Greenhouse gas (GHG) accounting for individual cities is confounded by spatial scale and boundary effects that impact the allocation of regional material and energy flows. This paper develops a demand-centered, hybrid life-cycle-based methodology for conducting city-scale GHG inventories that incorporates (1) spatial allocation of surface and airline travel across colocated cities in larger metropolitan regions, and, (2) life-cycle assessment (LCA) to quantify the embodied energy of key urban materialssfood, water, fuel, and concrete. The hybrid methodology enables cities to separately report the GHG impact associated with direct end-use of energy by cities (consistent with EPA and IPCC methods), as well as the impact of extra-boundary activities such as air travel and production of key urban materials (consistent with Scope 3 protocols recommended by the World Resources Institute). Application of this hybrid methodology to Denver, Colorado, yielded a more holistic GHG inventory that approaches a GHG footprint computation, with consistency of inclusions across spatial scale as well as convergence of city-scale per capita GHG emissions (∼25 mt CO 2 e/person/year) with state and national data. The method is shown to have significant policy impacts, and also demonstrates the utility of benchmarks in understanding energy use in various city sectors.

A hybrid life cycle-based trans-boundary greenhouse gas (GHG) emissions footprint is elucidated at the city-scale and evaluated for 8 US cities. The method incorporates end-uses of energy within city boundaries, plus cross-boundary demand for airline/freight transport and embodied energy of four key urban materials [food, water, energy (fuels), and shelter (cement)], essential for life in all cities. These cross-boundary activities contributed 47% on average more than the in-boundary GHG contributions traditionally reported for cities, indicating significant truncation at city boundaries of GHG emissions associated with urban activities. Incorporating cross-boundary contributions created convergence in per capita GHG emissions from the city-scale (average 23.7 mt-CO(2)e/capita) to the national-scale (24.5 mt-CO(2)e/capita), suggesting that six key cross-boundary activities may suffice to yield a holistic GHG emission footprint for cities, with important policy ramifications. Average GHG contributions from various human activity sectors include buildings/facilities energy use (47.1%), regional surface transport (20.8%), food production (14.7%), transport fuel production (6.4%), airline transport (4.8%), long-distance freight trucking (2.8%), cement production (2.2%), and water/wastewater/waste processing (1.3%). Energy-, travel-, and key materials-consumption efficiency metrics are elucidated in these sectors; these consumption metrics are observed to be largely similar across the eight U.S. cities and consistent with national/regional averages.

A case study of Denver, Colorado explores the roles of three social actors-individual users, infrastructure designer-operators, and policy actors-in near-term greenhouse gas (GHG) mitigation in U.S. cities. Energy efficiency, renewable energy, urban design, price- and behavioral-feedback strategies are evaluated across buildings-facilities, transportation, and materials/waste sectors in cities, comparing voluntary versus regulatory action configurations. GHG mitigation impact depends upon strategy effectiveness per unit, as well as societal participation rates in various action-configurations. Greatest impact occurs with regulations addressing the vast existing buildings stock in cities, followed by voluntary behavior change in electricity use/purchases, technology shifts (e.g., to teleconferencing), and green-energy purchases among individual users. A portfolio mix of voluntary and regulatory actions can yield a best-case maximum of ~1% GHG mitigation annually in buildings and transportation sectors, combined. Relying solely on voluntary actions reduces mitigation rates more than five-fold. A portfolio analysis of climate action plans in 55 U.S. cities reveals predominance of voluntary outreach programs that have low societal participation rates and hence low GHG impact, while innovative higher-impact behavioral, technological, and policy/regulatory strategies are under-utilized. Less than half the cities capitalize on cross-scale linkages with higher-impact state-scale policies. Interdisciplinary field research can help address the mis-match in plans, actions, and outcomes.