Editing IPCC:AR6/SR15/Chapter-2 (section)

==== 2.3.3.2 Emissions of short-lived climate forcers and fluorinated gases ====

<div id="section-2-3-3-2-block-1"></div>

SLCFs include shorter-lived GHGs like CH <sub>4</sub> and some fluorinated gases as well as particles (aerosols), their precursors and ozone precursors. SLCFs are strongly mitigated in 1.5°C pathways, as is the case for 2°C pathways (Figure 2.7). SLCF emissions ranges of 1.5°C and 2°C pathway classes strongly overlap, indicating that the main incremental mitigation contribution between 1.5°C and 2°C pathways comes from CO <sub>2</sub> (Luderer et al., 2018; Rogelj et al., 2018) <sup>[[#fn:r227|227]]</sup> . CO <sub>2</sub> and SLCF emissions reductions are connected in situations where SLCF and CO <sub>2</sub> are co-emitted by the same process, for example, with coal-fired power plants (Shindell and Faluvegi, 2010) <sup>[[#fn:r228|228]]</sup> or within the transport sector (Fuglestvedt et al., 2010) <sup>[[#fn:r229|229]]</sup> . Many CO <sub>2</sub> -targeted mitigation measures in industry, transport and agriculture (Sections 2.4.3–4) hence also reduce non-CO <sub>2</sub> forcing (Rogelj et al., 2014b; Shindell et al., 2016) <sup>[[#fn:r230|230]]</sup> .

Despite the fact that methane has a strong warming effect (Myhre et al., 2013; Etminan et al., 2016) <sup>[[#fn:r231|231]]</sup> , current 1.5°C-consistent pathways still project significant emissions of CH <sub>4</sub> by 2050, indicating only a limited CH <sub>4</sub> mitigation potential in IAM analyses (Gernaat et al., 2015) <sup>[[#fn:r232|232]]</sup> (Sections 2.3.1.2, 2.4.4, Table 2.SM.2). The AFOLU sector contributes an important share of the residual CH <sub>4</sub> emissions until mid-century, with its relative share increasing from slightly below 50% in 2010 to around 55–70% in 2030, and 60–80% in 2050 in 1.5°C-consistent pathways (interquartile range across 1.5°C-consistent pathways for projections). Many of the proposed measures to target CH <sub>4</sub> (Shindell et al., 2012; Stohl et al., 2015) <sup>[[#fn:r233|233]]</sup> are included in 1.5°C-consistent pathways (Figure 2.7), though not all (Sections 2.3.1.2, 2.4.4, Table 2.SM.2). A detailed assessment of measures to further reduce AFOLU CH <sub>4</sub> emissions has not been conducted.

Overall reductions of SLCFs can have effects of either sign on temperature depending on the balance between cooling and warming agents. The reduction in SO <sub>2</sub> emissions is the dominant single effect as it weakens the negative total aerosol forcing. This means that reducing all SLCF emissions to zero would result in a short-term warming, although this warming is ''unlikely'' to be more than 0.5°C (Section 2.2 and Figure 1.5 (Samset et al., 2018) <sup>[[#fn:r234|234]]</sup> ). Because of this effect, suggestions have been proposed that target the warming agents only (referred to as short-lived climate pollutants or SLCPs instead of the more general short-lived climate forcers; e.g., Shindell et al., 2012) <sup>[[#fn:r235|235]]</sup> , though aerosols are often emitted in varying mixtures of warming and cooling species (Bond et al., 2013) <sup>[[#fn:r236|236]]</sup> . Black carbon (BC) emissions reach similar levels across 1.5°C-consistent and 2°C-consistent pathways available in the literature, with interquartile ranges of emissions reductions across pathways of 16–34% and 48–58% in 2030 and 2050, respectively, relative to 2010 (Figure 2.7). Recent studies have identified further reduction potentials for the near term, with global reductions of about 80% being suggested (Stohl et al., 2015; Klimont et al., 2017) <sup>[[#fn:r237|237]]</sup> . Because the dominant sources of certain aerosol mixtures are emitted during the combustion of fossil fuels, the rapid phase-out of unabated fossil fuels to avoid CO <sub>2</sub> emissions would also result in removal of these either warming or cooling SLCF air-pollutant species. Furthermore, SLCFs are also reduced by efforts to reduce particulate air pollution. For example, year-2050 SO <sub>2</sub> emissions (precursors of sulphate aerosol) in 1.5°C-consistent pathways are about 75–85% lower than their 2010 levels. Some caveats apply, for example, if residential biomass use would be encouraged in industrialised countries in stringent mitigation pathways without appropriate pollution control measures, aerosol concentrations could also increase (Sand et al., 2015; Stohl et al., 2015) <sup>[[#fn:r238|238]]</sup> .

