Current trends and limitations of life cycle assessment applied to the urban scale: critical analysis and review of selected literature
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From 2010, more than half of human beings live in cities and global urbanization is growing at a fast pace. This leads to threats for the associated potential environmental burdens, but also to opportunities for cities to gain a leading role as hubs of interventions in favor of sustainability. The Life Cycle Thinking approach is suitable to account direct and indirect urban impacts, although methodological refinements are necessary to make it applicable at the urban level.
The goal of the present review is to provide a comprehensive insight in the application of Life Cycle Assessment (LCA) at the city scale, highlighting good and working points to properly identify and address the future research agenda to make LCA suitable to this challenge. The review considers a wide range of urban sub-sectors and activities (namely, built environment, energy systems, waste and water sector, transportation, consumption patterns, and urban ecosystems), as well as hybrid and upscaling approaches. The relevant papers were selected according to two criteria: (i) comprehensive impact assessment and (ii) and wide spatial scale of application. Subsequently, key features were screened and critically analyzed: (i) functional unit, (ii) system boundaries, (iii) data sources and granularity, and (iv) impact assessment methods.
Results and discussion
A short list of 65 papers published from 2010 was reviewed with no geographical restrictions. The analysis of the selected literature shows that no applications of a comprehensive LCA at the urban scale exist to date. Waste and water sub-sectors account for about the 20% of the coverage in literature. Transportation sectors and energy systems follow (about 10 and 9%, respectively), while a total of five studies take into account consumption patterns and urban ecosystems. Even if really relevant for the topic, the built environment is an unexplored sector yet. Methodological considerations are poorly addressed. First attempts of upscaling and hybrid approaches are available in literature, but most of the time still limited in scope, and only two researches try a full integration and propose methodological reflections.
The findings emerged from the present review trace the lines of a feature research agenda. Most of the applications to the different urban sectors are still immature for a transfer from product/process level to system level. Main research challenges include the definition of proper system boundaries and an appropriate functional unit, able to take into account the dynamics inherent to the city. An adequate data granularity and a proper organization of the life cycle inventory shall be time efficient and capable to detect in a precise way the potential hot spots at the macro- and micro-scale level. Furthermore, the urban context may require more specificity when applying impact assessment, as current impact assessment models have usually a coarse resolution. Proposals are made for an improved definition of the functional unit and data collection process.
KeywordsCity LCA Sustainable city Urban environmental assessment Urban metabolism Urban sustainability Urban systems
Cities are the major poles of aggregation of human beings, and, for the first time in human history, since 2010 more than one person out of two are living in an urban area (Yetano Roche et al. 2014; United Nations 2012). In the past decades, urban population increased at a high pace, especially in developing countries, and projections forecast further increase, up to 66% by 2050 (Yetano Roche et al. 2014). Comparing developed and developing countries, many drivers are responsible for this circumstance, e.g., the intersection between supply and offer of jobs and services, but also alleged better and safer living conditions or even a more attractiveness in terms of social lifestyles and opportunities.
The increasing of people living in urban area is extremely widespread in the globe, but with different characteristics. In the European Union, around 70% of the EU population lives in urban agglomeration of more than 5.000 inhabitants, with the biggest share in small and medium size cities (32.6%, according to the estimates of European Commission (2011)). Europe is characterized by a polycentric layout, and its urban areas are less dense than American or Asian ones. Indeed, one fourth of the population in the USA live in cities of over five million inhabitants compared to the 7% of the European context (European Commission 2011). In 2011, only two European cities out of 27 could be categorized as megacities,1 while the phenomenon is clearly predominant in Africa and Asia (Kennedy et al. 2015).
Despite the differences, the global growth of cities poses, globally, the challenge of environmental sustainability and makes the study of emissions, energy, and material consumption of paramount relevance within urban context. As cities concentrate most of the world’s economic activities, this convergence of human and economic capital could lead to even higher pressures in terms of environmental impacts, if not properly addressed. From the industrial revolution and the rise of capitalism, the resource use intensity and the massive deployment of fossil fuels to sustain urban life has resulted in a period of economic and social prosperity for cities in developed countries. Nevertheless, this implied a huge demand for resources, causing their continuous and increasing depletion and claiming for the oxymoron “unlimited resources in a limited world” (Pincetl et al. 2012). In low efficiency conversion systems, waste generation runs simultaneously to the resource and energy exploitation, and this combination further increases the environmental burden.
Developing countries, where the pace of population growth is higher, are experiencing a similar situation, but other significant social and economic challenges also occur, such as poverty, lack of appropriate housing, crime, and health-related issues. These will likely overwhelm efforts to address the environmental emergencies (Ferrão and Fernandez 2013), even if the consumption rates of food (e.g., meat) and goods are steeply increasing.
Worldwide, cities are the most complex anthropogenic system ever created by man, and several challenges need to be addressed, in order to act effectively, understand, and manage their impacts. Two of the most urgent thereof are intimately related, they are born within the semantic space to stray and develop within the technical space. Firstly, a proper definition of “city” is not available yet, but several definitions are possible according to the field of knowledge, i.e., urbanism, geography, economy, sociology, etc. Secondly, if defining the urban system is a hard task, defining its boundaries and functions is even more challenging, and this poses severe methodological choices for a proper urban environmental assessment. Various methods are available to establish the system boundaries of a city or urban area (Seto et al. 2014): (i) administrative approach, (ii) functional approach, and (iii) morphological approach. The first approach refers to the territorial or political boundaries of a city (Hartshorne 1933; Aguilar et al. 2003). The functional approach is based on the evaluation of connections and interactions between areas, such as economic activity, per capita income, or commuting zone (Brown and Holmes 1971; Douglass 2000; Hidle et al. 2009). Finally, the morphological approach is based on the form or structure of land use, land cover, or the built environment (Benediktsson et al. 2003; Rashed et al. 2003). Furthermore, as urban systems rely mainly on their surroundings and on natural ecosystems to sustain their activities, it is of paramount relevance taking into account the local and global reality in which they prosper and the transboundary processes, including both upstream and downstream flows. Indeed, cities are most likely to host consumption activities whose production chains are outside urban borders and potentially related to global and interconnected supply chains (Lenzen and Peters 2009).
Once the boundaries of a city are defined, its environmental performance may be considered from different perspectives: spatially limited geographic boundaries (territorial/production-based approach), or based on a broader consideration that also includes cross-boundary flows and/or indirect emissions (supply chain or consumption-based approach) (Kennedy et al. 2011; Yetano Roche et al. 2014). A comprehensive sustainability assessment should encompass the evaluation of what is happening at the local scale, affecting the local environment, as well as impact generated along supply chains and driven by local consumption patterns. Hence, holistic methodologies, allowing for a comprehensive overview and definition of appropriate guidelines, are necessary to tackle the urban environmental assessment challenge.
1.1 Current methods for an urban environmental assessment
At the urban level, procedural and quantitative approaches exist, and several frameworks, standards, and schemes of indicators (European Commission 2015; Albertí et al. 2017) proliferated in the past. Procedural approaches (such as Strategic Environmental Assessment or EMAS, the EU Eco-Management and Audit Scheme) are implemented since many years. However, they rely on indicators selected case by case and they barely accounts for shadow or embodied emissions related to production and consumption patterns at urban scale. Although holistic methodologies to evaluate the environmental footprint of cities are necessary, these are still lacking at the urban scale level. Few quantitative metrics exist to evaluate and improve the sustainability of cities from the environmental point of view. Two quantitative approaches are the most popular nowadays: mono-footprint methods (i.e., ecological, carbon, and water footprint), and the urban metabolism (UM) approach including the descending methodologies under its umbrella.
