The Dynamics and Trends of International Research on Urban Carbon Risk

Abstract

Research on Urban Carbon Risk (RUCR) is crucial for understanding the impact mechanisms of carbon emissions on urban environments and health, particularly in rapidly urbanizing areas. This paper conducted a bibliometric analysis of 2012 studies on RUCR indexed in the Web of Science (WOS) database from 1991 to June 2023. It reached the following conclusions: (1) The annual publication volume of RUCR has steadily increased since 2005, mainly focusing on environmental science and public health. A co-citation analysis of the literature indicates that RUCR research content is centered on carbon sink assessment, risk factor analysis, and response strategies. (2) RUCR has undergone four developmental stages: singular exploration, evaluation and construction, innovative breakthroughs, and technological synergy. (3) The key research issues of RUCR include carbon reduction and sink enhancement, integrating qualitative and quantitative planning methods, and multidisciplinary collaboration. (4) Current research hotspots in RUCR focus on urban pollution and health risks, ecological environment and land use change, carbon emissions and energy utilization, and pollution monitoring technologies. Future research trends are anticipated to center on source apportionment and monitoring of carbon emissions, the relationship between air pollution and health risks, and the governance and mitigation of carbon emissions. (5) Based on the analysis of critical issues and trends, it is recommended that future research prioritize spatial identification and scenario simulation of urban carbon risk. The conclusions of this paper facilitate researchers’ quick understanding of the current status and development trends of RUCR and propose future research directions from urban planning.

1. Introduction

The significant increase in carbon emissions has become one of the most widespread and urgent threats facing the world today [1]. Since the onset of the Industrial Revolution, carbon emissions from human activities, especially fossil fuel combustion, have been the primary driver of greenhouse gas emissions [2]. According to a report by the International Energy Agency (IEA), CO₂ emissions account for approximately 80% of total greenhouse gas emissions [3]. The IPCC Sixth Assessment Report highlights that managing carbon emissions is now a central theme in mitigating climate change [4]. Excessive carbon emissions contribute to global warming and exacerbate the risks of extreme meteorological, climatic, and hydrological events [5,6].

Cities are pivotal in managing carbon emissions and addressing climate risks [7]. Land use, economic activity density, and transportation patterns across various urban areas lead to spatial heterogeneity in carbon emissions, with specific urban spaces facing higher carbon emission threats [8]. For instance, the types and intensities of land use in industrial, commercial, and residential zones vary significantly, influencing carbon emissions within these areas, with built-up land as a significant carbon emission source [9]. Industrial zones have high levels of manufacturing activities and energy consumption, resulting in higher carbon emissions; however, the intensive utilization of industrial land can notably reduce CO₂ emissions [10]. While commercial zones have higher population densities, their activities produce lower carbon emissions. Residential zones’ carbon emissions are closely tied to household energy consumption patterns, building density, and greening rates. A stark contrast exists between agricultural land and the goal of low-carbon, sustainable, high-quality urban development, with imbalanced spatial distribution [11]. Therefore, in tackling urban carbon emission challenges, it is crucial to address the risks that gradually increasing urban carbon emissions effectively poses to cities’ future development.

Current research on Urban Carbon Risk (RUCR) remains in the exploratory phase, mainly focusing on corporate carbon risk [12,13], carbon finance risk [14,15], carbon asset risk [16], low-carbon industry transitions [17], and carbon emission risk [18]. Some researchers have analyzed and evaluated the uncertainties faced in developing and promoting low-carbon, zero-carbon, and negative-carbon technologies. For instance, Han et al. utilized an input-output approach to construct China’s “embedded carbon” network, identifying key nodes and pathways for carbon risk transmission during the low-carbon transition of high-energy industries [17]. Zhou et al. explored the influence of carbon risk awareness on carbon performance and its low-carbon innovation benefits [19]. However, few studies have focused on potential urban carbon risks, particularly in systematically identifying and addressing these risks in climate response and carbon reduction measures. With cities increasingly dominating CO₂ emissions, comprehensive identification and effective management of urban carbon risks have become crucial for reducing emissions and enhancing carbon sequestration capacity.

At present, there is no clear definition of urban carbon risk in the international academic community. By integrating the definitions and classifications of carbon risk (

Table 1. Definition and types of carbon risks.

Carbon Risk Definition *	Classification	Source
/	Systematic and unsystematic risk	[21]
Climate change-related risks	Regulatory risk, reputational risk, legal risk, physical risk, supply chain risk, product and technology risk	[22]
Any business risk related to climate change or fossil fuel use	/	[12]
/	Climate change-related risks consist of three independent but interrelated components: regulatory risk, physical risk, and business risk	[23]
/	Compliance risk, operational risk, strategic risk, reputational risk, and reporting risk	[24]
Uncertainty in the impact of carbon factors on business objectives	Environmental risk, technology risk, financial and information risk, management risk	[25]
A type of climate change-related risk, carbon risk represents potential future losses or current liabilities arising from increasingly stringent regulations on greenhouse gas emissions.	Measured by total carbon emissions, including direct and indirect emissions	[26]
Risks companies face in responding to climate change and related policies	Regulatory risk, reputational risk, risks from changes in customer demand, physical change risk, product and technology innovation, financial risk, and operational risk	[27],
An essential component of environmental risk related to carbon emissions	Technology risk, energy utilization risk, policy and regulatory risk, carbon emission risk	[19]
Uncertainty in political, technical, and regulatory aspects during the green transition phase and its impact on corporate value	/	[28]
Carbon risk is a component of climate risk, referring to the business impact caused by the transition from a high-carbon to a low-carbon economy required to address climate change.	/	[20]
Financial vulnerability of businesses transitioning from a fossil fuel-based economy to a low-carbon economy	/	[29]
Carbon risk typically refers to the impact on corporate value due to extensive policy, law, technology, market, and reputation changes during the societal shift to a low-carbon economy	/	[30]
The potential financial impact of tightening carbon emission policies	/	[31]
Regulatory and market risks arising during the transition of high-emission industries from high-carbon to low-carbon production systems	/	[13]
Negative impact of unexpected changes in carbon prices on businesses and stock portfolios	/	[32]

* “/” indicates that the referenced literature lacks a clear definition or classification of carbon risk.

) [20], this paper defines urban carbon risk as “the uncertainty and potential loss faced by cities in the process of responding to climate change and carbon emission regulations, due to factors such as policy changes, market demand shifts, technological innovations, environmental changes, and socio-economic structural adjustments”. This type of risk encompasses various fields, including urban energy use, infrastructure development, residents’ lifestyles, and urban planning, and has profound implications for the sustainable development of cities and the quality of life of their residents. Urban carbon risk reflects the complex risks induced at the urban level by carbon emission control and low-carbon transition, spanning policy, economic, technological, physical, and social dimensions. Compared to natural climate risks, carbon risks are also driven by policies and market actions involving legal, regulatory, market, or reputational factors. As a result, the timescale of carbon risk occurrence is significantly shorter than that of climate risks (Figure 1).

Currently, review papers on the progress of Research on Urban Carbon Risk (RUCR) are limited, with few studies employing bibliometric methods. This paper uses bibliometric analysis on 2012 RUCR-related publications indexed in the Web of Science (WoS) core database from 1991 to 2023 to reveal the distribution characteristics, key issues, research hotspots, and future development trends in this field. Specifically, (1) WoS data are used to analyze the distribution characteristics of RUCR literature, including annual publication volume, disciplinary composition, source journals, and country distribution; (2) CiteSpace is employed to conduct co-occurrence analysis of keywords, co-citation analysis, and temporal segmentation, to outline the research development trajectory of RUCR and its international collaboration; (3) CiteSpace’s keyword clustering and burst analysis help identify current key issues and research hotspots in RUCR; and (4) future trends in RUCR are predicted, with suggestions for future research from an urban planning perspective.

