TABLE 4.1 Various Pathways Through Which Urbanization Affects the Climate System


Urban Land Cover

Urban Aerosols

Anthropogenic Greenhouse Gas Emissions

Urban heat island (UHI) and mean surface temperature record

Surface energy budget

Insolation, direct aerosol effect

Radiative warming and feedbacks

Wind flow and turbulence

Surface energy budget, urban morphological parameters, mechanical turbulence, bifurcated flow

Direct and indirect aerosol effects and related dynamic/thermodynamic response

Radiative warming and feedbacks

Clouds and precipitation

Surface energy budget, UHI destabilization, UHI mesocirculations, UHI-induced convergence zones

Aerosol indirect effects on cloud-precipitation microphysics, insolation effects

Radiative warming and feedbacks

Land surface hydrology

Surface runoff, reduced infiltration, less evapotranspiration

Aerosol indirect effects on cloud-microphysical and precipitation processes

Radiative warming and feedbacks

Carbon cycle

Replacement of high net primary productivity land with impervious surface

Black carbon aerosols

Radiative warming and feedbacks, fluxes of carbon dioxide

Nitrogen cycle

Combustion, fertilization, sewage release, and runoff

Acid rain, nitrates

Radiative warming and feedback, NOx emissions

SOURCE: Seto and Shepherd (2009).

urbanization and global change scenarios. For example, the WaterSim Project7 simulates water supply and demand in a desert city. The simulation and forecast tools being developed in conjunction with this project can help cities evaluate policies to adapt to a water-constrained environment.

What are the impacts of accelerating and large-scale urbanization on local and regional climate patterns?

There is a growing scientific understanding of the relationship between urbanization and climate (Voogt and Oke, 2003; Shepherd, 2005). Urbanization alters climate through multiple pathways (Table 4.1).

Locally, the conversion of vegetated surfaces to urban areas modifies surface energy balance dynamics (Lo et al., 1997; Banta et al., 1998). Altering the exchange of heat, water, trace gases, aerosols, and momentum between the land surface and overlying atmosphere leads to the urban heat island effect, which is characterized by elevated daytime and nighttime temperatures in and near urban areas compared to non-urban or rural areas (Arnfield, 2003; Crutzen, 2004). The urban heat island effect is further affected by the interaction among building geometry, land use, and urban materials (Oke, 1973; Arnfield, 2003).

Urban areas have been shown to produce a warming trend over regional climate (Kalnay and Cai, 2003), and there is mounting evidence that urban areas also affect precipitation (Lowry, 1998; Shepherd and Jin, 2004). It has long been known that urban areas alter their regional and microclimates, resulting in an increase in rainfall downwind (Landsberg, 1970). Empirical evidence shows that in some regions, urban expansion and urban air pollution result in a decline in rainfall (Amanatidis et al., 1993; Rosenfeld, 2000; Kaufmann et al., 2007). In other places, urbanization has induced precipitation and possibly created thunderstorms, leading to significant anomalies in precipitation patterns (Figure 4.3) (Dixon and Mote, 2003). In places where it has been shown that urban areas increase rainfall, the difference can be as much as 5 percent to 25 percent (Mote et al., 2007). In addition to the quantity of rainfall, cities can also affect the timing and formation of thunderstorms and the severity of precipitation, as has been found for Tokyo (Yonetani, 1982), Beijing (Guo et al., 2006), Atlanta (Bornstein and Lin, 2000), Mexico City (Jauregui and Romales, 1996), among others. These changes imply


See (accessed January 20, 2010).

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