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

==== 5.2.2.4 Ocean and Inland Water Emissions and Sinks ====

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In AR5, the ocean CH <sub>4</sub> emissions were reported together with geological emissions, summing up to 54 (33–75) Tg yr <sup>–1</sup> . Coastal oceans, fjords and mud volcanos are major sources of CH <sub>4</sub> in the marine environment, but CH <sub>4</sub> flux measurements are sparse. [[#Saunois--2020|Saunois et al. (2020)]] estimate that the oceanic budget, including biogenic, geological and hydrate emissions from coastal and open ocean, is 6 (range 4–10) Tg yr <sup>–1</sup> for the 2000s, which is in good agreement with an air–sea flux measurement-based estimate of 6–12 Tg yr <sup>–1</sup> ( [[#Weber--2019|Weber et al., 2019]] ). When estuaries are included, the total oceanic budget is 9–22 Tg yr <sup>–1</sup> , with a mean value of 13 Tg yr <sup>–1</sup> . A recent synthesis suggests that CH <sub>4</sub> emissions from shallow coastal ecosystems, particularly from mangroves, can be as high as 5–6 Tg yr <sup>–1</sup> ( [[#Al-Haj--2020|Al-Haj and Fulweiler, 2020]] ). The reservoir emissions, including coastal wetlands and tidal flats, contribute up to 13 Tg yr <sup>–1</sup> ( [[#Borges--2011|Borges and Abril, 2011]] ; [[#Deemer--2016|Deemer et al., 2016]] ). Methane seepage from the Arctic shelf, possibly triggered by the loss of geological storage due to warming and thawing of permafrost and hydrate decomposition, has a wide estimated range of 0.0–17 Tg yr <sup>–1</sup> ( [[#Shakhova--2010|Shakhova et al., 2010]] , 2014, 2017; [[#Berchet--2016|Berchet et al., 2016]] ); advanced eddy covariance measurements put the best estimate at about 3 Tg yr <sup>–1</sup> from the East Siberian Arctic shelf ( [[#Thornton--2020|Thornton et al., 2020]] ). The current flux is expected to be a mix of pre-industrial and climate change-driven fluxes, CH <sub>4</sub> seepage is anticipated to increase in a warmer world ( [[#Dean--2018|Dean et al., 2018]] ).

All geological sources around the world, including the coastal oceans and fjords, are estimated to emit CH <sub>4</sub> in the range of 35–76 Tg yr <sup>–1</sup> ( [[#Etiope--2019|Etiope et al., 2019]] ). There is evidence that the ventilation of geological CH <sub>4</sub> is ''likely'' to be smaller than 15 Tg yr <sup>–1</sup> ( [[#Petrenko--2017|Petrenko et al., 2017]] ; [[#Hmiel--2020|Hmiel et al., 2020]] ). A lower geological CH <sub>4</sub> ventilation will reduce the gap between bottom-up and top-down estimates (Table 5.2), but widen the gap in the ratio of fossil fuel-derived sources to the biogenic sources for matching the D <sup>14</sup> C-CH <sub>4</sub> observations.

Inland water (lakes, rivers, streams, ponds, estuaries) emissions are proportionally the largest source of uncertainty in the CH <sub>4</sub> budget. Since AR5 ( [[#Ciais--2013|Ciais et al., 2013]] ), the inland water CH <sub>4</sub> source has been revised from 8–73 Tg yr <sup>–1</sup> (1980s) to 117–212 Tg yr <sup>–1</sup> (2000s) with the availability of more observational data and improved areal estimates ( [[#Bastviken--2011|Bastviken et al., 2011]] ; [[#Deemer--2016|Deemer et al., 2016]] ; [[#Stanley--2016|Stanley et al., 2016]] ; [[#DelSontro--2018|DelSontro et al., 2018]] ; [[#Saunois--2020|Saunois et al., 2020]] ). However, it is difficult to estimate bottom-up CH <sub>4</sub> emissions, due to the large spatial and temporal variation in lake and river CH <sub>4</sub> fluxes ( [[#Wik--2016|Wik et al., 2016]] ; [[#Crawford--2017|Crawford et al., 2017]] ; [[#Natchimuthu--2017|Natchimuthu et al., 2017]] ), uncertainties in their global area ( [[#Allen--2018|Allen and Pavelsky, 2018]] ), a relatively small number of observations, and varying measurement methods – for example, those neglecting ebullition, varying upscaling methods, and lack of appropriate processes ( [[#Sanches--2019|Sanches et al., 2019]] ; [[#Engram--2020|Engram et al., 2020]] ; L. [[#Zhang--2020|]] [[#Zhang--2020|]] [[#Zhang--2020|Zhang et al., 2020]] ). Accordingly, there is no clear accounting of inland waters in top-down budgets, which is the main reason for the large gap in bottom-up and top-down estimates of ‘other sources’ in the CH <sub>4</sub> budget (Table 5.2). Despite recent progress in separating wetlands from inland waters, there is double-counting in the bottom-up estimates of their emissions ( [[#Thornton--2016a|Thornton et al., 2016a]] ). Although there is evidence that regional human activities and global warming both increase inland water CH <sub>4</sub> emissions ( [[#Beaulieu--2019|Beaulieu et al., 2019]] ), the increase in the decadal emissions since AR5 ( [[#Ciais--2013|Ciais et al., 2013]] ) rather reflect improvements in the estimate ( ''medium confidence'' ), due to updates in the datasets and new upscaling approaches ( [[#Saunois--2020|Saunois et al., 2020]] ).

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