Megacities contribute significantly to national and global carbon dioxide emissions and are an increasingly important element of emissions mitigation efforts. Atmospheric monitoring helps to characterize urban emissions, but carbon dioxide measurements alone cannot distinguish between biogenic and fossil fuel contributions to observed carbon dioxide enhancements. For this study, John B. Miller and colleagues used measurements of carbon dioxide and the fossil fuel tracer 14 carbon dioxide to demonstrate that even for highly urbanized and arid environments such as Los Angeles, the managed urban biosphere contributes significantly to the local carbon budget. These findings highlight the need to understand and quantify urban biospheric carbon dioxide in order to more accurately measure and track fossil fuel–carbon dioxide emissions and the impact of urban greening campaigns, as needed to evaluate and optimize emissions mitigation strategies.