The future of cities - linear or circular?

As engineers and planners, and human beings, we have an innate desire to improve our society through modifying the built environment and the systems within it. But how can we continue to increase our standards of living, while building sustainable resource-use into our approach?

Globally, our current rate of resource use is unsustainable. This has been well established through decades of analysis in areas such as our ecological footprint (“the number of Earths” that we’re consuming) and by respected think-tanks such as the Club of Rome. However, none of this knowledge has really changed our development trajectory – we’ve greened a bit around the edges, but we’re still using natural resources much faster than they can be renewed.

Looking back

Historically, society operated within a more or less ‘circular’ metabolism; where waste could generally be returned to the land, and nutrients could be recycled into food, wood, and other useful mass goods.

Since large-scale urbanisation, the metabolism of society has become much more linear. Especially in cities where there is an increasing dependence on the rural hinterland for supply of food, construction materials, energy and waste disposal.

As our urban populations grow, hinterland resources are becoming more depleted and their ecological capacity to absorb pollution reduces as the environment becomes overloaded. And with globalisation, the hinterland (particularly for consumer products and services) has come to include the whole world.

What does this mean?

From a resource-use perspective, our current linear approach is unsustainable - the transportation of goods has a huge energy requirements and results in CO2 and other emissions. Furthermore, there is an adverse effect on the hinterland which eventually become repositories for landfills and other forms of pollution; causing cities to look further and further afield for their supply and waste management needs.

The most obvious way to address this imbalance, and to build sustainability into the fabric of society, is to engineer a re-circularisation of the metabolism in a systematic and informed way.

Urban metabolism and material flow analysis

Particularly in Europe and the US, urban metabolism [RN1] and material flow analysis techniques are increasingly being used to quantitatively measure the circularity of an area’s metabolism. These analyses depend on quality data, but provide a quantifiable basis for decision-making.

Urban metabolism looks at all the inputs to a city and reports on consumption of food, water, energy and materials consumption, and outputs of wastewater, solid waste and emissions – usually on a per-capita basis. It can provide a powerful insight into consumption patterns and trends over time, and informs planning for future infrastructure development, as well as benchmarking against similar cities.

Slightly differently but using the same mass-balance perspective, material flow analysis focusses on one specific substance (e.g. zinc or wood) and tracks the sourcing and use of this substance from cradle to grave. This can be useful in determining the fate and environmental impact of the material in question, and shows up inefficiencies or possible opportunities to implement recycling.

So… is a circular societal metabolism really possible?

With existing technology, it would be a stretch for everyone to operate in a fully sustainable manner. For example, even aluminium, one of our most readily recyclable materials, is only ~95% recyclable. So there will always be some losses, meaning that the circular metabolism is in reality more of a spiral of decreasing returns. However, huge amount of resources (and costs) can be saved by doing a lot more to reuse waste streams than we currently are, within our existing knowledge base.

Of course, technology is a big unknown and may surprise us yet, but people are slow to move away from the status quo. The more we can do as designers to integrate sustainable thinking into our projects, the sooner and the bigger impact we can have in bringing our sustainable future to life.

Looking forward

Cities all over the world are already starting to take on a more circular aspect. Europe is leading the way, with comprehensive recycling schemes that keep a large proportion of materials circulating within the urban economy. Japan is using hydroponic vertical gardens to help reduce transport emissions and costs, and provide local, fresher product to consumers.

Advances in energy storage are continually making renewable energy more viable. This includes solar panels which are now widespread in many countries such as Germany and Australia, providing cheap electricity and slowing the need to expand the national grid. Systems like these can provide significant benefits, but often require a longer-term view on return on investment to show off their full potential.

A sustainable society is built on millions of small individual choices. So when I approach a project, I consider how it fits within the vision of a sustainable future and the ongoing metabolism of the system being designing. For example:

  • What are the inputs – electricity, water, materials?
  • What are the outputs – waste products, emissions, wastewater?
  • Is there anything that could be reused or repurposed, either on the site in question or by giving (or even selling) the wastes to another party?
  • Where will matter be accumulating, and is this the best outcome?

 “How are you approaching your projects? Environmental and economic benefits frequently go hand in hand, but recognising where these opportunities exist requires thought and innovation.”


Kennedy, C., Cuddihy, J., & Engel-Yan, J. (2007). The changing metabolism of cities. Journal of Industrial Ecology, 11 (2), 43{59. DOI: 10.1162/jie.2007.1107

Matthews, E., Amann, C., Bringezu, S., Fischer-Kowalski, M., Hüttler, W., Kleijn, R., ... Weisz, H. (2000). The weight of nations: Material outflows from industrial economies (Tech. Rep.). World Resources Institute.

Meijer, M., Adriaens, F., van der Linden, O., & Schik, W. (2011). A next step for sustainable urban design in the Netherlands. Cities, 28 (6), 536-544. (Low Carbon Cities (45th ISOCARP World Congress Porto, Portugal 18-22 October 2009)) DOI: 10.1016/j.cities.2011.07.001

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