VIEWPOINT pubs.acs.org/est
Two Approaches to Greenhouse Gas Emissions Foot-Printing at the City Scale Anu Ramaswami,*,† Abel Chavez,† Jennifer Ewing-Thiel,‡ and Kara E. Reeve‡ † ‡
University of Colorado Denver, Colorado, United States ICLEI-Local Governments for Sustainability USA, Boston, Massachusetts, United States an entity and relates these to productivity metrics (e.g., GHG/ GDP). National GHG emissions are often normalized per unit population; however this GHG/capita metric does not truly represent an individual’s global impact, which must add/subtract carbon embodied in national imports/exports. For large nations, e.g., the United States, ∼90% of in-boundary GHG emissions are consumed within the nation; net carbon imports are relatively small at ∼7%.1 For smaller cities, however, where large carbon-intensive electricity inflows occur and where commuter travel is known to be regional (across cities), strict boundary-limited GHG accounting becomes unsuitable.
Rhonda Saunders
C
ommunity-wide greenhouse gas (GHG) accounting is confounded by the relatively small spatial size of cities compared to nations, due to which • Essential infrastructures—commuter and airline transport, energy supply, water supply, wastewater infrastructures, etc.—cross city boundaries, hence energy use to provide these services often occurs outside the boundary of the cities using them. • Significant trade of other goods and services also occurs across cities, with associated embodied GHGs. Consequently, human activity in cities—occurring in residential, commercial, and industrial sectors—stimulates in-boundary GHG emissions occurring within the geopolitical boundary of the community, as well as trans-boundary emissions (occurring outside). Allocating in-boundary and trans-boundary GHG emissions to communities can be achieved using different approaches described below.
’ PURELY GEOGRAPHIC PRODUCTION-BASED GHG ACCOUNTING Purely geographic accounting is exemplified in national-scale inventories and tracks all GHGs emitted within the boundary of r 2011 American Chemical Society
’ GEOGRAPHIC-PLUS INFRASTRUCTURE SUPPLY CHAIN GHG FOOTPRINTS To overcome the limitations of purely geographic approaches, GHG accounting protocols developed for corporations to address their trans-boundary supply chains,2 have been adapted to cities.3 This approach reports direct in-boundary GHG emissions (Scope 1), plus indirect GHG emissions from electricity (Scope 2) and from other key trans-boundary infrastructures serving cities (Scope 3), e.g., airline and commuter travel, water supply, transportation fuels, and other critical supply chains to cities.3 Such geographic-plus accounting yields an expanded infrastructure-based supply chain GHG emissions footprint for a city, considering energy and materials use for its homes, businesses, and industries, taken together as a whole. Utility. The geographic-plus approach is relevant to future infrastructure planning in cities addressing water, energy, and material needs of communities as a whole. The local spatialspecificity of Scope 1 inclusions help link in-boundary energy use and GHG emissions to local air pollution,4 urban heat islands, and local public health impacts, and can facilitate future linkages to emissions of short-lived climate forcers, such as black carbon, that impact climate at subglobal scales. The method is relevant to local infrastructure policies that shape buildings, transportation, and waste infrastructures within city boundaries. The trans-boundary inclusions enable analysis of regional cross-scale and cross-sector infrastructure efficiencies, including regional mass transit, transboundary water delivery, tele-commuting to displace airline travel, and local/regional waste reuse and industrial symbiosis.3 The transboundary inclusions also make visible supply chain risks posed to city economies, e.g., from water constraints on electricity imports. The separation of GHG emissions by Scopes makes the geographicplus method compatible with existing corporate protocols, as well Published: April 26, 2011 4205
dx.doi.org/10.1021/es201166n | Environ. Sci. Technol. 2011, 45, 4205–4206
Environmental Science & Technology
VIEWPOINT
as national inventories, on the basis of which carbon financing, previously reserved for nations, may be allocated directly to cities. Further, the method prevents shifting of GHG emissions “outside” as society transitions to new fuel infrastructures, like hydrogen, with zero tailpipe emissions within the city. Improved metrics are needed for intercity comparisons using the geographic-plus approach. Similar to purely geographic approaches, GHG/capita metrics are not suitable, per se, for intercity comparisons. GHG emissions normalized by residents plus employees in a city may be more appropriate. Alternatively, cities could be compared on a per capita basis only to peer-cities with similar relative proportions of commercial industrial activity relative to residential activity. Thus, a typology of cities as highly producing (industrial or resort communities), trade-balanced, and highly consuming (suburban towns dominated by homes) becomes useful.