Currently, considering the rising awareness related to climate change and water scarcity, carbon footprint and water footprint are widely used methods to evaluate the environmental performance of cities. These approaches can rely on a more consolidated politics’ and citizens’ concern and initiatives, as well as specific methodological guidelines relatively for their implementation (Hoekstra et al. 2011; WRI 2014; ADEME 2010; ISO 2006). These elements favored the proliferation of carbon and water footprint studies at the city level, despite their limited scope and identification of the hot spots. The UM concept uses a top-down approach and is one of the most widely applied models to gain insights in the material and energy flows to and from cities (Pincetl et al. 2012). UM clearly has it strengths in its established and scientifically recognized framework, it gives an overview of the local reality through the assessment of the flows into and out of the city (Ferrão and Fernandez 2013), but it does not provide a clear idea with what happens inside the city and it does not allow to interpret theflows in terms of environmental impacts (Goldstein et al. 2013). Furthermore, the focus of UM studies is constrained within the city boundaries, upstream flows are then omitted or hidden and it is hard to describe the relation between the flows and the specific urban or residents’ activities. Beloin-Saint-Pierre et al. (2017) investigated urban metabolism studies and the environmental assessment methods that have been used for the past 40 years. The authors suggested that a network system modeling approach, a global life-cycle perspective, and a multi-criteria assessment are strategic choices for environmental sustainability assessment.
The Life Cycle Thinking (LCT) and, specifically, Life Cycle Assessment (LCA) may be considered the most comprehensive and robust holistic methodology to face this challenge. Even if LCA was born as product-oriented, recent proposals have been made to broaden its object of analysis by studying larger scale systems (Pincetl et al. 2012; Chester et al. 2012; Loiseau et al. 2013; Hellweg and Mila’I Canals 2014) that could even take into account the surrounding context and potential rebound effects (UNEP 2011). The increasing attention towards this topic by the scientific community and policy makers is leading to an increasing number of LCA studies implemented to the system/infrastructure level (Hellweg and Mila’I Canals 2014).
Albertí et al. (2017) reviewed current existing LCA-based, non-LCA-based, and sustainability standards and guidelines available for cities or urban regions. The authors observed a lack of consensus in the definition of the city and its boundaries, which hinders the development of useful sustainability standards. Furthermore, it is concluded that current sustainability assessment tools miss, at least, one of these aspects: (i) holistic point of view, (ii) focus on various environmental impacts, (iii) LCT perspective, and (iv) the possibility to compare the results among different cities or urban regions. Petit-Boix et al. (2017) reviewed LCT studies related to urban issues and the proposed sustainability strategies to improve their environmental performance. In order to tackle urban complexity, the authors claimed for an enhanced integrated schemes able to LCT with other methodologies, the analysis of environmental, the social and economic dimensions, and the evaluation of multiple impact indicators for minimizing the potential trade-offs among impact categories. Nevertheless, the application of LCA to the entire urban scale is not a reality yet. This means that its application is limited in scope and applied to a portion of the city, geographical (e.g., neighborhood scale) or sector based (e.g., only the urban waste management sector is investigated). Even a combination of LCA with top-down methodologies is still not applied to the entire urban system, despite its advantages (Pincetl et al. 2012; Chester et al. 2012). A first attempt to fuse the UM approach with the LCA approach was done by Goldstein et al. (2013). The proposed UM-LCA translates the input-output flows within the city in terms of environmental impacts, but it considers only four environmental indicators and proper methodological refinements are claimed by the authors to adapt LCA at the city level.
As no complete urban LCA studies exist so far, the emphasis of the present review is on the current application of LCA at the city level, considering a wide range of urban activities. The aim of this contribution is twofold: firstly, it attempts to provide a comprehensive review of the existing literature in the field of LCA applied to the entire urban context, considering the different urban sectors (namely, built environment, transportation, waste, etc.), and, secondly, it attempts to highlight key features for intercross comparison. A critical analysis of the results and a proposal for future applications of LCA to urban systems are also provided. The paper is structured as follows: in section 2 materials and methods of the review, where goals and scope, selection criteria of the papers, and key features of analysis are presented; section 3 provides the most important results and key factors emerged from the review; section 4 discusses and critically analyses current methodological bottlenecks for implementation of LCA at the urban level; and finally, section 5 provides proposals to overcome the identified shortcomings, conclusions, and take-home messages emerged from the review.
2 Materials and methods
Whereas, a direct application of the methodology to the entire urban scale does not exist yet, an analysis of the existing literature for the different urban sub-sectors was carried out. The European Environment Agency (2017) defined the grey and green infrastructure necessary to sustain life in cities. The grey infrastructure was identified as main driver of urban impacts (European Environment Agency 2010) and covers the building environment, the mobility system, the consumption patterns, and the level of infrastructure for environmental management of, e.g., water and waste. The green one covers urban ecosystem services. Hence, the sub-sectors identified and reviewed are (i) built environment, (ii) energy systems, (iii) waste, (vi) water, (v) consumption patterns, (vi) transportation networks, and (vii) urban open spaces and green areas (including aspects related to land use and ecosystem services). The intention is to cover extensively the majority of the urban activities that enable to sustain the “life” of the city itself and its inhabitants, in terms of citizens, residents, and visitors (commuters, tourists, etc.), services, etc.
List of keywords and scientific literature reviewed for the study
LCA AND CITY
LCA AND URBAN
(urban) metabolism/built environment/ building/ waste/water/transportation/energy/land use/ecosystem services/consumption/lifestyle/household/input-output/material flow analysis/energy/remote sensing/BIM/GIS/neighborhood
Scientific journals and literature [nr of papers]
Clean Techn Environ Policy ; Journal of Environmental Engineering and Landscape Management ; Waste Management ; Eng Sanit Ambient ; Waste Management & Research ; J Mater Cycles Waste Manag ; Applied Energy ; Building and Environment ; Computers and Chemical Engineering ; Desalination and water treatment ; Ecological Indicators ; Ecological Modeling ; Energy Conversion and Management ; Energy Policy ; Environment International ; Environmental Research Letters ; Frontiers of Earth Science ; Geoforum ; Hungarian Journal of Industrial Chemistry ; Int J Life Cycle Assess ; International Building Performance Simulation Association ; J Mater Cycles Waste Manag ; Journal of Cleaner Production ; Journal of Environmental Management ; Journal of Geoscience and Environment Protection ; Journal of Industrial Ecology ; Journal of Infrastructure Systems ; Journal of Natural Gas Science and Engineering ; Land Use Policy ; LCA [avniR] Conf.; Natural Resources Forum ; Procedia Engineering ; Renewable Energy ; Resources, Conservation and Recycling ; Sustainability ; Sustainable Built Environment (SBE) regional conf. ; The Journal of Transport and Land Use ; Transportation Research Part D ; Urban Water Journal ; Water Research ; World Sustainable Building conf. 
According to Loiseau et al. (2013), Goldstein et al. (2013), and Albertí et al. (2017), the LCA methodology presents four major bottlenecks that needs refinements and adjustments for application at the territorial scale. These bottlenecks are (i) system boundaries, (ii) functional unit (FU), (iii) data gathering, and (iv) Life Cycle Impact Assessment (LCIA). As these considerations are mostly valid for application of LCA to the urban scale, these four bottlenecks plus other key elements are taken into account to provide the basis for a methodological discussion and propose new advancements.
Hence, section 4 presents a comparative analysis of the paper review based on the following LCA phases and elements. For phase 1, goal and scope, the bottlenecks analyzed are (i) system boundaries, (ii) functions and FU, and (iii) allocation procedures of cross boundaries processes/production and consumption. For phase 2, Life Cycle Inventory, the attention is focused on the data gathering, their granularity, quality, and representativeness. This is a crucial issue in urban studies due to the size of the systems investigated and the involvement of many entities. Moreover, an appropriate granularity of data is of paramount relevance for the precise identification of hot spots. Finally, the investigation examines the selected impact assessment methods and the address of considerations about the scale of impacts evaluated (phase 3, LCIA).
