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

== 8.1 Introduction ==

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=== 8.1.1 What Is New Since AR5? ===

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The Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC) was the first IPCC report that had a standalone chapter on urban mitigation of climate change. The starting point for that chapter was how the spatial organisation of urban settlements affects greenhouse gas (GHG) emissions and how urban form and infrastructure could facilitate mitigation of climate change. A main finding in AR5 was that urban form shapes urban energy consumption and GHG emissions.

Since AR5, there has been growing scientific literature and policy foci on urban strategies for climate change mitigation. There are three possible reasons for this. First, according to AR5 Working Group III (WGIII) [[IPCC:Wg3:Chapter:Chapter-12|Chapter 12]] on Human Settlements, Infrastructure, and Spatial Planning, urban areas generate between 71% and 76% of carbon dioxide (CO 2 ) emissions from global final energy use and between 67% and 76% of global energy (Seto et al. 2014). Thus, focusing on ‘urban systems’ (see Annex I: Glossary and Figure 8.15) addresses one of the key drivers of emissions. Second, more than half of the world population lives in urban areas, and by mid-century 7 out of 10 people on the planet will live in a town or a city ( [[#UN%20DESA--2019|UN DESA 2019]] ). Thus, coming up with mitigation strategies that are relevant to urban settlements is critical for successful mitigation of climate change. Third, beyond climate change, there is growing attention on cities as major catalysts of change and to help achieve the objectives outlined in multiple international frameworks and assessments.

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[[File:3ad214b95773938fed8dd3d42a849e8e IPCC_AR6_WGIII_Figure_8_1.png]]

'''Figure 8.1''' 4 ''': Relationship between urbanisation level and gross national income (GNI).''' There is a positive and strong correlation between the urbanisation level and gross national income. High-income countries have high levels of urbanisation, on average 80%. Low-income countries have low levels of urbanisation, on average 30%. Source: [[#UN%20DESA--2019|UN DESA 2019]] , p. 42.

Cities are also gaining traction within the work of the IPCC. The IPCC Special Report on Global Warming of 1.5°C (SR1.5 Chapter 4) identified four systems that urgently need to change in fundamental and transformative ways: urban infrastructure, land use and ecosystems, industry, and energy. Urban infrastructure was singled out but urban systems form a pivotal part of the other three systems requiring change ( [[#IPCC--2018a|IPCC 2018a]] ) (see ‘infrastructure’ in Glossary). The IPCC Special Report on Climate Change and Land (SRCCL) identified cities not only as spatial units for land-based mitigation options but also places for managing demand for natural resources including food, fibre, and water ( [[#IPCC--2019|IPCC 2019]] ).

Other international frameworks are highlighting the importance of cities. For example, the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) report on nature’s contribution to people is clear: cities straddle the biodiversity sphere in the sense that they present spatial units of ecosystem fragmentation and degradation while at the same time contain spatial units where the concentration of biodiversity compares favourably with some landscapes (IPBES 2019a). Cities are also featured as a key element in the transformational governance to tackle both climate change and biodiversity and ecosystem challenges in the first-ever IPCC-IPBES co-sponsored workshop report ( [[#Pörtner--2021|Pörtner et al. 2021]] ) ( [[#8.5|Section 8.5]] and see ‘governance’ in Glossary).

The UN Sustainable Development Goals (SDGs) further underscore the importance of cities in the international arena with the inclusion of SDG 11 on Sustainable Cities and Communities for ‘inclusive, safe, resilient and sustainable’ cities and human settlements ( [[#United%20Nations--2015|United Nations 2015]] ; [[#Queiroz--2017|Queiroz et al. 2017]] ; [[#United%20Nations--2019|United Nations 2019]] ). Additionally, UN-Habitat’s New Urban Agenda (NUA) calls for various measures, including integrated spatial planning at the city-regional scale, to address the systemic challenges included in greening cities, among which is emissions reduction and avoidance ( [[#United%20Nations--2017|United Nations 2017]] ).

