More to offer from the Montreal protocol: how the ozone treaty can secure further significant greenhouse gas emission reductions in the future

ABSTRACT

Action under the Montreal Protocol has contributed to climate change mitigation for almost 35 years. The phase-out of ozone-depleting substances (ODS) has set the ozone layer on a path to recovery, protecting the world’s biosphere from harmful ultraviolet radiation. The 2016 Kigali Amendment to the Montreal Protocol is expected to avoid 5.6–8.7 gigatonnes of carbon-dioxide equivalent (GtCO₂e) emissions of hydrofluorocarbons (HFC) per year by 2100, reducing the impact of HFCs on global average warming by up to 0.4°C. Despite its successes, unexpected emissions of phased out ODS – notably the chlorofluorocarbon, CFC-11 - have brought attention to shortcomings in the Protocol’s monitoring, reporting, verification and enforcement (MRV+E) which must be addressed to guarantee its controls are sustained. Meanwhile, additional significant mitigation could be achieved by accelerating the phase-down of HFCs under the Kigali Amendment, by tackling ODS and HFC emissions from leaking banks of equipment and products and by controlling feedstocks, which are not subject to Montreal Protocol phase-out controls. Recent scientific papers have linked almost 870 million tCO₂ per year of greenhouse gases (GHG) and ODS to fluorochemical industrial processes and illegal fluorochemical production. Expanding the scope of the Montreal Protocol to address nitrous oxide (N₂O), itself an ODS and GHG, would also contribute substantial ozone and climate benefits. This perspective essay discusses new and strengthened policy measures that governments can consider under the Montreal Protocol in order to maximize early, cost-effective reductions in emissions of non-CO₂ greenhouse gases and ensure future implementation.

Highlights

• Perspective essay examining non-CO₂ emission reductions under the Montreal Protocol.

• Significant ongoing emissions are linked to unregulated fluorochemical production.

• Strengthened institutions and processes will avoid illegal trade and sustain compliance.

• Addressing ODS and HFC banks and N₂O emissions present additional opportunities.

• The global HFC phase-down must be accelerated to meet the 1.5°C climate goal.

KEYWORDS:

• Montreal protocol
• climate change
• hydrofluorocarbons
• ozone depleting substances (ODS)
• fluorochemical emissions

1. The Montreal protocol – a successful climate treaty

The Montreal Protocol on Substances that Deplete the Ozone Layer was created in 1987 to regulate ozone depleting substances (ODS), the chemicals responsible for stratospheric ozone depletion. Initially, the Protocol mandated a 50% reduction in the consumption and production of five chlorofluorocarbons (CFC-11, CFC-12, CFC-113, CFC-114 and CFC-115) and a freeze in consumption and production of three halons (halon-1211, halon-1301 and halon-2402) (United Nations Environment Programme UNEP 1987). The agreement has since been strengthened multiple times to control new substances and accelerate consumption and production phase-out schedules (United Nations Environment Programme UNEP 2020).

Widely hailed as the world’s most successful international environmental treaty, the Montreal Protocol has phased out more than 99% of the production of controlled ozone depleting substances (ODS), setting the ozone layer on the path to recovery (World Meteorological Organization WMO 2022).

Although established to protect the ozone layer, the climate relevance of the Montreal Protocol has been recognized for over 15 years. Many ODS are potent greenhouse gases (GHG), and their phase-out under the Montreal Protocol has therefore contributed significantly to climate mitigation (World Meteorological Organization WMO 2022). A landmark paper (Velders et al. 2007) estimated that if the Montreal Protocol had not been created to address stratospheric ozone depletion, ODS emissions would have reached 15–18 GtCO₂e per year in 2010, equivalent to about half of global annual CO₂ emissions at that time. According to the latest Scientific Assessment of Ozone Depletion under the Montreal Protocol (World Meteorological Organization WMO 2022), this has avoided global warming of approximately 0.5–1°C by 2050, compared to a scenario where ODS use increased by 3–3.5% per year.

Additional climate benefits have resulted from mitigation of the damaging effect of UV radiation on the terrestrial biosphere, helping to preserve its capacity as a carbon sink. Young et al. (2021) estimate that, without the Montreal Protocol, plants and soils would carry 325–690 billion tonnes less carbon by the end of the century, leading to an additional 0.85°C warming on top of an estimated 1.7°C direct warming due to CFC emissions. Together, this represents a total of 2.5°C warming avoided by 2100 due to the Montreal Protocol.

