Editing IPCC:AR6/WGI/Chapter-8 (section)

==== 8.5.2.3 Volcanic Forcing ====

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Volcanic eruptions can affect climate projections in the near term (2021–2040; [[IPCC:Wg1:Chapter:Chapter-4#4.4.4|Section 4.4.4]] and Cross-Chapter Box 4.1). In this chapter, they are of interest because they can trigger a transient departure from the water cycle response to anthropogenic radiative forcing. Major volcanic eruptions temporarily reduce total global and wet tropical region precipitation ( ''high confidence'' ) ( [[#Iles--2014|Iles and Hegerl, 2014]] ), can weaken or shift the ITCZ ( [[#Iles--2014|Iles and Hegerl, 2014]] ; [[#Colose--2016|Colose et al., 2016]] ; [[#Liu--2016|Liu et al., 2016]] ), and reduce summer monsoon rainfall ( ''medium confidence'' ) (Pausata et al. , 2015b; [[#Zambri--2016|Zambri and Robock, 2016]] ; Zambri et al. , 2017; Zuo et al. , 2019; M. Singh et al. , 2020) . Monsoon precipitation in one hemisphere can be enhanced by the remote volcanic forcing occurring in the other hemisphere ( ''medium confidence'' ) (Pausata et al. , 2015a; Liu et al. , 2016; Zuo et al. , 2019) . Over the Sahel, the sign of hydrological changes depend on the hemisphere where the volcanic eruptions occur (J.M. [[#Haywood--2013|]] [[#Haywood--2013|Haywood et al., 2013]] ). Out of phase changes in the Sahel and the Amazonian basin are expected from the effect of volcanic aerosols on tropical Atlantic SST and the ITCZ ( [[#Hua--2019|Hua et al., 2019]] ). Over the last millennium, uncertainties remain in the symmetry/asymmetry of the monsoon response because it is difficult to estimate the exact latitude and season of past volcanic eruptions further back in time ( [[#Colose--2016|Colose et al., 2016]] ; [[#Fasullo--2019|Fasullo et al., 2019]] ).

Data for six major eruptions over the last century along with CMIP5 historical experiments indicate that volcanic eruptions cause a detectable decrease in streamflow in northern South America, Central Africa, high-latitude Asia and in wet tropical–subtropical regions, and a detectable increase in south-western North America and southern South America ( [[#Iles--2015|Iles and Hegerl, 2015]] ). Attempts to include volcanic forcing in future projections show enhanced precipitation variability on annual to decadal time scales with small reductions in Asian monsoon rainfall ( [[#Bethke--2017|Bethke et al., 2017]] ). The occurrence of volcanic eruptions in the coming century, either as single large events or clustered smaller ones, can alter the water cycle (see also Cross-Chapter Box 4.1), and regional drought events may be enhanced by co-occurring volcanic ( [[#Liu--2016|Liu et al., 2016]] ; [[#Gao--2017|Gao and Gao, 2017]] ; [[#Zambri--2017|Zambri et al., 2017]] ) and GHG (e.g., [[#Cook--2018|Cook et al., 2018]] ) forcing ( ''low confidence'' ). Volcanic eruptions may also lead to widespread precipitation anomalies up to several years following an eruption through their potential influence on the El Niño Southern Oscillation ( ''low confidence'' ) ( [[#Stevenson--2016|Stevenson et al., 2016]] ; [[#Dee--2020|Dee et al., 2020]] ; [[#McGregor--2020|McGregor et al., 2020]] ).

In summary, large volcanic eruptions reduce global mean precipitation, as well as precipitation in tropical wet regions ( ''high confidence'' ). There is ''low confidence'' in specific regional and seasonal responses, primarily due to the limitations of the observational record.

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