A shortlist of 67 papers were reviewed, including the following sources: 62 from scientific journals and 5 from conference proceedings. The number of scientific journals involved is 37. The results and most important features for each category analyzed are presented in the Electronic Supplementary Material and shown in Table S1. A brief overview is briefly presented in section 3.
3.1 Urban sub-sectors and activities
3.1.1 Built environment
The built environment is a crucial sector in the urban sustainability agenda. The share of impacts generated by the building industry is one of the most significant in the panorama of human activities. About 40% of the materials (i.e., three billion of ton) used in the global economy are consumed for construction purposes (Lavagna 2008) with a high rate of waste generation, with up to 30% of all waste generated in the European Union (European Commission 2016). Energy consumption in buildings is very significant too (Cabeza et al. 2014), also because buildings use direct and indirect energy throughout their life, i.e., from their construction to their demolition. Estimates report about 42% of the energy is consumed in buildings in Europe (Lavagna 2008). In urban areas, buildings represent a big portion of final energy use and GHG emissions and the increasing urbanization requires a better understanding of the embodied energy and material requirements for a reliable environmental assessment and informed decisions (Stephan and Athanassiadis 2016). For what concerns buildings, current urban environmental assessments tend to focus either on operational aspects or on material stocks, most of them using top-down or bottom-up techniques that excludes high spatial resolution or the three dimensional geometry of each building (Stephan and Athanassiadis 2016).
Numerous studies already applied LCT at the building level, focusing on their design, construction, materials, etc., but few studies apply LCA of built environment at the neighborhood level (see section 3.2 for further details) and no comprehensive LCA studies fit the selection criteria. Mastrucci et al. (2017b) reviewed a selection of bottom-up LCA studies evaluating the environmental impact of building stocks at several scales, from urban to transnational. According to the authors’ results, two main stock aggregation approaches, the archetype and building-by-building approach, are applied to model individual buildings and extrapolate results at stock level. The combination with GIS data, model calibration, and stock dynamics models is recommended to enhance these aggregation models. In particular, there is a potential for coupling LCA and GIS for spatially explicit inventories and LCIA.
3.1.2 Energy systems
Since the industrial revolution, energy consumption has sharply increased worldwide with economic development and population growth. It is estimated that energy consumption in urban areas accounts for more than 70% of the total global energy use and GHG emissions (UN Habitat 2016). Heating, cooling, electricity use, and mobility are the four components of final energy consumption in the urban systems. The majority of the studies available in literature focus either on one of the four components or on their energy carriers often for carbon emission accounting purposes. Few studies apply a holistic LCT approach to urban energy systems, and six studies were selected in the scope of this review, one at the national scale level and five at the urban scale level.
Atilgan and Azapagic (2015) analyzed the environmental impacts of electricity generation in Turkey from fossil fuel power plants, which supply three quarters of the national demand.
Moret et al. (2016) presented a methodology to assess the potential for integrating deep geothermal energy and woody biomass in an urban energy system. The city of Lausanne (Switzerland) is modeled in its entirety as a multi-period optimization problem with the total annual cost as an objective, assessing as well the environmental impact with LCA.
Chen et al. (2014) calculated the environmental impact of various types of energy resources (including coal, oil, natural gas, and electricity) used in urban regions and applied to the case study of Beijing (China). A second consecutive study propose a multi-objective optimization model at the urban sector scale (Su et al. 2016). The model considers the energy, economic, and environmental perspective, by integrating objectives of minimal energy consumption, energy cost, and environmental impact.
Ripa et al. (2017) evaluated the environmental performance of an enhanced mechanical and biological treatment demo plant installed in Mertesdorf (Germany). The plant under study is designed to concentrate the biodegradable part of the Municipal Solid Waste (MSW) and obtain a suitable biomass fuel with a final marketable quality, fulfilling the requirements for biomass power plants to generate urban decentralized production of heat and power (CHP). All materials, emissions, cost, energy consumption, and recovery levels were referred to the disposal of 100,000 t of waste (this amount is approximately generated by a medium-size city on yearly basis).
Bonamente et al. (2015) studied a single, vertical, stand-alone renewable energy plant designed to decrease the primary energy consumption from fossil fuels, to reduce GHG emissions, to maximize the energy production from renewable sources available in one place, and to minimize land use. A feasibility case study was performed for the city of Rome, Italy. Several technologies are exploited and integrated in a single system, including a photovoltaic plant, a geothermal plant, and a biomass digester for urban organic waste and sewage sludge.
Water is essential for the life of human beings, and this makes the urban water sector critical for stakeholders. In future, challenges related to water and sanitation are supposed to magnify due to demographic dynamics and climatic related impacts that may affect the availability of resource and the demand (UN Habitat 2016).
The sustainability of urban water management systems is one of the most mature fields of applications of the LCA method, as reported in this review. Thirteen studies were screened, both coming from developed and developing countries (eight and five studies, respectively). The urban water sector studies cover water and wastewater treatment, management, and supply. The majority of the reviewed studies deals with both water and wastewater systems, the remaining focus only on wastewater services. Comparative LCA is the main methodological approach observed across the studies, and traditional and innovative scenarios are set for strategic assessment or policy-making purposes.
Slagstad and Brattebø (2014) examined the system-wide life cycle potential environmental impact of operating the city’s water and wastewater system. The results of this study were used in the planning of a new carbon-neutral urban settlement in Trondheim (Norway).
Lane et al. (2015) generated the environmental impact profiles for two city-scale urban water systems in Australia: one typical of many urban centers, with a high reliance on freshwater extraction and the majority of treated wastewater being discharged to the sea; and one that adopts a more diverse range of water supply and wastewater recycling technologies.
Opher and Friedler (2016) compared the environmental impacts of four alternatives for a hypothetical city’s water-wastewater service system: one traditional linear scenario and three different scales of distribution of the wastewater treatment phase, along with urban irrigation and domestic non-potable water reuse (toilet flushing). All scenarios refer to a hypothetical future city (aka Model City) which, according to national statistics and demographic projections for the future, is assumed to be a common medium-sized city in Israel in 2050. The purpose of the study is to provide a policy support by means of comparative and consequential LCA.
Iasi (Romania) is the reference city for two studies because of its representativeness for Romanian water services. In the first study (Barjoveanu et al. 2014), the entire water services system in Iasi was investigated.
A second study on Iasi (Teodosiu et al. 2016) focused on an environmental assessment of a municipal wastewater treatment plant discharges by means of a number of approaches, namely: LCA, Water Footprint, Environmental Impact Quantification methods. The purpose was to understand their (methodological) weak and strong points in addressing the impacts.
Mahgoub et al. (2010) evaluated the environmental impacts of urban water system in Alexandria (Egypt). They identified and compared options to improve the sustainability of the system.
Pintilie et al. (2016) performed a comparative LCA to assess the environmental burdens of different wastewater post-treatment scenarios in Tarragona (Spain). The wastewater plant has mechanical and biological treatment for the wastewater line and sludge processing, and two options are considered after the conventional treatment: (a) direct discharge into a natural water stream and (b) tertiary treatment to facilitate water reuse in the nearby industrial area.
Li et al. (2016) compared water supply alternatives (i.e., water diversion, wastewater reclamation, and seawater desalination) in the water receiving areas of the South-to-North Water Diversion Project, a water diversion project that delivers water from water-sufficient southern China to water-deficient northern China. Beijing, Tianjin, Jinan, and Qingdao were studied as representative cities, because they are the primary water-receiving areas of the project.
Another sample of studies deals with a process-based approach to check the existing infrastructure and check for the major hot spots.
