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

=== 8.4.4 Urban Green and Blue Infrastructure ===

<div id="h2-18-siblings" class="h2-siblings"></div>

The findings of AR6 WGI and WGII have underscored the importance of urban green and blue infrastructure for reducing the total warming in urban areas due to its local cooling effect on temperature and its benefits for climate adaptation ( [[#IPCC--2021|IPCC 2021]] ; Cross-Working Group Box 2 in this chapter). Urban green and blue infrastructure in the context of nature-based solutions (NBS) involves the protection, sustainable management, and restoration of natural or modified ecosystems while simultaneously providing benefits for human well-being and biodiversity ( [[#IUCN--2021|IUCN 2021]] ) (see Glossary for additional definitions). As an umbrella concept, urban NBS integrates established ecosystem-based approaches that provide multiple ecosystem services and are important in the context of societal challenges related to urbanisation, climate change, and reducing GHG emissions through the conservation and expansion of carbon sinks ( [[#Naumann--2014|Naumann et al. 2014]] ; [[#Raymond--2017|Raymond et al. 2017]] ) ( [[#8.1.6.1|Section 8.1.6.1]] ).

Urban green and blue infrastructure includes a wide variety of options, from street trees, parks, and sustainable urban drainage systems ( [[#Davis--2017|Davis and Naumann 2017]] ), to building-related green roofs or green facades, including green walls and vertical forests ( [[#Enzi--2017|Enzi et al. 2017]] ). Figure 8.18 synthesises urban green and blue infrastructure based on urban forests, street trees, green roofs, green walls, blue spaces, greenways, and urban agriculture. Key mitigation benefits, adaptation co-benefits, and SDG linkages are represented by types of green and blue infrastructure. Local implementations of urban green and blue infrastructure can pursue these linkages while progressing toward inclusive sustainable urban planning (SDG 11.3) and the provision of safe, inclusive and accessible green and public spaces for all (SDG 11.7) ( [[#Butcher-Gollach--2018|Butcher-Gollach 2018]] ; [[#Pathak--2018|Pathak and Mahadevia 2018]] ; [[#Rigolon--2018|Rigolon et al. 2018]] ; [[#Anguelovski--2019|Anguelovski et al. 2019]] ; [[#Buyana--2019|Buyana et al. 2019]] ; [[#Azunre--2021|Azunre et al. 2021]] ) ( [[#8.2|Section 8.2]] ).

<div id="_idContainer00d" class="Basic-Text-Frame"></div>

[[File:bf885970b9cf3b74e41a7327b90027fd IPCC_AR6_WGIII_Figure_8_18a.png]] [[File:3e3403f793ff8fece611faae5f68677a IPCC_AR6_WGIII_Figure_8_18b.png]]

'''Figure 8.18: Key mitigation benefits, adaptation co-benefits, and SDG linkages of urban green and blue infrastructure.''' Panel '''(b)''' evaluates those strategies in the context of their mitigation benefits, adaptation co-benefits, and linkages to the SDGs. Urban forests and street trees provide the greatest mitigation benefit because of their ability to sequester and store carbon while simultaneously reducing building energy demand. Moreover, they provide multiple adaptation co-benefits and synergies based on the linkages to the SDGs (Figure 8.4). The assessments of mitigation benefits are dependent on context, scale, and spatial arrangement of each green and blue infrastructure type and their proximity to buildings. Mitigation benefits due to reducing municipal water use are based on reducing wastewater loads that reduce energy use in wastewater treatment plants. The sizes of the bars are illustrative and their relative size is based on the authors’ best understanding and assessment of the literature.

<div id="8.4.4.1" class="h3-container"></div>

<span id="the-mitigation-potential-of-urban-trees-and-associated-co-benefits"></span>
==== 8.4.4.1 The Mitigation Potential of Urban Trees and Associated Co-benefits ====

<div id="h3-13-siblings" class="h3-siblings"></div>

Due to their potential to store relatively high amounts of carbon compared to other types of urban vegetation, as well as their ability to provide many climate mitigation co-benefits ( ''high agreement, robust evidence'' ), natural area protection and natural forest management in urban areas is an important priority for cities looking to mitigate climate change. Globally, urban tree cover averages 26.5%, but varies from an average of 12% in deserts to 30.4% in forested regions ( [[#Nowak--2020|Nowak and Greenfield 2020]] ).

