A city's urban environment—the area that it occupies—is directly affected by the city's use of materials, air, water, and land. A city's ecological footprint is the space needed to support the city, equivalent to the physical and biological regions disturbed to provide resources to the city and accept its wastes. A city's metabolism is the sum total of materials, fuels, water, and goods that flow into a city to sustain its populace and economy. Cities vary in size, from agglomerated urban settings of 5,000 people or more to megacities, agglomerated urban settings with more than 10 million people. Global cities are a subset of megacities, cities that set global economic and cultural trends. All cities influence the environment. Large cities tend to have a disproportionately ...