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

=== 8.3.1 Trends in Urban Land Use and the Built Environment ===

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Urban land use is one of the most intensive human impacts on the planet ( [[#Pouyat--2007|Pouyat et al. 2007]] ; [[#Grimm--2008|Grimm et al. 2008]] ). Urban land expansion to accommodate a growing urban population has resulted in the conversion of agricultural land ( [[#Pandey--2018|Pandey et al. 2018]] ; [[#Liu--2019|Liu et al. 2019]] ), deforestation ( [[#van%20Vliet--2019|van Vliet 2019]] ), habitat fragmentation ( [[#Liu--2016b|Liu et al. 2016b]] ), biodiversity loss ( [[#McDonald--2018|McDonald et al. 2018]] , 2020), and the modification of urban temperatures and regional precipitation patterns ( [[#Li--2017|Li et al. 2017]] ; [[#Krayenhoff--2018|Krayenhoff et al. 2018]] ; [[#Liu--2019|Liu and Niyogi 2019]] ; [[#Zhang--2019|Zhang et al. 2019]] ).

Urban land use and the associated built environment and infrastructure shape urban GHG emissions through the demand for materials and the ensuing energy-consuming behaviours. In particular, the structure of the built environment (i.e., its density, form, and extent) have long-lasting influence on urban GHG emissions, especially those from transport and building energy use, as well as the embodied emissions of the urban infrastructure ( [[#Butler--2014|Butler et al. 2014]] ; [[#Salat--2014|Salat et al. 2014]] ; [[#Ramaswami--2016|Ramaswami et al. 2016]] ; [[#Seto--2016|Seto et al. 2016]] ; [[#d’Amour--2017|d’Amour et al. 2017]] ). Thus, understanding trends in urban land use is essential for assessing energy behaviour in cities as well as long-term mitigation potential (Sections 8.4 and 8.6, and Figure 8.21).

This section draws on the literature to discuss three key trends in urban land expansion, and how those relate to GHG emissions.

First, urban land areas are growing rapidly all around the world. From 1975 to 2015, urban settlements expanded in size approximately 2.5 times, accounting for 7.6% of the global land area ( [[#Pesaresi--2016|Pesaresi et al. 2016]] ). Nearly 70% of the total urban expansion between 1992 and 2015 occurred in Asia and North America ( [[#Liu--2020a|Liu et al. 2020a]] ). By 2015, the extent of urban and built-up lands was between 0.5% and 0.6% of the total 130 Mkm 2 global ice-free land use, taking up other uses such as fertile cropland and natural ecosystems.

Second, as Figure 8.5 shows, urban population densities are declining, with significant implications for GHG emissions. From 1970 to 2010, while the global urban settlement extent doubled in size ( [[#Pesaresi--2016|Pesaresi et al. 2016]] ), most regions (grouped by the AR6 WGIII 10-region aggregation) exhibited a trend of decreasing urban population densities, suggesting expansive urban growth patterns. Urban population densities have consistently declined in Australia, Japan and New Zealand, and Europe, North America, and Southern Asia regions, across all city sizes. North America consistently had the lowest urban population densities. Notably, the Middle East region appears to be the only region exhibiting an overall increasing trend across all city-size groups, while Latin America and Caribbean appears to be relatively stable for all city sizes. While the larger cities in Africa and South-East Asia and Pacific exhibit slightly stable urban population densities, the small and medium-sized cities in those regions trend toward lower urban population densities. In large urban centres of Eastern Asia and North America, rapid decreases in earlier decades seem to have tapered. Compared to larger cities, small-medium urban areas with populations of less than 2 million have more declines in urban population densities and higher rates of urban land expansion ( [[#Güneralp--2020|Güneralp et al. 2020]] ).

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'''Figure 8.5: Urban population density by decade (1970–2010) grouped by the AR6 WGIII 10-region aggregation''' '''.''' Panel '''(a)''' displays the results from all case study locations with a population &gt;300,000. Panels (b) and (c) show results grouped by city size: '''(b)''' cities with a population &gt;2 million (large urban centres), and '''(c)''' those with a population &gt;300,000 but &lt;2 million (small and medium urban centres). Box plots show the median, first and third quartiles, and lower and upper mild outlier thresholds of bootstrapped average urban population densities at the turn of each decade. The estimates are shown on a logarithmic scale. The data shows an overall trend of declining urban population densities among all but one region in the last four decades, at varying rates – although the Latin America and Caribbean region indicates relatively constant urban population density over time. The Middle East region is the only region to present with an increase in urban population density across all city sizes. Source: adapted from [[#Güneralp--2020|Güneralp et al. (2020)]] .

