Progress toward low carbon cities: approaches for transboundary GHG emissions’ footprinting

Abstract

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.

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Corrigendum

Figure 1.  Transboundary infrastructure supply chain footprint for the city and county of Denver.

Shows GHG emissions from direct in-boundary energy use (solid) and embodied energy in transboundary infrastructures (hatched). Per capita GHG emissions are within 10% of national per capita estimates showing boundary effects are largely overcome with the inclusion of these essential transboundary infrastructures.

TBIS: Transboundary infrastructure supply chain; WW: Wastewater.

Adapted with permission from [7].

Figure 2.  Denver’s GHG emissions footprint for 2005 and 2007, estimated using the transboundary infrastructure supply footprint method.

Both community-wide and per capita GHG emissions decreased despite the population increase of 0.95%. (A) Community-wide GHG emissions decreased by 1.1%; (B) Per capita GHG emissions decreased by 3.2%.

Figure 3.  Transboundary infrastructure supply and consumption-based footprinting.

Solid outline represents community boundary. Inflow arrows represent material and energy inputs into the community. Outflow arrow represents exports from the community (A) transboundary infrastructure supply footprint keeps the community together, accounting for all GHG emissions, and (B) consumption-based footprint divides the community, not accounting for GHG emissions from exports.

Figure 4.  Preliminary results for Denver’s consumption-based GHG emissions.

(A) Illustrating GHG emissions in various economic activity sectors (y-axis) representing final consumption in 2008, and sorted by scopes; (B) Consumption-based GHG emissions are mapped as in-boundary (solid) and transboundary energy use (hatched) serving final consumption.

Note: electricity imports are included within in-boundary energy use.

^†Serving final consumption.

FC: Final consumption; HH: Household; SUV: Sport utility vehicles; WW: Wastewater.

Figure 5.  Linking the methodological approaches described in this article to the reporting frameworks described in the ICLEI-USA draft protocol.

In 2007, for the first time in human history, more than half of the world’s population was living in urban settings [1]. According to the UN, the world’s urban population is projected to increase from 3.3 billion people in 2007, to 6.4 billion people in 2050. By 2025, 57% of the world’s population will be located in urban settings, and by 2050, this number will rise to 70%. The USA is also witnessing large rates of growth in urban population, particularly in western states such as Colorado and Arizona. US metropolitan areas are home to 80% of the total national population of 300 million people [2]. The percentage of the US urban population is forecasted to reach 86% by 2025, and 90% in 2050 [1].

Cities are are thus being recognized as major contributors to global GHG emissions [3,4], as well as a critical part of the solution, addressing both GHG mitigation and climate risk adaptation [5]. Cities worldwide have signed the Kyoto protocol, pledging to reduce GHGs by 7% by 2012 from 1990 baseline levels [6]. More recently, the mayor of Mexico City, Marcelo Ebrard, and International Council for Local Environmental Initiatives (ICLEI), convened 138 cities and signed the Global Cities Covenant on Climate – otherwise known as the Mexico City Pact. The pact promises to have cities report on their respective GHG emissions and climate mitigation activities [101]. In the USA, 1044 mayors have also committed their communities into some type of GHG mitigation [102]. However, such treaties must establish baseline GHG emission inventories that provide consistent, reproducible, comparable, and holistic (in-boundary and transboundary) GHG emission accounts with support for policy intervention.

There are three primary challenges in holistic GHG accounting at the city-scale that considers the full impact of urban activities on global GHG emissions. The three challenges are [7–9]:

	▪ Owing to the relatively small spatial scale of cities, important human activities such as commuter travel and air travel, are artificially truncated at the city’s geographic boundary;
	▪ Cities are also served by transboundary infrastructures such as electric power plants, oil refineries and pipelines, that extend beyond city boundaries;
	▪ Finally, beyond infrastructures, there are significant exchanges (i.e., trade) of goods and services across boundaries.

