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

==== 8.1.6.1 Urban Carbon Cycle ====

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In cities, carbon cycles through natural (e.g., vegetation and soils) and managed (e.g., reservoirs and anthropogenic – buildings, transportation) pools. The accumulation of carbon in urban pools, such as buildings or landfills, results from the local or global transfer of carbon-containing energy and raw materials used in the city ( [[#Churkina--2008|Churkina 2008]] ; [[#Pichler--2017|Pichler et al. 2017]] ; [[#Chen--2020b|Chen et al. 2020b]] ). Quantitative understanding of these transfers and the resulting emissions and uptake within an urban area is essential for accurate urban carbon accounting ( [[#USGCRP--2018|USGCRP 2018]] ). Currently, urban areas are a net source of carbon because they emit more carbon than they uptake. Thus, urban mitigation strategies require a twofold strategy: reducing urban emissions of carbon into the atmosphere, and enhancing uptake of carbon in urban pools ( [[#Churkina--2012|Churkina 2012]] ) (for a broader definition of ‘carbon cycle’ and related terms such as ‘carbon sink,’ ‘carbon stock,’ ‘carbon neutrality,’ ‘GHG neutrality,’ and others, see Glossary).

Burning fossil fuels to generate energy for buildings, transportation, industry, and other sectors is a major source of urban GHG emissions ( [[#Gurney--2015|Gurney et al. 2015]] ). At the same time, most cities do not generate within their boundaries all of the resources they use, such as electricity, gasoline, cement, water, and food needed for local homes and businesses to function ( [[#Jacobs--1969|Jacobs 1969]] ), requiring consideration of GHG emissions embodied in supply chains serving cities. Furthermore, urban vegetation, soils, and aquatic systems can both emit or remove carbon from the urban atmosphere and are often heavily managed. For example, urban parks, forests, and street trees actively remove carbon from the atmosphere through growing season photosynthesis. They can become a net source of carbon most often during the dormant season or heat waves. Some of the sequestered carbon can be stored in the biomass of urban trees, soils, and aquatic systems. Urban infrastructures containing cement also uptake carbon through the process of carbonation. The uptake of carbon by urban trees is at least two orders of magnitude faster than by cement-containing infrastructures ( [[#Churkina--2012|Churkina 2012]] ) ( [[#8.4.4|Section 8.4.4]] and Figures 8.17 and 8.18).

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