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

===== 7.3.3.1.1 Observation-based lines of evidence =====

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Estimating IRFari requires an estimate of industrial-era changes in aerosol optical depth (AOD) and absorption AOD, which are often taken from global aerosol model simulations. Since AR5, updates to methods of estimating IRFari based on aerosol remote sensing or data-assimilated reanalyses of atmospheric composition have been published. [[#Ma--2014|Ma et al. (2014)]] applied the method of [[#Quaas--2008|Quaas et al. (2008)]] to updated broadband radiative flux measurements from CERES, MODIS-retrieved AODs, and modelled anthropogenic aerosol fractions to find a clear-sky IRFari of −0.6 W m <sup>−2</sup> . This would translate into an all-sky estimate of about −0.3 W m <sup>−2</sup> based on the clear-sky to all-sky ratio implied by [[#Kinne--2019|Kinne (2019)]] . [[#Rémy--2018|Rémy et al. (2018)]] applied the methods of [[#Bellouin--2013a|Bellouin et al. (2013a)]] to the reanalysis by the Copernicus Atmosphere Monitoring Service, which assimilates MODIS total AOD. Their estimate of IRFari varies between −0.5 W m <sup>–2</sup> and −0.6 W m <sup>−2</sup> over the period 2003–2018, and they attribute those relatively small variations to variability in biomass-burning activity. [[#Kinne--2019|Kinne (2019)]] provided updated monthly total AOD and absorption AOD climatologies, obtained by blending multi-model averages with ground-based sun-photometer retrievals, to find a best estimate of IRFari of −0.4 W m <sup>−2</sup> . The updated IRFari estimates above are all scattered around the midpoint of the IRFari range of −0.35 ± 0.5 W m <sup>−2</sup> assessed by AR5 ( [[#Boucher--2013|Boucher et al., 2013]] ).

The more negative estimate of [[#Rémy--2018|Rémy et al. (2018)]] is due to neglecting a small positive contribution from absorbing aerosols above clouds and obtaining a larger anthropogenic fraction than [[#Kinne--2019|Kinne (2019)]] . [[#Rémy--2018|Rémy et al. (2018)]] also did not update their assumptions on black carbon anthropogenic fraction and its contribution to absorption to reflect recent downward revisions ( [[#7.3.3.1.2|Section 7.3.3.1.2]] ). [[#Kinne--2019|Kinne (2019)]] made those revisions, so more weight is given to that study to assess the central estimate of satellite-based IRFari to be only slightly stronger than reported in AR5 at –0.4 W m <sup>–2</sup> . While uncertainties in the anthropogenic fraction of total AOD remain, improved knowledge of anthropogenic absorption results in a slightly narrower ''very likely'' range here than in AR5. The assessed best estimate and ''very'' ''likely'' IRFari range from observation-based evidence is therefore –0.4 ± 0.4 W m <sup>–2</sup> , but with ''medium confidence'' due to the limited number of studies available ''.''

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