Since 2007, a greater percentage of the global population has been living in urban areas than in rural areas. Increased urbanization is expected to continue, with two-thirds of the world’s population projected to live in urban areas by 2050, a net urban influx of 2.5 billion people (1). Cities today are generally not equipped to address dramatic urban growth and strain on existing infrastructure in a sustainable way, especially with respect to their energy systems.
To be sustainable, cities must themselves, or in the resources that they command, become low-carbon, resilient, and livable (2). Although there can be considerable variation in methods for evaluating the emissions footprint of cities (3), with 54% of the population living in urban areas, it is estimated that cities are currently responsible for 60 to 70% of anthropogenic greenhouse gas emissions (4). The two main strategies for transitioning to a low-carbon city are to shift from fossil fuels to cleaner energy sources and to reduce urban energy consumption levels. The low-carbon transition can be accomplished through energy-efficiency measures, behavioral interventions, and incorporating carbon sinks such as urban parks. Cities and their energy systems should also be resilient to natural and human-made threats (2). The energy systems of cities are increasingly vulnerable to the effects of climate change and extreme weather, including storms, flooding, and sea-level rise, and also to natural and human-induced disasters. In addition, urban energy systems directly affect the well-being and happiness of urban inhabitants. Health conditions, economic competitiveness, cultural appeal, and social, gender, and racial equality are influenced by high-energy sectors such as transportation, food production, and water quality.
Here we evaluate some of the more promising recent technological advancements that could help urban areas become sustainable cities. Many opportunities exist, but focusing on city-integrated renewable energy—defined as distributed, non–fossil fuel energy generated locally in urban areas—has the potential to help cities meet several sustainability needs. Many of these renewable sources increase regional energy independence and can be redundant with other sources, thus increasing resiliency. Although there are several existing barriers to their adoption, solutions will involve increased power densities of renewable energy technologies, improved infrastructure capable of supporting widespread integrated energy generation systems, and increased urban energy efficiency, particularly in the buildings sector.