Since AR5, there has also been an increase in scientific literature on urban mitigation of climate change, including more diversity of mitigation strategies than were covered during AR5 ( [[#Lamb--2018|Lamb et al. 2018]] ), as well as a growing focus on how strategies at the urban scale can have compounding or additive effects beyond urban areas (e.g., in rural areas, land-use planning, and the energy sector).

There is more literature on using a systems approach to understand the interlinkages between mitigation and adaptation, and situating GHG emissions reduction targets within broader social, economic, and human well-being contexts and goals ( [[#Bai--2018|Bai et al. 2018]] ; [[#Ürge-Vorsatz--2018|Ürge-Vorsatz et al. 2018]] ; [[#Lin--2021|Lin et al. 2021]] ). In particular, the nexus approach, such as the water and energy nexus and the water-energy-food nexus, is increasingly being used to understand potential emissions and energy savings from cross-sectoral linkages that occur in cities ( [[#Wang--2016|Wang and Chen 2016]] ; [[#Engström--2017|Engström et al. 2017]] ; [[#Valek--2017|Valek et al. 2017]] ). There is also a growing literature that aims to quantify transboundary urban GHG emissions and carbon footprint beyond urban and national administrative boundaries ( [[#Chen--2016|Chen et al. 2016]] ; [[#Hu--2016|Hu et al. 2016]] ). Such a scope provides a more complete understanding of how local urban emissions or local mitigation strategies can have effects on regions’ carbon footprint or GHG emissions.

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==== 8.1.1.1 City Climate Action ====

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Moreover, cities around the world are putting increasing focus on tackling climate change. Since AR5:

• Climate leadership at the local scale is growing with commitment from city decision-makers and policymakers to implement local-scale mitigation strategies ( [[#GCoM--2018|GCoM 2018]] , 2019; [[#ICLEI--2019a|ICLEI 2019a]] ; [[#C40%20Cities--2020|C40 Cities 2020]] a).

'''•''' More than 360 cities announced at the Paris Climate Conference that the collective impact of their commitments will lead to a reduction of up to 3.7 GtCO 2 -eq (CO 2 -equivalent) of urban emissions annually by 2030 ( [[#Cities%20for%20Climate--2015|Cities for Climate 2015]] ).

'''•''' The Global Covenant of Mayors (GCoM), a transnational network of more than 10,000 cities, has made commitments to reduce urban GHG emissions by up to 1.4–2.3 GtCO 2 -eq annually by 2030 and 2.8–4.2 GtCO 2 -eq annually by 2050, compared to business-as-usual ( [[#GCoM--2018|GCoM 2018]] , 2019).

• More than 800 cities have made commitments to achieve net-zero GHG emissions, either economy-wide or in a particular sector (NewClimate Institute and Data-Driven EnviroLab 2020).

Although most cities and other subnational actors are yet to meet their net-zero GHG or CO 2 emissions commitments, the growing numbers of those commitments, alongside organisations enabled to facilitate reaching those targets, underscore the growing support for climate action by city and other subnational leaders.

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==== 8.1.1.2 Historical and Future Urban Emissions ====

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One major innovation in this Assessment Report is the inclusion of historical and future urban GHG emissions. Urban emissions based on consumption-based accounting by regions has been put forth for the time frame 1990–2100 using multiple datasets with projections given in the framework of the Shared Socio-economic Pathway (SSP)–Representative Concentration Pathway (RCP) scenarios. This advance has provided a time dimension to urban footprints considering different climate scenarios with implications for urban mitigation, allowing a comparison of the way urban emissions and their reduction can evolve given different scenario contexts (see Glossary for definitions of various ‘pathways’ and ‘scenarios’ in the context of climate change mitigation, including ‘SSPs’ and ‘RCPs’).

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==== 8.1.1.3 Sustainable Development Linkages and Feasibility Assessment ====

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Special emphasis is placed on the co-benefits of urban mitigation options, including an evaluation of linkages with the SDGs based on synergies and/or trade-offs. Urban mitigation options are further evaluated based on multiple dimensions according to the feasibility assessment (see [[#8.5.5|Section 8.5.5]] and Figure 8.19, and Section 8.SM.2) indicating the enablers and barriers of implementation. These advances provide additional guidance for urban mitigation.

