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

==== 7.3.4.6 Volcanic Aerosols ====

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There is large episodic negative radiative forcing associated with sulphur dioxide (SO <sub>2</sub> ) being ejected into the stratosphere from explosive volcanic eruptions, accompanied by more frequent smaller eruptions (Figure 2.2 and Cross-Chapter Box 4.1). From SO <sub>2</sub> gas, reflective sulphate aerosol is formed in the stratosphere where it may persist for months to years, reducing the incoming solar radiation. The volcanic SARF in AR5 ( [[#Myhre--2013b|Myhre et al., 2013b]] ) was derived by scaling the stratospheric aerosol optical depth (SAOD) by a factor of –25 W m <sup>–2</sup> per unit SAOD from [[#Hansen--2005b|Hansen et al. (2005b)]] . Quantification of the adjustments to SAOD perturbations from climate model simulations have determined a significant positive adjustment driven by a reduction in cloud amount (Figure 7.4; [[#Marshall--2020|Marshall et al., 2020]] ). Analysis of CMIP5 models provides a mean ERF of –20 W m <sup>–2</sup> per unit SAOD ( [[#Larson--2016|Larson and Portmann, 2016]] ). Single-model studies with successive generations of Hadley Centre climate models produce estimates between –17 and –19 W m <sup>–2</sup> per unit SAOD ( [[#Gregory--2016|Gregory et al., 2016]] ; [[#Marshall--2020|Marshall et al., 2020]] ), with some evidence that ERF may be non-linear with SAOD for large eruptions ( [[#Marshall--2020|Marshall et al., 2020]] ). Analysis of the volcanically active periods of 1982–1985 and 1990–1994 using the CESM1(WACCM) aerosol–climate model provided an SAOD-to-ERF relationship of –21.5 (± 1.1) W m <sup>–2</sup> per unit SAOD ( [[#Schmidt--2018|Schmidt et al., 2018]] ). Volcanic SO <sub>2</sub> emissions may contribute a positive forcing through effects on upper tropospheric ice clouds, due to additional ice nucleation on volcanic sulphate particles ( [[#Friberg--2015|Friberg et al., 2015]] ; [[#Schmidt--2018|Schmidt et al., 2018]] ), although one observational study found no significant effect ( [[#Meyer--2015|Meyer et al., 2015]] ). Due to ''low agreement'' , the contribution of sulphate aerosol effects on ice clouds to volcanic ERF is not included in the overall assessment.

Non-explosive volcanic eruptions generally yield negligible global ERFs due to the short atmospheric lifetimes (a few weeks) of volcanic aerosols in the troposphere. However, as discussed in ( [[#7.3.3.2|Section 7.3.3.2]] , the massive fissure eruption in Holuhraun, Iceland persisted for months in 2014 and 2015 and did in fact result in a marked and persistent reduction in cloud droplet radii and a corresponding increase in cloud albedo regionally ( [[#Malavelle--2017|Malavelle et al., 2017]] ). This shows that non-explosive fissure eruptions can lead to strong regional and even global ERFs, but because the Holuhraun eruption occurred in Northern Hemisphere winter, solar insolation was weak and the observed albedo changes therefore did not result in an appreciable global ERF ( [[#Gettelman--2015|Gettelman et al., 2015]] ).

The ERF for volcanic stratospheric aerosols is assessed to be –20 ± 5 W m <sup>–2</sup> per unit SAOD ( ''medium confidence'' ) based on the CMIP5 multi-model mean from the [[#Larson--2016|Larson and Portmann (2016)]] SAOD forcing efficiency calculations combined with the single-model results of [[#Gregory--2016|Gregory et al. (2016)]] , [[#Schmidt--2018|Schmidt et al. (2018)]] and [[#Marshall--2020|Marshall et al. (2020)]] . This is applied to the SAOD time series from ( [[IPCC:Wg1:Chapter:Chapter-2|Chapter 2]] [[IPCC:Wg1:Chapter:Chapter-2#2.2.2|Section 2.2.2]] ) to generate a time series of ERF and temperature response shown in ( [[IPCC:Wg1:Chapter:Chapter-2|Chapter 2]] (Figure 2.2 and Figure 7.8, respectively). The period from 500 BCE to 1749 CE, spanning back to the start of the record of [[#Toohey--2017|Toohey and Sigl (2017)]] , is defined as the pre-industrial baseline and the volcanic ERF is calculated using an SAOD anomaly from this long-term mean. As in AR5, a pre-industrial to present-day ERF assessment is not provided due to the episodic nature of volcanic eruptions.

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