In 2009, the Parties to the Montreal Protocol began formally discussing controlling production and consumption of hydrofluorocarbons (HFCs) (United Nations Environment Programme UNEP 2009). The negotiations culminated in the 2016 adoption of the Kigali Amendment, which came into force in 2019 (United Nations Environment Programme UNEP 2020). As of 19 January 2024, 156 Parties have ratified the Amendment, committing to legally binding targets mandating gradual reductions in HFC consumption and production (United Nations Environment Programme UNEP 2024). The Kigali Amendment will phase down HFC consumption and production, based on CO₂e, by 80–85% by 2045 (United Nations Environment Programme UNEP 2020). With full compliance, this will avoid 3.1–4.3 GtCO₂e per year emissions by 2050, reducing annual average surface warming from HFCs from 0.3–0.5°C to 0.04°C in 2100 (World Meteorological Organization WMO 2022).

2. Unexpected emissions and unregulated production

While the Montreal Protocol’s success should be celebrated, challenges to its ongoing work remain. In 2018, Montzka et al. (2018) reported unexpected CFC-11 emissions of 67 ± 3 Gg yr − 1 between 2014–16, an estimated 13 ± 5 Gg yr-1 increase (25 ± 13%) since 2012. Atmospheric studies pointed to east Asia as the source (Rigby et al. 2019), and emissions were traced to illegal production and use in the polyurethane foam sector in China (Environmental Investigation Agency 2018). A nationwide enforcement effort quickly followed (Environmental Investigation Agency 2021). Although recent CFC-11 emissions estimates of 45 ± 10 Gg in 2019 and 2020 suggest that most of the unexpected CFC-11 emissions have now been eliminated, it is nonetheless estimated that a cumulative 120–440 Gg of CFC-11 were emitted from 2012–19, equivalent to 0.77–2.82 GtCO₂e (World Meteorological Organization WMO 2021).¹ In addition, the bank of CFC-11 in foam insulation was increased by an estimated 146–1,320 Gg, equivalent to 0.94–8.46 GtCO₂e (World Meteorological Organization WMO 2022).²

The unexpected CFC-11 emissions brought into serious question the long-term sustainability of phase-out agreements under the Montreal Protocol and the efficacy of its monitoring, reporting, verification and enforcement (MRV+E) regime. It also prompted the authors of this paper to look more closely at recent published estimates of ODS that have either been phased out or are in the process of being phased out under the Montreal Protocol. This revealed a range of significant fluorochemical GHG emissions that are generally unreported, unaccounted for and in several cases, unexpected (see Table 1). Their primary sources appear to be feedstocks, by-products and intermediates in fluorochemical production processes, for example in the production of polytetrafluoroethylene (PTFE), a widely used fluoropolymer. These processes, and their emissions are not controlled by the Montreal Protocol.

Table 1. Emissions linked to fluorochemical production, illegal production or unknown sources.

