Technologies, policies and measures for GHG abatement at the urban scale

Abstract

As a greater fraction of the world's residents moves to cities, urban-scale policies will play a critical role in mitigating global climate change. Recognizing this role, many city governments around the world are developing greenhouse gas (GHG) reduction targets and action plans. Despite their ambition, however, these jurisdictions often lack the capability to prioritize actions based on the scale of GHG abatement potential and to use mitigation assessment to set and a plan on how to meet (often ambitious) emissions targets. This paper helps to address these gaps by (a) developing a general typology of urban-scale emission-reduction technologies and practices, (b) identifying policies and measures that can support their adoption, (c) assessing their relative abatement potential in the nearer (2020) and longer (2050) term and (d) examining the relative degree of influence that urban jurisdictions can wield with respect to realizing these potentials. Local jurisdictions can use this typology as an initial screening tool to identify technologies and practices with higher GHG abatement potential, especially those in the transport and buildings sectors, as well as policies and measures that may support them. Researchers can use the results to inform priorities for further development of standardized analytical methods, toolkits and indicators.

Keywords:

• urban climate change mitigation
• accounting frameworks GHG management
• mitigation measures
• methodologies
• protocols and standards

Introduction

Cities play a uniquely pivotal role in reducing global greenhouse gas (GHG) emissions and mitigating climate change. As Dhakal and Shrestha (2010), Hoornweg, Sugar, and Trejos Gomez (2011), Rosenzweig, Solecki, Hammer, and Mehrotra (2010) and others note, based on International Energy Agency (IEA) (2008), urban areas are currently responsible for two-thirds of the world's energy demand and carbon dioxide (CO₂) emissions, a share that is likely to grow with continued global urbanization United Nation (UN) (2011). Increasingly, major international assessments of GHG abatement include measures, such as bus rapid transit and compact development, that are squarely in the realm of urban planning and development (International Council on Clean Transportation (ICCT), 2012; IEA, 2012a). Indeed, decisions regarding spatial development patterns and transportation infrastructure can dramatically affect the GHG footprints of urban residents, with transportation emissions ranging from over 6 tons CO₂e (tCO₂e) per person in Denver to less than 1 tCO₂e per person in Barcelona (Kennedy et al., 2009).

Given the slow progress in addressing climate change at national and international levels, attention is increasingly focused on the opportunity for cities to play a leadership role (Mulugetta, Jackson, & van der Horst, 2010; Rosenzweig et al., 2010). Cities, for their part, have responded by announcing ambitious emission-reduction targets and action plans, several of which far exceeded the level of ambition of national targets. For example, a handful of cities, including Chicago, Hamburg, London, Portland, San Diego, Seattle, Taipei, Toronto and Yokohama, have all announced intentions to reduce GHG emissions 50% or more below 1990 levels by 2050 (Carbon Disclosure Project [CDP], 2012). Many of these targets are set at 80% reduction (or more), echoing Intergovernmental Panel on Climate Change (IPCC) guidance for national-scale Annex I reductions of 80–95% below 1990 levels by 2050 (Metz, Davidson, Bosch, Dave, & Meyer, 2007, p. 776). Blok, Höhne, van der Leun, and Harrison (2012) suggest that action by cities could help bridge the global emissions ‘gap’ between the level of abatement pledged by nations under the Cancun Agreements United Nations Framework Convention on Climate Change (UNFCCC) (2011) and that needed to maintain a chance of limiting global warming to 2°C, as cities control some unique policy levers (such as local land use planning) that are less available to nations. Cities are also motivated to address climate change, in part, because in some cases climate impacts, such as heat waves, may be worse in urban areas. Networks of cities have formed globally and regionally to share practices. For example, the C40 Cities Climate Leadership Group is comprised of about 60 cities, with a combined population of over 300 million people, that have joined together to reduce greenhouse emissions.

In addition to conducting emission inventories and establishing emission targets, many cities have also developed, and some have begun the implementation of urban-scale climate action plans (City of Cape Town, 2006; City of Melbourne, 2009; City of New York, 2007; City of Sao Paulo, 2009; Greater London Authority, 2007; Tokyo Metropolitan Government, 2007). City climate action plans typically identify a series of priority actions – policies and measures often selected based on stakeholder consultation processes – as well as implementation strategies and progress indicators. However, many of these climate action planning efforts lack a comprehensive assessment of the potential of various policies and measures in terms of long-term GHG abatement (Bassett & Shandas, 2010; Boswell, Greve, & Seale, 2010; Lazarus, Chandler, & Erickson, 2013).

