Conference Report: US–China Workshop on Pathways Toward Low Carbon Cities: quantifying baselines and interventions

The USA and China are the world’s two largest contributors of GHG emissions and together they are estimated to contribute approximately 41% of global GHG emissions [1]. A foreword to this special issue of Carbon Management outlines the broad trends in energy use and GHG emission in the USA and China, and describes various strategies and pathways for developing low-carbon cities in the two nations [2]. While international treaties to mitigate GHG emissions have not yet been ratified, more than 1026 cities in the USA alone (collectively >30% of the national population [101]) have pledged to reduce community-wide GHG emissions to less than 1990 levels, and an increasing number of cities in China, particularly 13 pioneering city and greater city regions (∼26% of the national population) showcased by China’s national development and reform commission, have set aggressive GHG emission reductions as part of their 11th 5-year plan [102]. Thus, examining various pathways toward low-carbon cities in the USA and China has great potential to contribute toward GHG mitigation globally.

However, there are several key measurement challenges in achieving the goal of low-carbon cities [3], such as the following:

	▪ First, there are no agreed upon protocols for measuring community-wide GHG emissions associated with cities. Such measurements are confounded by the transboundary nature of essential infrastructures serving cities, such as energy use in transportation (i.e., power and water supply often occur outside the boundary of the city using these services), and by the significant trade of goods and services that occurs across cities.
	▪ In addition to the boundary challenge, there is also the question of what must be measured to represent human impacts on our climate. Along with conventional long-lived GHG such as CO2, CH4, N2O and the three hydrochlorofluorocarbons, together, reported as CO2-equivalents (CO2e), studies are showing that short-lived climate forcers (SLCF), such as black carbon, can exert significant radiative forcing in the atmosphere, impacting climate regionally in the next few decades as well local air pollution and public health in cities [4]. Incorporating SLCF into traditional GHG accounting can transform a distanced debate about global climate change into one of regional and local air pollution with associated health impacts. Thus, quantifying not only GHG emission reductions, but also the other environmental, health and economic cobenefits of low-carbon development strategies becomes very important:
	▪ However, such quantification and measurement is not easy to accomplish. Even at the scale of buildings and neighborhoods, energy and GHG savings from various infrastructure engineering and design interventions vary widely based on local climate conditions, existing building/land-use characteristics (e.g., urban form and density), consideration of confounding factors (e.g., self selection bias impacting travel behaviors in high-density neighborhoods), as well as consideration of life cycle impacts;
	▪ The role of people (social actors) in shaping GHG mitigation associated with cities is also poorly understood. Much work is needed to understand the relative contribution to GHG mitigation from changes in the social system that address behavioral, institutional and regulatory pathways, as well as infrastructure and urban design interventions, and the interplay between the social and biophysical subsystems.

Workshop organization & participants

Given the key measurement challenges described above, a workshop sponsored by the US National Science Foundation was convened to bring together leading researchers from the USA and China to share and discuss cutting-edge research pertaining to the quantitative assessment of GHG emissions and mitigation actions at the city scale. The workshop was organized jointly by the University of Colorado, Denver (CO, USA) and the Hong Kong Polytechnic University (HKPU; Hong Kong) and was held in December 2010 on the HKPU campus.

The workshop took a unique inter-disciplinary approach, inviting researchers from multiple disciplines necessary for transitioning into low-carbon cities of the future, encompassing engineering, building sciences, architecture, urban planning, transportation, environmental sciences, atmospheric sciences and public affairs. In total, 30 speakers attended; 13 from the USA and 17 from China. The workshop was organized broadly into three groups of topics and sessions:

	▪ Integrative assessment of energy use, GHG emissions and mitigation goals in cities;
	▪ Infrastructure and urban design strategies, addressing buildings, transportation and materials-waste sectors;
	▪ Role of social actors in GHG mitigation, with emphasis on policy actors.

The following represent the key findings from the workshop presentations, arranged according to the three themes listed above.

