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

=== Cross-Chapter Box 8 | Carbon Dioxide Removal: Key Characteristics and Multiple Roles in Mitigation Strategies ===

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'''Authors:''' Oliver Geden (Germany), Alaa Al Khourdajie (United Kingdom/Syria), Christopher Bataille (Canada), Göran Berndes (Sweden), Holly Jean Buck (the United States of America), Katherine Calvin (the United States of America), Annette Cowie (Australia), Kiane de Kleijne (the Netherlands), Jan Christoph Minx (Germany), Gert-Jan Nabuurs (the Netherlands), Glen P. Peters (Norway/Australia), Andy Reisinger (New Zealand), Pete Smith (United Kingdom), Masahiro Sugiyama (Japan)

Carbon dioxide removal (CDR) is a necessary element of mitigation portfolios to achieve net zero CO 2 and GHG emissions both globally and nationally, counterbalancing residual emissions from hard-to-transition sectors such as industry, transport and agriculture. CDR is a key element in scenarios that limit warming to 2°C (&gt;67%) or lower, regardless of whether global emissions reach near-zero, net zero or net-negative levels (Sections 3.3, 3.4, 3.5 and 12.3). While national mitigation portfolios aiming at net zero or net-negative emissions will need to include some level of CDR, the choice of methods and the scale and timing of their deployment will depend on the ambition for gross emissions reductions, how sustainability and feasibility constraints are managed, and how political preferences and social acceptability evolve ( [[#12.3.3|Section 12.3.3]] ). This box gives an overview of CDR methods, presents a categorisation based on the key characteristics of removal processes and storage timescales, and clarifies the multiple roles of CDR in mitigation strategies. The term ‘negative emissions’ is used in this report only when referring to the net emissions outcome at a systems level (e.g., ‘net negative emissions’ at global, national, sectoral or supply chain levels).

'''Categorisation of the m''' '''ain CDR methods'''

CDR refers to anthropogenic activities that remove CO 2 from the atmosphere and store it durably in geological, terrestrial, or ocean reservoirs, or in products. It includes anthropogenic enhancement of biological, geochemical or chemical CO 2 sinks, but excludes natural CO 2 uptake not directly caused by human activities. Increases in land carbon sink strength due to CO 2 fertilisation or other indirect effects of human activities are not considered CDR (see Glossary). Carbon capture and storage (CCS) and carbon capture and utilisation (CCU) applied to CO 2 from fossil fuel use are not CDR methods as they do not remove CO 2 from the atmosphere. CCS and CCU can, however, be part of CDR methods if the CO 2 has been captured from the atmosphere, either indirectly in the form of biomass or directly from ambient air, and stored durably in geological reservoirs or products (Sections 11.3.6 and 12.3).

There are many different CDR methods and associated implementation options (Cross-Chapter Box 8, Figure 1). Some of these methods (including afforestation and improved forest management, wetland restoration and soil carbon sequestration (SCS)) have been practised for decades to millennia, although not necessarily with the intention of removing carbon from the atmosphere. Conversely, methods such as direct air carbon capture and storage (DACCS), bioenergy with carbon capture and storage (BECCS) and enhanced weathering are novel, and while experience is growing, their demonstration and deployment are limited in scale. CDR methods have been categorised in different ways in the literature, highlighting different characteristics. In this report, as in AR6 WGI, the categorisation is based on the role of CDR methods in the carbon cycle, that is, on the removal process ( ''land-based biological'' ; ''ocean-based biological'' ; ''geochemical'' ; ''chemical'' ) and on the timescale of storage ( ''decades to centuries'' ; ''centuries to millennia'' ; ''ten thousand years or longer'' ). The time scale of storage is closely linked to the storage medium: carbon stored in ocean reservoirs (through enhanced weathering, ocean alkalinity enhancement or ocean fertilisation) and in geological formations (through BECCS or DACCS) generally has longer storage times and is less vulnerable to reversal through human actions or disturbances such as drought and wildfire than carbon stored in terrestrial reservoirs (vegetation, soil). Furthermore, carbon stored in vegetation or through SCS has shorter storage times and is more vulnerable than carbon stored in buildings as wood products; as biochar in soils, cement and other materials; or in chemical products made from biomass or potentially through direct air ( [[#Fuss--2018|Fuss et al. 2018]] ; [[#Minx--2018|Minx et al. 2018]] ; [[#NASEM--2019|NASEM 2019]] ) capture ( [[IPCC:Wg3:Chapter:Chapter-11#11.3.6|Section 11.3.6]] ; AR6 WGI, Figure 5.36). Within the same category (e.g., land-based biological CDR) options often differ with respect to other dynamic or context-specific dimensions, such as mitigation potential, cost, potential for co-benefits and adverse side effects, and technology readiness level (Table 12.6).

