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

=== Potential Effects of SLCF Mitigation ===

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'''Over time scales of 10 to 20 years, the global temperature response to a year’s worth of current emissions of SLCFs is at least as large as that due to a year’s worth of CO''' <sub>2</sub> '''emissions''' ( ''high confidence'' '''). Sectors producing the largest SLCF-induced warming are those dominated by methane emissions: fossil fuel production and distribution, agriculture and waste management''' ( ''high confidence'' ''').''' On these time scales, SLCFs with cooling effects can significantly mask the CO <sub>2</sub> warming in the case of fossil fuel combustion for energy and land transportation, or completely offset the CO <sub>2</sub> warming and lead to an overall net cooling in the case of industry and maritime shipping (prior to the implementation of the revised fuel-sulphur limit policy for shipping in 2020) ( ''medium confidence'' ). Ten years after a one-year pulse of present-day aviation emissions, SLCFs induce strong but short-lived warming contributions to the GSAT response ( ''medium confidence'' ), while CO <sub>2</sub> both gives a warming effect in the near term and dominates the long-term warming impact ( ''high-confidence'' ). {6.6.1, 6.6.2}

'''The effects of SLCFs decay rapidly over the first few decades after pulse emission. Consequently, on time scales longer than about 30 years, the net long-term global temperature effects of sectors and regions are dominated by CO''' <sub>2</sub> ( ''high confidence'' ''').''' The global mean temperature response following a climate change mitigation measure that affects emissions of both short- and long-lived climate forcers depends on their atmospheric decay times, how fast and for how long the emissions are reduced, and the inertia in the climate system. For SLCFs including methane, the rate of emissions drives the long-term global temperature effect, as opposed to CO <sub>2</sub> for which the long-term global temperature effect is controlled by the cumulative emissions. About 30 years or more after a one-year emission pulse occurs, the sectors contributing the most to global warming are industry, fossil fuel combustion for energy and land transportation, essentially through CO <sub>2</sub> ( ''high confidence'' ). Current emissions of SLCFs, CO <sub>2</sub> and N <sub>2</sub> O from Eastern Asia and North America are the largest regional contributors to additional net future warming on both short ( ''medium confidence'' ) and long time scales ( ''high confidence'' ). {6.6.1, 6.6.2}

'''At present, emissions from the residential and commercial sectors (fossil and biofuel use for cooking and heating) and the energy sector (fossil fuel production, distribution and combustion) contribute the most to the world population’s exposure to anthropogenic fine PM''' ( ''high confidence'' '''), whereas emissions from the energy and land transportation sectors contribute the most to ozone exposure''' ( ''medium to'' ''high confidence'' ''').''' The contribution of different sectors to PM varies across regions, with the residential sector being the most important in Southern Asia and Africa, agricultural emissions dominating in Europe and North America, and industry and energy production dominating in Central and Eastern Asia, Latin America and the Middle East. Energy and industry are important PM <sub>2.5</sub> contributors in most regions, except Africa ( ''high confidence'' ). Sector contributions to surface ozone concentrations are similar for all regions. {6.6.2}

'''Assuming implementation and efficient enforcement of both the Kigali Amendment to the Montreal Protocol on Ozone Depleting Substances and current national plans to limit emissions (as in''' '''SSP1-2.6''' '''), the effects of HFCs on GSAT, relative to 2019, would remain below +0.02°C from 2050 onwards versus about +0.04°C to +0.08°C in 2050 and +0.1°C to +0.3°C in 2100 considering only national HFC regulations decided prior to the Kigali Amendment (as in''' '''SSP5-8.5''' ''')''' ( ''medium confidence'' ''').''' Further improvements in the efficiency of refrigeration and air-conditioning equipment during the transition to low-global-warming-potential refrigerants would bring additional greenhouse gas reductions ( ''medium confidence'' ) resulting in benefits for climate change mitigation and to a lesser extent for air quality due to reduced air pollutant emissions from power plants . {6.6.3, 6.7.3}

'''Future changes in SLCFs are expected to cause additional warming. This warming is stable after 2040 in scenarios leading to lower global air pollution as long as methane emissions are also mitigated, but the overall warming induced by SLCF changes is higher in scenarios in which air quality continues to deteriorate (induced by growing fossil fuel use and limited air pollution control)''' ( ''high confidence'' ''').''' If a strong air pollution control resulting in reductions in anthropogenic aerosols and non-methane ozone precursors was considered in SSP3-7.0 , it would lead to a ''likely'' additional near-term global warming of 0.08 [0.00 to 0.10] °C in 2040. An additional concomitant methane mitigation (consistent with SSP1’s stringent climate change mitigation policy implemented in the SSP3 world) would not only alleviate this warming but would turn this into a cooling of 0.07 °C with a ''likely'' range of [–0.02 to +0.14] °C (compared with SSP3-7.0 in 2040) . Across the SSPs, the collective reduction of methane, ozone precursors and HFCs can make a difference of 0.2°C with a ''very likely'' range of [0.1 to 0.4] °C in 2040 and 0.8°C with a ''very likely'' range of [0.5 to 1.3] °C at the end of the 21 st century (comparing SSP3-7.0 and SSP1-1.9 ), which is substantial in the context of the Paris Agreement. Sustained methane mitigation, wherever it occurs, stands out as an option that combines near- and long-term gains on surface temperature ( ''high confidence'' ) and leads to air-quality benefits by reducing surface ozone levels globally ( ''high confidence'' ). {6.6.3, 6.7.3, 4.4.4}

'''Rapid decarbonization strategies lead to air-quality improvements but are not sufficient to achieve, in the near term, air-quality guidelines set for fine PM by the World Health Organization (WHO), especially in parts of Asia and in some other highly polluted regions''' ( ''high confidence'' ''').''' Additional methane and BC mitigation would contribute to offsetting the additional warming associated with SO <sub>2</sub> reductions that would accompany decarbonization ( ''high confidence'' ). Strong air pollution control as well as strong climate change mitigation, implemented separately, lead to large reductions in exposure to air pollution by the end of the century ( ''high confidence'' ). Implementation of air pollution controls, relying on the deployment of existing technologies, leads more rapidly to air quality benefits than climate change mitigation ( ''high confidence'' ), which requires systemic changes. However, in both cases, significant parts of the population are projected to remain exposed to air pollution exceeding the WHO guidelines ( ''high confidence'' ). Additional policies envisaged to attain Sustainable Development Goals (SDGs; e.g., access to clean energy, waste management) bring complementary SLCF reduction. Only strategies integrating climate, air quality, and development goals are found to effectively achieve multiple benefits. {6.6.3, 6.7.3, Box 6.2}

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