Urban areas are hot spots that drive environmental change at multiple scales.  Material demands of production and human consumption alter land use and cover, biodiversity, and hydrosystems locally to regionally, and urban waste discharge affects local to global biogeochemical cycles and climate.  For urbanites, however, global environmental changes are swamped by dramatic changes in the local environment.  Urban ecology integrates natural and social sciences to study these radically altered local environments and their regional and global effects.  Cities themselves present both the problems and solutions to sustainability challenges of an increasingly urbanized world.