Low-carbon urban experiments from vision to reality: a systematic review of the literature from 2005 to 2020

ABSTRACT

In recent decades, many cities have launched low-carbon pilot schemes to explore feasible local policies and actions that can be replicated on a larger scale to reduce the intensity of carbon emissions. This synthesis analysis reviews the global literature to analyse the interplay of prescriptive visions, practical responses, and evaluation methods of low-carbon urban experiments. Before 2015, low-carbon urban experiments were regarded as a sub-branch of sustainable development, and prescriptive visions were influenced by sustainability. Low-carbon urban experiments were used as an urban planning tool, and such urban planning concepts impacted the practical response and evaluation methods. After 2015, low-carbon urban experiments were influenced by many more disciplines. Planning still occupies the largest proportion of low-carbon practices, but more advanced technologies – in energy and other fields – have also been adopted and influence the evolution of low-carbon urban experiments. We find several driving factors, including political events and developments, technology change, and contributions from various disciplines including academic research. As carbon neutrality is the long-term target of many nations, achieving net-zero carbon dioxide emissions through prescriptive visions, practical responses, and evaluation methods in urban experiments demands further investigation, especially in developing countries.

Key policy insights

• Low-carbon initiatives on a small scale are frequently practiced by local and national governments, which has reshaped the low-carbon agenda worldwide.

• Low-carbon development also demands efforts and commitment on the part of local urban governments to promote low-carbon development in its multiple dimensions.

• Policymakers at different levels of government and governance should focus on how to build on policy instruments in experimental pilots to take action at broader scales.

• Policymakers also need to adopt relevant indicators from social science in evaluation methods to reflect social sustainability and inclusiveness concerns in the evaluation of low-carbon urban development.

KEYWORDS:

• Low-carbon urban experiments
• prescriptive visions
• practical responses
• evaluation methods

1. Introduction

Climate change caused by increasing greenhouse gas (GHG) emissions is one of the world’s largest challenges. The IPCC sixth assessment report argued that ‘to limit global warming, strong, rapid, and sustained reductions in CO₂, methane, and other greenhouse gases are necessary. This would not only reduce the consequences of climate change, but also improve air quality’ (IPCC, 2021). To mitigate climate change, nations have set explicit targets for reducing GHG emissions, and local governments, communities, non-governmental organizations, and private actors have used urban experiments to realize these targets locally (Bulkeley et al., 2014). Low-carbon urban experiments can take place on different scales, including at the city, town, district, community, and zone level. The systematic approach offered by a low-carbon urban experiment enables a place to overcome the barriers of a single-project implementation in order to pursue an integrated low-carbon pathway (World Bank, 2014). Low-carbon development strategies also allow those running urban experiments to position themselves as major players in climate change mitigation and to set an example for developing national emission reduction policies (Wang et al., 2015). Low-carbon urban experiments may reach low-carbon targets themselves, to some extent, while also fostering replications and inspiring other places to institute the exemplary practices they demonstrate (Urrutia-Azcona et al., 2020).

The concept of low-carbon urban experiments is based on insights from the literature on climate change experiments (Castán Broto & Bulkeley, 2013), and it points to the ways in which experimentation forms part of the governance and contestation of socio-technical systems (Geels, 2012). Based on this concept, we define low-carbon urban experiments according to the following three criteria. First, an intervention is experimental when it is purposive and strategic but explicitly seeks to capture new forms of learning or experience. Second, an intervention is a low-carbon experiment where the purpose is to reduce emissions of GHGs (mitigation) and/or adapt to climate change impacts (adaptation). Third, low-carbon urban experiments tend to be delivered at city, district, town, zone, or community levels.

Most reviews can be classified as studying either evaluation indicators or practical responses in low-carbon urban experiments (Yu, 2014). In the first group of studies, much emphasis is placed on establishing key performance indicators to monitor and assess differences during and after the implementation of low-carbon urban experiment policies (Zhou et al., 2015). Tan et al. (2017) established low-carbon city indicators and applied them to ten global cities to rank their low-carbon levels. Rodriguez et al. (2020) found that energy and carbon intensity indicators are attractive to policymakers because they measure win-win sustainability. This shows that ecological modernization as economic growth can be decoupled from energy consumption when higher economic value can be produced with fewer natural resources (Lu et al., 2017). The indicators proposed by policymakers or other initiators also influence low-carbon practices.

In the second group of reviews, scholars’ study real-world responses to visions for low-carbon urban experiments by proposing low-carbon practices and evaluation methods. These studies cover urban experiments from various geographical areas. Castán Broto and Bulkeley (2013) studied urban climate change experiments in 100 global cities. They found that social and technical forms of experimentation were discernible, but they concluded that technical investigation is more common in urban infrastructure systems. Based on the OLS regression model, Croci et al. (2017) analyzed the emission reduction strategies and influencing factors of 124 cities that were signatories of the Covenant of Mayors. Using a comparative analysis as an evaluation method, Salvia et al. (2021) found that a quarter of 327 European cities aiming to reach carbon neutrality were members of the Climate Alliance or the Covenant of Mayors and showed more commitment than others. Both groups of reviews reflect the development of low-carbon urban experiments. However, prescriptive visions, practical responses, and evaluation methods are closely connected with each other, and they should not be considered separately.

In this research, prescriptive visions refer to future visions and the various indices established to guide the development of low-carbon urban experiments (Fu & Zhang, 2020; Zhou et al., 2015). Practical responses refer to the actions taken to reduce emissions of GHGs (mitigation) and/or adapt to climate change impacts (adaptation) (Castán Broto & Westman, 2020). Evaluation methods refer to methods adopted by scholars to investigate the results of low-carbon urban experiments during their implementation (Chen, 2020). These three perspectives should be further interpreted from research development in the low-carbon urban experiment literature. Therefore, this study contributes by answering the following questions: How have the prescriptive visions, practical responses, and evaluation methods within low-carbon urban experiments evolved so far? What are the relationships among prescriptive visions, practical responses, and evaluation methods? How can they be interpreted from the research development in the low-carbon urban experiment literature? We answer these questions through a thorough and systematic review of low-carbon urban experiments studies in recent decades.