Owing to the above three transboundary phenomena, it is being increasingly recognized that human activities in cities can stimulate emissions within their geographic boundary, as well as those outside (i.e., transboundary GHG emissions). Thus, measuring only energy use and GHG emission strictly within a city’s boundary can provide an incorrect and even misleading picture. In some cases, establishing a purely geographic measurement approach may create unintended incentives to simply move GHG emissions outside the boundary. As society considers new technologies, design strategies and policies for low-carbon cities, it is imperative that we have clearly defined methods for the holistic measurement of GHG emissions associated with cities, addressing both in-boundary and transboundary emissions.

In the past, the complexity of dealing with in-boundary and transboundary emissions has led to various inconsistent ways of GHG accounting at the city-scale. The objective of this article is twofold:

	▪ To provide an overview of past literature on GHG accounting, listing the in-boundary and transboundary inclusions.
	▪ To highlight two leading theoretical and emerging approaches for GHG emissions footprinting at the city scale, incorporating transboundary inclusions.

Where other works discuss GHG footprinting methods in a general sense, the added value of this article is in the exemplification of GHG footprinting methods and results from actual data from a Denver, Colorado case study.

We conclude by briefly discussing how ICLEI-USA is incorporating these leading approaches into the community-scale GHG emissions accounting and reporting protocol [10], which is a framework being developed to help standardize GHG emissions accounting in US cities. The value added of this article is the use of real case study data. Wright et al. provides a good overview of the theoretical framework for allocating GHG emission in cities, although it lacks actual case studies [11]. The same Wright et al. article provides an incomplete review of the literature as it misses key articles (e.g., Ramaswami et al.[7] and, Hillman and Ramaswami [8]).

Review of city-scale community-wide GHG emissions measurement

The lack of a standardized method for city-scale GHG emissions accounting to date has produced inconsistent accounting approaches for cities throughout the world. This inconsistency is seen both in the wide variation of inclusions in city-scale GHG emissions accounting in the peer-reviewed literature, and lack of explicit statements on what the unit of analysis is (i.e., who is the accounting being conducted for; is the unit of analysis, household consumption or community-wide energy use?).

An initial lack of clarity (or confusion) arises between GHG measurements produced for city municipal governments and those that attempt to measure cities as a whole (i.e., whole communities that comprise a city). This issue is easily dealt with by referring to the city-government emissions as local government operations [12], while the citywide analysis can be referred to as community-wide GHG accounting. This article is solely concerned with community-wide GHG accounting for cities; however, we mention this distinction to develop a consistent vocabulary in the literature going forward. Moreover, even the term ‘community-wide GHG accounting’ does not clearly address who the GHG accounting is being done for (and what the unit of analysis is). Sometimes it is carried out for the entire community encompassed within a city’s geopolitical boundary (i.e., residences, businesses and industries located within the geopolitical boundary), termed the ‘geographic-based approach’ in our review. At other times, GHG accounting appears to address primarily the consumption by households within a community – this is a subset of a full consumption-based approach, although many researchers are not explicit in such delineation in their papers. Finally, when GHG accounting addresses economic final consumption (i.e., households, government and capital expenditures within a community), this is termed full consumption-based accounting.

So, what is a GHG footprint? Broadly speaking, a ‘footprint’ describes the GHG emissions of an activity beyond the boundary of the organization or entity for which the footprint is being computed [8,9,11]. Thus, GHG emission footprints associated with cities seeks to measure and allocate the in-boundary and transboundary GHG emissions associated with cities in a manner that provides rigorous data and informs policy making. One way of describing in-boundary and transboundary GHG emissions is through the idea of scopes, developed by the World Resources Institute for corporate GHG emissions reporting [13]. The concept of scopes helps define organizational boundaries for GHG emissions, and can be mapped to activities associated with cities as shown below.

▪ Scope 1

GHG emissions include all direct GHG emissions resulting from in-boundary fossil fuel combustion (e.g., natural gas, fuel oil, gasoline or diesel), nonenergy industrial processes and waste.

▪ Scope 2

Indirect GHG emissions from imported electricity used within the community.

▪ Scope 3

All other indirect GHG emissions linked to supply chain life cycle of materials and energy carriers used within the boundary that is produced outside. It is worthy of note that in a consumption-based approach, one must also subtract the life cycle GHG emissions from products that are produced within the boundary that are exported for consumption elsewhere, and can be shown as a Scope 3 subtraction.