While even at the national level, city-level emission targets are often set in part as political statements and aspirational goals, with relatively little analysis of abatement potential. Similarly, and in contrast to national-level plans (Commonwealth of Australia, 2011; European Commission, 2008), urban-scale climate action plans are often adopted without an assessment of whether the measures selected are capable of meeting the stated target. Many urban-scale climate action plans have focused more on measures that provide incremental and visible reductions in GHG emissions, such as distribution of efficient lighting, upgrading of municipal vehicle fleets or tree planting, than on longer-term, transformative and potentially higher impact actions such as altering urban form or developing major new transportation systems (Arup & C40 Cities, 2011; Bassett & Shandas, 2010; Betsill & Bulkeley, 2007; Keirstead & Schulz, 2010). Unlike national-scale climate action planning and policy development, which can rely on a well-established literature and modelling toolkit (UNFCCC, 2006, 2012), for city-scale planning, there is a limited understanding of which option could yield the largest abatement benefit and limited availability of tools and frameworks to inform such an assessment (Zhou, Price, Ohshita, & Zheng, 2011). In the words of Rosenzweig et al. (2010), ‘What the world needs is the same science based foundation for cities that the IPCC provides for nations’. Similarly, Keirstead and Schulz (2010) state that ‘if urban energy policy is to be seen as a tool for addressing modern energy challenges, then there needs to be a more robust framework for the analysis of cities and their efforts’ (p. 4878).

This paper helps to address these gaps by (a) developing general typology of urban-scale emission-reduction technologies and practices, (b) identifying policies and measures that can support their adoption, (c) assessing their relative abatement potential in the nearer (2020) and longer (2050) term and (d) examining the relative degree of influence that urban jurisdictions can wield with respect to realizing these potentials. Developed based on review and analysis of existing literature, as well as a recent survey of city mayors, this typology and assessment can help to inform and structure urban climate action planning efforts and analysis, assisting planners to give greater weight to actions that bring significant abatement potential and influence and will be used by C40 Cities to help guide their priorities. The analysis presented here reflects, in most cases, global average conditions (or a sample of multiple cities), and as a result, individual municipalities will need to account for local circumstances by conducting their own mitigation assessments or, where that is not possible, by adjusting the estimates presented here. Furthermore, planners will need to embed this analysis within an assessment of other potential risks and benefits – such as local air quality improvement, increased resident mobility, economic development and social equity, among others – that are typically the focus of urban planning (Campbell, 1996).

Methodology

Our overall methodology builds upon the typologies of emission-reduction technologies and practices, along with associated national policies and measures, that have been developed over the course of the IPCC's assessment processes (Intergovernmental Panel on Climate Change (IPCC) (1996, 2007).¹ Broken down by sector – buildings, industry, energy supply, transport and so on – the IPCC's typologies have formed the foundation of other major analyses of national and international GHG abatement potential (IEA, 2012b; McKinsey & Company, 2009; United Nations Environment Programme [UNEP], 2012). For example, for road transportation, the IPCC's Fourth Assessment Report (Metz et al., 2007, chap. 5) identifies more efficient vehicles, biofuels and modal shifts to public transport systems as technologies and practices to reduce emissions, and land use planning, transport planning, pricing and regulation as among the policies and measures to increase their penetration and adoption. While the IPCC and other literature generally focus on policies and measures more relevant at national and regional levels, such as alternative fuel or GHG emissions standards, we focus here on the types of policies and measures more relevant for municipal jurisdictions such as vehicle registration fees tied to fuel economy or the provision of alternative fuel infrastructure (e.g. electric car charging stations).

Below is the further discussion of our approach to each of the four steps, (a) through (d), described above.

Technologies and practices

To develop a typology of urban-scale technologies and practices, we reviewed the IPCC's national assessments (IPCC, 1996, 2007) as well as prior categorizations of urban-scale GHG abatement (Kamal-Chaoui & Robert, 2009; Satterthwaite, 2009; Zhou et al., 2011), a number of studies that focus on urban transportation (Bongardt, Breithaupt, & Creutzig, 2010; Cambridge Systematics, 2009; Dalkmann & Brannigan, 2007; Dierkers, Silsbe, Stott, Winkelman, & Wubben, 2005), and studies that focus on shifts in consumption at the urban (Bioregional and the London Sustainable Development Commission, 2009; Erickson, Chandler, & Lazarus, 2012) and larger (Barrett & Scott, 2012; Brohmann & Barth, 2011; Erickson, Allaway, Lazarus, & Stanton, 2012; ICLEI – Local Governments for Sustainability USA, 2012; Stehfest et al., 2009) scales.

