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

==== 4.3.7.1 Bioenergy with carbon capture and storage (BECCS) ====

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BECCS has been assessed in previous IPCC reports (IPCC, 2005b, 2014b; P. Smith et al., 2014; Minx et al., 2017) <sup>[[#fn:r600|600]]</sup> and has been incorporated into integrated assessment models (Clarke et al., 2014) <sup>[[#fn:r601|601]]</sup> but also, 1.5°C-consistent pathways without BECCS have emerged (Bauer et al., 2018; Grubler et al., 2018; Mousavi and Blesl, 2018; van Vuuren et al., 2018) <sup>[[#fn:r602|602]]</sup> . Still, the overall set of  pathways limiting global warming to 1.5°C with limited or no overshoot indicates that 0–1, 0–8, and 0–16 GtCO <sub>2</sub> yr <sup>−1</sup> would be removed by BECCS by 2030, 2050 and 2100, respectively (Chapter 2, Section 2.3.4). BECCS is constrained by sustainable bioenergy potentials (Section 4.3.1.2, Chapter 5, Section 5.4.1.3 and Cross-Chapter Box 6 in Chapter 3), and availability of safe storage for CO <sub>2</sub> (Section 4.3.1.6). Literature estimates for BECCS mitigation potentials in 2050 range from 1–85 GtCO <sup>[[#fn:4|4]]</sup> . Fuss et al. (2018) <sup>[[#fn:r603|603]]</sup> narrow this range to 0.5–5 GtCO <sub>2</sub> yr <sup>−1</sup> ( ''medium agreement, high evidence'' ) (Figure 4.3), meaning that BECCS mitigation potentials are not necessarily sufficient for 1.5°C-consistent pathways. This is, among other things, related to sustainability concerns (Boysen et al., 2017; Heck et al., 2018; Henry et al., 2018) <sup>[[#fn:r604|604]]</sup> .

Assessing BECCS deployment in 2°C pathways (of about 12 GtCO <sub>2</sub> -eq yr <sup>−1</sup> by 2100, considered as a conservative deployment estimate for BECCS-accepting pathways consistent with 1.5°C), Smith et al. (2016b) <sup>[[#fn:r605|605]]</sup> estimate a land-use intensity of 0.3–0.5 ha tCO <sub>2</sub> -eq <sup>−1</sup> yr <sup>−1</sup> using forest residues, 0.16 ha CO <sub>2</sub> -eq <sup>−1</sup> yr <sup>−1</sup> for agricultural residues, and 0.03–0.1 ha tCO <sub>2</sub> -eq <sup>−1</sup> yr <sup>−1</sup> for purpose-grown energy crops. The average amount of BECCS in these pathways requires 25–46% of arable and permanent crop area in 2100. Land area estimates differ in scale and are not necessarily a good indicator of competition with, for example, food production, because requiring a smaller land area for the same potential could indicate that high-productivity agricultural land is used. In general, the literature shows ''low agreement'' on the availability of land (Fritz et al., 2011 <sup>[[#fn:r606|606]]</sup> ; see Erb et al., 2016b <sup>[[#fn:r607|607]]</sup> for recent advances). Productivity, food production and competition with other ecosystem services and land use by local communities are important factors for designing regulation. These potentials and trade-offs are not homogenously distributed across regions. However, Robledo-Abad et al. (2017) <sup>[[#fn:r608|608]]</sup> find that regions with higher potentials are understudied, given their potential contribution. Researchers have expressed the need to complement global assessments with regional, geographically explicit bottom-up studies of biomass potentials and socio-economic impacts (e.g., de Wit and Faaij, 2010; Kraxner et al., 2014; Baik et al., 2018) <sup>[[#fn:r609|609]]</sup> .

