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Article

Exploring the Global Research Trends of Cities and Climate Change Based on a Bibliometric Analysis

by
Yu-Ling Sun
1,2,*,
Chun-Hua Zhang
1,2,
Ying-Jie Lian
1,2 and
Jia-Min Zhao
1
1
National Science Library, Chinese Academy of Sciences, Beijing 100190, China
2
School of Economics and Management, University of Chinese Academy of Sciences, Beijing 100049, China
*
Author to whom correspondence should be addressed.
Sustainability 2022, 14(19), 12302; https://doi.org/10.3390/su141912302
Submission received: 19 July 2022 / Revised: 13 September 2022 / Accepted: 21 September 2022 / Published: 27 September 2022
(This article belongs to the Special Issue Climate Change and Natural Resources Economics)
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Abstract

:
Climate change is one of the great global challenges. Cities are both drivers and responders of climate change. In recent years, the literature associated with climate change and cities has grown rapidly, but few studies have used a bibliometric analysis and visualization approach to conduct deep mining and explore the current situation and development trends of this field. By using bibliometric and text mining methods, the authors conducted a knowledge map analysis of the research on cities and climate change. Moreover, this article attempts to identify the research hotspots and research gaps in this field. The following findings are distilled. First, research in this field is rapidly emerging, and the current research distribution is extremely uneven. China and the US contributed 36% of total paper output. Second, previous research focused on six topics: Impact of Climate Change and Urbanization, Urban Climate Change Adaptation, Urban Heat Island Effect, Urban Greenhouse Emission, Urban Climate Change and Water, and Urban Energy Systems. The first two topics are currently the most popular directions of research. Third, empirical research shows positive interest in big cities, while climate change research in small and medium-sized cities has been neglected. The results of this work will not only help researchers clarify the current situation in cities and climate change science but also provide guidance for future research.

1. Introduction

The relationship between urbanization and climate change is complex. A growing body of research shows that cities are affected by global climate change as well as the multiple effects of local climate change caused by urbanization itself. Related research shows that climate change causes cities to face many severe challenges, such as the sea-level rise and frequent extreme weather events, which cause more and more economic and health losses [1,2,3]. At the same time, the world has been undergoing a remarkable process of urbanization, especially in developing countries in recent years. By 2050, the proportion of the urban population is expected to rise from 55% in 2018 to 68% [4]. With the development of urbanization and industrialization, urban greenhouse gas (GHG) emissions and pollution are becoming increasingly serious, exacerbating regional climate change [5,6,7]. Obviously, from the perspective of climate change response, cities have become the main battlefield for future sustainable human development. There is growing evidence that cities and climate change will be major research topics facing mankind and an emerging frontier of climate change research [8].
Bibliometric analysis and text mining can effectively describe the overall trend of the development of a subject or field, and they have been widely used in the field of climate change [9]. Hou and Wang [10] analyzed the characteristics of publications, research foundations, research hotspots, and the evolutionary tracks of studies in the field of energy, the environment, and climate change by using a bibliometric method. Maretti et al. [11] used co-word analysis, co-author analysis, and co-citation analysis to discover the research hot topics and their distributions in environmental and climate migrations. In terms of text mining, the commonly used methods are K-means, Latent Dirichlet Allocation (LDA), etc. Fazle et al. [12] discovered research topics in the field of climate change by using the LDA topic model and found that the topic “climate change adaptation” appears on the rise, whereas “pollution” and other terms are declining. These studies demonstrate the current state and dynamics of research in the field of climate change.
There are also several relevant review articles using a bibliometric method in the field of city and climate change. Chen et al. [13] conducted a bibliometric review by using CiteSpace to elaborate changes in hotspots, frontiers, development processes, and applications of urban climate research. Qiu et al. [14] analyzed the development of urban floods under climate change and urbanization by using CiteSpace and VOSviewer to construct a scientific knowledge map. These studies used bibliometric methods to conduct research from a single perspective of urban and climate change research, but until now, there has been a lack of comprehensive examination of the research in the entire urban and climate change field. Compared with the current research, the characteristics and significance of this research are as follows: First, it covers the entire field of urban and climate change research and reveals current research hotspots and typical studies, which helps to better understand the main research context and provides an overview of the fast-growing research field of cities and climate change; Moreover, from the perspective of empirical research, it finds deficiencies in the current research, which is of great significance for future research gaps to be filled.
This paper attempts to answer the following questions: What are the current basic trends and global distribution patterns of research in this field? Which topics are receiving more research attention and which ones are neglected? First of all, the bibliometric method was used to describe the latest research status, including the trend of numbers of publications, distribution of the publications, and the frequency analysis of keywords. Then, natural language processing methods and text-mining methods such as the Term Frequency–Inverse Document Frequency model, the n-gram language model, and the K-means algorithm wear used for topic clustering and entity extraction to discover the most interesting research topics. Ultimately, the key research gaps are discussed, and several suggestions for research are provided in the discussion.

