The Montreal Protocol on Substances That Deplete the Ozone Layer is an international treaty designed to protect the ozone layer by phasing out the production of numerous substances that are responsible for ozone depletion. It was agreed on 16 September 1987, and entered into force on 1 January 1989. Since then, it has undergone several amendments and adjustments, with revisions agreed to in 1990 (London), 1992 (Copenhagen), 1995 (Vienna), 1997 (Montreal), 1999 (Beijing), 2007 (Montreal), 2016 (Kigali) and 2018 (Quito).^[2][3][4] As a result of the international agreement, the ozone hole in Antarctica is slowly recovering.^[5] Climate projections indicate that the ozone layer will return to 1980 levels between 2040 (across much of the world) and 2066 (over Antarctica).^[6][7][8][9] Due to its widespread adoption and implementation, it has been hailed as an example of successful international co-operation. Former UN Secretary-General Kofi Annan stated that "perhaps the single most successful international agreement to date has been the Montreal Protocol".^[10][11] In comparison, effective burden-sharing and solution proposals mitigating regional conflicts of interest have been among the success factors for the ozone depletion challenge, where global regulation based on the Kyoto Protocol has failed to do so.^[12] In this case of the ozone depletion challenge, there was global regulation already being installed before a scientific consensus was established. Also, overall public opinion was convinced of possible imminent risks.^[13][14]

The ozone treaty has been ratified by 198 parties (197 states and the European Union),^[15] making it the first universally ratified treaty in United Nations history.^[16]

This truly universal treaty has also been remarkable in the expedience of the policy-making process at the global scale, where only 14 years lapsed between a basic scientific research discovery (1973) and the international agreement signed (1985 and 1987).

The treaty^{[Notes 1]} is structured around several groups of halogenated hydrocarbons that lead to the catalytic destruction of stratospheric ozone. All of the ozone depleting substances controlled by the Montreal Protocol contain either chlorine or bromine (substances containing only fluorine do not harm the ozone layer, although they can be strong greenhouse gases). Some ozone-depleting substances (ODSs) are not yet controlled by the Montreal Protocol, including nitrous oxide (N₂O). For a table of ozone-depleting substances controlled by the Montreal Protocol see:^[17]

For each group of ODSs, the treaty provides a timetable on which the production of those substances must be reduced and eventually eliminated. This includes a 10-year phase-out for developing countries^[18] identified in Article 5 of the treaty.

The purpose of the treaty is that each signatory states:

Recognizing that worldwide emissions of certain substances can significantly deplete and otherwise modify the ozone layer in a manner that is likely to result in adverse effects on human health and the environment. Determined to protect the ozone layer by taking precautionary measures to control equitably total global emissions of substances that deplete it with the ultimate objective of their elimination on the basis of developments in scientific knowledge

Acknowledging that special provision is required to meet the needs of developing countries

shall accept a series of stepped limits on CFC use and production, including:

• from 1991 to 1992 its levels of consumption and production of the controlled substances in Group I of Annex A do not exceed 150 percent of its calculated levels of production and consumption of those substances in 1986;
• from 1994 its calculated level of consumption and production of the controlled substances in Group I of Annex A does not exceed, annually, twenty-five percent of its calculated level of consumption and production in 1986.
• from 1996 its calculated level of consumption and production of the controlled substances in Group I of Annex A does not exceed zero.

There was a faster phase-out of halon-1211, -2402, -1301, There was a slower phase-out (to zero by 2010) of other substances (halon 1211, 1301, 2402; CFCs 13, 111, 112, etc.)^{[contradictory]} and some chemicals were given individual attention (Carbon tetrachloride; 1,1,1-trichloroethane). The phasing-out of the less damaging HCFCs only began in 1996 and will go on until a complete phasing-out is achieved by 2030.

There were a few exceptions for "essential uses" where no acceptable substitutes were initially found (for example, metered dose inhalers commonly used to treat asthma and chronic obstructive pulmonary disease were previously exempt) or Halon fire suppression systems used in submarines and aircraft (but not in general industry).

