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

==== 5.2.2.5 Methane (CH <sub>4</sub>) Budget ====

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A summary of top-down and bottom-up estimates of CH <sub>4</sub> emissions and sinks for the period 2008–2017 is presented in Figure 5.14 (details in Table 5.2 and the associated text for the emissions). In addition to 483-682 Tg yr <sup>–1</sup> loss of CH <sub>4</sub> in the troposphere by reaction with OH, 1–35 Tg yr <sup>–1</sup> of CH <sub>4</sub> loss is estimated to occur in the lower troposphere due to Cl but are not included in the top-down models as shown in Table 5.2 ([[#Hossaini--2016|Hossaini et al., 2016]] ; [[#Gromov--2018|Gromov et al., 2018]] ; X. [[#Wang--2019|]] [[#Wang--2019|Wang et al., 2019]]). The decadal mean CH <sub>44</sub> burden/imbalance increased at the rate of 30, 12, 7 and 21 Tg yr <sup>–1</sup> in the 1980s (1980–1989), 1990s (1990–1999), 2000s (2000–2009) and the most recent decade (2008–2017), respectively (''virtually certain''), as can be estimated from observed atmospheric growth rate (Cross-Chapter Box 5.2, Figure 1).

Recent analysis using D <sup>14</sup> C-CH <sub>4</sub> in ice samples suggest that CH <sub>4</sub> emissions from fossil fuel exploitation are responsible for 30% of total CH <sub>4</sub> emissions ([[#Lassey--2007|Lassey et al., 2007]] ; [[#Hmiel--2020|Hmiel et al., 2020]]), which is largely inconsistent with sectorial budgets where fossil fuel emissions add up to 20% only ([[#Ciais--2013|Ciais et al., 2013]]). However, recent model simulations produce fairly consistent d <sup>13</sup> C-CH <sub>4</sub> values and trends, as observed in the atmospheric samples using 20% fossil fuel emissions fraction ([[#Ghosh--2015|Ghosh et al., 2015]] ; [[#Warwick--2016|Warwick et al., 2016]] ; [[#Fujita--2020|Fujita et al., 2020]] ; [[#Strode--2020|Strode et al., 2020]]). Further research is needed to clarify the relative roles of CH <sub>4</sub> emissions from fossil fuel exploitation and freshwater components. A key challenge is to accommodate the higher estimated emissions from these two components without a major increase in the sinks, in order to be consistent with the observed changes in the carbon and hydrogen isotopes.

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'''Figure 5.14 |''' '''Global methane (CH''' <sub>4</sub> ''') budget (2008–2017).''' Values and data sources as in Table 5.2 (in TgCH <sub>4</sub>). The atmospheric stock is calculated from mean CH <sub>4</sub> concentration, multiplying a factor of 2.75 ± 0.015 Tg ppb <sup>–1</sup> , which accounts for the uncertainties in global mean CH <sub>4</sub> ([[#Chandra--2021|Chandra et al., 2021]]). Further details on data sources and processing are available in the chapter data table (Table 5.SM.6).

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