Social Actors and Key Policy Levers for Mitigating the Greenhouse Gas Footprint of U.S. Cities

AbstractThis article links policy outputs in city climate action plans with environmental outcomes. This task is challenging because different human activities in cities vary in terms of their contributions to greenhouse gas (GHG) emissions and because the engineered infrastructures that support these activities extend well beyond the city scale. I present a generalizable quantitative approach that uses the transboundary infrastructure supply chain GHG emission footprints of cities to identify key actors and policy levers most effective in reducing the global GHG impact of cities. This infrastructure supply chain GHG emission footprint represents the life-cycle energy associated with provisioning key infrastructure services-water, energy, food, shelter, sanitation, mobility, connectivity, and public spaces-to support the activities of households, businesses, and industries in cities.Introduction: Greenhouse Gas Footprints of Cities and Mitigation PlansCities are hubs of human activity; the everyday actions of myriad households, businesses, and industries located within a city's geopolitical boundary. Measuring greenhouse gas (GHG) emissions associated with cities is confounded by the relatively small spatial scale of cities compared with the large-scale engineered infrastructures in which they are embedded; that is, the electricity grid, transportation networks, water-supply lines, and wastewater treatment networks that serve cities. As a result, human activities in cities are highly dependent on transboundary infrastructure provisions, defined formally as the provision of water, energy, food, shelter (building materials), sanitation/waste management, mobility, connectivity, and public spaces to homes, businesses, and industries located within the city (Chavez and Ramaswami, 2013; Ramaswami, in press). In addition to infrastructures, there is also the movement of other goods and services between cities resulting in the flow of embodied GHG between cities. To address these confounding factors, cities have started measuring not only direct energy use and GHG emissions within city boundaries (called a source-based GHG emissions inventory), but also transboundary, life-cycle-based GHG emission footprints of cities that are based upon human activities. First developed for the city of Denver in 2006, infrastructure supply chain GHG emissions footprints combine the life-cycle energy (inboundary and transboundary) associated with provisioning key infrastructure services with communitywide collective use of these infrastructures by homes, businesses, and industries colocated in cities (Ramaswami et al, 2008). Consider for example, water or electricity supply to a city that supports communitywide use of water and energy, respectively.The resulting transboundary infrastructure supply chain footprints have since been tested in more than 20 U.S. cities and show that direct inboundary GHG emissions can contribute less than onehalf of a city's overall infrastructure-related GHG emissions footprint, particularly in cities that import a significant percentage of their electricity, such as Denver (shown in exhibit la). Environmental Protection Agency (EPA) statistics (EPA, 2011) indicate that less than 5 percent of U.S. counties have significant electric power generation infrastructure within their geopolitical boundaries; thus, about 95 percent of U.S. cities import electricity similar to the case of Denver in exhibit la. Even after transboundary GHG emissions associated with imported electric power are allocated to cities, transboundary contributions from the provision of other infrastructures - such as energy supply (fuel) for transportation, fuel supply for the built environment, food supply, construction materials (for example, cement), water supply, and sanitation (wastewater treatment) - can be significant. See exhibit la, in which all the infrastructure sectors are mapped to human activities; for example, the use of cars in a city results in direct inboundary tailpipe emission (shown solid), whereas the transboundary energy to produce fuel used by the vehicles is shown hatched. …