The substances in Group I of Annex A are:

• CFCl₃ (CFC-11)
• CF₂Cl₂ (CFC-12)
• C₂F₃Cl₃ (CFC-113)
• C₂F₄Cl₂(CFC-114)
• C₂F₅Cl (CFC-115)

The provisions of the Protocol include the requirement that the Parties to the Protocol base their future decisions on the current scientific, environmental, technical, and economic information that is assessed through panels drawn from the worldwide expert communities. To provide that input to the decision-making process, advances in understanding on these topics were assessed in 1989, 1991, 1994, 1998 and 2002 in a series of reports entitled Scientific assessment of ozone depletion, by the Scientific Assessment Panel (SAP).^[19]

In 1990 a Technology and Economic Assessment Panel was also established as the technology and economics advisory body to the Montreal Protocol Parties.^[20] The Technology and Economic Assessment Panel (TEAP) provides, at the request of Parties, technical information related to the alternative technologies that have been investigated and employed to make it possible to virtually eliminate use of Ozone Depleting Substances (such as CFCs and Halons), that harm the ozone layer. The TEAP is also tasked by the Parties every year to assess and evaluate various technical issues including evaluating nominations for essential use exemptions for CFCs and halons, and nominations for critical use exemptions for methyl bromide. TEAP's annual reports are a basis for the Parties' informed decision-making.

Numerous reports have been published by various inter-governmental, governmental and non-governmental organizations to catalogue and assess alternatives to the ozone depleting substances, since the substances have been used in various technical sectors, like in refrigeration, air conditioning, flexible and rigid foam, fire protection, aerospace, electronics, agriculture, and laboratory measurements.^[21][22][23]

Under the Montreal Protocol on Substances that Deplete the Ozone Layer, especially Executive Committee (ExCom) 53/37 and ExCom 54/39, Parties to this Protocol agreed to set year 2013 as the time to freeze the consumption and production of HCFCs for developing countries. For developed countries, reduction of HCFC consumption and production began in 2004 and 2010, respectively, with 100% reduction set for 2020. Developing countries agreed to start reducing its consumption and production of HCFCs by 2015, with 100% reduction set for 2030.^[24]

Hydrochlorofluorocarbons, commonly known as HCFCs, are a group of human-made compounds containing hydrogen, chlorine, fluorine and carbon. They are not found anywhere in nature. HCFC production began to take off after countries agreed to phase out the use of CFCs in the 1980s, which were found to be destroying the ozone layer. Like CFCs, HCFCs are used for refrigeration, aerosol propellants, foam manufacture and air conditioning. Unlike the CFCs, however, most HCFCs are broken down in the lowest part of the atmosphere and pose a much smaller risk to the ozone layer. Nevertheless, HCFCs are very potent greenhouse gases, despite their very low atmospheric concentrations, measured in parts per trillion (million million).

The HCFCs are transitional CFCs replacements, used as refrigerants, solvents, blowing agents for plastic foam manufacture, and fire extinguishers. In terms of ozone depletion potential (ODP), in comparison to CFCs that have ODP 0.6–1.0, these HCFCs have lower ODPs (0.01–0.5). In terms of global warming potential (GWP), in comparison to CFCs that have GWP 4,680–10,720, HCFCs have lower GWPs (76–2,270).^{[citation needed]}

On 1 January 2019, the Kigali Amendment to the Montreal Protocol came into force.^[25] Under the Kigali Amendment countries promised to reduce the use of hydrofluorocarbons (HFCs) by more than 80% over the next 30 years.^[26] By 27 December 2018, 65 countries had ratified the Amendment.^[27] As of 31 October 2024, 160 states^[28] and the European Union^[29] have ratified the Amendment.

Produced mostly in developed countries, hydrofluorocarbons (HFCs) replaced CFCs and HCFCs. HFCs pose no harm to the ozone layer because, unlike CFCs and HCFCs, they do not contain chlorine. They are, however, greenhouse gases, with a high global warming potential (GWP), comparable to that of CFCs and HCFCs.^[30][31] In 2009, a study calculated that a fast phasedown of high-GWP HFCs could potentially prevent the equivalent of up to 8.8 Gt CO₂-eq per year in emissions by 2050.^[32] A proposed phasedown of HFCs was hence projected to avoid up to 0.5C of warming by 2100 under the high-HFC growth scenario, and up to 0.35C under the low-HFC growth scenario.^[33] Recognizing the opportunity presented for fast and effective phasing down of HFCs through the Montreal Protocol, starting in 2009 the Federated States of Micronesia proposed an amendment to phase down high-GWP HFCs,^[34] with the U.S., Canada, and Mexico following with a similar proposal in 2010.^[35]

After seven years of negotiations, in October 2016 at the 28th Meeting of the Parties to the Montreal Protocol in Kigali, the Parties to the Montreal Protocol adopted the Kigali Amendment whereby the Parties agreed to phase down HFCs under the Montreal Protocol.^[36] The amendment to the Montreal Protocol commits the signatory parties to reduce their HFC production and consumption by at least 85 per cent from the annual average value in the period from 2011 to 2013. A group of developing countries including China, Brazil and South Africa are mandated to reduce their HFC use by 85 per cent of their average value in 2020-22 by the year 2045. India and some other developing countries – Iran, Iraq, Pakistan, and some oil economies like Saudi Arabia and Kuwait – will cut down their HFCs by 85 per cent of their values in 2024-26 by the year 2047.

