In 1974, chemists Mario J. Molina and F. Sherwood Rowland warned that chlorofluorocarbons (CFCs), compounds widely used at the time as refrigerants and aerosol propellants, could destroy the stratospheric ozone layer [Molina and Rowland, 1974]. This layer protects Earth’s surface from harmful ultraviolet (UV) radiation that, in excess, can cause skin cancer and cataracts, suppress the human immune system, damage agricultural crops and natural ecosystems, and deteriorate the built environment. The following year, atmospheric scientist Veerabhadran Ramanathan further warned that CFCs and other chlorinated fluorocarbons are also powerful greenhouse gases [Ramanathan, 1975].
In the ensuing decade after these initial alarms, scientists measured and documented the buildup and long lifetimes in the atmosphere of CFCs and other ozone-depleting substances (ODSs). They also provided theoretical proof that ODSs chemically decompose in the stratosphere and catalytically deplete stratospheric ozone, and they quantified the adverse health, environmental, and economic effects of CFCs.
Warnings that ozone depletion from CFCs could increase the incidence of skin cancer sparked consumer boycotts of products made with and containing ODSs, such as aerosols and certain polystyrene foam food service containers. The boycotts then expanded into government prohibitions of specified products.
Reports that CFCs also acted as greenhouse gases sparked scientific investigations looking into what substances besides carbon dioxide (CO2) might destabilize the atmosphere and how the evolving cocktail of climate pollutants could be considered.
The scientific evidence was enough for the executive director of the United Nations Environment Programme, Mostafa K. Tolba, to persuade 25 countries and the European Union to sign the Vienna Convention on the Protection of the Ozone Layer in March 1985, setting the stage for international controls on ODSs [Benedick, 1998; Andersen and Sarma, 2002; Tolba and Rummel-Bulska, 2008].
Less than 2 months later, Joseph Farman and colleagues provided observational evidence of a stratospheric “ozone hole” over the Antarctic potentially linked to rising CFC concentrations [Farman et al., 1985], which ultimately led to international agreement on the 1987 Montreal Protocol on Substances that Deplete the Ozone Layer, better known simply as the Montreal Protocol.
Twenty years later, every United Nations (UN) member state had become a party to the Montreal Protocol, more than 99% of ODS production and consumption worldwide had been phased out, and the ozone layer was well on the way to recovery [World Meteorological Organization (WMO), 2018; Ajavon et al., 2015]. Accomplishments like these inspired former secretary-general of the United Nations Kofi Anan to state that the Montreal Protocol was “perhaps the single most successful international agreement to date” [Hunter et al., 2022].
As it has turned out, the protocol not only has been enormously successful in curtailing the depletion of stratospheric ozone, but it has also taken on a second life in recent years as a de facto—and highly beneficial—climate treaty. The protocol’s achievements in both respects hold valuable lessons for future climate- and environment-focused policymaking efforts. To better understand and apply these lessons in the future, it’s worthwhile to parse the science that went into formulating the Montreal Protocol and that led to its transformation into a climate treaty.
From an Ozone Treaty to a Climate Treaty
The Montreal Protocol is described as a “start and strengthen” treaty because after initially imposing controls on just two classes of chemicals (CFCs and halons), it has been strengthened subsequently by amendments, which require ratification by a certain number of parties, and by adjustments, which just require the consensus of all parties. Four amendments in the 1990s—crafted during meetings in London (1990), Copenhagen (1992), Montreal (1997), and Beijing (1999)—strengthened the treaty by adding additional ODSs to the list of those controlled. Six adjustments—agreed upon in London (1990), Copenhagen (1992), Vienna (1995), Montreal (1997), Beijing (1999), and Montreal (2007)—further strengthened the treaty by transitioning ODS phasedowns to phaseouts and then by accelerating those phaseouts.
The transition of the Montreal Protocol from a stratospheric ozone protection treaty to a climate treaty began in 2007. This is when atmospheric and climate scientist Guus J. M. Velders and colleagues, after analyzing historical and potential future ODS emissions, reported that the Montreal Protocol and its phaseout of ODSs had done more to reduce greenhouse forcing and mitigate climate change than any other treaty, including the Kyoto Protocol [Velders et al., 2007].
