Carbon cycle of urban ecosystems

Abstract

Urbanization is one of the most irreversible and visible anthropogenic forces on Earth. The fraction of urban population is growing and is predicted to reach 70% of the world’s population by 2050. Urban areas are both drivers and recipients of global environmental change. Carbon cycle of urban areas plays an important role in the feedbacks between urban development and global environmental change. On one side it is a driver of the global environmental change, because more than 70% of global CO2 emissions originate in urban areas. On the other side the global and regional environmental changes such as heat waves, water scarcity, and air pollution influence urban carbon cycle. Carbon cycling through natural (e.g., urban vegetation and soils) and anthropogenic components (e.g., buildings, furniture, landfills, etc.) is intrinsically coupled in urban areas. In cities not only green plants take up carbon, but also concrete buildings. Emissions of carbon from vegetation and soils are complemented by emissions from fossil fuel burning. Urban areas have a large variety of pools to store carbon: from vegetation and soil to buildings, furniture, and landfills. The natural and anthropogenic carbon fluxes through urban areas are controlled by common drivers such as climate and urban form. Three issues are identified as the most important ones for understanding and quantification of urban carbon cycle. They include: (i) the lateral flows of carbon between an urban area and its footprint; (ii) responses of urban vegetation to urban climate and pollution, and (iii) interactions between natural and anthropogenic components of the urban carbon cycle. Because both natural and anthropogenic components are equally important for understanding urban C cycle, they have to be considered simultaneously in the design of any observation strategy or numerical model development. Understanding of the urban C cycle and the whole spectrum of its C pools and fluxes would be beneficial not only for scientists, but also for city governments. It can be instrumental in choosing the optimal policy to reduce urban C footprint.