Editing IPCC:AR6/SR15/Chapter-4 (section)

=== 4.3.6 Short-Lived Climate Forcers ===

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The main short-lived climate forcer (SLCF) emissions that cause warming are methane (CH <sub>4</sub> ), other precursors of tropospheric ozone (i.e., carbon monoxide (CO), non-methane volatile organic compounds (NMVOC), black carbon (BC) and hydrofluorocarbons (HFCs); Myhre et al., 2013) <sup>[[#fn:r561|561]]</sup> . SLCFs also include emissions that lead to cooling, such as sulphur dioxide (SO <sub>2</sub> ) and organic carbon (OC). Nitrogen oxides (NOx) can have both warming and cooling effects, by affecting ozone (O <sub>3</sub> ) and CH <sub>4</sub> , depending on time scale and location (Myhre et al., 2013) <sup>[[#fn:r562|562]]</sup> .

Cross-Chapter Box 2 in Chapter 1 provides a discussion of role of SLCFs in comparison to long-lived GHGs. Chapter 2 shows that 1.5°C-consistent pathways require stringent reductions in CO <sub>2</sub> and CH <sub>4</sub> , and that non-CO <sub>2</sub> climate forcers reduce carbon budgets by about 2200 GtCO <sub>2</sub> per degree of warming attributed to them (see the Supplementary Material to Chapter 2).

Reducing non-CO <sub>2</sub> emissions is part of most mitigation pathways (IPCC, 2014c) <sup>[[#fn:r563|563]]</sup> . All current GHG emissions and other forcing agents affect the rate and magnitude of climate change over the next few decades, while long-term warming is mainly driven by CO <sub>2</sub> emissions. CO <sub>2</sub> emissions result in a virtually permanent warming, while temperature change from SLCFs disappears within decades after emissions of SLCFs are ceased. Any scenario that fails to reduce CO <sub>2</sub> emissions to net zero would not limit global warming, even if SLCFs are reduced, due to accumulating CO <sub>2</sub> -induced warming that overwhelms SLCFs’ mitigation benefits in a couple of decades (Shindell et al., 2012; Schmale et al., 2014) <sup>[[#fn:r564|564]]</sup> (and see Chapter 2, Section 2.3.3.2).

Mitigation options for warming SLCFs often overlap with other mitigation options, especially since many warming SLCFs are co-emitted with CO <sub>2</sub> . SLCFs are generally mitigated in 1.5°C- or 2°C-consistent pathways as an integral part of an overall mitigation strategy (Chapter 2). For example Section 2.3 indicates that most very-low-emissions pathways include a transition away from the use of coal and natural gas in the energy sector and oil in transportation, which coincides with emission-reduction strategies related to methane from the fossil fuel sector and BC from the transportation sector. Much SLCF emission reduction aims at BC-rich sectors and considers the impacts of several co-emitted SLCFs (Bond et al., 2013; Sand et al., 2015; Stohl et al., 2015) <sup>[[#fn:r565|565]]</sup> . The benefits of such strategies depend greatly upon the assumed level of progression of access to modern energy for the poorest populations who still rely on biomass fuels, as this affects the reference level of BC emissions (Rogelj et al., 2014) <sup>[[#fn:r566|566]]</sup> .

Some studies have evaluated the focus on SLCFs in mitigation strategies and point towards trade-offs between short-term SLCF benefits and lock-in of long-term CO <sub>2</sub> warming (Smith and Mizrahi, 2013; Pierrehumbert, 2014) <sup>[[#fn:r567|567]]</sup> . Reducing fossil fuel combustion will reduce aerosols levels, and thereby cause warming from removal of aerosol cooling effects (Myhre et al., 2013; Xu and Ramanathan, 2017; Samset et al., 2018) <sup>[[#fn:r568|568]]</sup> . While some studies have found a lower temperature effect from BC mitigation, thus questioning the effectiveness of targeted BC mitigation for climate change mitigation (Myhre et al., 2013; Baker et al., 2015; Stjern et al., 2017; Samset et al., 2018) <sup>[[#fn:r569|569]]</sup> , other models and observationally constrained estimates suggest that these widely-used models do not fully capture observed effects of BC and co-emissions on climate (e.g., Bond et al., 2013; Cui et al., 2016; Peng et al., 2016) <sup>[[#fn:r570|570]]</sup> .