The article is organized as follows: Section 2 explains the bibliometric method used to analyse the literature dataset in detail. Section 3 presents the prescriptive visions, practical responses, and evaluation methods of low-carbon urban experiments, as well as their application in discussing frequently-cited low-carbon cases in the literature. Section 4 interprets prescriptive visions, practical responses, and evaluation methods by analysing research hotspots and cluster changes. Section 5 concludes with key insights derived from the systematic review, current research limitations, and the research outlook in the future.

2. Research design

2.1. Data collection

A desk literature review was conducted as the primary data collection method to obtain insight into the low-carbon urban experiment literature. We followed the guidelines of the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) (Page et al., 2021), including the four stages of identification, screening, eligibility, and inclusion. We also considered the eligibility of articles and other document types in the screening process, and we merged the screening and eligibility stages into a single stage (see Figure 1).

Figure 1. Data collection process of low-carbon urban experiment research using the PRISMA method.

In this article, we used Scopus to compile our literature dataset. We were aware that our selection of the database Scopus would restrict the bibliography and, thus, limit our sample of low-carbon urban experiment research. However, Scopus is one of the most comprehensive and standardized literature databases for the export of data (Falagas et al., 2008). Consequently, we chose the complete database, increasing our research sample. Web of Science was abandoned because it offers less coverage of studies in the social sciences, arts, and humanities. It is worth noting that our topic can also be documented using grey literature, such as dissertations and governmental or institutional reports. However, article searches in Google or other databases that provide grey literature were not performed to maintain the replicability of this review and avoid selection bias (Piasecki et al., 2018). Figure 1 shows the flow of data collection with the complete steps followed, along with the query string input used in our search.¹ To systematically capture all studies on low-carbon urban experiments, we searched for articles and reviews in the database with varying combinations of urban experiment types.

We collected academic journal articles and reviews in the English language from the Scopus database from 2005 to 2020. We decided to include only articles and reviews in English, including most international scientific contributions. We chose 2005 as the starting date as only six relevant articles were published between 1991 and 2005. Since 2005, scholars have continuously published articles related to low-carbon urban experiment studies. The fifteen-year scale of the data sample allowed us to explore various aspects of low-carbon urban experiment development. Three bibliographic locations (title, abstract, and keywords) describe the essence of a study and summarize and represent the primary academic information provided in a publication. Therefore, we can assume that when a term features in these bibliographic locations, it is a key conceptual focus of the corresponding study. Consequently, our low-carbon urban experiment literature selection contains all scientific publications in English in Scopus, which make use of any search query in one of the bibliographic locations (title, abstract, keywords).

As a result of this filtering process, we ended up with a database of 531 articles. We then excluded duplicate records and those lacking abstract information. The next step was to delete those irrelevant to low-carbon urban experiments. At the conclusion of this process, 490 studies remained and are thus included in the present review.

The number of research articles and reviews on low-carbon urban experiments increased between 2005 and 2020 from three to 490 (see Figure 2). The numbers fluctuated between 2011 and 2015; after the 2015 Paris Agreement, a milestone event, research on low-carbon urban experiments increased rapidly, while academic interest began to wane after 2018. After 2020, scholarly attention on low-carbon urban experiments continued to grow. This fluctuation in article and review publishing trends is similar to the findings of Castán Broto and Westman (2020). It is also worth mentioning that 308 of the included articles were published after 2015, which accounts for 62.86% of the total included articles published before 2021. Articles published after 2015 occupy more than half of the articles in the database, further evidence of the 2015 Paris Agreements’ influence on low-carbon development. It is thus necessary to analyse the change in prescriptive visions, practical responses, and evaluation methods before and after 2015.

Figure 2. Number of articles related to low-carbon urban experiments from 2005 to 2020. (Academic attention on low-carbon urban experiments has increased greatly in the most recent decade.).

2.2. Review content

The review content of this research is shown in Figure 3. First, for the prescriptive visions of low-carbon urban experiments, we analyzed both visions and indicators used in low-carbon urban experiments. As our research focuses on the articles and reviews, the visions and indicators reflect academic opinions more than practical choices (Lou et al., 2019). The visions and indicators were summarized by collecting studies focusing on low-carbon urban experiment indicators.

Figure 3. Review content of the low-carbon urban experiment research.

Second, practical responses in low-carbon urban experiments are interventions or measures to adapt to or mitigate climate change. We coded the titles and abstracts of articles in the database into various categories. Wimbadi and Djalante (2020) classified the publications into electricity and heat, transportation, industry and buildings, while the non-energy sector consists of agriculture, forestry, land use, and waste. Castán Broto and Bulkeley (2013) classified low-carbon initiatives into building environment, transport, urban form, urban infrastructure, carbon sequestration, and adaption. Based on their categories and the features of publications in our dataset, the categories of practical response in this research include building, economy, energy, planning, policy, society, transport, and waste. As carbon emissions calculations and evaluations are not directly related to low-carbon practices, we deleted articles with these foci (102 total articles) from our analysis. Articles that could not be classified into the above categories were coded as other (109 articles). The topics and their placements in the various categories can be found in Table A1 in the Appendix. When an article covered more than one topic category, that article was counted once in each category.

Third, some research articles also further evaluate the performance of low-carbon pilots. We also reviewed the evaluation methods and results based on these articles. Fourth, we further explored prescriptive visions, practical responses, and evaluation methods in frequently cited cases. Frequently cited cases were determined according to the articles’ number of citations. As the amount of time since publication also influences an article’s number of citations, we calculated each article’s average number of citations by dividing the number of citations by the number of years since publication. The influential city cases were summarized according to the method detailed above. First, we collected articles and reviews that mentioned city names in their titles, abstracts, or keywords in a database. Subsequently, the cities’ names were recorded along with their frequency of appearance as cases in the articles in the database. City case frequency from all case-study articles was derived by counting each selection of a city as a case study in an article once. For instance, the case frequency for London would be three if it were adopted as a case study in three articles related to low-carbon urban experiments. The influential cases were determined by calculating city case frequency and multiplying it by the corresponding article’s (or articles’) average number(s) of citations per year.

