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

==== 5.6.2.2 Effects of Specific CDR Methods on Biogeochemical Cycles and Climate ====

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The AR5 WGI (Chapter 6) discussed the CDR methods, their implications and unintended side effects on carbon cycle and climate, including their time scales and potentials. Since then, three IPCC special reports (SR) have been published. First, SR1.5 ( [[IPCC:Wg1:Chapter:Chapter-4|Chapter 4]] ( [[#IPCC--2018|IPCC, 2018]] ) assessed the potentials and current understanding, including the side effects, of bioenergy with carbon capture and storage (BECCS), afforestation/reforestation, soil carbon sequestration, biochar, enhanced weathering, ocean alkalinization, direct air carbon capture with storage (DACCS) and ocean fertilization. Second, SRCCL ( [[IPCC:Wg1:Chapter:Chapter-6|Chapter 6]] ( [[#IPCC--2019a|IPCC, 2019a]] ) assessed the potentials, co-benefits and trade-offs of land-based mitigation options. It assessed with ''high confidence'' that land-based CDR options do not sequester carbon indefinitely, except for peatland restoration. Multiple co-benefits were identified in the deployment of CDR options, many of them with a potential to make positive contributions to sustainable development, enhancement of ecosystem functions and services and other societal goals. However, their potential was concluded to be context specific, and limits were identified in their contribution to global mitigation, such as competition for land. The third report, the Special Report on Ocean and the Cryosphere in a Changing Climate (SROCC) [https://www.ipcc.ch/report/ar6/wg1/chapter/chapter-5 Chapter 5] ( [[#IPCC--2019b|IPCC, 2019b]] ), assessed the potential of marine options for climate change mitigation. It concluded that the feasibility of open ocean fertilization and alkalinization approaches were negligible, due to their inconclusive influence on ocean carbon storage on long time scales, due to the unintended side effects on marine ecosystems, and the associated governance challenges. The assessment of blue carbon ecosystems concluded that they could contribute only minimally to atmospheric CO <sub>2</sub> reduction globally, but emphasized that the benefits of protecting and restoring coastal blue carbon extend beyond climate change mitigation (SROCC ( [[#5.5.1|Section 5.5.1]] 2).

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===== 5.6.2.2.1 Land-based biological CDR methods =====

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Biological CDR methods, introduced in Table 5.9, seek to increase carbon storage on land by enhancing net primary productivity and/or reducing CO <sub>2</sub> sources to the atmosphere.

Forest-based methods include afforestation, reforestation, and forest management (Table 5.9). Building on previous work that emphasized the global potentials of various options, more recent advances have focused on the limits of those global potentials in light of ecological and climate risks that can threaten the long-term permanence of carbon stocks ( [[#Boysen--2017b|Boysen et al., 2017b]] ; [[#Anderegg--2020|Anderegg et al., 2020]] ). Some of those risks arise from droughts, fires, insect outbreaks, diseases, erosion, and other disturbances ( [[#Thompson--2009|Thompson et al., 2009]] ).

Sustainable forest management can help to manage some of these vulnerabilities, while in some cases it can increase and maintain forest sinks through harvest, transfer of carbon to wood products and their use to store carbon and substitute emissions-intensive construction materials ( [[#Churkina--2020|Churkina et al., 2020]] ). Forest genomics techniques can increase the success of both reforestation and conservation initiatives, accelerating breeding for tree health and productivity ( [[#Isabel--2020|Isabel et al., 2020]] ).

In response to increasing risks to permanence of carbon stocks of some types of afforestation practices and the competition for land, there has been an increasing recognition that secondary forest regrowth and restoration of degraded forests and non-forest ecosystems can play a large role in carbon sequestration ( ''high confidence'' ). The rational for this focus builds on their high carbon stocks and rates of sequestration ( [[#Griscom--2017|Griscom et al., 2017]] ; [[#Lewis--2019|Lewis et al., 2019]] ; [[#Maxwell--2019|Maxwell et al., 2019]] ; [[#Pugh--2019|Pugh et al., 2019]] ), high resilience to disturbances ( [[#Dymond--2014|Dymond et al., 2014]] ; [[#Messier--2019|Messier et al., 2019]] ), and additional benefits such as enhanced biodiversity ( [[#Strassburg--2020|Strassburg et al., 2020]] ).

