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

==== 7.3.3.4 Overall Assessment of Total Aerosol ERF ====

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In AR5 ( [[#Boucher--2013|Boucher et al., 2013]] ), the overall assessment of total aerosol ERF (ERFari+aci) used the median of all ESM estimates published prior to AR5 of –1.5 [–2.4 to –0.6] W m <sup>–2</sup> as a starting point, but placed more confidence in a subset of models that were deemed more complete in their representation of aerosol–cloud interactions. These models, which included aerosol effects on mixed-phase, ice and/or convective clouds, produced a smaller estimate of –1.38 W m <sup>–2</sup> . Likewise, studies that constrained models with satellite observations (five in total), which produced a median estimate of –0.85 W m <sup>–2</sup> , were given extra weight. Furthermore, a longwave ERFaci of 0.2 W m <sup>–2</sup> was added to studies that only reported shortwave ERFaci values. Finally, based on higher resolution models, doubt was raised regarding the ability of ESMs to represent the cloud-adjustment component of ERFaci with fidelity. The expert judgement was therefore that aerosol effects on cloud lifetime were too strong in the ESMs, further reducing the overall ERF estimate. The above lines of argument resulted in a total aerosol assessment of –0.9 [–1.9 to –0.1] W m <sup>–2</sup> in AR5.

Here, the best estimate and range is revised relative to AR5 ( [[#Boucher--2013|Boucher et al., 2013]] ), partly based on updates to the above lines of argument. Firstly, the studies that included aerosol effects on mixed-phase clouds in AR5 relied on the assumption that anthropogenic black carbon (BC) could act as INPs in these clouds, which has since been challenged by laboratory experiments ( [[#Kanji--2017|Kanji et al., 2017]] ; [[#Vergara-Temprado--2018|Vergara-Temprado et al., 2018]] ). There is no observational evidence of appreciable ERFs associated with aerosol effects on mixed-phase and ice clouds ( [[#7.3.3.2.1|Section 7.3.3.2.1]] ), and modelling studies disagree when it comes to both their magnitude and sign ( [[#7.3.3.2.2|Section 7.3.3.2.2]] ). Likewise, very few ESMs incorporate aerosol effects on deep convective clouds, and cloud-resolving modelling studies report different effects on cloud radiative properties depending on environmental conditions ( [[#Tao--2012|Tao et al., 2012]] ). Thus, it is not clear whether omitting such effects from ESMs would lead to any appreciable ERF biases, or if so, what the sign of such biases would be. As a result, all ESMs are given equal weight in this assessment. Furthermore, there is now a considerably expanded body of literature which suggests that early modelling studies that incorporated satellite observations may have resulted in overly conservative estimates of the magnitude of ERFaci ( [[#7.3.3.2.1|Section 7.3.3.2.1]] ). Finally, based on an assessment of the longwave ERFaci in the CMIP5 models, the offset of +0.2 W m <sup>–2</sup> applied in AR5 appears to be too large ( [[#Heyn--2017|Heyn et al., 2017]] ). As in AR5, there is still reason to question the ability of ESMs to simulate adjustments in LWP and cloud cover in response to aerosol perturbation, but it is not clear that this will result in biases that exclusively increase the magnitude of the total aerosol ERf ( [[#7.3.3.2.2|Section 7.3.3.2.2]] ).

The assessment of total aerosol ERF here uses the following lines of evidence: satellite-based evidence for IRFari; model-based evidence for IRFari and ERFari; satellite-based evidence of IRFaci and ERFaci; and finally model-based evidence for ERFaci. Based on this, ERFari and ERFaci for 2014 relative to 1750 are assessed to be –0.3 ± 0.3 W m <sup>–2</sup> and –1.0 ± 0.7 W m <sup>–2</sup> , respectively. There is thus strong evidence for a substantive negative total aerosol ERF, which is supported by the broad agreement between observation-based and model-based lines of evidence for both ERFari and ERFaci that has emerged since AR5 ( [[#Gryspeerdt--2020|Gryspeerdt et al., 2020]] ). However, considerable uncertainty remains, particularly with regards to the adjustment contribution to ERFaci, as well as missing processes in current ESMs, notably aerosol effects on mixed-phase, ice and convective clouds. This leads to a ''medium confidence'' in the estimate of ERFari+aci and a slight narrowing of the uncertainty range. Because the estimates informing the different lines of evidence are generally valid for approximately 2014 conditions, the total aerosol ERF assessment is considered valid for 2014 relative to 1750.