Scopes 1 and 2 are required reporting for corporate accounting, although the Environmental Protection Agency and World Resources Institute recommend a small number of Scope 3 items to create win–win supply chain GHG mitigation strategies [14]. However, cities are not like corporations, and there is considerable variation on how to allocate in-boundary and transboundary GHG emissions to cities. Table 1 shows peer-reviewed studies that have accounted for various subsets of in-boundary (Scopes 1 and 2) and transboundary (Scope 3) GHG emissions relating to activities within cities. It is worthy of note that although we follow typical nomenclature by showing Scope 2 as in-boundary, most electricity used in cities is generated externally, thus potentially allowing it to be classified as transboundary.

Brown et al. inventoried GHG emissions for 100 US cities, and in their method, accounted for emissions resulting from in-boundary residential electricity use and fossil fuel (cooking and heating) use, and fuel combustion in road transport and freight within each city [15]. Neither commercial nor industrial activities within the boundary were included owing to ‘complex processing issues’, as stated by the authors. Parshall et al.[16] also considered multiple US cities, and sought to evaluate the GHG Vulcan data product and its ability to measure fossil fuel energy use in combustion in US urban areas. Owing to Vulcan’s focus on point of combustion, emissions from direct energy use within a community are accounted for, but imported electricity are not, which is significant in most US cities. Thus, both Brown et al.[15] and Parshall et al.[16], provide a partial accounting of in-boundary energy use and associated GHG emissions.

In Sovacool and Brown [17], the authors inventoried geographic-based GHG emissions of 12 international metropolitan areas. The study covered energy use in buildings (e.g., residential, commercial and industrial), road transport, agriculture within the boundary and waste, accounting for almost all in-boundary GHG emissions, nonenergy processes were not accounted for. In addition, no transboundary activities were accounted for.

The city of Denver (CO, USA) is the first known city to have included transboundary GHG emissions in their community-wide GHG emissions estimates [103], described by Ramaswami et al.[7]. The method accounted for all in-boundary emissions, and included transboundary emissions from airline travel, fuel refining, water/wastewater treatment, and production of cement and food for in-boundary use.

Ngo and Pataki [18], and McGraw et al.[19], accounted for GHG emissions for Los Angeles (CA, USA) and Chicago (IL, USA), respectively. Both accounted for a comprehensive set of in-boundary emissions; the former included transboundary emissions from food production and wastewater treatment, whereas the latter did not cover these categories, but did account for freight.

Kennedy et al.[20] inventoried GHG emissions for ten global cities, resulting from electricity, heating and industrial fuels, industrial processes, road transport, aviation, marine, and waste, in a method that fully accounted for in-boundary GHG emissions. Life cycle, upstream emissions from refining the fuels used within each city were the transboundary emissions considered. The authors cited the need to evaluate upstream GHG emissions from use of other critical materials in cities (e.g., food or buildings materials), which is now being addressed.

Hillman and Ramaswami [8] developed an approach that accounted for in-boundary GHG emissions, plus life cycle emissions associated with key transboundary infrastructures serving cities: water/wastewater pumping and treatment, fuel refining, and embodied emissions from cement and food production, and commuter, air, and freight travel. Applying their method across eight US cities elucidated that the in-boundary plus transboundary accounting methodology provides a more holistic account of GHG emissions approaching national per person GHG emissions of 25 mt-CO₂e/cap for large US metro cities, with a presumed balance of carbon in remaining imports and exports. Very small cities with disproportionately low industrial activity were found to be outliers.

More recently, certain US states (e.g., Oregon) and cities (e.g., King County, WA and San Francisco, CA) are embarking on full consumption-based approaches for GHG emissions footprinting that tracks trade of goods and services in and out of cities (i.e., all imports and exports). Such approaches based on economic input–output (IO) have been used at national scales [21], but city-scale applications have been sparse owing to challenges in obtaining and downscaling IO data to the city level. Table 1 presents a summary of all these above studies.