The global sequestration potential of forestation varies substantially depending on the scenario-assumptions of available land and of background climate (AR6 WGIII, [[IPCC:Wg1:Chapter:Chapter-7#7.5|Section 7.5]] ). Afforestation of native grasslands, savannas, and open-canopy woodlands leads to the undesirable loss of unique natural ecosystems with rich biodiversity, carbon storage and other ecosystem services ( [[#Veldman--2015|Veldman et al., 2015]] ; [[#IPBES--2018|IPBES, 2018]] ). Comprehensive approaches to assess the effectiveness of land-based carbon removal options need to be based on the whole carbon cycle, covering both carbon stocks and flows, and establishing the links between human activities and their impacts on the biosphere and atmosphere ( [[#Keith--2021|Keith et al., 2021]] ).

A range of mechanisms could enhance CO <sub>2</sub> sequestration of forest-based methods under future scenarios, including CO <sub>2</sub> fertilization, soil carbon enrichment due to enhanced litter input, or the northward shift of the tree line in future climate projection ( [[#Bathiany--2010|Bathiany et al., 2010]] ; [[#Sonntag--2016|Sonntag et al., 2016]] ; [[#Boysen--2017b|Boysen et al., 2017b]] ; [[#Harper--2018|Harper et al., 2018]] ). There is ''low'' ''confidence'' in the net direction of feedbacks of afforestation on global mean temperature. The feedbacks are highly region dependent. For instance, afforestation at high latitudes would decrease albedo and increase local warming, while at low latitudes, the cooling effect of enhanced evapotranspiration could exceed the warming effect due to albedo decrease ( [[#Pearson--2013|Pearson et al., 2013]] ; W. [[#Zhang--2013|]] [[#Zhang--2013|Zhang et al., 2013]] ; [[#Jia--2019|Jia et al., 2019]] , SRCCL, Section 2.6.1). Both afforestation and reforestation affect the hydrological cycle through increased volatile organic compound (VOC) emissions and cloud albedo ( [[#Teuling--2017|Teuling et al., 2017]] ; [[#Kalliokoski--2020|Kalliokoski et al., 2020]] ), enhanced precipitation ( [[#Ellison--2017|Ellison et al., 2017]] ) and increased transpiration, with potential effects on runoff and, especially in dry regions, on water supply (Figure 5.36 and Cross-Chapter Box 5.1; [[#Farley--2005|Farley et al., 2005]] ; [[#Smith--2016|Smith et al., 2016]] ; [[#Krause--2017|Krause et al., 2017]] ; [[#Teuling--2019|Teuling et al., 2019]] ). Forest-based methods can either raise or lower N <sub>2</sub> O emissions, depending on tree species, previous land use, soil type and climatic factors ( ''low confidence'' ) (Figure 5.36 and Supplementary Materials, Table 5.SM.4; [[#Benanti--2014|Benanti et al., 2014]] ; [[#Chen--2019|Chen et al., 2019]] ; [[#McDaniel--2019|McDaniel et al., 2019]] ). Afforestation will decrease biodiversity if native species are replaced by monocultures ( ''high'' ''confidence'' ), while there is ''medium confidence'' that biodiversity is improved when forests are introduced into land areas with degraded soils or intensive monocultures, or where native species are re-introduced into managed land (Figure 5.36, Supplementary Materials Table 5.SM.4; [[#Hua--2016|Hua et al., 2016]] ; [[#Williamson--2016|Williamson and Bodle, 2016]] ; [[#Smith--2018|P. Smith et al., 2018]] ; [[#Holl--2020|Holl and Brancalion, 2020]] ).