Integrative assessment of energy use, GHG emissions & mitigation goals in cities

▪ The challenges in measuring city-scale GHG emissions

The challenges in measuring city-scale GHG emissions were summarized in a presentation by Ramaswami and Chavez (University of Colorado, Denver) [5]. The presentation reviewed three different approaches to GHG accounting and footprinting for cities and described their advantages and disadvantages. The authors concluded that purely territorial GHG emissions accounting are not suited to cities, since most cities import key infrastructure services, such as electric power. Two more advanced methods (a transboundary infrastructure supply chain footprint method, as well as purely consumption based accounting) offer different but useful policy insights. The transboundary infrastructure supply chain method (as the name implies) supports infrastructure planning for cities, while the consumption-based approach allocates global GHG more rigorously to final consumption, primarily by households in cities. Obtaining city-scale data may be more challenging for the latter method. Ramaswami and Chavez indicated that in the purely territorial and in the transboundary methods, cities cannot be compared with each other based on per capita GHG emissions. Since cities contain varying proportions of both production and consumption activities, new metrics must be explored for improved inter-city comparisons by these methods. Alternatively, composite metrics of sector-specific efficiency indicators may be developed to compare cities.

The same points were reinforced in a presentation by Zhou (Lawrence Berkeley National Laboratory), who traced the history of energy and GHG intensities per unit GDP in USA and China and highlighted that after 2 decades of achieving a degree of decoupling, China’s energy and GHG intensity per unit GDP are rising [6]. Thus, new efforts are required to achieve energy efficiency gains, many focused on cities in China’s current 11th 5-year plan. Zhou presented overview data on energy use and GHG emissions in Chinese cities and highlighted that, unlike in US cities, the industrial sector contributes a large proportion of city-scale GHG emissions (50–70%) in China. Since the proportion of industrial activity can vary significantly across cities, Zhou also argued that a per capita GHG metric is not suited for inter-city comparisons. Zhou proposed a number of different weighted indicators that can help rank the GHG performance of cities, and introduced a tool for planning low-carbon interventions in cities.

The presentations by Ramaswami [5] and Zhou [6] demonstrate an urgent need to develop better measurement methods and metrics for comparing cities on the basis of energy efficiency and GHG emissions. Presentations on Shanghai by Bao (Tongji University, Shanghai, China) reinforced these insights [7].

▪ SLCF & black carbon

A presentation by Schauer (University of Wisconsin, WI, USA) introduced the importance of SLCF with emphasis on black carbon and highlighted that its impact on radiative forcing can be as high as 50% of that of CO₂ emissions [8]. Since the emissions of black carbon depend not only on the quantity of fuel combusted (e.g., diesel) but also on the specifics of the combustion technology and process (e.g., different types of engines have different fractions of organic carbon emitted as black carbon), Schauer recommended further field measurements in cities in Asia in order to better integrate SLCFs such as black carbon into city-scale GHG inventories. Workshop participants recognized the challenge of combining and reporting traditional GHGs (reported at CO₂e) and SLCFs (expressed as radiative forcing) in metrics that are readily understood by the public and by policy makers.

▪ Social context

Discussions around policy actors, presented by Lam (The Chinese University of Hong Kong, China) [9], Zhu (Tongji University) [10] and Wong (Environmental Protection Department, Hong Kong, China) [11], provided an overview of the Chinese government’s articulation of low-carbon development wherein economic development is articulated as a primary priority with reductions of carbon intensity per unit GDP representing the low-carbon goal as a secondary objective. A presentation by Amekudzi (Georgia Institute of Technology, GA, USA) provided a global framework for such multi objective decision-making for sustainability, placing low-carbon goals in the context of overall sustainable development priorities in communities [12]. Amekudzi argues that carbon mitigation cannot be the primary objective in all cities worldwide. For many cities in the USA that enjoy a high human development index (HDI), the primary goals of several sustainable energy plans are to reduce overall GHG emissions, while job creation and health are articulated as co-benefits. In China, economic development is articulated as a primary goal, while carbon reductions are motivated as secondary goals encapsulated in mitigating GHG/GDP intensity. For countries such as Haiti where basic services are lacking, primarily investing in carbon mitigation can come at the expense of HDI improvements.

These examples underscore that carbon mitigation benefits and co-benefits must be quantified to enable contextual prioritization in different nations.