GHG emissions accounting & footprinting methods

As seen in the literature review in Table 1, there are three methods that are emerging for city-scale GHG emissions accounting. The three methods include Geographic boundary limited accounting, transboundary infrastructure supply chain (TBIS) footprinting, and consumption-based footprinting. This section discusses each of the three methods within the context of their theoretical origins, followed by their advantages and disadvantages. The discussion builds upon recent articles by Wright et al.[11,22] who describe advantages and disadvantages of production- and consumption-based footprints, in general. Wright et al.[11] acknowledge city-scale footprints are in their infancy, making this article a timely addition by covering the newer TBIS method (not previously covered in [22]) and providing city-specific data as illustrative examples. We begin by discussing the geographic-based method first.

▪ Geographic-based accounting

Boundary-limited geographic approaches to GHG emissions accounting are those used in national inventories, which are largely considered ‘production-based’, even though they include GHG from fuel combustion by final consumption (i.e., in homes and personal vehicles). In other words, this method accounts for GHG emissions from all production activities within the nation’s geopolitical boundary, although direct GHG emissions from end-use of energy in households are also included. These national GHG accounts are typically related to metrics of productivity, particularly GDP, and is illustrated as GHG/$GDP [23]. Purely geographic-based accounting is not suited per se for reporting GHG/person; to truly represent an individual’s impact on global GHG emissions, carbon embodied in trade to and from the country (e.g., imports/exports) must be included. For larger nations such as the USA, approximately 90% of GHG emissions resulting from in-boundary production are consumed within the boundary, and net import GHG emissions (imports minus exports) are approximately 7% of the country’s GHG emissions [21]. Therefore, strictly geographic- and consumption-based methods may be numerically similar for large countries or populations. However, strictly geographic approaches are not really suited for small cities because many of their infrastructures (e.g., transport networks or power plants), extend well beyond the city. For example, more than 60% of workers in Denver commute from other cities in the region [7], electricity transmissions can exceed 200 miles in the USA [24], while freight travel averages 600 miles [25] and US food travel averages 1500 miles [26].

▪ TBIS footprint

The TBIS GHG footprint method is an innovative method developed by Ramaswami et al.[7], that recognizes that cities are not like large nations, in that energy use to provide essential infrastructures such as electricity, often occurs outside the geographic boundary of the city (e.g., food production). The TBIS method therefore borrows the concept of Scopes used in Corporate GHG accounting (described previously), to account for essential transboundary infrastructures serving cities. The method can be thought of as an infrastructure-based supply-chain footprint for cities, accounting for GHG emissions from buildings infrastructures (e.g., residential, commercial, and industrial) within the city (Scope 1) and transboundary electric power supply, transboundary transportation (e.g., road, air and freight), fuel supply, water supply, waste management and construction materials infrastructures serving cities (Scopes 2 and 3; Table 2).

▪ Sample results & policy impact

Results from applying the TBIS method to the City and County of Denver are shown in Figure 1. The method used bottom-up end-use of electricity and natural gas for buildings within the city, obtained from the local utility’s billing data. Energy use in surface transportation was computed using regional vehicle miles traveled (VMT) across the regional commuter-shed and allocated to Denver based on the origin destination of trips. Emission factors (EF) of energy carriers were consistent with IPCC. Often, material and energy flows (e.g., energy use in air travel and cement use in city) were obtained from local data such as airports or economic census. EF relating to the embodied energy of materials were obtained from regional scale life cycle analysis (LCA; for cement) and national economic IO (EIO)-LCA (for food production), as discussed in Ramaswami et al.[7] and Hillman and Ramaswami [8]. Results show that GHG emmisions from buildings sector corresponded to approximately 51%, of which Scope 2 electricity related GHG emissions are 36%, while GHG emissions from surface transport tailpipe emissions were approximately 19%. The additional Scope 3 emissions (hatched) were attributed to transboundary activities such as air travel, fuel processing, cement production, and food production. With these inclusions, Denver’s GHG emissions footprint approached a broader GHG footprint that is inline with the national average GHG emissions of 25 mt-CO₂e/person, suggesting the method is effective in capturing dominant transboundary emissions associated with Denver [7]. Similar convergence with national scale was seen in six other large US cities [8]. The TBIS method has been shown to be highly policy-relevant, resulting in innovative actions taken by the city [7,8]. In addition to focusing in energy efficiency and conservation within the boundary, cities are now also able to focus on cross-scale infrastructure efficiencies, such as those related to water supply, regional transport and the materials supply chain; for example, Denver using the TBIS method, has implemented a green concrete policy aimed at reducing GHG emissions embodied in concrete with the use of fly ash substitution for cement. Denver is also conducting a pilot project to evaluate conversion of food waste to energy. As a result of the TBIS method, cities such as Denver (in 2007) and more recently San Francisco (in 2009) are developing voluntary travel offset programs at their airports [27]. Cross-sector strategies such as telepresence that can displace airline travel are also particularly amenable for accounting in the TBIS method, wherein the trade-off between buildings energy use for tele-conferencing programs can be shown to offset airline travel emissions. Finally, as seen in Figures 2A & B and in Table 3, the TBIS method can be used in tracking GHG emissions over time, which further emphasizes the illustration of trade-offs.

