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

=== 2.3.3 Emissions Evolution in 1.5°C Pathways ===

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

This section assesses the salient temporal evolutions of climate forcers over the 21st century. It uses the classification of 1.5°C pathways presented in Section 2.1, which includes a Below-1.5°C class, as well as other classes with varying levels of projected overshoot (1.5°C-low-OS and 1.5°C-high-OS). First, aggregate-GHG benchmarks for 2030 are assessed. Subsequent sections assess long-lived climate forcers (LLCF) and short-lived climate forcers (SLCF) separately because they contribute in different ways to near-term, peak and long-term warming (Section 2.2, Cross-Chapter Box 2 in Chapter 1).

Estimates of aggregated GHG emissions in line with specific policy choices are often compared to near-term benchmark values from mitigation pathways to explore their consistency with long-term climate goals (Clarke et al., 2014; UNEP, 2016, 2017; UNFCCC, 2016) <sup>[[#fn:r210|210]]</sup> . Benchmark emissions or estimates of peak years derived from IAMs provide guidelines or milestones that are consistent with achieving a given temperature level. While they do not set mitigation requirements in a strict sense, exceeding these levels in a given year almost invariably increases the mitigation challenges afterwards by increasing the rates of change and increasing the reliance on speculative technologies, including the possibility that its implementation becomes unachievable (see Cross-Chapter Box 3 in Chapter 1 for a discussion of feasibility concepts) (Luderer et al., 2013; Rogelj et al., 2013b; Clarke et al., 2014; Fawcett et al., 2015; Riahi et al., 2015; Kriegler et al., 2018a) <sup>[[#fn:r211|211]]</sup> . These trade-offs are particularly pronounced in 1.5°C pathways and are discussed in Section 2.3.5. This section assesses Kyoto-GHG emissions in 2030 expressed in CO <sub>2</sub> equivalent (CO <sub>2</sub> e) emissions using 100-year global warming potentials. <sup>[[#fn:3|3]]</sup>

Appropriate benchmark values of aggregated GHG emissions depend on a variety of factors. First and foremost, they are determined by the desired likelihood to keep warming below 1.5°C and the extent to which projected temporary overshoot is to be avoided (Sections 2.2, 2.3.2, and 2.3.5). For instance, median aggregated 2030 GHG emissions are about 10 GtCO <sub>2</sub> e yr <sup>−1</sup> lower in 1.5°C-low-OS compared to 1.5°C-high-OS pathways, with respective interquartile ranges of 26–31 and 36–49 GtCO <sub>2</sub> e yr <sup>−1</sup> (Table 2.4). These ranges correspond to about 25–30 and 35–48 GtCO <sub>2</sub> e yr <sup>−1</sup> in 2030, respectively, when aggregated with 100-year Global Warming Potentials from the IPCC Second Assessment Report. The limited evidence available for pathways aiming to limit warming below 1.5°C without overshoot or with limited amounts of CDR (Grubler et al., 2018; Holz et al., 2018b; van Vuuren et al., 2018) <sup>[[#fn:r212|212]]</sup> indicates that under these conditions consistent emissions in 2030 would fall at the lower end and below the above mentioned ranges. Due to the small number of 1.5°C pathways with no overshoot in the report’s database (Table 2.4) and the potential for a downward bias in the selection of underlying scenario assumptions, the headline range for 1.5°C pathways with no or limited overshoot is also assessed to be of the order of 25–30 GtCO <sub>2</sub> e yr <sup>−1</sup> . Ranges for the 1.5°C-low-OS and Lower-2°C classes only overlap outside their interquartile ranges, highlighting the more accelerated reductions in 1.5°C-consistent compared to 2°C-consistent pathways.