Finally, prescriptive visions, practical responses, and evaluation methods are further interpreted in the low-carbon urban experiment literature. Keywords provide information about the core content of an article. Thus, keyword analysis can identify evolving research hotspots and clusters related to the low-carbon urban experiment domain. Citespace software was adopted to draw the keyword timeline view (see Figure 6) to study the hotspot of low-carbon urban experiment research. To understand research cluster changes in low-carbon urban experiments, we used VOSviewer to create the keyword co-occurrence knowledge map (see Figure 7).

3. Results: prescriptive visions, practical responses, and evaluation methods

3.1. Prescriptive visions

Researchers and policy practitioners worldwide have established various indexes to guide and evaluate the development of low-carbon urban experiments (Fu & Zhang, 2020; Zhou et al., 2015). With a systematic review of current evaluations of indexes for low-carbon urban experiments in the literature, we summarized the major aspects for low-carbon urban experimental visions and their corresponding indicators in Appendix Table A2. In general, the low-carbon urban experiment indexes encompass major aspects such as carbon emissions, economic aspects related to low-carbon performance, energy use, environmental impacts, urban mobility, urban planning, water use, and other social aspects (Lou et al., 2019).

For instance, low-carbon urban experiments may reshape their located cities in various other ways. Major changes may occur in transportation and urban mobility as more environmentally friendly and less carbon-intensive transportation modes are encouraged. The most commonly used indicators are buses per capita, the share of public transportation, and share of renewable energy vehicles (Tam et al., 2018).

We summarize the development history of low-carbon indicators in the literature and illustrate its trajectory in Figure 4. All low-carbon indicators are summarized into the following categories: Economic Aspect (i.e. economic indicators that are closely related to urban low-carbon performance), Energy (i.e. energy aspect indicators), Society (i.e. indicators in social aspects), Planning (i.e. low-carbon urban planning), Urban Mobility (i.e. urban transportation systems and mobility behaviours), Waste (i.e. waste management) and Water Use. Their weight in the low-carbon index is indicated by the size of the bubble, and repeated indicators are omitted in Figure 4. Only newly appearing indicators are added to the development timeline of low-carbon indicators.

Figure 4. A timeline of low-carbon indicator development.

The first wave of evaluating indicators for low-carbon urban experiments was developed before 2010, which mainly concerned the carbon intensity of economic activities and energy use, green space coverage, and the urbanization of cities. The first-generation index primarily addressed the most basic indicators of urban economy, carbon emission, social aspects, and urban planning in low-carbon urban experiment evaluation processes. However, from 2011 to 2015, the evaluation instruments evolved gradually as indicators of waste management, urban mobility, and transportation were gradually integrated into a low-carbon urban experiment index. This stressed the employment of environmental protection and urban planning in low-carbon indicators.

The 2015 Paris Agreement was a watershed moment for urban low-carbon indicators. The third wave of evaluation index emerged when more indicators about social development and inclusion were added into low-carbon urban experiment evaluation systems. Indicators of social inclusion, such as housing price, Gini coefficient, and citizens’ satisfaction, were included in the system as well. Moreover, indicators of energy use aspects were further categorized into subdivided energy types consumed by industrial sectors. More recently, the low-carbon urban experiment evaluation index has incorporated more indicators of urban mobility, water use, and economy, represented by the share of new fuel vehicles, water recycling, urban employment rate, etc.

In summary, the low-carbon urban experiment evaluation index has been evolving from the carbon emission and urban economy aspects to new aspects of urban mobility, social, water use, and urban land use, increasingly emphasizing a more inclusive low-carbon urban experiment index. Therefore, low-carbon urban experiments are at the same time affordable, equal, and smart places for residents. They require cities to provide accessible and inclusive low-carbon services to all residents and in almost all urban development realms, raising the benchmark for low-carbon urban experiment development.

3.2. Practical responses

It is also necessary to explore real-world responses to low-carbon urban experiments by means of their low-carbon practices. As mentioned above, we have classified low-carbon practices into nine categories by referring to previous research (Castán Broto & Bulkeley, 2013; Wimbadi & Djalante, 2020). The coverage of each category is shown in Table A1 in the Appendix. The planning category refers to low-carbon planning, design, and management measures, including low-carbon community, land use, etc. The energy category has been used to describe solutions for energy reduction in low-carbon urban experiments, including measures in energy provision and consumption. The policy category includes policy innovations or arrangements to achieve low-carbon development in urban experiments. The economy category here clarifies the economic measures in low-carbon urban experiments, such as industrial structure upgrading, low-carbon tourism development, etc. The transport category can be defined as transport measures to reduce carbon emissions. It encompasses measures such as public transport and sustainable mobility. The building category refers to measures and actions in the building sector, including green buildings, rooftop greenhouses, etc. The waste category refers to measures to reduce carbon emissions in waste management, such as solid-waste management systems. Lastly, the society category can refer to the measures or actions of citizens, including engagement initiatives, grassroots action, green behaviour adoption, etc.

Figure 5 shows the number of articles in various categories of low-carbon practices from 2005 to 2015, and from 2005 to 2020. Planning, energy, policy, and economy categories are adopted widely in both stages. Before 2015, the planning category is the dominant one, and the number of practices in policy and energy categories has continued to increase since 2015. Low-carbon urban experiments were closely related to urban planning before 2015. After 2015, the rapidly increasing number of practical responses is visible in the energy and policy categories. Compared with other practical response categories, energy and policy practice themes became more central and prominent according to number of articles, as low-carbon urban experiments began to address energy provision as well as integrated policy design.

