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

=== 2.3.2 Key Characteristics of 1.5°C Pathways ===

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1.5°C-consistent pathways are characterized by a rapid phase out of CO <sub>2</sub> emissions and deep emissions reductions in other GHGs and climate forcers (Section 2.2.2 and 2.3.3). This is achieved by broad transformations in the energy; industry; transport; buildings; and agriculture, forestry and other land-use (AFOLU) sectors (Section 2.4) (Bauer et al., 2018; Grubler et al., 2018; Holz et al., 2018b; Kriegler et al., 2018b; Liu et al., 2018; Luderer et al., 2018; Rogelj et al., 2018; van Vuuren et al., 2018; Zhang et al., 2018) <sup>[[#fn:r186|186]]</sup> . Here we assess 1.5°C-consistent pathways with and without overshoot during the 21st century. One study also explores pathways overshooting 1.5°C for longer than the 21st century (Akimoto et al., 2017) <sup>[[#fn:r187|187]]</sup> , but these are not considered 1.5°C-consistent pathways in this report (Chapter 1, Section 1.1.3). This subsection summarizes robust and varying properties of 1.5°C-consistent pathways regarding system transformations, emission reductions and overshoot. It aims to provide an introduction to the detailed assessment of the emissions evolution (Section 2.3.3), CDR deployment (Section 2.3.4), energy (Section 2.4.1, 2.4.2), industry (2.4.3.1), buildings (2.4.3.2), transport (2.4.3.3) and land-use transformations (Section 2.4.4) in 1.5°C-consistent pathways. Throughout Sections 2.3 and 2.4, pathway properties are highlighted with four 1.5°C-consistent pathway archetypes (LED, S1, S2, S5; referred to as P1, P2, P3, and P4 in the Summary for Policymakers) covering a wide range of different socio-economic and technology assumptions (Figure 2.5, Section 2.3.1).

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==== 2.3.2.1 Variation in system transformations underlying 1.5°C pathways ====

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Be it for the energy, transport, buildings, industry, or AFOLU sector, the literature shows that multiple options and choices are available in each of these sectors to pursue stringent emissions reductions (Section 2.3.1.2, Supplementary Material 2.SM.1.2, Chapter 4, Section 4.3). Because the overall emissions total under a pathway is limited by a geophysical carbon budget (Section 2.2.2), choices in one sector affect the efforts that are required from others (Clarke et al., 2014) <sup>[[#fn:r188|188]]</sup> . A robust feature of 1.5°C-consistent pathways, as highlighted by the set of pathway archetypes in Figure 2.5, is a virtually full decarbonization of the power sector around mid-century, a feature shared with 2°C-consistent pathways. The additional emissions reductions in 1.5°C-consistent compared to 2°C-consistent pathways come predominantly from the transport and industry sectors (Luderer et al., 2018) <sup>[[#fn:r189|189]]</sup> . Emissions can be apportioned differently across sectors, for example, by focussing on reducing the overall amount of CO <sub>2</sub> produced in the energy end-use sectors, and using limited contributions of CDR by the AFOLU sector (afforestation and reforestation, S1 and LED pathways in Figure 2.5) (Grubler et al., 2018; Holz et al., 2018b; van Vuuren et al., 2018) <sup>[[#fn:r190|190]]</sup> , or by being more lenient about the amount of CO <sub>2</sub> that continues to be produced in the above-mentioned end-use sectors (both by 2030 and mid-century) and strongly relying on technological CDR options like BECCS (S2 and S5 pathways in Figure 2.5) (Luderer et al., 2018; Rogelj et al., 2018) <sup>[[#fn:r191|191]]</sup> . Major drivers of these differences are assumptions about energy and food demand and the stringency of near-term climate policy (see the difference between early action in the scenarios S1, LED and more moderate action until 2030 in the scenarios S2, S5). Furthermore, the carbon budget in each of these pathways depends also on the non-CO <sub>2</sub> mitigation measures implemented in each of them, particularly for agricultural emissions (Sections 2.2.2, 2.3.3) (Gernaat et al., 2015) <sup>[[#fn:r192|192]]</sup> . Those pathways differ not only in terms of their deployment of mitigation and CDR measures (Sections 2.3.4 and 2.4), but also in terms of the resulting temperature overshoot (Figure 2.1). Furthermore, they have very different implications for the achievement of sustainable development objectives, as further discussed in Section 2.5.3.

