Comments on "The energetic metabolism of the European Union and the United States" by Haberl and colleagues - Theoretical and practical considerations on the meaning and usefulness of traditional energy analysis

This commentary responds to the study "The Energetic Metabolism of the European Union and the United States: Decadal Energy Input Time-Series with an Emphasis on Biomass" by Halberl and colleagues, published in this issue. Their article provides an analysis based on a set of data that could be very useful for discussing the sustainability of economic processes in terms of resource flows and societal relations to nature. The authors' choice to adopt a reductionist analysis of the metabolism of societies in energetic terms-that is, an analysis based on a single-scale and single-variable indicator such as "joules of energy input metabolized per year for the whole society"-is a controversial one. Such a choice implies the aggregation of different types of data (referring to nonequivalent categories of energy inputs) into a single overall assessment. That is, in their study the authors are adopting an old and controversial solution for aggregating different types of forms: applying a set of flat conversion factors (calorimetric equivalent) to the different types of energy inputs considered. This commentary discusses the trade-off entailed by any method of aggregation of energy forms of different quality: (i) compression-reducing the number of indices used-versus (ii) relevance-maintaining a diversity of categories needed for the usefulness of the analysis. A brief history of the main strategies adopted, so far, for dealing with the problem of aggregation suggests implications for the approach adopted by Haberl and colleagues.