Editing IPCC:AR6/WGIII/Chapter-12 (section)

=== 12.3.2 Consideration of Methods Assessed in Sectoral Chapters: A/R, Biochar, BECCS, Soil Carbon Sequestration ===

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'''Status:''' BECCS, afforestation/reforestation (A/R), soil carbon sequestration (SCS) and biochar are land-based biological CDR methods ( [[#Smith--2016|Smith et al. 2016]] ). BECCS combines biomass use for energy with CCS to capture and store the biogenic carbon geologically ( [[IPCC:Wg3:Chapter:Chapter-6#6.4.2.6|Section 6.4.2.6]] ); A/R and SCS involve fixing atmospheric carbon in biomass and soils, and biochar involves converting biomass to biochar and using it as a soil amendment. These CDR methods can be associated with both co-benefits and adverse side effects ( [[#Smith--2016|Smith et al. 2016]] ; [[#Hurlbert--2019|Hurlbert et al. 2019]] ; [[#Mbow--2019|Mbow et al. 2019]] ; [[#Olsson--2019|Olsson et al. 2019]] ; [[#Schleicher--2019|Schleicher et al. 2019]] ; [[#Smith--2019b|Smith et al. 2019b]] ; [[#Babin--2021|Babin et al. 2021]] ; [[#Dooley--2021|Dooley et al. 2021]] ) (Sections 7.4 and 12.5).

Among CDR methods, BECCS and A/R are most commonly selected by IAMs to meet the requirements of scenarios that limit warming to 2°C (&gt;67%) or lower. This is partially because of the long lead time required to refine IAMs to include additional methods and update techno-economic parameters. Currently, few IAMs represent SCS or biochar ( [[#Frank--2017|Frank et al. 2017]] ). Given the removal potential of SCS and biochar and some potential co-benefits, more efforts should be made to include these methods within IAMs, so that their mitigation potential can be compared to other CDR methods, along with possible co-benefits and adverse side effects ( [[#Smith--2016|Smith et al. 2016]] ; [[#Rogelj--2018|Rogelj et al. 2018]] ) ( [[#12.5|Section 12.5]] ).

'''Potential:''' The technical potential for BECCS by 2050 is estimated at 0.5–11.3 GtCO 2 -eq yr –1 (Table 7.3). These potentials do not include avoided emissions resulting from the use of heat, electricity and/or fuels provided by the BECCS system, which depend on substitution patterns, conversion efficiencies, and supply chain emissions for the BECCS and substituted energy systems (Box 7.7). The mitigation effect of BECCS also depends on how deployment affects land carbon stocks and sink strength ( [[IPCC:Wg3:Chapter:Chapter-7#7.4.4|Section 7.4.4]] ).

As detailed in Chapter 7, the technical potential for gross removals realised through A/R in 2050 is 0.5–10.1 GtCO 2 -eq yr –1 , and for improved forest management the potential is 1–2.1 GtCO 2 -eq yr –1 (including both CDR and emissions reduction). Technical potential for SCS in 2050 is estimated to be 0.6–9.4 GtCO 2 -eq yr –1 , for agroforestry it is 0.3–9.4 GtCO 2 -eq yr –1 , and for biochar it is 0.2–6.6 GtCO 2 -eq yr –1 . Peatland and coastal wetland restoration have a technical potential of 0.5–2.1 GtCO 2 -eq yr –1 in 2050, with an estimated 80% of the potential being CDR. Note that these potentials reflect only biophysical and technological conditions and become reduced when factoring in economic, environmental, socio-cultural and institutional constraints (Table 12.6).

