Cities and regions will move from linear to circular or closed-looped systems, where substantial amounts of their energy and material needs are provided from waste streams. Eco-efficient cities will reduce their ecological footprint by reducing wastes and reducing resource requirements.The fifth city model is the Eco-Efficient City (read about the first city model, the Renewable Energy City; the second city model, the Carbon Neutral City; the third city model, the Distributed City; and the fourth city model, the Photosynhetic City. A more integrated notion of energy and water as outlined above also entails seeing cities as complex metabolic systems (not unlike a human body) with flows and cycles and where, ideally the things that have traditionally been viewed as negative outputs (e.g. solid waste, wastewater) are re-envisioned as productive inputs to satisfy other urban needs, including energy. The sustainability movement has been advocating for some time for this shift away from the current view of cities as linear resource-extracting machines. This is often described as the eco-efficiency agenda. The eco-efficiency agenda has been taken up by the United Nations and the World Business Council on Sustainable Development, with a high target for industrialized countries of a 10-fold reduction in consumption of resources by 2040, along with rapid transfers of knowledge and technology to developing countries. While this eco-efficiency agenda is a huge challenge, it is important to remember that throughout the Industrial Revolution of the past 200 years, human productivity has increased by 20 000 per cent. The next wave of innovation has a lot of potential to create the kind of eco-efficiency gains that are required. The urban eco-efficiency agenda includes William McDonough’s ‘cradle to cradle’ concept for the design of all new products, and new systems like industrial ecology where industries share resources and wastes like an ecosystem. Good examples exist in Kalundborg, Germany and Kwinana, Australia. The view of cities as a complex set of metabolic flows might also help to guide cities dealing with those situations (especially in the shorter term) where considerable reliance on resources and energy from other regions and parts of the world still occurs. Policies can include sustainable sourcing agreements, region-to-region trade agreements, urban procurement systems based on green certification systems, among others. Embracing a metabolic view of cities and metropolitan areas takes global governance in some interesting and potentially very useful directions. This new paradigm of sustainable urban metabolism (seeing them as complex systems of metabolic flows), will require profound changes in the way cities and metropolitan regions are conceptualized as well as in the ways we plan and manage them. New forms of cooperation and collaboration between municipal agencies, and various urban actors and stakeholder groups will be required, for instance municipal departments will need to formulate and implement integrated resource flow strategies. New organizational and governance structures will likely be necessary as well as new planning tools and methods, for example cities that map the resource flows of their city and region, will need to see how these new data can be part of a comprehensive plan for integrating the green and brown agendas. Toronto has a trash-to-can program, which allows them to capture methane from waste to generate electricity. This not only reuses waste and provides an inexpensive energy source, but captures a significant amount of methane that would otherwise be released in the air. Before it reached capacity in its operation, it is estimated that the Keele Valley Landfill generated three to four million dollars annually, and provided enough power for approximately 24,000 homes (Clinton Climate Initiative best practices, www.c40cities.org/bestpractices/watse/toronto_organic.jsp). One extremely powerful example of how this eco-efficiency view can manifest in a new approach to urban design and building can be seen in the new dense urban neighborhood of Hammarby Sjöstad, in Stockholm. Here, from the beginning of the planning of this new district, an effort was made to think holistically, to understand the inputs, outputs and resources that would be required and that would result. For instance, about 1000 flats in Hammarby Sjöstad are equipped with biogas stoves that utilize biogas extracted from wastewater generated in the community. Biogas also provides fuel for buses that serve the area. Organic waste from the community is returned to the neighborhood in the form of district heating and cooling. There are many other important energy features in the design as well, most importantly perhaps is the close proximity to central Stockholm and the installation (from the beginning) of a high-frequency light rail system that makes it truly possible to live without a private automobile (there are also 30 car-sharing cars in the neighborhood). While not a perfect example, it represents a new and valuable way to see cities, and requires a degree of interdisciplinary and inter-sectoral collaboration in the planning system that is unusual in most cities. Eco-efficiency does not have to involve just new technology it can also be introduced into cities through intensive use of man-power as in Cairo’s famous Zabaleen recycling system (Box 6). There are many other examples of how cities across the third world have integrated waste management into local industries, buildings and food production. What do you think? Leave us a comment. ----- Peter Newman is Professor of Sustainability at Curtin University in Perth, Australia. He is the co-author of Cities as Sustainable Ecosystems, Green Urbanism Down Under, and Resilient Cities: Responding to Peak Oil and Climate Change.