<div id="section-2-3-3-2-block-2"></div>

<span id="table-2.4"></span>
<!-- START TABLE -->
'''Table 2.4'''

'''Emissions in 2030, 2050 and 2100 in 1.5°C and 2°C scenario classes and absolute annual rate''' '''s of change between 2010–2030, 2020–2030 and 2030–2050, respectively.''' Values show median and interquartile range across available scenarios (25th and 75th percentile given in brackets). If fewer than seven scenarios are available (*), the minimum–maximum range is given instead. Kyoto-GHG emissions are aggregated with GWP-100 values from IPCC AR4. Emissions in 2010 for total net CO <sub>2</sub> , CO <sub>2</sub> from fossil-fuel use and industry, and AFOLU CO <sub>2</sub> are estimated at 38.5, 33.4, and 5 GtCO <sub>2</sub> yr−1, respectively (Le Quéré et al., 2018) <sup>[[#fn:r239|239]]</sup> . Percentage reduction numbers included in headline statement C.1 in the Summary for Policymakers are computed relative to 2010 emissions in each individual pathway, and hence differ slightly from a case where reductions are computed relative to the historical 2010 emissions reported above. A difference is reported in estimating the ‘anthropogenic’ sink by countries or the global carbon modelling community (Grassi et al., 2017) <sup>[[#fn:r240|240]]</sup> , and AFOLU CO <sub>2</sub> estimates reported here are thus not necessarily comparable with countries’ estimates. Scenarios with year-2010 Kyoto-GHG emissions outside the range assessed by IPCC AR5 WGIII are excluded (IPCC, 2014b) <sup>[[#fn:r241|241]]</sup> , as are scenario duplicates that would bias ranges towards a single study.
<!-- TABLE -->
{| class="wikitable"
|-
|
| colspan="3"| Annual emissions/sequestration<br />
(GtCO <sub>2</sub> yr <sup>-1</sup> )
| colspan="3"| Absolute Annual Change<br />
(GtCO <sub>2</sub> /yr <sup>–1</sup> )
| Timing of Global Zero
|-
| Name
| Category
| #
| 2030
| 2050
| 2100
| 2010–2030
| 2020–2030
| 2030–2050
| Year
|-
| rowspan="6"| Total CO <sub>2</sub> (net)
| Below-1.5°C
| 5*
| 13.4
(15.4, 11.4)

| –3.0
(1.7, –10.6)

| –8.0
(–2.6, –14.2)

| –1.2
(–1.0, –1.3)

| –2.5
(–1.8, –2.8)

| –0.8
(–0.7, –1.2)

| 2044
(2037, 2054)

|-
| 1.5°C-low-OS
| 37
| 20.8
(22.2, 18.0)

| –0.4
(2.7, –2.0)

| –10.8
(–8.1, –14.3)

| –0.8
(–0.7, –1.0)

| –1.7
(–1.4, –2.3)

| –1.0
(–0.8, –1.2)

| 2050
(2047, 2055)

|-
| 1.5°C with no or limited OS
| 42
| 20.3
(22.0, 15.9)

| –0.5
(2.2, –2.8)

| –10.2
(–7.6, –14.2)

| –0.9
(–0.7, –1.1)

| –1.8
(–1.5, –2.3)

| –1.0
(–0.8, –1.2)

| 2050
(2046, 2055)

