Global Population Distribution and Urban Land Use in Geophysical Parameter Space

Abstract

The spatial distribution of human population on the land surface is a fundamental determinant of land-use impacts on Earth's ecosystems. Census enumerations and satellite-detected night lights provide two complementary, but distinct, representations of human population distribution. Census estimates indicate that 37% of Earth's enumerated land area is populated at densities greater than 1 person per square kilometer. Human populations are strongly clustered within this area. Spatial variations in human population density span more than six orders of magnitude with 50% of the 1990 population occupying less than 3% of the inhabited land area. Temporally stable lighted areas detectable from space provide an independent proxy for the spatial distribution of urban settlements and the intensive land-cover changes that accompany them. These ?60 000 lighted areas account for less than 2% of inhabited land area, and 50% of this lighted area is associated with the largest 5% of cities and conurbations. Urban land use associated with higher population densities can exert a disproportionate influence on environments both near and distant. The spatial distributions of population density and lighted areas relative to geophysical parameters (continental physiography and climate) highlight some similarities and differences in the relationship of urban and rural land use to different physical environments. The spatial distribution of urban land use is strongly localized with respect to continental physiography (coastal and fluvial proximity and elevation) but much less localized with respect to climatic parameters (annual mean and range of temperature and precipitation). These distributions quantify the extent to which spatially focused development of urban land use, with respect to the physical landscape, influences coastal and riparian ecosystems in particular. If future population distributions follow current patterns, then demographic momentum and increasing rates of urban migration will result in accelerated growth of urban areas in these environments.