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'''Cross-Chapter Box 8, Figure 1 | Carbon dioxide removal taxonomy.''' '''Methods are categorised based on removal process (grey shades) and storage medium (for which timescales of storage are given, yellow/brown shades).''' Main implementation options are included for each CDR method. Note that specific land-based implementation options can be associated with several CDR methods, for example, agroforestry can support soil carbon sequestration and provide biomass for biochar or BECCS. Source: adapted from [[#Minx--2018|Minx et al. (2018)]] .

'''Roles of CDR in mitiga''' '''tion strategies'''

Within ambitious mitigation strategies at global or national levels, CDR cannot serve as a substitute for deep emissions reductions but can fulfil multiple complementary roles: it can (i) further reduce net CO 2 or GHG emission levels in the near-term; (ii) counterbalance residual emissions from hard-to-transition sectors, such as CO 2 from industrial activities and long-distance transport (e.g., aviation, shipping), or methane and nitrous oxide from agriculture, in order to help reach net zero CO 2 or GHG emissions in the mid-term; (iii) achieve and sustain net-negative CO 2 or GHG emissions in the long-term, by deploying CDR at levels exceeding annual residual gross CO 2 or GHG emissions (Sections 2.7.3 and 3.5).

In general, these roles of CDR are not mutually exclusive and can exist in parallel. For example, achieving net zero CO 2 or GHG emissions globally might involve some countries already reaching net-negative levels at the time of global net zero, allowing other countries more time to achieve this. Equally, achieving net-negative CO 2 emissions globally, which could address a potential temperature overshoot by lowering atmospheric CO 2 concentrations, does not necessarily involve all countries reaching net-negative levels ( [[#Rajamani--2021|Rajamani et al. 2021]] ; [[#Rogelj--2021|Rogelj et al. 2021]] ) (Cross-Chapter Box 3 in Chapter 3).

Cross-Chapter Box 8, Figure 2 shows these multiple roles of CDR in a stylised ambitious mitigation pathway that can be applied to global and national levels. While such mitigation pathways will differ in their shape and exact composition, they include the same basic components: CO 2 emissions from fossil sources, CO 2 emissions from managed land, non-CO 2 emissions, and various forms of CDR. Cross-Chapter Box 8, Figure 2 also illustrates the importance of distinguishing between gross CO 2 removals from the atmosphere through deployment of CDR methods and the net emissions outcome (i.e., gross emissions minus gross removals).

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'''Cross-Chapter Box 8, Figure 2 | Roles of CDR in global or national mitigation strategies.''' Stylised pathway showing multiple functions of CDR in different phases of ambitious mitigation: (1) further reducing net CO 2 or GHG emissions levels in near-term; (2) counterbalancing residual emissions to help reach net zero CO 2 or GHG emissions in the mid-term; (3) achieving and sustaining net-negative CO 2 or GHG emissions in the long-term.