Soil carbon losses from human agriculture accounted for about 116 PgC in the last 12,000 years ( [[#5.2.1.2|Section 5.2.1.2]] ; [[#Sanderman--2017|Sanderman et al., 2017]] ). With best management practices, two-thirds of these losses may be recoverable, setting a theoretical maximum of 77 PgC that can be sequestered in soils. Methods to increase soil carbon content may be applied to the restoration of marginal or degraded land ( [[#Paustian--2016|Paustian et al., 2016]] ; [[#Smith--2016|Smith, 2016]] ), but may also be used in traditional agricultural lands. A simple practice is to increase the input of carbon to the soil by selecting appropriate varieties or species with greater root mass ( [[#Kell--2011|Kell, 2011]] ) or higher yields and net primary production (NPP) ( [[#Burney--2010|Burney et al., 2010]] ). In addition, improved agricultural practices also increase soil carbon content. These include using crop rotation cycles, increasing the amount of crop residues, using crop cover to prevent periods of bare soil ( [[#Poeplau--2015|Poeplau and Don, 2015]] ; [[#Griscom--2017|Griscom et al., 2017]] ), optimizing grazing ( [[#Henderson--2015|Henderson et al., 2015]] ) and residue management ( [[#Wilhelm--2004|Wilhelm et al., 2004]] ), using irrigation ( [[#Campos--2020|Campos et al., 2020]] ), employment of low-tillage or no-tillage (W. [[#Sun--2020|]] [[#Sun--2020|Sun et al., 2020]] ), agroforestry, cropland nutrient recycling, and avoiding grassland conversion ( [[#Paustian--2016|Paustian et al., 2016]] ; [[#Fargione--2018|Fargione et al., 2018]] ). With ''medium confidence,'' methods that seek soil carbon sequestration will diminish nitrous oxide (N <sub>2</sub> O) emissions and nutrient leaching, and improve soil fertility and biological activity (Figure 5.36; [[#Tonitto--2006|Tonitto et al., 2006]] ; [[#Fornara--2011|Fornara et al., 2011]] ; [[#Paustian--2016|Paustian et al., 2016]] ; [[#Smith--2016|Smith et al., 2016]] ; SRCCL, Section 2.6.1.3, [[#IPCC--2019a|IPCC, 2019a]] ). However, if improved soil carbon sequestration practices involve higher fertilization rates, N <sub>2</sub> O emissions would increase ( [[#Gu--2017|Gu et al., 2017]] ). Some soil carbon sequestration methods, such as cover crops and crop diversity, can increase biodiversity ( ''medium confidence'' ) ( [[#Paustian--2016|Paustian et al., 2016]] ; [[#Smith--2018|P. Smith et al., 2018]] ).

Biochar is produced by burning biomass at high temperatures under anoxic conditions (pyrolysis) and, when added to soils, can increase soil carbon stocks and fertility for decades to centuries ( [[#Woolf--2010|Woolf et al., 2010]] ; [[#Lehmann--2015|Lehmann et al., 2015]] ). Biochar application improves many soil qualities and increases crop yield ( ''medium confidence'' ) ( [[#Ye--2020|Ye et al., 2020]] ; SRCCL, Chapter 4.9.5), particularly in already degraded or weathered soils ( [[#Woolf--2010|Woolf et al., 2010]] ; [[#Lorenz--2014|Lorenz and Lal, 2014]] ; [[#Jeffery--2016|Jeffery et al., 2016]] ), increases soil water holding capacity ( ''medium'' ''confidence'' ) ( [[#Karhu--2011|Karhu et al., 2011]] ; [[#Liu--2016|Liu et al., 2016]] ; [[#Fischer--2019|]] [[#Fischer--2019|B.M.C. Fischer et al., 2019]] ; [[#Verheijen--2019|Verheijen et al., 2019]] ) and evapotranspiration ( ''low confidence'' )( [[#Fischer--2019|]] [[#Fischer--2019|B.M.C. Fischer et al., 2019]] ). The use of biochar reduces nutrient losses ( ''low confidence'' ) ( [[#Woolf--2010|Woolf et al., 2010]] ), enhances fertilizer nitrogen use efficiency and improves the bioavailability of phosphorus (Figure 5.36; [[#Clough--2013|Clough et al., 2013]] ; [[#Shen--2016|Shen et al., 2016]] ; [[#Liu--2017|Z. Liu et al., 2017]] ). Biochar addition may decrease methane (CH <sub>4</sub> ) emissions in inundated and acid soils such as rice fields ( ''low confidence'' )( [[#Jeffery--2016|Jeffery et al., 2016]] ; [[#Huang--2019|Huang et al., 2019]] ; [[#Wang--2019|]] [[#Wang--2019|Wang et al., 2019]] ; [[#Yang--2019|Yang et al., 2019]] ). In non-inundated, neutral soils, CH <sub>4</sub> uptake from the atmosphere is suppressed after biochar application ( ''low confidence'' ) ( [[#Jeffery--2016|Jeffery et al., 2016]] ), and soil N <sub>2</sub> O emissions decline ( ''medium confidence'' ) ( [[#Cayuela--2014|Cayuela et al., 2014]] ; [[#Kammann--2017|Kammann et al., 2017]] ). The potential risks of introducing harmful contaminants into the soil environment are not well understood ( [[#Lorenz--2014|Lorenz and Lal, 2014]] ). With ''low confidence'' , application of biochar can have co-benefits for soil microbial biodiversity ( [[#Smith--2018|P. Smith et al., 2018]] ), while the potential trade-offs for biodiversity are due to land requirements ( [[#Tisserant--2019|Tisserant and Cherubini, 2019]] ).