Advantages

The primary advantage of the TBIS method is that all activities within the city including residential, commercial, and industrial areas, are considered together, along with the transboundary infrastructures critical for these activities (Figure 3A). Thus, the method is relevant for city and regional planners who consider future transport, power, water and materials supply in the region as a whole. The manner in which the TBIS method addresses transboundary infrastructures serving the entire community is illustrated schematically in Figure 3A.

Thus, the advantages of the method are [28]:

	▪ Relevant to city and regional planning for whole communities – considering residences, businesses and industries together;
	▪ Well suited for showing impacts of infrastructure changes, linking local and regional actions;
	▪ Cross sector strategies, such as teleconferencing are visible;
	▪ Easy for public communication in that the major activities in home carbon calculators (e.g., airline travel) are now also included in city accounting;
	▪ The method yields sector-specific benchmarks developed for each city, useful for comparing sectoral efficiencies across cities;
	▪ The method is effective for tracking climate change impacts such as urban heat island effect that relate to direct in-boundary Scope 1 fuel combustion;
	▪ Metrics pertaining to risk, vulnerability and adaptation, can be quantified for both in-boundary infrastructures (e.g., urban heat island) and transboundary supply chain risks (e.g., risks to a city’s electricity system due to climate-water impacts);
	▪ The method is particularly useful in linking local Scope 1 GHG emissions with local health impacts, for example, increases in local ozone concentration [29], and in potential future inclusions of short-lived climate forcers;
	▪ As shown in Figure 2, the TBIS method used locally specific data and is suitable for tracking a city’s GHG emissions over time.

Indeed, with its capacity to address local health impacts of GHGs and short-lived climate forcers, and provide input on supply chain vulnerabilities, the TBIS method is well suited to address both GHG emissions and climate adaptation in cities.

Disadvantages

The primary shortcoming of this method is that it requires improved metrics for inter-city comparisons on a consistent basis. Since the TBIS method is based upon geographic production-based inventories, the often used per capita metric (which is the same as per resident) is not appropriate for inter-city comparisons using this method, particularly when a city with high industrial-commercial activity is compared with a solely residential community. GHG per unit gross regional product (or gross metropolitan product) is probably the best option; however, for many smaller cities and towns, such data are not reported. In such cases, normalizing community-wide emissions by residents plus jobs could be an option to compare cities. Per capita GHG emissions for this method may also be used if a typology of cities is created, representing producer-, consumer- and energy-balanced cities, such that cities are only compared within their peer group. Ongoing research at University of Colorado, Denver, is exploring these reporting metrics to promote better inter-city comparisons.

▪ Consumption-based footprinting

The consumption-based approach accounts for global GHG emissions resulting from economic final consumption (i.e., households, government and capital investments), within a city, including GHG emissions in imports, but excludes GHG from the production of exports within the boundary. This method traces GHG emissions fully upstream, outside of the community boundary, accounting for all transboundary activities that serve economic final consumption in the community. Of the economic final consumption sectors, households have been estimated to be responsible for the vast majority of the consumption [30] (i.e., 80% final demand in the USA), thus, the method becomes well suited for evaluating household impacts on GHG emissions. A schematic showing activities in a typical consumption-based application can be seen in Figure 3B. Household consumption surveys are often used to assess the impact of only household consumption on GHG emissions [30]. Applications wherein city-scale IO tables are downscaled from national IO tables must be used to address other components of final consumption (government and capital investments). However, the accuracy of downscaled IO tables in representing material and energy flows in cities remains to be explored.