Chemical	World Meteorological Organization (WMO) (2022) GWP	Estimated Emissions (Gg/yr)	Estimated Emissions (Million Tonnes CO2e/yr)	Observation Year(s)	Description of Emission Sources	Reference
HFC-23	14,700	17.20	252.84	2019	Top-down estimate of global emissions. By-product emissions from production of HCFC-22, as well as from pyrolysis of HCFC-22 to produce TFE and HFP. Potential by-product emissions from production of HFC-32, HFC-125 and other controlled substances. Also includes emissions from banks of niche refrigerant and fire suppression uses.	World Meteorological Organization (WMO) (2022)
CFC-12	12,500	18.30	228.75	2014–2016	Top-down estimate of unexpected emissions excluding emissions from banks. Emissions are linked to illegal production and use or other unknown sources.	Lickley et al. (2021)
CFC-11	6,410	23.20	148.71	2014–2016	Top-down estimate of unexpected emissions excluding emissions from banks. Emissions are linked to illegal production and use or other unknown sources.	Lickley et al. (2021)
CTC	2,150	34.00	73.10	2020	Top-down estimates of global CTC emissions are 44 ± 15 Gg/yr from 2016 and 2020. Once legacy emissions from landfills and contaminated soils (5-10Gg) are subtracted, total emissions from production and unexplained sources are 44–10 = 34Gg. Unexplained emissions are assumed to be from feedstock and chloromethane production or other unknown sources. CTC is a feedstock to various CFCs, HFCs, HFOs, & chloroform, which is used to make HCFC-22.	World Meteorological Organization (WMO) (2022) (Update to Sherry et al, 2018)
CFC-113	6,530	7.8	50.93	2014–2016	Top-down estimate of unexpected emissions excluding emissions from banks. CFC-113 is a common feedstock used to make HFC-134a, TFA, pesticides and chlorotrifluoroethylene (CTFE) which is a precursor used to make fluoropolymers.	Lickley et al. (2021)
HCFC-22	1,910	21.40	40.87	2019	Bottom-up estimate of emissions from feedstock production and use. Feedstock to TFE/HFP to produce PTFE and other fluoropolymers.	World Meteorological Organization (WMO) (2022)
PFC-318	10,600	2.50	26.50	2020	Top-down estimate. By-product of hexafluoropropylene (HFP) production, which is used to make fluoropolymers including PTFE (aka Teflon)	World Meteorological Organization (WMO) (2022)
CFC-115	9,630	n/a	14.30	2020	Top-down estimate of global emissions. No significant banks from end uses. By-product of HFC-125 production	Western et al. (2023)
CFC-113a	3,930**	n/a	14.00	2020	Top-down estimate of global emissions. No significant banks from end uses. Feedstock/By-product in HFC-125, HFC-134a, HFO-1334mzz production; feedstock in production of TFA & pesticides	Western et al. (2023)
CFC-13	16,300**	n/a	12.00	2020	Top-down estimate of global emissions. Unknown sources. Potential use as a feedstock for CFC-11, however emissions have not declined in recent years with CFC-11 emissions.	Western et al. (2023)
CFC-114a	7,410**	n/a	6.00	2020	Top-down estimate of global emissions. No significant banks from end uses. Feedstock/intermediate in production of HFC-125 and HFC-134a	Western et al. (2023)
HCFC-133a	378	2.30	0.87	2016–2019	Top-down estimate of global emissions. No known dispersive end-uses or banks. Feedstock to produce HCFC-123, CFC-113a.	Vollmer et al. (2021)
HCFC-132b	332	1.10	0.37	2019	Top-down estimate of global emissions. No known dispersive end-uses or banks. Likely by-product of HFC production.	Vollmer et al. (2021)
CFC-112a	3,550**	n/a	0.10	2020	Top-down estimate of global emissions. No significant banks from end uses. Unexplained, previous uses as a solvent and feedstock in fluorovinyl ether production	Western et al. (2023)
HCFC-31	85	0.71	0.06	2016–2019	Top-down estimate of global emissions. No known dispersive end-uses or banks. By-product of HFC production.	Vollmer et al. (2021)

These avoidable emissions of fluorochemical GHGs are significant from a climate perspective. If the 2014–16 estimates for unexpected CFC-11 and CFC-12 emissions are included as a proxy for illegal production and use, the total emissions from all unregulated production and use amount to almost 870 MtCO₂e per year. These emissions, with their observation years and potential sources, are summarized in Table 1.

3. Strengthening the Montreal protocol

Of the nine planetary boundaries deemed critical for maintaining a habitable Earth, stratospheric ozone depletion is one of only three that have not already been transgressed (Richardson et al. 2023). It is therefore worth considering what more the successful model of the Montreal Protocol can do to avoid catastrophic climate change. Solomon et al. (2020) referred to the “unfinished business” of the Montreal Protocol that needs to be addressed to secure its success in the 21^st century. These and other new and strengthened policy measures are considered here with a view to maximizing early, cost-effective reductions of non-CO₂ GHG emissions.

3.1. Monitoring, verification, reporting and enforcement (MRV+E)

The unexpected emissions of CFC-11 were a stark reminder that the sustainability of the ODS phase-outs and other controls under the Montreal Protocol cannot be taken for granted. This prompted a series of studies and discussions which highlighted limitations in the Protocol’s MRV+E and additional challenges expected in implementing the Kigali Amendment (United Nations Environment Programme UNEP 2019).

Central to assessing and improving the institutions and processes of the Montreal Protocol has been recognition of the need for enhanced global and regional atmospheric monitoring (United Nations Environment Programme UNEP 2022a). A 2023 workshop on strengthening the implementation and enforcement of the Montreal Protocol, identified a range of policy measures and tools that are required for effective MRV+E (United Nations Environment Programme UNEP 2023b). Along with the need to close gaps in global atmospheric monitoring, the workshop identified strengthened reporting requirements, implementation of effective licencing systems and third-party verification of importers and exporters as measures that could improve compliance (United Nations Environment Programme UNEP 2023b). Adoption of these measures, with support from the Protocol’s financial mechanism, could avoid significant future emissions due to illicit production, trade and use.

