Editing IPCC:AR6/WGIII/Chapter-12 (section)

==== 12.3.1.2 Enhanced Weathering ====

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Enhanced weathering involves (i) the mining of rocks containing minerals that naturally absorb CO 2 from the atmosphere over geological timescales (as they become exposed to the atmosphere through geological weathering), (ii) the comminution of these rocks to increase the surface area, and (iii) the spreading of these crushed rocks on soils (or in the ocean/coastal environments; [[#12.3.1.3|Section 12.3.1.3]] ) so that they react with atmospheric CO 2 ( [[#Schuiling--2006|Schuiling and Krijgsman 2006]] ; [[#Hartmann--2013|Hartmann et al. 2013]] ; [[#Beerling--2018|Beerling et al. 2018]] ; [[#Goll--2021|Goll et al. 2021]] ). Construction waste and waste materials from mining can also be used as a source material for enhanced weathering. Silicate rocks such as basalt, containing minerals rich in calcium and magnesium and lacking metal ions such as nickel and chromium, are most suitable for enhanced weathering ( [[#Beerling--2018|Beerling et al. 2018]] ); they reduce soil solution acidity during dissolution, and promote the chemical transformation of CO 2 to bicarbonate ions. The bicarbonate ions can precipitate in soils and drainage waters as a solid carbonate mineral ( [[#Manning--2008|Manning 2008]] ), or remain dissolved and increase alkalinity levels in the ocean when the water reaches the sea ( [[#Renforth--2017|Renforth and Henderson 2017]] ). The modelling study by [[#Cipolla--2021|Cipolla et al. (2021)]] found that rate of weathering is greater in high rainfall environments, and was increased by organic matter amendment.

'''Status:''' Enhanced weathering has been demonstrated in the laboratory and in small-scale field trials (TRL 3–4) but has yet to be demonstrated at scale ( [[#Beerling--2018|Beerling et al. 2018]] ; [[#Amann--2020|Amann et al. 2020]] ). The chemical reactions are well understood ( [[#Manning--2008|Manning 2008]] ; [[#Gillman--1980|Gillman 1980]] ; [[#Gillman--2001|Gillman et al. 2001]] ), but the behaviour of the crushed rocks in the field and potential co-benefits and adverse side effects of enhanced weathering require further research ( [[#Beerling--2018|Beerling et al. 2018]] ). Small-scale laboratory experiments have calculated weathering rates that are orders of magnitude slower than the theoretical limit for mass transfer-controlled forsterite ( [[#Renforth--2015|Renforth et al. 2015]] ; [[#Amann--2020|Amann et al. 2020]] ) and basalt dissolution ( [[#Kelland--2020|Kelland et al. 2020]] ). Uncertainty surrounding silicate mineral dissolution rates in soils, the fate of the released products, the extent of legacy reserves of mining by-products that might be exploited, location and availability of rock extraction sites, and the impact on ecosystems remain poorly quantified and require further research to better understand feasibility ( [[#Renforth--2012|Renforth 2012]] ; [[#Moosdorf--2014|Moosdorf et al. 2014]] ; [[#Beerling--2018|Beerling et al. 2018]] ). Closely monitored, large-scale demonstration projects would allow these aspects to be studied ( [[#Smith--2019a|Smith et al. 2019a]] ; [[#Beerling--2020|Beerling et al. 2020]] ).

'''Costs:''' [[#Fuss--2018|Fuss et al. (2018)]] , in a systematic review of the costs and potentials of CDR methods including enhanced weathering, note that costs are closely related to the source of the rock and the technology used for rock grinding and material transport ( [[#Renforth--2012|Renforth 2012]] ; [[#Hartmann--2013|Hartmann et al. 2013]] ; [[#Strefler--2018|Strefler et al. 2018]] ). Due to differences in the methods and assumptions between studies, literature ranges are highly uncertain and range from USD15–40 tCO 2 –1 to USD3460 tCO 2 –1 ( [[#Köhler--2010|Köhler et al. 2010]] ; [[#Taylor--2016|Taylor et al. 2016]] ). [[#Renforth--2012|Renforth (2012)]] reported operational costs in the UK of applying mafic rocks (rocks with high magnesium and iron silicate mineral concentrations) of USD70–578 tCO 2 –1 , and for ultramafic rocks (rocks rich in magnesium and iron silicate minerals but with very low silica content – the low silica content enhances weathering rates) of USD24–123 tCO 2 –1 . [[#Beerling--2020|Beerling et al. (2020)]] combined a spatially resolved weathering model with a techno-economic assessment to suggest costs of between USD54–220 tCO 2 –1 (with a weighted mean of USD118–128 tCO 2 –1 ). [[#Fuss--2018|Fuss et al. (2018)]] suggested an author judgement cost range of USD50–200 tCO 2 –1 for a potential of 2–4 GtCO 2 yr −1 from 2050, excluding biological storage.

