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

===== 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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