3.2. Feedstocks and production emissions

Ongoing ODS emissions from fluorochemical production processes should compel the Parties to review the exemption of feedstocks, intermediates and most by-product emissions from Montreal Protocol controls. An initial assessment by the Protocol’s Technology and Economic Assessment Panel (TEAP) identified 24 chemical pathways used in production of controlled substances that are likely to result in substantial emissions (defined as greater than 1,000 tonnes per year) (United Nations Environment Programme UNEP 2023a). The substances emitted can include feedstocks, intermediates, process agents, catalysts, and by-products or co-products (United Nations Environment Programme UNEP 2021b).

When used as raw materials to manufacture other chemicals, ODS and HFCs are exempt from Montreal Protocol controls, as feedstock emissions are assumed to be “insignificant” (United Nations Environment Programme UNEP 2020). The Protocol has thus only controlled production and consumption for emissive end uses, such as refrigerants, aerosols and foams. Meanwhile, reported feedstock use of ODS has increased, reaching approximately 1.5 million tonnes in 2020 (United Nations Environment Programme UNEP 2021a). Although decisions of the Protocol have urged Parties to take steps to minimize emissions, the most recent assessment estimated total emissions from a range of regulated ODS feedstocks to be 37.2–58.9Gg in 2019, equivalent to 88.1–145.8 MtCO₂e (World Meteorological Organization WMO 2022).³

HCFC-22 dominates global feedstock production, representing 48% of the total mass quantity of feedstocks produced (United Nations Environment Programme UNEP 2022b). According to United Nations Environment Programme (UNEP) (2022b), about 97% of the 713,536 tonnes of HCFC-22 produced in 2020 were used “to produce tetrafluoroethylene (TFE) and hexafluoropropene (HFP), both used primarily to manufacture fluoropolymers”. In turn, TFE/HFP production also generates potent by-product emissions, including perfluorocyclobutane (also known as c-C4F8 or PFC-318), which has a GWP of 10,200. Mühle et al. (2022) estimate that emissions of PFC-318 have almost tripled since the early 2000s, reaching 2,320 tonnes in 2020.

HCFC-22 production also results in unwanted by-product emissions of HFC-23, a highly potent GHG with a GWP of 14,700. Efforts to address HFC-23 emissions date back almost two decades, with destruction projects funded under the UNFCCC Clean Development Mechanism as early as 2006. Despite the CDM projects and subsequent pledges from fluorochemical companies and producer countries to capture and destroy the by-product, Stanley et al. (2020) reported all-time high global HFC-23 emissions of 15.9 ± 0:9 Gg in 2018. Updated top-down estimates show further increases to 17.2 ± 0.8 Gg/yr in 2019 and 16.5 ± 0.8 Gg/yr in 2020, some eight times larger than expected and likely to increase as feedstock production of HCFC-22 continues to expand (World Meteorological Organization WMO 2022).

The Kigali Amendment includes an obligation to destroy emissions of HFC-23 generated during the production of HCFCs or HFCs, “to the extent practicable using technology approved by the Parties” (World Meteorological Organization WMO 2022). Effective as of 2020, strict implementation of this measure will achieve substantial mitigation. Further emissions reductions could be achieved by applying similar obligations for all fluorochemical production emissions. Limiting the feedstock exemption to only substances for which there are no feasible alternatives would also produce significant co-benefits by reducing plastic pollution and emissions of per- and polyfluoroalkyl (PFAS) (Andersen et al. 2021).

3.3. Recovery and destruction of ODS and HFC banks

Emissions of ODS and HFCs from end-of-life equipment and products are significant, but are not addressed by the Montreal Protocol. CFC bank recapture and destruction, for example, is estimated to be “the single most effective ozone depletion and climate change mitigation option” for ODS (World Meteorological Organization WMO 2022). Dreyfus et al. (2024) suggest the destruction of banks could be undertaken to redress unexpected and unreported production of controlled substances.

