Editing IPCC:AR6/WGI/Chapter-6 (section)

==== 6.6.3.2 Recently Decided SLCF-relevant Global Legislation ====

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International shipping emissions regulation: from January 2020, a new global standard, proposed by the International Maritime Organisation, limits the sulphur content in marine fuels to 0.5% against the previous 3.5% ( [[#IMO--2016|IMO, 2016]] ). This legislation is considered in the SSP5 and SSP2-4.5 and with a delay of few years in SSP3-lowSLCF, SSP1-1.9, and SSP1-2.6, and in other SSP-emissions scenarios achieved by the mid-21st century. This global measure aims to reduce the formation of sulphate (and consequently PM <sub>2.5</sub> ) and largely reduce the health exposure to PM <sub>2.5</sub> , especially over India, east China and coastal areas of Africa, and the Middle East ( [[#Sofiev--2018|Sofiev et al., 2018]] ). [[#Sofiev--2018|Sofiev et al. (2018)]] used a high spatial-and-temporal resolution chemistry climate model and estimated a net total ERF of +71 mW m <sup>–2</sup> Associated with this measure and due to lower direct aerosol cooling (+3.9 mW m <sup>–2</sup> ) and lower cloud albedo (+67 mW m <sup>–2</sup> ). This value, which correponds to an 80% decrease of the cooling effect of shipping induced by about 8 Tg of SO <sub>2</sub> of avoided emissions, is consistent with older estimates which considered similar reduction of emitted sulphur. However, there is considerable uncertainty in the indirect forcing since small changes in aerosols, acting as CCNs in a clean environment, can have disproportionally large effects on the radiative balance. Since sulphate is by far the largest component of the radiative forcing ( [[#Fuglestvedt--2008|Fuglestvedt et al., 2008]] ) and of surface temperature effect (Figure 6.16) due to ship emissions over a short time scale, limiting the co-emitted SLCFs can not offset the warming by sulphur reductions. The reduction of sulphur emissions from shipping is assessed to lead to a slight warming mainly due to aerosol–cloud interactions ( ''medium evidence'' , ''medium agreement'' ).

The Kigali Amendment ( [[#UNEP--2016|UNEP, 2016]] ): with the adoption of the Kigali Amendment to the Montreal Protocol ( [[#UN--1989|UN, 1989]] ) in 2016, parties agreed to the phase-down of HFCs, substances that are not ozone depleting but are climate-forcing agents ( [[#Papanastasiou--2018|Papanastasiou et al., 2018]] ). Baseline scenarios, in the absence of controls or only pre-Kigali national legislation, projected increased use and emissions of HFCs. All recent baseline projections are significantly higher than those used in the Representative Concentration Pathways (RCP) scenarios (Figure 6.18; [[#Meinshausen--2011|Meinshausen et al., 2011]] ). There is ''low confidence'' that the high baseline (assuming absence of controls, lack of technical progress and high growth) as developed by [[#Velders--2009|Velders et al. (2009)]] , resulting in additional warming of about 0.5°C by 2100 ( [[#Xu--2013|Xu et al., 2013]] ; [[#WMO--2018|WMO, 2018]] ), is plausible. Evolution of HFC emissions along the baselines consistent with [[#Velders--2009|Velders et al. (2009)]] and [[#Velders--2015|Velders et al. (2015)]] would result in a global average warming, due to HFCs, relative to 2000, of about 0.1°C–0.12°C by 2050 and 0.35°C–0.5°C and 0.28°C–0.44°C by 2100, respectively (Xu et al., 2013). The baseline implementation considered in SSP5-8.5 (Section 6.7.1.1) is comparable to the lower bound of projections by Velders et al. (2015; Figure 6.18) and several other studies ( [[#Gschrey--2011|Gschrey et al., 2011]] ; [[#Purohit--2017|Purohit and Höglund-Isaksson, 2017]] ; [[#EPA--2019|EPA, 2019]] ; [[#Purohit--2020|Purohit et al., 2020]] ) and result in additional warming of 0.15°C–0.3°C by 2100 ( ''medium confidence'' ) (Figure 6.22).

Efficient implementation of the Kigali Amendment and national and regional regulations has been projected to reduce global average warming in 2050 by 0.05°C–0.07°C ( [[#Klimont--2017b|Klimont et al., 2017b]] ; [[#WMO--2018|WMO, 2018]] ) and by 0.2°C–0.4°C in 2100 compared with the baseline (see Figure 2.20 of [[#WMO--2018|WMO, 2018]] ). Analysis of SSP scenarios based on an emulator (Section 6.7.3) shows a comparable mitigation potential of about 0.02°C–0.07°C in 2050 and about 0.1°C–0.3°C in 2100 (Figure 6.22, SSP5-8.5 versus SSP1-2.6). Furthermore, the energy efficiency improvements of cooling equipment alongside the transition to low-global-warming potential alternative refrigerants for refrigeration and air-conditioning equipment could potentially increase the climate benefits from the HFC phasedown under the Kigali Amendment (Shah et al. ,2015; Höglund-Isaksson et al. , 2017; [[#Purohit--2017|Purohit and Höglund-Isaksson, 2017]] ; [[#WMO--2018|WMO, 2018]] ) . [[#Purohit--2020|Purohit et al. (2020)]] estimated that depending on the expected rate of technological development, improving the energy efficiency of stationary cooling technologies and compliance with the Kigali Amendment could bring future global electricity savings of more than 20% of the world’s expected electricity consumption beyond 2050 or cumulative reduction of about 75–275 Gt CO <sub>2-</sub> eq over the period 2018–2100 ( ''medium confidence'' ). This could potentially double the climate benefits of the HFC phase-down of the Kigali Amendment as well as result in small air-quality improvements due to reduced air pollutant emissions from the power sector (i.e., 8–16% reduction of PM <sub>2.5</sub> , SO <sub>2</sub> and NO <sub>x</sub> ; [[#Purohit--2020|Purohit et al., 2020]] ).

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