Quantifying GHG mitigation & co-benefits in infrastructure interventions

▪ Buildings

In the session on buildings, Zhai (University of Colorado, Boulder, CO, USA) illustrated financial co-benefits of conducting deep-building retrofits using the case study example of the Federal Office Building in Denver (CO, USA) [13], a pilot-to-portfolio approach is recommended enabling choice from a menu of retrofits for building operators. Wang (Hong Kong Polytechnic University) provided similar and parallel data for Hong Kong, using data from the International Commerce Centre building [14]. Matthews (Carnegie Mellon University) presented data from a forthcoming paper showing that the energy efficiency gains from building retrofits can vary by a factor of two to three across cities based on local climate conditions (e.g., heating and cooling degree days) [15]. Given such variability, he presented an optimization tool to maximize GHG mitigation given a fixed budget. Williams (Arizona State University, AZ, USA) demonstrated the role that ICT technologies can play in increasing energy efficiency in residential buildings, in particular highlighting low occupancy in most parts of buildings and homes during significant portions of the day when these unoccupied spaces are presently being heated or cooled by centralized heating, ventilation and air conditioning (HVAC) systems [16].

Bank (City College of New York , NY, USA) described the development of a building information model (BIM) integrated with a decision-making tool that addresses multiple objectives of sustainable building design, operations and retrofits; for example, not only energy and GHG savings, but also economic costs, human health considerations and others [17]. This integrated BIM-decision tool allows for sustainability trade-off analyses to be conducted with multiple objectives, in real time, over the life cycle of the building, incorporating the impact of design, maintenance, operations and occupant behavior modification decisions. In terms of renewable energy options, Lu (Hong Kong Polytechnic University) shared recent developments that integrate vertical axis wind turbines and photovoltaic solar technologies into buildings, resulting in on-site direct electricity generation [18]. Niu (Hong Kong Polytechnic University) presented potentials from thermal energy storage units, highlighting the advantages through quantified hard savings in energy and money [19].

Key conclusions from the buildings sector show that energy and GHG savings can be highly variable, with estimates varying widely both by local climate and consideration of life cycle impacts. It was noteworthy that the life span of buildings in China was reported to be much smaller than the 30–50 years for commercial buildings in the USA, explaining why the life cycle embodied energy of building materials are a large proportion of total energy use of buildings in China. Dynamic modeling and meta-analysis on energy savings and other sustainability benefits are essential to promote deep changes in the buildings sector.

▪ Transportation

Bagley (Kohn Pedersen Fox Associates) shared stunning visuals demonstrating the massive scale of new cities being built in China, a mix of very high-density and moderate density mixed-use serving younger professional working populations in upcoming Chinese cities [20]. In these new developments, large skyscrapers holding several tens of thousands of occupants (in mixed-use applications), are connected directly via mass transit from building to building across cities. Newer land development patterns were also presented by Ho (University Teknologi Malaysia) [21] for cities in Malaysia, where studies are now gathering data on travel behaviors and distances. Such information can inform the development of new low-carbon zoning codes in Asian cities – discussed by Yip (Arup, China) for the case study of Beijing, China [22].

Kockelman (University of Texas at Austin, TX, USA)presented a summary of several studies aimed at unraveling the connection between land use and transportation in US cities [23]. She described the challenges in accounting for confounding factors in such meta analyses, for example, self-selection bias for reduced automobile ownership among residents who choose urban high-density living. Kockelman compared regional features, such as accessibility to jobs, as well as neighborhood level design of streets and intersections, and compared such land-use interventions with other levers, for example, economic instruments such as carbon taxes and increased parking fees.

Workshop participants discussed the need to measure anticipated outcomes (e.g., reductions in VMT or transportation energy use intensity) of such land use and urban design interventions both in the USA and China to enable translation of these measures to other cities.

