Editing IPCC:AR6/WGII/Chapter-4 (section)

=== 4.5.4 Projected Risks to Urban and Peri-Urban Sectors ===

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AR5 reported with ''medium confidence'' that climate change would impact residential water demand, supply and management ( [[#Revi--2014|Revi et al., 2014]] ). According to AR5, water utilities are also confronted by changes to the availability of supplies, water quality and saltwater intrusion into aquifers in coastal areas due to higher ambient and water temperatures ( ''medium evidence, high agreement'' ), altered streamflow patterns, drier conditions, increased storm runoff, sea level rise and more frequent forest wildfires in catchments ( [[#Jiménez%20Cisneros--2014|Jiménez Cisneros et al., 2014]] ). SR1.5 found with ''medium confidence'' that constraining warming to 1.5°C instead of 2°C might mitigate risks for water availability, but socioeconomic drivers could affect water availability more than variations in warming levels, while the risks differ across regions ( [[#Hoegh-Guldberg--2018|Hoegh-Guldberg et al., 2018]] ).

In nearly a third of the world’s largest cities, water demand may exceed surface water availability by 2050, based on RCP6.0 projections and the WaterGAP3 modelling framework ( [[#Flörke--2018|Flörke et al., 2018]] ). Under all SSPs, the global volume of domestic water withdrawal is projected to reach 700–1500 km 3 yr –1 by 2050, indicating an increase of 50 to 250%, compared to the 2010 water use intensity (400–450 km 3 yr –1 ) ( [[#Wada--2016|Wada et al., 2016]] ). Increasing water demand by cities is already spurring competition between cities and agricultural users for water, which is expected to continue ( [[#Garrick--2019|Garrick et al., 2019]] ) ( [[#4.5.1|Section 4.5.1]] ). By 2030, South and Southeast Asia are expected to have almost three quarters of the urban land under high-frequency flood risk (10.4.6). South Asia, South America and mid-latitudinal Africa are projected have the largest urban extents exposed to floods and droughts ( [[#Güneralp--2015|Güneralp et al., 2015]] ). An analysis of 571 European cities from the Urban Audit database (using RCP8.5 projections without assessing urban heat island effects) found drought conditions are expected to intensify (compared to the historical period 1951–2000) in southern European cities, particularly in Portugal and Spain ( [[#Guerreiro--2018|Guerreiro et al., 2018]] ; Section [https://www.ipcc.ch/chapter/4#CCP4.3.3 CCP4.3.3] ). Changes in river flooding are projected to affect cities in northwestern European cities and the UK between 2051 and 2100 ( [[#Guerreiro--2018|Guerreiro et al., 2018]] ) (Sections 6.2.3.2, [https://www.ipcc.ch/chapter/4#CCP2.2 CCP2.2.1] , [https://www.ipcc.ch/chapter/4#CCP2.2 CCP2.2.3] ).

Globally, climate change is projected to exacerbate existing challenges for urban water services. These challenges include population growth, the rapid pace of urbanisation and inadequate investment, particularly in less developed economies with limited governance capacity ( ''high confidence'' ) ( [[#Ceola--2016|Ceola et al., 2016]] ; [[#van%20Leeuwen--2016|van Leeuwen et al., 2016]] ; [[#Reckien--2017|Reckien et al., 2017]] ; [[#Tapia--2017|Tapia et al., 2017]] ; [[#Veldkamp--2017|Veldkamp et al., 2017]] ). More specifically, in Arusha (Tanzania), a combination of urban growth modelling, satellite imagery and groundwater modelling projected that rapid urbanisation would reduce groundwater recharge by 23–44% of 2015 levels by 2050 (under business as usual and an RCP8.5 scenario), causing groundwater levels to drop up to 75 m ( [[#Olarinoye--2020|Olarinoye et al., 2020]] ). Flood risk modelling showed a median increase in flood risk of 183% in 2030 based on baseline conditions in Jakarta (Indonesia) with flood risks increasing by up to 45% due to land use changes alone ( [[#Budiyono--2016|Budiyono et al., 2016]] ). A probabilistic analysis of surface water flood risk in London (UK) using the UKCP09 Weather Generator (with 10th and 90th percentile uncertainty bounds) found that the annual damage is expected to increase from the baseline by 101% and 128% under 2030 and 2050 high-emission scenarios, respectively ( [[#Jenkins--2018|Jenkins et al., 2018]] ).

Modified streamflow is projected to affect the amount and variability of inflow to urban storage reservoirs ( ''high confidence'' ), which may exacerbate existing challenges to urban reservoir capacity, such as sedimentation and poor water quality ( [[#Goharian--2016|Goharian et al., 2016]] ; [[#Howard--2016|Howard et al., 2016]] ; [[#Yasarer--2016|Yasarer and Sturm, 2016]] ). For example, in Melbourne (Australia), a combination of stochastic hydro-climatological modelling, rainfall-runoff modelling and climate model data projects a mean precipitation shift over catchments by −2% at 1.5°C and −3.3% at 2°C, relative to 1961–1990. Considering an annual water demand of 0.75 of the mean yearly inflow, the median water supply shortage risk was calculated to be 0.6% and 2.9% at 1.5°C and 2°C warming levels, respectively. At the higher demand level of 0.85 of the mean annual inflow, the median water shortage risk is higher, between 9.6% and 20.4% at 1.5°C and 2° C warming, respectively, without supply augmentation desalination ( [[#Henley--2019|Henley et al., 2019]] ).

As climate change poses a substantial challenge to urban water management, further refinement of urban climate models, downscaling and correction methods (e.g., [[#Gooré%20Bi--2017|Gooré Bi et al., 2017]] ; [[#Jaramillo--2018|Jaramillo and Nazemi, 2018]] ) is needed. Additionally, given that 90% of urban growth will occur in less developed regions, where urbanisation is largely unplanned ( [[#UN-Habitat--2019|UN-Habitat, 2019]] ), further research is needed to quantify the water-related risks of climate change and urbanisation on informal settlements ( [[#Grasham--2019|Grasham et al., 2019]] ; [[#Satterthwaite--2020|Satterthwaite et al., 2020]] , 4.5.3).

In summary, rapid population growth, urbanisation, ageing infrastructure and changes in water use are responsible for increasing the vulnerability of urban and peri-urban areas to extreme rainfall and drought, particularly in less developed economies with limited governance capacity ( ''high confidence'' ). In addition, modified stream flows due to climate change ( [[#4.4.3|Section 4.4.3]] ) are projected to affect the amount and variability of inflows to storage reservoirs that serve urban areas and may exacerbate challenges to reservoir capacity, such as sedimentation and poor water quality ( ''high confidence'' ).

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