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

=== 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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