Researchers have, for the first time, mapped the rapid urban expansion that has occurred across the whole of East-Southeast Asia in the last decade.
In a study published in IOP Publishing's journal Environmental Research Letters, the researchers provide evidence to suggest that urban populations have grown more rapidly than the expansion of urban areas, leading to increased population densities in some of the most populated yet vulnerable regions in the world.
The study reveals that between 2000 and 2010, the population of East-Southeast Asia grew by 231 million people -- if this were the population of a single country, it would be the fifth most populous country in the world.
The study shows that at the same time, urban areas expanded by 34,000 km2 -- an area the size of Taiwan.
Contrary to previous findings, the new results reveal that the rate of population change has grown much more rapidly than the expansion of urban land. The population of cities in the studied region grew annually, on average, at 2.8%, in contrast to the rates of change for urban land average, which grew 2.0% annually.
Lead author of the study Professor Annemarie Schneider, from the University of Wisconsin-Madison, said: "Our results have shown that East-Southeast Asia is undergoing unprecedented urbanization and urban expansion.
"The assumption from past research has been that cities of all sizes will eventually decline in density, with greater amounts of urban expansion than population growth. This study reveals the opposite, and this could change how officials plan and adapt to urbanization in the future."
To arrive at their results, researchers at the University of Wisconsin-Madison calculated the difference in size of urban areas, from 2000 to 2010, by closely scrutinizing maps of urban extent and urban expansion developed from MODIS 250m satellite data.
The overall study area covered East-Southeast Asia and included 17 countries: China (including Hong Kong SAR, Macao SAR); Taiwan; Japan; Mongolia; North Korea; South Korea; Brunei Darussalam; Cambodia; Indonesia; Laos; Malaysia; Myanmar; Philippines; Singapore; Thailand; Timor-Leste; Vietnam.
An urban area, or urban agglomeration, was defined as a built-up area of a central city and any suburbs or small cities that were linked together by continuous urban land. In their study, the researchers analysed all cities with a population greater than 100,000, which gave a total of 1036 agglomerations.
The land data was then combined with estimates of population growth within each agglomeration taken from the most recent censuses in each country. The population maps were developed by the WorldPop team, led by Andy Tatem at the University of Southampton. The two groups worked closely together to develop their methodologies in tandem so that the data and overall results were consistent and as accurate as possible.
The researchers' analysis has shown that from 2000 to 2010, the urban land area in East-Southeast Asia increased by around 22% from 155,000 to 189,000 km2. The overall population increased by around 31%, from 738 to 969 million.
By explicitly measuring the urban density (persons per kilometre) change for each agglomeration, the researchers showed that, contrary to previous findings, urban growth has outpaced land expansion.
At the country level, Japan and South Korea -- both developed countries -- lead the region with highly urbanized populations (80-90%) spread across multiple large urban agglomerations covering 3-5% of each country's land area.
On average, population growth rates for large, middle-income countries, such as China, Indonesia, the Philippines and Thailand, were high (3.5%) relative to their average rates of urban expansion (2.6%).
The Pearl River Delta area in China surpassed Tokyo to become the largest urban agglomeration on Earth with over 41 million people inhabiting 6970 km2 of urban land.
Professor Schneider continued: "While there have been studies that have assessed urban expansion and urban population growth across large areas, this is the first study of its kind to use empirical maps and datasets for every settlement and population in the 17-country region. Historically, we've had virtually no empirical information on the built up areas of China's neighbours in the region, including Vietnam, the Philippines, Indonesia, Thailand, Malaysia, and Korea.
"We are currently working towards expanding this study to cover all agglomerations, cities, and human settlements on Earth. Our group here at the University of Wisconsin-Madison was recently awarded three years of NASA funding to develop global maps of urban expansion between 2000 and 2010. This will allow effective and accurate assessment of recent urban trends both within and across nations."
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