Urban ecosystems and the North American carbon cycle

TLDR

Urban areas now host 75–80 % of North America’s population, and modeling their fossil‑fuel emissions and land‑use changes can generate plausible city‑scale carbon scenarios, aided by existing land‑use/transportation models and growing data on vegetation and soil carbon dynamics. This review examines urban ecosystems as whole systems for carbon balance, focusing on fossil‑fuel emissions drivers and carbon cycling in urban vegetation and soils. The authors synthesize existing urban carbon balance models, atmospheric monitoring, and metabolic studies to evaluate drivers and validate urban carbon fluxes.

Abstract

Abstract Approximately 75–80% of the population of North America currently lives in urban areas as defined by national census bureaus, and urbanization is continuing to increase. Future trajectories of fossil fuel emissions are associated with a high degree of uncertainty; however, if the activities of urban residents and the rate of urban land conversion can be captured in urban systems models, plausible emissions scenarios from major cities may be generated. Integrated land use and transportation models that simulate energy use and traffic‐related emissions are already in place in many North American cities. To these can be added a growing dataset of carbon gains and losses in vegetation and soils following urbanization, and a number of methods of validating urban carbon balance modeling, including top down atmospheric monitoring and urban ‘metabolic’ studies of whole ecosystem mass and energy flow. Here, we review the state of our understanding of urban areas as whole ecosystems with regard to carbon balance, including both drivers of fossil fuel emissions and carbon cycling in urban plants and soils. Interdisciplinary, whole‐ecosystem studies of the socioeconomic and biophysical factors that influence urban carbon cycles in a range of cities may greatly contribute to improving scenarios of future carbon balance at both continental and global scales.