Editing IPCC:AR6/WGII/Chapter-6 (section)

==== 6.3.5.1 Urban Morphology and Built Form ====

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Urban morphology describes the overall status of cities as physical, environmental and cultural entities. Cities interact with surrounding environmental processes, for example, as documented in [[#6.2|Section 6.2]] by influencing urban temperature, but also precipitation and through coastal and riverine development fluvial and coastal sedimentary regimes of erosion and deposition that impact on flood risk. Rapid, increased urbanisation has contributed to observed flood risks in recent decades (see Section 5 4.2.4; Tramblay et al., 2019). The design process for physical infrastructure projects and significant construction (e.g., residential or industrial estates and large industrial development) typically includes risk assessments and social and environmental impact assessments that consider neighbouring land uses and connected infrastructure. Land use planning can consider diverse land uses and their interactions at the neighbourhood level ( [[#6.3.2|Section 6.3.2.1]] ). Resilience planning aims to bring together integrated, systemic views and enable joined-up planning at the city level (as well as lower scales) ( [[#6.3.2|Section 6.3.2.1]] ). There is however a lack of long-term studies that assess the climate change impacts on urban form, including informal settlements (Bai et al., 2018; Ramyar, Zarghami and Bryant, 2019), leading to impact assessments that often overlook urban form (Ramyar, Zarghami and Bryant, 2019). Additionally, context-specific spatial tools and community based approaches lack a precise connection to urban morphology. For example, there is a need for further studies that connect solar radiation, urban morphology (e.g., aspect and plot ratio), and the urban heat island spatio-temporal variability ( [[#Giridharan--2018|Giridharan and Emmanuel, 2018]] ; Li et al., 2019c).

Several tools and models have emerged in response to recommendations from AR5, including models that assess the impacts of urban heat island (Ramyar, Zarghami and Bryant, 2019), climatic uncertainty ( [[#Dhar--2017|Dhar and Khirfan, 2017]] ), flood vulnerability (Abebe, Kabir and Tesfamariam, 2018) and inundation (Barau et al., 2015; Ford et al., 2019). For example, findings from Kano, Nigeria, reveal that a lack of distribution of certain urban morphological features, including open spaces and streets (both pervious and impervious), roof and building materials (e.g., concrete and metallic) and urban ecological features (e.g., urban ponds and ecological basin), exacerbates inundations and their associated impacts (Barau et al., 2015). Also, findings about the urban forms of coastal settlements, particularly in small islands, reveal that they often experience severe beach erosion due to wave action, sea level rise and storm surge that leads to landward retreat of coastline which threatens their social and economic activities ( [[#Dhar--2016|Dhar and Khirfan, 2016]] ; Lane et al., 2015; [[#Khirfan--2019|Khirfan and El-Shayeb, 2019]] ). Despite these examples, very limited research is available to offer assessments of different urban scale morphologies and urban scale adaptation planning, including planning adaptation across supply chains and networked relationships with distant urban and rural places connected through trade and resource (financial, human and material) or waste flows.

Interventions in the morphology and built form of cities can contribute to the reduction of the urban heat island effect and reduce the consequences of urban heatwaves. These can include installing air conditioning, establishing public cooling centres (i.e., for use during heatwaves), pavement watering (Parison et al., 2020a) and increasing surface albedo through ‘cool roofs’ (i.e., with high-reflectance materials) and walls. Air conditioning can significantly increase the local urban heat island (Salamanca et al., 2014; Wang et al., 2019a) and the choice of refrigerant has a significant impact on global warming potential (McLinden et al., 2017). The relative efficiency of cool roofs compared with green roofs is variable, because while white roofs have similar potential to reduce the urban heat island (Li, Bou-Zeid and Oppenheimer, 2014), they can quickly turn grey due to dust and air pollution, losing their effectiveness (Gunawardena, Wells and Kershaw, 2017), although these effects are now well studied and newer performance standards should account for ageing and soiling effects on reflectivity (Paolini et al., 2014). Ageing of ‘cool pavements’ is more complex, which makes their long-term performance less reliable to predict (Lontorfos, Efthymiou and Santamouris, 2018). The cooling performance of green roofs is highly variable and depends on the actual water content of the green roof substrate, with dry vegetation performing poorly in terms of cooling (Parison et al., 2020b). This holds true for regular vegetation and NBS in general (Daniel, Lemonsu and Viguie, 2018). For all built environment adaptations, changes are locked-in for a long time, and are likely to be expensive so that care is needed to avoid potential negative impacts on social equity ( [[#Cabrera--2015|Cabrera and Najarian, 2015]] ; Romero-Lankao et al., 2018; Fried et al., 2020; Rode et al., 2017) and carbon-intensive construction (Bai et al., 2018; Seto et al., 2016).

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