Figure 5. Categories of low-carbon urban experiment practices (a) from 2005 to 2015; (b) from 2005 to 2020 (top two practices in planning, energy and policy categories are shown).

The top two practices in the planning, energy, and policy categories are also shown in Figure 5. The planning category is highly relevant to urban design, and low-carbon communities (low-carbon neighbourhoods or urban living labs) were the top practice even before 2015. Planning on the community or neighbourhood scale is featured in integrated low-carbon practices (Middlemiss & Parrish, 2010). After 2015, land use became important and occupied a large proportion of articles in this category. Before 2015, policy programmes and eco-city initiatives were the policy category’s primary practices. After 2015, governance or regulation practices became the main approach in this category, which indicates that the policies of low-carbon urban experiments became more specific (Song et al., 2020). The energy category is popular in the literature because energy consumption is the core issue in low-carbon urban experiments. Within the energy category, before 2015, energy systems and energy retrofitting were the primary practices in attempts to save energy. After 2015, district heating also became an essential low-carbon practice (Fu et al., 2021; Rezaei et al., 2021).

3.3. Evaluation methods

After the launch of low-carbon urban experiments, scholars began investigating city-level changes following the adoption of low-carbon urban experiment policies. Before 2015, to evaluate the low-carbon urban experiments based on one or multiple cases, some evaluation methods were adopted, such as the analytic hierarchy process (AHP) method, the driving forces-pressures-state-impacts-responses (DPSIR) framework, etc. These methods provide insights into the development of one or a few low-carbon urban experiments. Unfortunately, these methods do not greatly consider the social aspects of these methods. This may result from the difficulty of collecting data related to social aspects, including individual data. Still, the relationship between low-carbon urban experiments and the lives of individuals demands research attention. In particular, the social focus of low-carbon urban experiments varies in different political and economic contexts.

After 2015, in addition to evaluation based on a single or a few cases, scholars also tried to investigate the impact of low-carbon urban experiments using econometrics based on a larger sample. Particularly when low-carbon urban experiment policy is promoted as a national policy, such as in China, it is possible to evaluate the impact of the low-carbon urban experiments using a difference-in-different (DID) method. DID is typically used to estimate the effect of low-carbon urban experiments by comparing the changes in outcomes over time between low-carbon pilot cities and other cities. Current research adopting DID methods generally evaluates the economic or environmental perspectives of low-carbon urban experiments (Song et al., 2020; Zeng et al., 2019). Most research results show that low-carbon urban experiments have positively influenced cities’ energy efficiency or carbon intensity (Feng et al., 2021; Song et al., 2020). Additionally, a variety of evaluation methods have been adopted to analyse the carbon emissions of low-carbon urban experiments, such as simulation evaluation, optimization, and scenario analysis models. The popular ones include the Multi-Criteria Evaluation Model, Capability Maturity Model, Multi-Criteria Decision-Making, etc. (Azizalrahman & Hasyimi, 2018; Sala Benites et al., 2020). However, these evaluation methods have not considered the low-carbon urban experiment indicators of urban mobility, land use, and other factors. The evaluation of low-carbon urban experiments still demands more research focusing on the long-term impacts of environmental and social factors.

3.4. Prescriptive visions, practical response, and evaluation methods in frequently cited city cases

After summarizing influential low-carbon practices, the most cited low-carbon city cases and their practices show local responses in specific cities (Table 1). Beijing achieved the highest citation index (148.02) as a result of 30 research papers that adopted Beijing as a case study. The second and third most cited city cases were Shanghai (92.80) and Shenzhen (50.98). Although these cases are located only in China, they are selected because they are the cities most studied in the literature and thus have passed the citation threshold for case selection in this study. The high citation indexes of Chinese cities may be due to the increasing emphasis on CO₂ emissions reduction by the Chinese authorities and a large number of low-carbon research institutes based in Beijing, Shanghai, Shenzhen, and Tianjin. Tianjin followed with a citation index of 82.00, reflecting its influence and efforts on low-carbon development. As for citations per article, Tianjin was more frequently cited than other Chinese cities, which indicates that its low-carbon transformation attracted more attention from scholars and practitioners than the other Chinese cities cited here.

Table 1. Number of articles citing cities and aggregated article citation numbers.

	Prescriptive visions	Top low-carbon practice categories	Evaluation	Citation index (aggregated article citation numbers)^a	Citations per article per year
Beijing	To become carbon neutral by 2050	Energy and transport	Its liveability does not rank at the top level	195.35	6.3
Shanghai	To play an exemplary role in low-carbon development for other cities	Energy and planning	The most common example in comparative studies of low-carbon cities	149.8	6.24
Shenzhen	To build a low-carbon pilot in China	Economy and Policy	Performed well in low-carbon development	92.98	5.17
Tianjin	To become a low-carbon and ecologically liveable city	Policy	Performing well in CO2 emission intensity, air and water quality and low-carbon lifestyles	82	7.45

Note: ^aDetermined by the number of articles multiplied by the articles’ average yearly citation numbers.

Beijing is China's capital city and its vision is to become carbon neutral by 2050, which is ten years ahead of the carbon neutrality target of the nation. After 2015, literature covered a wider range of low-carbon development in the city. Generally, the largest number of papers are focused on the energy and transport categories from 2005 to 2020. As a megacity, Beijing has a high proportion of electricity transferred from other provinces, and hence, the import of green electricity is the most cost-effective means of reducing emissions (Cui et al., 2020). Regarding the transport category, Beijing has the largest fleet of motor vehicles among Chinese cities, with 6.57 million automobiles by 2021 (BTMB, 2022). As such, a considerable number of studies have focused on the city’s transport system (Gao et al., 2013; Qin & Han, 2013; Zhao & Lu, 2011). Regarding evaluation, compared with other low-carbon cities, the liveability of Beijing has not ranked at the top level among Chinese cities (Chen, 2020).