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=== 8.1.2 Preparing for the Special Report on Cities and Climate Change in AR7 ===

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At the 43rd Session of the IPCC in 2016, the IPCC approved a Special Report on Climate Change and Cities during the Seventh Assessment Cycle of the IPCC (AR7). To stimulate scientific research knowledge exchange, the IPCC and nine global partners co-sponsored the IPCC Cities and Climate Change Science Conference, which brought together over 700 researchers, policymakers, and practitioners from 80 countries.

The conference identified key research priorities including the need for an overarching systems approach to understanding how sectors interact in cities as drivers for GHG emissions and the relationship between climate and other urban processes, as well as achieving transformation towards low-carbon and resilient futures ( [[#Bai--2018|Bai et al. 2018]] ). The subsequent report on the global research and action agenda identifies scale, informality, green and blue infrastructure, governance and transformation, as well as financing climate action, as areas for scientific research during the AR6 cycle and beyond (WCRP 2019).

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<span id="the-scope-of-the-chapter-a-focus-on-urban-systems"></span>
=== 8.1.3 The Scope of the Chapter: A Focus on Urban Systems ===

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This chapter takes an urban systems approach and covers the full range of urban settlements, including towns, cities, and metropolitan areas. By ‘urban system’ (Figure 8.15), this chapter refers to two related concepts. First, an urban systems approach recognises that cities do not function in isolation. Rather, cities exhibit strong interdependencies across scales, whether it is within a region, a country, a continent, or worldwide. Cities are embedded in broader ecological, economic, technical, institutional, legal, and governance structures that often constrain their systemic function, which cannot be separated from wider power relations ( [[#Bai--2016|Bai et al. 2016]] ).

The notion of a system of cities has been around for nearly 100 years and recognises that cities are interdependent, in that significant changes in one city, such as economic activities, income, or population, will affect other cities in the system ( [[#Christaller--1933|Christaller 1933]] ; [[#Berry--1964|Berry 1964]] ; [[#Marshall--1989|Marshall 1989]] ). This perspective of an urban system emphasises the connections between a city and other cities, as well as between a city and its hinterlands ( [[#Hall--1980|Hall and Hay 1980]] ; [[#Ramaswami--2017b|Ramaswami et al. 2017b]] ; [[#Xu--2018c|Xu et al. 2018c]] ). An important point is that growth in one city affects growth in other cities in the global, national or regional system of cities ( [[#Gabaix--1999|Gabaix 1999]] ; [[#Scholvin--2019|Scholvin et al. 2019]] ; [[#Knoll--2021|Knoll 2021]] ).

Moreover, there is a hierarchy of cities ( [[#Taylor--1997|Taylor 1997]] ; [[#Liu--2014|Liu et al. 2014]] ), with very large cities at the top of the hierarchy concentrating political power and financial resources, but of which there are very few. Rather, the urban system is dominated by small and medium-sized cities and towns. With globalisation and increased interconnectedness of financial flows, labour, and supply chains, cities across the world today have long-distance relationships on multiple dimensions but are also connected to their hinterlands for resources.

The second key component of the urban systems lens identifies the activities and sectors within a city as being inter-connected – that cities are ecosystems ( [[#Rees--1997|Rees 1997]] ; [[#Grimm--2000|Grimm et al. 2000]] ; [[#Newman--2008|Newman and Jennings 2008]] ; [[#Acuto--2019|Acuto et al. 2019]] ; [[#Abdullah--2020|Abdullah and Garcia-Chueca 2020]] ; [[#Acuto--2021|Acuto and Leffel 2021]] ). This urban systems perspective emphasises linkages and interrelations within cities. The most evident example of this is urban form and infrastructure, which refer to the patterns and spatial arrangements of land use, transportation systems, and urban design. Changes in urban form and infrastructure can simultaneously affect multiple sectors, such as buildings, energy, and transport.

