Achieving Paris climate goals calls for increasing ambition of the Kigali Amendment

Hydrofluorocarbon emissions have increased rapidly and are managed by the Kigali Amendment to the Montreal Protocol. Yet the current ambition is not consistent with the 1.5 °C Paris Agreement goal. Here, we draw on the Montreal Protocol start-and-strengthen approach to show that accelerated phase-down under the Kigali Amendment could result in additional reductions of 72% in 2050, increasing chances of staying below 1.5 °C throughout this century. The current ambition for hydrofluorocarbon emissions reduction by the Kigali Amendment is not sufficient to meet the 1.5 °C Paris Agreement goal. The authors show that a more ambitious Kigali Amendment target could still help in achieving the Paris goal if more countries act early.

; ref. 6 ). Furthermore, unlike the Paris Agreement to the UNFCCC, the MP and its amendments are legally binding for countries that ratify them.
KA is a global agreement (in force since 1 January 2019) to phase-down consumption of HFCs of 80-85% by the late 2040s (Supplementary Table 1). Unlike previous MP amendments, which managed ODSs, the KA is primarily a climate treaty, therefore it is appropriate to evaluate the sufficiency of its ambition on the basis of its consistency with climate mitigation targets. The 2015 Paris Agreement established an ambitious target of limiting global mean temperature rise this century to well below 2 °C, preferably to 1.5 °C compared to pre-industrial levels, but did so in the context of broader international goals of sustainable development and poverty eradication. The 1.5 °C-consistent scenarios used in the IPCC Special Report on Global Warming of 1.5 °C (SR1.5) include a 75-80% reduction in HFC emissions by 2050, compared to 2010 levels 7 , along with deep and simultaneous reductions of CO 2 and all non-CO 2 climate-forcing emissions.
A recent IIASA study 8 used the Greenhouse Gas and Air Pollution Interactions and Synergies (GAINS) model 9 framework to develop a range of long-term scenarios for HFC emissions under varying degrees of stringency in climate policy and assessed co-benefits in the form of electricity savings and associated reductions in GHG and air pollutant emissions. Full compliance with the KA (Fig. 1a) is expected to achieve a 56% reduction in HFC emissions by 2050, compared to 2010 levels 8 . Hence, it will not achieve the 75-80% threshold set by 1.5 °C-consistent scenarios 7 . Full compliance with the KA phase-down schedule is estimated to avoid 0.2-0.4 °C additional warming by the end of this century 3,10 which is significant but insufficient to achieve a 1.5 °C-consistent pathway 7 . Despite the widely recognized success of the MP for phasing out ODSs faster 11 and at a lower cost than originally assumed, some observers question whether the HFC-reduction process under KA is taking place quickly enough to adequately address the urgency of the climate crisis 12 . Considering the role HFC mitigation plays in 1.5 °C-consistent scenarios 7 , enhancing the ambition of mitigation efforts by all parties to the MP is called for. In this study, we develop a series of alternative HFC phase-down scenarios ( Fig. 1) consistent with the MP's history and experience of a gradual increase in ambition.
KA defines HFC phase-down schedules for four different party groups. The first group (Article 5 Group 1) includes 136 primarily developing countries that make up all Article 5 countries with the exception of ten countries characterized by high ambient air temperatures forming a second group (Article 5 Group 2) and allowed less ambitious timing of targets. Non-Article 5 countries are primarily developed countries and under KA divided into two groups with 45 countries in a first group (non-Article 5, earlier start) and five in a second group allowed to start somewhat later (non-Article 5, later start).
We estimate HFC emissions (CO 2 eq using global warming potential GWP 100 from IPCC/AR5 (ref. 13 )) for all analysed scenarios (Fig. 2a). In a pre-KA baseline, HFC emissions increase to about 4.2 GtCO 2 eq by 2050, which is within the range of previous estimates (4.0-5.3 GtCO 2 eq) by Velders et al. 14 . With full KA compliance, global HFC emissions drop to 0.32 GtCO 2 eq by 2050, achieving 56% reduction compared to 2010 levels. Technology exists that, if deployed globally to a maximum extent, could achieve near-complete mitigation of HFC emissions one-decade sooner than the KA phase-down schedule, resulting in a cumulative reduction of ~77 GtCO 2 eq HFC emissions until 2050 (Supplementary  Table 2). Such a rapid reduction is, however, infeasible on practical grounds and also inconsistent with the MP's history of a phased step-wise approach to refrigerant management. Instead, we have analysed a set of more realistic reduction scenarios.