A life cycle assessment of the energy use of bus rapid transit (BRT) versus conventional bus travel in Xiamen City presented by Cui (Institute of Urban Environment, Chinese Academic Sciences) illustrated the importance of considering the full system life cycle energy use in analysis of infrastructure interventions [24]. Cui found vehicle operations energy saving wells-to-wheels (WTW) in the BRT system almost double that of the normal bus system due to the large volume, energy-saving vehicle type as well as exclusive right-of-way that improves fuel economy (reduced congestion). Interestingly, the vehicle operations energy in both BRT and conventional bus systems was a relatively small proportion (20 and 12%, respectively) of the life cycle energy use in the Chinese bus systems, while WTW vehicle fuel use is estimated to be a much greater proportion (70–80%) in the USA. Energy use to operate the BRT infrastructure (e.g., electricity use for elevators and escalators in the elevated BRT) and embodied energy of infrastructure materials were found to be equal or larger contributors to GHG emissions in China compared with conventional bus travel. These examples point to the need to look beyond vehicle-miles-traveled and vehicle-fuel-economy to full life cycle based analysis of transportation infrastructure.

▪ Materials, waste & symbiosis

LCA-based analysis was also cited as crucial in two important infrastructure interventions in the materials and waste sector. Han (City University of Hong Kong, China) presented case study data demonstrating potential for energy and GHG mitigation from industrial symbiosis in Tianjin Economic-Technological Development Area (TEDA), located 140 km South East of Beijing [25], while Ren (University of Colorado, Denver) showed the potential for energy savings and even energy generation in bench-scale demonstration units by using microbial fuel cells for desalination of water, a strategy that may be required in many water-scarce cities of the world [26]. Poon (Hong Kong Polytechnic University) evaluated the role of waste to energy incineration projects in reducing GHG emissions [27], while Li (Hong Kong Polytechnic University) examined the potential for carbon storage in urban soils [28].

▪ Social actors & policies

A final and most lively session examined the role of people in shaping low-carbon outcomes in cities. Weible (University of Colorado, Denver) and Davis identified three categories of actors, individual users who drive the need for water and energy in cities, infrastructure designers and operator provision essential services to urban residents, and the policy actors (e.g., government officials, policy makers, media, special interest groups and scientists) who govern urban infrastructures toward various outcomes [29]. They described a multi-theoretical approach to study the inter-dependent behaviors of the three actor categories, explaining how and why people engage in voluntary environmental programs, as well as collective action among policy actors that shapes how we govern energy use and GHG emissions associated with cities. Feiock et al. (Florida State University) further investigated the policy actor category studying more than 1000 municipal governments and a random sample of cities in the USA [30]. They examined how political and institutional factors, as well as various public entrepreneurs affect the adoption of GHG inventories in cities, and differences among policy directed toward community-wide emissions versus governmental operations. Their study results confirm that elected mayors and civic entrepreneurs promote community-wide GHG emissions reductions, while managers and bureaucratic entrepreneurs focus their efforts on carbon emissions of local governmental operations.

Chan (Hong Kong Polytechnic University) [31] and Ramaswami et al.[32] compared outcomes and participation rates in voluntary programs (e.g., voluntary building retrofits) in communities versus regulatory policies established by governments at different scales (city, state and national) in China and the USA. In the absence of regulations, participation rates in voluntary programs were typically too small in most Chinese cities and in typical US cities to significantly reduce GHG emissions. Thus, novel efforts, for example, interventions that use the power of social networks, to enhance participation in voluntary programs, or greater diffusion of innovative regulatory approaches are needed to generate meaningful GHG emissions reductions in cities, both in the USA and China.

Conclusion

The workshop brought together researchers from several disciplines, and demonstrated the different insights that can be gained from the broad-based study of low-carbon cities. Workshop participants concluded that integrated study of multiple infrastructures, social actors and system wide sustainability outcomes across the USA and Chinese cities would provide important insights and significantly advance our understanding of pathways and barriers for low-carbon city development worldwide. Long-term inter-disciplinary and international collaborations are essential for such efforts.

Acknowledgements

This report provides a synthesis of papers and presentations made at a Joint US–China Workshop on Pathways Toward Low Carbon Cities, held in Hong Kong in December, 2010, sponsored by the US National Science Foudnation (NSF CMMI-1045411).

Financial & competing interests disclosure

The authors have no 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. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties. No writing assistance was utilized in the production of this manuscript.