This chapter assesses urban systems beyond simply jurisdictional boundaries. Using an urban systems lens has the potential to accelerate mitigation beyond a single sector or purely jurisdictional approach ( [[#8.4|Section 8.4]] ). An urban systems perspective presents both challenges and opportunities for urban mitigation strategies. It shows that any mitigation option potentially has positive or negative consequences in other sectors, other settlements, cities, or other parts of the world, and requires more careful and comprehensive considerations on the broader impacts, including equity and social justice (see Glossary for a comprehensive definition of ‘equity’ in the context of mitigation and adaptation). This chapter focuses on cities, city regions, metropolitan regions, megalopolitans, mega-urban regions, towns, and other types of urban configurations because they are the primary sources of urban GHG emissions and tend to be where mitigation action can be most impactful.

There is no internationally agreed upon definition of ‘urban’, ‘urban population’, or ‘urban area’. Countries develop their own definitions of urban, often based on a combination of population size or density, and other criteria including the percentage of population not employed in agriculture, the availability of electricity, piped water, or other infrastructures, and characteristics of the built environment, such as dwellings and built structures. This chapter assesses urban systems, which includes cities and towns. It uses a similar framework to [[IPCC:Wg3:Chapter:Chapter-6|Chapter 6]] of AR6 WGII, referring to cities and urban settlements as ‘concentrated human habitation centres that exist along a continuum’ ( [[#Dodman--2022|Dodman et al. 2022]] ) (for further definitions of ‘urban’, ‘cities’, ‘settlements’ and related terms, see Glossary, and WGII Chapter 6).

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=== 8.1.4 The Urban Century ===

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The 21st century will be the urban century, defined by a massive increase in global urban populations and a significant building up of new urban infrastructure stock to accommodate the growing urban population. Six trends in urbanisation are especially important in the context of climate change mitigation. [[#footnote-005|4]]

First, the size and relative proportion of the urban population is unprecedented and continues to increase. As of 2018, approximately 55% of the global population lives in urban areas (about 4.3 billion people) ( [[#UN%20DESA--2019|UN DESA 2019]] ). It is predicted that 68% of the world population will live in urban areas by 2050. This will mean adding 2.5 billion people to urban areas between 2018 and 2050, with 90% of this increase taking place in Africa and Asia. There is a strong correlation between the level of urbanisation and the level of national income, with considerable variation and complexity in the relationship between the two ( [[#UN%20DESA--2019|UN DESA 2019]] ). In general, countries with levels of urbanisation of 75% or greater all have high national incomes, whereas countries with low levels of urbanisation under 35% have low national incomes ( [[#UN%20DESA--2019|UN DESA 2019]] ). In general, there is a clear positive correlation between the level of urbanisation and income levels (Figure 8.1 and Box 8.1).

Second, the geographic concentration of the world’s current urban population is in emerging economies, and the majority of future urban population growth will take place in developing countries and least-developed countries (LDCs). About half of the world’s urban population in 2018 lived in just seven countries, and about half of the increase in urban population through 2050 is projected to be concentrated in eight countries ( [[#UN%20DESA--2019|UN DESA 2019]] ) (Figure 8.2). Of these eight, seven are emerging economies where there will be a need for significant financing to construct housing, roads, and other urban infrastructure to accommodate the growth of the urban population. How these new cities of tomorrow will be designed and constructed will lock-in patterns of urban energy behaviour for decades if not generations (Sections 8.3.4 and 8.4). Thus, it is essential that urban climate change mitigation strategies include solutions appropriate for cities of varying sizes and typologies ( [[#8.6|Section 8.6]] and Figure 8.21).

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'''Figure 8.2: Urban population size in 2018 and increase in the projected urban population.''' In 2018, about half of the world’s urban population lived in seven countries, and about half of the increase in urban population through 2050 is forecasted to concentrate in eight countries. Source: [[#UN%20DESA--2019|UN DESA 2019]] , p. 44.

