<b>Gurney</b>, K. R., P. <b>Romero-Lankao</b>, S. <b>Pincetl</b>, M. Betsill, M. Chester, F. Creutzig, K. Davis, R. Duren, G. Franco, S. Hughes, L. R. Hutyra, C. Kennedy, R. Krueger, P. J. Marcotullio, D. Pataki, D. Sailor, and K. V. R. Schäfer, 2018: Chapter 4: Understanding urban carbon fluxes. In Second State of the Carbon Cycle Report (SOCCR2): A Sustained Assessment Report [Cavallaro, N., G. Shrestha, R. Birdsey, M. A. Mayes, R. G. Najjar, S. C. Reed, P. Romero-Lankao, and Z. Zhu (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, pp. 189-228, https://doi.org/10.7930/SOCCR2.2018.Ch4.
Understanding Urban Carbon Fluxes
Dozens of completed or underway studies on urban carbon flux are now reported in the peer-reviewed literature (see Table 4.1). Among these are intensive efforts testing different methods and approaches to understanding flux magnitudes, trends, driving activity, emissions mitigation guidance, and reduction performance tracking. Despite these efforts, consistent and comparable data on carbon fluxes in cities are still lacking, particularly at spatial resolutions below the whole-city level (Kennedy et al., 2015). Greater integration of these studies and greater exploration of whether and how this information can be used by stakeholders are needed. This will require continued efforts in interdisciplinary integration of existing subcommunities engaged in urban carbon research. For example, the use of sometimes singular reliance on atmospheric concentration observations common in inversion studies could move toward an assimilation framework in which all available observational constraints are incorporated with their accompanying uncertainties to arrive at optimized carbon fluxes, further integrating bottom-up and top-down approaches. Equally important are 1) the integration of information on CO2, CH4, and relevant local air pollution and 2) the continued trend toward data with higher space and time resolutions, particularly relevant to urban stakeholders. Finally, integration across ongoing urban studies will provide more insight into which research methods and approaches are successful under differing urban morphologies and social and physical constraints (e.g., urban density, data transparency, and topography). These advances could be achieved in part by integrating existing approaches with remote sensing of urban CO2 and other attributes relevant to the urban carbon cycle.
Urban carbon trends remain difficult to assess because of a lack of compatible and comparable data and limited historical information. Results from a number of intensive studies underway should begin to inform trend information in North America. Improvement to trend detection is critical to the assessment and prognostic capabilities important to urban stakeholders. Integration of urban trend detection with trend activity at larger scales could advance the ability of observing systems to systematically assess urban trends.
Urban carbon fluxes are dominated, directly and indirectly, by the human activities within the built environment that includes large infrastructural systems such as buildings, roads, and factories, along with their co-evolution with fossil fuel energy sources. The carbon fluxes associated with this co-evolved technological system are modulated by underlying climate and socioeconomic dynamics such as consumption, wealth, lifestyles, social norms, governance, and energy prices. A quantitative understanding of these drivers and flux outcomes remains difficult to generalize. This challenge is due to both the emergent properties of urban carbon fluxes and the idiosyncratic nature of cities and the studies performed thus far, which tend to focus on single urban domains. Particularly in Mexico, for example, little work has been accomplished outside the Mexico City metropolitan area. More research is needed that systematically explores multiple urban domains to better understand the relationships between emissions and the physical, social, and technological dynamics in cities.
The urban domain is a source of significant carbon mitigation potential evidenced by the rapid rise in individual urban-scale climate policy efforts. This mitigation, combined with the dominant role that cities play in total anthropogenic carbon emissions, implies that proposed emissions mitigation measures must be tested against documented success in urban areas. The ability of cities to manage carbon fluxes is determined by what control cities can exert over flux sources or their drivers. Cities and their carbon management efforts exist within a larger multilevel governance matrix that can both enable and hinder carbon mitigation efforts. For example, without control over energy supply systems, some cities have limited capability to mitigate emissions.
More targeted research evaluating how specific reductions in emissions are linked to specific policies would enhance the ability to design and implement effective policies in the future. There is limited evidence on the effects of urban climate policy on reducing community-wide emissions, advancing other urban policy goals, or contributing to a transition to low-carbon development. Attributing changes in urban carbon emissions to the actions of city governments also can be challenging, partly because of the complex networks of authority at play. Moreover, there has been little effort to study other effects of urban climate policy, such as cost-effectiveness, co-alignment with other goals and processes, and distributional effects on marginalized populations. Without common frameworks and comparable case studies, the extent to which local or distant political and economic factors shape these outcomes is unclear.
Given the increasing role that urban areas play in the total carbon fluxes within the three North American countries, there is a critical need to improve urban carbon flux projection capabilities in North American cities. Better information on fluxes and their drivers, combined with improved understanding of successful mitigation, would offer researchers and urban decision makers the means to bend urban flux trajectories toward low-carbon pathways. Continued work on the co-benefits and tradeoffs associated with carbon mitigation practices will further enrich carbon emissions planning to account for the important related issues of the UHI, urban air quality, and human health.
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