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

==== 5.2.2.3 Land Biospheric Emissions and Sinks ====

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Freshwater wetlands are the single largest global natural source of CH <sub>4</sub> in the atmosphere, accounting for about 26% of the total CH <sub>4</sub> source ( ''robust evidence, medium agreement'' ). Progress has been made since AR5 ( [[#Ciais--2013|Ciais et al., 2013]] ) in better constraining freshwater lake and river emissions and reducing double counting with wetland emissions. Bottom-up and top-down estimates for 2008–2017 are 149 and 180 Tg yr <sup>–1</sup> , respectively, with a top-down uncertainty range of 159–199 Tg yr <sup>–1</sup> (Table 5.2). The large uncertainties stem from challenges in mapping wetland area and temporal dynamics to landscape estimates, and in scaling methane production, transport and consumption processes that are measured with small chambers or flux towers ( [[#Pham-Duc--2017|Pham-Duc et al., 2017]] ). Both the top-down and bottom-up estimates presented in Table 5.2 indicate little increase in wetland CH <sub>4</sub> emissions during the last three decades, with the new estimates being slightly smaller than in AR5 due to updated wetland maps and ecosystem model simulations ( [[#Melton--2013|Melton et al., 2013]] ; [[#Poulter--2017|Poulter et al., 2017]] ). Wetland emissions show strong interannual variability due to the changes in inundated land area, air temperature and microbial activity ( [[#Bridgham--2013|Bridgham et al., 2013]] ). Present terrestrial ecosystem model simulated CH <sub>4</sub> emissions variability does not produce strong correlation with the El Niño–Southern Oscillation (ENSO) cycle (Cross-Chapter Box 5.2, Figure 2), although observation evidence is emerging for lower CH <sub>4</sub> emissions during El Niños and greater emissions during La Niña ( [[#Pandey--2017|Pandey et al., 2017]] ).

Trees in upland and wetland forests contribute to CH <sub>4</sub> emissions by abiotic production in the canopy, by the methanogenesis taking place in the stem, and by conducting CH <sub>4</sub> from soil into the atmosphere ( [[#Covey--2019|Covey and Megonigal, 2019]] ). There is emerging evidence of the important role of trees in transporting and conducting CH <sub>4</sub> from soils into the atmosphere, especially in tropics ( [[#Pangala--2017|Pangala et al., 2017]] ), whereas direct production of CH <sub>44</sub> by vegetation only has a minor contribution ( ''limited evidence, high agreement'' ) ( [[#Bruhn--2012|Bruhn et al., 2012]] ; [[#Covey--2019|Covey and Megonigal, 2019]] ). The contribution of trees in transporting CH <sub>4</sub> may further widen the gap between the bottom-up and top-down estimates in the global budget, particularly needing a re-assessment of emissions in the tropics and in forested wetlands of temperate and boreal regions ( [[#Pangala--2017|Pangala et al., 2017]] ; [[#Jeffrey--2019|Jeffrey et al., 2019]] ; [[#Welch--2019|Welch et al., 2019]] ; [[#Sjögersten--2020|Sjögersten et al., 2020]] ).

Microbial methane uptake by soil comprises up to 5% (30 Tg yr <sup>–1</sup> ) of the total CH <sub>4</sub> sink in 2008–2017 (Table 5.2). There is evidence from experimental and modelling studies of increasing soil microbial uptake due to increasing temperature ( [[#Yu--2017|Yu et al., 2017]] ), although evidence also exists for decreasing CH <sub>4</sub> consumption, possibly linked to precipitation changes ( [[#Ni--2018|Ni and Groffman, 2018]] ). The estimate of global methane loss by microbial oxidation in upland soils has been lowered marginally by 4 Tg yr <sup>–1</sup> , compared to 34 Tg yr <sup>–1</sup> in AR5, for the period 2000–2009. Termites, an infraorder of insects (Isoptera) found in almost all land masses, emitted about 9 Tg yr <sup>–1</sup> of CH <sub>4</sub> in 2000–2009. Increased emissions from insects and other anthropods are projected ( [[#Brune--2018|Brune, 2018]] ).

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