'''Potentials:''' In a systematic review of the costs and potentials of enhanced weathering, [[#Fuss--2018|Fuss et al. (2018)]] report a wide range of potentials ( ''limited evidence'' , ''low agreement'' ). The highest reported regional sequestration potential, 88.1 GtCO 2 yr −1 , is reported for the spreading of pulverised rock over a very large land area in the tropics, a region considered promising given the higher temperatures and greater rainfall ( [[#Taylor--2016|Taylor et al. 2016]] ). Considering cropland areas only, the potential carbon removal was estimated by [[#Strefler--2018|Strefler et al. (2018)]] to be 95 GtCO 2 yr −1 for dunite and 4.9 GtCO 2 yr −1 for basalt. Slightly lower potentials were estimated by [[#Lenton--2014|Lenton (2014)]] where the potential of carbon removal by enhanced weathering (including adding carbonate and olivine to both oceans and soils) was estimated to be 3.7 GtCO 2 yr –1 by 2100, but with mean annual removal an order of magnitude less at 0.2 GtC-eq yr –1 ( [[#Lenton--2014|Lenton 2014]] ). The estimates reported in [[#Smith--2016|Smith et al. (2016)]] are based on the potential estimates of [[#Lenton--2014|Lenton (2014)]] . [[#Beerling--2020|Beerling et al. (2020)]] estimate that up to 2 GtCO 2 yr –1 could be removed by 2050 by spreading basalt onto 35–59% (weighted mean 53%) of agricultural land of 12 countries. [[#Fuss--2018|Fuss et al. (2018)]] provide an author judgement range for potential of 2–4 GtCO 2 yr −1 for 2050.

'''Risks and impacts:''' Mining of rocks for enhanced weathering will have local impacts and carries risks similar to those associated with the mining of mineral construction aggregates, with the possible additional risk of greater dust generation from fine comminution and land application. In addition to direct habitat destruction and increased traffic to access mining sites, there could be adverse impacts on local water quality ( [[#Younger--2004|Younger and Wolkersdorfer 2004]] ).

'''Co-benefits:''' Enhanced weathering can improve plant growth by pH modification and increased mineral supply ( [[#Kantola--2017|Kantola et al. 2017]] ; [[#Beerling--2018|Beerling et al. 2018]] ), can enhance SCS in some soils ( [[#Beerling--2018|Beerling et al. 2018]] ) thereby protecting against soil erosion ( [[#Wright--1998|Wright and Upadhyaya 1998]] ), and increasing the cation exchange capacity, resulting in increased nutrient retention and availability ( [[#Gillman--1980|Gillman 1980]] ; [[#Baldock--2000|Baldock and Skjemstad 2000]] ; [[#Gillman--2001|Gillman et al. 2001]] ; [[#Manning--2010|Manning 2010]] ; [[#Guntzer--2012|Guntzer et al. 2012]] ; [[#Tubana--2016|Tubana et al. 2016]] ; [[#Yu--2017|Yu et al. 2017]] ; [[#Haque--2019|Haque et al. 2019]] ; [[#Smith--2019a|Smith et al. 2019a]] ). Through these actions, it can contribute to SDG 2 (zero hunger), SDG 15 (life on land) (by reducing land demand for croplands), SDG 13 (climate action) (through CDR), SDG 14 (life below water) (by ameliorating ocean acidification) and SDG 6 (clean water and sanitation) ( [[#Smith--2019a|Smith et al. 2019a]] ). To more directly ameliorate ocean acidification while increasing CDR and reducing impacts on land ecosystems, alkaline minerals could instead be directly added to the ocean ( [[#12.3.1.3|Section 12.3.1.3]] ). There are potential benefits in poverty reduction through employment of local workers in mining ( [[#Pegg--2006|Pegg 2006]] ).

'''Trade-offs and spillover effects:''' Air quality could be adversely affected by the spreading of rock dust ( [[#Edwards--2017|Edwards et al. 2017]] ), though this can partly be ameliorated by water-spraying ( [[#Grundnig--2006|Grundnig et al. 2006]] ). As noted above, any significant expansion of the mining industry would require careful assessment to avoid possible detrimental effects on biodiversity ( [[#Amundson--2015|Amundson et al. 2015]] ). The processing of an additional 10 billion tonnes of rock would require up to 3000 Terawatt-hours of energy, which could represent approximately 0.1–6 % of global electricity use in 2100. The emissions associated with this additional energy generation may reduce the net carbon dioxide removal by up to 30% with present-day grid average emissions, but this efficiency loss would decrease with low-carbon power ( [[#Beerling--2020|Beerling et al. 2020]] ).

'''Role in mitigation pathways:''' Only one study to date has included enhanced weathering in an integrated assessment model to explore mitigation pathways ( [[#Strefler--2021|Strefler et al. 2021]] ).

Status, costs, potentials, risk and impacts, co-benefits, trade-offs and spillover effects and the role in mitigation pathways of enhanced weathering are summarised in Table 12.6.

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