withdrawn for human use (NRC, 1999b). Of this amount, agriculture consumes 70 percent, much of which is accounted for by irrigation. Pollutants from industrial, agricultural, and urban areas contaminate water, making it less potable and posing health hazards (Steingraber, 1998).

Many of the large cities of the world are near or along coastlines. In the United States, 8 of the 10 largest metropolitan areas are situated along the oceans or the Great Lakes. The development of coastal zones, which puts more people and property at risk from natural hazards, produces extensive land-cover changes and disturbs fragile marine environments. These and other human-induced environmental changes contribute to climate change, loss of biotic diversity, and the reduced functioning of ecosystems (NRC, 2000c).

The USGS has the capability and range of expertise to view much of the biosphere and to appreciate the extent of human alteration of the planet. An important role for the USGS is to use its range of expertise to

Figure 3.1 Historical and projected human population growth (in billions) for less developed and more developed regions, 1950-2050. SOURCE: United Nations, 1999.