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Vincent Moreau and Guillaume Massard

The concept of metabolism takes root in biology and ecology as a systematic way to account for material flows in organisms and ecosystems. Early applications of the concept attempted to quantify the amount of water and food the human body processes to live and sustain itself. Similarly, ecologists have long studied the metabolism of critical substances and nutrients in ecological succession towards climax. With industrialization, the material and energy requirements of modern economic activities have grown exponentially, together with emissions to the air, water and soil. From an analogy with ecosystems, the concept of metabolism grew into an analytical methodology for economic systems. Research in the field of material flow analysis has developed approaches to modeling economic systems by assessing the stocks and flows of substances and materials for systems defined in space and time. Material flow analysis encompasses different methods: industrial and urban metabolism, input–output analysis, economy-wide material flow accounting, socioeconomic metabolism, and more recently material flow cost accounting. Each method has specific scales, reference substances such as metals, and indicators such as concentration. A material flow analysis study usually consists of a total of four consecutive steps: (a) system definition, (b) data acquisition, (c) calculation, and (d) interpretation. The law of conservation of mass underlies every application, which implies that all material flows, as well as stocks, must be accounted for. In the early 21st century, material depletion, accumulation, and recycling are well-established cases of material flow analysis. Diagnostics and forecasts, as well as historical or backcast analyses, are ideally performed in a material flow analysis, to identify shifts in material consumption for product life cycles or physical accounting and to evaluate the material and energy performance of specific systems. In practice, material flow analysis supports policy and decision making in urban planning, energy planning, economic and environmental performance, development of industrial symbiosis and eco industrial parks, closing material loops and circular economy, pollution remediation/control and material and energy supply security. Although material flow analysis assesses the amount and fate of materials and energy rather than their environmental or human health impacts, a tacit assumption states that reduced material throughputs limit such impacts.


Input–Output (I–O) models and analysis were originally conceived by the Nobel Prize winner Wassily Leontief in the 1930s as a tool that can be used by economists and economic policy makers to help in their decision process. The I–O models provide a “picture” of how the economy works, that is, what are the necessities to produce goods and services, how this production generates income, profits and taxes, and how this income is spent. In a simplified way the I–O models can be seen as the model implementation of the economy circular-flow diagrams usually shown in economics introductory courses. Associated with the theory behind I–O models and analysis, I–O tables contain the empirical information necessary to implement these models and theory. Taking, for example, the production of computer screens: • On the production side, the I–O models have information on: (a) how much is spent on the inputs, goods and services necessary to produce the screens; (b) whether these inputs have their origin in the domestic market or are imported; (c) how much was paid in tax to the government; (d) what was the total amount paid in wages and salaries; (e) what were the profits of the producing firms; (f) how many computer screens are sold on the domestic market or on the international market (exported); and (g) whether they are sold directly to the final consumer or are used as a production input, that is, incorporated into other goods, for example, a refrigerator with a computer screen; • On the demand side, the I–O models, taking into consideration the total income received by the different players in the economy, that is, households, firms, and government, have information on: (a) how the income of these players is spent on goods and services, and whether it is used for consumption or investment; (b) whether these goods and services were produced domestically or abroad (imported); and (c) how much consumer tax was paid. From the aforementioned structure of I–O models, and using economic mathematical models, it is possible to measure the direct and indirect inputs needed to produce goods and services in the economy, for example, to produce a car there is no need for agricultural goods as a direct input for production, but the fabric used in the car seats or on the car carpets could have come from cotton, which is an agricultural good, so, cotton is an indirect input used in car production. I–O models, by their capability to show a complete picture of the economic system, and tracing of the origin of direct and indirect inputs used in the production process, can be used in environmental studies by linking economic and environmental variables, on the production and consumption sides. From the production side it is possible to measure, by considering the direct and indirect inputs used, how many natural resources were used and how much pollution was generated in producing the goods and services. On the demand side it is possible to measure the environmental variables, natural resources, and pollution, embodied in the goods and services consumed in the economy. Expanding I–O models to a global scale, that is, using inter-country I–O models, it is possible to measure the environmental impacts, and contents, of the goods and services by country of origin of production and by countries of consumption.