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

==== 8.4.4.1 The Mitigation Potential of Urban Trees and Associated Co-benefits ====

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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]] ).

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