|-
| 1.5°C-high-OS
| 36
| 29.1
(36.4, 26.0)

| 1.0
(6.3, –1.2)

| –13.8
(–11.1, –16.4)

| –0.4
(0.0, –0.6)

| –1.1
(–0.5, –1.5)

| –1.3
(–1.1, –1.8)

| 2052
(2049, 2059)

|-
| Lower-2°C
| 54
| 28.9
(33.7, 24.5)

| 9.9
(13.1, 6.5)

| –5.1
(–2.6, –10.3)

| –0.4
(–0.2, –0.6)

| –1.1
(–0.8, –1.6)

| –0.9
(–0.8, –1.2)

| 2070
(2063, 2079)

|-
| Higher-2°C
| 54
| 33.5
(35.0, 31.0)

| 17.9
(19.1, 12.2)

| –3.3
(0.6, –11.5)

| –0.2
(–0.0, –0.4)

| –0.7
(–0.5, –0.9)

| –0.8
(–0.6, –1.0)

| 2085
(2070, post–2100)

|-
| rowspan="6"| CO <sub>2</sub> from fossil fuels and industry<br />
(gross)
| Below-1.5°C
| 5*
| 18.0
(21.4, 13.8)

| 10.5
(20.9, 0.3)

| 8.3
(11.6, 0.1)

| –0.7
(–0.6, –1)

| –1.5
(–0.9, –2.2)

| –0.4
(0, –0.7)

| –
|-
| 1.5°C-low-OS
| 37
| 22.1
(24.4, 18.7)

| 10.3
(14.1, 7.8)

| 5.6
(8.1, 2.6)

| –0.5
(–0.4, –0.6)

| –1.3
(–0.9, –1.7)

| –0.6
(–0.5, –0.7)

| –
|-
| 1.5°C with no or limited OS
| 42
| 21.6
(24.2, 18.0)

| 10.3
(13.8, 7.7)

| 6.1
(8.4, 2.6)

| –0.5
(–0.4, –0.7)

| –1.3
(–0.9, –1.8)

| –0.6
(–0.4, –0.7)

| –
|-
| 1.5°C-high-OS
| 36
| 27.8
(37.1, 25.6)

| 13.1
(17.0, 11.6)

| 6.6
(8.8, 2.8)

| –0.2
(0.2, –0.3)

| –0.8
(–0.2, –1.1)

| –0.7
(–0.6, –1.0)

| –
|-
| Lower-2°C
| 54
| 27.7
(31.5, 23.5)

| 15.4
(19.0, 11.1)

| 7.2
(10.4, 3.7)

| –0.2
(–0.0, –0.4)

| –0.8
(–0.5, –1.2)

| –0.6
(–0.5, –0.8)

| –
|-
| Higher-2°C
| 54
| 31.3
(33.4, 28.7)

| 19.2
(22.6, 17.1)

| 8.1
(10.9, 5.0)

| –0.1
(0.1, –0.2)

| –0.5
(–0.2, –0.7)

| –0.6
(–0.5, –0.7)

| –
|-
| rowspan="6"| CO <sub>2</sub> from fossil fuels and industry (net)
| Below-1.5°C
| 5*
| 16.4
(18.2, 13.5)

| 1.0
(7.0, 0)

| –2.7
(0, –9.8)

| –0.8
(–0.7, –1)

| –1.8
(–1.2, –2.2)

| –0.6
(–0.5, –0.9)

| –
|-
| 1.5°C-low-OS
| 37
| 20.6
(22.2, 17.5)

| 3.2
(5.6, –0.6)

| –8.5
(–4.1, –11.6)

| –0.6
(–0.5, –0.7)

| –1.4
(–1.1, –1.8)

| –0.8
(–0.7, –1.1)

| –
|-
| 1.5°C with no or limited OS
| 42
| 20.1
(22.1, 16.8)

| 3.0
(5.6, 0.0)

| –8.3
(–3.5, –10.8)

| –0.6
(–0.5, –0.8)

| –1.4
(–1.1, –1.9)

| –0.8
(–0.7, –1.1)

| –
|-
| 1.5°C-high-OS
| 36
| 26.9 (34.7, 25.3)
| 4.2 (10.0, 1.2)
| –10.7
(–6.9, –13.2)

| –0.3
(0.1, –0.3)

| –0.9
(–0.3, –1.2)

| –1.2
(–0.9, –1.5)

| –
|-
| Lower-2°C
| 54
| 28.2
(31.0, 23.1)

| 11.8
(14.1, 6.2)

| –3.1
(–0.7, –6.4)

| –0.2
(–0.1, –0.4)

| –0.8
(–0.5, –1.2)

| –0.8
(–0.7, –1.0)

| –
|-
| Higher-2°C
| 54
| 31.0
(33.0, 28.7)

| 17.0
(19.3, 13.1)

| –2.9
(3.3, –8.0)

| –0.1
(0.1, –0.2)

| –0.5
(–0.2, –0.7)

| –0.7
(–0.5, –1.0)

| –
|-
| rowspan="6"| CO <sub>2</sub> from AFOLU
| Below-1.5°C
| 5*
| –2.2
(–0.3, –4.8)

| –4.4
(–1.2, –11.1)

| –4.4
(–2.6, –5.3)

| –0.3
(–0.2, –0.4)

| –0.5
(–0.4, –0.8)

| –0.1
(0, –0.4)

| –
|-
| 1.5°C-low-OS
| 37
| –0.1
(0.8, –1.0)

| –2.3
(–0.6, –4.1)

| –2.4
(–1.2, –4.2)

| –0.2
(–0.2, –0.3)

| –0.4
(–0.3, –0.5)

| –0.1
(–0.1, –0.2)

| –
|-
| 1.5°C with no or limited OS
| 42
| –0.1
(0.7, –1.3)

| –2.6
(–0.6, –4.5)

| –2.6
(–1.3, –4.2)

| –0.2
(–0.2, –0.3)

| –0.4
(–0.3, –0.5)

| –0.1
(–0.1, –0.2)

| –
|-
| 1.5°C-high-OS
| 36
| 1.2
(2.7, 0.1)

| –2.1
(–0.3, –5.4)

| –2.4
(–1.5, –5.0)

| –0.1
(–0.1, –0.3)

| –0.2
(–0.1, –0.5)

| –0.2
(–0.0, –0.3)

| –
|-
| Lower-2°C
| 54
| 1.4
(2.8, 0.3)

| –1.4
(–0.5, –2.7)

| –2.4
(–1.3, –4.2)

| –0.2
(–0.1, –0.2)

| –0.3
(–0.2, –0.4)

| –0.1
(–0.1, –0.2)

| –
|-
| Higher-2°C
| 54
| 1.5
(2.7, 0.8)

| –0.0
(1.9, –1.6)

| –1.3
(0.1, –3.9)

| –0.2
(–0.1, –0.2)

| –0.2
(–0.1, –0.4)

| –0.1
(–0.0, –0.1)

| –
|-
| rowspan="6"| Bioenergy<br />
combined with carbon capture and storage (BECCS)
| Below-1.5°C
| 5*
| 0.4
(1.1, 0)

| 3.4
(8.3, 0)

| 5.7
(13.4, 0)

| 0
(0.1, 0)

| 0
(0.1, 0)

| 0.2
(0.4, 0)

| –
|-
| 1.5°C-low-OS
| 36
| 0.3
(1.1, 0.0)

| 4.6
(6.4, 3.8)

| 12.4
(15.6, 7.6)

| 0.0
(0.1, 0.0)

| 0.0
(0.1, 0.0)

| 0.2
(0.3, 0.2)

| –
|-
| 1.5°C with no or limited OS
| 41
| 0.4
(1.0, 0.0)

| 4.5
(6.3, 3.4)

| 12.4
(15.0, 6.4)

| 0.0
(0.1, 0.0)

| 0.0
(0.1, 0.0)

| 0.2
(0.3, 0.2)

| –
|-
| 1.5°C-high-OS
| 36
| 0.1
(0.4, 0.0)

| 6.8
(9.5, 3.7)