Table 4.5 provides an overview of three warming SLCFs and their emission sources, with examples of options for emission reductions and associated co-benefits.

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'''Table 4.5'''

Overview of main characteristics of three warming short-lived climate forcers (SLCFs) (core information based on Pierrehumbert, 2014 <sup>[[#fn:r571|571]]</sup> and Schmale et al., 2014 <sup>[[#fn:r572|572]]</sup> ; rest of the details as referenced)

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{| class="wikitable"
|-
! SLCF Compound
! Atmospheric Lifetime
! Annual Global Emission
! Main Anthropogenic Emission Sources
! Examples of Options to Reduce Emissions Consistent with 1.5°C
! Examples of Co-Benefits Based on Haines et al. (2017) <sup>[[#fn:r1537|1537]]</sup> Unless Specified Otherwise
|-
| Methane
| On the order of 10 years
| 0.3 GtCH <sub>4</sub> (2010)<br />
(Pierrehumbert, 2014)  <sup>[[#fn:r574|574]]</sup>
| Fossil fuel extraction and transportation;<br />
Land-use change;<br />
Livestock and rice cultivation; Waste and wastewater
| Managing manure from livestock; Intermittent irrigation of rice;<br />
Capture and usage of fugitive methane;<br />
Dietary change;<br />
For more: see Section 4.3.2
| Reduction of tropospheric ozone (Shindell et al., 2017a);  <sup>[[#fn:r575|575]]</sup><br />
Health benefits of dietary changes; Increased crop yields;<br />
Improved access to drinking water
|-
| HFCs
| Months to decades, depending on the gas
| 0.35 GtCO <sub>2</sub> -eq (2010)(Velders et al., 2015)  <sup>[[#fn:r576|576]]</sup>
| Air conditioning; Refrigeration; Construction material
| Alternatives to HFCs in air-conditioning and refrigeration applications
| Greater energy efficiency (Mota-Babiloni et al., 2017)  <sup>[[#fn:r577|577]]</sup>
|-
| Black Carbon
| Days
| ~7 Mt (2010) (Klimont et al., 2017)  <sup>[[#fn:r578|578]]</sup>
| Incomplete combustion of fossil fuels or biomass in vehicles (esp. diesel), cook stoves or kerosene lamps;<br />
Field and biomass burning
| Fewer and cleaner vehicles; Reducing agricultural biomass burning;<br />
Cleaner cook stoves, gas-based<br />
or electric cooking;<br />
Replacing brick and coke ovens;<br />
Solar lamps;<br />
For more see Section 4.3.3
| Health benefits of better air quality;<br />
Increased education opportunities;<br />
Reduced coal consumption for modern brick kilns;<br />
Reduced deforestation
|}
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A wide range of options to reduce SLCF emissions was extensively discussed in AR5 (IPCC, 2014b) <sup>[[#fn:r579|579]]</sup> . Fossil fuel and waste sector methane mitigation options have high cost-effectiveness, producing a net profit over a few years, considering market costs only. Moreover, reducing roughly one-third to one-half of all human-caused emissions has societal benefits greater than mitigation costs when considering environmental impacts only (UNEP, 2011; Höglund-Isaksson, 2012; IEA, 2017b; Shindell et al., 2017a) <sup>[[#fn:r580|580]]</sup> . Since AR5, new options for methane, such as those related to shale gas, have been included in mitigation portfolios (e.g., Shindell et al., 2017a) <sup>[[#fn:r581|581]]</sup> .