Shanghai is the most populous urban area in China and its vision is to play an exemplary role in low-carbon development for other cities. Most low-carbon papers concentrated on Shanghai's energy and planning categories from 2005 to 2020. Regarding energy, the technical and economic viability of introducing distributed energy network to reduce CO₂ emissions of the city is the most frequently discussed (Ren et al., 2010; Wu et al., 2016). Concerning planning, some studies compared its planning strategies for low-carbon development with its counterparts in Europe (Lehmann, 2012, 2013). As for evaluation, Shanghai is the most common example in comparative studies of low-carbon cities.

Shenzhen lies along the coast of the South China Sea, and its vision is to build a low-carbon pilot in China. One notable difference in low-carbon literature on Shenzhen after 2015 is the increasing number of articles that study economy and policy categories. A variety of research studied the Shenzhen International Low-Carbon City (ILCC) Programme. The specific topics are related to energy (Bin et al., 2015; Zhang et al., 2016), the service industry (Wang et al., 2019) and international collaboration (de Jong, Wang et al., 2013; De Jong, Yu et al., 2013). Concerning evaluation, Shenzhen performed well in low-carbon development due to its high proportion of advanced manufacturing and service industry.

Tianjin is a coastal metropolis in northern China, and its vision is to become a low-carbon and ecologically liveable city (TEEB, 2021). The green transformation method of Tianjin includes a massive deployment of photovoltaics and controlling the use of coal (TEEB, 2021). The literature, however, is more focused on the Sino-Singapore Eco-City in Tianjin (Lin, 2018; Xie et al., 2020), especially its policy implementation (Chang, 2018; Wang, 2013). The Sino-Singapore Tianjin Eco-City is a new town and Lin (2018) suggested that the Sino-Singapore Eco-City is successful in the evaluation parameters of CO₂ emission intensity, air, and water quality and low-carbon lifestyles.

4. Interpretation of research hotspots and research clusters

Referring to previous studies, we attempted to summarize the relationship between prescriptive visions, practical responses, and evaluation methods from research based on two perspectives (Hong et al., 2019; Wimbadi & Djalante, 2020). First, changes in low-carbon urban experiment research hotspots can indicate the popularity of topics and how these change over time. Second, the keyword co-occurrence knowledge map shows relationships between certain research clusters in the low-carbon urban experiment literature. As the Paris Agreement is an essential event in low-carbon development, we explain the how prescriptive visions, practical responses, and evaluation methods evolve in part by looking at research hotspots and cluster changes before and after 2015.

4.1. Changes in research hotspots

The trajectory of changes in low-carbon urban experiment hotspots is illustrated in a keyword timeline view, with time represented on the x-axis and keywords on the y-axis. Figure 6 offers a horizontal analysis of keywords and their progress in research; the historical analysis shows the differences and linkages of different clustering keywords. The keywords correspond to the nodes in the graph, and we have listed the top five keywords for each year.

Figure 6. Keyword timeline of low-carbon urban experiment research from 2005 to 2020.

The basic ideas related to low-carbon development emerged before 2015 and included such terms as climate change, low carbon city, carbon emission, low carbon, CO₂ emissions, and sustainability. They also appear throughout the entire research period by means of their relationships to other keywords, suggesting that they are the essential elements in low-carbon urban experiment research. Their nodes in Figure 6 are larger, indicating they appear more frequently than other keywords. It indicates the strong connections between sustainability concepts and low-carbon urban experiments in the early stage.

Before 2015, low-carbon urban experiments can be understood as a subset of sustainable cities (De Jong et al., 2015). Low-carbon urban experiments emphasize urban planning and energy use to reduce carbon emissions. Many scholars from the urban planning field have also attempted to envision a low-carbon urban future through planning strategy and built environment planning practices (Fu & Zhang, 2017). Keywords such as urban form, built environment, and low-carbon community emerged in this period. Additionally, low-carbon urban experiments are closely related to eco-city and compact city development in the early stage. Above all, the narratives from sustainable cities shape the fundamental prescriptive visions of low-carbon urban experiments, while urban planning and energy issues dictate practical responses and evaluation methods.

After 2015, when the United Nations’ Conference of the Parties (COP 21) resulted in the Paris Agreement to pursue net-zero GHG emissions, governments, especially local governments, committed more strongly to low-carbon urban experiments. As a result of the application of more advanced technologies in energy and other fields (e.g. energy transition or energy renovation), the visions and practices for low-carbon urban experiments changed in due course as well (Lu et al., 2018; Sun et al., 2021). Regarding evaluation methods, more complicated analysis methods have been adopted. Since 2015, the keywords have also included evaluation methods such as panel data model, scenario analysis, and difference in difference. This illustrates that, at this stage, scholars are paying more attention to the variety of evaluation methods.

Above all, research hotspot changes show the temporal impact of low-carbon urban experiment research. Before 2015, the prescriptive visions were mainly based on sustainability. Practical responses can be regarded as an urban planning tool, and low-carbon practices interact with urban planning on different scales. Additionally, low-carbon practices also shared some practices from eco-city or compact city development. After 2015, more advanced technologies in energy and other fields were tested and adopted, especially in energy fields. Additionally, more complicated evaluation methods were adopted in low-carbon urban experiment research.

4.2. Changes in research clusters

Research cluster changes can be shown by a keyword co-occurrence knowledge map. We used VOSviewer to create a keyword co-occurrence knowledge map, and nodes of the same colour represent research clusters (Yu et al., 2020). As Figure 7 shows, the keywords are divided into three clusters: green, red, and blue. The keywords in a single cluster are closely related, and the links between different clusters explore the relationships contained therein.

Figure 7. Research clusters of low-carbon urban experiment research from (a) 2005 to 2015 and (b) 2005 to 2020. (The colours in this figure mean different research clusters.).

