FIGURE 4.3 Annual precipitation anomalies, 1900–1988. Regions in blue show an increase in precipitation over the mean during the 1900–1988 period. Regions in red have become relatively drier during the same period. Areas without data are shown in white. SOURCE: Dai et al. (1997).

FIGURE 4.3 Annual precipitation anomalies, 1900–1988. Regions in blue show an increase in precipitation over the mean during the 1900–1988 period. Regions in red have become relatively drier during the same period. Areas without data are shown in white. SOURCE: Dai et al. (1997).

that urban land uses and urban expansion through land-cover change can affect local, regional, and global climate at diurnal, seasonal, and long-term scales (Stohlgren et al., 1998; Zhou et al., 2004).

A clearer picture is emerging that urban areas can affect climate at different scales. However, we lack a comprehensive understanding of the dynamics by which urban expansion will affect climate; nor do we understand the interactions between local-scale dynamics and regional and global patterns. We also have a fragmented picture of urban land-use patterns at a global scale. Most analyses of urban land use are based on individual case studies of city or metro regions, and there are few comparative, regional, or global studies (Seto and Shepherd, 2009). We have a good understanding of some of the ways that urban areas affect climate at several scales, but we lack comprehensive understanding, and more importantly, an understanding of how urban growth—or different forms of urban growth—will affect climate. Given the magnitude of the global urban transition of the 21st century, there is an urgent need to understand the impact of urban land-use change on precipitation and temperature, and to forecast scenarios of urban expansion and their possible impacts on precipitation changes. How will the growth of settlements affect rainfall and temperatures at local, regional, and global scales?

Continued advances in understanding the link between urban land use and regional climate will require mining a variety of data—from remote sensing of clouds, aerosols, and land to field-based meteorological data on temperature and precipitation. It will require modeling urban climates at fine spatial scales to understand the effect of building materials, street geometry, and building geometry on local temperatures (Oke, 1973, 1981), explicit treatment of urban land use in climate models (Bonan et al., 2002; Jin et al., 2005), and techniques for downscaling general circulation models (Wilby and Wigley, 1997).

SUMMARY

Urbanization in the 21st century will have far-reaching effects ranging from the local to the global. Understanding where people will live and how cities will develop in the future has implications for all aspects of human and environmental well-being discussed in this report, from the provision of food for a growing urban population to safeguarding our planet’s biodiversity and ecological services. New geographical data and emerging analytical methods, combined with existing research tools and techniques will help develop a more coherent and complete understanding of the patterns, implications, and uncertainties of urbanization.



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