2. Materials and Methods

2.1. Data Collection

This article used the SCI and SSCI literature from the Web of Science (WOS) core collection platform as the data source. WOS is a comprehensive database designed with relatively standardized and strict journal selection criteria to ensure the quality of the included articles to a certain extent. The selection of SCI and SSCI is more appropriate for the quality of the articles and the scope of the discipline. These data are considered suitable and reliable for bibliometric analysis due to the wide range of scientific articles and the high quality of the articles [15]. The following research string was applied: TS = (city or urban* or cities) near (“climate change” or “climate mitigation*” or “climate adaption*” or “extreme weather” or “climate risk” or “carbon neutral” or “extreme heat” or “extreme precipitation” or “extreme drought” or “extreme low temperature” or “heat island effect” or “global warming” or “atmospheric circulation*”). A total of 12,273 articles were retrieved, and a total of 8138 articles were obtained for later analysis after manual interpretation and deletion of low-relevance articles; the search time was 25 April 2022.

2.2. Data Processing

First, the was articles were cleaned according to the characteristics of the two fields of publishing journals and subject areas. We deleted the obvious irrelevant documents, and then sorted them by relevance. Then, we manually deleted the rest of the irrelevant documents and carried out a quantitative analysis of the remaining 8138 papers.
Second, to prepare topic clustering, natural language processing (NLP) was performed on the total papers by selecting the title and abstract of the article as text objects. Natural Language Toolkit 3.6.7 (NLTK, The data is available at https://github.com/nltk/nltk, accessed on 18 July 2022) in Python was used for word segmentation, including punctuation removal and stop wording and entity extraction. To improve the reliability and scientific interpretability of the word segmentation, we first used the keywords cleaned by domain experts as the dictionary for word segmentation. Specifically, the domain experts selected the important keywords in the dataset, and we used this keyword dictionary to perform the text objects’ first word segmentation. Then, the remaining text underwent a second dictionary segmentation using the NLTK and the snowball-stemming algorithm for NLP word extraction.
Last, we used an entity extraction algorithm to sample cities and countries that appear in the title. In the Name Entity Recognition (NER) session, the entity labeled as GPE and Location was extracted. To ensure the accuracy of the extraction, we manually removed the extracted non-country/region terms.

2.3. Topic Clustering and Bibliometric Analysis

Text mining of scientific literature can accurately capture the contextual structure of a topic, track research hotspots within a research field, and improve the availability of information about the literature [16]. This article used the text mining method to analyze the topics. First, a Term Frequency–Inverse Document Frequency model (TF-IDF) combined with the n-gram language model was used for keyword extraction. TF indicates the frequency of the word in the text, and IDF reflects the frequency of a word in all texts (the whole document). TF–IDF is a widely used text feature extraction technique in text mining to assess the importance of a word in a file or corpus, usually for keyword extraction. The n-gram model is based on the Markov hypothesis that the probability of each word in a sentence appearing is only related to the n-1 word that precedes it. The n-gram language model takes the influence of word context into account; so in word segmentation applications, fixed phrase collocations in English can be recognized. Considering cities and climate change domain vocabulary, such as “climate change”, “urban heat island”, “green infrastructure”, and many other fixed phrase collocations, this study uses unigram, bigram, and trigram models for word segmentation and combines TF–IDF technology to extract text keywords. Second, a topic clustering model was built, and the K-means algorithm was used for topic clustering. The K value was determined by calculating the within-cluster sum of squares (WCSS). Then Matplotlib and Gephi were used for visualization. Figure 1 shows the steps of the whole process.

3. Results

3.1. Basic Information of the Selected Articles

3.1.1. Trends in the Number of Annual Papers

Research papers on cities and climate change first appeared in the 1970s. However, until 2008, the annual number of articles published worldwide was below 100. The number of papers grew slowly, and the attention of the academic community was low. Since IPCC AR4, scientific research on urban climate change has received more attention and the number of publications has grown gradually. Between IPCC AR4 and IPCC AR5, extensive research on the risks related to urban climate change, involving drivers of urban climate change, urban exposure, and vulnerability in the context of global warming, etc., was carried out [17]. Since IPCC AR5, the number of papers in this field have been rapidly growing. In addition to an improved understanding of urban climate change science, the literature has extensively assessed the impacts, risks, and adaptations of climate change to cities, and the role of cities as non-state actors in addressing climate change has been highlighted by the international community [18]. From 2014 to 2021, the number of papers increased from 384 to 1366, with an average annual growth rate of 20% (Figure 2). A total of 8138 papers were cited 227,877 times in the WOS database, with an average of 28 citations.
The number of research outputs on cities and climate change is very small compared to the number of research outputs in the overall climate change field. In 2021, the number of SCI and SSCI papers in the field of climate change exceeded 260,000, but there were less than 2000 papers on cities and climate change research. On the one hand, research on cities and climate change started late, and the progress of related research was slow due to the limitations of city-level data, monitoring, and government participation.