On 17 November 2017, ahead of the 29th Meeting of the Parties of the Montreal Protocol, Sweden became the 20th Party to ratify the Kigali Amendment, pushing the Amendment over its ratification threshold ensuring that the Amendment would enter into force 1 January 2019.^[37]

In the 1970s, the chemists Frank Sherwood Rowland and Mario Molina, who were then at the University of California, Irvine, began studying the impacts of CFCs in the Earth's atmosphere. They discovered that CFC molecules were stable enough to remain in the atmosphere until they got up into the middle of the stratosphere where they would finally (after an average of 50–100 years for two common CFCs) be broken down by ultraviolet radiation releasing a chlorine atom. Rowland and Molina then proposed that these chlorine atoms might be expected to cause the breakdown of large amounts of ozone (O₃) in the stratosphere. Their argument was based upon an analogy to contemporary work by Paul J. Crutzen and Harold Johnston, which had shown that nitric oxide (NO) could catalyze the destruction of ozone. (Several other scientists, including Ralph Cicerone, Richard Stolarski, Michael McElroy, and Steven Wofsy had independently proposed that chlorine could catalyze ozone loss, but none had realized that CFCs were a potentially large source of chlorine.) Crutzen, Molina and Rowland were awarded the 1995 Nobel Prize for Chemistry for their work on this problem.

The environmental consequence of this discovery was that, since stratospheric ozone absorbs most of the ultraviolet-B (UV-B) radiation reaching the surface of the planet, depletion of the ozone layer by CFCs would lead to an increase in UV-B radiation at the surface, resulting in an increase in skin cancer and other impacts such as damage to crops and to marine phytoplankton.

The Rowland-Molina hypothesis was strongly disputed by representatives of the aerosol and halocarbon industries. The chair of the board of DuPont was quoted as saying that ozone depletion theory is "a science fiction tale...a load of rubbish...utter nonsense". Robert Abplanalp, the president of Precision Valve Corporation (and inventor of the first practical aerosol spray can valve), wrote to the Chancellor of UC Irvine to complain about Rowland's public statements (Roan, p. 56.)

After publishing their pivotal paper in June 1974, Rowland and Molina testified at a hearing before the U.S. House of Representatives in December 1974. As a result, significant funding was made available to study various aspects of the problem and to confirm the initial findings. In 1976, the U.S. National Academy of Sciences (NAS) released a report that confirmed the scientific credibility of the ozone depletion hypothesis.^[38] NAS continued to publish assessments of related science for the next decade.

In 1982, representatives from 24 countries met in Stockholm, Sweden to decide on a "Global Framework Convention for the Protection of the Ozone Layer." The following year, a group of countries, including the United States, Canada, the Nordic Countries, and Switzerland, proposed a worldwide ban on "nonessential" uses of CFCs in spray cans.^[39]

Then, in 1985, British Antarctic Survey scientists Joe Farman, Brian Gardiner and Jon Shanklin published results of abnormally low ozone concentrations above Halley Bay near the South Pole. They speculated that this was connected to increased levels of CFCs in the atmosphere. It took several other attempts to establish the Antarctic losses as real and significant, especially after NASA had retrieved matching data from its satellite recordings. This unforeseen phenomenon in the Antarctic, as well as NASA's scientific images of the ozone hole played an important role in the Montreal Protocol negotiations.^[40] The impact of these studies, the metaphor 'ozone hole', and the colorful visual representation in a time lapse animation proved shocking enough for negotiators in Montreal, Canada to take the issue seriously.^[41]

Also in 1985, 20 nations, including most of the major CFC producers, signed the Vienna Convention, which established a framework for negotiating international regulations on ozone-depleting substances.^[42] After the discovery of the ozone hole by SAGE 2 it only took 18 months to reach a binding agreement in Montreal, Canada. Mostafa Kamal Tolba, the head of the UNEP at the time, was considered the "father of the Montreal Protocol" for his role in bringing the nations together for an agreement.^[43]

In 1986, an assessment spearheaded by NASA and sponsored by the United Nationals Environment Program, the World Meteorological Organization, and various other organizations concluded that continued CFC emissions at the 1980 rate would "reduce global average ozone by about 9 percent by the latter half of the century." Based on these figures, the U.S. Environmental Protection Agency estimated that in the United States alone there could be "over 150 million new cases of skin cancer among people currently alive and born by the year 2075, resulting in over 3 million deaths."^[39]