Appropriate emissions benchmark values also depend on the acceptable or desired portfolio of mitigation measures, representing clearly identified trade-offs and choices (Sections 2.3.4, 2.4, and 2.5.3) (Luderer et al., 2013; Rogelj et al., 2013a; Clarke et al., 2014; Krey et al., 2014a; Strefler et al., 2018b) <sup>[[#fn:r213|213]]</sup> . For example, lower 2030 GHG emissions correlate with a lower dependence on the future availability and desirability of CDR (Strefler et al., 2018b) <sup>[[#fn:r214|214]]</sup> . On the other hand, pathways that assume or anticipate only limited deployment of CDR during the 21st century imply lower emissions benchmarks over the coming decades, which are achieved in models through further reducing CO <sub>2</sub> emissions in the coming decades. The pathway archetypes used in the chapter illustrate this further (Figure 2.6). Under middle-of-the-road assumptions of technological and socioeconomic development, pathway ''S2'' suggests emission benchmarks of 34, 12 and −8 GtCO <sub>2</sub> e yr <sup>−1</sup> in the years 2030, 2050, and 2100, respectively. In contrast, a pathway that further limits overshoot and aims at eliminating the reliance on negative emissions technologies like BECCS as well as CCS (here labelled as the ''LED'' pathway) shows deeper emissions reductions in 2030 to limit the cumulative amount of CO <sub>2</sub> until net zero global CO <sub>2</sub> emissions (carbon neutrality). The ''LED'' pathway here suggests emission benchmarks of 25, 9 and 2 GtCO <sub>2</sub> e yr <sup>−1</sup> in the years 2030, 2050, and 2100, respectively. However, a pathway that allows and plans for the successful large-scale deployment of BECCS by and beyond 2050 ( ''S5'' ) shows a shift in the opposite direction. The variation within and between the abovementioned ranges of 2030 GHG benchmarks hence depends strongly on societal choices and preferences related to the acceptability and availability of certain technologies.

Overall these variations do not strongly affect estimates of the 1.5°C-consistent timing of global peaking of GHG emissions. Both Below-1.5°C and 1.5°C-low-OS pathways show minimum–maximum ranges in 2030 that do not overlap with 2020 ranges, indicating the global GHG emissions peaked before 2030 in these pathways. Also, 2020 and 2030 GHG emissions in 1.5°C-high-OS pathways only overlap outside their interquartile ranges.

Kyoto-GHG emission reductions are achieved by reductions in CO <sub>2</sub> and non-CO <sub>2</sub> GHGs. The AR5 identified two primary factors that influence the depth and timing of reductions in non-CO <sub>2</sub> Kyoto-GHG emissions: (i) the abatement potential and costs of reducing the emissions of these gases and (ii) the strategies that allow making trade-offs between them (Clarke et al., 2014) <sup>[[#fn:r215|215]]</sup> . Many studies indicate low-cost, near-term mitigation options in some sectors for non-CO <sub>2</sub> gases compared to supply-side measures for CO <sub>2</sub> mitigation (Clarke et al., 2014) <sup>[[#fn:r216|216]]</sup> . A large share of this potential is hence already exploited in mitigation pathways in line with 2°C. At the same time, by mid-century and beyond, estimates of further reductions of non-CO <sub>2</sub> Kyoto-GHGs – in particular CH <sub>4</sub> and N <sub>2</sub> O – are hampered by the absence of mitigation options in the current generation of IAMs, which are hence not able to reduce residual emissions of sources linked to livestock production and fertilizer use (Clarke et al., 2014; Gernaat et al., 2015) <sup>[[#fn:r217|217]]</sup> (Sections 2.3.1.2, 2.4.4, Supplementary Material  2.SM.1.2). Therefore, while net CO <sub>2</sub> emissions are projected to be markedly lower in 1.5°C-consistent compared to 2°C-consistent pathways, this is much less the case for methane (CH <sub>4</sub> ) and nitrous-oxide (N <sub>2</sub> O) (Figures 2.6–2.7). This results in reductions of CO <sub>2</sub> being projected to take up the largest share of emissions reductions when moving between 1.5°C-consistent and 2°C-consistent pathways (Rogelj et al., 2015b, 2018; Luderer et al., 2018) <sup>[[#fn:r218|218]]</sup> . If additional non-CO <sub>2</sub> mitigation measures are identified and adequately included in IAMs, they are expected to further contribute to mitigation efforts by lowering the floor of residual non-CO <sub>2</sub> emissions. However, the magnitude of these potential contributions has not been assessed as part of this report.

As a result of the interplay between residual CO <sub>2</sub> and non-CO <sub>2</sub> emissions and CDR, global GHG emissions reach net zero levels at different times in different 1.5°C-consistent pathways. Interquartile ranges of the years in which 1.5°C-low-OS and 1.5°C-high-OS reach net zero GHG emissions range from 2060 to 2080 (Table 2.4). A seesaw characteristic can be found between near-term emissions reductions and the timing of net zero GHG emissions. This is because pathways with limited emissions reductions in the next one to two decades require net negative CO <sub>2</sub> emissions later on (see earlier). Most 1.5°C-high-OS pathways lead to net zero GHG emissions in approximately the third quarter of this century, because all of them rely on significant amounts of annual net negative CO <sub>2</sub> emissions in the second half of the century to decline temperatures after overshoot (Table 2.4). However, in pathways that aim at limiting overshoot as much as possible or more slowly decline temperatures after their peak, emissions reach the point of net zero GHG emissions slightly later or at times never. Early emissions reductions in this case reduce the requirement for net negative CO <sub>2</sub> emissions. Estimates of 2030 GHG emissions in line with the current NDCs overlap with the highest quartile of 1.5°C-high-OS pathways (Cross-Chapter Box 9 in Chapter 4).