Peatlands are less extensive than forests, croplands and grazing lands, yet per unit area, they hold high carbon stocks ( [[#Griscom--2017|Griscom et al., 2017]] ). Peatland restoration relies on back-conversion or building of high-carbon-density soils through flooding – that is, rewetting ( [[#Leifeld--2019|Leifeld et al., 2019]] ). High water level and anoxic conditions are prerequisites for restoring by returning drained and/or degraded peatlands back to their natural state as CO <sub>2</sub> sinks, but restoration also results in enhanced CH <sub>4</sub> emissions which are similar to or higher than the pre-drainage fluxes ( ''high'' ''confidence'' ) ( [[#Koskinen--2016|Koskinen et al., 2016]] ; [[#Wilson--2016a|Wilson et al., 2016a]] ; [[#Hemes--2019|Hemes et al., 2019]] ; [[#Renou-Wilson--2019|Renou-Wilson et al., 2019]] ; [[#Holl--2020|Holl et al., 2020]] ). In a multi-decadal time frame, the reduction in CO <sub>2</sub> emissions from rewetting more than compensates for the initial increase in radiative forcing due to enhanced CH <sub>4</sub> emissions ( [[#Günther--2020|Günther et al., 2020]] ). Rewetting drained peatlands will decrease N <sub>2</sub> O emissions ( ''medium confidence'' ) ( [[#Wilson--2016b|Wilson et al., 2016b]] ; H. [[#Liu--2020|]] [[#Liu--2020|]] [[#Liu--2020|Liu et al., 2020]] ; [[#Tiemeyer--2020|Tiemeyer et al., 2020]] ). Restored wetlands and peatlands act as buffer zones that provide infiltration and nutrient retention and offer protection to water quality ( [[#Daneshvar--2017|Daneshvar et al., 2017]] ; [[#Lundin--2017|Lundin et al., 2017]] ), particularly in nutrient-loaded agricultural catchments. Peatland restoration can also recover much of the original biodiversity ( ''medium confidence'' ) ( [[#Meli--2014|Meli et al., 2014]] ; [[#Smith--2018|P. Smith et al., 2018]] ).

