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

==== 6.3.4.1 Temperature Regulation ====

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Nature-based strategies, including street trees, green roofs, green walls and other urban vegetation, can reduce heat and extreme heat by cooling private and public spaces ( ''robust evidence'' , ''high agreement'' ). Shading and evapotranspiration are the primary mechanisms for vegetation-induced urban cooling (Coutts et al., 2016). Shading reduces mean radiant temperature, which is the dominant influence on outdoor human thermal comfort under warm, sunny conditions (Thorsson et al., 2014; Viguié et al., 2020). Outdoor green space and parks may also slightly reduce indoor heat hazard (Viguié et al., 2020). Apart from lowering temperature, NBS may also contribute to lower energy costs by reducing extra demand for conventional sources of cooling (e.g., air conditioning) (Viguié et al., 2020; Foustalieraki et al., 2017), especially during peak demand periods. Homes with shade trees that are located in cities where air conditioning systems are common can save over 30% of residential peak cooling demand (Zardo et al., 2017; Wang et al., 2015). Green roofs have been shown to significantly lower surface temperatures on buildings (Bevilacqua et al., 2017) and modelling suggests that green roofs, if employed widely throughout urban areas, have the potential to impact the regional heat profile of cities (Bevilacqua et al., 2017; Rosenzweig, Gaffin and Parshall, 2006). Community or allotment gardens, backyard greening and other types of low vegetation, as well as lakes, ponds, rivers and streams, can also provide local cooling benefits to nearby residents (Gunawardena, Wells and Kershaw, 2017; Larondelle et al., 2014; [[#Santamouris--2020|Santamouris, 2020]] ).

Urban climate models show that increased vegetation cover results in reducing both mean air temperatures and extreme temperatures during heatwaves (Heaviside, Cai and Vardoulakis, 2015; [[#Ferreira--2019|Ferreira and Duarte, 2019]] ; [[#Schubert--2013|Schubert and Grossman-Clarke, 2013]] ). Greater density and more canopy coverage relative to other built and paved surfaces increases shade provision and evapotranspiration (Hamstead et al., 2016; Grilo et al., 2020; Herath, Halwatura and Jayasinghe, 2018; Knight et al., 2021). However, local cooling by vegetation depends on regional climate context, geographic setting of the city, urban form, the density and placement of the trees, in addition to a variety of other ecological, technical, and social factors, such as local stewardship (Salmond et al., 2016). Green spaces less than 0.5–2.0 ha may have negligible cooling effects at regional scales, but impacts of shading can have microscale cooling benefits (Gunawardena, Wells and Kershaw, 2017; Zardo et al., 2017). Vegetation impacts on day versus night-time cooling varies (Imran et al., 2019) as does cooling potential in temperate versus tropical climates. The supply of cooler air from surrounding peri-urban and rural areas can impact cooling in the urban core suggesting that regional adaptation planning for NBS is important to maintain or extend ventilation paths from the urban fringe into the city centre (Schau-Noppel, Kossmann and Buchholz, 2020).

To maximize the adaptation benefits of NBS for regulating urban heat, it can be helpful to prioritise tree planting and other urban greening investments in areas where heat vulnerability and risk are the highest, especially communities that lack urban tree canopy or accessibility to parks to cool off during hot days or heatwaves (Ziter et al., 2019). Planting trees closely together or in partly permeable vegetated barriers along streets can improve local cooling benefits. Additionally, choosing tree species with leaves that have the greatest leaf area index or the largest leaves can improve cooling performance, as those trees have the greatest shading and evapotranspiration benefits that, in turn, provide the greatest cooling effects (Keeler et al., 2019). Drought-resistant trees, often native trees, are ideal to avoid high watering costs, though dry or water scarce areas may limit adoption of urban vegetation as an NBS strategy (Coutts et al., 2013). Native trees and permaculture can provide additional benefits for local biodiversity as shown in study in Melbourne, Australia which found that increasing vegetation from 10% to 30% increased occupancy of bats, birds, bees, beetles and bugs by up to 130% (Threlfall et al., 2017), with particularly high impact on native species.. Additionally, planting fruit or nut trees can provide co-benefits for local food production, and yet choice of species and placement is important to consider with respect to local cultural needs and norms ( [[#Adegun--2018|Adegun, 2018]] ; [[#Adegun--2017|Adegun, 2017]] ).

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