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'''Figure 2.5'''

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Evolution and break down of global anthropogenic CO <sub>2 </sub> emissions until 2100.
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[[File:dc806298e16950294c07c4ddda22564c Figure-2.5-1024x735.jpg]]

The top-left panel shows global net CO <sub>2</sub> emissions in Below-1.5°C, 1.5°C-low-overshoot (OS), and 1.5°C-high-OS pathways, with the four illustrative 1.5°C-consistent pathway archetypes of this chapter highlighted. Ranges at the bottom of the top-left panel show the 10th–90th percentile range (thin line) and interquartile range (thick line) of the time that global CO <sub>2</sub> emissions reach net zero per pathway class, and for all pathways classes combined. The top-right panel provides a schematic legend explaining all CO <sub>2</sub> emissions contributions to global CO <sub>2</sub> emissions. The bottom row shows how various CO <sub>2</sub> contributions are deployed and used in the four illustrative pathway archetypes (LED, S1, S2, S5, referred to as P1, P2, P3, and P4 in the Summary for Policymakers) used in this chapter (see Section 2.3.1.1). Note that the S5 scenario reports the building and industry sector emissions jointly. Green-blue areas hence show emissions from the transport sector and the joint building and industry demand sector, respectively.

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

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==== 2.3.2.2 Pathways keeping warming below 1.5°C or temporarily overshooting it ====

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This subsection explores the conditions that would need to be fulfilled to stay below 1.5°C warming without overshoot. As discussed in Section 2.2.2, to keep warming below 1.5°C with a two-in-three (one-in-two) chance, the cumulative amount of CO <sub>2</sub> emissions from 2018 onwards need to remain below a carbon budget of 420 (580) GtCO <sub>2</sub> ; accounting for the effects of additional Earth system feedbacks until 2100 reduces this estimate by 100 GtCO <sub>2</sub> . Based on the current state of knowledge, exceeding this remaining carbon budget at some point in time would give a one-in-three (one-in-two) chance that the 1.5°C limit is overshot (Table 2.2). For comparison, around 290 ± 20 (1 standard deviation range) GtCO <sub>2</sub> have been emitted in the years 2011–2017, with annual CO <sub>2</sub> emissions in 2017 around 42 ± 3 GtCO <sub>2</sub> yr <sup>−1</sup> (Jackson et al., 2017; Le Quéré et al., 2018) <sup>[[#fn:r193|193]]</sup> . Committed fossil-fuel emissions from existing fossil-fuel infrastructure as of 2010 have been estimated at around 500 ± 200 GtCO <sub>2</sub> (with about 200 GtCO <sub>2</sub> already emitted through 2017) (Davis and Caldeira, 2010) <sup>[[#fn:r194|194]]</sup> . Coal-fired power plants contribute the largest part. Committed emissions from existing coal-fired power plants built through the end of 2016 are estimated to add up to roughly 200 GtCO <sub>2</sub> , and a further 100–150 GtCO <sub>2</sub> from coal-fired power plants under construction or planned (González-Eguino et al., 2017; Edenhofer et al., 2018) <sup>[[#fn:r195|195]]</sup> . However, there has been a marked slowdown of planned coal-power projects in recent years, and some estimates indicate that the committed emissions from coal plants that are under construction or planned have halved since 2015 (Shearer et al., 2018) <sup>[[#fn:r196|196]]</sup> . Despite these uncertainties, the committed fossil-fuel emissions are assessed to already amount to more than two thirds (half) of the remaining carbon budget.