Energy production and land and water footprints show wide ranges in bottom-up assessments due to differences in technology, feedstock and other parameters (−1–150 EJ yr <sup>−1</sup> of energy, 109–990 Mha, 6–79 MtN, 218–4758 km <sup>3</sup> yr <sup>−1</sup> of water per GtCO <sub>2</sub> yr <sup>−1</sup> ; Smith and Torn, 2013; Smith et al., 2016b; Fajardy and Mac Dowell, 2017) <sup>[[#fn:r610|610]]</sup> and are not comparable to IAM pathways which consider system effects (Bauer et al., 2018) <sup>[[#fn:r611|611]]</sup> . Global impacts on nutrients and albedo are difficult to quantify (Smith et al., 2016b) <sup>[[#fn:r612|612]]</sup> . BECCS competes with other land-based CDR and mitigation measures for resources (Chapter 2).

There is uncertainty about the feasibility of timely upscaling (Nemet et al., 2018) <sup>[[#fn:r613|613]]</sup> . CCS (see Section 4.3.1) is largely absent from the Nationally Determined Contributions (Spencer et al., 2015) <sup>[[#fn:r614|614]]</sup> and lowly ranked in investment priorities (Fridahl, 2017) <sup>[[#fn:r615|615]]</sup> . Although there are dozens of small-scale BECCS demonstrations (Kemper, 2015) <sup>[[#fn:r616|616]]</sup> and a full-scale project capturing 1 MtCO <sub>2</sub> exists (Finley, 2014) <sup>[[#fn:r617|617]]</sup> ''',''' this is well below the numbers associated with 1.5°C or 2°C-compatible pathways (IEA, 2016a; Peters et al., 2017) <sup>[[#fn:r618|618]]</sup> . Although the majority of BECCS cost estimates are below 200 USD tCO  <sup>−1</sup> (Figure 4.2), estimates vary widely. Economic incentives for ramping up large CCS or BECCS infrastructure are weak (Bhave et al., 2017) <sup>[[#fn:r619|619]]</sup> . The 2050 average investment costs for such a BECCS infrastructure for bio-electricity and biofuels are estimated at 138 and 123 billion USD yr <sup>−1</sup> , respectively (Smith et al., 2016b) <sup>[[#fn:r620|620]]</sup> .

BECCS deployment is further constrained by bioenergy’s carbon accounting, land, water and nutrient requirements (Section 4.3.1), its compatibility with other policy goals and limited public acceptance of both bioenergy and CCS (Section 4.3.1). Current pathways are believed to have inadequate assumptions on the development of societal support and governance structures (Vaughan and Gough, 2016) <sup>[[#fn:r621|621]]</sup> . However, removing BECCS and CCS from the portfolio of available options significantly raises modelled mitigation costs (Kriegler et al., 2013; Bauer et al., 2018) <sup>[[#fn:r622|622]]</sup> .

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Panel A presents estimates based on a systematic review of the bottom up literature (Fuss et al., 2018) <sup>[[#fn:r623|623]]</sup> , corresponding to dashed blue boxes in Panel B. Dashed lines represent saturation limits for the corresponding technology. Panel B shows the percentage of papers at a given cost or potential estimate. Reference year for all potential estimates is 2050, while all cost estimates preceding 2050 have been included (as early as 2030, older estimates are excluded if they lack a base year and thus cannot be made comparable). Ranges have been trimmed to show detail (see Fuss et al., 2018 <sup>[[#fn:r624|624]]</sup> for the full range). Costs refer only to abatement costs. Icons for side-effects are allocated only if a critical mass of papers corroborates their occurrence

Notes: For references please see Supplementary Material Table 4.SM.3. Direct air carbon dioxide capture and storage (DACCS) is theoretically only constrained by geological storage capacity, estimates presented are considering upscaling and cost challenges (Nemet et al., 2018) <sup>[[#fn:r625|625]]</sup> . BECCS potential estimates are based on bioenergy estimates in the literature (EJ yr <sup>−1</sup> ), converted to GtCO <sub>2</sub> following footnote 4. Potentials cannot be added up, as CDR options would compete for resources (e.g., land). SCS – soil carbon sequestration; OA – ocean alkalinization; EW- enhanced weathering; DACCS – direct air carbon dioxide capture and storage; BECCS – bioenergy with carbon capture and storage; AR – afforestation

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