In Figure 7, we can observe the clusters of keywords adopted in the literature. Figure 7(a) shows that between 2005 and 2015, low-carbon cities were an essential keyword phrase in its research cluster. The other clusters included sustainable development and urban development. Sustainable development is a well-developed research cluster, which can in turn influence the development of low-carbon urban experiments. Inspired by the sustainable development concept, low-carbon urban experiment visions also cover economic, environmental, and social dimensions. Regarding urban planning, low-carbon urban experiments draw lessons from urban development on practical responses and evaluation methods.

Figure 7(b) shows that more articles about low-carbon urban experiments had been published by 2020, and the number of research clusters increased to four. The low-carbon city as a set of key words is no longer dominant (as it was in the earlier period shown in Figure 7(a)). Sustainable development and urban development remain significant, while the clusters have expanded to include other relevant keywords, such as energy efficiency and environmental management. Energy efficiency focuses on the effects of energy utilization, which can inspire scholars to develop evaluation methods for energy use in low-carbon urban experiments Additionally, carbon footprint and carbon emissions have also formed a research cluster in recent years, which influenced scholars to adopt prescriptive visions and specialized evaluation methods to tackle climate change. Finally, environmental management has become vital in the sustainable development cluster in the past five years, influencing the development of prescriptive visions, practical responses, and evaluation methods.

Research cluster change in the literature shows the interplay between visions, practical responses, and evaluation methods of low-carbon urban experiments, which are increasingly shaped by interdisciplinary influences. In the past five years, low-carbon urban experiment studies have become more than a research field with limited research topics. They have developed into an interconnected and systematic research field, thus promoting additional research topics such as carbon footprint, greenhouse gases, energy utilization, and energy efficiency. The dominant role of the keyword low-carbon cities has diminished, and each field has developed its own core keywords. On the one hand, low-carbon city studies have become less important as other research clusters have developed their own research agendas. On the other hand, low-carbon cities are still important experimental fields for these studies, and the connections between these research clusters remain. Research clusters’ diversity and inclusiveness significantly impact prescriptive visions, practical responses, and evaluation methods of low-carbon urban experiments.

5. Discussion

We further illustrate the conceptual relationships among prescriptive visions, practical responses, and evaluation methods in Figure 8. Based on the data collected in this study, these concepts reflect three different dimensions of low-carbon urban experiments, and their evolutionary trajectory, where there is interplay and a range of impacts and interactions from academic, political and technological factors.

Figure 8. The relationships among prescriptive visions, practical responses and evaluation methods in low-carbon urban experiment research.

5.1. The interplay among visions, practices and evaluation methods

Firstly, most of prescriptive visions are introduced from research, and they tend to attract scholars’ attention rather than practical responses. For instance, energy related concepts were adopted before 2015 as visions for low-carbon urban experiments, while practical responses related to energy became more prominent in low-carbon practices after 2015, suggesting that the practice usually follows the visions first introduced by researchers. To some extent, these prescriptive visions show the ideal expectations of researchers on low-carbon urban experiments (Cai et al., 2017). Prescriptive visions can also become more specific with the development of practical responses of low carbon urban experiments. As visions gradually materialized or became more specific, they may also promote the updating of evaluation methods.

Secondly, practical responses are the materialization of prescriptive visions in low-carbon urban experiments, which are bounded and guided by political priority, institutional limits and local resource capacities. Thus, the practical responses reflect the discrepancies of low-carbon development across different localities. Some scholars also argue that too much focus on how to conduct low-carbon urban experiments has also turned attention away from daily climate actions (Castán Broto & Westman, 2020). The variety of practical responses in low-carbon urban experiments also brings challenges to responding evaluation methods.

Thirdly, evaluation methods indicate the tension between economic concern and low-carbon development in reality (Lou et al., 2019). Evaluation methods are driven by the attention and resources of local urban governments to a large extent. For instance, although social aspects are included in the visions for low-carbon urban forms at the early stage, they have still not been widely adopted in evaluation systems. However, prescriptive visions and practical responses, under many circumstances, do help push the evaluation method to develop further, such as simulation evaluation, optimization, and scenario evaluation models. In sum, prescriptive visions show the expectations of scholars, practical responses reveal the local reality, and evaluations indicate current policymakers’ priorities in low-carbon urban experiments.

5.2. Impact of academic factors

From the research hotspot and cluster changes, we see that low-carbon urban experiments are influenced by multiple disciplines, from sustainability, urban planning, urban studies to public policies. Castán Broto and Westman (2020) argued that scholars also tend to study low-carbon urban experiments within the discipline of human geography. As the number of articles in human geography is limited, we classified these research items into urban studies.

Before 2015, low-carbon urban experiments were often a subdivision of research under the umbrella concept of sustainability and urban development (De Jong et al., 2015). Scholars and practitioners considered how to integrate low-carbon concepts into urban planning (Wang et al., 2013). After 2015, low-carbon development became so essential that it became a dominant research cluster with specific foci. Specially, scholars from urban studies still contribute to low-carbon urban experiments as they focus on the actions of low-carbon initiatives on different urban scales (Sun & Baker, 2021). As low-carbon urban experiments demand a variety of actions, integrated policy design is necessary for this process to advance. To some extent, low-carbon urban experiments can be understood as policy experiments implemented by governments. On the macro level, scholars paid attention to their governance mode, including enabling, provision, regulation, and self-governance (Castán Broto & Bulkeley, 2013). Later, scholars also paid attention to multi-level governance on the municipal or regional level (Liu & Qin, 2016; Sun & Baker, 2021). On the micro level, studies assessed organizational structures, grassroots initiatives at the community level (Lo, 2014), and policy learning in low-carbon urban experiments (van Doren et al., 2020).