The CFC industry continued pushing back against regulation as late as 1986, when the Alliance for Responsible CFC Policy (an association representing the CFC industry founded by DuPont) was still arguing that the science was too uncertain to justify any action. In 1987, DuPont testified before the US Congress that "We believe there is no imminent crisis that demands unilateral regulation."^[44] And even in March 1988, Du Pont Chair Richard E. Heckert would write in a letter to the United States Senate, "we will not produce a product unless it can be made, used, handled and disposed of safely and consistent with appropriate safety, health and environmental quality criteria. At the moment, scientific evidence does not point to the need for dramatic CFC emission reductions. There is no available measure of the contribution of CFCs to any observed ozone change..."^[45]

In an unexpected policy change, however, the Alliance for Responsible CFC Policy issued a statement in 1986 declaring that "large future increases...in CFCs...would be unacceptable to future generations," and that it would be "inconsistent with [industry] goals...to ignore the potential for risk to future generations." Three months before the protocol negotiations began, U.S. industry announced its support for new international controls on CFCs.^[39]

The main objective of the Multilateral Fund for the Implementation of the Montreal Protocol is to assist developing country parties to the Montreal Protocol whose annual per capita consumption and production of ozone depleting substances (ODS) is less than 0.3 kg to comply with the control measures of the Protocol. Currently, 147 of the 196 Parties to the Montreal Protocol meet these criteria (they are referred to as Article 5 countries).

It embodies the principle agreed at the United Nations Conference on Environment and Development in 1992 that countries have a common but differentiated responsibility to protect and manage the global commons.

The Fund is managed by an executive committee with an equal representation of seven industrialized and seven Article 5 countries, which are elected annually by a Meeting of the Parties. The Committee reports annually to the Meeting of the Parties on its operations. The work of the Multilateral Fund on the ground in developing countries is carried out by four Implementing Agencies, which have contractual agreements with the executive committee:^[46]

• United Nations Environment Programme (UNEP), through its OzonAction Programme.
• United Nations Development Programme (UNDP).
• United Nations Industrial Development Organization (UNIDO).
• World Bank.

Up to 20 percent of the contributions of contributing parties can also be delivered through their bilateral agencies in the form of eligible projects and activities.

The fund is replenished on a three-year basis by the donors. Pledges amount to US$3.1 billion over the period 1991 to 2005. Funds are used, for example, to finance the conversion of existing manufacturing processes, train personnel, pay royalties and patent rights on new technologies, and establish national ozone offices.

As of October 2022, all Member States of the United Nations, the Cook Islands, Niue, the Holy See, the State of Palestine as well as the European Union have ratified the original Montreal Protocol (see external link below),^[47] with the State of Palestine being the last party to ratify the agreement, bringing the total to 198. 197 of those parties (with the exception of the State of Palestine) have also ratified the London, Copenhagen, Montreal, and Beijing amendments.^[15]

Since the Montreal Protocol came into effect, the atmospheric concentrations of the most important chlorofluorocarbons and related chlorinated hydrocarbons have either leveled off or decreased.^[48] Halon concentrations have continued to increase, as the halons presently stored in fire extinguishers are released, but their rate of increase has slowed and their abundances are expected to begin to decline by about 2020. Also, the concentration of the HCFCs increased drastically at least partly because of many uses (e.g. used as solvents or refrigerating agents) CFCs were substituted with HCFCs. While there have been reports of attempts by individuals to circumvent the ban, e.g. by smuggling CFCs from undeveloped to developed nations, the overall level of compliance has been high. Statistical analysis from 2010 show a clear positive signal from the Montreal Protocol to the stratospheric ozone.^[49] In consequence, the Montreal Protocol has often been called the most successful international environmental agreement to date. In a 2001 report, NASA found the ozone thinning over Antarctica had remained the same thickness for the previous three years,^[50] however in 2003 the ozone hole grew to its second largest size.^[51] The most recent (2006) scientific evaluation of the effects of the Montreal Protocol states, "The Montreal Protocol is working: There is clear evidence of a decrease in the atmospheric burden of ozone-depleting substances and some early signs of stratospheric ozone recovery."^[52] However, a more recent study seems to point to a relative increase in CFCs due to an unknown source.^[53]

Reported in 1997, significant production of CFCs occurred in Russia for sale on the black market to the EU throughout the 90s. Related US production and consumption was enabled by fraudulent reporting due to poor enforcement mechanisms. Similar illegal markets for CFCs were detected in Taiwan, Korea, and Hong Kong.^[54]