2. Materials and Methods

CiteSpace enables the identification and visualization of emerging trends and profound changes within a research field by dynamically mapping the evolution of research frontiers and the knowledge base, thus revealing the dynamic characteristics of research development [33]. This paper utilizes CiteSpace software (version 5.6.R5) to conduct a scientific knowledge mapping analysis on “urban carbon risk” literature to uncover potential knowledge associations among research outputs (Figure 2). The data source is the Web of Science (WoS) Core Collection, the global English-language literature database. Data retrieval was conducted on 25 June 2023, with a search scope covering “all years” (default range 1975–2023). The search was conducted using the “Topic” + “Document Type” query mode with the search term TS (Topic Search) = (urban carbon risk), and documents were limited to English-language publications, yielding a total of 2012 relevant articles, with the earliest published in 1991.

Firstly, based on WoS data, this study analyzes the characteristics of literature in the “urban carbon risk” field, including annual publication volume, disciplinary composition, source publications, and country distribution. Subsequently, CiteSpace is used for co-citation analysis, research collaboration, keyword time-slicing, keyword clustering, and visualizations such as the keyword clustering landscape. This multi-dimensional, temporal visualization of knowledge mapping aims to reveal the distribution, development trajectory, key issues, and research hotspots of RUCR. Finally, based on these conclusions, future trends in RUCR are projected, and relevant research recommendations are proposed.

3. Results

3.1. The Literature Distribution Characteristics of RUCR

3.1.1. Literature Source Analysis

The volume of knowledge, disciplinary characteristics, and leading countries within a research field can be reflected through changes in the quantity of literature and its disciplinary and national distribution [34]. Based on the online analysis features of WoS, it can be observed that the annual publication volume before 2005 remained low (under 20 papers per year), indicating that RUCR was still in an exploratory phase and had yet to receive significant attention. Between 2005 and 2016, the publication volume steadily increased as research interest gradually increased. Post-2016, the annual publication volume experienced rapid growth, with 2022 showing an explosive rise (Figure 3). In 2022 alone, 264 papers were published, and it was anticipated that by 2023 the volume would exceed 300 papers. The increase in RUCR’s visibility suggests that this research area has gradually become a prominent research hotspot.

In terms of disciplinary distribution, RUCR is concentrated in fields such as Environmental Sciences (62.77%), Public Environmental Occupational Health (11.98%), and Engineering Environmental (10.09%). There is also involvement from Green Sustainable Science Technology (7.11%), Urban Studies (1.89%), and Construction Building Technology (1.84%), reflecting the interdisciplinary nature of RUCR (Figure 4).

As for source publications, 1635 journals have published RUCR-related papers. The top journals by publication volume are Science of The Total Environment and Atmospheric Environment, accounting for 4.06% and 2.73% of publications, respectively, with minor differences in the proportions of other journals (Figure 5). The top 10 journals primarily focus on themes related to environmental research, ecology, and sustainable development.

3.1.2. Co-Citation Analysis of Literature

Co-citation analysis of literature can reflect the research evolution and knowledge background at different stages [35]. The higher the co-citation frequency of an article, the more likely it is foundational or innovative within the field, representing a core aspect of the research domain. This paper conducted a visual analysis of 2012 documents from the core database using CiteSpace, yielding a co-citation network with 2479 nodes. In CiteSpace, nodes represent the citation status of literature within the core database, and links represent co-citation relationships between nodes [36]. The links connecting nodes indicate the strength of co-citation relationships between documents. The size of a node is proportional to the importance and novelty of the document; larger nodes indicate that a document is highly significant within the RUCR field. Figure 6 displays documents co-cited more than eight times, along with their primary authors and publication years, to generate the co-citation network. The 20 most highly co-cited documents were then selected for further analysis.

Table 2. RUCR’s top 20 most cited literature.

No.	Document Title	Citation Count	Year	Category
1	R: a language and environment for statistical computing [37]	31	2021	Carbon sequestration assessment and technological updates
2	Estimates and 25-year trends of the global burden of disease attributable to ambient air pollution: an analysis of data from the Global Burden of Diseases Study 2015 [38]	20	2017	Urban carbon risk factors and their impacts
3	A comparative risk assessment of burden of disease and injury attributable to 67 risk factors and risk factor clusters in 21 regions, 1990–2010: a systematic analysis for the Global Burden of Disease Study 2010 [39]	19	2012	Urban carbon risk factors and their impacts
4	Particulate Matter Air Pollution and Cardiovascular Disease [40]	13	2010	Urban carbon risk factors and their impacts
5	Bounding the role of black carbon in the climate system: A scientific assessment [41]	12	2013	Carbon sequestration assessment and technological updates
6	A review on polycyclic aromatic hydrocarbons: Source, environmental impact, effect on human health and remediation [42]	12	2016	Urban carbon risk factors and their impacts
7	Global estimates of mortality associated with long-term exposure to outdoor fine particulate matter [43]	11	2018	Urban carbon risk factors and their impacts
8	The contribution of outdoor air pollution sources to premature mortality on a global scale [44]	10	2015	Urban carbon risk factors and their impacts
9	Air pollution and health [45]	9	2002	Urban carbon risk factors and their impacts
10	Ambient (outdoor) air pollution [46]	9	2021	Strategies for mitigating carbon risk.
11	Critical review of black carbon and elemental carbon source apportionment in Europe and the United States [47]	8	2016	Carbon sequestration assessment and technological updates
12	PM2.5-bound oxygenated PAHs, nitro-PAHs, and parent-PAHs from the atmosphere of a Chinese megacity: Seasonal variation, sources, and cancer risk assessment [48]	8	2014	Urban carbon risk factors and their impacts
13	Lung Cancer, Cardiopulmonary Mortality, and Long-term Exposure to Fine Particulate Air Pollution [49]	8	2002	Urban carbon risk factors and their impacts
14	Characteristics and Source Identification of Polycyclic Aromatic Hydrocarbons (PAHs) in Urban Soils: A Review [50]	8	2017	Urban carbon risk factors and their impacts
15	Seasonal characteristics of aerosols (PM2.5 and PM10) and their source apportionment using PMF: A four-year study over Delhi, India [51]	8	2020	Urban carbon risk factors and their impacts
16	Polycyclic aromatic hydrocarbons (PAHs) in soils from a multi-industrial city in South Korea [52]	8	2014	Urban carbon risk factors and their impacts
17	The health risks and benefits of cycling in urban environments compared with car use: health impact assessment study [53]	7	2011	Strategies for mitigating carbon risk.
18	Spatiotemporally resolved black carbon concentration, schoolchildren’s exposure, and dose in Barcelona [54]	7	2016	Carbon sequestration assessment and technological updates
19	Polycyclic aromatic hydrocarbons (PAHs) in urban soils of the megacity Shanghai: Occurrence, source apportionment and potential human health risk [50]	7	2013	Urban carbon risk factors and their impacts
20	Levels of ambient air pollution according to mode of transport: a systematic review [55]	7	2017	Urban carbon risk factors and their impacts

lists the 20 most influential articles in the RUCR field. Combined with Figure 6, it is evident that the R Core Team authored the most frequently cited article [37], with the highest citation count at 31. The distribution of the top 20 highly-cited articles indicates that current RUCR research primarily focuses on carbon sequestration assessment and technological updates, urban carbon risk factors and their impacts, and strategies for mitigating carbon risk.

