Written by Helmut Haberl and Dominik Wiedenhofer

In this review article, we call them "socio-metabolic research" (SMR). We provide a primer on SMR, trace its ancestors, give an overview on its research traditions, and present some examples for current SMR. The take-home message: moving toward sustainability will require transformations of our collective resource consumption habits, re-arrangements of settlements and infrastructures, as well as of institutions governing patterns of production and consumption.

Recent high-level agreements such as the Paris Climate accord or the Sustainable Development Goals aim at reducing pressures on climate, ecosystems and biodiversity while pursuing key social goals such as the eradication of poverty and hunger or the provision of shelter, health care or hygiene & sanitation. However, many of these goals are approached in isolation and neither trade-offs nor synergies are properly taken into consideration. Real-world progress will therefore require a robust knowledge base derived from systemic approaches that bridge the gaps between social and natural sciences. Socio-metabolic research (SMR), the topic of this review article, is an approach that can help to systematically address these issues through interdisciplinary research on society-nature interaction.

When invited to write the review, we quickly discovered how difficult a task this would be. Where to draw the boundary around a heterogenous, interdisciplinary research community that has emerged from very different roots and works at different spatio-temporal levels and scales, from local to global, from snapshots to centennial timeframes? A community that is now active for several decades, even more than a century if the ancestors are properly acknowledged?

After much deliberation, we decided to start from a "family tree" summarizing the history of SMR, and then distinguish several research traditions using similar concepts or investigating similar problems: urban metabolism, the multi-scale integrated analysis of societal and ecosystem metabolism, biophysical economics, material and energy flow analysis (MEFA), and environmentally-extended input-output analysis (EE-IOA). They all - explicitly or implicitly - assume that social and economic processes need a biophysical basis for their functioning, that the pattern, magnitude and composition of resources used is driving environmental pressures, and that first principles of the natural sciences (e.g. the laws of thermodynamics) are fundamental to understand what's going on. They also all acknowledge that beyond the boundaries set by these first principles, societies, through institutions, actors, norms and power relations, have substantial agency in shaping their respective society-nature interactions and prospects for sustainability.

A comprehensive socio-metabolic perspective on resource use addresses the full range of materials and energy carriers extracted, processed and consumed by society. These are traced through material and energy cycles in socio-economic systems to provide fully mass-balanced assessments from extraction, to consumption and end-of-life waste phases. Efforts are ongoing to systematically integrate land-use change considerations as well as water uses, ultimately to provide integrated biophysical assessments of the ‘resource nexus’, which highlights the many linkages, synergies and trade-offs between materials, energy, land, water and socio-economic goals. Aggregate indicators from SMR such as measures of total materials and energy throughput consumed within a political boundary or across international supply chains, as well as comprehensive measurements of total waste and emissions are widely used in science and policy. For example, UNEP now maintains a global material flow database (see link below), whic provides several headline indicators for measuring progress towards SDGs (8.4., resource efficiency) and 12.2 (sustainable use of natural resources).

http://www.resourcepanel.org/global-material-flows-database

From the burgeoning literature we chose several recent examples to illustrate the analytic power of SMR. We demonstrate how indicators from SMR can be used to underpin attempts to link social and natural sciences when grappling with the Anthropocene concept. SMR already provides comprehensive long-term information on the unmitigated growth of global resource use, the dynamics of material stocks, and all resulting waste and emissions. This enables linking insights on socio-economic development (e.g. economic growth), with its biophysical basis in a comprehensive manner. Our review suggests that so far, attempts to "decouple" growth of GDP from that of resource use, measured in Joules or tons used per year, had limited success. While economic activity (i.e. GDP) rose indeed faster than the use of materials and energy, substantial and persistent reductions in resource use along with continued GDP growth were so far not observed. Hence the suspicion remains, that promoting eco-efficiency - while certainly worthwhile and necessary - will very likely not be sufficient to "bend the curve" towards absolute reductions of energy and materials use, and with them GHG emissions.

Perhaps one of the most daunting recent findings of SMR is that the material stocks created by humans, e.g. buildings, roads, factories, machinery and the like, now have almost the same mass as all plants (counted as dry matter) on the earth's lands. This corroborates what pioneers of sustainability in general and SMR specifically, such as Kenneth Boulding and Hermann Daly, have hypothesized many decades ago: that humanity is moving from an ‘empty world’ with abundant resources and endless possibilities, to a ‘full world’, where environmental limits are prevalent and ecological problems cannot be shifted in time and space anymore. Shockingly to us, these material stocks grew in almost perfect unison with GDP over the last century, indicating that the global economy seems to proportionally rely on ever expanding cities, infrastructures and factories, casting another dim light on efforts for limiting environmental degradation and respecting Planetary Boundaries. In our review, we also find that so far, these material stocks have not received the same level of attention in SMR as yearly flows of materials or energy, as the flows are immediately environmentally relevant: they are extracted from the environment, or released back to the environment as wastes and emissions. In both cases they are directly related with environmental pressures, e.g. from mining, biomass harvest or CO2 emissions.

A large fraction of materials used by humans serve to accumulate material stocks such as roads, railways and buildings. These, in turn, have long-lasting effects on resource use: they require energy to be used, require maintenance, and influence practices of production and consumption. Photo: (c) Helmut Haberl

Stocks are different: except for their area demand, they seem more or less "environmentally innocent", but only at first sight. Indeed they are major drivers of resource use. For example, the share of all materials used by humans to create new stocks has risen from approximately 20% in 1900 to over 50% today. Maintaining material stocks requires more resources, as stocks need to be repaired or replaced. Most material stocks require additional resources to deliver services, e.g. a car only drives when fuel is burned, or a house is only usable with adequate heating, cooling or electricity supply. Patterns of material stocks may lock societies into resource-guzzling practices, such as individual car-based mobility in the case of roads.

Changing social metabolism towards patterns that are compatible with Planetary Boundaries, e.g. by abolishing free-air combustion of fossil fuels or establishing circular material use patterns instead of the current linear ones, will require massive transformations of socio-economic systems. In order to help forging them, we need to understand the "stock-flow-service nexus", i.e. how key services such as shelter, mobility, nutrition, hygiene etc. can be delived in adequate quantities and qualities, while reducing flows to sustainable levels. This will need substantially different material stock patterns than those we have today.