Third, small and medium-sized cities and towns are a dominant type of urban settlement. In 2018, more than half (58%) of the urban population lived in cities and towns with fewer than 1 million inhabitants and almost half of the world’s urban population (48%) lived in settlements with fewer than 500,000 inhabitants (Figure 8.3). Although megacities receive a lot of attention, only about 13% of the urban population worldwide lived in a megacity – an urban area with at least 10 million inhabitants ( [[#UN%20DESA--2019|UN DESA 2019]] ). Thus, there is a need for a wide range of strategies for urban mitigation of climate change that are appropriate for cities of varying levels of development and size, especially smaller cities which often have lower levels of financial capacities than large cities.

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'''Figure 8.3: Population of the world, by area of residence and size class of urban settlement for 2018.''' As of 2018, 4.2 billion people or 55% of the world population reside in urban settlements while 45% reside in rural areas. The coloured stacked column for the urban population represents the total number of inhabitants for a given size class of urban settlements. Megacities of 10 million or more inhabitants have a total of only 529 million inhabitants, corresponding to 12.5% of the urban population. In contrast, about 1.8 billion inhabitants reside in urban settlements with fewer than 300,000 inhabitants, corresponding to 41.5% of the urban population. The pie chart represents the respective shares for 2018, with 42% of the urban population residing in settlements with more than 1 million inhabitants, and 58% of the urban population residing in settlements with fewer than 1 million inhabitants. Almost half of the world’s urban population (48%) live in settlements with fewer than 500,000 inhabitants. Source: adapted from [[#UN%20DESA--2019|UN DESA 2019]] , p. 56.

Fourth, another trend is the rise of megacities and extended metropolitan regions. The largest cities around the world are becoming even larger, and there is a growing divergence in economic power between megacities and other large cities ( [[#Kourtit--2015|Kourtit et al. 2015]] ; [[#Hoornweg--2017|Hoornweg and Pope 2017]] ; [[#Zhao--2017b|Zhao et al. 2017b]] ). Moreover, there is evidence that the largest city in each country has an increasing share of the national population and economy.

Fifth, population declines have been observed for cities and towns across the world, including in Poland, Republic of Korea, Japan, United States, Germany, and Ukraine. The majority of cities that have experienced population declines are concentrated in Europe. Multiple factors contribute to the decline in cities, including declining industries and the economy, declining fertility, and outmigration to larger cities. Shrinking urban populations could offer retrofitting opportunities ( [[#UNEP--2019|UNEP 2019]] ) and increasing greenspaces ( [[#Jarzebski--2021|Jarzebski et al. 2021]] ), but the challenges for these cities differ in scope and magnitude from rapidly expanding cities.

Sixth, urbanisation in many emerging economies is characterised by informality and an informal economy ( [[#Brown--2016|Brown and McGranahan 2016]] ). The urban informal economy includes a wide array of activities, including but not limited to street vending, home-based enterprises, unreported income from self-employment, informal commerce, domestic service, waste-picking, and urban agriculture. The urban informal economy is large and growing. Globally, about 44% of the urban economy is informal, although there is much variation between countries and regions ( [[#ILO--2018|ILO 2018]] ). Emerging and developing economies have the highest percentage of the urban informal economy, with Africa (76%) and the Arab States (64%) with the largest proportion ( [[#ILO--2018|ILO 2018]] ). Urban informality also extends to planning, governance and institutions ( [[#Roy--2009|Roy 2009]] ; [[#EU--2016|EU 2016]] ; [[#Lamson-Hall--2019|Lamson-Hall et al. 2019]] ). Given its prevalence, it is important for urban climate change mitigation strategies to account for informality, especially in emerging and developing countries ( [[#8.3.2|Section 8.3.2]] ).

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=== 8.1.5 Urbanisation in Developing Countries ===

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Urbanisation in the 21st century will be dominated by population and infrastructure growth in developing countries, and as such it is important to highlight three aspects that are unique and especially relevant for climate change mitigation. First, urbanisation will increase in speed and magnitude. Given their significant impact on emissions, mitigation action in Asian cities, especially the large and rapidly growing cities, will have significant implications on global ambitions ( [[#8.3.4|Section 8.3.4]] ).

