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

==== 9.7.2.1 Projected Risks ====

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By 2050, up to 921 million additional people in sub-Saharan Africa could be exposed to climate change-related water stress, while up to 459 million could experience reduced exposure ( [[#Dickerson--2021|Dickerson et al., 2021]] ). This large variance in numbers and direction of change is related to uncertainties in climate models and non-climate factors like population growth and water withdrawals ( [[#Dickerson--2021|Dickerson et al., 2021]] ). The baseline for most of the projected risks presented here is 1971–2000.

In west Africa, significant spatial variability in river flow is projected in the upper reaches of several rivers, with no clear pattern overall ( [[#Roudier--2014|Roudier et al., 2014]] ) and large uncertainties in estimations of change in runoff ( [[#Roudier--2014|Roudier et al., 2014]] ; [[#Bodian--2018|Bodian et al., 2018]] ). In some higher altitude regions, like the Niger Inland Delta in west Africa, river flows and water levels are expected to increase ( ''medium confidence'' ) ( [[#Aich--2014|Aich et al., 2014]] ; [[#Thompson--2017|Thompson et al., 2017]] ). In the Lower Niger Basin, combined average annual rainfall and erosivity for all the climatic models in all scenarios shows increasing rainfall amounts are projected to result in an increasing average change in rainfall-runoff erosivity of about 14%, 19% and 24% for the 2030s, 2050s and 2070s, with concomitant increase in soil loss of 12%, 19% and 21% ( [[#Amanambu--2019|Amanambu et al., 2019]] ). In the Volta River system, increasing wet season river flows (+36% by 2090s) and Volta lake outflow (+5% by 2090s) are anticipated under RCP8.5 ( ''medium confidence'' ) (Awotwi A et al., 2015; [[#Jin--2018|Jin et al., 2018]] ). In the Volta River basin, compared to 1976–2005, drought events are projected to increase by 1.2 events per decade at around 2°C to 1.6 events per decade at around 2.5°C global warming, and drought area extent is projected to increase by 24% to 34% ( [[#Oguntunde--2017|Oguntunde et al., 2017]] ). In central Africa, runoff in the Congo river system may increase by up to 50% (RCP8.5), especially in the wet season, enhancing flood risks in the entire Congo Basin, particularly in the central and western parts ( [[#CSC--2013|CSC, 2013]] ). Average river flows are expected to increase in most parts of central Africa, with expected increases in total potential hydropower production ( [[#Ludwig--2013|Ludwig et al., 2013]] ), but see Box 9.5.

In north Africa, in the upper White Nile basin, [[#Olaka--2019|Olaka et al. (2019)]] project a 25% and 5–10% (RCP4.5) increase in the intensification of future annual rainfall in the eastern and western parts of the Lake Victoria Basin, respectively, with corresponding variability in future river discharge ranging from 5% to 26%. In the upper Blue Nile basin, models also indicate up to 15% increase in runoffs in wet season and up to −24% decrease in dry season during 2021–2040 (RCP8.5) ( [[#Ayele--2016|Ayele et al., 2016]] ; [[#Siam--2017|Siam and Eltahir, 2017]] ; [[#Meresa--2018|Meresa and Gatachew, 2018]] ). The increase of precipitation in the wet season indicates a higher possibility of flash floods, while decreased runoffs in dry season further intensify existing shortage of irrigation water demand ( [[#Ayele--2016|Ayele et al., 2016]] ; [[#Siam--2017|Siam and Eltahir, 2017]] ; [[#Meresa--2018|Meresa and Gatachew, 2018]] ). The annual flow and revenues from hydropower production and irrigated agriculture of the Blue Nile River at Khartoum are projected to increase under maximum but are expected to decrease under minimum and median projected changes in streamflow for 2041–2070 and 2071–2100, respectively ( [[#Tariku--2021|Tariku et al., 2021]] ). The Middle Draa valley in Morocco is expected to experience more severe droughts and the estimation of the water balance suggests a lack of supply in the future ( [[#Karmaoui--2016|Karmaoui et al., 2016]] ).

