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

===== 5.6.2.2.3 Geochemical CDR methods =====

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Enhanced weathering (EW) is based on naturally occurring weathering processes of silicate and carbonate rocks, removing CO <sub>2</sub> from the atmosphere. Weathering is accelerated by spreading ground rocks on soils, coasts or oceans. EW increases the alkalinity and pH of natural waters, helps dampen ocean acidification and increases ocean carbon uptake ( [[#Beerling--2018|Beerling et al., 2018]] ). The dissolution of minerals stimulates biological productivity of croplands ( [[#Hartmann--2013|Hartmann et al., 2013]] ; [[#Beerling--2018|Beerling et al., 2018]] ), but can also liberate toxic trace metals (such as nickel, chromium, copper) into soil or water bodies ( [[#Keller--2018a|Keller et al., 2018a]] ; [[#Strefler--2018|Strefler et al., 2018]] ). EW can also contribute to freshwater salinization as a result of increased salt inputs and cation exchange in watersheds, and so adversely affecting drinking water quality ( ''low confidence'' ) ( [[#Kaushal--2018|Kaushal et al., 2018]] ). With a ''medium confidence'' , amendment of soils with minerals will have lower N <sub>2</sub> O emissions ( [[#Kantola--2017|Kantola et al., 2017]] ; [[#Blanc-Betes--2020|Blanc-Betes et al., 2020]] ) but will not have a marked effect on evapotranspiration or albedo ( [[#Fuss--2018|Fuss et al., 2018]] ; [[#de%20Oliveira%20Garcia--2020|de Oliveira Garcia et al., 2020]] ). The mining of minerals can cause adverse impacts on biodiversity, however, the use of waste materials such as concrete demolition or steel slags for EW can reduce the need for mining ( [[#Renforth--2019|Renforth, 2019]] ). The spreading of minerals on land has a neutral impact on biodiversity ( [[#Smith--2018|P. Smith et al., 2018]] ).

Ocean alkalinization, via the deposition of alkaline minerals (e.g., olivine) or their dissociation products (e.g., quicklime) at the ocean surface, can increase surface total alkalinity and thus increase CO <sub>2</sub> uptake and storage (Glossary; Supplementary Material Text 5.SM.3; AR6 WGIII Chapter 12; [[#GESAMP--2019|GESAMP, 2019]] ; [[#Keller--2019|Keller, 2019]] ). Ocean alkalinization ameliorates surface ocean acidification ( ''high confidence'' ) ( [[#Hauck--2016|Hauck et al., 2016]] ; [[#Tran--2020|Tran et al., 2020]] ), but there are also negative side effects on the marine ecosystem, most of which are poorly understood or quantified (Figure 5.36 and Supplementary Materials Table 5.SM.4; [[#Bach--2019|Bach et al., 2019]] ). Although ocean alkalinization could potentially sequester large amounts of carbon (≥1 PgC yr <sup>–1</sup> ; Figure 5.36; and Supplementary Materials Table 5.SM.5) there is no new evidence to revisit the SROCC (SROCC [[#5.5.1.2.4|Section 5.5.1.2.4]] ) conclusion that there is ''low confidence'' that ocean alkalinization is a viable climate change mitigation approach.

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