Jeong et al. (2015) performed an LCA study for the City of Atlanta’s centralized water system in order to understand the sources of the largest environmental impacts. The entire urban water system was taken into consideration, i.e., water supply, storm water collection system, and the wastewater collection and treatment system.
Uchea et al. (2013) investigated the impacts related to the urban water cycle in two Spanish areas: inland city (Zaragoza) and island area (Gran Canaria). The main objective of the study was to show which is the relative pollutant weights of the diverse water cycle stages in two different situations.
The study by Risch et al. (2015) is based on a comprehensive inventory including detailed construction and operation of sewer systems and wastewater treatment plants in Grabels, a small town belonging to Montpellier Conurbation (France), with a lifetime of 30 years. The attempt is to quantify the relative contributions of sewer systems to the total environmental impacts, taking into account construction, operation, and finally dismantling and end-of-life options of the sewer components and of the wastewater treatment with a mix of primary and secondary data.
Finally, two studies involved the support of simulation models to maximize the benefits of optimization and management of the resource.
Loubet et al. (2016) describe and apply a framework and an associated modeling tool, namely the WaLA model, to perform the LCA of urban water system. The model is implemented in Simulink/Matlab, and it is applied to a real case study, the urban water system of the Paris suburban area (France). The urban water system shall supply different categories of users and 1 m3 of drinking water delivered to the users is the related FU. The WaLA model combines components representing the different technologies, users, and resources, including drinking water production and distribution, users, storm water, and wastewater collection, wastewater treatment in the system boundaries. The operation, the infrastructure (production of construction materials and their end of life are considered), and the sludge disposal while the civil works associated with construction and deconstruction are not considered. The novel and comprehensive approach relies on the support of primary and secondary data (measurement flow meter, calculation from an external model, literature data, or result of a mass balance), and it involves also stakeholders. The authors apply the “territorial LCA approach” according to Loiseau et al. (2013), and the potential impacts are calculated accordingly to ILCD at midpoint and Impact 2002+ at endpoint. The ILCD category “water resource depletion” was replaced with the impact category “water deprivation,” in order to differentiate impacts due to withdrawal or release at different locations within the Seine river basin.
Cai et al. (2016). developed an integrated approach for supporting comprehensive decision-making in urban water allocation system (i.e., to effectively utilize water resources for satisfying multiple targets without causing too much environmental stress) through the incorporation of a fuzzy inexact two-stage programming model. The model was developed and combined with uncertainty and LCA. The research is applied to the situation of the city of Dalian (China).
As cities are often intensive producers in terms of waste, the related sector is one of the most monitored and present in literature, as proved by this review. Developing and developed countries are equally depicted and mainly deal with a comparison of alternative technological solutions, although with some differences. Studies performed in developing countries claim the urgency to replace or decrease the disposal by landfills.
A review study provides an overview of strategic options in critical situations, as the ones faced by developing countries, where an increasing rate of urbanization and products consumption and a lack of appropriate waste management systems, make the situation particularly harmful (Othman et al. 2013). The review is based on the selection of MSW management practices in some Asian countries with emphasis on LCA as integrated approach to select the most preferable and environmental friendly option. The results are specifically intended to be used as a basis for decisions on strategies and policies for waste management and investments for new waste treatment facilities by decision makers in local, regional, and national authorities or industries in the region considered.
A major portion of the studies selected report the environmental assessment of urban waste treatment options. Coventry et al. (2016) present findings from a comparative LCA of four solid waste treatment strategies for the city of Austin, Texas, which is set forth as a “typical” US city. Austin was selected as case study, because of its waste management and waste treatment technology options that could fairly represent the most common situations in North America. The four different scenarios collectively represent the fate of almost 100% of all non-recycled MSW in the USA and the general spectrum of available MSW treatment technologies for US cities ranging from “industry standard” to “pilot-scale.”
A similar approach is found in Ghinea et al. (2012) that compared four different scenarios elaborated as alternatives to the existing waste management system in Iasi (Romania), focusing the analysis on the influence of system boundaries. All the management scenarios consider the collection of the annual amount of MSW generated in Iasi City from residential areas, their transport, and different treatment alternatives (recycling, composting, incineration, landfilling).
A further comparison is proposed by Chi et al. (2015) to evaluate the current business-as-usual system in Hangzhou (China) with alternative waste management scenarios.
Finally, a study to determine the environmental aspects of a less impactful MSW management was conducted by Erses Yay (2015) for the city of Sakarya (Turkey).
A second set of studies examine and compare urban waste-to-energy options. A comparison of Asian waste-to-energy management treatments is proposed by Gunamantha and Sarto (2012), in the intercity area of Yogyakarta, Sleman, and Bantul (Yogyakarta province, Indonesia).
A comparative LCA of two different management options for food waste was realized by Grosso et al. (2012) based on the analysis of a real metropolitan situation (Milan, Italy). The baseline scenario, comprising the treatment of food waste in a waste-to-energy plant with the residual waste, is compared with an alternative scenario, which proposes a new collection scheme for household food waste, followed by its treatment in a new anaerobic digestion and post-composting facility.
A third waste-to-energy case study is proposed by Bezama et al. (2013), who technically investigated suitable and improving opportunities to compare with the original landfill project in Coyhaique (Patagonia, Argentina), analyzing the potential environmental impacts of these measures on the present design using a comparative LCA.
The last sample of studies screened attempted providing a more complete sustainability assessment, combining social and economic considerations or analyses.
Koroneos and Nanaki (2012) assessed the environmental performances of different MSW treatment strategies for the city of Thessaloniki. Landfill, recycling, and anaerobic digestion of the waste fractions produced in Thessaloniki are evaluated comparing four additional alternatives (for a total of seven scenarios) and computing the amount of waste fractions in Thessaloniki over 1 year.
Reichert and Mendes (2014) evaluated eight different scenarios for MSW management in Porto Alegre, but designed with the participation of social and technical municipal stakeholders. The LCA study included different collection (selective and containerized) and treatment (recycling, composting, anaerobic digestion, thermal processes, and landfill disposal) alternatives. The results were then coupled with social and economic qualitative indicators (e.g. quality and quantity of jobs created, investment and costs, etc.) proposed to enhance the results and provide further support to include social and economic sustainability.
A third study is focused on the economic and environmental evaluation of sludge management in Hong Kong (Lam et al. 2016). The eco-efficiency analysis framework proposed comprises environmental and economic criteria based on LCA and LCC. The purpose of the framework is provide reliable information for sustainable town planning based on the priorities of the decision makers.
Finally, the article by Teixeira et al. (2014) describe a methodological tool for an operational, economic, and environmental assessment of MSW collection. The proposed tool uses key performance indicators to evaluate independent operational and economic efficiency and performance of collection practices. These indicators are tested on a real case study (Porto, Portugal) and used as input for the LCI.
The last research topics involve the application of LCA methodology in support to waste prevention strategies and urban planning.
Cleary’s research (2013) confronts lightweight and refillable packaging alternatives to conventional single-use glass bottles in the city of Toronto.
In Trondheim, a new greenfield settlement with carbon-neutral ambitions was planned and the business-as-usual plus four different scenarios for the waste management system of the new settlement were compared (Slagstad and Brattebø 2012). The waste management system shall guarantee the collection, transport, and treatment of the waste streams of mixed waste, paper, plastic, glass, and metals from 1500 new households (3315 persons) over 1 year.
The transportation sector is a sensitive and often crucial issue in urban policies, especially in metropolitan or touristic cities, where large flows of people move. As transportation deals with dynamic fluxes in time and space, the collection of high quality data is often critical, and the more sophisticated studies rely on models to simulate real situations.
Regarding the impact assessment, energy and carbon footprint considerations are top priorities for the topic, especially in context of North America. Indeed, for this urban subsystem, only three out of seven studies apply LCA considering a wide range of impact categories, while the rest focus only on energy and/or GHGs emissions, applying an LCT approach.
