In 2012, officials in Dubai asserted that their city would rank among the most sustainable metropolises in the world by 2020. About the same time, Washington, D.C., Mayor Vincent Grey trumpeted greenest-city status by 2032. A glimpse of the cities' sustainability plans shows two different approaches to the same goal. For Dubai, it means supplying five percent of electricity photovoltaically and outlawing energy-hog buildings. While Washington also aims for renewable-energy use and efficient structures, it prioritizes cleaning up the Anacostia River and increasing urban agriculture.

In concept, tailoring one city's sustainability initiatives to reflect its climate, culture, and stage of development should benefit all cities—or at least maximize the environmental benefits. But even the best intentions will not necessarily yield positive results. What if Dubai's photovoltaics are sourced irresponsibly upstream? What if urban farming in Washington causes a spike in the insecticides and fertilizers that wash into the Anacostia? Because the causes and effects of environmental management are complex and far from linear, urban-scale sustainability is littered with possible backfirings: electrical vehicles that draw their power from coal-fired plants, local manufacturing initiatives that lead to transit inequity, and so on.