Reducing Urban Greenhouse Gas Footprints

Cities are economically open systems that depend on goods and services imported from national and global markets to satisfy their material and energy requirements. Greenhouse Gas (GHG) footprints are thus a highly relevant metric for urban climate change mitigation since they not only include direct emissions from urban consumption activities, but also upstream emissions, i.e. emissions that occur along the global production chain of the goods and services purchased by local consumers. This complementary approach to territorially-focused emission accounting has added critical nuance to the debate on climate change mitigation by highlighting the responsibility of consumers in a globalized economy. Yet, city officials are largely either unaware of their upstream emissions or doubtful about their ability to count and control them. This study provides the first internationally comparable GHG footprints for four cities (Berlin, Delhi NCT, Mexico City, and New York metropolitan area) applying a consistent method that can be extended to other global cities using available data. We show that upstream emissions from urban household consumption are in the same order of magnitude as cities’ overall territorial emissions and that local policy leverage to reduce upstream emissions is larger than typically assumed.


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Urban GHG accounting approaches and terminologies
Three broad approaches to urban GHG emissions accounting can be distinguished 16 : higher than for territorial accounts, but can be limited by reporting explicitly which consumption sectors are included and by presenting sectoral results in an available standardized disaggregation (household consumption, government consumption, gross fixed capital). In our paper we suggest that a combination between the multi-regional input-output model EROA, which has the highest available sectoral and regional resolution, and a focus on urban household consumption currently represents the best option for near term, widely applicable and comparable urban GHG footprints. 8 Territorial plus upstream emissions of key infrastructures: This approach was first proposed in 17 and applied to Denver, Colorado. It starts from territorial urban emission accounts and adds upstream emissions associated with various key infrastructures and materials that serve the city but occur outside the city's territory, such as e.g. airports, commuter travel, and upstream emissions of materials imported to the city (such as water, fuels, cement etc.). In this approach the added upstream emissions are often called extraterritorial or trans-boundary emissions.
The resulting composite indicator was originally termed "transboundary infrastructure supply chain footprint", but other names can be found in the literature 18,19 Following the published literature we can see that the allocation models used for estimating the upstream, extraterritorial emissions vary from city to city, as well as the included materials and infrastructures. Later extended indicators, such as e.g. "production based footprint" and "purely production footprint" have been proposed 19 . Clearly many more indicators could in principle be constructed, and some might be useful for local politicians. The main sources of uncertainty of the territorial plus infrastructures approach are the same as for territorial and consumption based accounting, but the lack of a comprehensive and consistent system boundary definition (i.e. a comprehensive list of infrastructures and materials to be included) adds additional uncertainty. Despite the fact that the consumption approach and the territorial plus infrastructure approach lead to composite indicators which are both denoted as "footprint", the conceptual differences between the two are so fundamental that a meaningful comparison of the numerical results is not possible.

Urban GHG reporting
Urban GHG reporting protocols fall into three camps, using various versions of the principle accounting approaches described above and different terminologies. http://www.c40.org/other/gpc-dashboard]. Concluding, we can state that while these initiatives recognize the importance of upstream emissions and of policy collaboration across jurisdictions, they rarely count, report, or expose a complete picture of those emissions, when evaluating cities' progress.
To the extent that these initiatives' theory of change is correct -that counting, transparency about plans and achievements do create pressure and competition for city leaders to mitigate GHG -they are essentially seeking an incomplete, potentially misleading change. Ignoring upstream emissions from household consumption also runs the risk of enabling misleading correlations. As a simple thought experiment, suppose that wealthier cities are first to undertake infrastructure changes that significantly lower Scope 1 & 2 emissions. The correlation between wealth and the headline "GHG emissions" would be negative even as the GHG footprints of these cities could still be increasing.