The concept of BECCS rests on the premise that bioenergy production is carbon neutral – that is, as much CO <sub>2</sub> is sequestered when growing biomass as feedstock as is released by its combustion. If these emissions are also captured and stored, the net effect is removal of CO <sub>2</sub> from the atmosphere ( [[#Fuss--2018|Fuss et al., 2018]] ). Sequestration potentials from BECCS depend strongly on the feedstock, climate, and management practices ( [[#Beringer--2011|Beringer et al., 2011]] ; [[#Kato--2014|Kato and Yamagata, 2014]] ; [[#Heck--2016|Heck et al., 2016]] ; [[#Smith--2016|Smith et al., 2016]] ; [[#Krause--2017|Krause et al., 2017]] ). If woody bioenergy plants replace marginal land, net carbon uptake increases, enriching soil carbon (Don et al., 2012; [[#Heck--2016|Heck et al., 2016]] ; [[#Boysen--2017a|Boysen et al., 2017a]] , b). However, replacing carbon-rich ecosystems with herbaceous bioenergy plants could deplete soil-carbon stocks and reduce the additional sink capacity of standing forests ( [[#Don--2012|Don et al., 2012]] ; [[#Harper--2018|Harper et al., 2018]] ). Furthermore, wood-based BECCS may not be carbon negative in the first decades, initially emitting more CO <sub>2</sub> than sequestering ( [[#Sterman--2018|Sterman et al., 2018]] ). BECCS has several trade-offs to deal with, including possible threats to water supply and soil nutrient deficiencies ( ''medium confidence'' ) (SRCCL Chapters 2 and 6, and Cross-Chapter Box 5.1; [[#Smith--2016|Smith et al., 2016]] ; [[#Krause--2017|Krause et al., 2017]] ; [[#de%20Coninck--2018|de Coninck et al., 2018]] ; [[#Heck--2018|Heck et al., 2018]] ; [[#Roy--2018|Roy et al., 2018]] ). Deployment of BECCS at the scales envisioned by many 1.5°C–2.0°C mitigation scenarios could threaten biodiversity and require large land areas, competing with afforestation, reforestation and food security ( [[#Anderson--2016|Anderson and Peters, 2016]] ; [[#Smith--2018|P. Smith et al., 2018]] ). Additional risks and side effects are related to geologic carbon storage ( [[#Fuss--2018|Fuss et al., 2018]] ; see also [[#5.6.2.2.4|Section 5.6.2.2.4]] ).

In conclusion, land-based CDR methods that rely on enhanced net biological uptake and storage of carbon, have a wide range of biogeochemical and biophysical side effects. These side effects can (directly or indirectly) strengthen or weaken the climate change mitigation effect of a given method, or affect water quality and quantity, food supply and biodiversity (Figure 5.36). With the exception of weakened ocean carbon sequestration, there is ''low confidence'' in the Earth system feedbacks of these methods. Most methods are associated with a range of biogeochemical and biophysical side effects and co-benefits and trade-offs, but these are often highly dependent on local context, management regime, prior land use, and scale ( ''high confidence'' ). Highest co-benefits are obtained with methods that seek to restore natural ecosystems and improve soil carbon sequestration (Figure 5.36) while highest trade-off possibilities (symmetry with the highest co-benefits) occur for reforestation or afforestation with monocultures and BECCS, again with strong dependence on scale and context ( ''medi'' ''um confidence'' ).

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===== 5.6.2.2.2 Ocean-based biological CDR methods =====

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Both ocean biological and physical processes drive the CO <sub>2</sub> exchange between the ocean and atmosphere. However, the ocean physical processes that remove CO <sub>2</sub> from the atmosphere, such as large-scale circulation, cannot be feasibly altered, so ocean CDR methods focus on increasing the productivity of ocean ecosystems, and subsequent sequestration of carbon ( [[#GESAMP--2019|GESAMP, 2019]] ). There has been no change to the assessment of SROCC (SROCC [[#5.5.1|Section 5.5.1]] ): there is ''low confidence'' that nutrient addition to the open ocean, either through artificial ocean upwelling or iron fertilization, could contribute to climate change mitigation, due to its inconclusive effect on carbon sequestration and risks of adverse side effects on marine ecosystems (Figure 5.36, Table 5.9; Supplementary Materials Text 5.SM.3 and Table 5.SM.4; AR6 WGIII Chapter 12; [[#Gattuso--2018|Gattuso et al., 2018]] ; [[#Boyd--2019|Boyd and Vivian, 2019]] ; [[#Feng--2020|Feng et al., 2020]] ). In addition, ocean fertilization is currently prohibited by the LondonProtocol ( [[#Dixon--2014|Dixon et al., 2014]] ; [[#GESAMP--2019|GESAMP, 2019]] ).

