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

==== 4.3.7.4 Enhanced weathering (EW) and ocean alkalinization ====

<div id="section-4-3-7-4-block-1"></div>

Weathering is the natural process of rock decomposition via chemical and physical processes in which CO <sub>2</sub> is spontaneously consumed and converted to solid or dissolved alkaline bicarbonates and/or carbonates (IPCC, 2005a) <sup>[[#fn:r656|656]]</sup> . The process is controlled by temperature, reactive surface area, interactions with biota and, in particular, water solution composition. CDR can be achieved by accelerating mineral weathering through the distribution of ground-up rock material over land (Hartmann and Kempe, 2008; Wilson et al., 2009; Köhler et al., 2010; Renforth, 2012; ten Berge et al., 2012; Manning and Renforth, 2013; Taylor et al., 2016) <sup>[[#fn:r657|657]]</sup> , shorelines (Hangx and Spiers, 2009; Montserrat et al., 2017) <sup>[[#fn:r658|658]]</sup> or the open ocean (House et al., 2007; Harvey, 2008; Köhler et al., 2013; Hauck et al., 2016) <sup>[[#fn:r659|659]]</sup> . Ocean alkalinization adds alkalinity to marine areas to locally increase the CO <sub>2</sub> buffering capacity of the ocean (González and Ilyina, 2016; Renforth and Henderson, 2017) <sup>[[#fn:r660|660]]</sup> .

In the case of land application of ground minerals, the estimated CDR potential range is 0.72–95 GtCO <sub>2</sub> yr <sup>−1</sup> ( ''low evidence, low agreement'' ) (Hartmann and Kempe, 2008; Köhler et al., 2010; Hartmann et al., 2013; Taylor et al., 2016; Strefler et al., 2018a) <sup>[[#fn:r661|661]]</sup> . Marine application of ground minerals is limited by feasible rates of mineral extraction, grinding and delivery, with estimates of  1–6 GtCO <sub>2</sub> yr <sup>−1</sup> ( ''low evidence, low agreement'' ) (Köhler et al., 2013; Hauck et al., 2016; Renforth and Henderson, 2017) <sup>[[#fn:r662|662]]</sup> . Agreement is low due to a variety of assumptions and unknown parameter ranges in the applied modelling procedures that would need to be verified by field experiments (Fuss et al., 2018) <sup>[[#fn:r663|663]]</sup> . As with other CDR options, scaling and maturity are challenges, with deployment at scale potentially requiring decades (NRC, 2015a) <sup>[[#fn:r664|664]]</sup> , considerable costs in transport and disposal (Hangx and Spiers, 2009; Strefler et al., 2018a) <sup>[[#fn:r665|665]]</sup> and mining (NRC, 2015a; Strefler et al., 2018a) <sup>[[#fn:r666|666]]</sup> <sup>[[#fn:6|6]]</sup> .

Site-specific cost estimates vary depending on the chosen technology for rock grinding (an energy-intensive process; Köhler et al., 2013; Hauck et al., 2016) <sup>[[#fn:r667|667]]</sup> , material transport, and rock source (Renforth, 2012; Hartmann et al., 2013) <sup>[[#fn:r668|668]]</sup> , and range from 15–40 USD tCO <sub>2</sub> <sup>−1</sup> to 3,460 USD tCO <sub>2</sub> <sup>−1</sup> ( ''limited evidence, low agreement'' ; Figure 4.2) (Schuiling and Krijgsman, 2006; Köhler et al., 2010; Taylor et al., 2016) <sup>[[#fn:r669|669]]</sup> . The evidence base for costs of ocean alkalinization and marine enhanced weathering is sparser than the land applications. The ocean alkalinization potential is assessed to be 0.1–10 GtCO <sub>2</sub> yr <sup>−1</sup> with costs of 14– &gt;500 USD tCO <sub>2</sub> <sup>−1</sup> (Renforth and Henderson, 2017) <sup>[[#fn:r670|670]]</sup> .

The main side effects of terrestrial EW are an increase in water pH (Taylor et al., 2016) <sup>[[#fn:r671|671]]</sup> , the release of heavy metals like Ni and Cr and plant nutrients like K, Ca, Mg, P and Si (Hartmann et al., 2013) <sup>[[#fn:r672|672]]</sup> , and changes in hydrological soil properties. Respirable particle sizes, though resulting in higher potentials, can have impacts on health (Schuiling and Krijgsman, 2006; Taylor et al., 2016) <sup>[[#fn:r673|673]]</sup> ; utilization of wave-assisted decomposition through deployment on coasts could avert the need for fine grinding (Hangx and Spiers, 2009; Schuiling and de Boer, 2010) <sup>[[#fn:r674|674]]</sup> . Side effects of marine EW and ocean alkalinization are the potential release of heavy metals like Ni and Cr (Montserrat et al., 2017) <sup>[[#fn:r675|675]]</sup> . Increasing ocean alkalinity helps counter ocean acidification (Albright et al., 2016; Feng et al., 2016) <sup>[[#fn:r676|676]]</sup> . Ocean alkalinization could affect ocean biogeochemical functioning (González and Ilyina, 2016) <sup>[[#fn:r677|677]]</sup> . A further caveat of relates to saturation state and the potential to trigger spontaneous carbonate precipitation. <sup>[[#fn:7|7]]</sup> While the geochemical potential to remove and store CO <sub>2</sub> is quite large, ''limited evidence'' on the preceding topics makes it difficult to assess the true capacity, net benefits and desirability of EW and ocean alkalinity addition in the context of CDR.

<div id="section-4-3-7-5"></div>

<span id="direct-air-carbon-dioxide-capture-and-storage-daccs"></span>