Combining the lines of evidence and adding uncertainties in quadrature, the ERFari+aci estimated for 2014 relative to 1750 is assessed to be –1.3 [–2.0 to –0.6] W m <sup>–2</sup> ( ''medium confidence'' ) ''.'' The corresponding range from Bellouin et al. (2019) is –3.15 to –0.35 W m <sup>–2</sup> , thus there is agreement for the upper bound while the lower bound assessed here is less negative. A lower bound more negative than –2.0 W m <sup>–2</sup> is not supported by any of the assessed lines of evidence. There is ''high confidence'' that ERFaci contributes most (75–80%) to the total aerosol effect (ERFari+aci). In contrast to AR5 ( [[#Boucher--2013|Boucher et al., 2013]] ), it is now ''virtually certain'' that the total aerosol ERF is negative. Figure 7.5 depicts the aerosol ERFs from the different lines of evidence along with the overall assessments.

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[[File:0c87fc40a5bd234b7f89a8f0e96755a5 IPCC_AR6_WGI_Figure_7_5.png]]

'''Figure 7.5''' '''|''' '''Net aerosol effective radiative forcing (ERF) from different lines of evidence.''' The headline AR6 assessment of –1.3 [–2.0 to –0.6] W m <sup>–2</sup> is highlighted in purple for 1750–2014 and compared to the AR5 assessment of –0.9 [–1.9 to –0.1] W m <sup>–2</sup> for 1750–2011. The evidence comprising the AR6 assessment is shown below this: energy balance constraints [–2 to 0 W m <sup>–2</sup> with no best estimate]; observational evidence from satellite retrievals of –1.4 [–2.2 to –0.6] W m <sup>–2</sup> ; and climate model-based evidence of –1.25 [–2.1 to –0.4] W m <sup>–2</sup> . Estimates from individual CMIP5 ( [[#Zelinka--2014|Zelinka et al., 2014]] ) and CMIP6 ( [[#Smith--2020b|Smith et al., 2020b]] and Table 7.6) models are depicted by blue and red crosses respectively. For each line of evidence the assessed best-estimate contributions from ERFari and ERFaci are shown with darker and paler shading respectively. The observational assessment for ERFari is taken from the IRFari. Uncertainty ranges are represented by black bars for the total aerosol ERF and depict ''very likely'' ranges. Further details on data sources and processing are available in the chapter data table (Table 7.SM.14).

As most modelling and observational estimates of aerosol ERF have end points in 2014 or earlier, there is '''limited evidence''' available for the assessment of how aerosol ERF has changed from 2014 to 2019. However, based on a general reduction in global mean AOD over this period ( [[IPCC:Wg1:Chapter:Chapter-2#2.2.6|Section 2.2.6]] and Figure 2.9), combined with a reduction in emissions of aerosols and their precursors in updated emissions inventories ( [[#Hoesly--2018|Hoesly et al., 2018]] ), the aerosol ERF is assessed to have decreased in magnitude from about 2014 to 2019 ( ''medium confidence'' ). Consistent with Figure 2.10, the change in aerosol ERF from about 2014 to 2019 is assessed to be +0.2 W m <sup>–2</sup> , but with ''low confidence'' due to '''limited evidence''' . Aerosols are therefore assessed to have contributed an ERF of –1.1 [–1.7 to –0.4] W m <sup>–2</sup> over 1750–2019 ( ''medium confidence'' ).

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