3.1.2. National (Regional) Distribution of the Research Papers

In this paper, the distribution of the countries (regions) where the first author of research papers was located was calculated (shown in Figure 3). There were 117 countries worldwide that have published research papers in the area of cities and climate change. And the distribution of the first author’s country was counted. China and the U.S. are far ahead of other countries in the number of papers published in this field, with China publishing 1515 papers, accounting for 18.2% of all papers, followed by the U.S. with 1474 papers accounting for 17.7%, followed by the UK with 492 papers. The top 10 countries also include Australia, Germany, Canada, Italy, the Netherlands, and Spain, the vast majority of which are developed economies. The top 10 countries account for 84% of all papers published.

3.2. Analysis of Research Topics

3.2.1. High-Frequency Keywords Distribution

High-frequency keywords with word frequencies greater than 200 are shown in Table 1. The high-frequency words reflect the hot directions of the research to some extent. In addition to the words “climate change” and “urban”, the word “impact” appears most frequently, indicating that the impact of climate change on cities is a hot topic. This is followed by the words “simulation”, “temperature”, “adaptation”, “management”, “heat island effect” and “vulnerability”, which also have a higher frequency. It can be inferred from the high-frequency terms that urban climate change modeling, urban adaptation and management systems, urban energy systems, resilience development, sustainable development, and urban planning are the hot topics in climate change and urban research. In terms of the study area, the word “China” appears in the list of high-frequency words, indicating that China is receiving more academic attention.