▪ Sample results & policy impact

The preliminary GHG emissions results from final consumption in Denver were computed using commercially available downscaled IO data from IMPLAN, which estimates monetary transactions and expenditures throughout the local economy across 440 economic sectors. The monetary expenditures were then converted to GHG emissions using a single region model and GHG emissions by the economic sector (in mt-CO₂e/million$) from the EIO-LCA tool [104]. The results separated out into multiple consumption categories, similar to those in Weber and Matthews [30], are shown in Figure 4A. GHG emissions from the direct consumption of energy in households (energy/utilities), along with combustion GHG from personal transport (cars/SUV’s), dominate. Furthermore, the total life cycle emissions associated with each consumption category can be separated out to show the contribution by scope for each consumption category (i.e., in-boundary fuel combustion [Scope 1], electricity imports [Scope 2], other infrastructure imports denoted in TBIS [Scope 3]), with the remainder denoting imports of other goods and services to meet final consumption.

▪ Advantages

The primary policy relevance of this method is that it makes the full transboundary impact of household consumption visible. Since most of the final consumption comes from households, this is theoretically the most rigorous method for comparing per person GHG emissions from household expenditures. Furthermore, the method can help illustrate how the supply chain of government operations can be made more green. With detailed and accurate IO data, imports and exports to/from a community can be traced.

▪ Disadvantages

However, the full consumption-based IO method is valuable only if accurate IO analysis and data are available at the city scale. Misallocations in local IO tables can occur when physical flows of energy and materials do not match the flow of economic activity; often occurring when large corporate headquarters in a city report economic activity well outside the city boundaries. For many US cities, IO data are not even published at a scale smaller than the county scale. Furthermore, unlike the TBIS method, the consumption-based method divides the community in two, with commercial-industrial activities for exports not included in the unit of analysis (Figure 3B). For some communities (e.g., resort towns and industrial towns), this excludes a sizable proportion of their local economy that could be shaped by local policies. The application of IO tables for GHG emission accounting is new at the small city level, and researchers are learning about its application to smaller spatial scales where downscaling national data poses challenges. The difficulty of tracking GHG emissions via this method is triggered by the low publishing frequency of national IO tables; usually every 5–7 years for the USA.

Update on protocol development

In summary, and as seen in Figures 3A & 3B and in the discussions in this article, the TBIS and consumption-based footprints are two different approaches that give distinctly different estimates of a communities GHG emissions, and inform policies differently. The TBIS method accounts for all in-boundary emissions within the geographic boundary of a city, along with key transboundary infrastructures serving the entire community taken together. The method is suited to future infrastructure planning that address the whole community, and to address regional cross-scale and cross-infrastructure strategies across city boundaries. Consumption-based GHG emissions footprinting accounts for all (in-boundary and transboundary) GHG emissions resulting from economic final consumption in the community, while the in-boundary commercial industrial activities exported elsewhere are excluded, even though these local activities generate jobs and may also be shaped by local regulations. The method is especially suited to educate households about the global nature of their consumption.

Recognizing that both methods provide useful and different information, ICLEI-USA has published a draft framework for community-scale GHG emissions accounting and reporting [10]. The framework aims to help local US governments in planning and demonstrating GHG emissions reductions, by establishing standardized approaches that communities can use to create holistic baseline GHG emissions measures. Owing to the fact that the protocol is in development, it is subject to future revisions.