'''Costs:''' Costs across technologies vary substantially ( [[#Smith--2016|Smith et al. 2016]] ) and were estimated to be USD15–400 tCO 2 –1 for BECSS, USD0–240 tCO 2 –1 for A/R, –USD45 to +USD100 tCO 2 –1 for SCS and USD10–345 tCO 2 –1 for biochar. [[#Fuss--2018|Fuss et al. (2018)]] estimated abatement cost ranges for BECCS, A/R, SCS and biochar to be 100–200, 5–50, 0–100, and 30–120 tCO 2 -eq −1 respectively, corresponding to 2100 potentials. Ranges for economic potential (&lt;USD100 tCO 2 –1 ) reported in [[IPCC:Wg3:Chapter:Chapter-7|Chapter 7]] are 0.5–3.0 GtCO 2 yr –1 (A/R); 0.6–1.9 GtCO 2 yr –1 (improved forest management); 0.7–2.5 GtCO 2 yr –1 (SCS); 0.4–1.1 GtCO 2 yr –1 (agroforestry); 0.3–1.8 GtCO 2 yr –1 (biochar); and 0.2–0.8 GtCO 2 yr –1 (peatland and coastal wetland restoration).

'''Risks, impacts, and co-benefits:''' a brief summary of risks, impacts and co-benefits is provided here and more detail is provided in [[IPCC:Wg3:Chapter:Chapter-7|Chapter 7]] and [[#12.5|Section 12.5]] . A/R and biomass production for BECCS and biochar potentially compete for land, water and other resources, implying possible adverse outcomes for ecosystem health, biodiversity, livelihoods and food security ( ''medium evidence'' , ''high agreement'' ) ( [[#Smith--2016|Smith et al. 2016]] ; [[#Heck--2018|Heck et al. 2018]] ; [[#Hurlbert--2019|Hurlbert et al. 2019]] ; [[#Mbow--2019|Mbow et al. 2019]] ) (Chapter 7). SCS requires the addition of nitrogen and phosphorus to maintain stoichiometry of soil organic matter, leading to a potential risk of eutrophication ( [[#Fuss--2018|Fuss et al. 2018]] ). Apart from possible negative effects associated with biomass supply, adverse side effects from biochar are relatively low if the biomass is uncontaminated ( [[#Tisserant--2019|Tisserant and Cherubini 2019]] ).

Possible climate risks relate to direct and/or indirect land carbon losses (A/R, BECCS, biochar), increased N 2 O emissions (BECCS, SCS), saturation and non-permanence of carbon storage (A/R, SCS) ( [[#Jia--2019|Jia et al. 2019]] ; [[#Smith--2019b|Smith et al. 2019b]] ) (Chapter 7), and potential CO 2 leakage from deep geological reservoirs (BECCS) (Chapter 6). Land cover change associated with A/R and biomass supply for BECCS and biochar may cause albedo changes that reduce mitigation effectiveness ( [[#Fuss--2018|Fuss et al. 2018]] ; [[#Jia--2019|Jia et al. 2019]] ). Potentially unfavourable albedo change resulting from biochar use can be minimised by incorporating biochar into the soil ( [[#Fuss--2018|Fuss et al. 2018]] ) (Chapter 7).

Concerning co-benefits, A/R and biomass production for BECCS or biochar could improve soil carbon, nutrient and water cycling ( ''robust evidence'' , ''high agreement'' ), and contribute to market opportunities, employment and local livelihoods, economic diversification, energy security, and technology development and transfer ( ''medium evidence'' , ''high agreement'' ) ( [[#Fuss--2018|Fuss et al. 2018]] ) (Chapter 7). It may contribute to reduction of other air pollutants, health benefits, and reduced dependency on imported fossil fuels. A/R can improve biodiversity if native and diverse species are used ( [[#Fuss--2018|Fuss et al. 2018]] ). For biochar, additional co-benefits include increased crop yields, reduced drought impacts, and reduced CH 4 and N 2 O emissions from soils ( [[#Joseph--2021|Joseph et al., 2021]] ) ( [[IPCC:Wg3:Chapter:Chapter-7#7.4.5.2|Section 7.4.5.2]] ). SCS can improve soil quality and resilience and improve agricultural productivity and food security ( [[#Frank--2017|Frank et al. 2017]] ; [[#Smith--2019b|Smith et al. 2019b]] ).