(1) Carbon Sequestration Assessment and Technological Updates. This area mainly addresses the calculation of black carbon’s carbon storage [41], source analysis [47], and advancements in identification technology [37]. Bond’s research highlights that black carbon aerosols play a significant role in the Earth’s climate system, primarily from fossil fuel combustion in industrial and residential settings. High CO₂ emissions exert a positive forcing effect on the climate, and reducing black carbon emissions could significantly impact climate change [41]. Some scholars have also performed source analyses of black carbon in the air, categorizing its origins [47], and examining its impact on specific populations [54].

(2) Urban Carbon Risk Factors and Their Impacts. Scholars have investigated carbon risk issues arising from air pollution [38], incomplete combustion of fuels [42], and their extended impact on urban elements such as soil [50] and groundwater [56]. Cohen calculated the temporal and spatial trends of mortality and disease burden caused by ambient air pollution at global, regional, and national levels from 1990 to 2015, revealing that environmental air pollution increases mortality and morbidity rates, thereby reducing life expectancy [38]. Lim compared 67 risk factors and associated hazards from 1990 to 2010, identifying that increased particulate matter from household cooking, vehicle emissions, and agricultural burning has emerged as a new risk over the past two decades [39]. Brook’s research revealed that particulate air pollution elevates cardiovascular disease incidence and mortality rates [40], findings consistent with those of Burnett, Brunekreef, and others [43,45,49]. Lelieveld expanded the study of air pollution, discovering that in China and India, emissions from residential energy use for heating and cooking significantly impact global premature mortality [44]. Additionally, other scholars have examined the critical impacts of pollutants from incomplete combustion and human activity emissions on environmental and human health [42], analyzing their effects on various population groups [55] and their seasonal variations [48,51]. Recently, attention has turned to the pollutants produced by urban combustion and their contamination of soils in different urban areas [50], with vehicle emissions and industry identified as primary sources of soil pollution [52]. Soil contamination may also lead to further groundwater quality degradation [56].

(3) Strategies for Reducing Carbon Risk. This area is mainly reflected in the strategies issued by the World Health Organization (WHO) to address environmental (outdoor) air pollution. Policies and investments supporting clean transportation, energy-efficient housing, power generation, industry, and improved urban waste management can reduce the primary sources of outdoor air pollution. Access to clean household energy would also significantly reduce ambient air pollution in certain regions [46]. Urban areas need to become greener and more compact by enhancing building energy efficiency, utilizing cleaner heavy-duty diesel vehicles, and adopting low-emission vehicles and fuels. Additionally, some scholars have examined the effectiveness of carbon reduction actions in mitigating carbon risks. Rojas-Rueda’s research found that bike-sharing programs could reduce CO₂ emissions and lower mortality rates [53].

3.1.3. Research Collaboration Analysis

Collaboration analysis in CiteSpace can reveal the cooperation dynamics among different academic institutions in the field of RUCR. This paper employs CiteSpace to calculate the collaboration among various institutions, as shown in Figure 7 and

Table 3. RUCR’s top 20 partner institutions.

Count	Centrality *	Year	Institutions
164	0.25	2004	Chinese Academy of Sciences
63	0.02	2014	University of Chinese Academy of Sciences
49	0.19	1996	University of California System
44	0.09	2002	N8 Research Partnership
42	0.08	2003	UDICE-French Research Universities
38	0.12	2003	Centre National de la Recherche Scientifique
33	0.02	2010	Beijing Normal University
33	0.15	2004	Helmholtz Association
29	0.05	2013	Peking University
28	0.04	1998	Indian Institute of Technology System
25	0.01	2004	Guangzhou Institute of Geochemistry
25	0.09	2016	University of London
24	0.01	2014	Tsinghua University
24	0.07	2004	Imperial College London
24	0.05	2008	Consejo Superior de Investigaciones Cientificas
24	0.01	2005	Research Center for Eco-Environmental Sciences
21	0.02	2013	Institute of Earth Environment
17	0.03	2013	Universidade de Sao Paulo
17	0.04	2014	United States Department of Agriculture
17	0.01	2014	University of Wisconsin System

* Centrality is a metric used to assess the importance of a document or node within a citation network. Through a centrality analysis of institutional collaboration, it is possible to identify institutions with key collaborative positions within the citation network.

. The number of collaborations among RUCR institutions is substantial, with close connections among various organizations. The Chinese Academy of Sciences leads with 164 research articles, followed by the University of Chinese Academy of Sciences and the University of California System, with 63 and 49 research papers, respectively. The leading publishing institutions are from China, the United States, and the United Kingdom. Chinese research institutions account for 55% of the publications among the top ten collaborating institutions, indicating China’s emphasis on RUCR and its significant research contributions in this field.

Figure 8 illustrates the collaboration relationships among countries, with the radius size corresponding to publication volume. China has the highest publication count (598 papers), followed by the United States (538 papers). The purple ring around nodes represents centrality; higher centrality indicates more substantial influence, with papers from that country having higher citation rates and potentially serving as foundational knowledge in the field. Centrality is a metric for assessing node importance within the network, and there is a notable gap in publication volume among different countries. The United States and France exhibit the strongest centrality, each with a centrality score of 0.19, reflecting their high overall citation rates.

As shown in

Table 4. RUCR’s top 20 cooperation countries.

Count	Centrality *	Year **	Countries
635	0.12	2002	China (PEOPLES R CHINA)
453	0.19	1991	USA
172	0.16	1991	England
138	0.03	1997	India
127	0.12	1999	Germany
115	0.1	1997	Australia
109	0.04	1998	Canada
102	0.11	1992	Italy
86	0.06	2001	Spain
84	0.19	2003	France
68	0.03	2003	Brazil
55	0.05	2003	Japan
52	0.08	1996	Sweden
43	0.03	2006	Netherlands
41	0.07	2005	Denmark
40	0.06	1992	Switzerland
38	0	2009	South Korea
38	0.04	2003	South Africa
36	0.01	2011	Soland
33	0.01	2012	Portugal

* Centrality is a metric for assessing the importance of a document or node within a citation network. Analyzing centrality in national collaborations makes it possible to identify countries that hold key collaborative positions within the overall citation network. ** “Year” indicates the year when the country first started RUCR cooperation.

, the United States was the earliest country to engage in RUCR research, starting in 1991. The publication volume of the United States and China far exceeds that of other countries, each having over 500 papers. The United Kingdom, India, Germany, Australia, Canada, and Italy form the second tier, each with an average annual publication volume exceeding 100 papers. Additionally, Spain and France, although having published fewer than 100 papers in the past 30 years, have also contributed to advancing urban carbon risk research.

The top five countries account for over 70% of global publications in this area, demonstrating their strong focus on “urban carbon risk” research. These countries have all implemented carbon reduction policies, including setting greenhouse gas reduction targets, establishing policies and measures, promoting energy transition, addressing climate change impacts, participating in international cooperation and commitments, and providing financial support.

Figure 9 illustrates the changes in annual publication volume for the top five countries/regions. It shows that research on urban carbon risk (RUCR) in China, the United States, the United Kingdom, India, and Germany has gradually increased. During the 1990s and early 2000s, China’s publication volume lagged behind that of the United States. However, since 2017, China’s publication output has rapidly increased, surpassing other countries to rank first globally. This trend indicates that Chinese scholars researching urban carbon risk are making increasingly significant contributions. Overall, RUCR has seen substantial growth over the past five years.

3.2. Development Stages of RUCR

The division of RUCR into four stages is primarily based on the temporal distribution of keywords (Figure 10) and changes in high-frequency keywords (

Table 5. High-frequency keywords of RUCR from 1991 to 2023 (first 6 months).