François et al. (2017) provide a comprehensive urban mobility environmental assessment, through the evaluation of the transport system and travel habits, by applying a LCT approach to the results of dynamic mobility simulations that were produced by a Land Use and Transport Interactions model, namely SIMBAD. The model is designed on a city commuting scale, and it is able to estimate economic, environmental, and social impacts of alternative public policies in urban and transport planning. The model simulates the location changes for households and companies over a 25-year timeframe, in interaction with a complete urban transport system (public transport, car and non-motorized modes for individuals, and goods movements due to economic activities). For this study, the authors define urban mobility as the activity that enables people living or working within an urban area to travel during a working day; therefore, the FUs considered are person per kilometer and per trip daily. A second interesting point of the study is the disaggregation of results by mode of transport, by life cycle phase, or by class of household that allows for a more conscious identification of the hotspots and enables ad hoc policies that could be more effective to target the correct offenders and/or practices in need of change.
Another application of LCA for policy support is the one investigated by Vedrenne et al. (2014), who evaluated the potential benefits of policy measures to renovate the municipal taxi fleet in Madrid. To model the use phase of the vehicles, data from the nine traffic management zones of Madrid and from a specific traffic model in combination with GIS were collected. The impact assessment phase was conducted using the ILCD method, comparing the different scenarios and providing specific spatial impact results for each management zones.
Simon et al. (2010) attempted investigation the environmental impact of the public bus transportation system used in Hungarian cities on annual basis, taking Budapest as a reference city.
Liu et al. (2016), Nichols and Kockelman (2015), Fraser and Chester (2016), and Shahraeeni et al. (2015) performed LCAs of urban transportation focused only on energy consumption and/or carbon footprint, but they attempted to model urban transportation patterns in an innovative and comprehensive way.
Liu et al. (2016) compared various and different means (across aviation, intercity bus, and automobiles) for intercity passenger transportation in the USA with respect to energy consumption, CO2 emissions, and criteria pollutant emissions (CO, VOC, SOX, NOX, and PM2.5). The analyses focus on short-distance to medium-distance 165 intercity trips (ranging between 200 and 1600 km) among 79 cities in USA and Canada.
Nichols and Kockelman (2015) modeled urban transportation habits and estimated life-cycle energy demands for residents and workers in different urban settings, combining daily operations and their embodied energy demands. Different neighborhood types in Austin (Texas, USA) were taken into account in the analysis, considering five different city types that reflect the actual form of chosen North American cities, in terms of accessibility and density profiles for residents and employments. The data were collected by means of surveys, literature, and models and translated in energy consumption. Modeled energy use by source and phase are evaluated and compared to infer the built environment’s impact on larger-scale energy demands.
Fraser and Chester (2016) developed a framework and an operational LCA tool (City Road Network) to assess the extent to which roadway commitments result in ongoing and increasing environmental and economic impacts on the long run. The proposed framework attempts at improving the understanding of the long-term costs associated roadway deployment and sustainability and can be generalizable to other permanent urban infrastructure systems. Los Angeles County is used as a case study to explore the relationship between historic infrastructure deployment decisions and the emergent behavior of vehicle travel, due to its extensive road network and automobile reliance. Literature and models are used as data sources.
Finally, Shahraeeni et al. (2015) compared the environmental performances of light duty commercial vehicles powered by compressed natural gas and diesel under typical North American conditions in a relatively large, semi-urban city, using statistical data and models.
3.1.6 Consumption patterns
With the rise of industrialization era and capitalism, huge amount of commodities are produced and consumed worldwide, and international trade is stronger than ever. The European Union-28, China, and the USA have been the three largest global players for international trade, and the European share was valued at 3.517 billion € in 2015 (Eurostat 2016). In a liberal economy, consumption is also claimed as one of the best medicines to fight economic crisis, a medicine that often results in consumerism. Anyway, it is evident that people need to consume household products in order to sustain their life and their activities, and researchers argue that the coupling between a great density of economic activities, retailers, shops, etc., with average higher household income establish also a higher expenditure for household products (UN Habitat 2016; Heinonen et al. 2013b). According to Ivanova et al. (2016), the environmental pressure arising from households is responsible for more than 60% of the global GHG emissions and between 50 and 80% of the total resource use. A significant portion of the emissions and resource use is embodied in internationally traded commodities. Sociology of consumption made progress in identifying and dissecting a series of mechanisms, which maintain and expand demand for goods and services, but supporting tools are still lacking for an appropriate environmental evaluation and comparison of goods and scenarios (Padovan et al. 2015).
The consumption sector is then strategic, and addressing and acting on current consumption habits could be a way to drive cities and their inhabitants towards a more sustainable lifestyle. Nevertheless, comprehensive LCA studies focused on urban consumption patterns that can hardly be found in literature, and only three publications were found to be relevant for this topic.
Heinonen et al. (2013a, b) investigated the urban form–lifestyle relationships in Finland and the resulting GHG emissions embedded in different goods and services, employing both monetary expenditure and time use data to portray lifestyles in different basic urban forms: metropolitan, urban, semi-urban, and rural. For the four types of living areas, statistical household and time data about a comprehensive basket of consumption goods (including 12 categories and divided in 52 sectors) for one citizen over 1 year were collected. The second part of the study (Heinonen et al. 2013b) focuses only on the middle-income segment and looks for differences in the lifestyles when the budget constraints are equal. Here, the authors add into the assessment the variables housing type and motorization. To model the citizens’ consumption behavior, the authors privilege the “consumption approach” and propose the concept of “parallel consumption” defined as concurrent consumption of service spaces in different locations.
Kalbar et al. (2016) analyzed consumer lifestyles by different means. A systematic commodity consumption, commodity disposal, and lifestyle survey of 1281 persons living in urbanized Danish areas was conducted (namely, Personal Metabolism (PM) patterns, consumption approach). Extending the PM analysis with LCA provided a picture of the per capita environmental and human health burdens, as well as resource consumptions, and the exact origin hereof. The authors inventoried direct and indirect flows related to consumption habits as food, accommodation, energy use, road transport, and air travel, with a mix of primary and secondary data referring to the consumption of one citizen per year.
3.1.7 Urban green
Cities both impact and depend on hinterlands for resources, as provision of food, energy, services, etc. Urban planners are increasingly acknowledging that cities have an important role as stewards of the ecosystems (Elmqvist et al. 2013). There is, however, a disconnect between using resources for urban areas and preserving or conserving ecosystem services that are outside of urban areas. The ability of the LCA methodology to capture both direct and indirect impacts can help to overwhelm this gap, but unfortunately, the topic is really poorly represented and two contributions were selected for the scope of this review.
Spatari et al. (2011) examined the avoided energy and greenhouse gas emissions of selected Low Impact Development (LID) strategies applied to a neighborhood in New York using LCA and a stochastic urban watershed model.
The provision of another ecosystem service is evaluated by Rothwell et al. (2015). The authors proposed a method for integrating housing and food production land uses in peri-urban regions, based on relative environmental impacts, taking into account a hypothetical city in the Australian context as case study.
3.2 Upscaling approaches
As briefly introduced in section 2, recent advancements in LCA focus on methodological proposals and approaches for application to larger-scale systems. Here, current attempts to upscale environmental impacts to the urban or territorial level are reported. A total of nine contributions relying on LCA were selected in the scope of this review.
