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The Phoenix metropolitan area (Arizona, USA) provides an excellent case study for examining the sustainability of a rapidly growing urban ecosystem in an and region, having grown six-fold in population in 50 years (to 3 million in 2000). The example is important, because the world's urban population will nearly double in 30 years, and most of this growth will occur in warm, and regions. Urbanization has warmed the city by 3 degrees C, increasing heat stress to humans (doubling the number of "misery hours per day"; hours over 37 degrees C), with generally negative impact on humans and their support systems. Water and land management practices have resulted in the accumulation of salts (> 70% of input) and nitrogen (15-20% of input). Accumulation has increased' salt and nitrate concentrations in groundwater and may have increased the salinity of soils in the urban area. Under normal hydrologic conditions, humans may adapt to gradual environmental degradation. However, we hypothesize that the resilience of the system has declined, making it vulnerable to disturbances such as severe droughts.