Energy is a resource of strategic importance for cities, a fortiori global cities that rely on tremendous indirect energy embodied in interregional trades of service industries. The assessment of embodied energy in service industries is thus vital to the committed sustainable development of global cities and fundamental to tailor-made local policymaking. This paper applies Multi-Regional Input-Output analysis to the assessment of total embodied energy in service industries in six global cities, i.e., Beijing, Chongqing, Hong Kong, Shanghai, Singapore, and Tianjin. It is found that Hong Kong and Singapore have relatively lower energy use intensities compared with the four cities located in Mainland China. Service industries consume 17.02∼46.40 % of total embodied energy in the six cities. Coastal cities like Tianjin, Shanghai, Hong Kong, and Singapore have higher proportions of transportation consumption. The four Chinese mainland cities consume a larger proportion of coal than Hong Kong and Singapore. The method and the findings of this paper are expected to facilitate both the government and the industries in energy policymaking for a smarter and more sustainable urban economy.

Rapid urban metabolism is causing many resources to flow from consumption to waste. But many of these wastes could be secondary resources, and cities could become urban mines and an increasing supply of future resources. Hong Kong, one of the most developed and populated cities in the world, has demonstrated a completely metabolic evolution to be an urban mine, since the 1970s. Covering 14 types of e-waste and eight types of end-of-life vehicles, this study first investigates Hong Kong’s evolution as an urban mine. The potential output weight of the urban mine quickly grew from 117 kt in 2000 to 368 kt in 2014, and it is estimated to remain in the range of 300–350 kt over the years 2015–2050, with 40–50 kg/cap/year. The economic potential of urban mining, for 18 metals, plastic, glass, and rubber tires, will be approximately US$2 billion annually, mainly contributed by precious and rare metals. All the obtained results contribute to Hong Kong’s waste management and promise to have positive impact on urban mining and circular economy for other, less-developed cities or regions.

Cities are leading actions against climate change through global networks. More than 360 global cities announced during the 2015 Paris Climate Conference that the collective impact of their commitments will deliver over half of the world's urban greenhouse gas emissions reductions by 2020. Previous studies on multi-city carbon footprint networks using sub-national, multi-region input-output (MRIO) modelling have identified additional opportunities for addressing the negative impacts of climate change through joint actions between cities within a country. However, similar links between city carbon footprints have not yet been studied across countries. In this study we focus on inter-city and inter-country carbon flows between two trading partners in a first attempt to address this gap. We construct a multi-scale, global MRIO model to describe a transnational city carbon footprint network among five Chinese megacities and the five largest Australian capital cities. First, we quantify city carbon footprints by sectors and regions. Based on the carbon map concept we show how local emissions reductions influence other regions' carbon footprints. We then present a city emissions 'outsourcing hierarchy' based on the balance of emissions embodied in intercity and international trade. The differences between cities and their position in the hierarchy emphasize the need for a bespoke treatment of their responsibilities towards climate change mitigation. Finally, we evaluate and discuss the potentially significant benefits of harmonising and aligning China's carbon trading schemes with Australia's cap and trade policy.

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.

Data from urban metabolism studies from eight metropolitan regions across five continents, conducted in various years since 1965, are assembled in consistent units and compared. Together with studies of water, materials, energy, and nutrient flows from additional cities, the comparison provides insights into the changing metabolism of cities. Most cities studied exhibit increasing per capita metabolism with respect to water, wastewater, energy, and materials, although one city showed increasing efficiency for energy and water over the 1990s. Changes in solid waste streams and air pollutant emissions are mixed. The review also identifies metabolic processes that threaten the sustainability of cities. These include altered ground water levels, exhaustion of local materials, accumulation of toxic materials, summer heat islands, and irregular accumulation of nutrients. Beyond concerns over the sheer magnitudes of resource flows into cities, an understanding of these accumulation or storage processes in the urban metabolism is critical. Growth, which is inherently part of metabolism, causes changes in water stored in urban aquifers, materials in the building stock, heat stored in the urban canopy layer, and potentially useful nutrients in urban waste dumps. Practical reasons exist for understanding urban metabolism. The vitality of cities depends on spatial relationships with surrounding hinterlands and global resource webs. Increasing metabolism implies greater loss of farmland, forests, and species diversity; plus more traffic and more pollution. Urban policy makers should consider to what extent their nearest resources are close to exhaustion and, if necessary, appropriate strategies to slow exploitation. It is apparent from this review that metabolism data have been established for only a few cities worldwide, and interpretation issues exist due to lack of common conventions. Further urban metabolism studies are required.

The Hong Kong Special Administrative Region is a highly developed modern city where technical and economic advances have made it possible to support 7 million people on 120 km2 of built-up land, resulting in 58 000 people per km2, one of the highest population densities in the world. This population depends on a continual supply of materials, energy and information to function, and these resources are mainly supplied from outside Hong Kong's own geographical boundaries. The ecological footprint (EF) of Hong Kong due to its direct and indirect consumption of renewable resources and waste generation is presented. Additionally, the paper traces the spatial patterns of Hong Kong's EF and examines the implications of this ecosystem appropriation. The study finds an EF for Hong Kong of about 6 ha per capita, with the largest appropriation occurring for marine ecosystems. If the impacts of fish farming are included, Hong Kong appropriates a marine area nearly 2000 times its own built-up city area. Current resource consumption and waste generation patterns in Guangdong, China — where much of Hong Kong's terrestrial ecosystem appropriation occurs — are also discussed.

Urban metabolism measures quantitatively a city's load on the natural environment. We update the Newcombe et al. (3) pioneering study of Hong Kong's urban metabolism in 1971, highlighting trends in resource consumption and waste generation. Per capita food, water and materials consumption have surged since the early 1970s by 20%, 40%, and 149%, respectively. Tremendous pollution has accompanied this growing affluence and materialism, and total air emissions, CO2 outputs, municipal solid wastes, and sewage discharges have risen by 30%, 250%, 245%, and 153%. As a result, systemic overload of land, atmospheric and water systems has occurred. While some strategies to tackle deteriorating environmental quality have succeeded, greater and more far-reaching changes in consumer behavior and government policy are needed if Hong Kong is to achieve its stated goal of becoming “a truly sustainable city” in the 21st century.

Demographic trends suggest that up to 5000 new cities of half a million in population each may be required between 1975 and 2000. In order to examine the implications of this for demands on resources, a detailed assessment has been made of the patterns of flow and the end-use of energy and of some selected materials in the city-state of Hong Kong. This assessment is used as a basis for extrapolating the future resource requirements of other urban development along similar lines. The calculations indicate that the capital and recurrent energy costs of the predicted urbanization would amount to more than five times the 1973 world consumption of energy.