Loiseau et al. (2013) proposed a revised LCA framework to assess the environmental performances of a territory. The authors discuss in deep the methodological bottlenecks existing and propose adaptations to make the LCA methodology suitable to be applied at the territorial level. One of the major adaptations necessary concerns the goal and scope definition phase. Henceforth, the association of a territory and the studied land planning scenario, defined by its geographical boundaries and its interactions with other territories, will be designated as the reference flow in LCA. The revised framework is then applied to a theoretical territory in Southern France. The system boundaries of the revised framework shall include all activities present in the territorial boundaries, i.e., both domestic production and domestic consumption. The concept of the FU is substituted by land use functions (environmental, social, economic perspective), and the territory is the reference flow. In order to support a better decision-making, the inventory of the activities is built to distinguish a “foreground” and a “background” system: the “foreground system” refers to processes under the influence of the decision makers that have commissioned, whereas decision makers have no direct influence on “background” system. The territorial LCA framework was then expanded by the authors (Loiseau et al. 2014) and applied to the existing case study of Bassin de Thau (France), with the support of primary and secondary data, mainly activity data, the stakeholders’ communication, and statistics. The LCIA method applied is Recipe v. 1.07 at midpoint and Impact World+ for sensitivity analyses.
Trigaux et al. (2017) assessed the life cycle financial and environmental impact of road infrastructure in residential neighborhoods and analyze the relative contribution of road infrastructure in the total impact of neighborhoods. The study is performed on a hypothetical Belgian neighborhood. The life cycle stages included are production of building materials (including raw material extraction and transport to the production site), transport to the construction site, and construction activities. The use stage includes processes related to cleaning, maintenance, replacement of components, and energy use. Finally, the EOL stage covers the demolition activities, waste transport, and waste treatment. Two FUs are selected, according to the analysis of the paper: (i) meter road of the entire road section (including one or more lanes) and (ii) square meter of floor area of the buildings.
Lotteau et al. (2015a) reviewed all papers related to LCA of the built environment at the neighborhood scale. A focus is carried out on 21 existing case studies which are analyzed according to criteria derived from the four phases of LCA international standards. It sums up current practices in terms of goal and scope definition, LCI, and LCIA. The papers selected include case studies dealing with at least two out of four identified classes of built environment (buildings, open spaces (i.e., roads, green spaces), networks (water, telecommunications, sewage, heating distribution, electricity distribution), and mobility. The authors proposed also a decision support LCA based tool designed for the built environment at the neighborhood scale, namely NEST (Neighborhood Evaluation for Sustainable Territories) (Lotteau et al. 2015b). NEST addresses early design stages, and uses 3D models of neighborhood projects to quantitatively assess a set of environmental impacts. NEST was also applied to three districts in the city of Donostia (Spain) (Oregi et al. 2016).
Peuportier and Roux (2013) propose a tool based on LCA to compare alternative designs of buildings and urban settlement projects, taking as example scenarios in Paris and Champs-sur-Marne (France).
Schiopu et al. (2014) combined principles from the Urban Metabolism/Material Flow Analysis (UM/MFA), LCA, and Environmental Risk Assessment (ERA) methodologies and used a hypothetical French eco-neighborhood as reference.
Mailhac et al. (2016) identified key elements that may improve the systemic simulation of districts and thus ensure a better assessment of their environmental performances.
Sibiude et al. (2016) explored enhancement perspectives to facilitate LCA scaling up from building to territory.
3.3 Hybrid methodologies and supporting spatial tools
This last section presents the results regarding hybrid approaches available at urban scale level, and the use of supporting spatial tools in combination with LCA.
Goldstein et al. (2013) focused on the application of LCA framework to the urban metabolism (UM) concept. The intended aim was to address existing shortcomings in the UM’s ability to capture the embedded environmental loading in goods consumed by a city and, therefore, fully quantify a city’s (un)sustainability. A hybrid UM-LCA model was developed and applied to five case cities (Beijing, Cape Town, Hong Kong, London, and Toronto) using the product system modeling software GaBi 4.4, in conjunction with the ecoinvent 2.0 database and national statistics, considering the metabolic flows of food, construction materials, energy, buildings, and industry. A cradle-to-grave approach, from resource extraction to end-of-life, is applied and the FU considers the material, social, and institutional needs of a single resident in the city over the period of 1 year.
Clark and Chester (2017) explored the value of integrating UM with LCA, using vehicle transportation in the Phoenix metropolitan area (USA) as an illustrative case study. A UM framework is used to study the direct energy flows of fuel and vehicles in Phoenix and is later joined with a process-based LCA to evaluate indirect flows during the fuel feedstock and refining stages as well as the vehicle manufacturing stages that occur outside the city. To estimate inbound and outbond trips and related data, the authors take advantage of models and estimations. The impact assessment is limited to GHG emissions and energy, applying the GREET model (Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation), a process-based LCA model for both fuel and vehicle developed by the US Department of Energy.
Gerber et al. (2013) proposed a systematic methodology for sustainable process systems design, combining the principles of industrial ecology, process design and process integration, LCA, and multi-objective optimization. The model is illustrated by an application to the environmental and economic design of an urban energy system, in La Chaux-de-Fonds (Switzerland).
Padeyanda et al. (2016) conducted MFA and LCA studies to examine different recycling facilities of food waste to find environmentally sustainable options for the Daejeon Metropolitan City (South Korea).
Ulgiati et al. (2011) used a multi-method multi-scale assessment procedure (namely, SUMMA) LCA based in order to generate consistent performance indicators based on the same set of input data and to highlight the importance of a multiple perspective point of view for the proper evaluation of a system’s environmental and resource use performance.
Kissinger et al. (2013) focus on the investigation of the material consumption component of the urban ecological footprint and identify the so-called solid waste LCA approach as one that overcomes data limitations. The approach uses data that many cities regularly collect regarding municipal, solid waste composition data, used as a proxy for material consumption.
Mastrucci et al. (2017a) propose the development of a framework for the characterization of building material stocks and the assessment of the potential environmental impacts associated with the end-of-life of buildings at the urban scale to support decision on waste management strategies. The methodology combines a bottom-up material stock model based on geographical information systems (GIS) and a spatial–temporal database with LCA for the evaluation of end-of-life scenarios. The framework is applied to the housing stock of in Esch-sur-Alzette (Luxembourg).
Cousins and Newell (2015) propose a political–industrial ecology approach to explore the urban water metabolism of Los Angeles, combining theory and method from urban political ecology and industrial ecology. Spatiality is infused into the traditional LCA approach by coupling it with GIS. The historical and political background of the city are considered to question the feasibility of more environmental friendly options.
Liu et al. (2014) refer to the conversion of the results from site-generic or site-dependent LCIAs into smaller spatial units (regionalization of LCA). This is achieved using GIS, because according to the authors, GIS has the potential to easily allocate the potential impacts into smaller spatial units through the overlay analysis of fate, exposure, and effect layers.
Davila and Reinhart (2013) propose an analytical framework and a new CAD tool for Rhino3d to estimate the cumulative embodied energy content of an urban design proposal or retrofit scenario.
4 Discussion, outlook, and research needs
For what concerns Fig. 1, a continuous increment of LCA studies conducted at the urban level is observed from 2010 onwards, with a peak of studies published in 2016 and 2015 (19 and 15, respectively), and one publication already included in the early months of 2017. It is remarkable that from 2012 to 2013, the number of publications were more than doubled (total number of studies published in 2013 was 12 against 5 in 2012), and in few years, the urgency to upscale the LCT approach to a higher scale level seems to grow. In particular, only in 2016 every sector analyzed is covered at least by one contribution.
Regarding the geographical coverage (Fig. 2), European cities are the most represented with half of the publications (32), but these are sometimes strictly localized to a specific area (such as France for attempts of upscaling and Northern Europe for the consumption sector).
Asia and North America follow the ranking (13 and 11 cities, respectively), but if North America could be considered more homogenous, it is important to remark that Asia has a higher urban heterogeneity across the different countries, in terms of urban forms, culture, and lifestyles of the inhabitants. For example, Chinese cities are usually the biggest and normally exceed the millions of inhabitants, but similarities can be found in Israel population’s lifestyle and European or North America ones.