In east Africa, [[#Liwenga--2015|Liwenga et al. (2015)]] project warmer and wetter conditions in the Great Ruaha River region and with increasing seasonal variation and extremes towards the end of the century. A similar observation is made for the River Pangani, with mean river flow being about 10% higher in the 2050s relative to the 1980–1999 period, associated with a 16–18% increase in rainfall in its upper catchment ( [[#Kishiwa--2018|Kishiwa et al., 2018]] ). However, at more local scales, the projections cover a range of slight declines to significant increases in mean annual rainfall amounts ( [[#Gulacha--2017|Gulacha and Mulungu, 2017]] ). In the Tana River basin in Kenya, water yield is projected to increase progressively under RCP4.5 and RCP8.5 relative to the baseline period 1983–2011 but is characterised by distinct spatial heterogeneity ( [[#Muthuwatta--2018|Muthuwatta et al., 2018]] ).

In southern Africa, reductions in rainfall over the Limpopo and Zambezi river basins under 1.5°C and 2°C global warming could have adverse impacts on hydropower generation, irrigation, tourism, agriculture and ecosystems (Figure Box 9.5.1) ( [[#Maúre--2018|Maúre et al., 2018]] ), although model projections of strong early summer drying trends remain uncertain ( [[#Munday--2019|Munday and Washington, 2019]] ).

Changes in the amplitude, timing and frequency of extreme events such as droughts and floods will continue to affect lake levels, rates of river discharge and runoff and groundwater recharge ( ''high confidence'' ) ( [[#Gownaris--2016|Gownaris et al., 2016]] ; [[#Darko--2019|Darko et al., 2019]] ), but with disparate effects at regional, basin and sub-basin scales, and at seasonal, annual and longer timescales. The increased frequency of extreme rainfall events under climate change ( [[#Myhre--2019|Myhre et al., 2019]] ) is projected to amplify groundwater recharge in drylands ( [[#Jasechko--2015|Jasechko and Taylor, 2015]] ; [[#Cuthbert--2019|Cuthbert et al., 2019]] ). However, declining trends in rainfall and snowfall in some areas of north Africa ( [[#Donat--2014b|Donat et al., 2014b]] ) are projected to continue in a warming world ( [[#Seif-Ennasr--2016|Seif-Ennasr et al., 2016]] ), restricting groundwater recharge from meltwater flows, exacerbating the salinisation and depletion of groundwater ( [[#Hamed--2018|Hamed et al., 2018]] ) and increasing the risk of reduced soil moisture ( [[#Petrova--2018|Petrova et al., 2018]] ) in this region where groundwater abstraction is greatest ( [[#Wada--2014|Wada et al., 2014]] ).

Lake surface temperatures across Africa are expected to rise in tandem with increasing global warming. Lake heatwaves, periods of extreme warm lake surface water temperature, are projected to become hotter and longer (Figure 9.21), with heatwaves more than 300 days per year in many lakes for global warming of 4.2°C ( [[#Woolway--2021|Woolway et al., 2021]] ). Lake warming is expected to have adverse consequences for aquatic biodiversity, habitats, water quality and disruption of current lake physical processes and circulation patterns ( [[#Kraemer--2021|Kraemer et al., 2021]] ).

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[[File:dd51904b20954d199ccfce4042dca12c IPCC_AR6_WGII_Figure_9_021.png]]

'''Figure 9.21 |''' '''Climate change is projected to increase the intensity of lake heatwaves across Africa.''' Projected increases in average intensity of lake heatwaves (°C) under

'''(a)''' 1.8°C global warming (RCP2.6 in 2070–2099) and

'''(b)''' 4.2°C global warming (RCP8.5 in 2070–2099). Each lake is represented by a point. Data were extracted from [[#Woolway--2021|Woolway et al. (2021)]] .

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