Second, a number of cities in developing countries lack institutional, financial and technical capacities to enable local climate change action ( [[#Sharifi--2017|Sharifi et al. 2017]] ; [[#Fuhr--2018|Fuhr et al. 2018]] ). While these capacities differ across contexts ( [[#Hickmann--2017|Hickmann et al. 2017]] ), several governance challenges are similar across cities ( [[#Gouldson--2015|Gouldson et al. 2015]] ). These factors also influence the ability of cities to innovate and effectively implement mitigation action ( [[#Nagendra--2018|Nagendra et al. 2018]] ) (Chapter 17).

Third, there are sizable economic benefits in developing country cities that can provide an opportunity to enhance political momentum and institutions ( [[#Colenbrander--2016|Colenbrander et al. 2016]] ). The co-benefits approach ( [[#8.2|Section 8.2]] ), which frames climate objectives alongside other development benefits, is increasingly seen as an important concept justifying and driving climate change action in developing countries ( [[#Sethi--2018|Sethi and Puppim de Oliveira 2018]] ).

Large-scale system transformations are also deeply influenced by factors outside governance and institutions, such as private interests and power dynamics ( [[#Jaglin--2014|Jaglin 2014]] ; [[#Tyfield--2014|Tyfield 2014]] ). In some cases, these private interests are tied up with international flows of capital. In India, adaptation plans involving networks of private actors and related mitigation actions have resulted in the dominance of private interests. This has led to trade-offs and adverse impacts on the poor ( [[#Chu--2016|Chu 2016]] ; [[#Mehta--2019|Mehta et al. 2019]] ).

When planning and implementing low-carbon transitions, it is important to consider the socio-economic context. An inclusive approach emphasises the need to engage non-state actors, including businesses, research organisations, non-profit organisations and citizens ( [[#Lee--2015|Lee and Painter 2015]] ; [[#Hale--2020|Hale et al. 2020]] ). For example, engaging people in defining locally relevant mitigation targets and actions has enabled successful transformations in China ( [[#Engels--2018|Engels 2018]] ), Africa ( [[#Göpfert--2019|Göpfert et al. 2019]] ) and Malaysia ( [[#Ho--2015|Ho et al. 2015]] ). An active research and government collaboration through multiple stakeholder interactions in a large economic corridor in Malaysia led to the development and implementation of a low-carbon blueprint for the region ( [[#Ho--2013|Ho et al. 2013]] ). Many cities in LDCs and developing countries lack adequate urban infrastructure and housing. An equitable transformation in these cities entails prioritising energy access and basic services, including safe drinking water and sanitation, to meet basic needs of their populations.

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=== 8.1.6 Urban Carbon Footprint ===

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Urban areas concentrate GHG fluxes because of the size of the urban population, the size and nature of the urban economy, the energy and GHGs embodied in the infrastructure (see ‘embodied emissions’ in Glossary), and the goods and services imported and exported to and from cities ( [[#USGCRP--2018|USGCRP 2018]] ).

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==== 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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==== 8.1.6.2 Urban Emissions Accounting ====

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Urban GHG emissions accounting can determine critical conceptual and quantitative aspects of urban GHG emissions. The accounting framework chosen can therefore predetermine the emissions responsibility, the mitigation options available, and the level of effort required to correctly account for emissions ( [[#Afionis--2017|Afionis et al. 2017]] ).

Two main urban carbon accounting advances have occurred since AR5. The first includes efforts to better understand and clarify how the different urban GHG accounting frameworks that have emerged over the past 15 years are interrelated, require different methodological tools, and reflect differing perspectives on emissions responsibility and quantification effort. The second main advance lies in a series of methodological innovations facilitating practical implementation, emissions verification, and scaling-up of the different GHG accounting approaches. This section provides an overview of the most used GHG urban accounting frameworks followed by a review of the advances since AR5.