5.3. Impact of political and technological drivers

In addition to various academic contributions to low-carbon urban experiments, we found that political drivers also influence prescriptive visions, practical responses, and evaluation methods. First, political factors, such as the Paris Agreement, provoked new global low-carbon initiatives in multiple cities by placing pressure on governmental and other institutions. This has been illustrated by keywords in research on hotspot issues, such as public private partnership and local governments. An underlying reason for political impact is the acceptance of ecological modernization, which argues that low-carbon and economic development can be achieved simultaneously (Bayulken and Huisingh, 2015; Geng et al., 2016); some also argue that ecological modernization reduces policymakers’ concerns over the economic burdens of low-carbon policies (Li et al., 2017).

In addition to political drivers, innovations in technologies and management have also contributed to thriving practical responses and evaluation methods. The upgrading of energy infrastructure and the application of new energy technologies have enabled cities to embrace more sustainable and affordable energy resources. Low-carbon urban experiments thus became more closely related to the energy field. As energy infrastructure, demand and supply is directly related to carbon emissions, renewable energy provision and energy system restructuring are widely discussed and adopted in low-carbon urban experiments (Li & Yu, 2020; Roberts et al., 2019). In this process, low-carbon urban experiments inspired technology development, such as smart energy cities (Thornbush & Golubchikov, 2021). In sum, the role of technology in low-carbon development goes beyond a technological fix: it includes co-evolution and interactions between market, policy, and society; it touches on the socio-technical regime and on the socio-technical landscape (Geels, 2012).

6. Conclusion

We have examined prescriptive visions, practical responses, and evaluation methods of low-carbon urban experiments through a systematic review of the global literature from 2005 to 2020. We find that 2015 was the milestone in low-carbon urban experiments, and we further interpret the changes before and after the Paris Agreement based on the shifts in prescriptive visions, practical reponses, and evaluation methods. We further explain shifts in research hotspots and thematic clusters, and summarize the relationships and interplay from three perspectives.

First, a review of research hotspots and research clusters shows that prescriptive visions have been evolving from the carbon emission and urban economy fields to new aspects of social dimensions. The shift in prescriptive visions is reflected in the fact that low-carbon urban experiments increasingly emphasize a more inclusive low-carbon development in which environmental and socio-economic distributional inequality and injustice is highlighted as a result of responses tackling climate change. Similar to prescriptive visions, low-carbon practices in urban experiments are utilized as an urban planning tool to attract more investment and attention from different stakeholders in the beginning; in later stages, these actions extend to test and adopt more advanced technologies in energy and other fields. The shift in practical response shows that low-carbon urban practices have become more hybrid and driven by interactions between technology change, politics and policy orientation, and markets among other factors. Regarding evaluation methods before 2015, the focus was on one or multiple cases, and the social dimension was not greatly taken into account. After 2015, scholars explored a larger set of locations and adopted more research methods, but less evaluation still occurs around social aspects. Still, the shift in evaluation methods shows an increasing number of types of evaluation methods and an expansion in the range of subjects being evaluated.

Second, at a conceptual level, driving factors influencing prescriptive visions, practical response, and evaluation methods include academic contributions, political and technological changes. Regarding academic contributions, low-carbon urban experiments gradually emerged from being a subtopic under urban planning and sustainable development to becoming a complex research topic related to urban studies and public policy. The Paris Agreement of 2015 is a benchmark political driver, which provoked new low-carbon initiatives in multiple cities globally by placing pressure on governmental and other institutions. Concerning the governance of low carbon, the literature also covers multi-level governance, various local governance modes, and grassroot initiatives at the community level. For influence of technology innovation, the literature points to examples, including upgrading energy infrastructure and applying other new low carbon technologies help low-carbon urban experiments optimize their energy structure and usage.

Finally, the literature shows that in the relationship to academic research where prescriptive visions attract more attention from scholars than do practical responses or evaluation methods, and this literature tends to reflect the ideal expectations of scholars for low-carbon urban experiments. By comparison, practical responses in low-carbon urban experiments are shown to be driven by political needs and low-carbon development priorities, while the evaluation methods literature shows the tension between economic concerns and low-carbon development in reality.

The results of this literature review also have some policy implications related to local practices and evaluation methods. Concerning low-carbon practices, small-scale initiatives in low-carbon urban practices are popular in current research, as they illustrate urban living labs and low-carbon communities. This indicates that low-carbon initiatives on a small scale, and their potential for replication, a point which is recognized by many scholars and practitioners. Policymakers at different levels of governments could usefully focus on how to replicate successful cases, drawing on evidence from small-scale experiments in larger scale low-carbon actions.

Regarding policy relevant issues, a number of other findings emerge. On evaluation methods, this review shows they still lack the signficant attention to social and inclusive policy perspectives, despite growning attention to such issues in the work of visioning and practice. Policymakers can further adopt relevant indicators from social science to reflect the performance of low-carbon urban experiments on social development and inclusiveness. Also, different levels of government and governance could pay more attention to academic research and technological change, including to make use of them in their political agenda.

The research limitation of this study is that we merely focused on low-carbon development in urban experiments at various levels, without including carbon neutrality urban experiments, such as zero-carbon urban experiments. Thus, some of the relevant and emerging literature on these topics is not covered here. As carbon neutrality is the long-term target of many nations, the evaluation indicators and practical responses to achieve net-zero carbon dioxide emissions through urban experiments demand further investigation. Additionally, in the current global battle against COVID-19, actions taken by various countries and cities also impact their energy systems and low-carbon practices. Beyond this, most of the current research focuses on urban experiments in Europe and Asia. Thus future research should investigate these issues in more cities, including from large emitter countries (e.g. United States, Brazil, Indonesia).

Additional information

Funding

This research is supported by the Humanities and Social Sciences Fund of the Ministry of Education in China [grant number: 21YJC630023]; the 14th five-year plan of philosophy and social sciences project of Guangdong [GD22XGL54]; the 2022 Planning Project of Philosophy and Social Sciences of Shenzhen (SZ2022B023), Shenzhen Peacock Plan, Guangdong Basic and Applied Basic Research Foundation [grant number: 2022A1515110755; 2023A1515012414] This paper is also supported by the Shenzhen Humanities & Social Sciences Key Research Bases (Carbon Emission Peaking and Carbon Neutral Technology, Policy and Management, Harbin Institute of Technology, Shenzhen; Greater Bay Area-ASEAN Research Institute, Shenzhen University).