Recognizing that local governments have distinct reasons for measuring GHG emissions, the protocol has varying tiers of reporting. The reporting approaches for community-wide GHG emissions are basic reporting standard (basic), expanded community impact reporting (expanded), and consumption-based reporting. The below schematic illustrates the ICLEI reporting framework, and how it links with the methodological approaches described in this article. The basic reporting standard is expected to describe a minimum level of inclusions for community GHG emission accounting, to establish consistency across cities. The expanded community impact reporting provides guidance on measuring energy use and GHG emissions more holistically, by incorporating all key transboundary infrastructures as described in the TBIS method. Finally, the protocol allows for an optional and separate accounting of GHG emissions from community final consumption, using the consumption-based method.

Conclusion: GHG, carbon & other sustainability metrics

Establishing a goal to develop low-carbon cities requires good measurement tools for GHG accounting in cities. While it is obvious that a low carbon city must improve the energy efficiency of its buildings and transport system within the boundary, this article asks what other transboundary sectors are important in considering and defining a low-carbon city. Many of the sectors that are transboundary may in fact offer further and more innovative opportunities for GHG mitigation (e.g., waste and industrial symbiosis and innovative technologies such as tele-presence). Changing the nature of consumption in communities (e.g., changing food-diets), also becomes a part of the low-carbon strategy toolkit. Recalling the adage ‘What gets measured gets done’, measurement tools play a major role in shaping the available strategy set, and vice versa. Recent advances in transboundary GHG accounting (Table 1), and the inclusions of such emerging knowledge into community-wide GHG protocols being developed by ICLEI-USA and others, is a major step in developing improved measurement tools (Figure 5).

The discussion presented in this article demonstrates that one single metric (e.g., GHG/person) will probably not be suitable to represent GHG emissions associated with cities. A combination of variables such as GHG per unit city residents plus city employees or the totality of economic output may all serve as potential metrics for defining a low-carbon city. In addition to aggregate city-wide metrics, such as GHG/person or GHG/GRP, sector specific efficiency and consumption measures are also useful. Hillman and Ramaswami [8] have quantified efficiency and consumption measures in buildings, transport and materials sectors in cities, at no additional cost or effort beyond TBIS. Table 4 illustrates some of these efficiency metrics.

Particular metrics will need to be ranked and weighted across cities. Efficiency benchmarks already existing in the literature [8] could be expanded on. It is likely to take a combination of various metrics, together, to help define a low-carbon city both for rigor and for policy relevance [31].

Other sustainability metrics such as health and well being, Amartya Sen’s concepts of human capabilities approach reflected in the Human Development Index [32], and emerging metrics of risk and vulnerability must also be considered in defining the low-carbon goal.

Future perspective

The TBIS and the consumption-based footprints reviewed in this article, together are expected to enhance cross-scale governance to reduce GHG emissions associated with cities.

Table 1.  Differences in city-scale GHG emissions in peer reviewed literature.

Authors	In-boundary energy use								Transboundary energy use
	Scope 1 (direct energy use & GHG emissions)						Scope 2 (indirect)		Scope 3 (indirect)
	Residential	Commercial	Industrial	Non-energy processes (e.g. waste)	Surface transport		Electricity imports		From other infrastructure imports serving entire community						From other import–export trade balance
									Water	Fuel	Cement	Food	Air travel	Freight
Brown et al.[15]^†	✓				Personal only		Residential only							✓
Parshall et al.[16]^†	✓	✓	✓	✓	✓									Unclear
Sovacool and Brown [17]^†	Only where available	Only where available	Only where available	✓	Personal and mass transit		✓					✓			Not usually considered in geographic approaches
Ramaswami et al.[7]^‡	✓	✓	✓	✓	✓		✓		✓	✓	✓	✓	✓
Ngo and Pataki [18]^‡	✓	✓	✓	✓	✓		✓		✓			✓
McGraw et al.[19]^‡	✓	✓	✓	✓	✓		✓							✓
Kennedy et al.[20]^‡	✓	✓	✓	✓	✓		✓			✓			✓
Hillman and Ramaswami [8]^‡	✓	✓	✓	✓	✓		✓		✓	✓	✓	✓	✓	✓
Oregon, King County, San Francisco^§	Ongoing. A few cities and one state are conducting GHG emissions accounting using the consumption-based method. Embodied GHG emissions in imports are included, but energy used in exports are excluded.

^†Geographic.

^‡Transboundary infrastructure supply chain.