Numerous studies have reviewed urban GHG accounting frameworks and methods with somewhat different nomenclatures and categorical divisions ( [[#Lin--2015|Lin et al. 2015]] ; [[#Lombardi--2017|Lombardi et al. 2017]] ; [[#Chen--2019b|Chen et al. 2019b]] ; [[#Arioli--2020|Arioli et al. 2020]] ; [[#Heinonen--2020|Heinonen et al. 2020]] ; [[#Hachaichi--2021|Hachaichi and Baouni 2021]] ; [[#Ramaswami--2021|Ramaswami et al. 2021]] ). Furthermore, accounting frameworks are reflected in multiple protocols used by urban practitioners ( [[#BSI--2013|BSI 2013]] ; [[#Fong--2014|Fong et al. 2014]] ; [[#ICLEI--2019b|ICLEI 2019b]] ). Synthesis of these reviews and protocols, as well as the many individual methodological studies available, point to four general frameworks of urban GHG accounting: (i) territorial accounting (TA); (ii) community-wide infrastructure supply chain footprinting (CIF); and (iii and iv) consumption-based carbon footprint accounting (CBCF; [[#Wiedmann--2008|Wiedmann and Minx 2008]] ). The last, CBCF, can be further divided into accounting with a focus on household or personal consumption (iii: the personal carbon footprint, or PCF); and an approach in which one includes final consumption in an area by all consumers (iv: the areal carbon footprint, or ACF) ( [[#Heinonen--2020|Heinonen et al. 2020]] ). A number of small variations to these general categories are found in the literature ( [[#Lin--2015|Lin et al. 2015]] ; [[#Chen--2020a|Chen et al. 2020a]] ), but these four general frameworks capture the important distinctive (i.e., policy-relevant) features of urban GHG accounting.

All these approaches are foundationally rooted in the concept of urban metabolism, that is, the tracking of material and energy flows into, within, and out of cities ( [[#Wolman--1965|Wolman 1965]] ). These frameworks all aim to quantify urban GHG emissions but reflect different perspectives on where the emission responsibility is allocated in addition to how much and which components of the GHG emissions associated with the import and export of goods and services to and from a city (‘transboundary embedded/embodied GHG emissions’) are included in a given urban emissions account. The four frameworks share some common, overlapping GHG emission quantities and their interrelationships have been defined mathematically ( [[#Chavez--2013|Chavez and Ramaswami 2013]] ).

A key advance since AR5 lies in understanding the different GHG accounting frameworks in terms of what they imply for responsibility – shared or otherwise – and what they imply for the depth and breadth of GHG emission reductions. TA focuses on in-city direct emission of GHGs to the atmosphere (e.g., combustion, net ecosystem exchange, methane (CH 4 ) leakage) within a chosen geographic area ( [[#Sovacool--2010|Sovacool and Brown 2010]] ; [[#Gurney--2019|Gurney et al. 2019]] ). CIF connects essential infrastructure use and demand activities in cities with their production, by combining TA emissions with the transboundary supply chain emissions associated with imported electricity, fuels, food, water, building materials, and waste management services used in cities ( [[#Ramaswami--2008|Ramaswami et al. 2008]] ; [[#Kennedy--2009|Kennedy et al. 2009]] ; [[#Chavez--2013|Chavez and Ramaswami 2013]] ).

CBCF considers not only the supply-chain-related GHG emissions of key infrastructure, but also emissions associated with all goods and services across a city, often removing emissions associated with goods and services exported from a city ( [[#Wiedmann--2016|Wiedmann et al. 2016]] , 2021). The distinction between the PCF and ACF variants of the CBCF is primarily associated with whether the agents responsible for the final demand are confined to only city residents (PCF) or all consumers in a city (ACF), which can include government consumers, capital formation, and other final demand categories ( [[#Heinonen--2020|Heinonen et al. 2020]] ).

A recent synthesis of these frameworks in the context of a net-zero GHG emissions target suggests that the four frameworks contribute to different aspects of decarbonisation policy and can work together to inform the overall process of decarbonisation ( [[#Ramaswami--2021|Ramaswami et al. 2021]] ). Furthermore, the relative magnitude of GHG emissions for a given city resulting from the different frameworks is often a reflection of the city’s economic structure as a ‘consumer’ or ‘producer’ city ( [[#Chavez--2013|Chavez and Ramaswami 2013]] ; [[#Sudmant--2018|Sudmant et al. 2018]] ).

