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Sustainable development arises from the interplay of many different strategies. These strategies include technological solutions like renewable energy and electric vehicles, economic incentives like taxes and subsidies, legal instruments like prescriptions and standards, and lifestyle changes or new social norms. The group’s central research theme is to assess the possible future implications of different sustainable development strategies.

Policy makers need quantitative information on the possible future effect that sustainable development strategies can have on people, nature, and the industrial system. They also need insights on how the different strategies may impact each other. Those effects that occur only after large-scale implementation or after several decades are of special importance. They are important for policy makers, because they heavily impact the overall contribution of the strategy to sustainable development. They are important for researchers, because they are hard to grasp and require sophisticated modelling and collaboration across disciplines. An example for an effect with a long time horizon is the scarcity of certain metal resources as a consequence of the large-scale distribution of photovoltaics or wind turbines, both of which require large amounts of critical metals to function. An example for interaction between strategies is the net climate benefit of electric vehicles, which heavily depends on the share of renewable energies in the electricity mix of region where it is driven.

To understand the system-wide effects of implementing sustainable development strategies on the large scale and over long periods of time, researchers need methods and models that describe the global industrial system several decades ahead in time. The group for sustainable material and energy flow management and industrial ecology in Freiburg contributes to the development of these models. It applies these models to determine the possible overall potential of energy and material efficiency strategies in end-use sectors like buildings or transportations as well as in different industrial sectors, especially in the metal industries.

The research activities of the group for sustainable energy and material flow management and industrial ecology in Freiburg cover three topics. Below, each topic is briefly described and some of our own contributions are listed. A complete list of Stefan’s publications plus the self-archived manuscripts can be found here, and a list of citations by other authors can be found on Google Scholar.


1) Prospective analysis an assessment of sustainable development strategies, with focus on metal cycles.

 Material production accounts for more than 50% of industrial greenhouse gas emissions, and recycling of metals is therefore an important strategy to reduce greenhouse gas emissions from industry. The extent of recycling, however, is not determined by climate policy but by the amount of available scrap. To estimate the potential contribution of recycling to climate change mitigation, one therefore needs to understand how scrap is generated and what amounts of scrap can be expected in the future. To understand how scrap supply evolves over time one needs to model the turnover of the in-use stocks of metals in buildings, infrastructure, vehicles, consumer products, and machines in great detail and several decades into the future. In our group, we develop and apply models of global metal cycles that comprise the in-use stocks of metals contained in buildings, products, and major industrial assets. These models not only allow us to estimate the future extent of recycling, but also to study the possible future contribution of material efficiency strategies like light-weighting, material substitution, or lifetime extension to global and EU-wide climate targets.

The figure below illustrates the challenge that lies ahead of humanity. Part (A) shows that the richest countries (red) have emitted about 50 tons of CO2 to build up their current in-use stocks of steel, cement, and aluminium. The light blue area corresponds to the CO2 emissions that would occur if all countries would increase their stock levels of those of the richest countries by 2050 using current technology. Part (B) shows that the greenhouse gas emissions associated only with producing the three basic materials cement, steel, and aluminium already make up for about one third of the emissions budget that is left to humanity to reach the 2°C-target. This number only includes material production, no manufacturing or use phase emissions were considered. This estimate contains significant uncertainty regarding actual economic development and technological progress, but it is reasonable to assume that in an emissions-constrained world not all humans can have access to the same levels of material stocks that we see in the richest countries today. The result is a conflict between the wish for further expansion of material stocks to facilitate human development and the constraints set by the climate system. To solve this conflict, society needs to promote and implement material efficiency strategies to decouple the services provided by products and buildings from the materials used, and it needs to increase the share of renewable energy in material production and recycling.