’ PURE CONSUMPTION-BASED ACCOUNTING The unique characteristics of communities that makes some develop into resort towns and others into industrial cities, is “evened out” in purely consumption-based GHG accounting, which allocates trade of all goods and services across cities to “economic final demand-consumption” in communities exerted primarily through household consumption.5 In this approach, commercial energy-use in an office building or in a resort is allocated not to the city where these facilities are located but to households in other cities that purchase and consume these services. Most consumption-based accounting uses household expenditure data (e.g., ref 5). More recently, economic input output (I O) tables down-scaled from national data are being tested to track trade across cities, similar to global multi-region input output models.1 Utility. Conceptually, pure consumption-based accounting provides the most rigorous GHG/capita estimate, incorporating trade between cities, including international trade. Such information can empower households and governments to redirect purchasing toward less GHG-intensive sectors and/or regions of the world. However, down-scaled I O tables are unavailable in many nations and at scales smaller than the county-scale, precluding use in smaller communities. Further, financial flows may not match the physical flow of materials and energy. Lastly, local commercial industrial production exported elsewhere is no longer allocated to the local community, even though these activities generate local jobs and can be shaped by local governments to reduce GHG emissions. ’ OUTCOMES A multistakeholder process to develop guidelines for communitywide GHG accounting and reporting for U.S. cities, convened by ICLEI-Local Governments for Sustainability USA, concluded the following: Local governments are uniquely positioned to shape GHGs from household consumption as well as industrial commercial production activities within their communities, even if the latter are exported for consumption elsewhere. Consequently, both geographic-plus and consumption-based approaches can provide different sets of useful GHG footprint information to communities based on their typology (Figure 1). Therefore, ICLEIUSA’s draft Protocol provides multiple reporting frameworks—a basic reporting standard along with optional expanded-impact reports—based upon the geographic-plus approach.3 Consumptionbased reporting is offered as an optional and separate approach in the draft Protocol. Trade-offs between the availability, uncertainty, and cost of gathering requisite input data versus the policy
Figure 1. Comparing GHG emission footprint methods for two U.S. communities. In the metro (Denver), both methods yielded similar aggregate footprints at ∼25 ((10%) mt CO2e/cap. For the resort community (Routt), the consumption-based number remained at ∼25, typical of U.S. households, while geographic-plus (at 40) reflects disproportionately high commercial activity in that community.
insights gained from the two approaches were recognized. Fieldwork comparing both approaches side-by-side is recommended to enhance city-scale GHG accounting in future years.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ BIOGRAPHY Anu Ramaswami is a Professor and Director of the IGERT Program on Sustainable Urban Infrastructure at the University of Colorado Denver, where Abel Chavez is a PhD candidate. Ramaswami’s team has conducted and published peer-reviewed research on city-scale carbon accounting. Jennifer Ewing-Thiel, Director of Tools and Technical Innovation, and Kara Reeve, Technical Innovation Manager, at ICLEI-Local Governments for Sustainability USA, recently convened a multistakeholder process to develop a GHG emissions accounting and reporting protocol for community-scale emissions. ICLEI-Local Governments for Sustainability is a nongovernmental organization with more than 1200 local government members globally committed to climate action and sustainability. ’ REFERENCES (1) Peters, G.; Hertwich, E. CO2 Embodied in International Trade with Implications for Global Climate Policy. Environ. Sci. Technol. 2008, 42 (5), 1401–1407. (2) WRI. The Greenhouse Gas Protocol: A Corporate Accounting and Reporting Standard; World Resources Institute and World Business Council for Sustainable Development: Washington, DC, 2004. (3) Hillman, T.; Ramaswami, A. Greenhouse Gas Emission Footprints and Energy Use Benchmarks for Eight U.S. Cities. Environ. Sci. Technol. 2010, 44 (6), 1902–1910. (4) Jacobson, M. Z. The enhancement of local air pollution by urban CO2 domes. Environ. Sci. Technol. 2010, 44, 2497–2502. (5) Weber, C.; Matthews, H. Quantifying the global and distributional aspects of American household carbon footprint. Ecol. Econ. 2008, 66, 379–391. 4206
dx.doi.org/10.1021/es201166n |Environ. Sci. Technol. 2011, 45, 4205–4206