Adapting the IPCC's approach, we categorize urban GHG abatement options according to the sectors, such as buildings or transportation, in which emissions occur. Sector distinctions are widely used for economic and GHG abatement analysis, and often line up well with organization of city government, where different departments focus in areas such as transportation infrastructure or building codes. Here, we adopt the IPCC sector definitions (e.g. energy supply, buildings and transport), but expand the definition of industry to include goods and materials, and the definition of agriculture to include food (Figure 1). Making this change helps broadens the focus to include additional practices – especially shifts in consumption – that reduce global GHGs through actions at the local scale.

Figure 1. Urban-scale technologies, policies and measures for GHG abatement (see above for rating scales).

Figure 1. Figure 1. Continued.

Figure 1. Figure 1. Continued.

Within each sector, we identify the groups of technologies and practices that can be used to reduce GHG emissions, such as the provision of low-carbon electricity, energy retrofits of buildings or switching to lower-GHG fuels. We aggregated technologies and practices into groups that tend to act on a single emissions driver (activity, energy intensity and carbon intensity of fuel) and with a common set of supporting policies and measures. For example, utility-scale low-carbon electricity supply includes major wind, solar and other low-carbon electricity installations, all of which could be accessed through the application of city-scale policies such as renewable portfolio standards or feed-in tariffs.

The typology focused on technologies and practices that, if applied locally, would reduce global (not merely local) GHG emissions. We excluded options that might simply shift emissions across jurisdictional lines or sectoral boundaries, but not result in global emission reductions. For example, reducing or ceasing local production of a good (e.g. cement) or service (e.g. air travel) within a community's political border may reduce that community's (production-based) GHG inventory, but could simply shift that emission-causing activity elsewhere, causing emissions leakage (Erickson & Lazarus, 2012; Peters & Hertwich, 2007). The focus on global emissions also enables the inclusion of activities implemented within an urban area, such as electricity savings programmes or support for low-GHG diets, that may reduce GHG emissions largely outside the urban area itself (where electricity is generated or food produced) (Chavez & Ramaswami, 2011; Dhakal & Shrestha, 2010; Erickson et al., 2012; Grubler & Fisk, 2013; Kennedy et al., 2010; UN-Habitat, 2011).² We do not explicitly consider economic costs or benefits as a criterion for inclusion, but note that we have excluded especially high-cost or speculative technologies or practices (such as carbon capture and storage), instead relying on options that are routinely included in abatement studies and are generally well under 100 US$ per tCO₂e; many, especially energy efficiency measures, are cost-negative.

The result is a comprehensive categorization of technologies and practices for GHG abatement at the urban scale (Figure 1 in the Results section).

Policies and measures

We compiled a list of policies and measures to increase development, diffusion and implementation of each technology and practice, drawing from major international assessments (Metz et al., 2007, chap. 13; IEA, 2012a, UNEP, 2012) and a number of urban and sector-focused studies (Baeumler, Ijjasz-Vasquez, & Mehndiratta, 2012; Bongardt et al., 2010; Cambridge Systematics, 2009; Dalkmann & Brannigan, 2007; Dierkers et al., 2005; Erickson et al., 2012; Grubler & Fisk, 2013; Hammer, Kamal-Chaoui, Robert, & Plouin, 2011; Hickman, Ashiru, & Banister, 2011; Hoornweg et al., 2011; Ramaswami et al., 2012; Ürge-Vorsatz et al., 2012). Policies and measures include types of regulations and standards, financial and market-based incentives (e.g. taxes and charges), subsidies, information and advocacy, research and development and provision of infrastructure (Hammer et al., 2011; Metz et al., 2007, p. 767; WRI, 2012). We identify policies over which local jurisdictions have relatively unique and direct influence, as well as complementary national or international options. Local and national policy influence can differ sharply (Keirstead & Schulz, 2010), and both will be needed for cities to meet ambitious GHG abatement goals (Hammer et al., 2011; Lazarus et al., 2013).

