Editing IPCC:AR6/WGI/Chapter-7 (section)

==== 7.3.2.2 Methane (CH <sub>4</sub> ) ====

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The SARF for methane (CH <sub>4</sub> ) has been substantially increased due to updates to spectroscopic data and inclusion of shortwave absorption ( [[#Etminan--2016|Etminan et al., 2016]] ). Adjustments have been calculated in nine climate models by [[#Smith--2018b|Smith et al. (2018b)]] . Since CH <sub>4</sub> is found to absorb in the shortwave near infrared, only adjustments from those models including this absorption are taken into account. For these models the adjustments act to reduce the ERF because the shortwave absorption leads to tropospheric heating and reductions in upper tropospheric cloud amounts. The adjustment is –14% ± 15%, which counteracts much of the increase in SARF identified by [[#Etminan--2016|Etminan et al. (2016)]] . [[#Modak--2018|Modak et al. (2018)]] also found negative forcing adjustments from a methane perturbation including shortwave absorption in the NCAR CAM5 model, in agreement with the above assessment. The uncertainty in the shortwave component leads to a higher radiative modelling uncertainty (14%) than for CO <sub>2</sub> ( [[#Etminan--2016|Etminan et al., 2016]] ). When combined with the uncertainty in the adjustment, this gives an overall uncertainty of ±20%. There is ''high confidence'' in the spectroscopic revision but only ''medium confidence'' in the adjustment modification. CH <sub>4</sub> concentrations have increased from 729 ppb in 1750 to 1866 ppb in 2019 [[IPCC:Wg1:Chapter:Chapter-2#2.2.3.3|Section 2.2.3.3]] ). The historical ERF estimate from AR5 of 0.48 ± 0.10 W m <sup>–2</sup> (1750–2011) is revised to 0.54 ± 0.11 W m <sup>–2</sup> (1750 to 2019) in this assessment from a combination of spectroscopic radiative efficiency revisions (+0.12 W m <sup>–2</sup> ), adjustments (–0.08 W m <sup>–2</sup> ) and the 63 ppb rise in atmospheric CH <sub>4</sub> concentrations between 2011 and 2019 (+0.03 W m <sup>–2</sup> ). As the adjustments are assessed to be small, there is ''high confidence'' in the overall assessment of ERF from methane. Increased methane leads to tropospheric ozone production and increased stratospheric water vapour, so that an attribution of forcing to methane emissions gives a larger effect than that directly from the methane concentration itself. This is discussed in detail in ( [[IPCC:Wg1:Chapter:Chapter-6|Chapter 6]] (Section 6.4.2) and shown in Figure 6.12.

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