Carbon fluxes in urban areas

Although urban areas are widely recognized as important sources of atmospheric carbon dioxide (CO2) little attention has, as yet, been directed to measuring the magnitude of the releases of CO2 nor to quantifying rates of uptake by urban vegetation. Attempts to quantify the role of urban areas on the global carbon budget have focused largely on inventories of emissions (from estimates of fossil fuel consumption, cement production, etc) and sinks (the amount of carbon sequestered in urban vegetation based on biomass estimates). Surface-atmosphere exchanges of carbon dioxide can be measured directly using micrometeorological techniques (eddy covariance equipment mounted on tall towers). This approach has been employed for other ecosystems (notably grasslands, forests and wetlands) as part of the global FLUXNET program and important data is emerging on the role of different ecosystems, their variability, and controls. This paper will present an overview of this approach and discuss some of the inherent problems in urban settings, and then present results of a series of short and long-term measurements conducted in contrasting urban environments: vegetated suburbs of Baltimore, MD (part of the NSF ULTER Baltimore ecosystem study); residential neighborhoods in Chicago, IL; and the densely urbanized core of Marseille, France. Particular attention will be directed to spatial and temporal variability in carbon releases and uptake within and between cities, and the effects of human activities and urban vegetation as controls on release and uptake, respectively.