Stage	Year	Keywords	Frequency	Centrality
1991–2001	1991	exposure	160	0.15
	1992	emissions	87	0.02
	1994	air pollution	247	0.13
	1995	mortality	88	0.08
	1996	management	55	0.05
	1997	volatile organic compounds	33	0.02
	2000	accumulation	23	0.01
	2001	particulate matter	243	0.1
2002–2009	2002	health	94	0.07
	2003	temperature	22	0.01
	2004	aerosol	31	0.02
	2005	water	63	0.02
	2006	risk assessment	78	0.04
	2007	size distribution	10	0.01
	2008	black carbon	181	0.03
	2008	PM2.5	92	0.02
	2008	urban soils	69	0.01
	2008	carbon sequestration	16	0.02
	2009	source apportionment	199	0.05
2010–2017	2010	climate change	100	0.04
	2010	land use	23	0.01
	2010	biodiversity	16	0.01
	2011	quality	68	0.01
	2012	health risk assessment	62	0.01
	2013	benefits	14	0.01
	2013	waste water	12	0.01
	2014	human health	15	0.01
	2015	ecosystem services	53	0.01
	2015	greenhouse gas emissions	20	0
	2015	sustainable development	19	0
	2016	ecological risk assessment	19	0.01
	2016	variability	14	0.01
	2017	spatial distribution	31	0.01
2018–2023	2018	urbanization	26	0
	2019	mitigation	13	0
	2019	vulnerability	10	0
	2020	soil organic carbon	9	0
	2021	energy consumption	9	0
	2021	land use change	8	0
	2021	nature-based solutions	6	0
	2022	indicators	4	0
	2023	carbon neutrality	3	0

), combined with the evolving research hotspots and themes over different periods. RUCR demonstrates distinct stage-specific clustering, characterized by a focus on different research topics, content, and elements. The centrality and frequency of keywords reveal shifts in research priorities. Additionally, the logic of this division is influenced by the phased impacts of international policies, academic research priorities, and technological advancements. The single exploration stage (pre-2001) focused primarily on air pollution and particulate matter as singular risk factors, with fragmented research themes. The assessment and construction stage (2002–2009), driven by international policies like the Kyoto Protocol, expanded research to urban elements and formed initial collaborative networks. The innovation and breakthrough stage (2010–2017), influenced by the Paris Agreement, introduced more diverse methodologies and expanded to multi-dimensional perspectives such as ecosystems and health. The technological synergy stage (2018–present) reflects the rise of carbon neutrality goals, marked by significant depth and breadth in research, interdisciplinary collaboration, and the application of new technologies. Thus, RUCR is categorized into the four stages of single exploration, assessment and construction, innovation and breakthrough, and technological synergy, with a summary of the key research achievements and characteristics of each stage. This evolution illustrates RUCR’s progression from analyzing singular issues to developing multi-dimensional response strategies.

3.2.1. Single Exploration Stage (Pre-2001): Fragmented and Isolated Exploration

The First World Climate Conference in 1979 explicitly stated that if atmospheric CO₂ concentrations continued to rise, a significant warming effect would emerge by the mid-21st century, prompting countries worldwide to prioritize reducing CO₂ emissions. Following this, the United Nations (UN) and individual countries addressed the risks of rising greenhouse gas concentrations by proposing measures to control atmospheric greenhouse gases and reduce emissions. Subsequently, scholars began exploring the field of urban carbon risk, or RUCR; however, due to the absence of a theoretical framework and methodology, research in this stage was primarily empirical, focusing on the impacts of specific carbon risks such as particulate matter in air pollution.

During this stage, high-frequency keywords in RUCR included air pollution (247 occurrences), particulate matter (243 occurrences), exposure (160 occurrences), mortality (88 occurrences), and emissions (87 occurrences) (Table 5). Research primarily focused on the impact of carbon risks, such as air pollution, on cities, with an emphasis on singular risk elements related to air pollution, and few studies addressed other urban factors or sources of risk.

Studies on air pollution risks concentrated mainly on their consequences. Keywords like exposure and air pollution showed high centrality, with scores of 0.15 and 0.13, respectively, highlighting them as critical research focuses of the time. Burnett identified that environmental air pollution from fossil fuel combustion, including nitrogen dioxide and carbon monoxide, was a risk factor for premature mortality among populations in 11 Canadian cities [57]. Klouda used radiocarbon analysis to trace the origins of urban carbon monoxide, finding fossil fuel combustion as the primary source [58]. Burnett further found that ambient carbon monoxide increased mortality rates in Toronto’s urban population, suggesting that carbon monoxide should be considered a potential public health risk in urban areas [59]. Most studies in this stage focused on individual subjects, such as police officers [60,61], drivers [62], and outdoor exercisers [63], who are frequently exposed to air pollution. Research also analyzed different demographic groups based on region, ethnicity, gender, and age [64,65,66]. Less attention was given to the impact of increased carbon emissions on urban elements, though some scholars advocated for using renewable energy to reduce CO₂ emissions and mitigate carbon risk due to concerns about CO₂’s role in exacerbating urban warming [67].

In RUCR’s early phase, research predominantly concentrated on the consequences and influencing factors of air pollution risks, especially at the individual impact level. The number of publications was relatively low (accounting for only 2.1% of the total publication volume), focusing primarily on microsystems, such as the risk factors for various population groups and urban inhabitants. Few studies addressed the impact or causes of urban carbon risks on other urban elements, with limited discussion on urban carbon risk response. Nonetheless, scholars had already begun to recognize the importance of CO₂ emissions [68], marking this period as the single exploration stage of RUCR’s development.

3.2.2. Assessment and Construction Stage (2002–2009): The Rise of Macro-Level and Broad-Based Research

With the release of the IPCC’s Third Global Climate Assessment Report in 2002 and the official enactment of the Kyoto Protocol in 2005, greenhouse gas emissions became further restricted. China, Japan, and EU member states introduced related policies to control and reduce greenhouse gases generated by human activities. During this stage, researchers began recognizing greenhouse gases from human activities as a significant factor contributing to carbon risk. RUCR started accumulating knowledge and experience, with research expanding its focus on the factors influencing urban carbon risk and methods for mitigating it. The field entered the assessment and construction stage, marked by a surge in research outcomes, growing segmentation within the discipline, and forming academic exchange and collaboration networks. Scholars began examining the impact of carbon risk on various urban elements and investigating the cumulative contributions of urban sectors—such as buildings, energy, and transportation—to carbon reduction, revealing the roles these sectors play in mitigating climate change.

High-frequency keywords in this stage included source apportionment (199 occurrences), black carbon (181 occurrences), health (92 occurrences), and PM2.5 (92 occurrences) (see Table 5). Health had high centrality (0.07), reflecting it as a significant research focus. Research during this phase can be divided into two main areas: (1) assessing the factors and extent of urban carbon risk and (2) exploring methods and measures for mitigating carbon risk.

Scholars have evaluated the effectiveness of reducing carbon emissions and analyzed influencing factors from various perspectives, such as building energy efficiency [69], micro-transport environments [70,71], and household fuel consumption [72]. (1) Building Energy Efficiency: Buildings are the most cost-effective sector for reducing carbon dioxide (CO₂) emissions. Sunikka studied the potential for buildings to reduce CO₂ emissions and mitigate carbon risk, using the Netherlands as a case to analyze the effects of improving building energy efficiency, potential risks, and policy response strategies [69]. (2) Micro-Transport Environment: Kaur assessed the carbon risk posed by increased particulate matter and carbon monoxide in urban micro-transport environments and considered measures such as fixed monitoring stations and optimized routes to manage and mitigate these risks [70]. Zegras, on the other hand, enhanced the benefits of greenhouse gas emission reduction in transportation by improving transport efficiency, thereby reducing transportation carbon risk [73]. (3) Household Fuel Consumption: Montoya analyzed the issue of increased carbon monoxide from household fuel use and suggested focusing on reducing household carbon monoxide exposure to decrease carbon risk [72].