CDR methods currently deployed on managed land, such as afforestation or reforestation and improved forest management, lead to CO 2 removals already today, even when net emissions from land use are still positive, for example, when gross emissions from deforestation and draining peatlands exceed gross removals from afforestation or reforestation and ecosystem conservation (Sections 2.2 and 7.2; Cross-Chapter Box 6 in Chapter 7). As there are currently no removal methods for non-CO 2 gases that have progressed beyond conceptual discussions ( [[#Jackson--2021|Jackson et al. 2021]] ), achieving net zero GHG implies gross CO 2 removals to counterbalance residual emissions of both CO 2 and non-CO 2 gases, applying 100-year global warming potential (GWP100) as the metric for reporting CO 2 -equivalent emissions, as required for emissions reporting under the Rulebook of the Paris Agreement (Cross-Chapter Box 2 in Chapter 2).

Net zero CO 2 emissions will be achieved earlier than net zero GHG emissions. As volumes of residual non-CO 2 emissions are expected to be significant, this time-lag could reach one to several decades, depending on the respective size and composition of residual GHG emissions at the time of net zero CO 2 emissions. Furthermore, counterbalancing residual non-CO 2 emissions by CO 2 removals will lead to net-negative CO 2 emissions at the time of net zero GHG emissions (Cross-Chapter Box 3 in Chapter 3).

While many governments have included A/R and other forestry measures in their NDCs under the Paris Agreement ( [[#Moe--2018|Moe and Røttereng 2018]] ; [[#Fyson--2019|Fyson and Jeffery 2019]] ; [[#Mace--2021|Mace et al. 2021]] ), and a few countries also mention BECCS, DACCS and enhanced weathering in their mid-century low emission development strategies ( [[#Buylova--2021|Buylova et al. 2021]] ), very few are pursuing the integration of a broad range of CDR methods into national mitigation portfolios so far ( [[#Schenuit--2021|Schenuit et al. 2021]] ) (Box 12.1). There are concerns that the prospect of large-scale CDR could, depending on the design of mitigation strategies, obstruct near-term emissions reduction efforts ( [[#Lenzi--2018|Lenzi et al. 2018]] ; [[#Markusson--2018|Markusson et al. 2018]] ), mask insufficient policy interventions ( [[#Geden--2016|Geden 2016]] ; [[#Carton--2019|Carton 2019]] ), might lead to an overreliance on technologies that are still in their infancy ( [[#Anderson--2016|Anderson and Peters 2016]] ; [[#Larkin--2018|Larkin et al. 2018]] ; [[#Grant--2021|Grant et al. 2021]] ), could overburden future generations ( [[#Lenzi--2018|Lenzi 2018]] ; [[#Shue--2018|Shue 2018]] ; [[#Bednar--2019|Bednar et al. 2019]] ) might evoke new conflicts over equitable burden-sharing ( [[#Pozo--2020|Pozo et al. 2020]] ; [[#Lee--2021|Lee et al. 2021]] ; [[#Mohan--2021|Mohan et al. 2021]] ), could impact food security, biodiversity or land rights ( [[#Buck--2016|Buck 2016]] ; [[#Boysen--2017|Boysen et al. 2017]] ; [[#Dooley--2018|Dooley and Kartha 2018]] ; [[#Hurlbert--2019|Hurlbert et al. 2019]] ; [[#Dooley--2021|Dooley et al. 2021]] ), or might be perceived negatively by stakeholders and broader public audiences (Royal Society and Royal Academy of Engineering 2018; [[#Colvin--2020|Colvin et al. 2020]] ). Conversely, without considering different timescales of carbon storage ( [[#Fuss--2018|Fuss et al. 2018]] ; [[#Hepburn--2019|Hepburn et al. 2019]] ) and implementation of reliable measurement, reporting and verification of carbon flows ( [[#Mace--2021|Mace et al. 2021]] ), CDR deployment might not deliver the intended benefit of removing CO 2 durably from the atmosphere. Furthermore, without appropriate incentive schemes and market designs ( [[#Honegger--2021b|Honegger et al. 2021b]] ), CDR implementation options could see under-investment. The many challenges in research, development and demonstration of novel approaches, to advance innovation according to broader societal objectives and to bring down costs, could delay their scaling up and deployment ( [[#Nemet--2018|Nemet et al. 2018]] ). Depending on the scale and deployment scenario, CDR methods could bring about various co-benefits and adverse side effects (see below). All this highlights the need for appropriate CDR governance and policies ( [[#12.3.3|Section 12.3.3]] ).

