As we struggle to get our collective arms around the concept of urban sustainability, various ways of understanding 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 advance our understanding of urban 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 implementing community-wide urban energy studies in a bottom-up manner for greenhouse gas (GHG) accounting (Bailey 2007).2 Mayors 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 transportation fuels used by homes, businesses, and industries within the city and computed associated GHG emissions. Since most cities import some share of their electricity, 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 beyond 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 materials 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 services 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 productivity 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 industries within a city, plus the embodied energy and GHGs associated 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 infrastructure. What about furniture, backpacks, and other consumer goods? This created a dilemma. Should the furniture's GHG impact be assigned to the consumer in city A, or to city B where energy used in its industrial–commercial production is accounted in the infrastructure footprint?
A focus on households emerged and urban consumption-based footprints were developed to describe the global GHG impact of all goods and services purchased by a city's households.5 Some researchers are exploring using downscaled input-output (IO) tables that will help identify to what extent local businesses (e.g., grocer, dry cleaner, movie theater, and any factories within the city) serve local households—this could help reveal the extent to which local business efficiencies translate to reduced consumption footprints. The downscaling of IO tables, however, has so far been quite challenging; new methods to develop high-quality local IO data are essential to understand local material and energy flows both in the context of households and future business development strategies in the city.
As an alternative, consumption-based footprinting employs the more familiar approach of combining household consumer expenditure surveys with a life cycle assessment describing the production of goods and services in the larger global/national economy. Consequently some local specificity is lost because, while local improvements in household energy use and personal mobility are captured, local improvements in production to serve local consumption are lost within the average production of the global economy. Further, local businesses serving visitors or producing goods for export would also not be visible in a local consumption account.
Despite some loss of local specificity, many cities seek to report consumption-based footprints to educate households about their global impacts, while the infrastructure footprint informs local and regional infrastructure planning for all local homes, businesses, and industries. Cities and urban researchers realize that a community-wide infrastructure footprint and a separate consumption-based footprint enable complementary views of a city as producer and consumer, respectively (e.g., Baynes et al. 2011). The dual view of cities offered by the two different footprints is being standardized in GHG accounting protocols developed by ICLEI-USA6 (in the United States) and the British Standards Institute7 (United Kingdom).
Integrating the discourse that occurred during GHG footprint development can help inform discussions around what trans-boundary and embodied components to include in models of coupled material and energy flows in urban systems that seek to address sustainability. This can also further our understanding of the metabolism of cities.
This discourse raises three overarching questions: First, what flows best describe the environmental impacts of cities? Clearly, direct flows arising from the use of energy, water, and materials within city boundaries are important because they are influenced locally. Inclusion of trans-boundary flows in the form of resource footprints (water, energy, and materials) not only informs understanding of the broader environmental impact of cities, but experience with GHG footprints shows that it also stimulates additional innovative strategies across spatial scale to promote GHG mitigation—for example, supply chain substitutions (e.g., fly-ash concrete), service substitutions (e.g., airline travel with telepresence), changes in consumption (e.g., healthy diets campaigns), and improvements in the larger transboundary infrastructures (e.g., commuter or freight rail). Direct plus indirect material accounting in MFA is likewise useful to describe the global impact of local recycling efforts. For intimately connected resources such as water and energy, inclusion of the embodied energy in water flows and the water embodied in energy flows is particularly important to address the water–energy nexus.
However, even if we could account for all direct and indirect flows of water, energy, and materials to cities, what would we do with all that information? Discussions among the GHG accounting community (figure 1) offer useful guidance on aggregating flows into different footprints that represent cities from infrastructure and consumption perspectives, each with distinct policy levers. Methods are also emerging to classify cities as net producers, net consumers, and trade-balanced, based on energy embodied in exports and imports (Chavez and Ramaswami 2012). Connecting such a typology with MFA can be very useful because the large material stock accumulation in cities inadvertently results in their portrayal as “unsustainable parasites” that do not produce anything useful (Barles 2010). Combining a materials plus energy/economic view of cities enables their environmental impact to be reported in a more nuanced manner reflecting the diverse and vibrant functions that cities play in the world.
What flows are important to address scarcity? For material resources, high-quality local data on direct urban material inflows and outflows can help development of precise estimates of stocks important to address scarcity and supply chain concerns. For fossil fuels that often do not have significant storage within the city, the trans-boundary view offered by community-wide infrastructure footprints will be more important. The case of water is likely mixed. Both water storage within the city (e.g., as groundwater) as well as trans-boundary water flows embodied in various material and energy carriers (fuel, food, electricity, and their own storage outside the city boundary) are likely to be important.
In sum, both the diversity of cities and the diverse nature of resources serving cities are important considerations in the analysis of urban material-energy flows. Integrating learning from the carbon accounting and MFA communities is essential to advance urban sustainability.