During this stage, scholars also investigated the impact of different scenarios and technological measures on carbon risk reduction. Woodcock used comparative risk assessment to estimate carbon reduction outcomes for two different transport scenarios in the United Kingdom and India, finding that carbon reduction measures in the transport sector positively impacted public health [74]. Nabuurs analyzed the effects of various management practices on forest carbon sequestration, considering options that maximize carbon storage under high carbon loss risk scenarios. Patz examined how elevated greenhouse gas levels and other risk factors associated with climate change adversely affect public health [75]. Regarding technological measures, some researchers utilized Ambient Carbon Particle Monitors to track particulate carbon in the air to facilitate the development of more appropriate carbon reduction strategies [70].

Overall, scholars in this stage conducted preliminary analyses of methods, measures, and the roles of various sectors in reducing carbon risk, proposing initial ideas for urban carbon risk response. The volume of published research increased (accounting for 8.3% of total publications), reflecting growing academic attention and an increased focus on carbon reduction for urban elements. With an expanded scope, research began to consider the influence of different sectors on carbon reduction policies, though policy and mechanism responses to carbon risk remained underexplored. This stage represents the assessment and construction phase of RUCR development.

3.2.3. Innovation and Breakthrough Stage (2010–2017): Field Expansion and Diversification of Research Methods

Building on the previous stage’s substantial theoretical and practical advancements, RUCR accumulated extensive knowledge and experience. The signing of the Cancun Agreements and the Paris Agreement heightened global awareness of the impact of greenhouse gases, including CO₂. Countries worldwide introduced measures such as the Clean Energy and Security Act, and national climate change response plans to curb carbon emissions. High-frequency keywords during this period included climate change (100 occurrences), quality (68 occurrences), health risk assessment (62 occurrences), and ecosystem services (53 occurrences), with climate change having high centrality (0.04), highlighting its prominence in the research context. Based on keyword trends, RUCR witnessed significant innovations and breakthroughs, with foundational theories, methods, and technologies emerging. In this phase, RUCR expanded its policy measures to address carbon risk, incorporating risk assessments in health, ecology, and other domains. The research scope extended beyond urban land, considering carbon from spatial distributions such as biotic and land use factors, marking this as a stage of innovative breakthroughs in RUCR research.

Research contents of RUCR transitioned from simply promoting carbon risk reduction to developing mechanisms and strategies for controlling carbon emissions to mitigate carbon risk. (1) Transportation: Fahmi, through a case study of the Banda Aceh Integrated Land Use and Transport Planning (BILT), introduced new long-term strategic planning ideas for high-risk cities to reduce both disaster and carbon emission risks, aiming to lower urban risks and achieve transportation sustainability [76]. Other scholars analyzed carbon reduction strategies in transportation, focusing on reducing motor vehicles, improving public transit, and using biofuels [77]. (2) Buildings: Yalazi developed the “Sustainable Urban Regeneration” standard for residential disaster risk reduction in Turkey, which reduced household and government expenses and lowered carbon emissions through footprint reduction. By creating carbon footprint calculation tools, this project provided technical support for promoting sustainable development and reducing risk [78]. Colombier’s research highlighted that significantly improving building energy efficiency is essential for reducing emissions and achieving long-term low-carbon building goals [79]. Other studies focused on methods for reducing household energy carbon emissions, finding that low-carbon behaviors in high-risk areas made substantial contributions to carbon reduction [80]. (3) Policy: Nakamichi examined the interaction between climate change mitigation and adaptation measures, with the Japanese government using an integrated assessment model to evaluate CO₂ reduction co-benefits in urban planning. The model demonstrated that the interplay of mitigation and adaptation measures could effectively lower carbon emissions [81]. Additionally, some scholars proposed eco-financing to promote the shift to low-carbon development from an economic perspective [82].

In this phase, the role of the ecological environment in enhancing carbon storage and reducing carbon risk gained increasing attention. Changes in urban tree cover and forest structure were found to impact carbon storage, reduce air pollution, and conserve energy [83,84]. Friess discovered that tropical forest ecosystems provide crucial carbon storage services in urban areas; however, due to urbanization and land reclamation in Singapore, mangroves faced degradation, reducing their carbon storage capacity and posing a risk to urban carbon storage services [85]. Niemelä emphasized the importance of ecosystem services in urban areas and the risks to these services from urban development. Urban vegetation could be increased through rational land use planning, promoting carbon sequestration and mitigating climate change [86].

This stage saw further development in mechanisms for addressing carbon risk, with studies on health risk assessment [87] and network governance [88] showing potential in reducing carbon risk. An increasing number of scholars focused on the importance of urban factors in mitigating carbon risk, underscoring the critical role of urban carbon reduction in addressing global warming [89]. The volume of publications overgrew compared to previous stages (accounting for 29.6% of total publications), with a substantial increase in co-cited literature (Figure 6), indicating a surge in research issues. Meanwhile, RUCR’s response measures improved and were characterized by innovative approaches and expanded methodologies.

3.2.4. Technological Synergy Stage (2018–Present): Application of New Technologies and Multi-Perspective Collaboration

During this stage, as the urgency of carbon emissions intensifies, the international community has strengthened carbon reduction actions to meet carbon mitigation and temperature control targets. The approach now includes nature-based solutions (NbS) and efforts to enhance urban resilience against carbon risks. As research has progressed, there has been increasing recognition of this field’s interconnected and cross-disciplinary challenges. RUCR has begun integrating with other disciplines, utilizing diverse theories, methods, and resources to address complex challenges. Multi-perspective collaboration broadens research horizons and drives innovation and advances in the field. Key terms in this stage include urbanization (26 occurrences), mitigation (13 occurrences), and vulnerability (10 occurrences). RUCR research has diversified, with new concepts and technologies, such as carbon neutrality and NbS, becoming integrated within a collaborative framework.

In terms of new technology applications, scholars have employed advanced methods such as multi-factor overlays [90], remote sensing [91], machine learning [92], and NbS [93] to reduce carbon emissions and mitigate carbon risk. Wang et al. discovered, through simulations of various technology innovation models, that technology stacking can gradually reduce carbon footprints but may also limit the scale of technological diffusion, thus reducing overall carbon reduction potential. Economic risks must be addressed to fully realize the carbon reduction potential of technological innovation, and agricultural product prices must be increased [90]. Dewa utilized remote sensing to assess the impact of rapid urbanization on carbon emissions and land surface temperature in Indonesia, highlighting the correlation between increased urbanization, carbon emissions, and land surface temperature. This study suggests that spatial planning and new strategies should be prioritized to mitigate environmental risks [91]. NbS offers immediate, cost-effective opportunities to reduce net CO₂ emissions while providing significant co-benefits [93]. Lan applied artificial neural networks to predict future urban heat island effects, which could guide climate–socio-economic targets to mitigate public health impacts [92].

In terms of multi-perspective collaboration, factors such as co-benefits [94], land use change [95], land expansion [96], and carbon financing [97] have been incorporated into efforts to reduce carbon risk. Zhang evaluated carbon sequestration from urban greening and rainwater harvesting systems under different climate scenarios, aiming to reduce environmental risks and enhance urban vegetation’s carbon sequestration rate [94]. Zhong conducted a dynamic analysis of land use changes in Shangdongsheng, developing risk zoning strategies based on carbon emissions [95]. Meyer and Schwarze found that carbon pricing alone does not sufficiently incentivize private investment, as market prices fail to account fully for the costs and benefits associated with carbon risk and climate resilience investments. A broader approach involving social and political factors and mechanisms, like international agreements and urban networks, is necessary to generate the commitment required for climate resilience investments [97].

