A new urban landscape in East–Southeast Asia, 2000–2010

We have entered the urban era: cities now form the basis of the human experience for the majority of the Earth's population (UN 2012). Cities today must meet the needs of growing populations and expanding economies, while at the same time minimizing their environmental impacts (Grimm et al 2008, Montgomery 2008). Expansion of built-up land is often the most direct environmental impact associated with urban growth, with far-reaching implications for climate, hydrology, and biogeochemical cycles that extend beyond municipal boundaries (Seto et al 2010). While remote sensing has proven especially useful for characterizing broad-scale land changes, detailed monitoring of urban land use change remains costly and challenging due to the highly heterogeneous nature of cities, the spectral similarity between new urban land and other land cover types, and the lack of cloud-free data in locations where estimates are most needed (e.g. tropics, Mertes et al 2015). As a result, there has been little information on the building boom that is accompanying population growth in many developing countries (China, India, etc) other than case-study analysis of individual cities (Schneider and Woodcock 2008, Angel et al 2011), or country-level assessments (Liu et al 2005, Wang et al 2012). Comparing urban populations has also been notoriously difficult due to differences in census timing, data availability/quality, and most critically, the considerable variability in how cities are defined, whether by population threshold, functional area, or administrative boundaries (Cohen 2004). One of the few studies reporting transnational urban land and population trends concluded that cities are universally spreading out and declining in density (Angel et al 2005). While there is evidence to contradict this in East Asia (Murakami et al 2005, Bagan and Yamagata 2012), there has been no systematic way to compare trends across cities, nations, or regions.

To describe urban trajectories across East and Southeast Asia¹⁰ systematically, we characterize urban extent and urban expansion 2000–2010 using Moderate Resolution Imaging Spectroradiometer (MODIS) satellite observations (Mertes et al 2015). In these maps, urban land refers to places dominated by the 'built environment', which includes all non-vegetative, human-constructed elements (e.g. roads, buildings) with >50% coverage of a landscape unit (here, a 250 m pixel). We synthesize this information with population density maps developed using demographic data at the finest administrative unit available and empirically-tested population-land cover relationship-based methods (Tatem et al 2007). To address the issue of comparability, we conduct our analysis of regional urbanization trends using the urban agglomeration as the unit of analysis. We perform a comparative analysis to understand within-nation and across-nation trends in East–Southeast Asia (figure 1, A1) recognizing that such a regional approach cannot account for each city's circumstances or individual drivers/impacts. Our results likely produce a conservative estimate of urban change in the region, and may differ from 'official' statistics (World Bank 2015) as a result of necessary choices regarding definitions, spatial scale, and data sources. Our aim is not to replace national estimates, but to offer a consistent approach for regional comparability of all cities >100 000 in the region.

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Great strides have been made to map population distribution using consistent data and methods (Balk et al 2006, Tatem et al 2007), but they depict population as measured at one point in time, and at best adjust only for changing population growth rates at the country level. Similarly, urban maps from remote sensing data have been limited to either static global maps that sacrifice detail to provide areal coverage (Potere et al 2009), or local maps of metropolitan growth that forego coverage in favor of spatial detail (Seto et al 2010). Moreover, of the many global urban maps now available (Elvidge et al 2007, Schneider et al 2009), none characterize changes in urban land.

Recent comparative work on urbanization has found a middle ground by focusing on local maps (typically 30 m resolution) for a sample of cities (Seto et al 2011, Angel et al 2012, Taubenbock et al 2012, Schneider and Mertes 2014). These studies point to several key findings: (1) cities in developing countries are consistently smaller or more compact than those in developed nations (Huang et al 2007, Schneider and Woodcock 2008); (2) developing country cities typically undergo some decrease in population density in the core during their development trajectory (Murakami et al 2005); and (3) cities are declining in density (Angel et al 2011, 2012).

Although they represent advances in our empirical understanding of urbanization, these studies (and their conclusions) suffer from several limitations. Many rely on a limited very city sample (Murakami et al 2005, Taubenbock et al 2012); most exclude small-and medium-sized cities (100 000-1 million) where the majority of new growth is taking place (UN 2012). Definitional issues also jeopardize comparability: nearly all utilize municipal boundaries to clip the city extent, making it nearly impossible to compare trends across places. In addition, administrative divisions often under-bound the built-up extent, so new growth in fringe areas is not captured. Finally, many studies rely only on population or remote sensing, failing to connect the two to provide a complete picture of urban trends meaningful for environmental assessment, land use planning, and regional policy implementation.

3.1. Satellite-based maps of urban expansion

3.2. Population density maps

3.3. Analysis

4.1. Regional and country-level results

Across the region, the total net increase in urban land area was >34 000 km² from 2000 to 2010, expanding from 155 000 to 189 000 km². While urban land area increased >22%, urban populations climbed >31%, adding 231 million persons in just ten years (from 738 to 969 million). The rapid pace of population change is clear in the average rates of change for each country (table A4): cities in the region grew annually at 2.8%, with Malaysia, Vietnam, Cambodia, and Laos all observing rates well above this average (4.0–7.8%). In contrast, the rates of change for urban land average 2.0% annually, with only China, the Philippines, Cambodia, and Laos having rates above this level (2.2–3.2%).