This decline in urban densities is paralleled by an increase in ‘sprawl’, or ‘outward’ urban development. Urban expansion occurs in either one of three dimensions: (i) outward in a horizontal manner; (ii) upward, by way of vertical growth; or (iii) infill development, where unused, abandoned, or underutilised lands within existing urban areas are developed or rehabilitated (Figure 8.20). Outward expansion results in more urban land area and occurs at the expense of other land uses (i.e., the conversion and loss of cropland or forests). Vertical expansion results in more multi-storey buildings and taller buildings, more floor space per area, and an increase in urban built-up density. Every city has some combination of outward and upward growth in varying degrees ( [[#Mahtta--2019|Mahtta et al. 2019]] ) (Figure 8.6). That each city is comprised of different and multiple urban growth typologies suggests the need for differentiated mitigation strategies for different parts of a single city ( [[#8.6|Section 8.6]] and Figure 8.21). Recent research shows that the relative combination of outward versus upward growth is a reflection of its economic and urban development ( [[#Lall--2021|Lall et al. 2021]] ). That is, how a city grows – whether upward or outward – is a function of its economic development level. Upward growth, or more tall buildings, is a reflection of higher land prices ( [[#Ahlfeldt--2018|Ahlfeldt and McMillen 2018]] ; [[#Ahlfeldt--2020|Ahlfeldt and Barr 2020]] ).

An analysis of 478 cities with populations of more than 1 million people found that the predominant urban growth pattern worldwide is outward expansion, suggesting that cities are becoming more expansive than dense ( [[#Mahtta--2019|Mahtta et al. 2019]] ) (Figure 8.6). The study also found that cities within a geographic region exhibit remarkably similar patterns of urban growth. Some studies have found a mix of urban forms emerging around the world; an analysis of 194 cities identified an overall trend (from 1990 to 2015) toward urban forms that are a mixture of fragmented and compact ( [[#Lemoine-Rodriguez--2020|Lemoine-Rodriguez et al. 2020]] ). The exception to this trend is a group of large cities in Australia, New Zealand, and the United States that are still predominantly fragmented. The same study also identified small to medium-sized cities as the most dynamic in terms of their expansion and change in their forms.

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'''Figure 8.6: (a) Distribution of growth typologies across 10 cities, and (b) sample of 64 cities by region with different patterns of urban growth.''' The empirical data is based on the Global Human Settlement Layer and backscatter power ratio for different patterns of urban growth across the sample of cities. In (b), the blue arrows indicate outward urban growth. Other urban patterns indicate stabilised (orange), mature upward (light blue), budding outward (green), and upward and outward (red). Note that with few exceptions, each city is comprised of multiple typologies of urban growth. Source: [[#Mahtta--2019|Mahtta et al. (2019)]] .

A third trend in is urban land growth taking place on agricultural land, carbon stocks, and other land uses (see ‘carbon stock’ and ‘AFOLU’ – agriculture, forestry, and other land uses – in Glossary). As Figure 8.7 shows, over 60% of the reported urban expansion (nearly 40,000 km 2 ) from 1970 to 2010 was formerly agricultural land ( [[#Güneralp--2020|Güneralp et al. 2020]] ). This percentage increased to about 70% for global urban expansion that occurred between 1992 and 2015, followed by grasslands (about 12%) and forests (about 9%) ( [[#Liu--2020a|Liu et al. 2020a]] ). In terms of percent of total urban land expansion, the largest conversion of agricultural lands to urban land uses from 1970 to 2010 took place in the Eastern Asia, and South-East Asia and Pacific regions; the largest proportional losses of natural land cover were reported for the North America and Australia, Japan and New Zealand regions ( [[#Güneralp--2020|Güneralp et al. 2020]] ). At a sub-regional level, agricultural land constituted the largest proportion of land converted to urban areas in China, India, Europe, Southeast Asian countries and the central United States between 1995 and 2015; in the eastern United States, most new urban land was converted from forests ( [[#Liu--2020a|Liu et al. 2020a]] ). Urban expansion through 2040 may lead to the loss of almost 65 Mt of crop production – a scenario that underscores the ongoing relationship between urbanisation and AFOLU ( [[#van%20Vliet--2017|van Vliet et al. 2017]] ) (Chapter 7).

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'''Figure 8.7: Percent of total urban land expansion from other land covers, sorted by the AR6 WGIII 10-region aggregation (1970–2010).''' As urban land has expanded outward, other forms of land cover, including agriculture, ‘nature’ (e.g., forest, grassland, shrubland, water, and bare soil, all of which are disaggregated to the bottom half of the plot), and other land covers, have been displaced. Globally, agriculture comprises the majority (about 60%) of the land displaced by urban expansion since 1970. Forests and shrubland vegetation – important carbon stocks – also make up a significant proportion of displacement. The loss of carbon-sequestering land like forests and shrubland independently impacts climate change by reducing global carbon stocks. Eurasia is omitted because there are no case studies from that region that report land conversion data. Source: adapted from [[#Güneralp--2020|Güneralp et al. (2020)]] .

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