Montreal Protocol: Have Efforts to Save the Ozone Layer Been Successful?

Treaty is still considered one of the most effective environmental actions ever

View from 40,000 feet above the earth
Grant Faint / Getty Images

The Montreal Protocol is an international agreement designed to phase out the production and consumption of chemicals that deplete the ozone layer. Signed in 1987 and put into effect in 1989, the Montreal Protocol treaty stemmed from increased global concern about the harmful impact that chemicals such as chlorofluorocarbons (CFCs) were having on the planet's protective ozone layer.

In the early 1970s, scientific research began finding evidence of CFC chemicals depleting the ozone layer, increasing the levels of ultraviolet radiation on the planet’s surface. Dr. F. Sherwood Rowland, a chemistry professor at UC Irvine, and Dr. Mario Molina, a postdoctoral fellow, were credited with the first demonstrations of CFCs' damaging effect on atmospheric ozone.

In a 1974 paper titled “Stratospheric sink for chlorofluoromethanes: chlorine atom-catalysed destruction of ozone,” Molina and Rowland hypothesized that the chlorofluoromethanes could remain in the atmosphere between 40 and 150 years. Their studies concluded that when the chemicals reached the stratosphere, it led to the destruction and thinning of the atmospheric ozone layer. The findings were revolutionary (and eye-opening) at the time, and the team later received a Nobel Prize for their work.

Yearly evolution of the whole in the ozone layer
Yearly evolution of the whole in the ozone layer between 1979 and 1990, as illustrated by NASA.

Jupiterimages / Getty Images

More than 10 years later, in 1985, a British science team from Cambridge discovered massive ozone depletion over the Antarctic during the region’s spring months. They attributed the depletion to low midwinter temperatures, making the stratosphere even more sensitive to inorganic chlorine growth. At the time, chlorofluorocarbons in particular were widely used in common products like refrigerants and aerosol sprays. 

After that, multiple countries began advocating for stronger controls on the chemicals. The same year scientists discovered the thinning Antarctic ozone “hole,” nations came together at the Vienna Convention for the Protection of the Ozone Layer to discuss efforts to protect it. The Vienna Convention didn’t require participating countries to take concrete actions to control ozone depleting substances, but instead provided the framework for what would later become the Montreal Protocol on Substances that Deplete the Ozone Layer (often simply referred to as the Montreal Protocol). 

What Is the Ozone Layer, Exactly?

The Earth’s ozone layer is there to protect living things from the sun’s radiation; when it's damaged, more harmful ultraviolet (UV) light can seep through. Too much UV light has negative effects on humans by increasing the chance of skin cancer and cataracts, but can also damage crops and harm marine life. Our atmosphere is composed of several layers, including a lower troposphere layer where most human activity occurs and a stratosphere level where most commercial airlines fly.

Illustration of the ozone layer and UV rays

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While airplanes tend to stay on the lower portion of the stratosphere, most atmospheric ozone is concentrated on the mid to higher end. This ozone layer in the stratosphere is responsible for absorbing a portion of the sun’s radiation, and more specifically, the portion of UV light that has been linked to the most harmful effects. Although ozone concentrations vary, reducing and recovering during natural cycles, the research conducted throughout the 1970s revealed ozone depletion far exceeding the natural process.

The Montreal Protocol International Agreement

Today, the Montreal Protocol is signed by 197 countries, making it the first in United Nations history to achieve universal ratification. It is considered to be one of the most successful global environmental actions and an achievement that has continued to inspire future policies decades later.

The treaty’s structure developed and implemented ozone-depleting substances phase-out plans and deadlines, setting goals for both developed and developing countries. Most importantly, it established flexibility and room for growth as new scientific research arises in the future.

Ozone-Depleting Substances (ODS)

Chlorine and bromine atoms destroy ozone molecules when they come into contact with the ozone layer; even just one chlorine atom can kill 100,000 ozone molecules before it is removed from the stratosphere — meaning that ozone is destroyed more quickly than nature can regenerate. Some compounds, including chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), carbon tetrachloride, methyl chloroform, halons, and methyl bromide, release chlorine or bromine when they’re exposed to UV light in the stratosphere. Scientists call these compounds ozone-depleting substances, or ODSs.

Member States

The Montreal Protocol became the first treaty of any kind in the history of the United Nations to achieve universal participation in 2009. The agreement required all 197 countries to begin phasing out CFCs at the time of signing, initially committing to a 20% reduction by 1994 and a 50% reduction by 1998. More developed countries agreed to reduce their production and consumption of halons, as well.

The Multilateral Fund

In 1991, the Multilateral Fund was established to assist developing countries in meeting their commitments to the treaty, performing tasks like technical assistance and training. The protocol also has advisory bodies known as Assessment Panels that are responsible for issuing regular reports on phasing out progress and assessing alternatives to ODSs. Protocol parties meet annually to adjust, amend, or make necessary decisions to help enable effective implementation of the treaty, but they are also authorized to respond quickly should new, relevant scientific findings arise.


Since its creation, the protocol has been adjusted or amended five times. The first amendment, the London Amendment of 1990, required the complete phaseout of CFCs, halons, and carbon tetrachloride by the year 2000 in developed countries and by 2010 in developing countries. It also added methyl chloroform to the list of controlled substances, with phaseout targeted in developed countries targeted by 2005 and in developing countries by 2015. Just two years later, the Copenhagen Amendment accelerated the ODS phaseout, targeting a total ban of CFCs, halons, carbon tetrachloride, and methyl chloroform by 1996 in developed countries. It also incorporated hydrochlorofluorocarbons (HCFC) phaseout for 2004.

