Short-lived Climate Forcers

and span a range from very high to very low emissions.In the scenarios without climate change mitigation (SSP3-7.0 and SSP5-8.5) the likely range of the estimated warming due to SLCFs in 2100 relative to 2019 is 0.4°C-0.9°C{6.7.3, 6.7.4}.In SSP3-7.0 there is a near-linear warming due to SLCFs of 0.08°C per decade, while for SSP5-8.5 there is a more rapid warming in the first half of the century.For the scenarios considering the most stringent climate and air pollution mitigations (SSP1-1.9 and SSP1-2.6), the reduced warming from reductions in methane, ozone and HFCs partly balances the warming from reduced aerosols, and the overall SLCF effect is a likely increase in GSAT of 0.0°C-0.3°C in 2100, relative to 2019.The SSP2-4.5 scenario (with moderate climate change and air pollution mitigations) results in a likely warming of 0.2°C-0.5°C in 2100 due to SLCFs, with the largest warming from reductions in aerosols.{6.7.3} Potential Effects of SLCF MitigationOver 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 2 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 2 warming in the case of fossil fuel combustion for energy and land transportation, or completely offset the CO 2 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 2 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 2 (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 2 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 2 (high confidence).Current emissions of SLCFs, CO 2 and N 2 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 2.5 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 OzoneDepleting 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-globalwarming-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 21st 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}