Global urban tree carbon storage is approximately 7.4 billion tonnes (GtC) given 363 million hectares of urban land, 26.5% tree cover, and an average carbon storage density of urban tree cover of 7.69 kgC m –2 (kilograms carbon per square metre) ( [[#Nowak--2013|Nowak et al. 2013]] ; World Bank et al. 2013). Estimated global annual carbon sequestration by urban trees is approximately 217 million tonnes (MtC) given an average carbon sequestration density per unit urban tree cover of 0.226 kgC m –2 ( [[#Nowak--2013|Nowak et al. 2013]] ). With an average plantable (non-tree and non-impervious) space of 48% globally ( [[#Nowak--2020|Nowak and Greenfield 2020]] ), the carbon storage value could nearly triple if all this space is converted to tree cover. In Europe alone, if 35% of the urban surfaces (26,450 km 2 ) were transformed into green surfaces, the mitigation potential based on carbon sequestration would be an estimated 25.9 MtCO 2 yr −1 with the total mitigation benefit being 55.8 MtCO 2 yr −1 , including an energy saving of about 92 TWh yr −1 ( [[#Quaranta--2021|Quaranta et al. 2021]] ). Other co-benefits include reducing urban runoff by about 17.5% and reducing summer temperatures by 2.5°C–6°C ( [[#Quaranta--2021|Quaranta et al. 2021]] ).

Urban tree carbon storage is highly dependent on biome. For example, carbon sequestered by vegetation in Amazonian forests is two to five times higher compared to boreal and temperate forests ( [[#Blais--2005|Blais et al. 2005]] ). At the regional level, the estimated carbon storage density rates of tree cover include a range of 3.14–14.1 kgC m –2 in the United States, 3.85–5.58 kgC m –2 in South Korea, 1.53–9.67 kgC m –2 in Barcelona, Spain, 28.1–28.9 kgC m –2 in Leicester, England, and an estimated 6.82 kgC m –2 in Leipzig, Germany and 4.28 kgC m –2 in Hangzhou, China ( [[#Nowak--2013|Nowak et al. 2013]] ). At the local scale, above- and below-ground tree carbon densities can vary substantially, as with carbon in soils and dead woody materials. The conservation of natural mangroves has been shown to provide urban mitigation benefits through carbon sequestration, as demonstrated in the Philippines ( [[#Abino--2014|Abino et al. 2014]] ). Research on urban carbon densities from the Southern Hemisphere will contribute to better estimates.

On a per-tree basis, urban trees offer the most potential to mitigate climate change through both carbon sequestration and GHG emissions reduction from reduced energy use in buildings ( [[#Nowak--2017|Nowak et al. 2017]] ). Maximum possible street tree planting among 245 world cities could reduce residential electricity use by about 0.9–4.8% annually ( [[#McDonald--2016|McDonald et al. 2016]] ). Urban forests in the United States reduce building energy use by 7.2%, equating to an emissions reduction of 43.8 MtCO 2 annually ( [[#Nowak--2017|Nowak et al. 2017]] ).

Urban trees can also mitigate some of the impacts of climate change by reducing the UHI effect and heat stress, reducing stormwater runoff, improving air quality, and supporting health and well-being in areas where the majority of the world’s population resides ( [[#Nowak--2007|Nowak and Dwyer 2007]] ). Urban forest planning and management can maximise these benefits for present and future generations by sustaining optimal tree cover and health (also see SDG linkages in Figure 8.4). Urban and peri-urban agriculture can also have economic benefits from fruit, ornamental, and medicinal trees ( [[#Gopal--2014|Gopal and Nagendra 2014]] ; [[#Lwasa--2017|Lwasa 2017]] ; [[#Lwasa--2018|Lwasa et al. 2018]] ).

<div id="box-8.2:-urban-carbon-storage:-an-example-from-new-york-city" class="h2-container box-container"></div>

<span id="box-8.2-urban-carbon-storage-an-example-fro-m-new-york-city"></span>