An important question is to what extent the nationally determined contributions (NDCs) under the Paris Agreement are aligned with the remaining carbon budget. It was estimated that the NDCs, if successfully implemented, imply a total of 400–560 GtCO <sub>2</sub> emissions over the 2018–2030 period (considering both conditional and unconditional NDCs) (Rogelj et al., 2016a) <sup>[[#fn:r197|197]]</sup> . Thus, following an NDC trajectory would already exhaust 95–130% (70–95%) of the remaining two-in-three (one-in-two) 1.5°C carbon budget (unadjusted for additional Earth system feedbacks) by 2030. This would leave no time ( 0–9 years) to bring down global emissions from NDC levels of around 40 GtCO <sub>2</sub> yr <sup>−1</sup> in 2030 (Fawcett et al., 2015; Rogelj et al., 2016a) <sup>[[#fn:r198|198]]</sup> to net zero (further discussion in Section 2.3.5).

Most 1.5°C-consistent pathways show more stringent emissions reductions by 2030 than implied by the NDCs (Section 2.3.5) The lower end of those pathways reach down to below 20 GtCO <sub>2</sub> yr <sup>−1</sup> in 2030 (Section 2.3.3, Table 2.4), less than half of what is implied by the NDCs. Whether such pathways will be able to limit warming to 1.5°C without overshoot will depend on whether cumulative net CO <sub>2</sub> emissions over the 21st century can be kept below the remaining carbon budget at any time. Net global CO <sub>2</sub> emissions are derived from the gross amount of CO <sub>2</sub> that humans annually emit into the atmosphere reduced by the amount of anthropogenic CDR in each year. New research has looked more closely at the amount and the drivers of gross CO <sub>2</sub> emissions from fossil-fuel combustion and industrial processes (FFI) in deep mitigation pathways (Luderer et al., 2018) <sup>[[#fn:r199|199]]</sup> , and found that the larger part of remaining CO <sub>2</sub> emissions come from direct fossil-fuel use in the transport and industry sectors, while residual energy supply sector emissions (mostly from the power sector) are limited by a rapid approach to net zero CO <sub>2</sub> emissions until mid-century. The 1.5°C pathways with no or limited (&lt;0.1°C) overshoot that were reported in the scenario database project remaining FFI CO <sub>2</sub> emissions of 610–1260 GtCO <sub>2</sub> over the period 2018–2100 (5th–95th percentile range; median: 880 GtCO <sub>2</sub> ). Kriegler et al. (2018b) <sup>[[#fn:r200|200]]</sup> conducted a sensitivity analysis that explores the four central options for reducing fossil-fuel emissions: lowering energy demand, electrifying energy services, decarbonizing the power sector and decarbonizing non-electric fuel use in energy end-use sectors. By exploring these options to their extremes, they found a lowest value of 500 GtCO <sub>2</sub> (2018–2100) gross fossil-fuel CO <sub>2</sub> emissions for the hypothetical case of aligning the strongest assumptions for all four mitigation options. The two lines of evidence and the fact that available 1.5°C pathways cover a wide range of assumptions (Section 2.3.1) give a robust indication of a lower limit of about 500 GtCO <sub>2</sub> remaining fossil-fuel and industry CO <sub>2</sub> emissions in the 21st century.

