space has tripled over a 27-year period, from 8.1 m2 in 1978 to 26 m2 in 2005 (National Bureau of Statistics of China, 2009).

It follows that we need to understand how 10 billion people will be allocated among households and distributed geographically across the world over the next 40 years. What processes create diverse patterns of human settlement? How is accelerating urbanization changing social, environmental, and economic conditions? The environmental challenges, resource requirements, infrastructure needs, energy demands, and governance issues associated with the unfolding growth in urban population raise fundamental, policy-relevant questions—and pose unprecedented opportunities for moving toward sustainability—that cannot be ignored as society navigates the urbanizing world of the 21st century. Thus, one of the biggest challenges facing humanity is how and where 10 billion people will live so as to reduce their environmental footprint. Given that most of these 10 billion people will live in cities, what are the consequences of an urbanizing Earth, and how can we reduce the negative impacts, while enhancing the positive impacts?

ROLE OF GEOGRAPHICAL SCIENCES

The study of human settlements is inherently an investigation of human–environment interactions, which requires spatially explicit data and analysis, an understanding of the interaction among places and across scales, and knowledge of the trade-offs among different land uses. Studies of the city, urban growth, urban-land-use theory, and the development of human settlements all have long traditions in the geographical sciences (Marsh, 1864). Much of the work on urban areas, their form and function in urban planning, urban economics, urban geography, and urban sociology, has drawn on the spatial land-use models of von Thünen (1826/1966), Burgess (1925), Muth (1961), Alonso (1964), and others.

Although cities have always depended on complex linkages with their immediate surroundings, as well as with more distant places, the speed, reach, and impact of these interconnections have become truly global, and will continue to be so. Understanding the processes and consequences of accelerating urbanization therefore requires tracing the web of connections and interactions that link people, places, and processes together. These linkages, which span environmental, cultural, social, economic, and political realms, mean that, for example, urbanization in one location affects demand for resources or waste disposal in another.

The geographical sciences have a long tradition of examining where, within urban areas, various kinds of people live, of investigating the processes that help to create such patterns, and of assessing the implications of residential patterning for variation in access to various opportunities such as jobs, medical care, or recreation. Since the 1960s, for example, studies have documented the ways in which urban settlements are distinguished by segregation along the lines of stage in the life course (e.g., singles, couples without children, families with children, and so on), socioeconomic status, and race and ethnicity. The spatial patterning of these dimensions is different in different regions of the world (Abu-Lughod, 1969), but in every place, the patterns of where people live within cities are the outcome of a mix of public policies and household preferences.

Modern geographical approaches and methods are generating insights into urban land-use patterns, from intracity to regional and global scales. The routine collection of imagery for most of Earth’s land areas by satellites provides an invaluable historical record covering more than three decades. This revolutionary development makes it possible to monitor human modification and urbanization of Earth’s surface across a range of spatial resolutions, from <1 m to the global scale (Sawaya et al., 2003; Zhang et al., 2004). Satellites such as Terra, Aqua, Landsat, and Tropical Rainfall Measuring Mission all provide data on the urban environment (Box 4.1).

The growing inventory of geographically indexed data makes it possible to combine satellite images with census and other information to develop analytically useful maps that show the distribution of human population around the world. For example, the Global Rural-Urban Mapping Project4 has generated a globally consistent and spatially explicit dataset of urban population distribution. Furthermore, advances in the development of analytical methods for geovisualization, geosimulation, and spatially explicit process models have simulated urban growth (Torrens, 2006); shown the linkages between places and scales over time (Kwan,

4

See sedac.ciesin.columbia.edu/gpw (accessed January 20, 2010).



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