Editing IPCC:AR6/WGI/Chapter-2 (section)

=== 2.2.2 Volcanic Aerosol Forcing ===

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The AR5 concluded that, on interannual time scales, the radiative effects of volcanic aerosols are a dominant natural driver of climate variability, with the greatest effects occurring within the first 2–5 years following a strong eruption. Reconstructions of radiative forcing by volcanic aerosols used in the Paleoclimate Modelling Intercomparison Project Phase III (PMIP3) simulations and in AR5 featured short-lived perturbations of a range of magnitudes, with events of greater magnitude than –1 W m <sup>–2</sup> (annual mean) occurring on average every 35–40 years, although no associated assessment of confidence was given. This section focuses on advances in reconstructions of stratospheric aerosol optical depth (SAOD), whereas ( [[IPCC:Wg1:Chapter:Chapter-7|Chapter 7]] focuses on the ERF of volcanic aerosols, and [[IPCC:Wg1:Chapter:Chapter-5|Chapter 5]] assesses volcanic emissions of CO <sub>2</sub> and CH <sub>4</sub> ; tropospheric aerosols are discussed in [[#2.2.6|Section 2.2.6]] . Cro ss-Ch apter Box 4.1 undertakes an integrative assessment of volcanic effects including potential for 21st century effects.

Advances in analysis of sulphate records from the Greenland Ice Sheet (GrIS) and AIS have resulted in improved dating and completeness of SAOD reconstructions over the past 2.5 kyr ( [[#Sigl--2015|Sigl et al., 2015]] ), a more uncertain extension back to 10 ka ( [[#Kobashi--2017|Kobashi et al., 2017]] ; [[#Toohey--2017|Toohey and Sigl, 2017]] ), and a better differentiation of sulphates that reach high latitudes via stratospheric (strong eruptions) versus tropospheric pathways ( [[#Burke--2019|]] [[#Burke--2019|A. Burke et al., 2019]] ; [[#Gautier--2019|Gautier et al., 2019]] ). The PMIP4 volcanic reconstruction extends the period analysed in AR5 by 1 kyr (Figure 2.2c; [[#Jungclaus--2017|Jungclaus et al., 2017]] ) and features multiple strong events that were previously misdated, underestimated or not detected, particularly before about 1500 CE. The period between successive large volcanic eruptions (Negative ERF greater than –1 W m <sup>–2</sup> ), ranges from 3–130 years, with an average of 43 ± 7.5 years between such eruptions over the past 2.5 kyr (data from [[#Toohey--2017|Toohey and Sigl, 2017]] ). The most recent such eruption was that of Mt Pinatubo in 1991. Century-long periods that lack such large eruptions occurred once every 400 years on average. Systematic uncertainties related to the scaling of sulphate abundance in glacier ice to radiative forcing have been estimated to be about 60% ( [[#Hegerl--2006|Hegerl et al., 2006]] ). Uncertainty in the timing of eruptions in the proxy record is ± 2 years (95% confidence interval) back to 1.5 ka and ± 4 years before ( [[#Toohey--2017|Toohey and Sigl, 2017]] ).

SAOD averaged over the period 950–1250 CE (0.012) was lower than for the period 1450–1850 CE (0.017) and similar to the period 1850–1900 (0.011). Uncertainties associated with these inter-period differences are not well quantified but have little effect because the uncertainties are mainly systematic throughout the record. Over the past 100 years, SAOD averaged 14% lower than the mean of the previous 24 centuries (back to 2.5 ka), and well within the range of centennial-scale variability ( [[#Toohey--2017|Toohey and Sigl, 2017]] ).

Direct observations of volcanic gas-phase sulphur emissions (mostly SO <sub>2</sub> ), sulphate aerosols, and their radiative effects are available from a variety of sources ( [[#Kremser--2016|Kremser et al., 2016]] ). New estimates of SO <sub>2</sub> emissions from explosive eruptions have been derived from satellite (beginning in 1979) and in situ measurements ( [[#Höpfner--2015|Höpfner et al., 2015]] ; [[#Carn--2016|Carn et al., 2016]] ; [[#Neely%20III--2016|Neely III and Schmidt, 2016]] ; [[#Brühl--2018|Brühl, 2018]] ). Satellite observations of aerosol extinction after recent eruptions have uncertainties of about 15–25% ( [[#Vernier--2011|Vernier et al., 2011]] ; [[#Bourassa--2012|Bourassa et al., 2012]] ). Additional uncertainties occur when gaps in the satellite records are filled by complementary observations or using statistical methods ( [[#Thomason--2018|Thomason et al., 2018]] ). Merged datasets ( [[#Thomason--2018|Thomason et al., 2018]] ) and sparse ground-based measurements ( [[#Stothers--1997|Stothers, 1997]] ) allow for volcanic forcing estimates back to 1850. In contrast to the CMIP5 historical volcanic forcing datasets ( [[#Ammann--2003|Ammann et al., 2003]] ), updated time series (Figure 2.2d; [[#Luo--2018|Luo, 2018]] ) feature a more comprehensive set of optical properties including latitude-, height- and wavelength-dependent aerosol extinction, single scattering albedo and asymmetry parameters. A series of small-to-moderate eruptions since 2000 resulted in perturbations in SAOD of 0.004–0.006 ( [[#Andersson--2015|Andersson et al., 2015]] ; [[#Schmidt--2018|Schmidt et al., 2018]] ).

To conclude, strong individual volcanic eruptions cause multi-annual variations in radiative forcing. However, the average magnitude and variability of SAOD and its associated volcanic aerosol forcing since 1900 are not unusual in the context of at least the past 2.5 kyr ( ''medium confidence'' ).

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