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The expected urbanization of the planet in the coming century coupled with aging infrastructure in developed regions, increasing complexity of man-made systems, and pressing climate change impacts have created opportunities for reassessing the role of infrastructure and technologies in cities and how they contribute to greenhouse gas (GHG) emissions. Modern urbanization is predicated on complex, increasingly coupled infrastructure systems, and energy use continues to be largely met from fossil fuels. Until energy infrastructures evolve away from carbon-based fuels, GHG emissions are critically tied to the urbanization process. Further complicating the challenge of decoupling urban growth from GHG emissions are lock-in effects and interdependencies. This paper synthesizes state-of-the-art thinking for transportation, fuels, buildings, water, electricity, and waste systems and finds that GHG emissions assessments tend to view these systems as static and isolated from social and institutional systems. Despite significant understanding of methods and technologies for reducing infrastructure-related GHG emissions, physical, institutional, and cultural constraints continue to work against us, pointing to knowledge gaps that must be addressed. This paper identifies three challenge themes to improve our understanding of the role of infrastructure and technologies in urbanization processes and position these increasingly complex systems for low-carbon growth. The challenges emphasize how we can reimagine the role of infrastructure in the future and how people, institutions, and ecological systems interface with infrastructure.

Independent lines of research on urbanization, urban areas, and carbon have advanced our understanding of some of the processes through which energy and land uses affect carbon. This synthesis integrates some of these diverse viewpoints as a first step toward a coproduced, integrated framework for understanding urbanization, urban areas, and their relationships to carbon. It suggests the need for approaches that complement and combine the plethora of existing insights into interdisciplinary explorations of how different urbanization processes, and socio-ecological and technological components of urban areas, affect the spatial and temporal patterns of carbon emissions, differentially over time and within and across cities. It also calls for a more holistic approach to examining the carbon implications of urbanization and urban areas, based not only on demographics or income but also on other interconnected features of urban development pathways such as urban form, economic function, economic-growth policies, and other governance arrangements. It points to a wide array of uncertainties around the urbanization processes, their interactions with urban socio-institutional and built environment systems, and how these impact the exchange of carbon flows within and outside urban areas. We must also understand in turn how carbon feedbacks, including carbon impacts and potential impacts of climate change, can affect urbanization processes. Finally, the paper explores options, barriers, and limits to transitioning cities to low-carbon trajectories, and suggests the development of an end-to-end, coproduced and integrated scientific understanding that can more effectively inform the navigation of transitional journeys and the avoidance of obstacles along the way. Why Urbanization, Urban Areas, and Carbon?In recent years, the relationships between urbanization, urban areas, and the carbon cycle have generated increased interest in research and policy circles for a variety of reasons. We have urbanized our planet to an unprecedented level. The concentrations of infrastructure, economic and social activities, and populations in cities create growing demands for fossil fuels and carbon-intensive materials to build and power domestic services, commercial buildings, industrial processes, telecommunications systems, water COMMENTARY 10.1002/2014EF000258Special Section: Urbanization, carbon cycle, and climate change Key Points:• We need integrated, coproduced approaches to urbanization, urban areas, and carbon • Urbanization uncertainties are of similar magnitude to carbon uncertainties • Lock-ins in urbanization, cities, and carbon constrain low-carbon transitions provision, waste production, travel, and a seemingly endless array of other uses. By 2050, the global urban population is expected to increase from 3.6 billion to over 6 billion, mainly in low-and middle-income countries [United Nations Department of Economic and Social Affairs, 2010]. With urban extent forecast to triple between 2000 and 2030, more urban land e...

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