Notes

1 (TITLE-ABS-KEY ("low-carbon cit*") OR TITLE-ABS-KEY ("low carbon cit*") OR TITLE-ABS-KEY ("low-carbon pilot*") OR TITLE-ABS-KEY ("low carbon pilot*") OR TITLE-ABS-KEY ("low-carbon district*") OR TITLE-ABS-KEY ("low carbon district*") OR TITLE-ABS-KEY ("low-carbon communit*") OR TITLE-ABS-KEY ("low carbon communit*") OR TITLE-ABS-KEY ("low-carbon zone*") OR TITLE-ABS-KEY ("low carbon zone*") OR TITLE-ABS-KEY ("low-carbon town*") OR TITLE-ABS-KEY ("low carbon town*") OR TITLE-ABS-KEY ("climate change cit*") OR TITLE-ABS-KEY ("climate change pilot*") OR TITLE-ABS-KEY ("climate change district*") OR TITLE-ABS-KEY ("climate change communit*") OR TITLE-ABS-KEY ("climate change zone*") OR TITLE-ABS-KEY ("climate change town*") OR TITLE-ABS-KEY ("zero-carbon cit*") OR TITLE-ABS-KEY ("zero carbon cit*") OR TITLE-ABS-KEY ("zero-carbon pilot*") OR TITLE-ABS-KEY ("zero carbon pilot*") OR TITLE-ABS-KEY ("zero-carbon district*") OR TITLE-ABS-KEY ("zero carbon district*") OR TITLE-ABS-KEY ("zero-carbon communit*") OR TITLE-ABS-KEY ("zero carbon communit*") OR TITLE-ABS-KEY ("zero-carbon zone*") OR TITLE-ABS-KEY ("zero carbon zone*") OR TITLE-ABS-KEY ("zero-carbon town*") OR TITLE-ABS-KEY ("zero carbon town*")) AND PUBYEAR < 2021 AND PUBYEAR > 2004 AND (LIMIT-TO (DOCTYPE, "ar") OR LIMIT-TO (DOCTYPE, "re")) AND (LIMIT-TO (LANGUAGE, "English")).

Appendix

Table A1. Classification of low-carbon practices in each category.

Category	Content	Example references
Planning	Low-carbon city experiments, low-carbon community, planning and design, planning and management, built environment, city construction planning, compact city, development strategy, environmental planning strategy, infrastructure, etc.	(Lehmann, 2013, 2012; Middlemiss & Parrish, 2010)
Energy	Energy consumption, cooling system, distributed energy resources, distributed energy systems, district heating, electricity demand, electricity utilities and residential consumers, energy circulation systems, energy feedback, energy management, and etc.	(Bin et al., 2015; Zhang et al., 2016)
Policy	Institutional arrangement, policy innovation, action plans, behavior of local government leaders/policy innovation, circular economy promotions, Climate Challenge Fund, etc.	(de Jong, Wang et al., 2013; De Jong, Yu et al., 2013)
Economy	Industrial ecosystem, industrial structure upgrading, low-carbon tourism, tourism, urban-industrial symbiosis, affordable, business strategies, carbon trading, circular economy promotions, cleantech firms, etc.	(Song et al., 2020; Zeng et al., 2019)
Transport	Urban travel, transportation, daily travel, GHG accounting for public transport, travel behaviours, motorised travel, public bicycle systems, shared mobility, sustainable mobility, transport mode, transport system, transportation carbon emission, transportation mode, transportation policies, transportation sector, urban transport, etc.	(Gao et al., 2013; Qin & Han, 2013; Zhao & Lu, 2011)
Building	Building sector, built environment, construction industry, green building, green roof, green space construction, mass-retrofitting, nearby buildings, residential area, residential buildings, retrofit actions, reuse of heritage buildings, rooftop greenhouses, etc.	(Chen & Chen, 2019)
Society	Engagement initiative, grassroots action, green behavior adoption, human health individual behaviour, individual environmental behavior, individual’s behaviour, lifestyle, public awareness and acceptance, scholars and the public, social learning, etc.	(Cai et al., 2017; Liu et al., 2017)
Waste	Waste flow, solid-waste management system, waste-related greenhouse gas, waste-to-energy, etc.	(Sun et al., 2018)

Table A2. The major assessment aspects and corresponding indicators of low-carbon urban experiment development.

Major aspects	Core indicators	Example references
Economic aspects (E)	Per capita GDP; ratio of commercial final energy to tertiary sector employees; ratio of environmental protection spending to GDP; tertiary industry/GDP ratio; carbon productivity, etc.	(Tan et al., 2017; Zhou et al., 2015)
Energy	Carbon emission per power produced; proportion of renewable energy; rate of energy-efficient appliance usage; proportion of renewable electricity; energy consumption per capita; energy consumption per unit of GDP, etc.	(Rodriguez et al., 2020)
Society	Percentage of affordable housing; noise level compliance area coverage; health care practitioners per 1000 persons; urban disaster prevention and reduction; urbanization rate; Gini coefficient, etc.	(Cai et al., 2017; Liu et al., 2017)
Urban mobility (UM)	Buses per capita; cars per capita; average distance between public transportation and residential building; percentage of built area within 500 m of public transit, etc.	(Tam et al., 2018)
Planning	Green urban land per capita; agricultural cultivation area per capita; urban land use per capita; forest coverage, etc.	(Dovey et al., 2017)
Waste	Solid waste per capita; proportion of waste collected and disposed; proportion of waste to energy; solid waste landfill rate, Noise level compliance area coverage; etc.	(Sun et al., 2018)
Water use	Recycling of water usage; Water consumption per capita per day; Per capita waste water generation; Drinking water quality, etc.	(Deakin et al., 2012)