GHG abatement potential

Methods for assessing urban-scale GHG abatement are still in their infancy, and very few reviews or meta-analyses of urban-scale GHG abatement exist. To help fill this gap, this assessment reviewed studies that focus on urban-scale GHG abatement or which could be applied at the urban scale. Although several city-specific studies exist (Bioregional and the London Sustainable Development Commission, 2009; Gomi, Shimada, & Matsuoka, 2010; Hickman et al., 2011; Lazarus et al., 2013; Phdungsilp, 2010; Zhang, Feng, & Chen, 2011), they generally do not use consistent methods or assumptions as each other, and it is difficult to determine whether abatement potential in one particular city could be broadly applicable across cities.

As a result, to develop a globally consistent set of abatement potential estimates for urban-scale technologies and practices, we relied first on widely cited international studies of global GHG abatement potential, especially the International Energy Agency's Energy Technology Perspectives (IEA, 2012a). Using IEA provides a consistent set of abatement potential estimates for several of the technologies and practices in the energy, buildings, transport and industry sectors that would directly apply in the urban setting (e.g. more-efficient vehicles and home appliances). Other studies were used if they had a more specific urban focus. For example, for improvements in urban building heating and cooling energy, a recent study prepared for the Global Buildings Performance Network (Ürge-Vorsatz et al., 2012) was used. For transportation mode shift and measures that avoid vehicle trips – both of which are distinctly urban – we used a recent study by the International Council on Clean Transportation (ICCT, 2012) that included significant detail on these measures and reviewed other, city-specific studies (Hickman et al., 2011; Lazarus et al., 2013). An appendix to this paper details all literature sources used to develop abatement estimates and provides further discussion, including limitations, of the methodology.

All abatement potential was estimated relative to the average world resident's carbon footprint in the years 2020 and 2050, which was calculated based on forecasts of CO₂ in the International Energy Agency's reference 4°C scenario (IEA, 2012a) plus an extension of forecasts of non-CO₂ emissions by the US Environmental Protection Agency (US EPA, 2011). For example, the IEA estimates that global abatement potential of more-efficient vehicles is 4.0 Gt CO₂ in 2050, or nearly 7% of global reference case emissions in that year,³ and it is assumed here that this relative potential would also apply at the urban scale, given the lack of more specific information on differences in vehicle stocks and travel distances between urban and rural areas.

As shown in Table 1, we assign one of the three ratings based on the potential of a suite of technologies and/or practices to reduce the average resident carbon footprint by <1, 1–5 or >5%.

Table 1. Scale for rating the abatement potential of urban-scale technologies and practices.

Rating symbol	Relative, average abatement potential	Fraction of average resident's carbon footprint	Equivalent abatement potential for a world average resident (tCO2e per resident per year)
	High	>5%	>0.3
	Medium	1–5%	0.06–0.3
	Low	<1%	<0.06

These broad rating categories are meant, in part, to address uncertainty, and are intended to be used on relative rather than absolute (e.g. total tons per capita) basis, given that cities across the world are on different paths or at different stages of development. For example, average per capita emissions can vary by a factor of 20 or more across cities, from >20 tCO₂e per resident (e.g. Denver) to 1 or less (Kolkata) (Hoornweg et al., 2011). For illustration, Table 1 shows that for a global average resident with a carbon footprint of 6 tCO₂e per year, 1% would correspond to roughly 0.06 tCO₂e per year and 5% to 0.3 tCO₂e per year. While the 1 and 5% thresholds are somewhat arbitrary, they provide a clear basis for ratings and help to distinguish options with high and low abatement potential.

Furthermore, these ranges represent average, relative abatement potential across a range of the world's cities and relative, as well as absolute, potentials in individual jurisdictions may vary. For example, cities that already have (or have planned) low-carbon electricity supply will have greater relative potential abatement in other sectors, and might therefore place greater emphasis on policies and measures in these sectors than is indicated by Figure 1.

