ABSTRACT
More than 50% of the Earth’s population now lives in cities. Cities consume enormous resources, the by-products of urban activity and land use are numerous (Foley et al., 2005), and a variety of recent studies demonstrate that the ecological footprint of many cities is significant and not sustainable (Rees and Wackernagel, 1996; Kareiva et al., 2007). Cities are also emerging as an important source of uncertainty in regional-to global-scale biogeophysical processes in the climate system. For example, the impact of urban areas on atmospheric chemistry and aerosols is both pronounced and well documented (Atkinson, 2000; Arnfield, 2003). Urban land use influences
local to regional climates through urban heat islands (Oke, 1982; Zhou et al., 2004), with concomitant impacts on human health (Patz et al., 2005) and ecosystems (Zhang et al., 2004; Alberti, 2005), and recent evidence has suggested that cities may also significantly affect local precipitation regimes (Kalnay and Cai, 2003; Kaufmann et al., 2007). At larger scales, recent studies have demonstrated that accurate representation of urban land use is both important and poorly captured in current models (Jin et al., 2005; Feddema et al., 2006; Oleson et al., 2008). As the global population grows and societies in developing countries become more urbanized, the environmental impact of cities will become even more pronounced (Mills, 2007; Grimmond, 2007). Accurate and timely information related to the global distribution and nature of urban areas is therefore critical to a wide array of environmental and geophysical research questions related to the effect of humans on the global environment (Sala et al., 2000; Peters-Lidard et al., 2004; Kaye et al., 2006; Pataki et al., 2006).
