Progress toward low carbon cities: Approaches for transboundary GHG emissions' footprinting

Cities are home to a large proportion of the world's population and as a result, are being recognized as major contributors to global GHG emissions. There is a need to establish baseline GHG emission accounting protocols that provide consistent, reproducible, comparable and holistic GHG accounts that incorporate in-boundary and transboundary GHG impacts of urban activities and support policy intervention. This article provides a synthesis of previously published GHG accounts for cities by organizing them according to their in-boundary and transboundary considerations, and reviewing three broad approaches that are emerging for city-scale GHG emissions accounting: geographic accounting, transboundary infrastructure supply chain (TBIS) footprinting, and consumption-based footprinting. The TBIS and consumption-based footprints are two different approaches that result in different estimates of a community's GHG emissions, and inform policies differently, as illustrated with a case study of Denver, CO, USA. The conceptual discussions around TBIS and consumption-based footprints indicate that one single metric (e.g., GHG/person) will probably not be suitable to represent GHG emissions associated with cities, and it will take a combination of variables for defining a low-carbon city.