As countries around the world set carbon neutrality targets and pathways, predicting emission reduction effects under different scenarios and balancing multiple goals has become a new focus of RUCR [98]. This period has seen a rapid expansion of RUCR, with approximately 60.1% of total publications released during this stage, reflecting explosive growth in research scope and methodology. Scholars have increasingly shifted their focus to zero-carbon targets, with extensive research on CO₂ removal technologies. Carbon neutrality research is now extending from climate change into multiple disciplines. Studies on RUCR’s development patterns, improvement strategies, and technological measures have emerged, reflecting a trend toward multi-perspective and intelligent approaches, marking this stage as a deepening phase of technological synergy in RUCR.

3.3. Key Issues of RUCR

3.3.1. Keyword Cluster Analysis

The keyword clustering analysis feature in CiteSpace generates clusters of research issues based on the network relationships of keywords across numerous documents, revealing critical themes within the field [34,99]. CiteSpace utilizes the log-likelihood ratio algorithm and evaluates keyword clusters based on two metrics: modularity (Q) and mean silhouette (S) [100]. The modularity (Q) value assesses the structural integrity of keyword clusters, ranging from 0 to 1, with a Q value above 0.3 typically indicating a relatively complete clustering structure. The mean silhouette (S) value, ranging from −1 to 1, evaluates the similarity of keywords within clusters.

Using CiteSpace’s keyword clustering feature, this paper divides RUSR into five primary research clusters (see Figure 11 and

Table 6. RUCR keyword clustering map from 2019 to 2023.

Cluster Name	Size	Silhouette *	Year	Main Keywords
0. air pollution	163	0.768	2006	air pollution; mortality; particulate matter; personal exposure; carbon monoxide
1. climate change	151	0.722	2015	climate change; ecosystem services; urbanization; sustainable development; air pollution
2. heavy metals	147	0.73	2012	heavy metals; urban soils; sediment; air pollution; risk assessment
3. source apportionment	128	0.747	2011	source apportionment; pahs; health risk; chemical composition; positive matrix factorization
4. pregnancy	55	0.949	1996	pregnancy; low birth weight; alcohol; occupational diseases; lung cancer

* Silhouette is an indicator to measure the degree of clustering of keywords.

). The modularity value (Q) of RUSR is 0.46, and the mean silhouette value (S) is 0.78, indicating relatively complete clustering. The five research clusters in RUSR are “0. air pollution”, “1. climate change”, “2. heavy metals”, “3. source apportionment”, and “4. pregnancy”. The largest cluster by size (number of documents) is “0. air pollution”, comprising 193 documents with keywords such as “mortality”, “particulate matter”, and “carbon monoxide”. The cluster with the highest silhouette value is “4. pregnancy” (S = 0.964), with keywords including “low birth weight”, “alcohol”, “occupational diseases”, and “lung cancer”.

The Landscape feature in CiteSpace is often used to analyze the evolution of research clusters over a given period [33], allowing the identification of each cluster’s start and end dates as well as its research popularity. As shown in Figure 12, the keyword clusters “0. air pollution” (1991 to 2023) and “4. pregnancy” (1991 to 2013) are the earliest research clusters. However, while the former spans the entire development of RUSR, interest in the latter significantly declined after 2001. The clusters “1. climate change” and “2. heavy metals” emerged slightly later but have maintained high interest levels since 2000.

The term “pollution” broadly encompasses clusters 0, 1, and 2, with related keywords such as “air pollution”, “particulate matter”, “carbon monoxide”, “climate change”, “ecosystem services”, and “heavy metals”, reflecting RUSR’s emphasis on issues related to urban air, soil, and ecosystem pollution caused by carbon emissions. Additionally, keywords within cluster 3, such as “source apportionment”, “health risk”, and “chemical composition”, indicate the academic community’s focus on tracing the origins of carbon risk. Keywords in cluster 4, including “pregnancy”, “low birth weight”, “occupational diseases”, and “lung cancer”, highlight the significant health impacts of carbon risk on urban residents.

3.3.2. Summary of Key Issues

There is a foundational body of international research on urban carbon risk. Based on keyword clustering, this paper classifies and synthesizes the core issues of RUCR, focusing on objectives, planning methods, and strategies. Analysis of published literature reveals that RUCR’s key objectives primarily include carbon reduction and sequestration; planning methods involve a combination of qualitative and quantitative analyses; and planning strategies emphasize the importance of multidisciplinary collaboration, international cooperation, and technological innovation to advance research and strengthen urban resilience to carbon risk and climate change.

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Objectives and Content

The main issues in urban carbon risk focus on various methods, techniques, and technologies for reducing carbon risk, enhancing urban carbon sequestration capacity and reducing carbon emissions to drive energy transition, sustainability, and renewable energy development. Researchers have explored scenario analysis [94], land use change [95], and technological innovation [90] to assess the positive role of urban greening in urban carbon sequestration under different simulation scenarios [94]. Some studies aim to predict the impact of land use change on urban carbon emission risk and classify areas by carbon emission risk levels [95], facilitating the development of suitable low-carbon policies. Additionally, RUCR research covers a wide range of perspectives, including urban form [101,102], urbanization [103], industrial transformation [104], low-carbon land management [105], and land cover change [106,107].

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Planning Methods

To mitigate urban carbon risk, researchers have proposed various planning evaluation methods, such as carbon pricing to assess climate risk mitigation and resilience [97] and using random forest and neural network models to evaluate the impact of urban expansion on carbon storage [96]. RUCR planning methods can be divided into qualitative and quantitative approaches. Qualitative methods focus on the management and governance of urban carbon risk, while quantitative methods emphasize carbon risk identification and quantification to reduce urban carbon risk and achieve carbon balance.

In qualitative studies, researchers have used methods such as the Delphi technique and semi-structured expert interviews to assess the EU’s governance mechanisms for reducing carbon emissions [108]; they have applied risk assessment methods to evaluate the impact of urban development and industrial activities on urban carbon risk, considering natural and human factors like climate change, population growth, and industrialization [109]. Other studies utilize survey questionnaires to assess ways to reduce carbon risk [80] and establish and optimize carbon management systems to manage and mitigate urban carbon risk effectively. Quantitative studies employ data analysis to model the impacts of different land use practices on carbon allocation, modeling, and accumulation [110,111]; analyze the urban carbon sequestration processes of carbon emissions, absorption, and cycles to evaluate the nonlinear impacts of compound factors on urban carbon balance [112,113]; and compare carbon balance models with observational data to analyze the spatiotemporal variation, long-term trends, and risk levels of global carbon emissions [114]. Qualitative and quantitative research methods provide scientific foundations for formulating effective urban carbon reduction strategies.

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Planning Strategies

Governments and researchers worldwide have explored numerous strategies addressing critical issues in urban carbon risk. Research indicates that variations in urban clusters, environmental resource endowments, pollution levels, and the intensity of low-carbon policies affect the effectiveness of urban carbon risk strategies [115]. Consequently, nations and international organizations have developed diverse strategies based on regional and temporal differences.

Table 7. Major “urban carbon risk” related policies and reports in various countries around the world from 1991 to 2024.

