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

==== 4.3.7.5 Direct air carbon dioxide capture and storage (DACCS) ====

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Capturing CO <sub>2</sub> from ambient air through chemical processes with subsequent storage of the CO <sub>2</sub> in geological formations is independent of source and timing of emissions and can avoid competition for land. Yet, this is also the main challenge: while the theoretical potential for DACCS is mainly limited by the availability of safe and accessible geological storage, the CO <sub>2</sub> concentration in ambient air is 100–300 times lower than at gas- or coal-fired power plants (Sanz-Pérez et al., 2016) <sup>[[#fn:r678|678]]</sup> thus requiring more energy than flue gas CO <sub>2</sub> capture (Pritchard et al., 2015) <sup>[[#fn:r679|679]]</sup> . This appears to be the main challenge to DACCS (Sanz-Pérez et al., 2016; Barkakaty et al., 2017) <sup>[[#fn:r680|680]]</sup> .

Studies explore alternative techniques to reduce the energy penalty of DACCS (van der Giesen et al., 2017) <sup>[[#fn:r681|681]]</sup> . Energy consumption could be up to 12.9 GJ tCO <sub>2</sub> -eq <sup>−1</sup> ; translating into an average of 156 EJ yr <sup>−1</sup> by 2100 (current annual global primary energy supply is 600 EJ); water requirements are estimated to average 0.8–24.8 km <sup>3</sup> GtCO <sub>2</sub> -eq <sup>−1</sup> yr <sup>−1</sup>  (Smith et al., 2016b, based on Socolow et al., 2011) <sup>[[#fn:r682|682]]</sup> .

However, the literature shows ''low agreement'' and is fragmented (Broehm et al., 2015) <sup>[[#fn:r683|683]]</sup> . This fragmentation is reflected in a large range of cost estimates: from 20–1,000 USD tCO <sub>2</sub> <sup>−1</sup> (Keith et al., 2006; Pielke, 2009; House et al., 2011; Ranjan and Herzog, 2011; Simon et al., 2011; Goeppert et al., 2012; Holmes and Keith, 2012; Zeman, 2014; Sanz-Pérez et al., 2016; Sinha et al., 2017) <sup>[[#fn:r684|684]]</sup> . There is lower agreement and a smaller evidence base at the lower end of the cost range. Fuss et al. (2018) <sup>[[#fn:r685|685]]</sup> narrow this range to 100–300 USD tCO <sub>2</sub> <sup>-1</sup> .

Research and efforts by small-scale commercialization projects focus on utilization of captured CO <sub>2</sub> (Wilcox et al., 2017) <sup>[[#fn:r686|686]]</sup> . Given that only a few IAM scenarios incorporate DACCS (e.g., Chen and Tavoni, 2013; <sup>[[#fn:r687|687]]</sup> Strefler et al., 2018b) <sup>[[#fn:r688|688]]</sup> its possible role in cost-optimized 1.5°C scenarios is not yet fully explored. Given the technology’s early stage of development (McLaren, 2012; NRC, 2015a; Nemet et al., 2018) <sup>[[#fn:r689|689]]</sup> and few demonstrations (Holmes et al., 2013; Rau et al., 2013; Agee et al., 2016) <sup>[[#fn:r690|690]]</sup> , deploying the technology at scale is still a considerable challenge, though both optimistic (Lackner et al., 2012) <sup>[[#fn:r691|691]]</sup> and pessimistic outlooks exist (Pritchard et al., 2015) <sup>[[#fn:r692|692]]</sup> .

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