Understanding the drivers of energy and material flows of cities is important for addressing global environmental challenges. Accessing, sharing, and managing energy and material resources is particularly critical for megacities, which face enormous social stresses because of their sheer size and complexity. Here we quantify the energy and material flows through the world’s 27 megacities with populations greater than 10 million people as of 2010. Collectively the resource flows through megacities are largely consistent with scaling laws established in the emerging science of cities. Correlations are established for electricity consumption, heating and industrial fuel use, ground transportation energy use, water consumption, waste generation, and steel production in terms of heating-degree-days, urban form, economic activity, and population growth. The results help identify megacities exhibiting high and low levels of consumption and those making efficient use of resources. The correlation between per capita electricity use and urbanized area per capita is shown to be a consequence of gross building floor area per capita, which is found to increase for lower-density cities. Many of the megacities are growing rapidly in population but are growing even faster in terms of gross domestic product (GDP) and energy use. In the decade from 2001–2011, electricity use and ground transportation fuel use in megacities grew at approximately half the rate of GDP growth.The remarkable growth of cities on our planet during the past century has provoked a range of scientific inquires. From 1900–2011, the world’s urban population grew from 220 million (13% of the world’s population) to 3,530 million (52% of the world’s population) (
1,
2). This phenomenon of urbanization has prompted the development of a science of cities (
3,
4), including interdisciplinary contributions on scaling laws (
5,
6), networks (
7), and the thermodynamics of cities (
8,
9). The growth of cities also has been strongly linked to global challenges of environmental sustainability, making the study of urban energy and material flows, e.g., for determining greenhouse gas emissions from cities and urban resource efficiency (
10–
19), important.At the pinnacle of the growth of cities is the formation of megacities, i.e., metropolitan regions with populations in excess of 10 million people. In 1970, there were only eight megacities on the planet (
SI Appendix, Fig. S1). By 2010, the number had grown to 27, and a further 10 megacities likely will exist by 2020 (
20). In 2010, 460 million people (6.7% of the global population) lived in the 27 megacities. The sheer size and complexity of megacities gives rise to enormous social and environmental challenges. Megacities often are perceived to be areas of high global risk (i.e., threatened by economic, environmental, geopolitical, societal, and technological risks with potential impacts across entire countries) with extreme levels of poverty, vulnerability, and social–spatial fragmentation (
21–
24). To provide adequate water and wastewater services, many megacities require massive technical investment and appropriate institutional development (
25,
26). Many inhabitants of megacities also suffer severe health impacts from air pollution (
27). However, these factors present only one side; the megacities include some of the wealthiest cities in the world (albeit with large disparities between citizens). Even the poorer megacities are seen by some as potential centers of innovation, where high levels of resource efficiency might reduce global environmental burdens (
21,
28,
29). Whether megacities can develop as sustainable cities depends to a large extent on how they obtain, share, and manage their energy and material resources.The aims of our study are first to quantify the energy and material flows for the world’s 27 megacities, based on 2010 population, and second to identify physical and economic characteristics that underlie these resource flows at multiple scales. This goal entailed developing a common data-collection process applied to all the megacities. The cities were identified based on Brinkhoff’s database of metropolitan regions (
www.citypopulation.de/world/Agglomerations.html;
SI Appendix, Fig. S2). The megacities are essentially common commuter-sheds of more than 10 million people; most are contiguous urban regions, but a contiguous area is not a requirement; for example, the London megacity includes a ring of commuter towns outside the Greater London area. Megacities can spread across political borders. They include large tracts of suburban regions, which can have higher per capita resource flows than central areas (
30,
31). We quantify energy flows for the dominant direct forms of consumption in megacities. A wide and complex range of materials flow through cities; here the focus is on water, concrete, steel, and waste. We show how values of aggregate resource use of all megacities generally are consistent with the scaling laws that have been developed for cities (
5,
6). We then analyze factors correlated with energy and material flow at macro- and microscales; discuss megacities with low, high, and efficient use of resources; and examine changes over time.
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