The results suggest that urban population growth has outpaced land expansion, a trend we measure explicitly using urban density. Here we estimate persons per square kilometer of built-up land since the conventional measure, persons per square kilometer within an administrative region, does not account for the vastly different sizes of municipal boundaries. While the results show a great degree of variability (figure 2), there are two common trends across nations: (a) urban densities are high (mean 2010, 5850 persons/km²), and (b) urban densities increase 15–30% from 2000 to 2010, adding between 270 and 2020 persons/km² in ten years. Although on average, urban densities are decreasing in China (from 6150 to 5290 persons/km² across 677 cities), there is considerable variability here as well: roughly half of Chinese agglomerations are decreasing in density, while the remaining half witnessed no change or an increase in urban density, similar to other agglomerations in the region.

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4.2. City-level results: the view from above

More than one-third of all urban land and urban population in East–Southeast Asia falls into 30 large agglomerations (figure 3). By 2010, the Pearl River Delta agglomeration climbed to >41 million inhabitants and 6970 km² of urban land, surpassing Tokyo (31 million persons, 5570 km² urban land) as the largest urban agglomeration on Earth. An additional 12 of the top agglomerations are located in China, including Shanghai and Beijing, with 3480 and 2720 km² of urban land, and populations of 24 and 16 million persons, respectively, in 2010. China also contains the agglomerations with the greatest urban land expansion, 2000–2010, with a median increase of 463 km², compared to a median of 217 km² for all 30 cities. The Chinese agglomerations have witnessed significant population increases as well, adding a median 2.5 million persons to each large agglomeration during the last decade. Several large agglomerations outside of China have major population increases (Tokyo, Jakarta, Manila), but not surprisingly, none have the scale of new development witnessed in China.

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The growth of these 'mega-agglomerations' is not the whole story, however. The region has an additional 101 large agglomerations, each with populations between 1 and 5 million persons, totaling >207 million. Although rates of expansion in these areas are on par with the 30 large agglomerations (>3%), the average rates of population increase surpass those of the top 30, at >3.4%. These trends are also apparent in agglomerations 100 000–1 million: small cities in Myanmar, Indonesia, Vietnam and the Philippines, especially, have added population without much expansion (figure 4). Nearly all trajectories are headed in the same general direction, with an average increase of 970 persons/km² for the 2000–2010 period.

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4.3. City-level results: governance and policy perspectives

While the agglomeration provides a consistent way to compare metropolitan areas since they are defined by built-up extent, many agglomerations are comprised of a large number of independently-governed cities. For instance, the Manila agglomeration has 17 cities in its administrative core (where resources and planning are concentrated) and another 15 cities on the outskirts. Alternatively, many large agglomerations have a small core area governed as one unit, with expansion that spills into the jurisdiction of nearby county-or city-level governments (e.g. Shanghai, Seoul, Hanoi). To understand how cities within an agglomeration view and govern themselves, we measure urban expansion for all established cities within 120 km of the city core for the top 30 agglomerations. Here we delineate each core according to its 2010 municipal area, and standardize the size of each small city extent using adaptive radial zones corresponding to each city's 2010 population.

On average, >60% of 2010 urban land and >71% of new development 2000–2010 are located outside the core administrative area, but within the urban agglomeration defined by this study (figure 5). The results also highlight three distinct urban typologies for large agglomerations: (1) a core surrounded by rapidly growing cities, with expansion rates that decline with distance (e.g. Hangzhou, Guangzhou, Chengdu, Jakarta); (2) a core with numerous nearby cities, but with limited growth due to geophysical factors (e.g. Manila, Bangkok, Kuala Lumpur); and (3) a core with few nearby cities (Hanoi, Bangkok). Some of these latter areas are witnessing peri-urbanization (expansion up to 100 km from the core, Kontgis et al 2014), but this trend may not be fully captured in satellite-based estimates or census data due to its small, patchy nature.

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This research presents new evidence that East–Southeast Asia is undergoing unprecedented urbanization and urban expansion, coincident with well-established trends of rapid industrialization, economic growth, and globalization. These results were generated using directly comparable, spatially-detailed datasets derived from multiple sources of remote sensing and disaggregated census data, with close attention to how urban land, urban expansion, urban population and agglomeration boundaries were defined and operationalized. When the factors limiting comparative urban analysis are addressed, the results reveal that urban agglomerations across East Asia are experiencing increasing urban densities. While these trends are not surprising for some scholars and local experts, they do contradict established empirical work that shows—with similar attention to consistency in data and definitions but with results modeled using static c. 2000 urban maps—that cities are universally declining in density (Angel et al 2011, 2012).

