Citywide physical and virtual carbon balances as significant leverage points for global urban decarbonization

Activities in cities are important drivers of global carbon fluxes. It is crucial to understand direction and magnitude of carbon flows embedded in urban metabolic processes for the decarbonization of cities. Current efforts for carbon flow inventories for cities mostly concentrate on CO2 emissions. In addition to gaseous GHG emissions, the tracking of other physical carbon flows, stocks and sinks of urban settlements under the umbrella of urban metabolism provides another important strand of literature. Yet the two accounting approaches are usually not combined. In this paper, we develop an integrated approach to account for urban carbon flows of 16 global cities. We assess the urban total carbon balance through inflows and outflows of both physical carbon-content of products, which has not yet been released into the atmosphere, and gaseous or virtual carbon associated with urban production and consumption. To illustrate our approach, we develop profiles of carbon balances for 16 global cities by integrating material flow analysis, life-cycle analysis, and input-output analysis. We find a huge carbon divide among cities per capita, ranging from 2.7 t of carbon for Sao Paulo to 12.2 t for Singapore. While a large proportion of imported physical carbon is immediately released as CO2, the remaining 46% to 64% (1.6-3.0 t C/capita) end up in solid waste, stored in durable household goods and other stocks or are exported. Carbon emissions embodied in upstream supply chains account for 33% to 78% of cities’ total carbon footprints. For cities in developed countries embodied carbon emissions are mainly driven by household and government consumption, while for cities in developing nations larger shares of carbon are caused by capital formation, mainly building of infrastructure, but also production for exports. Urban carbon mitigation policies need to keep pace with the changing structure of urban demand.