Changing Urban Carbon Metabolism over Time: Historical Trajectory and Future Pathway.

Cities are expected to play a major role in carbon emissions mitigation. A key step in decoupling urban economy from carbon emissions is to understand the full impact of socioeconomic development on urban metabolism over time. Herein, we establish a system-based framework for modeling the variation of urban carbon metabolism through time by integrating a metabolic flow inventory, input-output model, and network analysis. Using Beijing as a case study, we track the historical trajectory of carbon flows embodied in urban final consumption over 1985-2012. We find that while the tendency of increase in direct carbon emission continues within this time frame, consumption-based carbon footprint might have peaked around 2010. Significant transitions in emission intensity and roles sectors play in transferring carbon over the period are important signs of decoupling urban development from carbonization. Our further analysis of driving factors reveals a strong competition between efficiency gains and consumption level rise, showing a cumulative contribution of -584% and 494% to total carbon footprint, respectively. Projection into a future pathway suggests there is still a great potential for carbon mitigation for the city, but a strong mitigation plan is required to achieve such decarbonization before 2030. By bridging temporal metabolic model and socioeconomic planning, this framework fills one of the main gaps between monitoring of urban metabolism and design of a low-carbon economy.