For each technology and practice, we assess abatement potential assuming that no other measures within the sector have been undertaken. However, some technologies and practices may target the same emissions source: for example, trip avoidance and vehicle efficiency. In such cases, we take into account overlaps only when estimating sector-wide abatement potentials.⁴

Degree of influence

In addition to rating the abatement potential of each technology or practice, we also assign preliminary ratings of the average degree of influence of local jurisdictions. C40 Cities recently conducted a survey of mayors of major cities to help assess their capacity to deliver GHG-reducing initiatives and actions, rating influence as strong, limited or mixed (Arup & C40 Cities, 2011).⁵ We adapt their assessment here by developing ratings based on the fraction of cities C40 assessed as having strong powers (Table 2), noting that this assessment is based largely on mayors’ own perception of their powers, which may be influenced by factors beyond regulatory, financial or planning authority, such as political priorities. Where C40 Cities’ prior research did not clearly address a particular technology or practice in the proposed typology, we gave a low rating except where available evidence suggested otherwise.⁶ For example, their research did not assess city influence over resident diets or food waste practices, which therefore receive low ratings here. Similarly, their research did not focus on industry, goods or materials other than a rating for energy in ‘industrial buildings’. By industry, here we refer to large industrial producers of major commodities, such as steel, cement, paper, chemicals, glass and aluminium, which represent the great majority of global industrial energy demand and for which energy use occurs in distinct industrial processes, not necessarily in ‘buildings’ (IEA, 2011). Low local influence is assumed for these processes, but we give low-GHG cement and steel each medium ratings because mayors did report influence over building codes, which could influence the characteristics (and presumably GHG-intensity) of these materials when used in construction.

Table 2. Scale for rating degree of influence of local jurisdictions.

Rating symbol	Relative, average influence of local jurisdictions	Fraction of C40 cities with strong ‘powers’ over each technology or practice
	High	>50%
	Medium	10–50%
	Low	<10%

Table 2 shows the rating scale used for this assessment of the degree of influence.

Results and discussion

Figure 1 presents the typology of technologies and practices for GHG abatement at the urban scale, along with the assessment of abatement potential, local influence and supporting policies and measures.

This assessment is one of the first attempts to develop a standard set of technologies and practices for urban-scale GHG abatement, building on the foundational work of the IPCC (IPCC, 2007) and city-focused efforts (Arup & C40 Cities, 2011; Baeumler et al., 2012; Kamal-Chaoui & Robert, 2009; Satterthwaite, 2009; UN-Habitat, 2011). Specifically, this assessment indicates that

• Transportation and buildings have both high abatement potential and high local influence, making them prime candidates for concerted effort by cities. In the building sector, abatement potential is spread across upgrades to heating and cooling equipments; appliances, lighting and other plug loads; and building thermal integrity. A number of urban-scale policies and measures are available to capture this potential, from building energy codes to incentives for building retrofits. In the transportation sector, even greater potential exists, especially in the long term. Higher impact options include higher-efficiency cars, electric cars, low-GHG biofuels, public transportation and policies that help avoid or shorten trips.

• Energy supply has a high abatement potential, but generally low reported local influence. Although the provision of utility-scale low-carbon electricity has a high potential for GHG abatement (the single greatest abatement potential in Figure 1), fewer cities report a strong ability to influence their utility-scale electricity supply. Major exceptions include cities with city-owned public utilities, as well as the ability of some cities to develop district energy infrastructure or provide incentives for distributed solar technologies. Given the high potential for reducing emissions from energy supply, further research is needed on policies that cities can employ to influence resource decisions made by utility-scale electricity and heat providers.

• Food choice and reducing food waste generation could bring significant abatement potential and have received increased attention in local jurisdictions, although local influence may be limited. Reducing or avoiding resident consumption of high-GHG foods such as red meat and dairy can have significant abatement potential, similar to reducing food waste, where it leads to reduced food purchases. These practices have been the subject of increasing research and planning at local scales,⁷ although relatively few policies have so far been adopted.

Results of the assessment above can inform priorities for further development of standardized analytical methods, as well as toolkits to support more in-depth city-scale analysis. The results indicate that transportation and buildings should be considered as priority sectors and areas of focus for further work to review existing methods, develop analytical guidance and develop tracking metrics. They also suggest that special attention may be warranted on individual technologies and practices that have high city influence and medium-to-high abatement potential, including each of the technologies or practices that address building energy (heating/cooling equipment, building thermal envelopes, appliances and lighting and space usage) as well as avoiding transportation trips and reducing trip length. This last practice, supported through local policies and measures such as land use and transportation planning, parking policies and road pricing, has often been overlooked in national and international assessment of GHG abatement, further suggesting the potential importance of new research and/or analytical methods.