To compare these numbers with the remaining carbon budget, CO <sub>2</sub> emissions from agriculture, forestry and other land use (AFOLU) need to be taken into account. In many of the 1.5°C-consistent pathways, AFOLU CO <sub>2</sub> emissions reach zero at or before mid-century and then turn to negative values (Table 2.4). This means human changes to the land lead to atmospheric carbon being stored in plants and soils. This needs to be distinguished from the natural CO <sub>2</sub> uptake by land, which is not accounted for in the anthropogenic AFOLU CO <sub>2</sub> emissions reported in the pathways. Given the difference in estimating the ‘anthropogenic’ sink between countries and the global integrated assessment and carbon modelling community (Grassi et al., 2017) <sup>[[#fn:r201|201]]</sup> , the AFOLU CO <sub>2</sub> estimates included here are not necessarily directly comparable with countries’ estimates at global level. The cumulated amount of AFOLU CO <sub>2</sub> emissions until the time they reach zero combine with the fossil-fuel and industry CO <sub>2</sub> emissions to give a total amount of gross emissions of 650–1270 GtCO <sub>2</sub> for the period 2018–2100 (5th–95th percentile; median 950 GtCO <sub>2</sub> ) in 1.5°C pathways with no or limited overshoot. The lower end of the range is close to what emerges from a scenario of transformative change that halves CO <sub>2</sub> emissions every decade from 2020 to 2050 (Rockström et al., 2017) <sup>[[#fn:r202|202]]</sup> . All these estimates are above the remaining carbon budget for a one-in-two chance of limiting warming below 1.5°C without overshoot, including the low end of the hypothetical sensitivity analysis of Kriegler et al. (2018b) <sup>[[#fn:r203|203]]</sup> , who assumes 75 Gt AFOLU CO <sub>2</sub> emissions adding to a total of 575 GtCO <sub>2</sub> gross CO <sub>2</sub> emissions. As almost no cases have been identified that keep gross CO <sub>2</sub> emissions within the remaining carbon budget for a one-in-two chance of limiting warming to 1.5°C, and based on current understanding of the geophysical response and its uncertainties, the available evidence indicates that avoiding overshoot of 1.5°C will require some type of CDR in a broad sense, e.g., via net negative AFOLU CO <sub>2</sub> emissions ( ''medium confidence'' ). (Table 2.2).

Net CO <sub>2</sub> emissions can fall below gross CO <sub>2</sub> emissions, if CDR is brought into the mix. Studies have looked at mitigation and CDR in combination to identify strategies for limiting warming to 1.5°C (Sanderson et al., 2016; Ricke et al., 2017) <sup>[[#fn:r204|204]]</sup> . CDR, which may include net negative AFOLU CO <sub>2</sub> emissions, is deployed by all 1.5°C-consistent pathways available to this assessment, but the scale of deployment and choice of CDR measures varies widely (Section 2.3.4). Furthermore, no CDR technology has been deployed at scale yet, and all come with concerns about their potential (Fuss et al., 2018) <sup>[[#fn:r205|205]]</sup> , feasibility (Nemet et al., 2018) <sup>[[#fn:r206|206]]</sup> and/or sustainability (Smith et al., 2015; Fuss et al., 2018) <sup>[[#fn:r207|207]]</sup> (see Sections 2.3.4, 4.3.2 and 4.3.7 and Cross-Chapter Box 7 in Chapter 3 for further discussion). CDR can have two very different functions in 1.5°C-consistent pathways. If deployed in the first half of the century, before net zero CO <sub>2</sub> emissions are reached, it neutralizes some of the remaining CO <sub>2</sub> emissions year by year and thus slows the accumulation of CO <sub>2</sub> in the atmosphere. In this first function it can be used to remain within the carbon budget and avoid overshoot. If CDR is deployed in the second half of the century after carbon neutrality has been established, it can still be used to neutralize some residual emissions from other sectors, but also to create net negative emissions that actively draw down the cumulative amount of CO <sub>2</sub> emissions to return below a 1.5°C warming level. In the second function, CDR enables temporary overshoot. The literature points to strong limitations to upscaling CDR (limiting its first abovementioned function) and to sustainability constraints (limiting both abovementioned functions) (Fuss et al., 2018; Minx et al., 2018; Nemet et al., 2018) <sup>[[#fn:r208|208]]</sup> . Large uncertainty hence exists about what amount of CDR could actually be available before mid-century. Kriegler et al. (2018b) <sup>[[#fn:r209|209]]</sup> explore a case limiting CDR to 100 GtCO <sub>2</sub> until 2050, and the 1.5°C pathways with no or limited overshoot available in the report’s database project 40–260 GtCO <sub>2</sub> CDR until the point of carbon neutrality (5th to 95th percentile; median 110 GtCO <sub>2</sub> ). Because gross CO <sub>2</sub> emissions in most cases exceed the remaining carbon budget by several hundred GtCO <sub>2</sub> and given the limits to CDR deployment until 2050, most of the 1.5°C-consistent pathways available to this assessment are overshoot pathways. However, the scenario database also contains nine non-overshoot pathways that remain below 1.5°C throughout the 21st century (Table 2.1).

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