SUSTAINABLE URBAN SYSTEMS Carbon Footprinting of Cities and Implications for Analysis of Urban Material and Energy Flows Anu Ramaswami, Abel Chavez, and Marian Chertow As we struggle to get our collective arms around the concept of urban sustainability, various ways of under- standing material and energy flows associated with cities have emerged in the literature. Of course, this is not new. Historians have noted that, one hundred years ago New York City was dealing with streets covered Integrating learning from the GHG accounting and MFA communities is essential to ad- vance our understanding of ur- ban sustainability. with horse manure and coal ash. In Europe, concerns about supplying materials to cities were discussed in the early 1900s, and continued (after a hiatus) into the late twentieth century from a new perspective of environmental impact, leading to the development of methods for economy-wide material flow analysis (MFA) and their application to cities (Barles 2010). This method, used in many current studies of urban metabolism, allows for tracking of material inputs, changes in stock, export of goods, and release of waste and pollution; indirect material requirements to support these flows can also be computed. While the MFA methodology also draws on energy analysis and is considered to be readily adaptable to include energy, in practice there is wide variation in the inclusion of embodied energy components. At the start of the 21st century, concerns about climate change prompted several U.S. cities to adopt the U.S. Mayor’s Climate Protection Agreement, 1 and cities began implement- ing community-wide urban energy studies in a bottom-up man- ner for greenhouse gas (GHG) accounting (Bailey 2007). 2 May- ors in the European Union (EU) adopted similar covenants by 2011. 3 Through organizations such as ICLEI, 4 such city-scale GHG accounting efforts spread worldwide, including to cities in the developing world. In the early years, cities took a boundary-limited view, tracking community-wide use of electricity, natural gas, and © 2012 by Yale University DOI: 10.1111/j.1530-9290.2012.00569.x Volume 16, Number 6 transportation fuels used by homes, businesses, and indus- tries within the city and computed associated GHG emis- sions. Since most cities import some share of their electric- ity, the embodied GHGs in electric power generation were intuitively recognized early on. However, cities realized that the issue of imports also extended to other infrastructures. Many cities were using wastewater treatment services provided by a central plant located be- yond their boundary in the larger urban region. The same issues arose with airline travel, where jet fuel is used by large airports serving several surrounding cities. Furthermore, water in many cities is pumped over long distances, requiring energy outside the city boundary to provide this very basic necessity. This observation in turn prompted questions about food, fossil fuels, and other basic ma- terials such as concrete needed for constructing the urban built environment. Thus, as the process of GHG accounting evolved, cities focused on energy initially and then began incorporating the impact of materials use, while in the MFA community, the reverse occurred. Cities began engaging with researchers to examine flows of energy plus water and other essential goods and ser- vices that meet basic needs of water, energy, food, shelter, and mobility for the community as a whole (Ramaswami et al. 2008). Many of these are related to infrastructure provision and are also critical for the economic productiv- ity of cities, and result in what is now represented as a community-wide infrastructure footprint for cities (Chavez and Ramaswami 2012). This infrastructure footprint accounts for GHGs from direct energy use by homes, businesses, and indus- tries within a city, plus the embodied energy and GHGs associ- ated with providing key infrastructures—electricity; fuel; water; food; building materials; airline, commuter, and freight travel; and waste management—to the community as a whole (see figure 1); the method has the advantage that it helps avoid double counting with in-boundary GHGs. During these discussions, questions arose about all the other “stuff’” used in society, not accounted for above as part of www.wileyonlinelibrary.com/journal/jie Journal of Industrial Ecology 783