This assessment, as summarized in Figure 1, can serve as a starting point and framework for communities’ own assessment of abatement potential. For example, city planners could begin climate action planning processes by sharing this list of abatement technologies, practices, policies and measures, with city agencies and stakeholders, gathering input and revising (and improving) estimates of abatement potential in their communities. By focusing initial attention on the technologies and practices with highest (average) abatement potential, this assessment may help increase the ambition and effectiveness of community-scale climate action plans. Further work to develop analytical tools with standardized inputs and assumptions (e.g. to support forecasts of business-as-usual emissions growth) is underway (World Bank, 2011), including the works by these authors.

Conclusions

As a greater fraction of the world's residents moves to cities, urban-scale policies will play a critical role in mitigating global climate change. City governments around the world are increasingly realizing their important roles, and are developing GHG reduction targets and action plans. Despite their ambition, these efforts often lack assessments of the greatest GHG abatement opportunities, or how the jurisdictions plan to meet their emissions targets. Clearly, individual circumstances vary, and most cities would benefit from analysis specific to their local conditions. Yet, resources and analytical capacity are often in short supply. This paper has attempted to help in filling this gap by developing a typology intended to assist cities, and other analysts of urban areas, in identifying technologies and practices for further consideration, while providing an initial assessment of the relative GHG abatement potential of each. Local jurisdictions can use this typology as an initial screening tool to identify technologies and practices with high average GHG abatement potential, as well as policies and measures that may support them, for further analysis.

Clearly, more research is needed, both within individual cities and across them. A quantitative meta-analysis (Clapp, Karousakis, Buchner, & Château, 2009) may be ideal, but would be difficult at this time given the relative paucity of city-scale abatement studies and lack of standardized methods. In the meantime, further efforts to standardize analytical methods for cities (perhaps building from or linking to prior and existing efforts, such as city-scale GHG inventory methods and the World Resources Institute's GHG Protocol Mitigation Accounting initiative), development of analytical tools and rigorous studies of GHG abatement potential – particularly for under-studied urban-scale practices such as combined land use and transportation planning – would be particularly helpful.

Acknowledgements

The authors wish to thank C40 Cities for envisioning and commissioning this research. Special thanks are due to Rit Aggarwala and Amanda Eichel at C40 Cities for helpful conversations and insights.

Notes

1. For example, see Tables 1 and 5 in IPCC (1996) and Table SPM.3 in IPCC's Fourth Assessment Report (Metz et al., 2007).

2. Some practices included in urban-scale climate action plans are not included in the typology at this time, because they have not been shown to reliably reduce global GHGs. For example, farmer's markets and localized food production were not included as practices because the research reviewed did not indicate that, overall, local food had a reliably lower GHG footprint (Cleveland et al., 2011; Coley, Howard, & Winter, 2009; Weber & Matthews, 2008).

3. All estimates from IEA's ETP are for abatement potential in their 2°C scenario (2DS) relative to their 4° scenario (4DS), where the 4DS is intended to be consistent with recent country-scale pledges to limit emissions and also broadly consistent with the IEA's New Policies Scenario in World Energy Outlook (IEA, 2011).

4. There is a small degree of overlap between sector-wide potentials in the buildings and energy sectors, accounting for about 10% of the abatement potential in the buildings sector. Removing this double counting from the buildings sector would not change the overall buildings-sector rating here.

5. C40 administered a questionnaire to each city to evaluate the extent to which it owned or operated particular infrastructure, set or enforced policies and regulation, controlled budgets, levied fees and charges or set the overall vision for an activity. Based on these ratings, C40 developed ratings for what constituted ‘strong’ power for dozens of city ‘assets’, such as ‘centralized power generation’ or ‘city roads’. We then matched, as closely as possible, C40's ratings of these assets with the technologies and practices identified here. Ratings for sector-wide influence are weighted averages, where the weighting factors are the abatement potentials of the individual technologies or practices within the sector.

6. An additional consideration is that C40's assessment of mayoral powers focuses on the powers of city mayors. For some jurisdictions where a city legislative or other body has significant influence, these ratings may not accurately reflect the degree of influence.

7. For example, analyses or climate action plans in C40 cities London (Bioregional and the London Sustainable Development Commission, 2009), Seattle (Lazarus, Chandler, and Erickson 2013), and Portland (City of Portland & Multnomah County, 2009) have all looked at low-GHG diets in their climate action planning; San Francisco has adopted a meatless Monday resolution, and ICLEI's new US protocol (ICLEI – Local Governments for Sustainability USA, 2012) includes a focus on food.

