THE struggle to slow global warming will be won or lost in cities, which emit 80 percent of the world’s greenhouse gases. So “greening” the city is all the rage now. But if policy makers end up focusing only on those who can afford the low-carbon technologies associated with the new environmental conscientiousness, the movement for sustainability may end up exacerbating climate change rather than ameliorating it. 

While cities like Portland, Seattle and San Francisco are lauded for sustainability, the challenges faced by Phoenix, a poster child of Sunbelt sprawl, are more typical and more revealing. In 2009, Mayor Phil Gordon announced plans to make Phoenix the “greenest city” in the United States. Eyebrows were raised, and rightly so. According to the state’s leading climatologist, central Arizona is in the “bull’s eye” of climate change, warming up and drying out faster than any other region in the Northern Hemisphere. The Southwest has been on a drought watch 12 years and counting, despite outsized runoff last winter to the upper Colorado River, a major water supply for the subdivisions of the Valley of the Sun. 

Across that valley lies 1,000 square miles of low-density tract housing, where few signs of greening are evident. That’s no surprise, given the economic free fall of a region that had been wholly dependent on the homebuilding industry. Property values in parts of metro Phoenix have dropped by 80 percent, and some neighborhoods are close to being declared “beyond recovery.” 

In the Arizona Legislature, talk of global warming is verboten and Republican lawmakers can be heard arguing for the positive qualities of greenhouse gases. Most politicians are still praying for another housing boom on the urban fringe; they have no Plan B, least of all a low-carbon one. Mr. Gordon, a Democrat who took office in 2004, has risen to the challenge. But the vast inequalities of the metro area could blunt the impact of his sustainability plans. 

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. 

Andrew Ross is a professor of social and cultural analysis at New York University and author of “Bird on Fire: Lessons From the World’s Least Sustainable City.”