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

=== 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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