Achieving Paris climate goals calls for increasing ambition of the Kigali Amendment
Pallav Purohit 1 ✉ , Nathan Borgford-Parnell 2 , Zbigniew Klimont 1 and Lena Höglund-Isaksson 1 First, we analyse whether aligning Article 5 Group 2 countries with the higher ambition level of the Article 5 Group 1 (Article 5 Groups 1 and 2 alignment scenario) would result in achieving the Paris Agreement targets. The results show that this would not be the case (Fig. 2b, where the orange box indicates the 75-80% threshold set by 1.5 °C-consistent scenarios) 7 . Next, we increase the ambitions of both Article 5 and non-Article 5 parties, resulting in achieving the Paris Agreement targets globally by 2050, however with different cumulative emissions until 2050 due to variations in the timing of adapted KA targets. If Article 5 and non-Article 5 party groups follow the KA phase-down schedules but step-up ambitions in their final phase-down step (starting in 2036 for non-Article 5 with Article 5 following only in year 2050) to 95% below baseline in the year 2050 (95% scenario), then the resulting cumulative reduction is 61 GtCO 2 eq (Supplementary Table 2). If the Article 5 Groups 1 and 2 alignment scenario and the 95% scenario are combined (combined scenario), the resulting cumulative reduction is 63 GtCO 2 eq. Finally, if the combined scenario is accelerated with a more ambitious target timeline (accelerated combined scenario), with non-Article 5 countries achieving 95% reduction already in 2036 and Article 5 countries starting earlier and achieving 95% in 2045, then a cumulative HFC-reduction potential of 69 GtCO 2 eq can be achieved. The last scenario follows the example of the accelerated phase-out of HCFCs under the MP from 2007. In particular, for the period 2021-2030, the cumulative emissions are lower in the accelerated combined scenario compared to other scenarios.
The developing countries are less than 3 years away from the first HFC consumption compliance obligation of the KA applicable to Article 5 Group 1 countries. Much still needs to be done to ensure that all these countries are ready to comply with the 2024 cap on HFC consumption and production. In countries where HFC consumption is projected to exceed their baselines by 2024, there is an urgent need to implement actions towards a rapid transition to low-GWP refrigerants. In countries where HFC consumption is projected lower than their baselines by the agreed freeze year, there are opportunities for faster implementation of the KA to achieve HFC emission reductions earlier than strictly required under KA.
KA provides an important opportunity and framework to control the production and consumption of HFCs resulting in reductions of both direct and indirect emissions from the cooling sector. Combining benefits from energy efficiency and climate-friendly cooling is vital to developing markets with rising cooling demand. Harnessing such opportunities by ensuring the transition to low-GWP refrigerants, combined with the adoption of energy-efficient cooling equipment, can potentially double the climate benefits of the HFC phase-down 3 and save as much as 20% of the expected future global electricity consumption 8 . Lower electricity consumption also offsets the need to build new power plants and increases energy access across emerging economies. For example, transitioning to low-GWP refrigerants with enhanced energy efficiency in room air-conditioners in China could avoid the construction of ~300 coal-fired power plants (500 MW each) by 2050 15 . Therefore, an early HFC phase-down will foster sustainable growth with energy-efficient, innovative technologies that provide jobs, increase energy access and reduce air pollution while reducing consumer energy bills.
KA is a work in progress, but one that needs to be embraced and expanded upon in the global interests of mitigating climate change, just as the original MP has been instrumental in the recovery of the stratospheric ozone layer. An example of progressive legislation could be the HFC-reduction steps under the European Union F-gas regulation that is more ambitious than what is included in the KA 16 . Finally, if parties to the MP do not align early HFC phase-down policies with their economic transformation plans in the post-COVID era, they might not only become more vulnerable to climate shocks but also miss out on new technologies, investment and market access in a rapidly shifting global economy.

Online content
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methods
In developing the baseline HFC emissions under the IIASA study 8 , the historical consumption of HFCs for major sources, that is, stationary and mobile air-conditioning and domestic refrigerators, have been derived in a consistent manner across countries, starting from a compilation of data on underlying drivers; for example, the number of vehicles by type, commercial floor space area, cooling degree days, per capita income, average household sizes, current equipment penetration rates and so on. Estimating HFC consumption in commercial, industrial and transport refrigeration, foams and other smaller HFC sources (for example, aerosols, fire extinguishers and solvents) is more challenging since it varies greatly between countries due to differences in industrial structures and consumption patterns 17 . For these sectors, historical HFC consumption, as reported by industrialized countries to the UNFCCC, has been adopted when available.
For developing countries, information on HFC consumption in these sectors has been compiled from various published sources, alternatively, derived consistently from underlying activity data using default factors from literature 8,18 . For the development of the baseline scenarios until 2040 we use the existing model setup in GAINS 9 , which for global scenarios uses drivers consistent with macroeconomic and energy sector projections from the International Energy Agency 19 . The extension in demand for cooling services between 2040 and 2100, expressed in tonnes of HFC consumed 8 , is consistent with the growth in population and macroeconomic indicators of the third Shared Socioeconomic Pathway (SSP3) 20 and the expected future increase in regional cooling degree days.
In addition to the KA and Maximum Technically Feasible Reduction (MTFR) scenarios as explained in the IIASA study 8 , we analyse four scenarios for HFC phase-down to achieve the Paris Agreement targets by 2050. In the Article 5 Groups 1 and 2 alignment scenario, we assume that Article 5 Group 2 countries join the Article 5 Group 1 phase-down schedule immediately as shown in Fig. 1b. Note that Article 5 Group 2 countries have a later freeze date (Supplementary Table 1) and delayed phase-down steps compared with Article 5 Group 1 under the KA. In the 95% scenario, we assume that all party groups will follow the KA phase-down schedules but, in addition, the final phase-down step by 2050 will be 95% of baseline, valid for all party groups, as shown in Fig. 1c. In the combined scenario, we assume that Article 5 Group 2 countries join the Article 5 Group 1 phase-down schedule immediately as in the case of the Article 5 Groups 1 and 2 alignment scenario; in addition, the final phase-down step by 2050 will be 95% of baseline and, just like in the case of the 95% scenario, be valid for all party groups, as shown in Fig. 1d. Finally, the accelerated combined scenario is designed following the example of the accelerated phase-out of HCFC in 2007 (Extended Data Fig. 1) as shown in Fig. 1e. In the case of the HCFC example, the accelerated phase-out was agreed upon 11 years after the freeze date set for non-Article 5 countries and 3 years after the first phase-out step but before the freeze date set for Article 5 countries.