Impacts of urbanization on regional and global carbon cycling: A complete carbon budget of Phoenix metropolitan region as a case study

Background/Question/Methods Cities support a majority of the world's population and have been shown to have significant influence on carbon cycling and global climate change. To date, analyses of carbon dynamics in urban systems have not been completed at an appropriate scale or are not comprehensive enough to develop useful policies or planning strategies. Although numerous studies have attempted to quantify some sources and sinks of carbon in urban areas, construction of a complete carbon budget has yet to be accomplished. Our goal was to estimate all of the carbon inputs, outputs, and storage pools within the Phoenix metropolitan region, while creating a reproducible methodology applicable to cities across the nation. We used data sources available from the literature, models, and municipal and state databases to construct this budget, and conducted life-cycle analysis of building materials to estimate carbon costs of construction and transportation. Results/Conclusions Carbon inputs include building materials, fuel, food and other consumables. These inputs exceeded outputs, which include energy and transportation emissions, as well as plant and soil respiration, indicating annual accumulation of carbon within the system. Storage in built infrastructure rivals that in vegetation and soils in the Phoenix metropolitan region; however, life-cycle analyses of building materials revealed that any of the benefits associated with long-term carbon storage in hard infrastructure are minimal compared to the carbon dioxide released in their development. Furthermore, our results indicate that fluxes associated with the life cycle of materials imported into the region may outweigh emissions due to transportation and energy use. Similar analyses in other cities are needed before we can identify the major drivers of carbon dynamics in metropolitan regions across the continent and globally, but we expect that the variables affecting carbon flux and storage at a national scale will be very different than those across a city. For instance, drivers like population density may be more important at a local scale, while regional carbon budgets are highly influenced by climate and energy source. We therefore conclude that analyses are needed at multiple scales to truly inform the policy and planning of cities.