The different life cycle stages typically covered in an LCA each with its own associated environmental exchanges in terms of energy and mass requirements and waste and pollutant emissions

<p><strong>Figure 1.</strong> The different life cycle stages typically covered in an LCA each with its own associated environmental exchanges in terms of energy and mass requirements and waste and pollutant emissions. The gray box indicates that only the use stage is typically accounted for in traditional UM studies and the up- and downstream burdens tend to remain unaccounted for.</p> <p><strong>Abstract</strong></p> <p>Cities now consume resources and produce waste in amounts that are incommensurate with the populations they contain. Quantifying and benchmarking the environmental impacts of cities is essential if urbanization of the world's growing population is to occur sustainably. Urban metabolism (UM) is a promising assessment form in that it provides the annual sum material and energy inputs, and the resultant emissions of the emergent infrastructural needs of a city's sociotechnical subsystems. By fusing UM and life cycle assessment (UM–LCA) this study advances the ability to quantify environmental impacts of cities by modeling pressures embedded in the flows upstream (entering) and downstream (leaving) of the actual urban systems studied, and by introducing an advanced suite of indicators. Applied to five global cities, the developed UM–LCA model provided enhanced quantification of mass and energy flows through cities over earlier UM methods. The hybrid model approach also enabled the dominant sources of a city's different environmental footprints to be identified, making UM–LCA a novel and potentially powerful tool for policy makers in developing and monitoring urban development policies. Combining outputs with socioeconomic data hinted at how these forces influenced the footprints of the case cities, with wealthier ones more associated with personal consumption related impacts and poorer ones more affected by local burdens from archaic infrastructure.