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

==== 5.2.2.1 Atmosphere ====

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Since the start of direct measurements of CH <sub>4</sub> in the atmosphere in the 1970s (Figure 5.13), the highest growth rate was observed from 1977 to 1986 at 18 ± 4 ppb yr <sup>–1</sup> (multi-year mean and 1 standard deviation) ( [[#Rice--2016|Rice et al., 2016]] ). This rapid CH <sub>4</sub> growth followed the green revolution with increased crop production and a fast rate of industrialization that caused rapid increases in CH <sub>4</sub> emissions from ruminant animals, rice cultivation, landfills, oil and gas industry and coal mining ( [[#Ferretti--2005|Ferretti et al., 2005]] ; [[#Ghosh--2015|Ghosh et al., 2015]] ; [[#Crippa--2020|Crippa et al., 2020]] ). Due to increases in oil prices in the early 1980s, emissions from gas flaring declined significantly ( [[#Stern--1996|Stern and Kaufmann, 1996]] ). This explains the first reduction in CH <sub>4</sub> growth rates from 1985 to 1990 ( [[#Steele--1992|Steele et al., 1992]] ; [[#Chandra--2021|Chandra et al., 2021]] ). Further emissions reductions occurred following the Mt Pinatubo eruption in 1991 that triggered a reduction in CH <sub>4</sub> growth rate through a decrease in wetland emissions driven by lower surface temperatures due to the light scattering by aerosols ( [[#Bândă--2016|Bândă et al., 2016]] ; [[#Chandra--2021|Chandra et al., 2021]] ). In the late 1990s through to 2006 there was a temporary pause in the CH <sub>4</sub> growth rate, with higher confidence on its causes than in AR5: emissions from the oil and gas sectors declined by about 10 Tg yr <sup>–1</sup> through the 1990s, and atmospheric CH <sub>4</sub> loss steadily increased ( [[#Dlugokencky--2003|Dlugokencky et al., 2003]] ; [[#Simpson--2012|Simpson et al., 2012]] ; [[#Crippa--2020|Crippa et al., 2020]] ; [[#Höglund-Isaksson--2020|Höglund-Isaksson et al., 2020]] ; [[#Chandra--2021|Chandra et al., 2021]] ). The methane growth rate began to increase again at 7 ± 3 ppb yr <sup>–1</sup> during 2007–2016, the causes of which are highly debated since AR5 ( [[#Rigby--2008|Rigby et al., 2008]] ; [[#Dlugokencky--2011|Dlugokencky et al., 2011]] ; [[#Dalsøren--2016|Dalsøren et al., 2016]] ; [[#Nisbet--2016|Nisbet et al., 2016]] ; [[#Patra--2016|Patra et al., 2016]] ; [[#Schaefer--2016|Schaefer et al., 2016]] ; [[#Schwietzke--2016|Schwietzke et al., 2016]] ; [[#Turner--2017|Turner et al., 2017]] ; [[#Worden--2017|Worden et al., 2017]] ; [[#He--2020|He et al., 2020]] ); studies disagree on the relative contribution of thermogenic, pyrogenic and biogenic emission processes and variability in tropospheric OH concentration. The renewed CH <sub>4</sub> increase is accompanied by a reversal of d <sup>13</sup> C trend to more negative values post 2007; opposite to what occurred in the 200 years prior ( [[#Ferretti--2005|Ferretti et al., 2005]] ; [[#Ghosh--2015|Ghosh et al., 2015]] ; [[#Schaefer--2016|Schaefer et al., 2016]] ; [[#Schwietzke--2016|Schwietzke et al., 2016]] ; [[#Nisbet--2019|Nisbet et al., 2019]] ), suggesting an increasing contribution from animal farming, landfills and waste, and a slower increase in emissions from fossil fuel exploitation since the early 2000s ( [[#Patra--2016|Patra et al., 2016]] ; [[#Jackson--2020|Jackson et al., 2020]] ; [[#Chandra--2021|Chandra et al., 2021]] ). Atmospheric concentrations of CH <sub>4</sub> reached 1866.3 ppb in 2019 (Figure 5.14). A comprehensive assessment of the CH <sub>4</sub> growth rates over the past four decades is presented in Cross-Chapter Box 5.2.

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[[File:8633e13d2dceadf54aefa89175d5574e IPCC_AR6_WGI_Figure_5_13.png]]

'''Figure 5.13 |''' '''Time series of CH''' <sub>4</sub> '''concentrations, growth rates and isotopic composition. (a)''' CH <sub>4</sub> concentrations; '''(b)''' CH <sub>4</sub> growth rates; '''(c)''' d <sup>13</sup> -CH <sub>4</sub> . Data from selected site networks operated by the National Oceanic and Atmospheric Administration (NOAA; [[#Dlugokencky--2003|Dlugokencky et al., 2003]] ), Advanced Global Atmospheric Gases Experiment (AGAGE; [[#Prinn--2018|Prinn et al., 2018]] ) and Portland Airport (PDX, Portland State University; [[#Rice--2016|Rice et al., 2016]] ). To maintain clarity, data from many other measurement networks are not included here, and all measurements are shown in the World Metereological Organization X2004ACH <sub>4</sub> global calibration standard. Global mean values of XCH <sub>4</sub> (total-column), retrieved from radiation spectra measured by the Greenhouse Gases Observing Satellite (GOSAT) are shown in panels (a) and (b). Cape Grim Observatory (CGO; 41°S, 145°E) and Trinidad Head (THD; 41°N, 124°W) data are taken from the AGAGE network. NOAA global and northern hemispheric (NH) means for d <sup>13</sup> C are calculated from 10 and 6 sites, respectively. The PDX data adjusted to NH (period: 1977–2000) are merged with THD (period: 2001–2019) for CH <sub>4</sub> concentration and growth rate analysis, and PDX and NOAA NH means of d <sup>13</sup> C data are used for joint interpretation of long-term trends analysis. The multivariate El Niño–Southern Oscillation (ENSO) index (MEI) is shown in panel (b). Further details on data sources and processing are available in the chapter data table (Table 5.SM.6).

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