3.2.2. Topic Clustering Analysis

The K-means algorithm was used to cluster the extracted keywords, and the six classes were judged as the most suitable clusters by calculating the within-cluster sum of squares (WCSS) between the K-means groups. Table 2 shows the specific clustering results. The themes of each class cluster can be broadly summarized as Impact of Climate Change and Urbanization, Urban Climate Adaption, Urban Heat Island Effect, Urban Greenhouse Emission, Climate Change and Water, and Urban Energy System, and the corresponding papers under each topic were 2719, 1671, 1599, 817, 764, and 568, respectively. Figure 4 shows the time-trend analysis for each category of papers. Figure 5 shows the visualization of clustering. The number of papers on each topic shows an increasing trend year by year. In particular, the topic of urbanization and climate change has the fastest growth rate, while urban energy system has a slower growth rate.
(1)
Impact of Climate Change and Urbanization. This topic included 2719 research papers, the largest number of papers in the category of the six topics. The complex interaction mechanisms between cities and climate change are a hot topic of the scientific research. The main scientific issues of interest are following. (1) The first major focus is the assessment and prediction of the impact of urbanization on regional or urban climate. The research focuses on projecting high temperature and heat waves observed in urban areas, extreme events such as heavy precipitation-flooding, changes in air pollution, and future changes caused by the “urbanization effect”. In recent years, model simulation methods and data science methods using satellite/remote sensing have emerged. Especially in the past five years, model simulation research has increased rapidly. Using various urbanization and climate models, the impact of past urbanization on climate change has been assessed, and future urbanization scenarios and the impact of future urbanization on climate change are also predicted [19,20] Some scholars have developed regional or inter-city comparisons. Kim et al. [21] compared future urban growth and flood risk in Amsterdam and Houston in the context of climate change. Yan et al. [22] assessed surface temperature changes in urbanization and agriculture in three of the most developed urban agglomerations in China (Beijing–Tianjin–Hebei, Jinjiang, the Yangtze River Delta, and the Pearl River Delta) based on satellite data. (2) The second major focus is urban climate change investigation, along with its impact and vulnerability assessment. Ziska et al. [23] examined the possible public health consequences of temperature/CO2 increases associated with regional urbanization versus projected global climate change. Sun et al. [24] and Gál et al. [25] studied the mitigation of the urban heat island effect by different types of urban green spaces. Manish et al. [26] assessed climate change impact on precipitation extremes over Indian cities. (3) The third focus is the simulation of coupled population, urban, and climate change relationships. Effective action to address climate change needs to be based on the scientific understanding of urban climate change and its impacts. Research in recent years has begun to focus on more diversified factors, such as social, economic, and demographic factors. Salman et al. [27] examined flood risk simulation and assessment under multiple factors, such as climate change, population growth, increased urbanization, and infrastructure decline. Liu et al. [28] studied the impact of climate change and human activities together on the hydrological characteristics of urban rivers. Castells-Quintana et al. [29] studied the relationship between changes in weather patterns and the spatial distribution of population and economic activity within countries. Since IPCC AR5, significant progress has been made in the research on the impact and prediction of urbanization effects in the context of global warming. According to the assessment results of IPCC AR6, urbanization has an important impact on the changes in local climate and related extreme events. It is pointed out that under the background of global climate change, cities have an aggravating effect on high temperature and heat waves, heavy precipitation and flood disasters, and air pollution. On the one hand, it benefits from the deepening of climate change scientific research, which shows the expansion from focusing on global climate change to regional impact assessment; on the other hand, it also reflects the deepening of climate change assessment work oriented to solve scientific problems.
(2)
Urban Climate Change Adaptation. This topic included 1671 research papers focusing on how cities are adapting to climate change from scientific, management, and practical perspectives. Early responses to climate change focused on mitigation measures, and it was not until IPCC AR3 that the theme of adaptation began to receive independent and adequate attention in assessments. There is growing attention to integrating adaptation as part of a development process addressing the structural condition causing social and urban vulnerability. The main scientific issues of concern are: (1) The first main scientific issue of concern is research on urban climate adaptation strategies and policies. There have been diverse and useful disciplinary contributions and experiences to building adaptation strategies during the last decade, including both practical and theoretical approaches [30,31,32,33,34]. Mukheibir et al. [30] discussed an overarching framework that would facilitate the development of a municipal adaptation plan. Rosenzweig et al. [32] presented the adaptation framework and the sea-level rise and storm projections related to coastal risks developed through the stakeholder process. (2) The second issue is research on pathways and development models for urban climate adaptation. This research has developed a variety of technological, political, or ecological solutions. The ones that have received more attention in recent years are green infrastructure (GI), ecosystem-based adaptation (EbA), nature-based solutions (NbS), and their combined effects [35,36,37]. Synergistic solutions for urban climate adaptation and mitigation [38,39,40] have also been widely discussed. At the same time, urban development forms and models based on climate adaptation have received academic attention, and in this regard, eco-cities [41], sustainable cities [42], climate-resilient cities [43], low-carbon cities [44], and resilient cities [45,46,47,48] were discussed. (3) The third issue is urban climate governance study. Cities play an important role in the global governance of climate change and are increasingly recognized as a crucial component of the post-Paris climate regime. Since the early 1990s, the research field has grown and diversified geographically, theoretically, and methodologically and now encompasses a wide range of topics, including governing techniques, limitations and challenges, central–local relations, municipal networks, network governance, and grassroots initiatives. These studies attempt to answer how climate change is being governed in the city and the implications for urban governance, socio-environmental justice, the relationship between climate change and global governance, and the reconfiguration of political authority. In [49,50,51,52,53], the authors examined the conceptual paradigm, framework, and system of climate adaptive urban governance. Bulkeley et al. [54,55] and Broto et al. [56] studied the relationship between urban political ecology and climate governance systematically. In recent years, collaborative urban climate governance, data governance, urban climate adaptation practices, inclusive governance, and innovation in the context of sustainable development have attracted much attention [57,58,59,60]. In the past 20 years, some areas of developed countries, such as New York [61], London [62], and Toronto [63], have successively carried out urban adaptation to climate change. In some developed countries, urban climate adaptation is gradually being incorporated into city planning. In February 2017, the Chinese government issued the “Notice on Printing and Distributing the Pilot Work of Climate-adaptive City Construction” [64], and 28 regions, including Hohhot in Inner Mongolia Autonomous Region and Dalian in Liaoning Province, were selected as pilot projects for climate-adaptive city construction.
(3)
Urban Heat Island Effect. Urban Heat Island (UHI) is a phenomenon where urban areas experience a higher temperature than their surrounding non-urban areas and is considered a critical factor contributing to global warming, heat-related mortalities, and unpredictable climatic changes. This topic included 1599 research papers, mainly focusing on urban heat effect model simulation, assessment and monitoring, spatial and temporal factors of heat island effect formation, and heat island effect mitigation policies and technologies. The main scientific issues of concern are following. (1) The first issue is the specific regional or urban heat island intensity, magnitude estimations and spatio-temporal evolution simulations. Researchers have carried out extensive research from different temporal and spatial scales involving multiple dimensions such as microclimate, local climate, mesoclimate, and macroclimate. Driven by remote sensing data, geographic information data, and artificial intelligence methods, in the past five years, the research on urban heat island simulation has integrated more historical data, and the simulation scale has developed more microscopically. Meng et al. [65] characterized spatiotemporal changes in surface urban heat islands (SUHIs) using 12 years of satellite data in Beijing. Huang et al. [66] and Trimmel et al. [67] predicted urban heat waves and heat stress under different future urban expansion scenarios. Some recent studies refine the assessment of heat island effects at a more micro