Year	Policy/Convention	International Institution/Country	Main Content
1992	United Nations Framework Convention on Climate Change	United Nations	Stabilize greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system [119].
1995	Decision on Joint Implementation of the Convention	United Nations	Calls for the broadest possible cooperation between industrialized and developing countries to reduce global greenhouse gas emissions [120].
1997	Kyoto Protocol	Third United Nations Climate Change Conference	Developed and transitional economies committed to reducing greenhouse gas emissions by at least 5% from 1990 levels during the commitment period from 2008 to 2012 [121].
2002	Act on Promotion of Global Warming Countermeasures	Japan	Promote international cooperation to combat global warming by establishing a development mechanism compatible with economic and environmental needs, fulfilling a 6% emission reduction commitment [122].
2003	European Union Emission Trading Scheme (EU ETS)	European Union	Implements a “cap and trade” system to limit total greenhouse gas emissions through a permit trading mechanism, aiming for an 8% reduction in emissions from 1990 levels by 2008–2012 [123].
2006 (revised 2009)	Renewable Energy Law of the People’s Republic of China	People’s Republic of China	Promotes the development and utilization of renewable energy, prioritizes renewable energy in the national energy development strategy, and ensures unified management of renewable energy development across China.
2007	China’s Special Action Plan on Climate Change Science and Technology	People’s Republic of China	Addresses scientific issues of climate change, technology development for greenhouse gas control, and climate change mitigation [124].
2007	Bali Road Map	13th United Nations Climate Change Conference	Outlines measures for climate adaptation, emission reductions, technology advancement for climate change mitigation, and increased funding support for adaptation and mitigation [125].
2008	Energy Conservation Law of the People’s Republic of China	President of the People’s Republic of China	Promotes nationwide energy conservation, improves energy efficiency, and aims to reduce consumption, losses, and pollutant emissions from energy production to consumption [126].
2008	Climate Change Act	United Kingdom	Aims to reduce UK greenhouse gas emissions by at least 80% by 2050 compared to 1990 levels, establishing five-year “carbon budgets” [127].
2009	Copenhagen Accord	United Nations	Extends the Bali Road Map negotiations, authorizing two working groups under the UNFCCC and Kyoto Protocol to continue discussions [128].
2009	American Clean Energy and Security Act	United States	Proposes a greenhouse gas cap-and-trade system to reduce emissions and promote clean energy (not passed into law) [129].
2010	Cancun Agreements	16th United Nations Climate Change Conference	Sets a long-term global goal of limiting global average temperature rise below 2 °C compared to pre-industrial levels, with consideration for a 1.5 °C goal based on best available science [130].
2014	National Climate Change Plan (2014–2020)	State Council of the People’s Republic of China	Aims to meet greenhouse gas control targets by 2020, advance low-carbon pilot projects, and enhance climate change adaptation capacity [131].
2015	Paris Agreement	21st United Nations Climate Change Conference	Establishes a global goal of balancing greenhouse gas emissions and absorption in the century’s second half [132].
2016	13th Five-Year Plan for Economic and Social Development of the People’s Republic of China	State Council of the People’s Republic of China	Proactively controls carbon emissions, fulfills emission reduction commitments, enhances climate adaptation capabilities, and contributes to global climate governance [133].
2021	Opinions on Fully and Accurately Implementing New Development Concepts for Achieving Carbon Peaking and Carbon Neutrality	State Council of the People’s Republic of China	Focuses on green transition in economic and social development, industrial restructuring, a clean and efficient energy system, low-carbon transportation, green urban-rural development, major technological breakthroughs, and carbon sequestration enhancement [134].
2021	Carbon Peaking Action Plan by 2030	State Council of the People’s Republic of China	Implements “Ten Actions for Carbon Peaking”, including green and low-carbon energy transformation, energy efficiency and emission reduction, industrial carbon peaking, urban-rural low-carbon development, and carbon sequestration capacity enhancement [135].
2021	Net Zero by 2050: A Roadmap for the Global Energy Sector	International Energy Agency	Examines key uncertainties, such as the roles of bioenergy, carbon capture, and behavioral change in achieving net zero emissions [136].
2022	Risk Prevention for Carbon Neutrality Under the New Development Concept	China Council for International Cooperation on Environment and Development	Proposes risk prevention measures and research directions in the carbon neutrality transition [137].
2022	IPCC Sixth Assessment Report	IPCC	Carbon management is a core theme in mitigating climate change [4].
2024	China’s Carbon Peaking and Carbon Neutrality Policies and Actions (2023)	Ministry of Ecology and Environment of the People’s Republic of China	Summarizes national, local, ministerial, and corporate carbon peaking and neutrality policies, emphasizing the role of urban-rural development in green and low-carbon growth [138].
2024	Five Major Risks in Low-Carbon Transition	Learning Times (China)	Emphasizes the need to prevent potential risks in the low-carbon transition, analyzing impacts on industrial and energy transformations [139].
2024	Energy Conservation and Carbon Reduction Action Plan (2024–2025)	State Council of the People’s Republic of China	Sets specific energy conservation and carbon reduction targets for 2024, promotes transformation in key sectors, and ensures fulfillment of the 14th Five-Year Plan’s energy and carbon reduction goals [140].

presents various strategies that countries and international organizations have established to address urban carbon risk. Carbon mitigation plans commonly involve specific reduction targets for urban carbon sources, including enhanced carbon reduction measures in industrial and transport sectors, energy structure transformation, promotion of clean energy, and advocacy for low-carbon lifestyles [116,117]. Health risk management strategies include air pollution prevention measures to improve urban air quality, safeguard public health, and implement precautionary measures during peak pollution periods [118]. For ecosystem restoration, strategies focus on enhancing urban green spaces and ecological conservation to increase ecosystem carbon sequestration capacity, thereby reducing urban carbon emissions [86].

Future urban carbon risk management strategies should establish a multidisciplinary collaborative research platform that integrates observational, modelling, and experimental data, facilitating cooperation and dialogue among scientists, policymakers, and communities. Carbon risk governance requires collaboration across multiple sectors, with joint participation from the government, businesses, and the public, to create a unified effort that promotes comprehensive implementation of urban carbon risk management [108,141]. Additionally, international cooperation should be strengthened to set global carbon reduction targets and policy frameworks, driving worldwide carbon neutrality efforts. It is also essential to advance the research and application of sustainable energy and carbon cycle technologies to address the challenges of carbon risk.

3.4. Research Hotspots and Trends of RUCR

3.4.1. Identification of Research Hotspots

Based on CiteSpace’s analytical framework, keywords that experience a rapid increase in frequency within a certain period in research literature are referred to as burst keywords [33]. Through burst analysis of keywords in research literature from 1991 to the first half of 2023, this paper has selected the top 47 RUCR keywords with the highest burst intensity (Figure 13 and

Table 8. Forty-seven burst keywords in RUSR research from 1991 to 2023 (first 6 months).

Keywords	Year	Strength *	Bursting Begin	Bursting End
carbon monoxide	1994	10.79	1994	2009
exposure	1991	8.04	1997	2011
spatial-distribution	2018	7.8	2018	2019
urbanization	2018	7.29	2022	2023
global burden	2015	6.69	2015	2018
ultrafine particles	2005	6.49	2005	2014
sorption	2008	6.46	2008	2013
energy	2019	6.33	2022	2023
association	2000	6.31	2000	2011
mortality	1995	5.85	1995	2013
street dust	2020	5.84	2020	2021
asthma	2004	5.56	2004	2011
disease	2001	5.15	2017	2018
volatile organic compounds	1997	5.14	2014	2017
surface sediments	2016	5.13	2018	2020
particulate air pollution	1996	5	1996	2009
trends	2021	4.82	2021	2022
ambient air	2004	4.81	2014	2015
diesel exhaust	1991	4.79	1991	2014
air pollution	1994	4.69	2005	2013
elemental carbon	1998	4.68	2007	2015
polychlorinated-biphenyls	2016	4.61	2016	2018
personal care products	2018	4.61	2018	2020
children	1994	4.59	1994	2015
persistent organic pollutants	2014	4.52	2014	2018
health	2005	4.46	2005	2012
atmosphere	2011	4.44	2011	2015
risk-assessment	2009	4.37	2015	2019
organochlorine pesticides	2014	4.12	2014	2019
energy consumption	2021	3.94	2021	2022
phosphorus	2018	3.84	2018	2021
mitigation	2019	3.7	2019	2020
human exposure	2019	3.7	2019	2020
area	2005	3.68	2009	2011
carbon emissions	2016	3.67	2021	2023
heavy metal	2018	3.64	2018	2019
cancer	1991	3.62	1991	2013
polycyclic aromatic hydrocarbons	2004	3.6	2014	2017
oxidative stress	2011	3.58	2020	2022
agricultural soils	2011	3.52	2017	2019
lung-cancer	2005	3.51	2005	2012
land use change	2021	3.5	2021	2022
greenhouse gas emissions	2015	3.46	2018	2019
aerosol	2004	3.29	2004	2012
pollutants	2009	3.27	2009	2010
transport	2009	3.27	2019	2021
nitrogen dioxide	2001	3.25	2013	2017

* Strength is a metric that measures the frequency with which a specific topic or keyword appears within a given period.

