The UN expects that "urban areas will absorb all the population growth over the next four decades" (UN 2007b). A turning point was reached in 2008: more than half of the planet now lives in cities. Correspondingly, Earth's surface is becoming increasingly urban. The conversion of Earth's land surface to urban uses will be one of the biggest environmental challenges of the 21st century. Although cities have existed for centuries, urbanization processes today are different from urban transitions of the past in terms of the scale of global urban land area, the rapidity at which landscapes are being converted to urban uses, and the location of new cities, which will primarily be in Asia and Africa. As urban areas expand, transform, and envelop the surrounding landscape, they impact the environment at multiple spatial and temporal scales through climate change, loss of wildlife habitat and biodiversity, and greater demand for natural resources. The size and spatial configuration of an urban extent directly impacts energy and material flows, such as carbon emissions and infrastructure demands, and thus has consequences on Earth system functioning. Intensification and diversification of land use and advances in technology have led to rapid changes in biogeochemical cycles, hydrologic processes, and landscape dynamics (Melillo et al. 2003).

Between 2007 and 2050, the urban population is expected to increase by 3.1 billion: from 3.3 billion to 6.4 billion. This scenario assumes fertility reductions in developing countries. If fertility remains at current levels, then the urban population in 2050 will increase by 4.8 billion to reach 8.1 billion. In short, urban populations may increase by 3-5 billion over the next 42 years. Most of this growth will occur in Asia (58%) and Africa (29%), with China and India combined housing nearly one-third of the world's urban population by 2030.

If we assume urban population densities of middle and low income countries (7500/km2), and an additional 3 billion urbanities, by 2050 the world will add an additional 400,000 km2 of urban land, roughly the size of Germany (357,000 km2). Under the constant fertility scenario, global urban areas will increase by 666,000 km2 by 2050, roughly the size of Texas (678,000 km2). However, if current trends continue and urban densities move toward the global average of 3500/km2, the demand for new urban land will range from 857,000 km2 to 1,429,000 km2, or roughly twice Texas.

New urban expansion is likely to take place in prime agricultural land, as human settlements have historically developed in the most fertile areas (Seto et al. 2000; del Mar Lopez et al. 2001). In turn, the conversion of existing agricultural land to urban uses will place additional pressures on natural ecosystems. There is evidence that urban growth is indeed taking a toll on agricultural lands and that loss of fertile plains and deltas are being accompanied by the conversion of other natural vegetation to farmland (Doos 2002).

Building cities on previously vegetated surfaces modifies the exchange of heat, water, trace gases, aerosols, and momentum between the land surface and overlying atmosphere (Crutzen 2004). In addition, the composition of the atmosphere over urban areas differs from undeveloped areas (Pataki et al. 2003). These changes imply that urbanization can affect local, regional, and possibly global climate at diurnal, seasonal, and long-term scales (Zhou et al. 2004; Zhang et al. 2005). The urban heat island effect is well documented around the world and is generated by the interaction between building geometry, land use, and urban materials (Oke 1976; Arnfield 2003). Recent studies show that there is also an "urban rainfall effect" (Shepherd et al. 2009), with urbanization increasing rainfall in some areas (Hand and Shepherd 2009; Shepherd 2006) while decreasing rainfall in others (Trusilova et al. 2008; Kaufmann et al. 2007).

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