Urbanization and the carbon cycle: Current capabilities and research outlook from the natural sciences perspective

This paper explores the urban carbon cycle from the natural sciences perspective, identifying key knowledge gaps and priority areas for future research. The combination of large, concentrated carbon fluxes and rapid change makes cities key elements of the carbon cycle and offers the potential for them to serve as “first responders” for climate action. Estimates of urban-scale carbon fluxes are significantly more uncertain than at larger spatial scales, in part because past studies have mostly avoided local/urban scales where the mix of anthropogenic and natural fluxes is complex and difficult to observationally isolate. To develop effective emission reduction policies, we need to understand emission sources and how they may be changing. Such improved quantification and understanding of underlying processes at the urban scale will not only provide policy-relevant information and improve the understanding of urban dynamics and future scenarios, but will also translate into better global-scale anthropogenic flux estimates, and advance our understanding of carbon cycle and climate feedbacks across multiple scales. Understanding the relationship between urbanization and urban carbon flows requires intellectual integration with research communities beyond the natural sciences. Cities can serve as interdisciplinary process laboratories that are sufficiently constrained in both spatial and governance scale to support truly integrated research by the natural sciences, social sciences, and engineering. A thoughtfully crafted science research agenda that is grounded in sustained, dense observations relevant to estimating urban carbon fluxes and their controlling processes and is focused on a statistically significant sample of cities will advance our understanding of the carbon cycle. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made. 1. Motivation The relationship between urbanization, urban areas, and the carbon cycle has received increased attention from the natural science community in recent years driven by several intersecting interests and priorities. Foremost among these is the recognition that urban areas represent the dominant source of energy-related CO2 emissions and a significant share of CH4 emissions, proportions that are expected to climb as the global urban extent and the urban population grow disproportionately in the coming decades [Seto et al., 2012a; United Nations Department of Economic and Social Affairs (UNDESA), 2012]. Furthermore, urbanization constitutes an important land-use and land-cover change process, with impacts on both terrestrial and aquatic carbon pools. The combination of large, concentrated carbon fluxes and rapid change in pools makes cities a large and dynamic element of the global carbon cycle. Between 2002 and 2011, global fossil fuel and cement production emissions averaged 8.3 ± 0.4 PgC yr−1 [Le Quéré et al., 2013], with over 70% of fossil fuel CO2 emissions attributable to urban areas [Energy Information Administration (EIA), 2013]. Annual urban CO2 emissions are more than double the net terrestrial or ocean carbon sinks [Le Quéré et al., 2013]. Understanding and quantifying carbon flows in cities offers a powerful lens into urban ecosystems and provides a compact metric of urban sustainability [Bettencourt et al., 2007; Fragkias et al., 2013].