The TA framework is unique in that it can be independently verified through direct measurement of GHGs in the atmosphere, offering a check on the integrity of emission estimates ( [[#Lauvaux--2020|Lauvaux et al. 2020]] ; [[#Mueller--2021|Mueller et al. 2021]] ). It is traditionally simpler to estimate by urban practitioners given the lower data requirements, and it can be relevant to policies aimed specifically at energy consumption and mobility activities within city boundaries. However, it will not reflect electricity imported for use in cities or lifecycle emissions associated with in-city consumption of goods and services.

The CIF framework adds to the TA framework by including GHG emissions associated with electricity imports and the lifecycle GHG emissions associated with key infrastructure provisioning activities in cities, serving all homes, businesses, and industries. This widens both the number of emitting categories and the responsibility for those emissions by including infrastructure-related supply chain emissions. The CIF framework enables individual cities to connect community-wide demand for infrastructure and food with their transboundary production, strategically aligning their net-zero emissions plans with larger-scale net-zero efforts ( [[#Ramaswami--2013|Ramaswami and Chavez 2013]] ; [[#Ramaswami--2021|Ramaswami et al. 2021]] ; [[#Seto--2021|Seto et al. 2021]] ).

The PCF version of the CBCF shifts the focus of the consumption and associated supply chain emissions to only household consumption of goods and services ( [[#Jones--2014|Jones and Kammen 2014]] ). This both reduces the TA emissions considered and the supply chain emissions, excluding all emissions associated with government, capital formation, and exports. The ACF, by contrast, widens the perspective considerably, including the TA and supply chain emissions of all consumers in a city, but often removing emissions associated with exports.

An additional distinction is the ability to sum up accounts from individual cities in a region or country, for example, directly to arrive at a regional or national total. This can only be done for the TA and PCF frameworks. The ACF and CIF frameworks would require adjustment to avoid double-counting emissions ( [[#Chen--2020a|Chen et al. 2020a]] ).

A second major area of advance since AR5 has been in methods to implement, verify and scale up the different GHG footprinting approaches. Advances have been made in six key areas: (i) advancing urban metabolism accounts integrating stocks and flows, and considering biogenic and fossil-fuel-based emissions ( [[#Chen--2020b|Chen et al. 2020b]] ); (ii) improving fine-scale and near-real-time urban use-activity data through new urban data science ( [[#Gately--2017|Gately et al. 2017]] ; [[#Gurney--2019|Gurney et al. 2019]] ; [[#Turner--2020|Turner et al. 2020]] ; [[#Yadav--2021|Yadav et al. 2021]] ); (iii) using atmospheric monitoring from the ground, aircraft, and satellites combined with inverse modelling to independently quantify TA emissions ( [[#Lamb--2016|Lamb et al. 2016]] ; [[#Lauvaux--2016|Lauvaux et al. 2016]] , 2020; [[#Davis--2017|Davis et al. 2017]] ; [[#Mitchell--2018|Mitchell et al. 2018]] ; [[#Sargent--2018|Sargent et al. 2018]] ; [[#Turnbull--2019|Turnbull et al. 2019]] ; [[#Wu--2020a|Wu et al. 2020a]] ); (iv) improving supply chain and input-output modelling, including the use of physically based input-output models ( [[#Wachs--2018|Wachs and Singh 2018]] ); (v) establishing the global multi-region input-output models ( [[#Lenzen--2017|Lenzen et al. 2017]] ; [[#Wiedmann--2021|Wiedmann et al. 2021]] ); and (vi) generating multi-sector use and supply activity data across all cities in a nation, in a manner where data aggregate consistently across city, province, and national scales ( [[#Tong--2021|Tong et al. 2021]] ) ( [[#8.3|Section 8.3]] ).

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