level. Meng proposed a new scheme to quantify the warming effect of large, heat-emitting urban objects versus complex surroundings, and the IHI effect was accordingly defined at a finer scale [68]. (2) The second issue is the study of the drivers of the heat island effect. Deilami et al. [69] reviewed studies on drivers of the urban heat island effect in the related literature and showed that the most common factors affecting the UHI effect included vegetation cover (44%), season (33%), built-up area (28%), day/night (25%), population density (14%), and water body (12%) together with others. Huang et al. [70] explored the comprehensive effect of 2D and 3D urban morphology on LST in different urban functional zones (UFZs). Yang et al. [71] explored the impacts of PM2.5 on the wintertime UHII in the Beijing–Tianjin–Hebei megalopolis of China during 2013–2017. The study improves the understanding of the urban climate affected by air pollution and provides a scientific basis for the mitigation of UHI impacts. (3) The third issue is urban population vulnerability and exposure to the heat island effect and heat waves. Tuholske et al. [72] estimated daily urban population exposure to extreme heat for 13,115 urban settlements from 1983 to 2016. Rathi et al. [73] assessed the extreme heat vulnerability of the population of four cities with different characteristics across. (4) The fourth issue is urban heat island mitigation technologies. Studies have shown that future urban land expansion substantially intensifies heat stress. It exists with or without climate change induced by rising concentrations of GHGs. Heat burden cannot easily be reduced by measures concerning buildings within the city itself [66,67]. Measures such as planting trees, regional water-sensitive planning, and the global reduction of GHGs emission are required. So, diverse urban heat island mitigation technologies, such as highly reflective materials, cool and green roofs, cool pavements, and urban greening for urban heat island mitigation and application cases, are receiving attention [74].
(4)
Urban Greenhouse Emission. Cities contribute 75–80% of global GHG emissions, and GHG emissions from urban spatial systems are related to urban density, urban form, transportation, industrial layout, and other factors. Urban carbon emission accounting is important for formulating emission reduction policies and building low-carbon cities. This research topic included 816 research papers, mainly focusing on urban GHG emission accounting and assessment, scenario simulation, influencing factors, and emission reduction policies and approaches. (1) The first main scientific issues of concern is urban GHG emission accounting methods and inventory tools development. Urban GHG inventory research began in the 1990s. Most of the early urban GHG inventory methods followed the national inventory method of the Intergovernmental Panel on Climate Change (IPCC). ICLEI (the International Council for Local Environmental Issues) explored and established a greenhouse gas inventory compilation system and method suitable for urban characteristics, which has been widely accepted by cities around the world and has become the mainstream method for compiling GHG inventories. Due to the large differences in the characteristics and management models of cities around the world and the differences in the understanding and definition of cities, the methodology for the compilation of GHG inventories at the city scale is still not unified and standardized. Many scholars have also researched and explored urban GHG inventories. Ghaemi et al. [75] reviewed studies of life-cycle GHG emissions at the city scale and concluded that there are difficulties in calculating city-scale GHG emissions using the Life Cycle Assessment (LCA) approach at three stages. Yu et al. [76] used a hybrid life-cycle approach to assess the full life-cycle GHG emissions in urban areas and thus quantified the reduction potential of different emission reduction measures. (2) The second issue of concern is the assessment of the contribution of various sectors, such as transportation, industry, urban waste, infrastructure, and energy systems, to greenhouse gas emissions. Kennedy et al. [77] examined how the balance between geophysical factors (climate, resource access, and gateway status) and technological factors (power generation, urban design, and waste treatment) in global cities is attributable to urban GHG emissions. Liu et al. [78] assessed the contribution of green infrastructure (GI) to low-carbon urban drainage systems and cities in Dongying, China, and concluded that GI optimizes hydrological processes and corresponding greenhouse gas (GHG) emissions during rainfall events. (3) The third is greenhouse gas emission reduction pathways and policies. Churkina et al. [79] investigated the possibility of improving building materials to achieve emission reductions and carbon storage. Hsu et al. [80] evaluated mitigation effectiveness and performance determinants in more than 1000 European cities, showing that greater reductions are associated with programs targeting energy efficiency.
(5)
Urban Climate Change and Water. Urban climate change and water is one of the key issues in the current climate change field. Urbanization and climate change are together exacerbating water scarcity, which includes increasing the risk and vulnerability of urban water systems and challenging the sustainability of urban water resources. This topics included 764 research papers focusing on the impact of urbanization on water security and the resulting challenges of urban water management. (1) The first main scientific issues of concern is the impact and challenges of climate change and urbanization on the sustainability of water resources. In 2008, IPCC published a special research report on “Climate Change and Water”, pointing out that climate change has brought catastrophic risks to urban water systems. Since then, many countries and international organizations have strengthened research on climate change and water systems, and the depth and breadth of research have continued to increase. Yigzaw et al. [81] studied water sustainability of large cities in the United States from the perspectives of population increase, anthropogenic activities, and climate change. He et al. [82] quantified global urban water scarcity in 2016 and 2050 under four socioeconomic and climate change scenarios and explored potential solutions. Florke et al. [83] studied water competition between cities and agriculture driven by climate change and urban growth. (2) The second major issue is urban water management for climate change adaptation. Nair et al. [84] integrated the dynamics of multiple water–energy–GHG linkages in urban water systems to propose appropriate urban water strategies. Paez-Curtidor et al. [85] analyzed the application of the water–energy–food nexus approach to the climate-resilient water safety plan of Leh Town. (3) The third major issue is climate-resilient water sustainability and water-sensitive cities. Water-sensitive urban design and practice have received widespread attention as a mainstream solution for managing water resources in cities in response to climate change. In the past ten years, from theoretical design to practical case study, water-sensitive cities research made meaningful progress. Wong et al. [86] provided an overview of the emerging research and practice focused on system resilience and principles of sustainable urban water management. Then, they proposed three key pillars that need to underpin the development and practice of a water sensitive city. Sullivan et al. [87] studied water-sensitive cities in the Colorado River Basin, and Nguyen et al. [88] assessed the practice of sponge cities in China. Bichai [89] examined the water-sensitive urban management paradigm and the implications of its globalization.
(6)
Urban Energy Systems. Urbanization, climate change, and energy transition have become mainstream trends in global development. Clarifying the intricate relationship between the three will help effectively deal with the risks of urbanization and climate change. This topic included 698 research papers focusing on the impact of climate change on urban energy demand, urban energy system planning and low-carbon transition, and the interaction of complex systems such as urban energy, economy, and climate. (1) The first main scientific issues of concern is assessing the impact of climate change on urban energy demand and consumption. Hooyberghs et al. [90] studied the impact of climate change on summer cooling costs and heat stress in office buildings. Perera et al. [91] quantified the impacts of 13 climate change scenarios on the energy systems of 30 Swedish cities. Shen et al. [92] have also studied the future energy demand of single-sector cities in different urbanization and climate change scenarios. (2) The second issue is optimizing urban energy planning and governance for low-carbon transition. Creutzig et al. [93] assessed the potential of urban energy systems to mitigate climate change. The conclusion is that energy use will triple by 2050 in the urban expansion scenario, and urban planning and transport policies can contribute to climate mitigation. Kammen et al. [94] explored options for establishing sustainable energy systems in urban buildings and transportation. Luca et al. [95] studied a renewable energy system for an almost zero greenhouse city using a small city in southern Italy as a case study. (3) The third issue is the coupled interaction of complex systems, such as climate change, urbanization, energy, and economy. Fu et al. [96] assessed the relationship between population trends, historical energy consumption, changes in average electricity prices, average annual temperatures, and extreme weather events for three selected cities, New York, Chicago, and Los Angeles. Wang et al. [97] studied the dynamic interrelationship between urbanization rate, energy use, economic growth, and GHG emissions China.