The trend toward increasing urban densities is clear in nearly all countries, and at multiple scales. At the country level, Japan and South Korea lead the region with highly urbanized populations (80–90%) spread across multiple large urban agglomerations covering 3–5% of each country's land area. Although growth has tapered off in these countries, their aggregate urban densities are still climbing. On average, population growth rates for large, middle-income countries (China, Indonesia, the Philippines, Thailand) are high (3.5%) relative to their average rates of urban expansion (2.6%). Cities of all sizes are growing in these countries, with higher rates of population growth for small cities than for large agglomerations during the last decade. China is clearly a unique case, however. At the country level, Chinese cities appear to be decreasing in density, a result that is expected given the central government's planning and policy initiatives focused on small cities outside major metropolitan areas (Lin 1999). Results at the city level reveal that half of all Chinese cities have urban densities that increase or remain unchanged.

East–Southeast Asia is also home to several low or low-middle income countries with 30% of their total populations living in urban areas, including Vietnam, Myanmar, Laos, and Cambodia. During the last decades, these countries have witnessed major shifts from predominantly subsistence agrarian economies to increasingly commercialized agriculture, leading to rapid urbanization of rural populations (Hall et al 2011). The rates of urban population growth at the country level average 4.6% annually, primarily due to the extraordinary growth of just a few large cities (>1 million). Ho Chi Minh City, Hanoi, Yangon, and Vientiane, for example, have all witnessed rapid population growth, adding an average 1.4 million persons, 2000–2010. The results here reveal limited urban expansion, though, which has led to an average increase in urban density of 870 persons/km².

Finally, this work also examines how differences in administrative boundaries and urban definitions impact how we characterize, monitor, and understand urban change. We defined 30 large agglomerations by contiguous urban land, but evaluated rates/amounts of growth within the core administrative area and for the individual cities comprising these agglomerations (figure 5). Most administrative cores contain multiple cities on average, while an additional 2–21 cities exist within the built-up area of the agglomeration, but outside the jurisdiction of the core. It is in these outer cities where the majority of urban growth is concentrated. From these results, we therefore conclude that cities as they are experienced on the ground (i.e. contiguous built-up regions) are often not the same as how they are governed. Given rising urban densities, continued expansion, and a lack of coordinated governance, the question for governments and planners becomes whether adequate services, infrastructure, housing, and employment are available or can be provided to incoming populations.

There are several potential sources of uncertainty in this study that should be noted. With respect to the remote sensing data, the 250 m pixel size combined with the population threshold of 100 000 makes it difficult to capture all small settlements. In China, Indonesia, and Vietnam, villages are spectrally distinct and sufficiently large (>1 km²), and disaggregated population data are available. Accordingly, they are well-mapped with our methods (figure 1). In Laos, Cambodia, and North Korea, villages are small and comprised of local materials that are spectrally similar to surrounding land cover types. These countries have no reliable village population estimates, and consequently, the results may under-report urban land or growth. On average, the total land area and population of these settlements is a fraction of the urban extent and urban population in each country, and should therefore have a limited effect on interpreting the results of this study. Finally, the urban extent does not include low-density settlements (e.g. 30–40% built-up), although these areas may function as urban space. If we relax the 50% threshold, higher rates and amounts of urban land would be likely.

One additional area of uncertainty is related to the availability of population data. Locations with less-than-ideal data include Malaysia, Thailand, Laos, Myanmar, and North Korea (table A3); results for these countries should be considered in light of this bias. In addition, population estimates have greater uncertainty when the administrative unit is large relative to urban extent, and rural populations within the unit are dense (Hay et al 2005). In these areas (e.g. Indonesia), population densities may be overestimated. Finally, the approach here does not capture growth within existent urban areas, including redevelopment or vertical growth. The lack of within-city monitoring remains a critical limitation of both population data sources and remote sensing for land use planning. New datasets (crowd sourcing, social media, etc) and advances in radar/lidar have the potential to significantly change how we monitor urban change (Frolking et al 2013, Tsou et al 2013).

Urban growth has increased in scope, scale, and complexity in recent decades, and has become one of the most important challenges of the 21st century. The urban expansion and urban growth datasets¹¹ presented here provide a valuable, practical, and consistent way to monitor a broad range of issues, including impacts to local-regional climate (Kaufmann et al 2007), pollution levels (Grimm et al 2008), water quality/availability (McDonald et al 2011), arable land (Lambin and Meyfroidt 2011) as well as the livelihoods and vulnerability of populations in the region (Solecki et al 2011). These datasets are unique in that they represent the first comprehensive mapping of urban expansion and growth for all cities >100 000 in East–Southeast Asia, and they also form the basis of ongoing work to examine land and population trends globally for all cities and agglomerations. While uncertainties may always be present no matter the data source, spatially-and temporally-detailed maps of urban expansion and population growth based on the best available data are nevertheless critical for researchers, urban planners, land managers, and government officials interested in a sustainable urban future.

AS acknowledges funding support from the World Bank for preparation of datasets. AJT acknowledges funding support from the RAPIDD program of the Science and Technology Directorate, Department of Homeland Security, and the Fogarty International Center, National Institutes of Health, and is also supported by grants from the Bill and Melinda Gates Foundation (#49446, #1032350). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. The authors also wish to thank Caitlin Kontgis, Mutlu Ozdogan, and four anonymous reviewers for their helpful comments on an earlier draft of this manuscript.