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

==== Cross-cutting issues and uncertainties across carbon dioxide removal (CDR) options, aspects and uncertainties ====

{| class="wikitable"
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! Area of Uncertainty
! Cross-Cutting Issues and Uncertainties
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| Technology upscaling
| * CDR options are at different stages of technological readiness (McLaren, 2012) <sup>[[#fn:r712|712]]</sup> and differ with respect to scalability.
* Nemet et al. (2018)  <sup>[[#fn:r713|713]]</sup> find &gt;50% of the CDR innovation literature concerned with the earliest stages of the innovation process (R&amp;D), identifying a dissonance between the large CO <sub>2</sub> removals needed in 1.5°C pathways and the long -time periods involved in scaling up novel technologies.
* Lack of post-R&amp;D literature, including incentives for early deployment, niche markets, scale up, demand, and public acceptance.

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| Emerging and niche technologies
| * For BECCS, there are niche opportunities with high efficiencies and fewer trade-offs, for example, sugar and paper processing facilities (Möllersten et al., 2003) <sup>[[#fn:r714|714]]</sup> , district heating (Kärki et al., 2013; Ericsson and Werner, 2016) <sup>[[#fn:r715|715]]</sup> ,  and industrial and municipal waste (Sanna et al., 2012) <sup>[[#fn:r716|716]]</sup> . Turner et al. (2018)  <sup>[[#fn:r717|717]]</sup> constrain potential using sustainability considerations and overlap with storage basins to avoid the CO <sub>2</sub> transportation challenge, providing a possible, though limited entry point for BECCS.
* The impacts on land use, water, nutrients and albedo of BECCS could be alleviated using marine sources of biomass that could include aquacultured micro and macro flora (Hughes et al., 2012; Lenton, 2014) <sup>[[#fn:r718|718]]</sup> .
* Regarding captured CO <sub>2</sub> as a resource is discussed as an entry point for CDR. However, this does not necessarily lead to carbon removals, particularly if the CO <sub>2</sub> is sourced from fossil fuels and/or if the products do not store the CO <sub>2</sub> for climate-relevant horizons (von der Assen et al., 2013)  <sup>[[#fn:r719|719]]</sup> (see also Section 4.3.4.5).
* Methane <sup>[[#fn:8|8]]</sup> is a much more potent GHG than CO <sub>2</sub> (Montzka et al., 2011) <sup>[[#fn:r720|720]]</sup> , associated with difficult-to-abate emissions in industry and agriculture and with outgassing from lakes, wetlands, and oceans (Lockley, 2012; Stolaroff et al., 2012) <sup>[[#fn:r721|721]]</sup> . Enhancing processes that naturally remove methane, either by chemical or biological decomposition (Sundqvist et al., 2012) <sup>[[#fn:r722|722]]</sup> , has been proposed to remove CH <sub>4</sub> . There is ''low confidence'' that existing technologies for CH <sub>4</sub>  removal are economically or energetically suitable for large-scale air capture (Boucher and Folberth, 2010) <sup>[[#fn:r723|723]]</sup> . Methane removal potentials are limited due to its low atmospheric concentration and its low chemical reactivity at ambient conditions.

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| Ethical aspects
| * Preston (2013)  <sup>[[#fn:r724|724]]</sup> identifies distributive and procedural justice, permissibility, moral hazard (Shue, 2018) <sup>[[#fn:r725|725]]</sup> , and hubris as ethical aspects that could apply to large-scale CDR deployment.
* There is a lack of reflection on the climate futures produced by recent modelling and implying very different ethical costs/risks and benefits (Minx et al., 2018) <sup>[[#fn:r726|726]]</sup> .

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| Governance
| * Existing governance mechanisms are scarce and either targeted at particular CDR options (e.g., ocean-based) or aspects (e.g., concerning indirect land-use change (iLUC)) associated with bioenergy upscaling, and often the mechanisms are at national or regional scale (e.g., EU). Regulation accounting for iLUC by formulating sustainability criteria (e.g., the EU Renewable Energy Directive) has been assessed as insufficient in avoiding leakage (e.g., Frank et al., 2013) <sup>[[#fn:r727|727]]</sup> .
* An international governance mechanism is only in place for R&amp;D of ocean fertilization within the Convention on Biological Diversity (IMO, 1972, 1996; CBD, 2008, 2010) <sup>[[#fn:r728|728]]</sup> .
* Burns and Nicholson (2017)  <sup>[[#fn:r729|729]]</sup> propose a human rights-based approach to protect those potentially adversely impacted by CDR options.

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| Policy
| * The CDR potentials that can be realized are constrained by the lack of policy portfolios incentivising large-scale CDR (Peters and Geden, 2017) <sup>[[#fn:r730|730]]</sup> .
* Near-term opportunities could be supported through modifying existing policy mechanisms (Lomax et al., 2015) <sup>[[#fn:r731|731]]</sup> .
* Scott and Geden (2018)  <sup>[[#fn:r732|732]]</sup> sketch three possible routes for limited progress, (i) at EU-level, (ii) at EU Member State level, and (iii) at private sector level, noting the implied paradigm shift this would entail.
* EU may struggle to adopt policies for CDR deployment on the scale or time-frame envisioned by IAMs (Geden et al., 2018) <sup>[[#fn:r733|733]]</sup> .
* Social impacts of large-scale CDR deployment (Buck, 2016) <sup>[[#fn:r734|734]]</sup> require policies taking these into account.

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| Carbon cycle
| * On long time scales, natural sinks could reverse (C.D. Jones et al., 2016)  <sup>[[#fn:r735|735]]</sup>
* No robust assessments yet of the effectiveness of CDR in reverting climate change (Tokarska and Zickfeld, 2015; Wu et al., 2015; Keller et al., 2018) <sup>[[#fn:r736|736]]</sup> , see also Chapter 2, Section 2.2.2.2.

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