3.2.3. The High Profile Cities Research and Topic Distribution

To understand the global cities of interest in this field, we used entity extraction techniques in NLP to perform text mining on the retrieved documents and extract the specific cities involved in each document. If more than one city appears in a document, it is marked as multiple, so we can calculate which cities receive high academic attention and which receive average or low attention. Then, we conducted a correlation analysis between cities and topics to further understand what relevant research has been conducted by academics in cities that have received high and average attention. The results are shown in Figure 6.
The statistics found that the literature surveyed involved relevant studies from nearly 2000 cities worldwide. We have listed the top 1% of high-interest cities in terms of frequency of occurrence and the countries where they are located. To understand which areas of research focus on these cities, we analyzed the literature studying these cities in association with the clustering results. Similar to the national distribution pattern of the literature, the degree of attention varies very much between cities. Fifty percent of the top twenty cities receiving the most attention are capitals of countries, from eight developed and four developing countries. Countries with high vulnerability to climate risk are concentrated in small island developing states, the Arctic, South Asia, Central and South America, and much of sub-Saharan Africa, and cities in these regions have not received sufficient attention. Cities in African countries have generally received less attention, with several coastal cities such as Cape Town, Maputo, Mombasa, and Accra appearing in the titles of research papers in the retrieved literature, but only 5–6 times in the literature. Most of the top 20 cities are global megacities, with the top 10 cities in order being Beijing, New York, Shanghai, Hong Kong, London, Mexico City, Seoul, Dhaka, Singapore, and Barcelona. These cities share common characteristics of being economically developed and densely populated, with high climate risk and vulnerability.
The correlation analysis of the city and topic clustering results shows that the city case studies were concentrated in the two topic areas of climate change adaptation and mitigation, particularly in Asian countries such as China and Japan. There is a relative lack of research on urban climate change impacts, heat island, and urban energy systems. Due to limitations in data collection, monitoring, and measurement, there is a relative lack of evidence of urban-scale climate change, which to a certain extent limits urban climate change risk prediction and impact-related research, so it is important for cities to establish long-term climate change monitoring systems. City government networks such as ICLEI and C40, in collaboration with individual cities, can make important advances in evidence collection, target setting, emission reductions, and improved adaptive capacity [98,99].