South America and Africa are very populated areas with a high urbanization rate and constraints, but they are still poorly represented (two contributions each). Finally, the least populated continent, Oceania, closes the ranking with two Australian cities.
4.1 Comparative analysis of LCA studies across the different urban sub-sectors
The literature review shows that no applications of LCA at the entire urban scale exist to date, and upscaling approaches are still on their way of development. Waste and water sub-sectors seem to be the most mature in performing the transition from the product/process level to the higher scale systems, with a share of 21 and 20%, respectively. LCA is a widespread methodology for environmental sustainability assessment in these fields both in developed and developing countries. The main use found is as supporting tool for policy making, and strategic and comparative analysis are conducted to find the most suitable scenarios in given conditions. Contributions related to the waste sector mainly focus on municipal solid waste management and often LCA is used both as mean to evaluate environmental performances and to provide a comparison between different technologies. Regarding the water sector, the majority of the studies deal with the whole urban water system, but wastewater management and treatment is the area that rises more attention and it is considered more source of concerns. In terms of literature coverage, transportation sectors and energy systems follow, but here the impact assessment is mainly focused on assessing and comparing emissions and energy demands or technology systems, respectively. For household consumption, only one contribution considered a wide range of categories for the impact assessment and the topic is still unexplored in an integrated way. For the building sector several papers focus on energy performances or construction materials of either one single building or a cluster of buildings or a sample (Cabeza et al. 2014), but until to date none is dealing with a comprehensive assessment at the city level. Considering the mentioned responsibility of the built environment to contribute to the urban environmental impacts, this absence could be unexpected, but reasonable considering some possible explanations. First of all, buildings are highly heterogeneous systems with an additional heterogeneity of materials, practices, technologies involved in their design, construction, refurbishment, etc. Huge amount of data and stakeholders are involved even at the building level. It is likely then, that no studies were found yet because the upscale to the macro-scale needs to rely solidly on the previous micro-scale analysis, and these are still on their way of development. Secondly, the specificity of the built environment and its relationship with the city requires peculiar methodological advancements and supporting tools, which are in their first phases of exploration (see sections 3.2 and 3.3). Thirdly, the interest is still more focused on technological innovations than on the existing building stock. Moreover, such interdisciplinary matter of research requires the collaboration of different expertise between urbanists, architects, and engineers that are still involved to solve other challenges at a lower level of resolution. From the decision makers’ point of view, the interest is growing but, at the top of previous considerations, the application of LCA methodology to the urban building stock is also very time-consuming and expensive.
For what concerns new methodological approaches for upscaling, first attempts are available in literature. The most comprehensive and established is the Territorial LCA approach (Loiseau et al. 2013, 2014), while other upscaling approaches are applied to a limited spatial extent, e.g., neighborhood scale, or they focus on specific urban issues. Part of these upscaling proposals have a good level of maturity (Loiseau et al. 2013, 2014; Trigaux et al. 2017), while the remaining contributions are available only through conference proceedings and are supposed to be still under development.
Finally, the use of hybrid methodologies is not usually applied at the entire urban level. A unique attempt to fuse LCA with UM is available now for five cities (Goldstein et al. 2013), but some issues are still open and need further research (e.g., FU, proper modeling of the flows, cutoff criteria, etc.). The integration with other methodologies (such as LCA-MFA, and LCA-EF) is identified to be still limited in scope when applied to the urban context. It is interesting to notice that there is a claim for integration of LCA with spatial planning and ICT tools. Indeed, these tools have a big potential in terms of collection and management of urban data and support to the planning process. Another strong point is their ability to enable the spatialization and visualization of the hot spots and the overall environmental performances, a point that could be helpful also for communication purposes.
A comparative analysis based on key methodological features of LCA, as pointed by Loiseau et al. (2013), is presented in the following sections. It is important to remark that the high heterogeneity and the specific properties of the studies do not allow to make a consistent and detailed comparison across the selected investigation areas, but rather, they have the intention to be a basis of discussion to identify current limitations and propose methodological steps forward for the application of LCA at the entire urban level.
4.2 Goal and scope
The first phase of LCA sets the crucial basis for the correct development of the study, and it can be considered the compass that enables to navigate through the next three steps. Here, key features related to the objectives and the system under evaluations are defined. All the studies screened in this review involve an attributional LCA, and only one includes considerations for consequential LCA, despite its relevant support for policy making. About 70% of the studies is performed for comparative purposes, while the rest performs process-based LCA. Regarding the goal and scope phase, it is remarkable to report that few studies (less than 10%) address specific allocation procedures to take into account the cross boundaries processes occurring at the urban level. Then, the concept of “multi-functionality,” inherent to the urban systems for definition, is not explored in the majority of the selected contributions, because of the design of goal and scope phase and, if present, it is mainly restricted to the upscaling approaches. The majority of the studies are found to be modeled as “black box,” with no or limited considerations about the potential exchange of flows the surroundings or the other components of the urban system.
The purpose of the FU is to provide a reference unit to address and refer the inventory flows and quantifies the function of the system investigated. Hence, the definition of the FU is a crucial step in LCA because it can influence the final results of the study. Furthermore, as a proper and unique definition of city does not exist yet, its application to the urban context poses further methodological challenges. A rough comparison across the studies (Table S1) shows that the majority of the authors (about 57%) select a mass, energy, or distance unit as reference, i.e., ton of waste or cubic meter of water, etc., in compliance with the scope of the studies and sometimes normalized to the number of residents during a reference year. About a quarter of the studies evaluated prefer to focus on the yearly total performance of the system. It is questionable and object of debate what could be a proper definition to evaluate such a heterogeneous and dynamic system as the urban context. Relating the inventory and the results to a reference unit could be advantageous to better manage and evaluate the overall city performances, e.g., showing the environmental assessment or impacts of an average citizen and/or providing a ranking of cities to identify the most virtuous and stimulating sustainable policies. This choice, however, could hide some inherent urban dynamics and fluctuations that is necessary to take into account for a fair allocation of the fluxes and a more precise identification of the hot spots. Loiseau et al. (2013, 2014) provides the only methodological advancement in this regard, proposing a FU as a vector of services which can be assessed in a qualitative and quantitative way and that aims at capturing the multi-functionality of the urban system.
4.3 Life cycle inventory
During the Life Cycle Inventory phase, the source and the quality of data gathered and collected to perform the study is important to provide reliable results and a significant assessment. In this regard, for every system modeled, the inputs and outputs shall be as much representative as possible to comply with data quality requirements regarding technological, geographical, and time-related representativeness and appropriateness. A proper discussion should be included in LCA studies, and these requirements should be checked by means of uncertainty analysis in order to provide a fair understanding of the validity of the results.
For what concerns the papers examined in the scope of this review, no in-deep discussion about data quality and related uncertainty was found. The majority of the authors however claim to apply the best available data in terms of representativeness, paying special attention to the geographical issue and the availability of local data. A combination of primary and secondary data is the most frequent case across all the categories: on-site measures, industrial and activity data, questionnaires, and surveys represent the main primary set, often completed by literature studies, databases, statistics, etc., as secondary source. Nevertheless, some differences between the categories analyzed are found and reported. Studies coming from the waste and water make the most extensive use of on-site and activity data, supported by the use of databases and models. The transportation and energy sectors rely more on secondary data regarding statistics focused on fuel consumption and emissions, with the complementary use of models and/or GIS data in the most sophisticated studies. In the consumption sector, two different data gathering approaches are explored: a top-down approach that links expenditures data and GHG emissions and a bottom-up approach with the collection and analysis of responses from surveys. Finally, it is noted that the more the scope of studies is enlarged and the authors zoom out to a higher level of evaluation (hybrid and upscaling approaches), the more the use of secondary and more aggregated data is common, but in some cases, the support and expertise of the local stakeholders are also searched to get more in touch with the local reality.
