Those looking for ecotopia can find pockets of it in the prosperous upland enclaves of Scottsdale, Paradise Valley and North Phoenix. Hybrid vehicles, LEED-certified custom homes with solar roofs and xeriscaped yards, which do not require irrigation, are popular here, and voter support for the preservation of open space runs high. By contrast, South Phoenix is home to 40 percent of the city’s hazardous industrial emissions and America’s dirtiest ZIP code, while the inner-ring Phoenix suburbs, as a legacy of cold-war era industries, suffer from some of the worst groundwater contamination in the nation.

Whereas uptown populations are increasingly sequestered in green showpiece zones, residents in low-lying areas who cannot afford the low-carbon lifestyle are struggling to breathe fresh air or are even trapped in cancer clusters. You can find this pattern in many American cities. The problem is that the carbon savings to be gotten out of this upscale demographic — which represents one in five American adults and is known as Lohas, an acronym for “lifestyles of health and sustainability” — can’t outweigh the commercial neglect of the other 80 percent. If we are to moderate climate change, the green wave has to lift all vessels.

Solar chargers and energy-efficient appliances are fine, but unless technological fixes take into account the needs of low-income residents, they will end up as lifestyle add-ons for the affluent. Phoenix’s fledgling light-rail system should be expanded to serve more diverse neighborhoods, and green jobs should be created in the central city, not the sprawling suburbs. Arizona has some of the best solar exposure in the world, but it allows monopolistic utilities to impose a regressive surcharge on all customers to subsidize roof-panel installation by the well-heeled ones. Instead of green modifications to master-planned communities at the urban fringe, there should be concerted “infill” investment in central city areas now dotted with vacant lots.

In a desert metropolis, the choice between hoarding and sharing has consequences for all residents. Their predecessors — the Hohokam people, irrigation farmers who subsisted for over a thousand years around a vast canal network in the Phoenix Basin — faced a similar test, and ultimately failed. The remnants of Hohokam canals and pit houses are a potent reminder of ecological collapse; no other American city sits atop such an eloquent allegory.