4. Discussion

Although the quantitative gap in research outputs is huge compared to climate change research, the trend in the number of papers indicated that the field of cities and climate change is increasingly becoming an emerging research frontier. So far, most countries have paid little attention to this field. The regional distribution of existing urban and climate change research was extremely unbalanced. China and the U.S. have jointly led the research. A large number of diverse city types in China provides cases for conducting research. However, most developing countries and undeveloped countries give little attention to this research field. At least two reasons are causing this imbalance and inadequacy of the research. For one thing, cities around the world are complex and diverse, the definition and scope of cities are not uniform internationally, and their management models and governance methods are quite different. Many pieces of research focused on the case of certain cities, and the generalizability is limited. On the other hand, cities are complex and dynamic metasystems in which technological and social components interact with each other. Limited data availability and scientific capacity remain significant challenges, especially for cities in developing countries. Therefore, the global city level needs to focus on establishing climate data monitoring, acquisition, and analysis.
Current research topics reflect the hotspots of scientific issues related to cities and climate change. It involves interdisciplinary and multi-scale research on impact and risk prediction, climate modeling, adaptation and mitigation, strategies and policies, etc. From the clustering results of topics, it can be concluded that the research on the impact of climate change and urbanization and urban adaptation was of greater concern. By reviewing the literature on the topic, we found a comprehensive geographic perspective that a couple of multiple factors is increasingly becoming a trend. The research not only involved high temperature and heat waves observed in urban areas, extreme events such as flooding, changes in air pollution, and future changes caused by the “urbanization effect” and climate change, but also increasingly focused on economic and social dimensions, such as population and economic activity, social vulnerability, and human health. However, in addition to the above factors, technological progress seems to be ignored, especially the huge impact of disruptive technologies. In fact, ICT technology-driven smart city construction has been practiced in many countries and has resulted in sustainable smart cities and climate-neutral smart cities combined with low-carbon and sustainable development. The EU plans to build 100 climate-neutral smart cities within the EU by 2030. In the future, the mode of urban response to climate change in the context of technological progress should be evaluated.
The topic of urban energy-related research has declined in recent years. Studies have shown that energy transition is critical to address climate change and help local government to achieve sustainable development goals [100,101]. Existing research focuses on the impact of urban climate change on energy consumption and the optimization of renewable energy systems, etc. While the transition to a sustainable energy system is subject to multiple influences and constraints, such as economy, technology, society, and culture at the regional level, the transformation process will also influence local employment and income. There have been some successful empirical studies. Pasquale et al. [102] investigated the roles and perspectives of stakeholders in support of local bioenergy transition processes taking Porto Marghera and Gela in North Italy as an example. However, there is no single model for sustainable energy development in all cities in the world, and policy makers need to choose the appropriate strategy based on the specific circumstances of the city and country. How to coordinate the relationship between the policy framework and technological innovation and how to improve the ability of local governments to effectively implement sustainable energy policies and the dialogue and coordination of other energy stakeholders are still challenges facing the large-scale transformation of urban energy. In the future, we can focus on the above issues, provide policy makers with policy paths, and promote the practice of urban energy transformation.
The research on climate risk and adaptation in different regions and types of cities is imbalanced. Cities in developed countries and cities in economically and socially developed regions have received more research attention, while underdeveloped countries and regions, as well as small and medium-sized cities, have received less attention. Although climate risk is higher in large cities, the vulnerability of small and medium-sized cities cannot be ignored. The analysis of climate change risks in major urban clusters in China reveals that the risks are not only concentrated in large cities with highly developed economies and societies, but the climate change risks in small and medium-sized cities and small town regions are also rising rapidly. In addition, for developing countries, such as China, how to achieve coordinated development of pollution reduction and carbon reduction is very important. Recent studies have systematically evaluated the progress of climate change mitigation and air governance at the city level in China and found that their development trends are not consistent [103]. Future research should pay more attention to small and medium-sized cities, especially those in developing and underdeveloped areas. Growing urbanization and climate change pose compounding risks, especially for cities with poor planning, high levels of poverty and unemployment, and a lack of basic infrastructures.

5. Conclusions

Both climate change and urbanization are hotspot issues of concern to the international community. Obtaining a clear picture of the status of urban and climate change research is of great practical significance to a global response to climate change and the sustainable development of cities. Considering these factors, this paper conducted a statistical analysis of the data on SCI/SSCI papers selected in this field through topic retrieval, with main data in some data fields properly cleaned, and then developed a clustering analysis atlas. Research findings include:
(1)
The publication volume has risen quickly since 2015. Trends in the number of articles correlate with IPCC AR4 and IPCC AR5 releases. The national distribution characteristics of papers reflect a high degree of concentration. The top 10 countries account for 84% of all papers published. China and the U.S. are far ahead of other countries in the number of papers published in this field.
(2)
Regional and urban distribution of existing urban and climate change research was unbalanced. Existing case studies focused on large cities in economically developed regions and gave insufficient attention to small and medium-sized cities, which are in ecologically vulnerable regions and climate-change-sensitive regions. Future research should pay more attention to small and medium-sized cities, especially those in developing and underdeveloped areas.
(3)
The clustering analysis and frequency analysis of the keyword shows that the hot topic categories in the field of cities and climate change. It can mainly be found in six categories, such as Impact of Climate Change and Urbanization, Urban Climate Change Adaptation, Urban Heat Island Effect, Urban Greenhouse Emission, Urban Climate Change and Water, and Urban Energy Systems. The first two topics are currently the most popular directions of research.
(4)
Through the analysis of the hot topics, this paper also explored the changing trends of hot topics which concern researchers in each area of cities and climate change. The following research should be strengthened based on the paper: the application of multidisciplinary methods such as humanities, economics, and geography to better understand the combined risks of cities and climate change and the impact of disruptive technological factors on urban climate change response and policy research on urban energy transition.
The conclusions of this study will help researchers master the development and trends in cities and climate change research, and provide guidance and reference for future research in this field. There are some limitations in the paper. The objectctive of this study focuses on research articles in a scientific and technological evaluation database and does not include reports issued by international organizations and think tanks in the research field of cities and climate change. In addition, to identify future research directions, the literature metrology described here was just a research perspective. It is deemed necessary to undertake a more detailed literature review around the most interesting current research topics and methods in the field of climate change.