The Montreal Amendment of 1997 followed, including the HCFC phaseout in developing countries by 2005 and the phaseout of methyl bromide in developed and developing countries by 2005 and 2015, respectively. In 1999, the Beijing Amendment tightened restrictions on the production of HCFCs and added bromochloromethane to the list.

The most recent amendment, known as the Kigali Amendment, phased out hydrofluorocarbons (HFCs) in 2016. HFCs were used as a replacement substance for one of the ODSs banned by the original protocol, and while not shown to deplete the ozone, they are potent greenhouse gases that cause damage to the Earth’s climate.

Recovery of the Ozone Layer

By 2015, it was clear that the Montreal Protocol had already made a substantial difference in the ozone layer. Environmental studies showed that, without the Montreal Protocol, the Antarctic ozone hole would have increased in size by 40% by 2013, while the decline over the Northern Hemisphere's middle latitudes would have more than doubled to about 15%. According to the U.S. Environmental Protection Agency, Americans born between 1890 and 2100 will avoid over 280 million cases of skin cancer, 1.6 million skin cancer deaths, and over 45 million cases of cataracts thanks to the treaty.

The 2018 World Meteorological Organization (WMO) scientific assessment of ozone depletion showed that the ozone layer had a chance to recover to pre-1980 levels by 2060 thanks to the Montreal Protocol banning ozone depleting chemical emissions. For the first time since the year 2000, there were emerging indications that the Antarctic ozone hole has shrunk in size and depth. Even outside polar regions, the upper stratospheric ozone had increased by 1% to 3% each decade since 2000.

So, why did the Montreal Protocol succeed where other environmental initiatives failed? Ronald Reagan, who was president at the time of its finalization, said that the treaty was all about mutual cooperation. “It is a product of the recognition and international consensus that ozone depletion is a global problem, both in terms of its causes and its effects,” he said. “The protocol is the result of an extraordinary process of scientific study, negotiations among representatives of the business and environmental communities, and international diplomacy.” It presented global investment in alternative technologies to ODSc and included hard science in the negotiations.

The treaty has served as a model for other global issues, such as marine plastic pollution. For example, a 2017 study published in Marine Policy proposed a reduction in the production of virgin plastic material within the plastic industry and regulating polymers and chemical additives at a global level, similar to the Montreal Protocol phaseout process.

Since ODSs like CFCs are greenhouse gases, the treaty has also helped reduce negative future environmental concerns beyond the ozone layer. The Montreal Protocol has since been attributed to an estimated 27% reduction in global sea level rises by the year 2065 and the Earth’s hydroclimate would have only been half as strong in the coming decade without the treaty.

Antarctic Ozone Hole in 2020
Antarctic ozone hole in 2020 intensified by persistent cold temperatures and strong cirumpolar winds, according to NASA..

NASA Earth Observatory

Even considering the success of the Montreal Protocol so far, its relevance is far from over. In 2020, the WMO recorded one of the largest and deepest holes in the ozone layer in the last 40 years. The hole grew from mid-August, peaking in September at nearly 25 million square kilometers (close to 10 million square miles) in size, spreading over a majority of the Antarctic continent. When the hole finally closed on December 28th, the WMO reported that the ozone hole is still subject to annual variations, even though the ozone layer has slowly been recovering thanks to Montreal Protocol regulations.

View Article Sources
  1. Molina, Mario J., and F. S. Rowland. "Stratospheric Sink for Chlorofluoromethanes: Chlorine Atom-Catalysed Destruction of Ozone." Nature, vol. 249, no. 5460, 1974, pp. 810-812., doi:10.1038/249810a0

  2. Farman, J. C., et al. "Large Losses of Total Ozone in Antarctica Reveal Seasonal ClOx/NOx Interaction." Nature, vol. 315, no. 6016, 1985, pp. 207-210., doi:10.1038/315207a0

  3. Bornman, Janet F., et al. "Linkages Between Stratospheric Ozone, UV Radiation and Climate Change and Their Implications for Terrestrial Ecosystems." Photochemical & Photobiological Sciences, vol. 18, no. 3, 2019, pp. 681-716., doi:10.1039/c8pp90061b

  4. "Basic Ozone Layer Science." U.S. Environmental Protection Agency.

  5. "International Treaties and Cooperation About the Protection of the Stratospheric Ozone Layer." U.S. Environmental Protection Agency.

  6. Chipperfield, M. P. et al. "Quantifying the Ozone and Ultraviolet Benefits Already Achieved by the Montreal Protocol." Nature Communications, vol. 6, no. 1, 2015, doi:10.1038/ncomms8233

  7. "Stratospheric Ozone Protection: 30 Years of Progress and Achievements." U.S. Environmental Protection Agency, 2017.

  8. "Scientific Assessment of Ozone Depletion: 2018." World Meteorological Organization, 2019.

  9. Raubenheimer, Karen, and Alistair McIlgorm. "Is the Montreal Protocol a Model That Can Help Solve the Global Marine Plastic Debris Problem?" Marine Policy, vol. 81, 2017, pp. 322-329., doi:10.1016/j.marpol.2017.04.014

  10. Previdi, Michael, and Lorenzo M. Polvani. "Impact of the Montreal Protocol on Antarctic Surface Mass Balance and Implications for Global Sea Level Rise." Journal of Climate, vol. 30, no. 18, 2017, pp. 7247-7253., doi:10.1175/jcli-d-17-0027.1

  11. Wu, Yutian et al. "The Importance of the Montreal Protocol in Protecting Earth’s Hydroclimate." Journal of Climate, vol. 26, no. 12, 2013, pp. 4049-4068., doi:10.1175/jcli-d-12-00675.1