Editing IPCC:AR6/SRCCL/Chapter-5 (section)

=== 5.6.1 Land-based carbon dioxide removal (CDR) and bioenergy ===

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Large-scale deployment of negative emission technologies (NETs) in emission scenarios has been identified as necessary for avoiding unacceptable climate change (IPCC 2018b <sup>[[#fn:r950|950]]</sup> ). Among the available NETs, carbon dioxide removal (CDR) technologies are receiving increasing attention. Land-based CDRs include afforestation and reforestation (AR), sustainable forest management, biomass energy with carbon capture and storage (BECCS), and biochar (BC) production (Minx et al. 2018 <sup>[[#fn:r951|951]]</sup> ). Most of the literature on global land-based mitigation potential relies on CDRs, particularly on BECCS, as a major mitigation action (Kraxner et al. 2014 <sup>[[#fn:r952|952]]</sup> ; Larkin et al. 2018 <sup>[[#fn:r953|953]]</sup> and Rogelj et al. 2018, 2015, 2011). BECCS is not yet deployable at a significant scale, as it faces challenges similar to fossil fuel carbon capture and storage (CCS) (Fuss et al. 2016 <sup>[[#fn:r954|954]]</sup> ; Vaughan and Gough 2016 <sup>[[#fn:r955|955]]</sup> ; Nemet et al. 2018 <sup>[[#fn:r956|956]]</sup> ). Regardless, the effectiveness of large-scale BECCS to meet Paris Agreement goals has been questioned and other pathways to mitigation have been proposed (Anderson and Peters 2016 <sup>[[#fn:r957|957]]</sup> ; van Vuuren et al. 2017, 2018; Grubler et al. 2018 <sup>[[#fn:r958|958]]</sup> ; Vaughan and Gough 2016 <sup>[[#fn:r959|959]]</sup> ).

Atmospheric CO <sub>2</sub> removal by storage in vegetation depends on achieving net organic carbon accumulation in plant biomass over decadal time scales (Kemper 2015 <sup>[[#fn:r960|960]]</sup> ) and, after plant tissue decay, in soil organic matter (Del Grosso et al. 2019 <sup>[[#fn:r961|961]]</sup> ). AR, BECCS and BC differ in the use and storage of plant biomass. In BECCS, biomass carbon from plants is used in industrial processes (e.g., for electricity, hydrogen, ethanol, and biogas generation), releasing CO <sub>2</sub> , which is then captured and geologically stored (Greenberg et al. 2017 <sup>[[#fn:r962|962]]</sup> ; Minx et al. 2018 <sup>[[#fn:r963|963]]</sup> ).

Afforestation and reforestation result in long-term carbon storage in above and belowground plant biomass on previously unforested areas, and is effective as a carbon sink during the AR establishment period, in contrast to thousands of years for geological carbon storage (Smith et al. 2016 <sup>[[#fn:r964|964]]</sup> ).

Biochar is produced from controlled thermal decomposition of biomass in absence of oxygen (pyrolysis), a process that also yields combustible oil and combustible gas in different proportions. Biochar is a very stable carbon form, with storage on centennial time scales (Lehmann et al. 2006 <sup>[[#fn:r965|965]]</sup> ) (Chapter 4). Incorporated in soils, some authors suggest it may lead to improved water-holding capacity, nutrient retention, and microbial processes (Lehmann et al. 2015 <sup>[[#fn:r966|966]]</sup> ). There is, however, uncertainty about the benefits and risks of this practice (The Royal Society 2018).

Land-based CDRs require high biomass-producing crops. Since not all plant biomass is harvested (e.g., roots and harvesting losses), it can produce co-benefits related to soil carbon sequestration, crop productivity, crop quality, as well improvements in air quality, but the overall benefits strongly depend on the previous land-use and soil management practices (Smith et al. 2016 <sup>[[#fn:r967|967]]</sup> ; Wood et al. 2018 <sup>[[#fn:r968|968]]</sup> ). In addition, CDR effectiveness varies widely depending on type of biomass, crop productivity, and emissions offset in the energy system. Importantly, its mitigation benefits can be easily lost due to land-use change interactions (Harper et al. 2018 <sup>[[#fn:r969|969]]</sup> ; Fuss et al. 2018 <sup>[[#fn:r970|970]]</sup> ; Daioglou et al. 2019 <sup>[[#fn:r971|971]]</sup> ).

Major common challenges of implementing these large-scale CDR solutions, as needed to stabilise global temperature at ‘well-below’ 2°C by the end of the century, are the large investments and the associated significant changes in land use required. Most of the existing scenarios estimate the global area required for energy crops in the range of 109–990 Mha (IPCC 2018a <sup>[[#fn:r972|972]]</sup> ), most commonly around 380–700 Mha (Smith et al. 2016 <sup>[[#fn:r973|973]]</sup> ), reaching net area expansion rates of up to 23.7 Mha yr–1 (IPCC 2018b <sup>[[#fn:r974|974]]</sup> ). The upper limit implies unprecedented rates of area expansion for crops and forestry observed historically, for instance, as reported by FAO since 1961 (FAOSTAT 2018 <sup>[[#fn:r975|975]]</sup> ). By comparison, the sum of recent worldwide rates of expansion in the harvested area of soybean and sugarcane has not exceeded 3.5 Mha yr–1 on average. Even at this rate, they have been the source of major concerns for their possible negative environmental and food security impacts (Boerema et al. 2016 <sup>[[#fn:r976|976]]</sup> ; Popp et al. 2014 <sup>[[#fn:r977|977]]</sup> ).