Reducing BC emissions and co-emissions has sustainable development co-benefits, especially around human health (Stohl et al., 2015; Haines et al., 2017; Aakre et al., 2018) <sup>[[#fn:r582|582]]</sup> , avoiding premature deaths and increasing crop yields (Scovronick et al., 2015; Peng et al., 2016) <sup>[[#fn:r583|583]]</sup> . Additional benefits include lower likelihood of non-linear climate changes and feedbacks (Shindell et al., 2017b) <sup>[[#fn:r584|584]]</sup> and temporarily slowing down the rate of sea level rise (Hu et al., 2013) <sup>[[#fn:r585|585]]</sup> . Interventions to reduce BC offer tangible local air quality benefits, increasing the likelihood of local public support (Eliasson, 2014; Venkataraman et al., 2016) <sup>[[#fn:r586|586]]</sup> (see Chapter 5, Section 5.4.2.1). Limited interagency co-ordination, poor science-policy interactions (Zusman et al., 2015) <sup>[[#fn:r587|587]]</sup> , and weak policy and absence of inspections and enforcement (Kholod and Evans, 2016) <sup>[[#fn:r588|588]]</sup> are among barriers that reduce the institutional feasibility of options to reduce vehicle-induced BC emissions. A case study for India shows that switching from biomass cook stoves to cleaner gas stoves (based on liquefied petroleum gas or natural gas) or to electric cooking stoves is technically and economically feasible in most areas, but faces barriers in user preferences, costs and the organization of supply chains (Jeuland et al., 2015) <sup>[[#fn:r589|589]]</sup> . Similar feasibility considerations emerge in switching from kerosene wick lamps for lighting to solar lanterns, from current low-efficiency brick kilns and coke ovens to cleaner production technologies; and from field burning of crop residues to agricultural practices using deep-sowing and mulching technologies (Williams et al., 2011; Wong, 2012) <sup>[[#fn:r590|590]]</sup> .

The radiative forcing from HFCs are currently small but have been growing rapidly (Myhre et al., 2013) <sup>[[#fn:r591|591]]</sup> . <sub> </sub> The Kigali Amendment (from 2016) to the Montreal Protocol set out a global accord for phasing out these compounds (Höglund-Isaksson et al., 2017) <sup>[[#fn:r592|592]]</sup> . HFC mitigation options include alternatives with reduced warming effects, ideally combined with improved energy efficiency so as to simultaneously reduce CO <sub>2</sub> and co-emissions (Shah et al., 2015) <sup>[[#fn:r593|593]]</sup> . Costs for most of HFC’s mitigation potential are estimated to be below USD <sub>2010</sub> 60 tCO <sub>2</sub> -eq <sup>−1</sup> , and the remainder below roughly double that number (Höglund-Isaksson et al., 2017) <sup>[[#fn:r594|594]]</sup> .

Reductions in SLCFs can provide large benefits towards sustainable development, beneficial for social, institutional and economic feasibility. Strategies that reduce SLCFs can provide benefits that include improved air quality (e.g., Anenberg et al., 2012) <sup>[[#fn:r595|595]]</sup> and crop yields (e.g., Shindell et al., 2012) <sup>[[#fn:r596|596]]</sup> , energy access, gender equality and poverty eradication (e.g.,Shindell et al., 2012; Haines et al., 2017) <sup>[[#fn:r597|597]]</sup> . Institutional feasibility can be negatively affected by an information deficit, with the absence of international frameworks for integrating SLCFs into emissions accounting and reporting mechanisms being a barrier to developing policies for addressing SLCF emissions (Venkataraman et al., 2016) <sup>[[#fn:r598|598]]</sup> . The incentives for reducing SLCFs are particularly strong for small groups of countries, and such collaborations could increase the feasibility and effectiveness of SLCF mitigation options (Aakre et al., 2018) <sup>[[#fn:r599|599]]</sup> .

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