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

==== 5.2.2.2 Anthropogenic Methane (CH <sub>4</sub> ) Emissions ====

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The positive gradient between CH <sub>4</sub> at Cape Grim, Australia (41°S) and Trinidad Head, USA (41°N), and the bigger difference between Trinidad Head and global mean CH <sub>4</sub> compared to that between global mean CH <sub>4</sub> and Cape Grim, strongly suggest that the Northern Hemisphere is the dominant origin of anthropogenic CH <sub>4</sub> emissions (Figure 5.13). The loss rate of CH <sub>4</sub> in troposphere does not produce a large positive north–south hemispheric gradient in CH <sub>4</sub> due to parity in hemispheric mean OH concentration ( [[#Patra--2014|Patra et al., 2014]] ), or in the case of greater OH concentrations in the northern rather than the Southern Hemisphere as simulated by the chemistry-climate models ( [[#Naik--2013|Naik et al., 2013]] ). Coal mining contributed about 35% of the total CH <sub>4</sub> emissions from all fossil fuel-related sources. Top-down estimates of fossil fuel emissions (106 Tg yr <sup>–1</sup> ) are smaller than bottom-up estimates (115 Tg yr <sup>–1</sup> ) during 2008–2017 (Table 5.2). Inventory-based estimates suggest that CH <sub>4</sub> emissions from coal mining increased by 17 Tg yr <sup>–1</sup> between the periods 2002–2006 and 2008–2012, with a dominant contribution from China ( [[#Peng--2016|Peng et al., 2016]] ; [[#Crippa--2020|Crippa et al., 2020]] ; [[#Höglund-Isaksson--2020|Höglund-Isaksson et al., 2020]] ). Inventory-based estimates suggest that CH <sub>4</sub> emissions from coal mining increased by 17 Tg yr <sup>–1</sup> between the periods 2002–2006 and 2008–2012, with a dominant contribution from China ( [[#Peng--2016|Peng et al., 2016]] ; [[#Crippa--2020|Crippa et al., 2020]] ; [[#Höglund-Isaksson--2020|Höglund-Isaksson et al., 2020]] ). Recent country statistics and detailed inventory-based estimates show that CH <sub>4</sub> emissions from coal mining in China declined between 2012 and 2016 ( [[#Sheng--2019|Sheng et al., 2019]] ; [[#Gao--2020|Gao et al., 2020]] ), while atmospheric-based estimates suggest a continuation of CH <sub>4</sub> emissions growth but at a slower rate to the year 2015 ( [[#Miller--2019|Miller et al., 2019]] ) and 2016 ( [[#Chandra--2021|Chandra et al., 2021]] ). Emissions from oil and gas extraction and use decreased in the 1980s and 1990s, but increased in the 2000s and 2010s ( [[#Dlugokencky--1994|Dlugokencky et al., 1994]] ; [[#Stern--1996|Stern and Kaufmann, 1996]] ; [[#Howarth--2019|Howarth, 2019]] ; [[#Crippa--2020|Crippa et al., 2020]] ). The attribution to multiple CH <sub>4</sub> sources using spatially aggregated atmospheric d <sup>13</sup> C data remained underdetermined to infer the global total emissions from the fossil fuel industry, biomass burning and agriculture ( [[#Rice--2016|Rice et al., 2016]] ; [[#Schaefer--2016|Schaefer et al., 2016]] ; [[#Schwietzke--2016|Schwietzke et al., 2016]] ; [[#Worden--2017|Worden et al., 2017]] ; [[#Thompson--2018|Thompson et al., 2018]] ).

In the agriculture and waste sectors (Table 5.2), livestock production has the largest emissions source (109 Tg yr <sup>–1</sup> in 2008–2017) dominated by enteric fermentation by about 90%. Methane is formed during the storage of manure, when anoxic conditions are developed ( [[#Hristov--2013|Hristov et al., 2013]] ). Emissions from enteric fermentation and manure have increased gradually from about 87 Tg yr <sup>–1</sup> in 1990–1999 to 109 Tg yr <sup>–1</sup> in 2008–2017 mainly due to the increase in global total animal numbers. Methane production in livestock rumens (cattle, goats, sheep, water buffalo) are affected by the type, amount and quality of feeds, energy consumption, animal size, health and growth rate, meat and milk production rate, and temperature ( [[#Broucek--2014|Broucek, 2014]] ; S.R.O. [[#Williams--2020|]] [[#Williams--2020|Williams et al., 2020]] ; SRCCL [[#5.4.3|Section 5.4.3]] ). Waste management and landfills produced 64 Tg yr <sup>–1</sup> in 2008–2017, with global emissions increasing steadily since the 1970s and, despite significant declines in the USA, western Europe and Japan ( [[#Crippa--2020|Crippa et al., 2020]] ; [[#Höglund-Isaksson--2020|Höglund-Isaksson et al., 2020]] ).

Emissions from rice cultivation decreased from about 45 Tg yr <sup>–1</sup> in the 1980s to about 29 Tg yr <sup>–1</sup> in the decade 2000–2009, but increased again slightly to 31 Tg yr <sup>–1</sup> during 2008–2017, based on inventories data. However, ecosystem models showed a gradual increase with time due to climate change ( ''limited evidence, low agreement'' ) ( [[#Crippa--2020|Crippa et al., 2020]] ; [[#Höglund-Isaksson--2020|Höglund-Isaksson et al., 2020]] ; [[#Ito--2020|Ito, 2020]] ).

Biomass burning and biofuel consumption (including natural and anthropogenic processes) caused at least 30 Tg yr <sup>–1</sup> emissions during 2008–2017 and constituted up to about 5% of global anthropogenic CH <sub>4</sub> emissions. Methane emissions from open biomass burning decreased during the past two decades mainly due to reduction of burning in savanna, grassland and shrubland ( [[#van%20der%20Werf--2017|van der Werf et al., 2017]] ; [[#Worden--2017|Worden et al., 2017]] ). There is recent evidence from the tropics that fire occurrence is non-linearly related to precipitation, implying that severe droughts will increase CH <sub>4</sub> emissions from fires, particularly from the degraded peatlands ( [[#Field--2016|Field et al., 2016]] ).

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