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/su141912302/s1.

Author Contributions

Conceptualization and methodology: Y.-L.S.; data collection, analysis and visualization: C.-H.Z. and Y.-J.L.; writing—original draft preparation: Y.-L.S.; writing—review and editing: C.-H.Z. and J.-M.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Chinese Academy of Sciences Strategic Research Project (GHJ-ZLZX-2022-13); Chinese Academy of Sciences Literature and Information Capacity Building Project (E1290428).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data is contained within the article and Supplementary Materials.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Methods and processes of this study.
Figure 1. Methods and processes of this study.
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Figure 2. Trend chart of annual publications.
Figure 2. Trend chart of annual publications.
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Figure 3. Distribution of the top 10 countries (regions).
Figure 3. Distribution of the top 10 countries (regions).
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Figure 4. Time trends for papers on each topic.
Figure 4. Time trends for papers on each topic.
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Figure 5. Topic-clustering visualization.
Figure 5. Topic-clustering visualization.
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Figure 6. City–topic distribution map.
Figure 6. City–topic distribution map.
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Table
Table 1. List of high-frequency words (word frequency >200).
Table 1. List of high-frequency words (word frequency >200).
KeywordsFrequencyKeywordsFrequency
Climate change2879Emission321
Impact1613Perform312
City1516Land-use304
Urban1250Veget288
Model818Area288
Temperature763Health287
Adaption648Ecosystem service287
Management566Variable273
Urban heat island544Trend268
Climate521Framework265
Vulnerability471Environment258
Energy449Water256
System441Precipitation256
Resilience432Consumption242
Sustainability410Urban plan235
China393Pattern235
Policy387Challenge224
Mitigation386Urban heat-island222
Govern385Strategy220
Risk344Design218
Mortal334Green infrastructure213
Heat-island333Thermal comfort211
Simulation332Transport201
Table
Table 2. List of cities and climate-change research topics clustering.
Table 2. List of cities and climate-change research topics clustering.
No.ClustersKeywords (Partial)Number of
Papers
1Impact of Climate Change and
Urbanization		City, climate, model, change, develop, urban, risk, climate change, data, land, impact, future,
management, increase, plan, level, approach,
analysis, region, scenario		2719
2Urban Climate AdaptionClimate change, adaption, city, plan, policy, risk,
develop, strategy mitigation, future, approach, level, challenge, process, assess, management, effect, measure		1671
3Urban Heat Island EffectHeat island, heat, temperature, island, urban heat, urban heat island, surface, city, air, land, model, increase, climate, effect, build, condition,
environment, data, impact		1599
4Urban Greenhouse EmissionEmission, greenhouse, city, energy, policy,
mitigation, model, air, develop, scenario, climate, data, impact, analysis, level, contribution,
measure		817
5Climate Change and WaterWater, climate, management, climate change, city, model, develop, impact, future, scenario, surface, increase, challenge, approach, popular, region, condition, assess, urban, risk764
6Urban Energy
System		Energy, build, city, emission, heat, climate, model, greenhouse, develop, change, policy, climate change, right, scenario, mitigation, analysis, data, increase, strategy, plan, environment568
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Sun, Y.-L.; Zhang, C.-H.; Lian, Y.-J.; Zhao, J.-M. Exploring the Global Research Trends of Cities and Climate Change Based on a Bibliometric Analysis. Sustainability 2022, 14, 12302. https://doi.org/10.3390/su141912302

AMA Style

Sun Y-L, Zhang C-H, Lian Y-J, Zhao J-M. Exploring the Global Research Trends of Cities and Climate Change Based on a Bibliometric Analysis. Sustainability. 2022; 14(19):12302. https://doi.org/10.3390/su141912302

Chicago/Turabian Style

Sun, Yu-Ling, Chun-Hua Zhang, Ying-Jie Lian, and Jia-Min Zhao. 2022. "Exploring the Global Research Trends of Cities and Climate Change Based on a Bibliometric Analysis" Sustainability 14, no. 19: 12302. https://doi.org/10.3390/su141912302

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