| 14.9
(16.3, 12.1)

| 0.0
(0.0, 0.0)

| 0.0
(0.0, 0.0)

| 0.3
(0.4, 0.2)

| –
|-
| Lower-2°C
| 54
| 0.1
(0.3, 0.0)

| 3.6
(4.6, 1.8)

| 9.5
(12.1, 6.9)

| 0.0
(0.0, 0.0)

| 0.0
(0.0, 0.0)

| 0.2
(0.2, 0.1)

| –
|-
| Higher-2°C
| 47
| 0.1
(0.2, 0.0)

| 3.0
(4.9, 1.6)

| 10.8
(15.3, 8.2) [46]

| 0.0
(0.0, 0.0)

| 0.0
(0.0, 0.0)

| 0.1
(0.2, 0.1)

| –
|-
| rowspan="6"| Kyoto GHG (AR4) [GtCO <sub>2</sub> e]
| Below-1.5°C
| 5*
| 22.1
(22.8, 20.7)

| 2.7
(8.1, –3.5)

| –2.6
(2.7, –10.7)

| –1.4
(–1.3, –1.5)

| –2.9
(–2.1, –3.3)

| –0.9
(–0.7, –1.3)

| 2066
(2044, post–2100)

|-
| 1.5°C-low-OS
| 31
| 27.9
(31.1, 26.0)

| 7.0
(9.9, 4.5)

| –3.8
(–2.1, –7.9)

| –1.1
(–0.9, –1.2)

| –2.3
(–1.8, –2.8)

| –1.1
(–0.9, –1.2)

| 2068
(2061, 2080)

|-
| 1.5°C with no or limited OS
| 36
| 27.4
(30.9, 24.7)

| 6.5
(9.6, 4.2)

| –3.7
(–1.8, –7.8)

| –1.1
(–1.0, –1.3)

| –2.4
(–1.9, –2.9)

| –1.1
(–0.9, –1.2)

| 2067
(2061, 2084)

|-
| 1.5°C-high-OS
| 32
| 40.4
(48.9, 36.3)

| 8.4
(12.3, 6.2)

| –8.5
(–5.7, –11.2)

| –0.5
(–0.0, –0.7)

| –1.3
(–0.6, –1.8)

| –1.5
(–1.3, –2.1)

| 2063
(2058, 2067)

|-
| Lower-2°C
| 46
| 39.6
(45.1, 35.7)

| 18.3
(20.4, 15.2)

| 2.1
(4.2, –2.4)

| –0.5
(–0.1, –0.7)

| –1.5
(–0.9, –2.2)

| –1.1
(–0.9, –1.2)

| post–2100
(2090 post–2100)

|-
| Higher-2°C
| 42
| 45.3
(48.5, 39.3)

| 25.9
(27.9, 23.3)

| 5.2
(11.5, –4.8)

| –0.2
(–0.0, –0.6)

| –1.0
(–0.6, –1.2)

| –1.0
(–0.7, –1.2)

| post–2100
(2085 post–2100)

|}
<!-- END TABLE -->

<div id="section-2-3-3-2-block-3"></div>

Emissions of fluorinated gases (IPCC/TEAP, 2005; US EPA, 2013; Velders et al., 2015; Purohit and Höglund-Isaksson, 2017) <sup>[[#fn:r242|242]]</sup> in 1.5°C-consistent pathways are reduced by roughly 75–80% relative to 2010 levels (interquartile range across 1.5°C-consistent pathways) in 2050, with no clear differences between the classes. Although unabated hydrofluorocarbon (HFC) emissions have been projected to increase (Velders et al., 2015) <sup>[[#fn:r243|243]]</sup> , the Kigali Amendment recently added HFCs to the basket of gases controlled under the Montreal Protocol (Höglund-Isaksson et al., 2017) <sup>[[#fn:r244|244]]</sup> . As part of the larger group of fluorinated gases, HFCs are also assumed to decline in 1.5°C-consistent pathways. Projected reductions by 2050 of fluorinated gases under 1.5°C-consistent pathways are deeper than published estimates of what a full implementation of the Montreal Protocol including its Kigali Amendment would achieve (Höglund-Isaksson et al., 2017) <sup>[[#fn:r245|245]]</sup> , which project roughly a halving of fluorinated gas emissions in 2050 compared to 2010. Assuming the application of technologies that are currently commercially available and at least to a limited extent already tested and implemented, potential fluorinated gas emissions reductions of more than 90% have been estimated (Höglund-Isaksson et al., 2017) <sup>[[#fn:r246|246]]</sup> .