Most land area available for CDR is currently pasture, estimated at 3300 Mha globally (FAOSTAT 2018 <sup>[[#fn:r978|978]]</sup> ). However, there is ''low confidence'' about how much low-productivity land is actually available for CDR (Lambin et al. 2013 <sup>[[#fn:r979|979]]</sup> and Gibbs and Salmon 2015). There is also ''low confidence'' as to whether the transition to BECCS will take place directly on low-productivity grasslands (Johansson and Azar 2007 <sup>[[#fn:r980|980]]</sup> ), and uncertainty on the governance mechanisms required to avoid unwanted spill-over effects, for instance causing additional deforestation (Keles et al. 2018 <sup>[[#fn:r981|981]]</sup> ).

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Further, grasslands and rangelands may often occur in marginal areas, in which case, they may be exposed to climate risks, including periodic flooding. Grasslands and especially rangelands and savannas tend to predominate in less-developed regions, often bordering areas of natural vegetation with little infrastructure available for transport and processing of large quantities of CDR-generated biomass (O’Mara 2012 <sup>[[#fn:r982|982]]</sup> ; Beringer et al. 2011 <sup>[[#fn:r983|983]]</sup> ; Haberl et al. 2010 <sup>[[#fn:r984|984]]</sup> ; Magdoff 2007 <sup>[[#fn:r985|985]]</sup> ).

CDR-driven reductions in the available pastureland area is a scenario of constant or increasing global animal protein output as proposed by Searchinger et al. (2018) <sup>[[#fn:r986|986]]</sup> . However, despite the recent reduction in meat consumption in western countries, this will require productivity improvements (Cohn et al. 2014 <sup>[[#fn:r987|987]]</sup> ; Strassburg et al. 2014 <sup>[[#fn:r988|988]]</sup> ). It would also result in lower emission intensities and create conditions for increased soil carbon stocks (de Oliveira Silva et al. 2016 <sup>[[#fn:r990|990]]</sup> ; Searchinger et al. 2018 <sup>[[#fn:r991|991]]</sup> ; Soussana et al. 2019 <sup>[[#fn:r992|992]]</sup> , 2013). At the same time, food security may be threatened if land-based mitigation displaced crops elsewhere, especially if to regions of lower productivity potential, higher climatic risk, and higher vulnerability.

There is low agreement about what are the more competitive regions of the world for CDRs. Smith et al. (2016) <sup>[[#fn:r993|993]]</sup> and Vaughan et al. (2018) <sup>[[#fn:r994|994]]</sup> identify as candidates relatively poor countries in Latin America, Africa and Asia (except China and India). Others indicate those regions may be more competitive for food production, placing Europe as a major BECCS exporter (Muratori et al. 2016 <sup>[[#fn:r995|995]]</sup> ). Economically feasible CDR investments are forecast to be directed to regions with high biomass production potential, demand for extra energy production, low leakage potential for deforestation and low competition for food production (Vaughan et al. 2018 <sup>[[#fn:r996|996]]</sup> ). Latin America and Africa, for instance, although having high biomass production potential, still have low domestic energy consumption (589 and 673 MTOE – 24.7 and 28.2 EJ, respectively), with about 30% of primary energy from renewable sources (reaching 50% in Brazil), mainly hydropower and traditional biomass.

There is ''high confidence'' that deployment of BECCS will require ambitious investments and policy interventions (Peters and Geden 2017 <sup>[[#fn:r997|997]]</sup> ) with strong regulation and governance of bioenergy production to ensure protection of forests, maintain food security and enhance climate benefits (Burns and Nicholson 2017 <sup>[[#fn:r998|998]]</sup> ; Vaughan et al. 2018 <sup>[[#fn:r999|999]]</sup> ; Muratori et al. 2016 <sup>[[#fn:r1000|1000]]</sup> ), and that such conditions may be challenging for developing countries. Increased value of bioenergy puts pressure on land, ecosystem services, and the prices of agricultural commodities, including food ( ''high confidence'' ).

There is ''medium confidence'' for the impact of CDR technologies on increased food prices and reduced food security, as these depend on several assumptions. Nevertheless, those impacts could be strong, with food prices doubling under certain scenario combinations (Popp et al. 2017 <sup>[[#fn:r1001|1001]]</sup> ). The impacts of land-mitigation policies on the reduction of dietary energy availability alone (without climate change impacts) is estimated at over 100 kcal per person per day by 2050, with highest regional impacts in South Asia and Sub-Saharan Africa (Hasegawa et al. 2018 <sup>[[#fn:r1002|1002]]</sup> ) (Section 5.2). However, only limited pilot BECCS projects have been implemented to date (Lenzi et al. 2018 <sup>[[#fn:r1003|1003]]</sup> ). Integrated assessment models (IAMs) use theoretical data based on high-level studies and limited regional data from the few on-the-ground BECCS projects.

Furthermore, it has been suggested that several BECCS IAM scenarios rely on unrealistic assumptions regarding regional climate, soils and infrastructure suitability (Anderson and Peters 2016 <sup>[[#fn:r1004|1004]]</sup> ), as well as international bioenergy trade (Lamers et al. 2011 <sup>[[#fn:r1005|1005]]</sup> ). Current global IAMs usually consider major trends in production potential and projected demand, overlooking major challenges for the development of a reliable international market. Such a market will have to be created from scratch and overcome a series of constraints, including trade barriers, logistics, and supply chains, as well as social, ecological and economic impacts (Matzenberger et al. 2015 <sup>[[#fn:r1006|1006]]</sup> ).

In summary, there is high agreement that better assessment of BECCS mitigation potential would need to be based on increased regional, bottom-up studies of biomass potentials, socio-economic consequences (including on food security), and environmental impacts in order to develop more realistic estimates (IPCC 2018a <sup>[[#fn:r1007|1007]]</sup> ).

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