Assessing urban low-carbon performance from a metabolic perspective

How to mitigate anthropogenic carbon emissions in cities determines to a large degree whether global temperature targets in this century are to be met. Using 12 cities in or outside China as case studies, we quantified the critical processes of carbon metabolism based on the urban carbon metabolism assessment framework (CMAF) proposed. The differences of sector contribution, and per capita and intensity among carbon throughflows, carbon inflows and carbon emissions were evaluated. Furthermore, we established an indicator system for CMAF consisting of flow-based and structural indicators to compare the low-carbon performances of cities. The results showed that the total carbon throughflow (TCT) and total carbon inflow (TCI) in Chinese cities were 7-12% higher than in European and American cities regarding the manufacturing and services sector on average, but 6-9% lower in the household consumption sector. Beijing, Tianjin, Nanjing and Guangzhou had lower per capita TCT and TCI than in European and American cities such as Paris and Los Angeles, while their carbon intensities were about three times as much. The per capita TCT in a city was found significantly correlated with per capita energy consumption and had a certain correlation with per capita building or housing area. This study found that TCT, TCI and carbon dioxide emission each provided unique information to measure the potential climatic impact of cities. The difference in the ranking of low-carbon performance between the investigated cities was significant both in terms of flow-based and structural indicators. We suggest these assessment indicators of carbon metabolism be integrated into urban resources management to reflect both the decarbonization status and future emission reduction potential more accurately and to provide systemic decision support for achieving the goal of carbon neutrality in cities.