Restoration of vegetated coastal ecosystems (sometimes referred to as ‘blue carbon’ – see Glossary) refers to the potential for increasing carbon sequestration by plant growth and burial of organic carbon in the soil of coastal wetlands (including salt marshes and mangroves) and seagrass ecosystems. Wider usage of the term blue carbon occurs in the literature, for example, including seaweeds (macroalgae), shelf sea sediments and open ocean carbon exchanges. However, such systems are less amenable to management, with many uncertainties relating to the permanence of their carbon stores ( [[#Windham-Myers--2018|Windham-Myers et al., 2018]] ; [[#Lovelock--2019|Lovelock and Duarte, 2019]] ; SROCC, [[#5.5.1.1|Section 5.5.1.1]] ).

Coastal wetlands and seagrass meadows store significant amounts of carbon and are among the most productive ecosystems per unit area ( [[#Griscom--2017|Griscom et al., 2017]] , 2020; [[#Ortega--2019|Ortega et al., 2019]] ; [[#Serrano--2019|Serrano et al., 2019]] ). These rates could be reduced in the future, since these habitats are vulnerable to changing conditions, such as temperature, salinity, sediment supply, storm severity and continued coastal development ( [[#Bindoff--2019|Bindoff et al., 2019]] ; [[#NASEM--2019|NASEM, 2019]] ). These ecosystems are under threat from anthropogenic conversion and degradation and are being lost at rates between 0.7% and 7% per annum with consequent CO <sub>2</sub> emissions (e.g., [[#Atwood--2017|Atwood et al., 2017]] ; [[#Howard--2017|Howard et al., 2017]] ; [[#Hamilton--2018|Hamilton and Friess, 2018]] ; [[#Sasmito--2019|Sasmito et al., 2019]] ). Although sea level rise might lead to greater carbon sequestration in coastal wetlands ( [[#Rogers--2019|Rogers et al., 2019]] ), there is ''high confidence'' that the frequency and intensity of marine heatwaves will increase (Cross-Chapter Box 9.1; [[#Frölicher--2018|Frölicher and Laufkötter, 2018]] ; [[#Laufkötter--2020|Laufkötter et al., 2020]] ),which poses a more immediate threat to the integrity of coastal carbon stocks ( [[#Smale--2019|Smale et al., 2019]] ). Blue carbon restoration seeks to increase the rate of carbon sequestration, although restoration may be challenging, because of ongoing use of coastal land for human settlement, conversion to agriculture and aquaculture, shoreline hardening and port development.

Biogeochemical factors affecting reliable quantification of the climatic benefits of coastal vegetation include the variable production of CH <sub>4</sub> and N <sub>2</sub> O by such ecosystems ( [[#Adams--2012|Adams et al., 2012]] ; [[#Keller--2018|Keller, 2018]] ; [[#Rosentreter--2018|Rosentreter et al., 2018]] ), uncertainties regarding the provenance of the carbon that they accumulate ( [[#Macreadie--2019|Macreadie et al., 2019]] ), and the release of CO <sub>2</sub> by biogenic carbonate formation in seagrass ecosystems ( [[#Kennedy--2018|Kennedy et al., 2018]] ). While coastal habitat restoration potentially provides significant mitigation of national emissions for some countries ( [[#Taillardat--2018|Taillardat et al., 2018]] ; [[#Serrano--2019|Serrano et al., 2019]] ), the global sequestration potential of blue carbon approaches is &lt;0.02 PgC yr <sup>–1</sup> ( ''medium confidence'' ) (Figure 5.36; SROCC, [[#5.5.1.2|Section 5.5.1.2]] ; [[#Griscom--2017|Griscom et al., 2017]] ; [[#Gattuso--2018|Gattuso et al., 2018]] ; [[#GESAMP--2019|GESAMP, 2019]] ; [[#NASEM--2019|NASEM, 2019]] ).

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===== 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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===== 5.6.2.2.4 Chemical CDR methods =====