There is a general agreement across 1.5°C-consistent pathways that until 2030 forcing from the warming SLCFs is reduced less strongly than the net cooling forcing from aerosol effects, compared to 2010. As a result, the net forcing contributions from all SLCFs combined are projected to increase slightly by about 0.2–0.3 W m <sup>−2</sup> , compared to 2010. Also, by the end of the century, about 0.1–0.3 W m <sup>−2</sup> of SLCF forcing is generally currently projected to remain in 1.5°C-consistent scenarios (Figure 2.8). This is similar to developments in 2°C-consistent pathways (Rose et al., 2014b; Riahi et al., 2017) <sup>[[#fn:r247|247]]</sup> , which show median forcing contributions from these forcing agents that are generally no more than 0.1 W m <sup>−2</sup> higher. Nevertheless, there can be additional gains from targeted deeper reductions of CH <sub>4</sub> emissions and tropospheric ozone precursors, with some scenarios projecting less than 0.1 W m <sup>−2</sup> forcing from SLCFs by 2100.

<div id="section-2-3-3-2-block-4"></div>

<span id="figure-2.7"></span>
<!-- START IMG -->
<!-- IMG TITLE -->
'''Figure 2.7'''

<span id="section-5"></span>
<!-- IMG CAPTION -->
Global characteristics of a selection of short-lived non-CO <sub>2 </sub> emissions until mid-century for five pathway classes used in this chapter.
<!-- IMG FILE -->
[[File:50f492894a82ac749d8e7191b365c452 Figure-2.7-1024x670.jpg]]

Data are shown for (a) methane (CH4), (b) fluorinated gases (F-gas), (c) black carbon (BC), and (d) sulphur dioxide (SO2) emissions. Boxes with different colours refer to different scenario classes. Icons on top the ranges show four illustrative pathway archetypes that apply different mitigation strategies for limiting warming to 1.5°C. Boxes show the interquartile range, horizontal black lines the median, and whiskers the minimum–maximum range. F-gases are expressed in units of CO <sub>2</sub> -equivalence computed with 100-year Global Warming Potentials reported in IPCC AR4.

Original Creation for this Report using IAMC 1.5°C Scenario Data hosted by IIASA

<!-- END IMG -->
<div id="section-2-3-3-2-block-5"></div>

<span id="figure-2.8"></span>
<!-- START IMG -->
<!-- IMG TITLE -->
'''Figure 2.8'''

<span id="estimated-aggregated-effective-radiative-forcing-of-slcfs-for-1.5c-and-2c-pathway-classes-in-2010-2020-2030-2050-and-2100-as-estimated-by-the-fair-model-smith-et-al.-2018-248-."></span>
<!-- IMG CAPTION -->
'''Estimated aggregated effective radiative forcing of SLCFs for 1.5°C and 2°C pathway classes in 2010, 2020, 2030, 2050, and 2100, as estimated by the FAIR model (Smith et al., 2018) <sup>[[#fn:r248|248]]</sup> .'''
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Aggregated short-lived climate forcer (SLCF) radiative forcing is estimated as the difference between total anthropogenic radiative forcing and the sum of CO <sub>0</sub> and N2 <sub>0</sub> radiative forcing over time, and is expressed relative to 1750. Symbols indicate the four pathways archetypes used in this chapter. Horizontal black lines indicate the median, boxes the interquartile range, and whiskers the minimum–maximum range per pathway class. Because very few pathways fall into the Below-1.5°C class, only the minimum–maximum is provided here.

Original Creation for this Report using IAMC 1.5°C Scenario Data hosted by IIASA

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