The same year, the protocol’s usefulness in further protecting climate inspired parties to approve an acceleration of the ongoing hydrochlorofluorocarbon (HCFC) phaseout, which had started with the 1999 Beijing Amendment. The transition to a climate treaty continued for 9 more years, culminating with the fifth Montreal Protocol Amendment (2016 Kigali Amendment) controlling hydrofluorocarbons (HFCs), which are safe for stratospheric ozone but are strong greenhouse gases. Markets for HFCs had grown rapidly, and they came into widespread use as necessary replacements for some CFCs, allowing the rapid phaseout of ODSs, but in some cases, HFCs trap thousands of times more heat than CO2 [WMO, 2018; Zaelke et al., 2018; Hunter et al., 2022].
The Kigali Amendment, named for the capital city of Rwanda, where the agreement was reached, not only phases down HFCs but also encourages greater energy efficiency in next-generation equipment and the use of alternate chemicals to replace HFCs in applications such as air-conditioning, refrigeration, and thermal insulating foam.
Timelines for initial HFC consumption and production freezes (i.e., no further increases in consumption and production allowed) and phasedowns are ambitious, yet parties took a leap of faith, consistent with the precautionary principal, that alternatives to HFCs would be available and affordable. (Substitutes are, indeed, now available in most applications. Many also offer higher energy efficiency than HFCs [Technology and Economic Assessment Panel, 2021].)
For most developing countries, the freeze begins in 2024, and phasedown begins in 2029, whereas for “high ambient temperature” countries, the freeze begins in 2029 and phasedown begins in 2032. The timeline for developed countries was even more aggressive, with the phasedown intended to begin in 2019 for most parties and in 2020 for the Russian Federation and a few other countries of the former Soviet Union. In the United States, where the Senate recently approved the Kigali Amendment, a plan to reduce HFC use was codified in late 2020 with the passage of the American Innovation and Manufacturing Act.
Ultimately, the dangers to society, economies, agriculture, and public health posed by expected climate change and the links between this change and the continued use of HFCs convinced parties to the Montreal Protocol that fast action was necessary. These links were spelled out clearly in a series of scientific studies carried out from 2007 to 2016.
Twelve Papers That Justified Phasing Down HFCs
In consultation with scientists and other colleagues, we set out to identify core scientific papers published during deliberations for the 2007 adjustment that accelerated the phaseout of HCFCs and for the 2016 Kigali Amendment. The purpose was to recognize those who worked to understand the contribution of CFCs, HCFCs, and HFCs to climate change and to inform policymakers and the public. Demonstrating the important contribution of published scientific research to groundbreaking environmental policy may help inspire and motivate others to publish their own research and make it available to decisionmakers.
Although many quality studies have quantified and assessed the climate impacts of HFCs, here we highlight studies that were particularly influential in accelerating the HCFC phasedown and HFC phasedown. Specifically, we evaluated papers on the basis of whether they were crafted to inform Montreal Protocol and other government policymakers; focused on quantifying the potential effects of HFC reductions on climate forcing (i.e., in equivalent CO2 emissions); and written by authors, contributors, or reviewers of the 2014 and 2018 reports of the Montreal Protocol Scientific Assessment Panel (SAP), an advisory panel composed of hundreds of international experts who periodically evaluate atmospheric ozone conditions and related issues.
The Distinguished Dozen
Chipperfield, M. P., et al. (2015), Quantifying the ozone and ultraviolet benefits already achieved by the Montreal Protocol, Nat. Commun., 6, 7233, https://doi.org/10.1038/ncomms8233.
Molina, M., and D. Zaelke (2013), A Comprehensive Approach for Reducing Anthropogenic Climate Impacts Including Risk of Abrupt Climate Changes, Fate of Mountain Glaciers in the Anthropocene, Proceedings of the Working Group, 2-4 April 2011, edited by P. J. Crutzen, L. Bengtsson, and V. Ramanathan, Scripta Varia 118, Pontifical Acad. of Sci., Vatican City, www.pas.va/content/dam/casinapioiv/pas/pdf-volumi/scripta-varia/sv118/sv118-molina-zaelke.pdf.
Molina, M., et al. (2009), Reducing abrupt climate change risk using the Montreal Protocol and other regulatory actions to complement cuts in CO2 emissions, Proc. Natl. Acad. Sci. U. S. A., 106, 20,616–20,621, https://doi.org/10.1073/pnas.0902568106.
Montzka, S. A., et al. (2015), Recent trends in global emissions of hydrochlorofluorocarbons and hydrofluorocarbons—Reflecting on the 2007 adjustments to the Montreal Protocol, J. Phys. Chem. A, 119, 4,439–4,449, https://doi.org/10.1021/jp5097376.
