Editing IPCC:AR6/WGII/Chapter-9 (section)

==== 9.5.3.2 Precipitation ====

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===== 9.5.3.2.1 Observations =====

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Negative trends in rainfall accompanied by increased rainfall variability were observed between 1960s–1980s over west Africa ( [[#Nicholson--2018|Nicholson et al., 2018]] ; [[#Thomas--2018|Thomas and Nigam, 2018]] ), caused by a combination of anthropogenic aerosols and GHGs emitted between the 1950s and1980s ( [[#Booth--2012|Booth et al., 2012]] ; [[#Wang--2016|Wang et al., 2016]] ; [[#Giannini--2019|Giannini and Kaplan, 2019]] ; [[#Douville--2021|Douville et al., 2021]] ). Declining rainfall trends ended by 1990 due to the growing influence of GHGs and reduced cooling effect of aerosol emissions, with a trend to wetter conditions emerging in the mid-1990s accompanied by more intense, but fewer precipitation events ( [[#Sanogo--2015|Sanogo et al., 2015]] ; [[#Sylla--2016|Sylla et al., 2016]] ; [[#Kennedy--2017|Kennedy et al., 2017]] ; [[#Barry--2018|Barry et al., 2018]] ; [[#Bichet--2018a|Bichet and Diedhiou, 2018a]] ; 2018b; [[#Thomas--2018|Thomas and Nigam, 2018]] ). A shift to a later onset and end of the west African monsoon is also reported in west Africa and Sahel ( ''low confidence'' ) ( [[#Chen--2021|Chen et al., 2021]] ; [[#Ranasinghe--2021|Ranasinghe et al., 2021]] ). Between 1981–2014 the Gulf of Guinea and the Sahel have experienced more intense precipitation events ( [[#Panthou--2014|Panthou et al., 2014]] ; [[#Bichet--2018a|Bichet and Diedhiou, 2018a]] ; [[#Panthou--2018|Panthou et al., 2018]] ) and the frequency of mesoscale storms has tripled ( [[#Taylor--2017|Taylor et al., 2017]] ; [[#Callo-Concha--2018|Callo-Concha, 2018]] ). Extreme heavy precipitation indices show increasing trends from 1981–2010 ( [[#Barry--2018|Barry et al., 2018]] ), increasing high flow events in large Sahelian rivers as well as small to mesoscale catchments leading to pluvial and riverine flooding ( [[#Douville--2021|Douville et al., 2021]] ). Meteorological, agricultural and hydrological drought in the region has increased in frequency since the 1950s ( ''medium confidence'' ) ( [[#Seneviratne--2021|Seneviratne et al., 2021]] ).

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===== 9.5.3.2.1 Projections =====

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West African rainfall projections show a gradient of precipitation decrease in the west and increase in the east ( ''medium confidence'' ) (Figure 9.14; [[#Dosio--2021|Dosio et al., 2021]] ; [[#Gutiérrez--2021|Gutiérrez et al., 2021]] ; [[#Ranasinghe--2021|Ranasinghe et al., 2021]] ). This pattern is evident at 1.5°C of global warming and the magnitude of change increases at higher warming levels (Figure 9.16c; [[#Schleussner--2016b|Schleussner et al., 2016b]] ; [[#Kumi--2018|Kumi and Abiodun, 2018]] ; [[#Sylla--2018|Sylla et al., 2018]] ). A reduction in length of the rainy season is projected over the western Sahel through delayed rainfall onset by 4–6 days at global warming levels of 1.5°C and 2°C ( [[#Kumi--2018|Kumi and Abiodun, 2018]] ; [[#Douville--2021|Douville et al., 2021]] ; [[#Gutiérrez--2021|Gutiérrez et al., 2021]] ). Although there are uncertainties in rainfall projections over the Sahel ( [[#Klutse--2018|Klutse et al., 2018]] ; [[#Gutiérrez--2021|Gutiérrez et al., 2021]] ), CMIP6 models project monsoon rainfall amounts to increase by approximately 2.9% per degree of warming ( [[#Jin--2020|Jin et al., 2020]] ; [[#Wang--2020a|Wang et al., 2020a]] ), therefore, at higher levels of warming and towards the end of the century, a wetter monsoon is projected in the eastern Sahel ( ''medium confidence'' ).

The frequency and intensity of extremely heavy precipitation are projected to increase under mid- and high-emission scenarios (Figures 9.13a; 9.14; [[#Sylla--2015b|Sylla et al., 2015b]] ; [[#Diallo--2016|Diallo et al., 2016]] ; [[#Akinsanola--2019|Akinsanola and Zhou, 2019]] ; [[#Giorgi--2019|Giorgi et al., 2019]] ; [[#Dosio--2021|Dosio et al., 2021]] ; [[#Li--2021|Li et al., 2021]] ; [[#Seneviratne--2021|Seneviratne et al., 2021]] ). However, heavy rainfall statistics from global and regional climate models may be conservative as very-high-resolution, convection-permitting climate models simulate more intense rainfall than these models ( [[#Stratton--2018|Stratton et al., 2018]] ; [[#Berthou--2019|Berthou et al., 2019]] ; [[#Han--2019|Han et al., 2019]] ; [[#Kendon--2019|Kendon et al., 2019]] ).

At 2°C global warming, west Africa is projected to experience a drier, more drought-prone and arid climate, especially in the last decades of the 21st century ( [[#Sylla--2016|Sylla et al., 2016]] ; [[#Zhao--2016|Zhao and Dai, 2016]] ; [[#Klutse--2018|Klutse et al., 2018]] ). The duration of meteorological drought in the western parts of West Africa is projected to increase from approximately 2 months during 1950–2014 to approximately 4 months in the period 2050–2100 under RCP8.5 and SSP5-8.5 ( [[#Ukkola--2020|Ukkola et al., 2020]] ). Increased intensity of heavy precipitation events combined with increasing drought occurrences will substantially increase the cumulative hydroclimatic stress on populations in west Africa during the late 21st century ( [[#Giorgi--2019|Giorgi et al., 2019]] ).

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