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

==== 2.3.4.2 Sustainability implications of CDR deployment in 1.5°C pathways ====

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Strong concerns about the sustainability implications of large-scale CDR deployment in deep mitigation pathways have been raised in the literature (Williamson and Bodle, 2016; Boysen et al., 2017b; Dooley and Kartha, 2018; Heck et al., 2018) <sup>[[#fn:r301|301]]</sup> , and a number of important knowledge gaps have been identified (Fuss et al., 2016) <sup>[[#fn:r302|302]]</sup> . An assessment of the literature on implementation constraints and sustainable development implications of CDR measures is provided in Chapter 4, Section 4.3.7 and the Cross-chapter Box 7 in Chapter 3. An initial discussion of potential environmental side effects of CDR deployment in 1.5°C-consistent pathways is provided in this section. Chapter 4, Section 4.3.7 then contrasts CDR deployment in 1.5°C-consistent pathways with other branches of literature on limitations of CDR. Integrated modelling aims to explore a range of developments compatible with specific climate goals and often does not include the full set of broader environmental and societal concerns beyond climate change. This has given rise to the concept of sustainable development pathways (Cross-Chapter Box 1 in Chapter 1) (van Vuuren et al., 2015) <sup>[[#fn:r303|303]]</sup> , and there is an increasing body of work to extend integrated modelling to cover a broader range of sustainable development goals (Section 2.6). However, only some of the available 1.5°C-consistent pathways were developed within a larger sustainable development context  (Bertram et al., 2018; Grubler et al., 2018; Rogelj et al., 2018; van Vuuren et al., 2018) <sup>[[#fn:r304|304]]</sup> . As discussed in Section 2.3.4.1, those pathways are characterized by low energy and/or food demand effectively limiting fossil-fuel substitution and alleviating land competition, respectively. They also include regulatory policies for deepening early action and ensuring environmental protection (Bertram et al., 2018) <sup>[[#fn:r305|305]]</sup> . Overall sustainability implications of 1.5°C-consistent pathways are assessed in Section 2.5.3 and Chapter 5, Section 5.4.

Individual CDR measures have different characteristics and therefore would carry different risks for their sustainable deployment at scale (Smith et al., 2015) <sup>[[#fn:r306|306]]</sup> . Terrestrial CDR measures, BECCS and enhanced weathering of rock powder distributed on agricultural lands require land. Those land-based measures could have substantial impacts on environmental services and ecosystems (Cross-Chapter Box 7 in Chapter 3) (Smith and Torn, 2013; Boysen et al., 2016; Heck et al., 2016; Krause et al., 2017) <sup>[[#fn:r307|307]]</sup> . Measures like afforestation and bioenergy with and without CCS that directly compete with other land uses could have significant impacts on agricultural and food systems (Creutzig et al., 2012, 2015; Calvin et al., 2014; Popp et al., 2014b, 2017; Kreidenweis et al., 2016; Boysen et al., 2017a; Frank et al., 2017; Stevanović et al., 2017; Strapasson et al., 2017; Humpenöder et al., 2018) <sup>[[#fn:r308|308]]</sup> . BECCS using dedicated bioenergy crops could substantially increase agricultural water demand (Bonsch et al., 2014; Séférian et al., 2018) <sup>[[#fn:r309|309]]</sup> and nitrogen fertilizer use (Bodirsky et al., 2014) <sup>[[#fn:r310|310]]</sup> . DACCS and BECCS rely on CCS and would require safe storage space in geological formations, including management of leakage risks (Pawar et al., 2015) <sup>[[#fn:r311|311]]</sup> and induced seismicity (Nicol et al., 2013) <sup>[[#fn:r312|312]]</sup> . Some approaches like DACCS have high energy demand (Socolow et al., 2011) <sup>[[#fn:r313|313]]</sup> . Most of the CDR measures currently discussed could have significant impacts on either land, energy, water, or nutrients if deployed at scale (Smith et al., 2015) <sup>[[#fn:r314|314]]</sup> . However, actual trade-offs depend on a multitude factors (Haberl et al., 2011; Erb et al., 2012; Humpenöder et al., 2018) <sup>[[#fn:r315|315]]</sup> , including the modalities of CDR deployment (e.g., on marginal vs. productive land) (Bauer et al., 2018) <sup>[[#fn:r316|316]]</sup> , socio-economic developments (Popp et al., 2017) <sup>[[#fn:r317|317]]</sup> , dietary choices (Stehfest et al., 2009; Popp et al., 2010; van Sluisveld et al., 2016; Weindl et al., 2017; van Vuuren et al., 2018) <sup>[[#fn:r318|318]]</sup> , yield increases, livestock productivity and other advances in agricultural technology (Havlik et al., 2013; Valin et al., 2013; Havlík et al., 2014; Weindl et al., 2015; Erb et al., 2016b) <sup>[[#fn:r319|319]]</sup> , land policies (Schmitz et al., 2012; Calvin et al., 2014; Popp et al., 2014a) <sup>[[#fn:r320|320]]</sup> , and governance of land use (Unruh, 2011; Buck, 2016; Honegger and Reiner, 2018) <sup>[[#fn:r321|321]]</sup> .