Rogelj, J., et al. (2014), Disentangling the effects of CO2 and short-lived climate forcer mitigation, Proc. Natl. Acad. Sci. U. S. A., 111, 16,325–16,330, https://doi.org/10.1073/pnas.1415631111.
Solomon, S., et al. (2010), Persistence of climate changes due to a range of greenhouse gases, Proc. Natl. Acad. Sci. U. S. A., 107, 18,354–18,359, https://doi.org/10.1073/pnas.1006282107.
Velders, G. J. M., et al. (2007), The importance of the Montreal Protocol in protecting climate, Proc. Natl. Acad. Sci. U. S. A., 104, 4,814–4,819, https://doi.org/10.1073/pnas.0610328104.
Velders, G. J. M., et al. (2009), The large contribution of projected HFC emissions to future climate forcing, Proc. Natl. Acad. Sci. U. S. A., 106, 10,949–10,954, https://doi.org/10.1073/pnas.0902817106.
Velders, G. J. M., et al. (2012), Preserving Montreal Protocol climate benefits by limiting HFCs, Science, 335, 922–923, https://doi.org/10.1126/science.1216414.
Velders, G. J. M., et al. (2015), Future atmospheric abundances and climate forcings from scenarios of global and regional hydrofluorocarbon (HFC) emissions, Atmos. Environ., 123A, 200–209, https://doi.org/10.1016/j.atmosenv.2015.10.071.
Xu, Y., et al. (2013), The role of HFCs in mitigating 21st century climate change, Atmos. Chem. Phys., 13, 6,083–6,089, https://doi.org/10.5194/acp-13-6083-2013.
Zaelke, D., S. O. Andersen, and N. Borgford-Parnell (2012), Strengthening ambition for climate mitigation: The role of the Montreal Protocol in reducing short-lived climate pollutants, Rev. Eur. Compliance Int. Environ. Law, 21(3), 231–242, https://doi.org/10.1111/reel.12010.
From our evaluation, we identified 12 papers (see sidebar) that formed the scientific foundation for the Montreal Protocol parties to take bold steps to phase down HFCs via the Kigali Amendment. These thoroughly researched and clearly presented scientific papers, which were among those contributing to SAP presentations at Meetings of the Parties and were directly read and considered by treaty negotiators from party countries, made the link between HFCs and climate change apparent and persuaded skeptics and stakeholders to take action. All told, the coauthors of these dozen papers include about 40 scientists from 10 countries, reflecting the substantial degree of international attention to the problems posed by HFCs and scientific collaboration to address them.
Other scholars of the Montreal Protocol may have different opinions about which studies were most significant in informing the Kigali Amendment or about what criteria should be applied in evaluating studies. We welcome such differences of opinion because they will spur discussions that help trace the evolution of scientific understanding and its links to policy resolve in this case—and perhaps offer useful insights in future cases.
Following their groundbreaking 2007 study showing the benefits to climate change mitigation of ODS drawdowns, Velders and the same group of colleagues published another prominent study in 2009. In it, they found that regulatory controls on ozone-safe HFC greenhouse gases could significantly reduce anthropogenic climate forcing even as CO2 reductions are aggressively pursued for long-term success [Velders et al., 2009]. For example, a scenario in which HFC consumption levels were frozen and then gradually drawn down would have resulted in reduced global warming potential equivalent to 106–171 gigatons of CO2 from 2013 to 2050 and reduced global radiative forcing of 0.18–0.30 watt per square meter by 2050. This paper started the debate among Montreal Protocol parties that culminated in the 2016 Kigali Amendment.
Subsequently, more papers validated, extended, and enhanced these 2009 findings and were incorporated into Montreal Protocol SAP reports. We have identified 10 papers in addition to Velders et al. [2007, 2009] that provided primary warnings about hazards to environmental and human health from HFCs, clear elaboration of the emerging problem, and guidance about what must be done—and how fast—to avoid existential threats and catastrophic consequences.
For example, Montzka et al.  reported that global atmospheric measurements of HFCs from 2007 to 2012 were consistent with modeled projections by Velders et al.  but were twice as large as the amount of HFC emissions reported to the U.N. Framework Convention on Climate Change, likely reflecting the rapid growth in the use of these chemicals as substitutes for HCFCs, which were being phased out under the Montreal Protocol.
Earlier, Solomon et al.  illustrated the complexity of atmospheric processes and showed how warming effects can extend beyond the time needed for greenhouse gases to degrade. These authors emphasized the need to act quickly to prevent long-lasting and heat-amplifying impacts, such as the transfer of heat to the oceans.
