Developing carbon budgets for cities: Phoenix metropolitan region as a case study

Background/Question/Methods

Studies have shown that cities can alter regional carbon dynamics through changing ecosystem productivity, overall carbon cycling rate, and total carbon storage in vegetation and soils.  Furthermore, people in urban regions import a large amount of carbon in food and fuel, as well as release an exceptional amount of CO₂ into the atmosphere. Numerous studies have attempted to quantify some sources and sinks of carbon in urban areas, although a complete carbon budget for a city that accounts for total inputs, outputs, and storage within the ecosystem has yet to be fully accomplished.  One challenge is associated with attaining the data necessary to accurately account for all carbon dynamics in these heterogeneous and complex systems.  Our goal was to estimate a budget for the Phoenix metropolitan area while developing methodology to calculate carbon dynamics in urban systems that can be applied to cities across the US. Only with comparable carbon budgets for multiple cities will we finally begin to understand the influence of urbanization on carbon dynamics. 

Results/Conclusions

Many calculations can be determined from both bottom-up and top-down methodologies, and although these approaches should provide similar results, the data source can restrict how the results are interpreted and the spatial scale at which they are relevant.  Like other studies have suggested transportation is associated with the highest output of carbon from the Phoenix system, and city wide vegetation can do little to offset emissions; however, it is important to recognize there are many other sources and sinks where small changes can accumulate to be a large contribution to reducing our citywide CO₂ footprint.