The volumes of future global CDR deployment assumed in IAM-based mitigation scenarios are large compared to current volumes of deployment, which presents a challenge since rapid and sustained upscaling from a small base is particularly difficult ( [[#de%20Coninck--2018|de Coninck et al. 2018]] ; [[#Nemet--2018|Nemet et al. 2018]] ; [[#Hanna--2021|Hanna et al. 2021]] ). All Illustrative Mitigation Pathways (IMPs) that limit warming to 2°C (&gt;67%) or lower use some form of CDR. Across the full range of similarly ambitious IAM scenarios (scenario categories C1 to C3; see [[IPCC:Wg3:Chapter:Chapter-3#3.3|Section 3.3]] ), the reported annual CO 2 removal from AFOLU (mainly A/R) reaches 0.86 [0.01–4.11] GtCO 2 yr –1 by 2030, 2.98 [0.23–6.38] GtCO 2 yr –1 by 2050, and 4.19 [0.1–6.91] GtCO 2 yr –1 by 2100 (values are the medians and bracketed values denote the 5–95th percentile range [[#footnote-003|1]] ). The annual BECCS deployment is 0.08 [0–1.09] GtCO 2 yr –1 , 2.75 [0.52–9.45] GtCO 2 yr –1 , and 8.96 [2.63–16.15] GtCO 2 yr –1 for these years, respectively. The annual DACCS deployment eaches 0 [0–0.02] GtCO 2 yr –1 by 2030, 0.02 [0–1.74] GtCO 2 yr –1 by 2050, and 1.02 [0–12.6] GtCO 2 yr –1 by 2100 (Figure 12.3). [[#footnote-002|2]] Reported cumulative volumes of BECCS, CO 2 removal from AFOLU, and DACCS reach 328 [168–763] GtCO 2 , 252 [20–418] GtCO 2 , and 29 [0–339] GtCO 2 for the 2020–2100 period, respectively. Reaching the higher end of CDR volumes is subject to issues regarding their feasibility (see below), especially if achieved with only a limited number of CDR methods. Recent studies have identified some drivers for large-scale CDR deployment in IAM scenarios, including insufficient representation of variable renewables, a high discount rate that tends to increase initial carbon budget overshoot and therefore inflates usage of CDR to achieve net-negative emissions at later times, omission of CDR methods aside from BECCS and A/R ( [[#Emmerling--2019|Emmerling et al. 2019]] ; [[#Hilaire--2019|Hilaire et al. 2019]] ; [[#Köberle--2019|Köberle 2019]] ), and limited deployment of demand-side options ( [[#Grubler--2018|Grubler et al. 2018]] ; [[#van%20Vuuren--2018|van Vuuren et al. 2018]] ; [[#Daioglou--2019|Daioglou et al. 2019]] ). The levels of CDR in IAMs in modelled pathways would change depending on the allowable overshoot of policy targets such as temperature or radiative forcing and the costs of non-CDR mitigation options ( [[#Johansson--2020|Johansson et al. 2020]] ; [[#van%20der%20Wijst--2021|van der Wijst et al. 2021]] ) ( [[IPCC:Wg3:Chapter:Chapter-3#3.2.2|Section 3.2.2]] ).