Turning Beneficial Science into Beneficial Policy
Science often informs major, environmentally beneficial policy shifts—think of the research that initiated efforts to draw down the use of hazardous pesticides or lead in gasoline—but rarely does it do so as rapidly as it did in the case of the Kigali Amendment. As we face many other ongoing Earth system and global health challenges, persuasive science will continue to be necessary to build the confidence of policymakers to act according to the precautionary principle, which requires action to avoid possibly irreversible effects long before all the scientific details of an issue are certain [Willi et al., 2021].
The overarching lesson from the Kigali Amendment is that research, analysis, and publication by scientists focusing on current and emerging environmental threats is essential to successful and timely policy action to address them. As Sherwood Rowland said at a White House climate change roundtable in 1997, paraphrasing others before him, “If not us, who; if not now, when?”
The authors wish to thank Timothy Oleson, senior science editor, Eos, for his careful review and valuable editing.
Ajavon, A.-L., et al. (2015), Synthesis of the 2014 reports of the Scientific, Environmental Effects, and Technology & Economic Assessment Panels of the Montreal Protocol, U.N. Environ. Programme, Nairobi, ozone.unep.org/sites/default/files/2019-05/SynthesisReport2014_0.pdf.
Andersen, S. O., and K. M. Sarma (2002), Protecting the Ozone Layer: The United Nations History, edited by Lani Sinclair, Earthscan, London, digitallibrary.un.org/record/474462.
Benedick, R. E. (1998), Ozone Diplomacy: New Directions in Safeguarding the Planet, enlarged ed., Harvard Univ. Press, Cambridge, Mass., www.jstor.org/stable/j.ctv1smjv7m.
Farman, J. C., B. G. Gardiner, and J. D. Shanklin (1985), Large losses of total ozone in Antarctica reveal seasonal ClOx/NOx interaction, Nature, 315, 207–210, https://doi.org/10.1038/315207a0.
Hunter, D., J. Salzman, and D. Zaelke (2022), International Environmental Law and Policy, 6th ed., chap. 10, section V, Foundation, St. Paul, Minn.
Molina, M. J., and F. S. Rowland (1974), Stratospheric sink for chlorofluoromethanes: Chlorine atom-catalysed destruction of ozone, Nature, 249, 810–812, https://doi.org/10.1038/249810a0.
Ramanathan, V. (1975), Greenhouse effect due to chlorofluorocarbons: Climatic implications, Science, 190(4209), 50–52, https://doi.org/10.1126/science.190.4209.50.
Technology and Economic Assessment Panel (2021), Continued provision of information on energy-efficient and low-global-warming-potential technologies, volume 4: Decision XXXI/7, U.N. Environ. Programme, Nairobi, eta-publications.lbl.gov/sites/default/files/teap-eetf-report-may2021.pdf.
Tolba, M. K., and I. Rummel-Bulska (2008), Global Environmental Diplomacy: Negotiating Environmental Agreements for the World, 1973–1992, MIT Press, Cambridge, Mass.
Velders, G. J. M., et al. (2007), The importance of the Montreal Protocol in protecting climate, Proc. Natl. Acad. Sci. U. S. A., 104, 4,814–4,819, https://doi.org/10.1073/pnas.0610328104.
Willi, K. et al. (2021), The precautionary principle and the environment: A case study of an immediate global response to the Molina and Rowland warning, ACS Earth Space Chem., 5(11), 3,036–3,044, https://doi.org/10.1021/acsearthspacechem.1c00244.
World Meteorological Organization (WMO) (2018), Scientific assessment of ozone depletion: 2018, Global Ozone Res. Monit. Proj. Rep. 58, 588 pp., Geneva, Switzerland, csl.noaa.gov/assessments/ozone/2018/.
Zaelke, D., et al. (2018), Primer on HFCs: Fast action under the Montreal Protocol can limit growth of hydrofluorocarbons (HFCs), prevent 100 to 200 billion tonnes of CO2-eq by 2050, and avoid up to 0.5°C of warming by 2100, Inst. for Governance and Sustainable Dev., Washington, D.C., www.igsd.org/wp-content/uploads/2018/01/HFC-Primer-v11Jan18.pdf.
Stephen O. Andersen, Institute for Governance and Sustainable Development, Washington, D.C.; Marco Gonzalez, Montreal Protocol Technology and Economic Assessment Panel, Alajuela, Costa Rica; and Nancy J. Sherman (firstname.lastname@example.org), Institute for Governance and Sustainable Development, Washington, D.C.