Figure 2.11 shows the land requirements for BECCS and afforestation in the selected 1.5°C-consistent pathway archetypes, including the LED (Grubler et al., 2018) <sup>[[#fn:r322|322]]</sup> and S1 pathways (Fujimori, 2017; Rogelj et al., 2018) <sup>[[#fn:r323|323]]</sup> following a sustainable development paradigm. As discussed, these land-use patterns are heavily influenced by assumptions about, among other things, future population levels, crop yields, livestock production systems, and food and livestock demand, which all vary between the pathways (Popp et al., 2017) <sup>[[#fn:r324|324]]</sup> (Section 2.3.1.1). In pathways that allow for large-scale afforestation in addition to BECCS, land demand for afforestation can be larger than for BECCS (Humpenöder et al., 2014) <sup>[[#fn:r325|325]]</sup> . This follows from the assumption in the modelled pathways that, unlike bioenergy crops, forests are not harvested to allow unabated carbon storage on the same patch of land. If wood harvest and subsequent processing or burial are taken into account, this finding can change. There are also synergies between the various uses of land, which are not reflected in the depicted pathways. Trees can grow on agricultural land (Zomer et al., 2016) <sup>[[#fn:r326|326]]</sup> , and harvested wood can be used with BECCS and pyrolysis systems (Werner et al., 2018) <sup>[[#fn:r327|327]]</sup> . The pathways show a very substantial land demand for the two CDR measures combined, up to the magnitude of the current global cropland area. This is achieved in IAMs in particular by a conversion of pasture land freed by intensification of livestock production systems, pasture intensification and/or demand changes (Weindl et al., 2017) <sup>[[#fn:r328|328]]</sup> , and to a more limited extent, cropland for food production, as well as expansion into natural land. However, pursuing such large-scale changes in land use would pose significant food supply, environmental and governance challenges, concerning both land management and tenure (Unruh, 2011; Erb et al., 2012, 2016b; Haberl et al., 2013; Haberl, 2015; Buck, 2016) <sup>[[#fn:r329|329]]</sup> , particularly if synergies between land uses, the relevance of dietary changes for reducing land demand, and co-benefits with other sustainable development objectives are not fully recognized. A general discussion of the land-use transformation in 1.5°C-consistent pathways is provided in Section 2.4.4.

An important consideration for CDR which moves carbon from the atmosphere to the geological, oceanic or terrestrial carbon pools is the permanence of carbon stored in these different pools (Matthews and Caldeira, 2008; NRC, 2015; Fuss et al., 2016; Jones et al., 2016) <sup>[[#fn:r330|330]]</sup> (see also Chapter 4, Section 4.3.7 for a discussion). Terrestrial carbon can be returned to the atmosphere on decadal time scales by a variety of mechanisms, such as soil degradation, forest pest outbreaks and forest fires, and therefore requires careful consideration of policy frameworks to manage carbon storage, for example, in forests (Gren and Aklilu, 2016) <sup>[[#fn:r331|331]]</sup> . There are similar concerns about outgassing of CO <sub>2</sub> from ocean storage (Herzog et al., 2003) <sup>[[#fn:r332|332]]</sup> , unless it is transformed to a substance that does not easily exchange with the atmosphere, for example, ocean alkalinity or buried marine biomass (Rau, 2011) <sup>[[#fn:r333|333]]</sup> . Understanding of the assessment and management of the potential risk of CO <sub>2</sub> release from geological storage of CO <sub>2</sub> has improved since the IPCC Special Report on Carbon Dioxide Capture and Storage (IPCC, 2005) <sup>[[#fn:r334|334]]</sup> with experience and the development of management practices in geological storage projects, including risk management to prevent sustentative leakage (Pawar et al., 2015) <sup>[[#fn:r335|335]]</sup> . Estimates of leakage risk have been updated to include scenarios of unregulated drilling and limited wellbore integrity (Choi et al., 2013) <sup>[[#fn:r336|336]]</sup> and find that about 70% of stored CO <sub>2</sub> would still be retained after 10,000 years in these circumstances (Alcalde et al., 2018) <sup>[[#fn:r337|337]]</sup> . The literature on the potential environmental impacts from the leakage of CO <sub>2</sub> – and approaches to minimize these impacts should a leak occur – has also grown and is reviewed by Jones et al. (2015) <sup>[[#fn:r338|338]]</sup> . To the extent that non-permanence of terrestrial and geological carbon storage is driven by socio-economic and political factors, there are parallels to questions of fossil-fuel reservoirs remaining in the ground (Scott et al., 2015) <sup>[[#fn:r339|339]]</sup> .

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'''Land-use changes in 2050 and 2100 in the illustrative 1.5°C-consistent pathway archetypes.'''
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[[File:8c674c48ea97cf8131c4724563a33782 Figure-2.11-1024x408.jpg]]

Land-use changes in 2050 and 2100 in the illustrative 1.5°C-consistent pathway archetypes (Fricko et al., 2017; Fujimori, 2017; Kriegler et al., 2017; Grubler et al., 2018; Rogelj et al., 2018) <sup>[[#fn:r340|340]]</sup> . Changes in land for food crops, energy crops, forest, pasture and other natural land are shown, compared to 2010.

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

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