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42 ON TERRITORIAL METABOLISM INTRODUCTION City administrations from Beijing, Amsterdam, Paris, and Brussels have in common that they claim to use the principles of “circular economy” as their compass for navigating through economic and environmental challenges. The policy roadmaps they produce in this context often describe the circularization of urban systems in terms of their “metabolism.” The two central concepts underpinning these policies—the urban “metabolism” and its “circularization”—are, however, often very poorly characterized. Regarding the notion of “metabolism,” the metabolic overviews that have been commissioned by metropolises are largely confined to discussing flows to, from, and within a given urban agglomeration (Ecores et al., 2015). In most cases, this is achieved through quantitative accounts in the form of Material and Energy Flow Analysis (MEFA) or Sankey diagrams. The intensity of flows is, however, not the only dimension of a city’s metabolism. The first objective of this article is to discuss the implications of two other dimensions of urban metabolism: the spatial structure in which these flows are organized and the socio-technical agents that govern them. However, current uses of the notion of “circularization” are also questionable. Arnsperger and Bourg (2017) recently pointed out that many of the policies and promises churned out by governments, consultancies, and corporations are, in fact, not “authentically” circular. Although they are steeped in the language and ideology of economic growth, these circular economy initiatives may eventually fall short of expectations. As an extension to the critical stance of Arnsperger and Bourg, our second objective is to ask about the theoretical implications of circularization for the intensity, spatial structure, and socio-technical agents of urban metabolism. A better understanding of the different dimensions of urban metabolism and their circularization is not only of theoretical interest; we argue that issues of intensity, spatial structure, and socio-technical agents are also relevant in the practical context of making plans and strategies aimed at improving metabolic flows in the urban landscape. To be sure, previous research on planning for circular economy, and in particular contributions based on research by design (Grulois et al., 2015), have already touched upon all three of the dimensions of urban metabolism that we highlight in this paper. However, a critical approach that frontally and explicitly addresses the multidimensional character of (circular) urban metabolism is still lacking in the literature. The concluding section will return to the practical relevance of the theoretical considerations developed in this paper by assessing their implications for planning and design. INTENSITY The intensity of stocks and flows of water, construction materials, nitrogen, food, fuel, final products, municipal waste, etc. is arguably the most explored aspect of urban metabolism in industrial ecology and neighboring fields (Weisz and Steinberger, 2010). The analysis of metabolic intensity relies on quantitative indicators such as the primary and final consumption within a given territory. The literature has also developed tools that bring several quantitative indicators of metabolic intensity together, such as Material and Energy Flow Analysis (MEFA), Life Cycle Analysis (LFA), or Sankey diagrams. These approaches have the merit of allowing more systemic analyses of the relationships between different flows (Haberl et al., 2004). Following quantitative indicators over time has led to the observation that the flows of many substances have intensified in most cities over the XIX th and XX th centuries (Barles, 2015; McNeill, 2001).

43 On the Circularization of Territorial Metabolism How might circularization affect the intensity of urban metabolism? Few advocates of circular economy have addressed this question directly. To be sure, many would agree that the intensity of dangerous or otherwise clearly undesirable flows should be reduced. McDonough and Braungart (2002), for instance, propose to phase out all toxic materials, i.e., substances that pose a more or less immediate threat to living organisms. But what about the intensity of all other flows, say, the urban throughput of aluminum, polyethylene, or timber? Does the possibility of recycling or reusing these materials mean that we should not be concerned with the intensity of these flows? The answer to this question marks a clear divide between, on the one hand, those who see circular economy as a “Third Industrial Revolution” harboring the prospect of renewed economic growth and those, on the other hand, who argue that the circularization of material flows necessarily entails a drastic reduction of their intensity. The biggest and loudest driver among the former group has been the Ellen MacArthur Foundation, a lobbying organization that misses no opportunity to tell the world’s largest corporations that they can grow bigger and faster by embracing the principles of circular economy. Among the leading voices taking a more critical stance are Christian Arnsperger and Dominique Bourg, whose recent work summarizes convincing arguments to then conclude that an “authentically circular economy” is incompatible with strong economic growth (2017). Indeed, even the most well-designed system of circular flows will inevitably give rise to losses and waste. Some of these can be traced back to unsurmountable constraints such as the second law of thermodynamics that heterodox economists and system thinkers have identified as ultimate limits to unrestricted economic growth (Georgescu-Roegen, 1971; Daly, 1996). In a context of growing throughput, even small increases in entropy will ultimately lead to the complete depletion of raw materials. The seminal analysis by Grosse (2010) provides clear examples of this phenomenon. Take, for instance, the case of steel, which is one of the materials with the highest recycling rate in the world (currently around 62% globally according to Grosse). Even the systematic, widespread, and relatively cheap recycling of steel by no means precludes the depletion of iron ore. First, as much as 38% of steel production is not recycled and lost due to some form of entropy. And second, even if the entire production of steel could be recycled, this would not suffice to keep up with the rising demand for steel, which in turn is due to economic growth. The combined effect of entropy and growth sheds a rather pessimistic light on the efficacy of recycling efforts. In the case of steel, the calculations by Grosse—assuming that the global production of steel continues to grow at an average of 3.5%—suggest that the overall cumulative effect of steel recycling postpones the depletion of iron ore by only twelve years. In other words, if the world completely stopped recycling steel today, the stock of iron ore would be in 2050 at the level it would have reached in 2062 with recycling. What is more, the benefits of circular business practices could be more than offset if they were to free up materials for other uses (“rebounding effects”), or allow materials to be exploited over a longer but nevertheless finite time period (“reporting effects”). To the extent that economic growth and material throughput continue to be highly correlated, at least on larger scales, the critical stance developed by Arnsperger, Bourg, and others offers a sobering message: the circularization of the urban metabolism not only implies purging toxic materials but also a general reduction of throughput intensity of all other substances whose reproduction cannot keep up with the pace of economic growth. This calls for reducing the throughput of virtually