Legend: (A) Greenhouse gas emissions required to build up the 2008 levels of the in-use stocks of cement, steel, and aluminium in different countries, and possible future emissions that would occur if the developing countries would eventually build up the same stock levels as we see today in the richer countries. (B) Emissions required to build up in-use stocks of materials in relation to the total CO2-Budgets associated with the 2°C-target. Source: Carbon Emissions of Infrastructure Development. Müller, Liu, Løvik, Modaresi, Pauliuk, Steinhoff, and Brattebø (2013). DOI: 10.1021/es402618m. (c) American Chemical Society Publications, 2013.  This figure can also be found in a slightly modified version in the Fifth Assessment Report of the IPCC, in the chapter on urbane settlements of the working group III.


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2) Accounting and integrated economic and physical modelling of industrial systems and in-use stocks

Since the establishment of the three-pillar model of sustainable development, there has been continuous tension between its economic, environmental, and social aspects. An important contribution to a more balanced assessment of sustainability and sustainable development strategies, which takes into account all three aspects, is the supply of consistent physical and socio-economic data on human activities and the impact of these activities on the environment.

For example, if a production facility reaches the end of its lifetime, its economic value goes to zero, but its material content remains close to what it was when the facility was new. Estimating the potential for material recycling using economic statistics alone is therefore not possible, and therefore, continuous efforts are made to establish harmonized economic and physical accounts of society’s biophysical basis.

We contribute to the global database on socioeconomic metabolism and in-use stocks by determining in-use stocks of metals in different countries and sectors. We also develop theoretical frameworks for integrated economic and physical bookkeeping of material flows and stocks. Moreover, we use monetary and physical input-output tables to trace material and waste flows through the world economy.

The figure below illustrates the direct and indirect material and energy flows that were necessary to produce all final products that were consumed in Germany in 2007, including the treatment of the waste flows that accrued. Part of the material and energy flows abroad, for example, flows form mining activities in Australia or steel production and manufacturing in China, which are part of the supply chain of final consumption in Germany. On average, every German consumed about 600 kg of food in 2007. Materials like steel, concrete, plastics, or copper, however, represent much larger flows than food consumption. The material cycle of German final consumption is far from being closed, and the in-use and obsolete stocks in households, public buildings and infrastructure as well as industries continue to grow. Total consumption of manufactured goods was about nine tones per capita and year, which is roughly 25 % of the in-use stocks of steel, cement, and aluminium together.



Legend: Aggregated representation of the socioeconomic metabolism an in-use stocks in Germany, 2007. Source: Data were taken from a preliminary and unpublished physical version of the multiregional input-output-table EXIOBASE, which was compiled as part of the EU-CREEA-project.


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3) Theory of environmental systems analysis, integration of existing assessment tools and methods, development of open-source-software and databases

Environmental systems analysis is a young research field, which feeds from several academic traditions and was developed at several places across the globe. The main quantitative assessment methods of the field include life cycle assessment, material and energy flow analysis, and input-output analysis. These methods are conceptually similar, and large synergies between these methods and the underlying data exist. The integration of the different methods into a common framework to study society’s bio-physical basis allows researchers to realize the potential from the synergies. The integrated environmental assessment methods can help us to explore new research questions, make more robust recommendations to consumers, industry, and policy makers, and utilize research time and money more efficiently.

We contribute to the development of a common theoretical framework, which is important part of method integration. Our work also comprises integrated assessment models. It is not purely theoretical, but includes the development of open source software and database structures that span different methods.

An important aspect of our theoretical work on sustainable development is to embed the concepts socioeconomic metabolism, society’s biophysical structures, and human colonization of the Earth’s ecosystems into the larger framework of socioecological systems. The relation between these concepts is illustrated in the figure below.



Legend: Relation between the natural environment, the bio-physical sphere of causation, and the social sphere of causation in the global socio-ecological system (left). The relation between socioeconomic metabolism, society’s bio-physical structures, and human colonization of the Earth's ecosystems (right). Source: Socioeconomic metabolism as paradigm for studying the biophysical basis of human societies, Stefan Pauliuk and Edgar G Hertwich, 2015, Ecological Economics, Volume 119, November 2015, Pages 83–93.


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