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

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

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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).

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