While many CDR methods are gradually being explored, IAM scenarios have focused mostly on BECCS and A/R ( [[#Tavoni--2013|Tavoni and Socolow 2013]] ; [[#Fuhrman--2019|Fuhrman et al. 2019]] ; [[#Rickels--2019|Rickels et al. 2019]] ; [[#Calvin--2021|Calvin et al. 2021]] ; [[#Diniz%20Oliveira--2021|Diniz Oliveira et al. 2021]] ). Although some IAM studies have also included other methods such as DACCS ( [[#Chen--2013|Chen and Tavoni 2013]] ; [[#Marcucci--2017|Marcucci et al. 2017]] ; [[#Realmonte--2019|Realmonte et al. 2019]] ; [[#Fuhrman--2020|Fuhrman et al. 2020]] ; [[#Akimoto--2021|Akimoto et al. 2021]] ; [[#Fuhrman--2021a|Fuhrman et al. 2021a]] ), enhanced weathering ( [[#Strefler--2021|Strefler et al. 2021]] ), SCS and biochar ( [[#Holz--2018|Holz et al. 2018]] ) there is much less literature compared to studies on BECCS ( [[#Hilaire--2019|Hilaire et al. 2019]] ). A large-scale coordinated IAM study on BECCS (‘EMF-33’) has been conducted ( [[#Muratori--2020|Muratori et al. 2020]] ; [[#Rose--2020|Rose et al. 2020]] ) but none exists for other CDR methods. A recent review proposes a combination of various CDR methods ( [[#Fuss--2018|Fuss et al. 2018]] ) but more in-depth literature on such a portfolio approach is limited ( [[#Strefler--2021|Strefler et al. 2021]] ). A multi-criteria analysis has identified pathways with CDR portfolios different from least-cost pathways often dominated by BECCS and A/R ( [[#Rueda--2021|Rueda et al. 2021]] ).

At the national and regional levels, the role of land-based biological CDR methods has long been analysed, but there is little detailed techno-economic assessment of the role of other CDR methods. There is a small but emerging literature providing such assessments for developed countries ( [[#Kraxner--2014|Kraxner et al. 2014]] ; [[#Baik--2018|Baik et al. 2018]] ; [[#Daggash--2018|Daggash et al. 2018]] ; [[#Patrizio--2018|Patrizio et al. 2018]] ; [[#Sanchez--2018|Sanchez et al. 2018]] ; [[#Breyer--2019|Breyer et al. 2019]] ; [[#Kato--2019|Kato and Kurosawa 2019]] ; [[#Larsen--2019|Larsen et al. 2019]] ; [[#McQueen--2020|McQueen et al. 2020]] ; [[#Bistline--2021|Bistline and Blanford 2021]] ; [[#García-Freites--2021|García-Freites et al. 2021]] ; [[#Jackson--2021|Jackson et al. 2021]] ; [[#Kato--2021|Kato and Kurosawa 2021]] ; [[#Negri--2021|Negri et al. 2021]] ) while the literature outside developed countries is limited ( [[#Alatiq--2021|Alatiq et al. 2021]] ; [[#Fuhrman--2021b|Fuhrman et al. 2021b]] ; [[#Weng--2021|Weng et al. 2021]] ).

In IAMs, CDR is contributed mainly by the energy sector (through BECCS) and AFOLU (through A/R) (Figure 12.3). IAMs are starting to include other CDR methods, such as DACCS and enhanced weathering ( [[#12.3.1|Section 12.3.1]] ), which are yet to be attributed to specific sectors in IAMs. Following IPCC guidance for UNFCCC inventories, A/R and SCS are reported in land use, land-use change and forestry (LULUCF), while BECCS would be reported in the sector where the carbon capture occurs, that is, the energy sector in the case of electricity and heat production, and the industry sector for BECCS linked to manufacturing (e.g., steel or hydrogen) ( [[#Tanzer--2020|Tanzer et al. 2020]] ; Bui et al. 2021; [[#Tanzer--2021|Tanzer et al. 2021]] ).

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'''Figure 12.3 | Sequestration through three predominant CDR methods: BECCS, CO''' 2 '''removal from AFOLU (mainly A/R), and DACCS (upper panels) annual sequestration and (lower panels) cumulative sequestration.''' The IAM scenarios described in the figure correspond to those that limit warming to 2°C (&gt;67%) or lower. The black line in each of the upper panels indicates the median of all the scenarios in categories C1 to C3. Hinges in the lower panels represent the interquartile ranges while whiskers extend to 5th and 95th percentiles. The IMPs are highlighted with colours, as shown in the key. The number of scenarios is indicated in the header of each panel. The number of scenarios with a non-zero DACCS value is 146.

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