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Direct air carbon capture with carbon storage (DACCS) is a combination of two techniques, direct capture of CO <sub>2</sub> from ambient air (DAC) and carbon storage. DAC entails contacting the air, capturing the CO <sub>2</sub> on a liquid solvent or solid sorbent, and regenerating the solvent or sorbent. Different DAC methods have been proposed, which differ by the chemical process used to capture the CO <sub>2</sub> and to recover it from the sorbent or solvent ( [[#NASEM--2019|NASEM, 2019]] ). The captured CO <sub>2</sub> may be either stored geologically as a high-pressure gas or sequestered by a mineral carbonation process. Storage is potentially permanent in both pressurised gas and mineral form ( [[#Fuss--2018|Fuss et al., 2018]] ). DACCS has significant requirements of energy and, (depending on the type of technology), water and materials ( [[#Smith--2016|Smith et al., 2016]] ; [[#NASEM--2019|NASEM, 2019]] ). Compared to other CDR methods, it has a small land footprint ( [[#Smith--2016|Smith et al., 2016]] ; [[#NASEM--2019|NASEM, 2019]] ). Side effects of DACCS include CO <sub>2</sub> -depleted air leaving the air contactor, which could have adverse effects on crop and ecosystem productivity, and VOC emissions ( [[#NASEM--2019|NASEM, 2019]] ). Additional risks and side effects are related to the high pressure at which CO <sub>2</sub> is stored in geologic formations ( [[#Fuss--2018|Fuss et al., 2018]] ). DACCS is assessed in detail in WGIII Chapter 12.

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===== 5.6.2.2.5 Methane removal =====

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Proposals to remove CH <sub>4</sub> from the atmosphere are emerging ( [[#de%20Richter--2017|de Richter et al., 2017]] ; [[#Jackson--2019|Jackson et al., 2019]] ). CH <sub>4</sub> removal methods seek to capture CH <sub>4</sub> directly from ambient air, similarly to DACCS for CO <sub>2</sub> using, for example, zeolite trapping, but instead of storing it, CH <sub>4</sub> would be chemically oxidized to CO <sub>2</sub> ( [[#Jackson--2019|Jackson et al., 2019]] ). Methane can be also removed microbially by supporting naturally occurring processes, such as by enhancing the soil microbial uptake through afforestation (J. [[#Wu--2018|]] [[#Wu--2018|Wu et al., 2018]] ) or by directing the venting air from a cow barn into the soil bed of a nearby greenhouse, utilizing microbial CH <sub>4</sub> oxidation ( [[#Nisbet--2020|Nisbet et al., 2020]] ). Microbial CH <sub>4</sub> oxidation could also be used for removal of CH <sub>4</sub> leaked from point sources by building biocatalytic polymers which include methane-oxidizing enzymes ( [[#Blanchette--2016|Blanchette et al., 2016]] ). Methane removal is, however, still in its infancy and the available literature is insufficient for an assessment.

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'''Figure 5.36 |''' '''Characteristics of carbon dioxide removal (CDR) methods, ordered according to the time scale of carbon storage.''' The first column shows biogeophysical (for open-ocean methods) or technical (for all other methods) sequestration potentials (i.e., the sequestration potentials constrained by biological, geophysical, geochemical limits and thermodynamics and, for technical potentials, availability of technologies and practices; technical potentials for some methods also consider social or environmental factors if these represent strong barriers for deployment; see Glossary, Annex VII), classified into low (&lt;0.3 GtCO <sub>2</sub> yr <sup>–1</sup> ), moderate (0.3−3 GtCO <sub>2</sub> yr <sup>–1</sup> ) and large (&gt;3 GtCO <sub>2</sub> yr <sup>–1</sup> ) (details underlying this classification are provided in Supplementary Materials Table 5.SM.5). The other columns show Earth system feedbacks that deployment of a given CDR method would have on carbon sequestration and climate, along with biogeochemical, biophysical, and other side effects of a given method. Earth system feedbacks do not include the direct effect of CO <sub>2</sub> sequestration on atmospheric CO <sub>2</sub> , only secondary effects. For Earth system feedbacks, the colours indicate whether the feedbacks strengthen or weaken carbon sequestration and the climate cooling effect of a given CDR method. For biogeochemical and biophysical side effects the colours indicate whether the deployment of a CDR method increases or decreases the magnitude of the effect, whereas for co-benefits and trade-offs the colour indicates whether deployment of a CDR method results in beneficial (co-benefits) or adverse side effects (trade-offs) for water quality and quantity, food production and biodiversity. The details and references underlying the Earth system feedback and side effect assessment are provided in Supplementary Materials Table 5.SM.4. Further details on data sources and processing are available in the chapter data table (Table 5.SM.6).

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