The water-energy-carbon nexus is complex but important to understand because energy use in the water sector is growing and its importance is under-recognized. The implications of its linkages are also evident in three key contemporary policy objectives, namely, climate change mitigation, energy security, and water security. Past research and our understanding on drivers, processes and implications of this nexus is limited, especially for cities. Cities are key places to analyze this nexus since urban settlements now constitute more than half of the global population, and improving how efficiently cities manage this nexus will greatly aid our sustainability efforts. Water use in cities is typically lower than agricultural and other sectoral uses but its socio-economic importance is high and the embodied energy, and thus carbon, is often very high.

The Global Carbon Project and the Center for Global Environmental Research jointly organized a workshop on 1–2 March 2012 at Tokyo Garden Place Hotel. The workshop was aimed to enhance our knowledge and understanding of the water-energy-carbon nexus in cities, to facilitate much needed interactions amongst key scholars, and to build a network with emphasis on Asian cities where such knowledge is in infancy stage.

The workshop was attended by about thirty experts from Japan, the United Kingdom, the United States of America, Norway, Canada, China, Singapore, India, Thailand, and Australia representing key centers of excellence, and global institutions on the topic such as The University of Tokyo, Asia Pacific Network for Global Change Research, Hiroshima University, National Institute for Environmental Studies Japan, Lawrence Berkeley National Laboratory, University of California Santa Barbara, University of East Anglia, University of Toronto, University of Colorado Denver, Nanyang Technological University, TERI University, Indira Ganghi Institute for Development Research, Chinese Academy of Sciences, Norwegian University of Science and Technology, University of Queensland, Durakaji Pandit University Thailand, and many others. Key questions that were addressed in this workshop were:

• How can we characterize the water-energy-carbon nexus in cities? What are the key indicators? What are the key drivers, processes and implications of this nexus in cities?
• What do we know about the energy and carbon footprints of water systems in cities? What are the frameworks that are in use and what are their strengths and limitations?
• What is our image of water-energy-carbon efficient cities? What are the key opportunities, especially the role of technology and the potential for technological innovations and for behavioral change?
• What are the barriers to and opportunities for water-energy-carbon efficient systems in cities? How do these barriers differ between the types of water management (e.g. public and private)? What are the potential roles/impacts of energy and carbon related prices and other policies in this nexus?
• What are the key commonalities and differences of the above across cities?

In the workshop discussions several indicators were identified to characterize the nexus, but the lack of a framework and scope of the issues were acknowledged. Such a framework needs consideration of factors other than purely environmental, must address system specific issues and local differences reasonably, and needs to address upstream supply chain connected issues. The need for more data and comparative analyses is evident, especially for cities outside the United States, and especially for Asian cities where the implications of the nexus are important given the growth in cities and urbanization as well as rising water use, energy use and carbon emissions. The implications of the nexus are evident in designs of waste water and other urban infrastructure systems and in enhancing the efficiency of inter-connected systems of energy, water and carbon.

It was also acknowledged that drawing the system boundary of the nexus was complex, and an optimized or efficient system for the nexus probably should first aim for energy as a starting point. A systems thinking with the full set of costs and benefits and economies and diseconomies of the scale is needed. While long-term perspectives are needed for an efficient system (short-term thinking leads to sub-optimal solutions), paradoxically, a too long-term thinking has also the danger of investment locking the system into one particular set of assumptions in the rapidly changing technology landscape. For example, the key trend is toward increasing opportunities for decentralization in water and energy. Harvesting the savings from end uses, energized water and from wastewater are the key issues now. Therefore, a careful balance is needed which allows enough flexibility in the long-term planning. Essentially, there is no benchmarking of an efficient city and how cities compare and can or should optimize in given natural, urban metabolic and other settings as of yet.

The participants identified that some of the key barriers for energy-water-carbon efficient cities were lack of data, infrastructure lock-in, physical differences, lack of a system boundary against which to optimize, lack of knowledge of the key links to drivers and how the carbon market and energy price link directly and indirectly in different settings. Further, key barriers are primarily institutional and behavioral, and less technical. However, there are opportunities to create more knowledge to support an efficient system. Some of these are: the use of integrated assessment models, the connection to other system components such as waste and transport, and the opportunity for new modeling tools, such as I-O modeling and hybrid methods, to link indirect and economic connections.

There was a realization that cities differ from one another due to their local contexts. However, many cities are rapidly developing an infrastructure in face of growing urbanization pressures and thus create new opportunities for efficient systems too. It is necessary to develop a set of indicators and knowledge with some common and some specific ones. More knowledge is needed reflecting the local situation for drivers, processes and system boundaries reflecting the challenges and opportunities for guiding energy-water-carbon efficient cities.

The summary of the discussion mentioned above created an understanding amongst the participants to work together to bridge the key gaps in knowledge, especially for Asian cities. Several avenues for collaborations were discussed in the meeting and suggestions for enhancing the network of scholars were made in this workshop. Some of the possible outcomes from this to be further coordinated were identified as:

• Development of meta-analyses of the state-of-art knowledge so far,
• Possibility to develop special issues in journals to consolidate ongoing research activities and knowledge and to further engage the scholarly community,
• Conducting a set of well-coordinated case studies across cities following a common protocol and quantification framework and jointly approaching grant-making entities to enable such activities.

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