'''Role in mitigation pathways:''' Biomass use for BECCS in 2050 is 61 EJ yr –1 (13–208 EJ yr –1 , 5–95th percentile range) in scenarios limiting warming to 1.5°C (&gt;50%) with no or limited overshoot (C1, excluding traditional energy). This corresponds to 5.3 GtCO 2 yr –1 (1.1–18 GtCO 2 yr –1 ) CDR, if assuming 28 kg C GJ –1 biomass carbon content and 85% capture rate in BECCS systems. In scenarios that limit warming to 2°C (&gt;67%) (C3), biomass use for BECCS in 2050 is 28 EJ yr –1 (0–96 EJ yr –1 , 5–95th percentile range), corresponding to 2.4 GtCO 2 yr –1 (0–8.3 GtCO 2 yr –1 ) CDR. Cumulative CO 2 removal from AFOLU (mainly through A/R), as reported from models, in the period 2020 to 2100 is 262 GtCO 2 (17–397 GtCO 2 ) and 209 GtCO 2 (20–415 GtCO 2 ) in C1 and C3 scenarios, respectively (5–95th percentile range).

Uncertainties remain in two main areas: the availability of land and biomass, which is affected by many factors ( [[#Anandarajah--2018|Anandarajah et al. 2018]] ) (Chapter 7), and the role of other mitigation measures including CDR methods other than A/R and BECCS. Strong near-term climate change mitigation to limit overshoot, and deployment of CDR methods other than A/R and BECCS, may significantly reduce the contribution of these CDR methods in scenarios limiting warming to 1.5°C or 2°C ( [[#Köberle--2019|Köberle 2019]] ; [[#Hasegawa--2021|Hasegawa et al. 2021]] ).

'''Trade-offs and spillovers:''' Some land-based biological CDR methods, such as BECCS and A/R, demand land. Combining mitigation strategies has the potential to increase overall carbon sequestration rates ( [[#Humpenöder--2014|Humpenöder et al. 2014]] ). However, these CDR methods may also compete for resources ( [[#Frank--2017|Frank et al. 2017]] ). Land-based mitigation approaches currently propose the use of forests (i) as a source of woody biomass for bioenergy and various biomaterials and (ii) for carbon sequestration in vegetation, soils, and forest products. Forests are therefore required to provide both provisioning (biomass feedstock) and regulating (carbon sequestration) ecosystem services. This multifaceted strategy has the potential to result in trade-offs ( [[#Makkonen--2015|Makkonen et al. 2015]] ). Some land-based mitigation options could conflict with biodiversity goals, e.g., A/R using monoculture plantations can reduce species richness when introduced into (semi-)natural grasslands ( [[#Smith--2019a|Smith et al. 2019a]] ; [[#Dooley--2021|Dooley et al. 2021]] ). When trade-offs exist between biodiversity protection and mitigation objectives, biodiversity is typically given a lower priority, especially if the mitigation option is considered risk-free and economically feasible ( [[#Pörtner--2021|Pörtner et al. 2021]] ). Approaches that promote synergies, such as sustainable forest management, reducing deforestation rates, cultivation of perennial crops for bioenergy in sustainable farming practices, and mixed-species forests in A/R, can mitigate biodiversity impacts and even improve ecosystem capacity to support biodiversity while mitigating climate change ( [[#Pörtner--2021|Pörtner et al. 2021]] ) ( [[#12.5|Section 12.5]] ). Systematic land-use planning could help to deliver land-based mitigation options that also limit trade-offs with biodiversity ( [[#Longva--2017|Longva et al. 2017]] ) (Cross-Working Group Box 3: Mitigation and Adaptation via the Bioeconomy, in this chapter).

Status, costs, potentials, risk and impacts, co-benefits, trade-offs and spillover effects and the role in mitigation pathways of A/R, biochar, SCS, peatland and coastal wetland restoration, agroforestry and forest management are summarised in Table 12.6. See also [[#12.5|Section 12.5]] .

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