature reporting that springtime ozone levels above their Halley research station had dropped by roughly 40% since the late 1970s. The atmosphere above Antarctica was missing an enormous quantity of an unstable molecule, and the depletion was getting worse. The data had been sitting in their notebooks for years before they were sure enough of it to publish. NASA's Nimbus-7 satellite had been recording the same drop since 1979 but had been programmed to flag readings that low as instrument error and discard them.
What followed in the next two and a half years was unusual. A scientific finding from a remote field station was confirmed within months by satellite reanalysis and aircraft missions, debated within an international assessment process, and converted into a binding global treaty signed in September 1987 in Montreal. The treaty restricted, then phased out, the chemicals causing the depletion. By the early 2000s the ozone layer was no longer thinning. By the 2020s it was visibly recovering. The Montreal Protocol is the only major international environmental treaty that has clearly worked at the scale it set out to work.
It is also a treaty that climate negotiators have been trying, and largely failing, to replicate for thirty years. The reasons it succeeded are specific to the problem it addressed, and understanding those reasons matters more than treating Montreal as a generic template. This is the story of a chemist who refused to drop a question, an unexpected discovery in Antarctica, an industry that pivoted, and a treaty whose effects are still unfolding.
The question we're asking: why did this particular environmental treaty work, and what conditions allowed it that climate negotiations have not reproduced?
What we'll see: Vienna, the hole, Montreal, and what came after.
Table of contents
01The chemistry that nobody had checked
In 1974, two chemists at the University of California Irvine, Mario Molina and Sherwood Rowland, published a paper proposing that chlorofluorocarbons CFCs, the gases used in refrigeration, aerosol sprays and foam insulation would destroy stratospheric ozone. The chemistry was straightforward. CFCs were stable enough to drift up into the stratosphere, where ultraviolet light would break them apart, releasing chlorine atoms. Each chlorine atom could catalyze the destruction of thousands of ozone molecules. Damaging the ozone layer would let more UV through to the surface.
The chemical industry's response was sharp. DuPont, the largest CFC manufacturer, called the hypothesis a science fiction tale. CFCs had been considered miracle compounds for forty years because they were stable, non-toxic and non-flammable. They were used in inhalers, fire extinguishers, automotive air conditioning, and the foam in fast food packaging. Removing them would mean replacing an enormous industrial system. Until there was actual evidence of damage, the industry argued, restrictions were premature.
02Susan Solomon on the ice
In August 1986, an atmospheric chemist at the United States National Oceanic and Atmospheric Administration named Susan Solomon led a small expedition to McMurdo Station in Antarctica. She was thirty years old. The mission was to test why the ozone hole formed where it did. Solomon had proposed that the unique conditions of the Antarctic stratospheric vortex extremely cold temperatures, isolated air masses, the formation of polar stratospheric clouds were creating ice surfaces on which chlorine compounds could be activated. When sunlight returned in the southern spring, this stockpiled active chlorine would tear through ozone in a few weeks.
The equipment was modest. Solomon and her colleagues used spectrometers to measure chlorine dioxide concentrations as the polar dawn returned. The readings were exactly what her hypothesis predicted. Chlorine compounds were elevated. Ozone was being destroyed in real time. By the time she returned a year later for a second expedition, the chemistry was solid enough that the international assessment process could no longer treat the cause as uncertain. The polar stratosphere was the place where the damage showed up first because the conditions there made the chemistry run faster.
03DuPont changes its mind
In March 1988, six months after Montreal was signed, NASA released a report from its Ozone Trends Panel showing that depletion was happening over populated mid-latitude regions, not just over the poles. The damage was global. Two weeks later, DuPont the company that had spent fifteen years arguing CFCs were probably safe announced it would cease all CFC production by the end of the century. Part of the answer was the Ozone Trends Panel data. Part was that DuPont's R&D group had developed substitutes (HCFCs) and saw a commercial opportunity in being the first mover. Part was the realization that Montreal's trade provisions would lock the global market into the new chemistry whether DuPont participated or not.
The treaty was strengthened repeatedly through the 1990s. The London Amendment in 1990 moved the phase-out date to 2000 for developed countries. The Copenhagen Amendment in 1992 added other ozone-depleting substances and accelerated the schedule. The 1995 Vienna Adjustment shortened the timeline again. Each amendment was negotiated quickly, partly because the economic disruption from the previous round had been smaller than industry had warned, and partly because new scientific findings kept arriving. Mario Molina, Sherwood Rowland and Paul Crutzen three atmospheric chemists whose work had shaped the field shared the 1995 Nobel Prize in Chemistry.
04Why this one worked
The features that made Montreal work are worth being specific about, because they are also the features missing from climate negotiations. The first is that the affected industry was concentrated. CFC production was dominated by a small number of large chemical companies in a small number of countries. Negotiating with five or ten major firms is structurally different from negotiating with millions of fossil fuel users embedded in every sector of every economy. The second is that substitutes existed and could be developed on a timescale of years. CFCs were used for specific industrial purposes that other chemicals could perform. Replacing fossil fuels involves rebuilding energy systems.
The third is that the science was unusually unambiguous. Once Solomon's work confirmed the chemistry of the Antarctic depletion, there was a single tractable cause and a single tractable solution. Climate change is caused by carbon dioxide, methane, nitrous oxide, deforestation, agriculture, transportation, electricity generation and several other sources, each requiring its own intervention. The fourth is that the consequences of inaction were tangible and personal. People understood ultraviolet radiation, sunburn and skin cancer in a way they did not understand the abstract risks of climate change in the 1980s.
05Conclusion
The Montreal Protocol works in part because we forget about it. The Antarctic ozone hole is no longer a regular news item. The chemicals that caused it have been substantially replaced. The recovery is happening on the timescale of decades rather than centuries. A generation of children has grown up without the warnings that defined environmental conversation in the late 1980s. The treaty solved its problem and gradually became invisible which is what successful regulation tends to do.