^§Consumption-based.

Table 2.  Transboundary infrastructure activities accounted for in the transboundary infrastructure supply chain method.

Energy use and direct GHG emissions within the boundary (Scope 1)	Energy use in various transboundary infrastructure sectors serving cities (Scopes 2 and 3)
Buildings infrastructure – direct fuel combustion and GHG emissions from: Residential buildings Commercial buildings Industrial facilities	Water and waste sector – Water supply pumping – Water and wastewater treatment Landfill emissions from waste disposal energy sector – Fuel refining – Electric power production Construction sector – Cement production – Supply chain of other major materials to cities Transportation sector – Air travel – Long distance freight
	Food production

Table 3.  Demographic and per resident use trends in Denver, CO, USA.

Demographic trends				Per person use
Measure	Annual change (%)	Data source		Measure	Annual change (%)	Data source
Population	+ 0.95%	US census [2]		Electricity	+ 0.9%	Xcel energy [34]
New home stock	+ 1.26%	CCD assessor [33]		Natural gas	- 1.36%	Xcel energy [34]
New commercial area	+ 0.19%	CCD assessor [33]		Motor gasoline	- 0.7%	DOR [105]
				Diesel	+ 1.2%	DOR [105]

^†Calculated from 2000–2007.

CCD: City and County of Denver; DOR: Colorado Department of Revenue.

Table 4.  Examples of city-scale energy and material efficiency metrics.

Sector	End use efficiency metric
Household energy use	kWh/HH/month, therms/HH/month, kBTU/HH/month
Commercial building energy use	kWh/sq-ft/month, kBTU/sq-ft/month
Community transport	VMT/person/day
Material use	tons-municipal solid waste/capita/year, mt-cement/capita/year

BTU: British thermal units; HH: Household, VMT: Vehicle miles traveled.

In-boundary energy use

End use of energy occurring in city. Included are natural gas, petroleum fuels (e.g., gasoline and diesel) and electricity.

Transboundary energy use

Energy used in the provisioning of essential materials to cities.

Consumption-based

Defines the community as local residences, plus local and imported commercial and industrial activity serving local household consumption. Community is divided, as activity for exports is not accounted for locally.

Transboundary infrastructure supply chain

Defines a community as all local residences, commercial and industrial units. The community is considered as a whole, together.

Executive summary

Review of city-scale community-wide GHG emissions measurements

	▪ No standardized method for city-scale GHG emission accounting has produced inconsistent approaches for cities.
	▪ GHG emissions can be measured for local government operations, or community-wide activities.
	▪ Community-wide footprints could be geographic (for all activities from residences, commercial and industrial units in the city) or consumption (represented by household consumption in the city).

GHG emissions accounting & footprinting methods

	▪ Three emerging methods for city-scale GHG emissions accounting and footprinting.
	▪ Geographic boundary limited are considered production-based, and usually related to metrics of productivity (GHG/$GDP)
	▪ Transboundary infrastructure supply chain (TBIS) footprint estimates GHG emissions from in-boundary activities, plus essential transboundary infrastructures serving cities.
	▪ TBIS is policy relevant and cross-sector strategies and trade-offs are illustratable.
	▪ The consumption-based footprint estimates GHG emissions resulting from locally produced and imported production to meet local economic final consumption.
	▪ Illustrates full transboundary impacts from household consumption.

Update on protocol development

	▪ The TBIS and consumption-based approach are two distinct approaches, resulting in unique estimates of a city’s GHG emissions.
	▪ Each method informs policy differently: ▪ TBIS – the community as a whole is addressed, along with regional cross-scale and cross-infrastructure strategies across city boundaries. ▪ Consumption-based – activities relating to local household consumption are accounted for, and those for exports are not.

Acknowledgements

This paper was presented at a US–China Workshop on Pathways Toward Low Carbon Cities held in Hong Kong (December 2010), sponsored by the US National Science Foundation grant CMMI-1045411.

Financial & competing interests disclosure

This work was sponsored by a grant from the US National Science Foundation (IGERT Award No. DGE-0654378). The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

No writing assistance was utilized in the production of this manuscript.