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

=== 4.6.5 Adaptation in Urban and Peri-Urban Sectors ===

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AR5 reported that although case studies of the potential effectiveness of adaptation measures in cities are growing, not all considered how adaptation would be implemented in practice ( [[#Jiménez%20Cisneros--2014|Jiménez Cisneros et al., 2014]] ). Furthermore, AR5 concluded that more attention had been given to adaptations that help ensure sufficient water supplies than to increasing the capacity of sewage and drainage systems to adapt to heavier rainfall or sea level rise ( [[#Revi--2014|Revi et al., 2014]] ).

Since AR5 knowledge on urban adaptation has advanced, even though there is still limited documentation of water adaptation in urban contexts as compared to other adaptation responses (Figure 4.23.) The majority of case studies on urban adaptation are also from developed countries, most commonly in Europe and Australasia (Figure 4.24). Water-related urban and peri-urban climate change adaptation can involve ‘hard’ engineering structures (grey), managed or restored biophysical systems (green and blue) or hybrid approaches that combine these strategies ( [[#Ngoran--2015|Ngoran and Xue, 2015]] ; [[#Palmer--2015|Palmer et al., 2015]] ) (Figure 4.21, also see Figure 4.22 for types of urban adaptation options).

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[[File:874b9c2ec605731213e4dc9930214023 IPCC_AR6_WGII_Figure_4_022.png]]

'''Figure 4.22 |''' '''Decision tree, documenting the classification of water-related adaptation responses across 48 subcategories into 16 intermediate and 8 larger categories.''' We use the 16 intermediate categories of adaptation responses for further analysis in this section.

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[[File:6c6c877225b16479b251c17fedec9d90 IPCC_AR6_WGII_Figure_4_021.png]]

'''Figure 4.21 |''' '''Strategies for urban water adaptation.'''

'''(a)''' Green and blue strategies of urban water adaptation prioritise ecosystem restoration, such as wetlands restoration.

'''(b)''' Grey water strategies are hard engineering approaches to urban water adaptation, including infrastructure such as pipes and canals, with extensive areas of impervious surfaces.

'''(c)''' Hybrid approaches combine green, blue and grey adaptation strategies, such that ecosystem functions are complemented by engineered infrastructure, such as constructed wetlands, green roofs and riparian buffers. Green and blue and hybrid approaches are variously classified in terms of a circular economy, water sensitive urban design, nature-based solutions (NbS), integrated urban water management, and ecological infrastructure. Adapted from [[#Depietri--2017|Depietri and McPhearson (2017)]] .

In most regions, hybrid adaptation approaches are underway. For example, sustainable urban drainage systems (SUDS) are a common adaptation measure that can reduce flooding and improve stormwater quality while reducing the urban heat island effect (e.g., [[#Chan--2019|Chan et al., 2019]] ; [[#Loiola--2019|Loiola et al., 2019]] ; [[#Song--2019|Song et al., 2019]] ; [[#Huang--2020|Huang et al., 2020]] ; [[#Lin--2020|Lin et al., 2020]] ) (Box 4.6; 12.5.5.3.2; 12.7.1). Municipal, catchment and local community plans to minimise water-related climate risks are another form of adaptation ( [[#Stults--2018|Stults and Larsen, 2018]] ). Plans involve supply augmentation ( [[#Chu--2017|Chu, 2017]] ; [[#Bekele--2018|Bekele et al., 2018]] ), as well as floodplain management, land use planning, stakeholder coordination and water demand management ( [[#Andrew--2017|Andrew and Sauquet, 2017]] ; [[#Flyen--2018|Flyen et al., 2018]] ; [[#Robb--2019|Robb et al., 2019]] ; [[#Tosun--2019|Tosun and Leopold, 2019]] ), with some US cities including strategies to address social inequalities that climate change may exacerbate ( [[#Chu--2021|Chu and Cannon, 2021]] ).

Such adaptation measures are concentrated in more developed countries ( [[#Olazabal--2019|Olazabal et al., 2019]] ). For example, about 80% of European cities with more than 500,000 inhabitants have either mitigation and/or adaptation plans ( [[#Reckien--2018|Reckien et al., 2018]] ). In contrast, a survey of cities with more than one million inhabitants found 92% of Asian cities, 89% of African cities and 87% of Latin American cities did not report adaptation initiatives ( [[#Araos--2016|Araos et al., 2016]] ) (12.5.8.1). Autonomous adaptation measures (e.g., elevating housing and drainage maintenance) are pursued to reduce flood risk in urban Senegal ( [[#Schaer--2015|Schaer, 2015]] ), Kenya ( [[#Thorn--2015|Thorn et al., 2015]] ), Brazil ( [[#Mansur--2018|Mansur et al., 2018]] ) and Guyana ( [[#Mycoo--2014|Mycoo, 2014]] ) (Box 4.7; 9.8.5.1; 12.5.5.3; FAQ12.2).

Further studies are required to ascertain the effectiveness of adaptation measures implemented since AR5, particularly for the growing populations of informal and peri-urban settlements. For example, in urban Africa, such informal settlements are sites of political contestation as residents resist municipal relocation strategies for flood alleviation ( [[#Douglas--2018|Douglas, 2018]] ). In addition, the growing complexity of challenges facing urban water management, such as climate change, urbanisation and environmental degradation, warrants a transformative shift away from prevailing siloed approaches of water supply, sanitation and drainage to more integrated systems that enhance adaptive capacity ( [[#Ma--2015|Ma et al., 2015]] ; [[#Franco-Torres--2020|Franco-Torres et al., 2020]] ).

In summary, although water-related adaptation is underway in the urban, peri-urban and municipal sectors of some nations, governance, technical and economic barriers remain in implementing locally informed strategies, particularly in developing countries ( ''high confidence'' ).

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