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

<span id="emissions-of-long-lived-climate-forcers"></span>
==== 2.3.3.1 Emissions of long-lived climate forcers ====

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

Climate effects of long-lived climate forcers (LLCFs) are dominated by CO <sub>2</sub> , with smaller contributions of N <sub>2</sub> O and some fluorinated gases (Myhre et al., 2013; Blanco et al., 2014) <sup>[[#fn:r219|219]]</sup> . Overall net CO <sub>2</sub> emissions in pathways are the result of a combination of various anthropogenic contributions (Figure 2.5) (Clarke et al., 2014) <sup>[[#fn:r220|220]]</sup> : (i) CO <sub>2</sub> produced by fossil-fuel combustion and industrial processes, (ii) CO <sub>2</sub> emissions or removals from the agriculture, forestry and other land use (AFOLU) sector, (iii) CO <sub>2</sub> capture and sequestration (CCS) from fossil fuels or industrial activities before it is released to the atmosphere, (iv) CO <sub>2</sub> removal by technological means, which in current pathways is mainly achieved by BECCS and AFOLU-related CDR, although other options could be conceivable (see Chapter 4, Section 4.3.7). Pathways apply these four contributions in different configurations (Figure 2.5) depending on societal choices and preferences related to the acceptability and availability of certain technologies, the timing and stringency of near-term climate policy, and the ability to limit the demand that drives baseline emissions (Marangoni et al., 2017; Riahi et al., 2017; Grubler et al., 2018; Rogelj et al., 2018; van Vuuren et al., 2018) <sup>[[#fn:r221|221]]</sup> , and come with very different implication for sustainable development (Section 2.5.3).

All 1.5°C pathways see global CO <sub>2</sub> emissions embark on a steady decline to reach (near) net zero levels around 2050, with 1.5°C-low-OS pathways reaching net zero CO <sub>2</sub> emissions around 2045–2055 (Table 2.4; Figure 2.5). Near-term differences between the various pathway classes are apparent, however. For instance, Below-1.5°C and 1.5°C-low-OS pathways show a clear shift towards lower CO <sub>2</sub> emissions in 2030 relative to other 1.5°C and 2°C pathway classes, although in all 1.5°C classes reductions are clear (Figure 2.6). These lower near-term emissions levels are a direct consequence of the former two pathway classes limiting cumulative CO <sub>2</sub> emissions until carbon neutrality in order to aim for a higher probability of limiting peak warming to 1.5°C (Section 2.2.2 and 2.3.2.2). In some cases, 1.5°C-low-OS pathways achieve net zero CO <sub>2</sub> emissions one or two decades later, contingent on 2030 CO <sub>2</sub> emissions in the lower quartile of the literature range, that is, below about 18 GtCO <sub>2</sub> yr <sup>−</sup> <sup>1</sup> . Median year-2030 global CO <sub>2</sub> emissions are of the order of 5–10 GtCO <sub>2</sub> yr <sup>−</sup> <sup>1</sup> lower in Below-1.5°C compared to 1.5°C-low-OS pathways, which are in turn lower than 1.5°C-high-OS pathways (Table 2.4). Below-1.5°C and 1.5°C-low-OS pathways combined show a decline in global net anthropogenic CO <sub>2</sub> emissions of about 45% from 2010 levels by 2030 (40–60% interquartile range). Lower-2°C pathways show CO <sub>2</sub> emissions declining by about 25% by 2030 in most pathways (10–30% interquartile range). The 1.5°C-high-OS pathways show emissions levels that are broadly similar to the 2°C-consistent pathways in 2030.