The absence of comprehensive global data to quantify banks and their emissions is a key issue to be addressed. A global inventory, where amounts available for recovery and any stockpiles can be reported, would help facilitate end-of-life management (Environmental Investigation Agency 2019). As a first step, the Executive Committee of the Multilateral Fund for the Implementation of the Montreal Protocol recently established a “funding window for the preparation of national inventories of banks of used or unwanted controlled substances and a plan for the collection, transport and disposal of such substances, including consideration of recycling, reclamation and cost-effective destruction” (Multilateral Fund for the Implementation of the Montreal Protocol 2022). Such policies and regulations may, for example, include implementation of Extended Producer Responsibility schemes and other approaches to incentivize ODS and HFC recovery at end of life. This places the Montreal Protocol in a strong position to lead development of a global framework to recover and destroy ODS and HFC banks, in coordination with other stakeholders and institutions.

3.4. Nitrous oxide emissions

The Montreal Protocol could also play a role in mitigating emissions of nitrous oxide (N₂O), the most significant ODS in the world today in terms of ozone-depleting potential (ODP) emissions (Compton 2021), and the third most prevalent GHG on a CO₂e basis (Intergovernmental Panel on Climate Change IPCC 2022). Although listed under the Vienna Convention as a substance with the potential to modify the ozone layer (United Nations Environment Programme UNEP 1985), no controls on N₂O have been agreed by the Parties to the Montreal Protocol and only limited action on N₂O has been proposed by some Parties to the UNFCCC under their Nationally Determined Contributions (United Nations Framework Convention on Climate Change 2023). By 2018, atmospheric concentrations of N₂O had reached 331 parts per billion (ppb), a 20% increase compared to pre-industrial levels with further increases of 2% per decade expected throughout this century (Tian et al. 2020).

The majority of N₂O emissions come from agriculture, largely through the overuse of fertilizers, with wastewater, biomass burning, transport and industrial emissions also contributing significantly (Kanter et al. 2013). In 2020, industrial N₂O process emissions from the manufacture of nitric acid, adipic acid and caprolactam, were estimated at 47.4 MtCO₂e in industrialized countries alone. These industrial emissions are occurring despite the fact that existing, cost-effective abatement technologies have been proven to eliminate more than 80% of emissions from these sources (Kanter et al. 2020). Tackling these industrial emissions represents a first step that the Montreal Protocol should consider, following the Protocol’s approach to HFC-23 emissions with financial support for abatement technologies in developing countries facilitated by the Multilateral Fund.

3.5. Accelerating the Kigali amendment

Global HFC emissions have continued to grow rapidly, reaching 1.22 ± 0.05 GtCO₂e per year in 2020, a 19% increase since 2016 when the Kigali Amendment was adopted. Critically, the current phase-down schedules are not consistent with scenarios developed by the Intergovernmental Panel on Climate Change (IPCC) that can achieve the UNFCCC Paris Agreement goal of limiting warming to 1.5°C by 2100. Purohit et al. (2022) estimate that full compliance will achieve a 56% reduction in HFC emissions by 2050, from 2010 levels. This falls short of the IPCC’s 1.5°C scenarios, which require HFC emission reductions of between 75–80% by mid-century (Rogelj et al. 2018). An acceleration of the global HFC phase-down, through an adjustment to the Kigali Amendment, alongside early action policies to avoid HFC growth in countries that are yet to freeze HFC consumption, would avoid significant additional emissions and be firmly in the interest of all Parties that are pursuing the Paris Agreement goal.

4. Conclusion

The Vienna Convention and the Montreal Protocol are the only environmental agreements that have achieved universal ratification, unmatched in their success as instruments of global environmental governance. But the need to take greater climate action that is rapid and meaningful is now more critical than ever.

With its strong track record in protecting the ozone layer, and its evolution into a powerful climate treaty, the Montreal Protocol has shown its consistent ability to increase ambition and deliver on its promises. As the world’s most successful environmental treaty, the Protocol must be harnessed to secure every possible tonne of mitigation that it can deliver. By strengthening the institutions and processes of the Montreal Protocol and implementing a series of new and strengthened policy measures, what began as the ozone treaty can achieve highly significant additional reductions in emissions of non-CO₂ GHGs, now, and in the future.

Disclosure statement

The authors declare that they have no relevant or material financial interests that relate to the research described in this paper.

Data availability statement

The authors confirm that the data supporting the findings of this study are available within the article.

Additional information

Funding

The work was supported by the Children’s Investment Fund Foundation and the Clean Cooling Collaborative .

Notes

1. CO₂-equivalence calculated by authors on basis of GWPs in World Meteorological Organization (WMO) (2022).

2. CO₂-equivalence calculated by authors on basis of GWPs in World Meteorological Organization (WMO) (2022).

3. CO₂-equivalence calculated by authors on basis of GWPs in World Meteorological Organization (WMO) (2022).