In conclusion, as much as feasible, small, and medium granularity of the data is then found, with a limited use of aggregated data. This level of granularity is an added value of LCA and an important step forward to take into account for the assessment of the environmental impacts at the urban scale, because it allows for a more precise identification of the hot spots and more effective proposals of beneficial measures, compared to other approaches currently used.
4.4 Life cycle impact assessment
The Life Cycle Impact Assessment phase has the purpose to translate the elementary flows from the Life Cycle Inventory into their potential contributions to the environmental impacts that are considered in the LCA and thus to support the interpretation phase where the questions posed in the goal definition are answered and support to decision-making is provided (Hauschild and Huijbregts 2015). The discussion around the significance and the uncertainties related to the LCIA models started in the 1990s, but if significant scientific progresses were made on the methodology, the science behind LCIA did not run at the same pace, especially for what concerns the issue of spatial differentiation. Spatial differentiation is an important issue in the scope of the present review to capture local emission that directly affect the urban environment and it can be a strategic significant development for application of LCA at city level and its use for urban planning purposes.
From the analysis of the selected contributions, few and very limited considerations about this issue emerge. Frequently, considerations frequently miss also regarding the selection of the methods or the set of impact categories analyzed. However, some authors proposed and added specific impact categories to address impacts missing in the selected methods.
In order to be thorough, a brief overview of the different LCIA choices across the sectors studied is provided and some trends in the categories are identified (see Table S1, Electronic Supplementary Material). The most selected LCIA method is CML 2001 (22% of the occurrences), especially for the subsector of waste treatment and management. Eco-indicator 99 and Recipe follow with an equal share of 11% of the studies covered, while the use of other methods is never above the 5%. It is remarkable that in the most recent studies is more often the use of ILCD. Finally, a total of 12 studies relied on the results provided by various methods, for the following purposes: (i) evaluation and comparison of different results, (ii) highlight of specific impact categories, and (iii) assessment of midpoint and endpoint impacts.
5 Conclusions and prospects
Since the dawn of humanity and the emergence of the first settlements, cities have always had a leading role. They were centers of creation and innovation, crucial junctions of trades, and points of exchange and dialogue among peoples.
In recent decades, the continuous and growing urban migration of people, resources, and energy is leading the cities to be main characters of the political debate in a new role, as key players in the challenge to sustainability. The attention of policy makers and researchers is looking to the cities as preferred hubs of attention and intervention, bearers of a critical mass which ideally optimizes and maximizes the effectiveness of measures to reduce the environmental impacts of the activities that allow the city to survive and prosper. These ambitious goals require appropriate tools, able to understand and manage the drivers and take appropriate actions without the risk of burdens shifting. Numerous concepts, methodologies, and frameworks exist and are used currently, but none of them is able to respond to the need to evaluate urban environmental impacts in a holistic manner. Considering that cities, because of their nature, are one of the most complex anthropogenic systems existing to date, it is clear that responding to this need is anything but trivial.
Among existing methods, the LCT approach seems to date the most promising to comply this challenge, but appropriate methodological refinements are necessary to achieve this target. This paper reviews current applications of LCA methodology at the city scale and provides a critical analysis of the emerging trends and limitations, to highlight working points and delineate proposals for methodological improvements. These are summarized as follows.
First of all, it is very important providing a proper definition of the city. This is not only a semantic issue to solve, but also a technical one, as it involves the definition of system boundaries for the object investigated. It is hard, however, to define the urban context univocally and many definitions of city are possible, according to the field of knowledge. The intrinsic uniqueness of each city represents a great challenge to manage, understand, and quantify in a proper and robust way its environmental impacts. Moreover, from a policy-making perspective, it is also relevant determining whether an attributional LCA or a consequential LCA could be most appropriate for each research targets. At the current research stage, attributional LCA is likely to be the most feasible approach to use for the assessment of urban environmental, analysis of the related hot spots, and identification of areas of improvements. In a second stage, the comparison of different scenarios could be performed either with an attributional LCA (for punctual interventions, such as retrofit strategies for buildings or comparison of different water management technologies) or with a consequential LCA (structural interventions, such as circular economy practices that can affect the market). Indeed, consequential LCA is designed to generate information on the consequences of actions and can be more accurate and comprehensive in evaluating the effect of a policy and/or scenarios at the urban scale and beyond, especially for large cities.
Secondly, the definition of system boundaries shall be carefully done, as it is important to understand the predominant inherent dynamics of the urban context examined and take them into account without incurring in the black box approach or excessive truncation errors. It is likely that geographical and administrative boundaries are the most feasible choices, as they are objective, legal, political ways to define the urban context and they support better the data gathering process. Anyway, it is essential to consider appropriate allocation procedures for the existing transboundary processes, especially for what concerns mobility and consumption activities. Furthermore, as the influence of cities go far beyond their boundaries, effective policies able to lead measurable and long-term benefits need to consider both production and consumption perspectives. A “fair accounting” of production and consumption activities in cities would support effective sustainable actions, as the first acknowledges for impacts within the city boundaries, while the former accounts for impacts generated by activities in the city. Thirdly, an adequate functional unit shall describe and account the heterogeneous urban space. Referring the yearly flows to a single citizen or to the overall urban population are popular ways identified in the current studies. Based on this finding, we propose a more promising and advantageous concept of population equivalent able to take into account not only the permanent residents, but also the share of people that takes advantages of the urban services (e.g., tourists, commuters, etc.). This enhanced functional unit could better capture the issue of multifunctionality inherent to cities, identifying and acknowledging the permanent/full-time portion of residents, but also the variable/part-time portion of people that daily, monthly, or yearly live, work, or visit cities. The accounting of population equivalent concept shall encompass a specific study and analysis of the social, historical, and economic urban conditions in order to identify main flows of people and their habits and subsequently define appropriate allocation rules.
Regarding the LCI phase, its definition is usually time consuming and often struggling, due to restrictions, lack or unavailability or low quality of data. Considering the high level of dimension and complexity of the urban system, the problem assumes a macroscopic size. Then, another working point is to explore and define a feasible level of data quality and granularity that could maximize the efforts and provide robust and consistent results in a reasonable time. Learning from both approaches, an innovative way could be the combination of common “low-tech” data (statistics, literature, etc.), with “high-tech” data, e.g., spatial data from GIS or big data to track movement of people and goods. Finally, appropriate models and LCIA methods shall be carefully considered to address better the spatialization of impacts and the identification of global, regional, and local impacts for a more effective and conscious decision-making process.
Urban sustainability is recognized as a global challenge, involving both developed and developing countries. These, however, are found to be more vulnerable twice, risk-wise (i.e., effects of climate change), but also technical-wise for the (i) lack of adequate urban infrastructure, (ii) lack of financial and technical support, (iii) additional areas of priority (health, safety, etc.), and (iv) lack or absence of appropriate data collection.
The environmental assessment of the urban context is a highly interdisciplinary topic, and it is remarkable to recognize that the integration with other disciplines, as ones in the realm of urban ecology, sociology, and economics is essential to provide a good overview and effective analysis. Hence, as future outlook towards urban sustainability, specific methodological refinements should be proposed also for Social-LCA and LCC, in order to provide a beneficial integration and a complete Life Cycle Sustainability Assessment applicable to the city level.
To conclude, the present review aims also at addressing the steps forward towards future researches to propose a revised environmental assessment framework, namely City Environmental Footprint, able to (i) quantify the impact of urban contexts in a holistic way, (ii) identify hot spots and most responsible actors of the environmental burdens, and (iii) test and compare proposed sustainable solutions for improvement.
Megacities are defined as metropolitan regions with populations in excess of 10 million people.
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