The development of CO <sub>2</sub> emissions in the second half of the century in 1.5°C pathways is characterized by the need to stay or return within a carbon budget. Figure 2.6 shows net CO <sub>2</sub> and N <sub>2</sub> O emissions from various sources in 2050 and 2100 in 1.5°C pathways in the literature. Virtually all 1.5°C pathways obtain net negative CO <sub>2</sub> emissions at some point during the 21st century, but the extent to which net negative emissions are relied upon varies substantially (Figure 2.6, Table 2.4). This net withdrawal of CO <sub>2</sub> from the atmosphere compensates for residual long-lived non-CO <sub>2</sub> GHG emissions that also accumulate in the atmosphere (like N <sub>2</sub> O) or cancels some of the build-up of CO <sub>2</sub> due to earlier emissions to achieve increasingly higher likelihoods that warming stays or returns below 1.5°C (see Section 2.3.4 for a discussion of various uses of CDR). Even non-overshoot pathways that aim at achieving temperature stabilization would hence deploy a certain amount of net negative CO <sub>2</sub> emissions to offset any accumulating long-lived non-CO <sub>2</sub> GHGs. The 1.5°C overshoot pathways display significantly larger amounts of annual net negative CO <sub>2</sub> emissions in the second half of the century. The larger the overshoot the more net negative CO <sub>2</sub> emissions are required to return temperatures to 1.5°C by the end of the century (Table 2.4, Figure 2.1).

N <sub>2</sub> O emissions decline to a much lesser extent than CO <sub>2</sub> in currently available 1.5°C pathways (Figure 2.6). Current IAMs have limited emissions-reduction potentials (Gernaat et al., 2015) <sup>[[#fn:r222|222]]</sup> (Sections 2.3.1.2, 2.4.4, Supplementary Material  2.SM.1.2), reflecting the difficulty of eliminating N <sub>2</sub> O emission from agriculture (Bodirsky et al., 2014) <sup>[[#fn:r223|223]]</sup> . Moreover, the reliance of some pathways on significant amounts of bioenergy after mid-century (Section 2.4.2) coupled to a substantial use of nitrogen fertilizer (Popp et al., 2017) <sup>[[#fn:r224|224]]</sup> also makes reducing N <sub>2</sub> O emissions harder (for example, see pathway ''S5'' in Figure 2.6). As a result, sizeable residual N <sub>2</sub> O emissions are currently projected to continue throughout the century, and measures to effectively mitigate them will be of continued relevance for 1.5°C societies. Finally, the reduction of nitrogen use and N <sub>2</sub> O emissions from agriculture is already a present-day concern due to unsustainable levels of nitrogen pollution (Bodirsky et al., 2012) <sup>[[#fn:r225|225]]</sup> . Section 2.4.4 provides a further assessment of the agricultural non-CO <sub>2</sub> emissions reduction potential.

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

<span id="figure-2.6"></span>
<!-- START IMG -->
<!-- IMG TITLE -->
'''Figure 2.6'''

<span id="section-4"></span>
<!-- IMG CAPTION -->
Annual global emissions characteristics for 2020, 2030, 2050, 2100.
<!-- IMG FILE -->
[[File:9943509a36d49b42308928cfd6843289 Figure-2.6-766x1024.jpg]]

Data are shown for (a) Kyoto-GHG emissions, and (b) global total CO <sub>2</sub> emissions, (c) CO <sub>2</sub> emissions from the agriculture, forestry and other land use (AFOLU) sector, (d) global N2O emissions, and (e) CO <sub>2</sub> emissions from fossil fuel use and industrial processes. The latter is also split into (f) emissions from the energy supply sector (electricity sector and refineries) and (g) direct emissions from fossil-fuel use in energy demand sectors (industry, buildings, transport) (bottom row). Horizontal black lines show the median, boxes show the interquartile range, and whiskers the minimum–maximum range. Icons indicate the four pathway archetypes used in this chapter. In case less than seven data points are available in a class, the minimum–maximum range and single data points are shown. Kyoto-GHG, emissions in the top panel are aggregated with AR4 GWP-100 and contain CO <sub>2</sub> , CH4, N2O, HFCs, PFCs, and SF6. NF3 is typically not reported by IAMs. Scenarios with year-2010 Kyoto-GHG emissions outside the range assessed by IPCC AR5 WGIII assessed are excluded (IPCC, 2014b) <sup>[[#fn:r226|226]]</sup> .

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

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

<span id="emissions-of-short-lived-climate-forcers-and-fluorinated-gases"></span>
==== 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> .'''
<!-- IMG FILE -->
[[File:1c68da6b4ecab0e7d0a8880a88f12464 Figure-2.8-1024x572.jpg]]

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

<!-- END IMG -->
<span id="cdr-in-1.5c-pathways"></span>