).

By analyzing the 47 significant burst keywords from 1991 to the first half of 2023 in Table 8 and related literature, this paper categorizes them into four main research hotspots. These reflect the diversification of RUCR across fields such as climate change, ecosystem management, pollution control, and health risks.

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Urban Pollution and Health Risk Research

Keywords like “carbon monoxide”, “exposure”, “air pollution”, “asthma”, and “mortality” have high burst frequencies, indicating that the impact of urban pollution on human health is a primary focus in RUCR. For instance, “carbon monoxide” had a burst strength of 10.79 from 1994 to 2009, highlighting its significant impact on air quality and human health. Similarly, “exposure” exhibited high burst strength (8.04) between 1997 and 2011, reflecting growing public concern over health risks related to pollutant exposure. Research in this area examines the relationship between pollutant exposure, health risks, and disease burden, particularly emphasizing factors like “global burden” and “ambient air”, which focus on reducing the adverse health effects of pollutant exposure on residents.

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Ecological Environment and Land Use Change Research

Keywords like “spatial distribution”, “land use change”, “agricultural soils”, and “heavy metal” indicate the influence of ecological environment and land use change on urban carbon risk. Notably, “spatial distribution” showed a burst (Strength = 7.8) between 2018 and 2019, reflecting scholars’ interest in the role of spatial patterns in pollution distribution and risk assessment. Meanwhile, “heavy metal” emerged as a research hotspot in 2018–2019. These studies further highlight the relationship between ecosystem stability and pollutant management, emphasizing the impact of human activities on ecological environments and their effects on spatial distribution and land use changes.

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Carbon Emissions and Energy Utilization Research

Keywords like “carbon emissions”, “energy”, “mitigation”, and “greenhouse gas emissions” underscore RUCR’s focus on carbon emissions and strategies for addressing climate change. “Carbon emissions” burst (strength = 3.67) from 2021 to 2023, indicating rising interest in emissions, particularly in energy consumption and urbanization. Additionally, “urbanization” peaked in 2022–2023 (strength = 7.29), reflecting the dual impact of rapid urbanization on carbon emissions and energy management. Since transportation is a major source of carbon emissions, the keyword “transport” showed a burst from 2019 to 2021, indicating a heightened focus on the transport sector’s role in urban carbon risk management.

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Pollutant Dispersion and Monitoring Technology Research

Keywords such as “ultrafine particles”, “polycyclic aromatic hydrocarbons”, “volatile organic compounds”, “personal care products”, and related literature highlight an interest in pollutant dispersion pathways and monitoring technologies in urban settings. For example, ultrafine particles showed a burst from 2005 to 2014 (strength = 6.49), reflecting the potential risks of ultrafine particles to air quality and health. Similarly, “volatile organic compounds” burst from 2014 to 2017, indicating significant attention to their dispersion and monitoring. This research includes investigations into pollutant sources, composition, and dispersion patterns, such as “elemental carbon” and “street dust”, and the application of emerging technologies like “remote sensing” for pollutant monitoring and spatial distribution analysis.

3.4.2. Prediction of RUCR Research Trends

Based on a bibliometric analysis of RUCR literature from 1991 to 2023, this paper has identified key clusters, critical issues, and research hotspots within this field. Consequently, three research trends are projected for the future development of RUCR.

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Studies on the source apportionment and monitoring of urban carbon emissions within the context of urbanization

With the rapid acceleration of urbanization, the source apportionment and control of urban carbon emissions have become focal points for academics and policymakers. The high-frequency emergence of keywords such as “carbon emissions”, “urbanization”, “energy consumption”, and “greenhouse gas emissions” reflects the current research emphasis on carbon emission issues. “Carbon emissions” showed significant emergence from 2021 to 2023 (intensity = 3.67), while “urbanization” reached its peak from 2022 to 2023 (intensity = 7.29), indicating that the issue of carbon emissions in the context of rapid urbanization has become an urgent challenge [118].

Given the complex and diverse sources of urban carbon emissions, source apportionment in an accelerated urbanization context is crucial. Building models for various emission sources based on analyzing major carbon emission pathways can provide scientific support for urban policy development. Zheng et al. emphasize that the core of urban carbon source apportionment lies in identifying different carbon emission sources, including residential areas, infrastructure, industrial sectors, and commercial zones [142]. Tian et al. argue that accurate source apportionment of carbon emissions within complex urban environments aids in formulating targeted emission reduction strategies, with positive matrix factorization (PMF) and similar techniques providing scientific evidence for analyzing primary carbon emission pathways [143]. Parsa et al. note that reducing urban carbon emissions through improved ecosystem services is a critical research topic within global climate change strategies [144]. De la Sota’s findings suggest that urban green infrastructure not only absorbs carbon emissions but also regulates the urban climate through natural mechanisms, thereby supporting carbon neutrality goals [145].

In summary, future research will likely focus on precise data collection and analytical methods, such as positive matrix factorization and remote sensing technology, to accurately locate the sources and pathways of urban carbon emissions [143,146]. With advancements in sensor and big data technologies, dynamic monitoring and spatiotemporal distribution analysis of carbon sources may further reveal the daily fluctuation patterns and long-term trends of carbon emissions. Additionally, researchers can explore enhancing urban carbon sink functions by improving ecosystem services and introducing green infrastructure, effectively alleviating carbon emission pressures and achieving sustainable urban development.

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Research on the impacts of urban carbon emissions related to air pollution and health risk

Research on air pollution and health risks is emerging as a critical domain within RUCR, aiming to uncover the potential threats of urban air pollutants to residents’ health. The high-frequency emergence of keywords associated with urban carbon emissions and other pollutants (such as “carbon monoxide”, “black carbon”, and “volatile organic compounds”) highlights the dual threats these substances pose to climate change and public health. Among these, the high-frequency keyword “carbon monoxide” reached an emergence intensity of 10.79 between 1994 and 2009, while “exposure” peaked with an intensity of 8.04 from 1997 to 2011, indicating an increasing focus on pollutant exposure and associated health risks.

As urbanization and industrialization advance, the rising threat to public health posed by carbon emissions and related pollutants (e.g., carbon monoxide, black carbon, and volatile organic compounds) has become a prominent urban air pollution issue [147]. Dong et al. report that the long-term adverse effects of carbon emissions on public health are evidenced by a 0.298% increase in outpatient cases and a 0.162% rise in hospitalizations for every 1% increase in carbon emissions [147]. Carbon emissions directly impact human physiology; high concentrations of CO₂ reduce pulse rates [148] and induce symptoms like headache, dizziness, and fatigue [149]. Studies show that increased carbon emissions drive climate change and global warming, making urban residents more susceptible to digestive, neurological, cardiovascular, and respiratory diseases [150]. Thus, urban carbon risk is not only an ecological issue but is also closely related to human well-being, with carbon reduction offering substantial health benefits for residents [151]. