Editing IPCC:AR6/WGIII/Chapter-8 (section)

==== 8.4.4.2 Benefits of Green Roofs, Green Walls, and Greenways ====

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Green roofs and green walls have potential to mitigate air and surface temperature, improve thermal comfort, and mitigate UHI effects ( [[#Jamei--2021|Jamei et al. 2021]] ; [[#Wong--2021|Wong et al. 2021]] ), while lowering the energy demand of buildings ( [[#Susca--2019|Susca 2019]] ) (Figure 8.18). Green roofs have the highest median cooling effect in dry climates (3°C) and the lowest cooling effect in hot, humid climates (1°C) ( [[#Jamei--2021|Jamei et al. 2021]] ). These mitigation potentials depend on numerous factors and the scale of implementation. The temperature reduction potential for green roofs when compared to conventional roofs can be about 4°C in winter and about 12°C during summer conditions ( [[#Bevilacqua--2016|Bevilacqua et al. 2016]] ). Green roofs can reduce building heating demands by about 10–30% compared to conventional roofs ( [[#Besir--2018|Besir and Cuce 2018]] ), 60–70% compared to black roofs, and 45–60% compared to white roofs ( [[#Silva--2016|Silva et al. 2016]] ). Green walls or facades can provide a temperature difference between air temperature outside and behind a green wall of up to 10°C, with an average difference of 5°C in Mediterranean contexts in Europe ( [[#Perini--2017|Perini et al. 2017]] ). The potential of saving energy for air conditioning by green facades can be around 26% in summer months. Considerations of the spatial context are essential given their dependence on climatic conditions ( [[#Susca--2019|Susca 2019]] ). Cities are diverse and emissions savings potentials depend on several factors, while the implementation of green roofs or facades may be prevented in heritage structures.

Green roofs have been shown to have beneficial effects in stormwater reduction ( [[#Andrés-Doménech--2018|Andrés-Doménech et al. 2018]] ). A global meta-analysis of 75 international studies on the potential of green roofs to mitigate runoff indicate that the runoff retention rate was on average 62% but with a wide range (0–100%) depending on a number of interdependent factors ( [[#Zheng--2021|Zheng et al. 2021]] ). These factors relate to the characteristics of the rainfall event (e.g., intensity) and characteristics of the green roof (e.g., substrate, vegetation type, and size), and of the climate and season type. A hydrologic modelling approach applied to an Italian case demonstrated that implementing green roofs may reduce peak runoff rates and water volumes by up to 35% in a 100% green roof conversion scenario ( [[#Masseroni--2016|Masseroni and Cislaghi 2016]] ).

Greenways support stormwater management to mitigate water runoff and urban floods by reducing the water volume (e.g., through infiltration) and by an attenuation or temporal shift of water discharge ( [[#Fiori--2020|Fiori and Volpi 2020]] ; [[#Pour--2020|Pour et al. 2020]] ). Using green infrastructure delays the time to runoff and reduces water volume but depends on the magnitude of floods ( [[#Qin--2013|Qin et al. 2013]] ). Measures are most effective for flood mitigation at a local scale; however, as the size of the catchment increases, the effectiveness of reducing peak discharge decreases ( [[#Fiori--2020|Fiori and Volpi 2020]] ). Reduction of water volume through infiltration can be more effective with rainfall events on a lower return rate. Overall, the required capacity for piped engineered systems for water runoff attenuation and mitigation can be reduced while lowering flow rates, controlling pollution transport, and increasing the capacity to store stormwater ( [[#Srishantha--2017|Srishantha and Rathnayake 2017]] ). Benefits for flood mitigation require a careful consideration of the spatial context of the urban area, the heterogeneity of the rainfall events, and characteristics of implementation ( [[#Qiu--2021|Qiu et al. 2021]] ). Maintenance costs and stakeholder coordination are other aspects requiring attention ( [[#Mguni--2016|Mguni et al. 2016]] ).

Providing a connected system of greenspace throughout the urban area may promote active transportation ( [[#Nieuwenhuijsen--2016|Nieuwenhuijsen and Khreis 2016]] ), thereby reducing GHG emissions. Soft solutions for improving green infrastructure connectivity for cycling is an urban NBS mitigation measure, although there is ''low evidence'' for emissions reductions. In the city of Lisbon, Portugal, improvements in cycling infrastructure and bike-sharing system resulted in 3.5 times more cyclists within two years ( [[#Félix--2020|Félix et al. 2020]] ). In Copenhagen, the cost of cycling (0.08 EUR km -1 ) is declining and is about six times lower than car driving (Euro 0.50/km) ( [[#Vedel--2017|Vedel et al. 2017]] ). In addition, participants were willing to cycle 1.84 km longer if the route has a designated cycle track and 0.8 km more if there are also green surroundings. Changes in urban landscapes, including through the integration of green infrastructure in sustainable urban and transport planning, can support the transition from private motorised transportation to public and physically active transportation in carbon-neutral, more liveable and healthier cities ( [[#Nieuwenhuijsen--2016|Nieuwenhuijsen and Khreis 2016]] ; [[#Nieuwenhuijsen--2020|Nieuwenhuijsen 2020]] ). Car infrastructure can be also transferred into public open and green space, such as in the Superblock model in Barcelona’s neighbourhoods ( [[#Rueda--2019|Rueda 2019]] ). Health impact assessment models estimated that 681 premature deaths may be prevented annually with this implementation ( [[#Mueller--2020|Mueller et al. 2020]] ) and the creation of greenways in Maanshan, China has stimulated interest in walking or cycling ( [[#Zhang--2020|Zhang et al. 2020]] ).

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