Appendix. Sources for abatement potential estimates

Table A1 provides sources and notes on the abatement potential estimates used in this assessment. As described in the main body of this paper, we rely especially on the International Energy Agency's Energy Technology Perspectives (IEA, 2012a) because it provides a consistent set of abatement potential estimates for technologies and practices in several sectors that would not differ sharply in urban versus non-urban contexts (e.g. more-efficient vehicles and home appliances). For technologies and practices that are strongly urban, we either adjusted IEA estimates or used other, urban-focused studies, as described below. All abatement potential estimates are presented relative to the average world resident's carbon footprint in the year 2020 and 2050. While the method helps in providing one of the first assessments of the relative abatement potential of urban-scale technologies and practices, it is also important to understand its limitations. Perhaps most significantly, by assessing abatement relative to an average world resident's carbon footprint, our analysis makes little distinction between fast-growing cities (largely in developing economies) characterized by rapid development and growth in vehicle travel and those cities (especially in industrialized countries) where urban form and transportation patterns are already well-established. For example, in the former, measures to avoid vehicle trips, reduce trip length and build public transit systems may have greater abatement potential than assessed here. Similarly, our assessment also does not distinguish between cities in different climates; for example, cities in mild climates may have relatively less abatement potential in building heating and cooling equipment and thermal integrity.

Table A1. Sources of abatement potential estimates in the proposed typology.

Sector	Technologies and practices	Source of abatement potential estimates
Energy supply	Utility-scale low-carbon electricity supply	IEA ETP (IEA, 2012a) Figure 11.11, including abatement from carbon capture and storage, nuclear, hydro, solar photovoltaic, concentrating solar polar, wind, other renewables, as well as fuel switching (e.g. coal to natural gas) and power plant efficiency improvements
	Distributed renewables/on-site generation	Grubler and Fisk (2013) estimate of annual maximum urban renewable energy supply of 0.2–0.5 W/m^2, average urban land area of 186 m^2/person from Angel et al. (2005) and assuming avoided emissions of 0.6 tCO2/MWh (e.g. natural gas-fired power) and realization of potential linearly between 2010 and 2050^a
	District heating and cooling	Assumed doubling of global district energy penetration to about 40% of heat demand in 2050 at CO2-intensity of 12 kg CO2 per GJ (IEA, 2012a) displacing natural gas at 56 kg CO2 per GJ, as applied to building heating and cooling demand forecasts of Ürge-Vorsatz et al.’s (2012) and assuming all district heating is in urban areas given high densities needed^b
	Transmission and distribution/‘smart grid’	IEA ETP Table 15.1 (IEA, 2012a), cut by half to reflect lower baseline in ETP's 4DS scenario compared with 6DS assumption of table
Residential, commercial and institutional buildings and infrastructure	Heating/cooling equipment (including water heating)	Ürge-Vorsatz et al. (2012, Figure 4) for reductions in urban heating and cooling energy in deep compared with moderate scenario^c
	Building thermal integrity
	Appliances/lighting/plug loads	IEA ETP Figure 14.12 for cooking, lighting, appliances and ‘other’ efficiency, adjusted upward to exclude effect of low-carbon electricity assumed in IEA's 2DS scenario
	Reduced average home size	Assumed 20% decrease in per-person floor area and corresponding emissions for new buildings, roughly consistent with reduced floor area scenarios in Ürge-Vorsatz et al. (2012); applied to all building emissions in Figure 14.12 of IEA ETP, after scaling to new urban dwellers (UN, 2011)
Transport	Reduced vehicle energy intensity	IEA ETP Figure 13.21 adjusted upward to include overlap between this and other technologies and practices
	Lower-GHG fuel sources	IEA ETP Figure 13.21 adjusted upward to include overlap between this and other technologies and practices
	Increased operational efficiency of road network/transport system	Cambridge Systematics (2009) estimates of relative potential of systems operation and management (including ‘ecodriving’ and various forms of traffic management)
	Increased share of public and non-motorized transport (mode shift)	ICCT (2012) Roadmap model and assuming that all reductions due to mode shift occur in urban areas^d
	Avoided trips and/or reduced trip length	ICCT (2012) Roadmap model and assuming that all trip reductions and reduced trip lengths occur in urban areas^d
Industry, goods and materials	Process change, energy efficiency and fuel switching (in-region industrial facilities)	IEA ETP Figure 12.6 (high demand) case, scaled down to reflect lower prevalence of industry in urban areas as indicated by cities’ responses to survey by the Carbon Disclosure Project^e
	Use of low-GHG cement	IEA ETP Figure 12.6 (high demand) case, scaled to reflect estimated use of cement in urban construction^f
	Use of low-GHG steel	IEA ETP Figure 12.6 (high demand) case, scaled to reflect estimated use of steel in urban construction^g
	Increased consumer product longevity	Average global GHG footprint of clothing and household furnishings from Eureapa.net (Hertwich & Peters, 2010), and assuming a doubling of the useful life of these items and associated emissions cut in half (Barrett & Scott, 2012)
Agriculture and food	Healthier, low-GHG diets	Stehfest, Solecki, Hammer, & Mehrotra (2009) for Harvard Medical School's ‘healthy diet’. Vegetarian or vegan diets would lead to greater abatement potentials in their study
	Reduced food waste generation	Average global GHG footprint of food from Eureapa.net (Hertwich & Peters, 2010), food waste rates from FAO (Gustavsson, Cederberg, Sonesson, van Otterdijk, & Meybeck, 2011) and UK WRAP (Quested & Johnson, 2009), and assuming that avoided food is not purchased in the first place and that emissions are therefore avoided
Forestry and land use	Increased and improved urban tree cover	USFS study of Chicago's urban forest (McPherson, Nowak, & Rowntree, 1994)
	Forest and agricultural land conservation/avoided conversion	Emissions from land clearing for residential development in King County, Washington in 2003 (Erickson, Stanton, et al., 2012), a time of intense development in a region with high carbon stocks and assuming that 90% of these could be avoided
Waste management	Capture gas and/or energy from waste	US EPA draft update to Global Mitigation of Non-CO2 Gases (US EPA, 2012)
	Increased recycling and composting	McKinsey (McKinsey & Company, 2010) and substituting 2030 for 2050
	Wastewater treatment	US EPA draft update to Global Mitigation of Non-CO2 Gases (US EPA, 2012)