The Montreal Protocol is also expected to have effects on human health. A 2015 report by the U.S. Environmental Protection Agency estimates that the protection of the ozone layer under the treaty will prevent over 280 million cases of skin cancer, 1.5 million skin cancer deaths, and 45 million cataracts in the United States.^[55]

However, the hydrochlorofluorocarbons, or HCFCs, and hydrofluorocarbons, or HFCs, contribute to anthropogenic global warming.^[56] On a molecule-for-molecule basis, these compounds are up to 10,000 times more potent greenhouse gases than carbon dioxide. The Montreal Protocol currently calls for a complete phase-out of HCFCs by 2030, but does not place any restriction on HFCs. Since the CFCs themselves are equally powerful greenhouse gases, the mere substitution of HFCs for CFCs does not significantly increase the rate of anthropogenic climate change, but over time a steady increase in their use could increase the danger that human activity will change the climate.^[57]

Policy experts have advocated for increased efforts to link ozone protection efforts to climate protection efforts.^[58][59][60] Policy decisions in one arena affect the costs and effectiveness of environmental improvements in the other.

In 2018, scientists monitoring the atmosphere following the 2010 phaseout date reported evidence of continuing industrial production of CFC-11, likely in eastern Asia, with detrimental global effects on the ozone layer.^[61][62] A monitoring study detected fresh atmospheric releases of carbon tetrachloride from China's Shandong province, beginning sometime after 2012, and accounting for a large part of emissions exceeding global estimates under the Montreal Protocol.^[63]

The year 2012 marked the 25th anniversary of the signing of the Montreal Protocol. Accordingly, the Montreal Protocol community organized a range of celebrations at the national, regional and international levels to publicize its considerable success to date and to consider the work ahead for the future.^[64] Among its accomplishments are:

• The Montreal Protocol was the first international treaty to address a global environmental regulatory challenge;
• the first to embrace the "precautionary principle" in its design for science-based policymaking;
• the first treaty where independent experts on atmospheric science, environmental impacts, chemical technology, and economics, reported directly to parties, without edit or censorship, functioning under norms of professionalism, peer review, and respect;
• the first to provide for national differences in responsibility and financial capacity to respond by establishing a multilateral fund for technology transfer; the first MEA with stringent reporting, trade, and binding chemical phase-out obligations for both developed and developing countries; and,
• the first treaty with a financial mechanism managed democratically by an executive board with equal representation by developed and developing countries.^[65]

Within 25 years of signing, parties to the MP celebrate significant milestones. Significantly, the world has phased-out 98% of the Ozone-Depleting Substances (ODS) contained in nearly 100 hazardous chemicals worldwide; every country is in compliance with stringent obligations; and, the MP has achieved the status of the first global regime with universal ratification; even the newest member state, South Sudan, ratified in 2013. UNEP received accolades for achieving global consensus that "demonstrates the world’s commitment to ozone protection, and more broadly, to global environmental protection".^[66]

• Action for Climate Empowerment
• Carbon footprint
• Copenhagen Accord
• Net capacity factor
• International Day for the Preservation of the Ozone Layer
• Paris Agreement
• R-134a
• Section 608
• Vienna Conference (1985)
• Fossil fuel phase-out
• Phase-out of fossil fuel vehicles
• Phase-out of gas boilers
• Plastic bans

 This article incorporates public domain material from The World Factbook (2024 ed.). CIA. (Archived 2003 edition.)(referred to as Ozone Layer Protection)

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• Multilateral Fund for the Implementation of the Montreal Protocol
• The Montreal Protocol
• The Vienna Convention
• Ozone-Depleting Substances (ODS) Controlled Under the Montreal Protocol
• U.S. EPA Ozone Layer Protection Information Home Page
• The Montreal Protocol Who's Who Archived 31 May 2010 at the Wayback Machine
• [1] by F.Sherwood Rowland and Mario J.Molina
• Has the Montreal Protocol been successful in reducing ozone-depleting gases in the atmosphere? (NOAA Aeronomy Lab)
• Doomsday Déjà vu: Ozone Depletion's Lessons for Global Warming Archived 10 September 2009 at the Wayback Machine by Ben Lieberman
• EIA reports: Reports on illegal trade and solutions.
• Introductory note by Edith Brown Weiss, procedural history note and audiovisual material on the Montreal Protocol on Substances that Deplete the Ozone Layer in the Historic Archives of the United Nations Audiovisual Library of International Law
• Green Cooling Initiative
• Green Cooling Initiative on alternative natural refrigerants cooling technologies