Since it was enacted in 1987, the Montreal Protocol has stood out as a remarkable example of international cooperation in response to a global environmental problem.
Officially known as the Montreal Protocol on Substances that Deplete the Ozone Layer, the treaty aims to protect the ozone layer by phasing out the production of ozone-depleting substances.
Ratified by all 197 UN member and non-member states and the European Union, it is the first universally ratified treaty in the history of the United Nations.
The late chemist and Nobel Prize winner Mario Molina, whose work contributed to the discovery of the Antarctic ozone hole, wrote in 2017 that “it can claim to be one of the most successful international treaties ever struck.”
But what made the Montreal Protocol so successful – and what lessons can we learn from it to tackle the climate crisis today?
The Antarctic ozone hole at its joint-largest extent in September 2006. Visualization by NASA via Wikimedia Commons
The Antarctic ozone hole
In 1985, scientists with the British Antarctic Survey made a disturbing discovery: ozone levels were dropping around Antarctica.
Ozone is a gas found in the atmosphere that absorbs most of the sun’s ultraviolet B (UVB) radiation. The growing hole in the ozone layer would have left us exposed to the harmful effects of UVB, including greater risks of skin cancer and cataracts.
The culprit? The widespread use of synthetic compounds like chlorofluorocarbons (CFCs) in aerosol sprays, refrigeration units and air conditioners.
In a 1974 study published in Nature, Molina and fellow chemist F. Sherwood Rowland recognized that CFCs were migrating to the Earth’s stratosphere. This was limiting its ability to provide oxygen, stabilize the climate and protect humanity from UVB radiation.
World leaders reacted surprisingly quickly. In 1985, countries signed the Vienna Convention, which would eventually be universally ratified and paved the way for later measures to phase out ozone-depleting substances.
Those included the Montreal Protocol, which was signed in 1987 and went further by establishing a series of binding measures to control and ultimately eliminate these substances.
What impact has the Montreal Protocol had?
Thanks to the Montreal Protocol, nearly 99 percent of the chemicals that once caused the severe thinning of the ozone layer are no longer in use.
After peaking around the turn of the century, the Antarctic ozone hole has been gradually healing. Last year, it was the fifth-smallest it had been since the Montreal Protocol took effect in 1992.
And according to a 2022 report from the World Meteorological Organization (WMO), the ozone layer is expected to recover to 1980 levels by 2040 across most of the world and by 2066 in Antarctica.
Without the protocol, “models show that there would have been catastrophic ozone depletion, exposing all living things on Earth to huge and ever increasing dangerous amounts of UV radiation,” says Stephanie Egger Haysmith, communications and information officer at the United Nations Environmental Programme’s Ozone Secretariat.
The ozone layer’s recovery, she adds, has averted millions of cases of skin cancer and tens of millions of cataract cases, as well as serious damage to crops, pollinators and global biodiversity in general.
The projected recovery of the Antarctic ozone hole. Visualization by NASA
Why was the Montreal Protocol successful?
For Solène Guggisberg, an independent researcher affiliated with Utrecht University, several factors played into the success of the Montreal Protocol meeting its goals.
“The fact that there was a strong lead by the United States at the time, or at least not an opposition, was a key factor,” she says.
Even DuPont, the world’s largest producer of CFCs, eventually came to support the treaty after having developed alternatives that did not deplete the ozone layer.
Another possible contributing factor, says Guggisberg, is that industry leaders preferred a global treaty over domestic anti-CFC measures that could put them at a disadvantage against their competitors in other countries.
Crucially, the Protocol also provided different timelines for developed and developing countries to lower the use of ozone-depleting substances.
This not only recognized that low-income countries needed more time to phase out their use but also included a financial and technology transfer mechanism to help them do so.
“The multilateral funding mechanism with technical advice was a very important carrot to encourage states to join,” says Guggisberg.
Perhaps the bedrock of the negotiations however was the attention paid to the science behind ozone depletion and its causes, says Guggisberg: “advice that was really was considered scientific and not already tainted by policy and politics.”
Even as the Montreal Protocol was being adopted, she co-wrote in a book chapter, “several reports and new scientific findings indeed further showed not only the worldwide seriousness of the human-caused ozone depletion issue, but also that the issue had actually been underestimated.” This brought about renewed debate about the adequacy of the protocol’s control measures.
The maximum size of the ozone hole each year since 1979. Visualization by NASA
“There was obviously clear, compelling science,” Egger Haysmith agrees. Even at the earlier Vienna Convention, she says, there was recognition “that while we may not fully understand it, there is sufficient cause for alarm.”
The parties to both treaties have held regular follow-up meetings since, and stricter measures have gradually been adopted as scientific reports and assessments called the adequacy of the applicable measures into question, says Guggisberg.
The Montreal Protocol has been revised and amended several times since its adoption, with the latest adjustments entering into force in 2019.
This “start and strengthen” approach, as Egger Haysmith calls it, meant “that there was a fundamental agreement that there will be modest commitments with a view to reviewing the treaty as more information and more scientific data comes to light, and we can tighten this treaty under amendments or agreements.”
This flexibility made it easier for countries to sign on.
“I think one of the strengths of the Montreal Protocol is that you have this body that negotiates, and negotiations can be fraught, tense and long,” she says.
“But there is a common understanding and acceptance that every effort will be made to bring everybody to the table and to find compromise so that there is this consensus.”
The 2016 Kigali Amendment, for example, was a legally binding agreement to gradually phase down the production and consumption of hydrofluorocarbons (HFCs), which replaced the use of CFCs but are themselves dangerous greenhouse gases. This amendment took seven years to negotiate.
The amendments and negotiations will continue. The four implementing agencies of the Montreal Protocol monitor progress and support member states with implementing it and meeting their obligations.
“In that respect, we help the parties facilitate their job because until the ozone layer is back to its pre-1980s levels, essentially, this job is not done,” she says.
A press conference in the buildup to the COP31 climate summit in February 2026. Photo: UN Climate Change, Flickr
What can we learn from the Montreal Protocol?
The quick, decisive action taken to protect the ozone layer are a far cry from the often acrimonious atmosphere at the annual UN climate negotiations. So, what can the Montreal Protocol teach us today?
Egger Haysmith believes the ‘start and strengthen’ approach is crucial. “I think that is where potentially consensus and the middle ground can be found in future treaties, even where groups and positions seem to be intractable.”
“I think it is possible to find a middle ground if you, for example, could get to a point where you agree, ‘okay, we will start with this and we will incrementally build on this,’” she adds.
That said, as Guggisberg points out, the Montreal Protocol worked because it focused on one set of substances with relatively limited industrial uses.
On the other hand, the climate crisis is “caused by “a basket of greenhouse gases connected to most human activities and therefore requiring a response at an immensely bigger scale.”
“The current system under the Paris Agreement of voluntary reductions isn’t necessarily a bad start, but it can’t stay there,” she says. “It needs to be accompanied by the ‘strengthen’ phase.”
“If there is one thing any regime really would benefit from, it is this regular in-depth review of how the regime’s adopted measures actually fit in with the goal.”