^aCalculation based on Hoogwijk's (2004) estimate of global potential of rooftop PV of 6 billion MWh/year yields a similar result.

^bThis method yields a potential average CO₂ abatement of 0.1 tCO₂ per person in 2050, or 2% of an average world resident's footprint. For comparison, a district energy feasibility study in the City of London (which currently has almost no district energy) estimated a CO₂ abatement potential of 0.22 tCO₂ per person in 2031, or 4% of London's emissions (Greater London Authority, 2011).

^cBy contrast, IEA ETP Figure 14.12 for fuel switching, building shell improvements, space cooling, solar thermal and heat pumps and cogeneration, adjusted upward to approximate emissions reductions from avoiding reference 4DS case electricity intensity rather than 2DS electricity intensity, shows somewhat lower potential in both 2020 and 2050.

^dWe use a version of the ICCT RoadMap model sent to us by the authors used to calculate the results through 2030 in their recent report (ICCT, 2012). That model also included placeholder, simple extrapolation results for 2050 that we use here, after reviewing other literature to gauge other results for 2050. More specifically, Cambridge Systematics (2009) and Hickman et al. (2011) estimate the effect of public transport and avoided and reduce trips at urban scales in the United States (Cambridge Systematics) and Delhi and London (Hickmam et al.) and find results somewhat greater. By contrast, IEA (2012a, Figure 13.21) finds results somewhat lower, assuming that all abatement potential is in the urban setting. We use ICCT results due to their detailed global analysis, but note that further analysis is needed for city-specific findings.

^eAverage industrial energy use in cities that reported to the Carbon Disclosure Project (CDP, 2012) is, on a per-person basis, about 47% of the average global industrial energy use. Accordingly, potentials here were scaled accordingly.

^fWe excluded cement use in intercity highways, estimated at 16% of the total per Kapur et al. (Kapur, Keoleian, Kendall, & Kesler, 2008), and assumed that remaining cement use was proportional to population (UN, 2011).

^gWe exclude the use of steel for non-construction uses, for example, for auto bodies or appliances, estimated at 61% of global total (Carbon Trust, 2011) and assume that construction uses are proportional to population (UN, 2011).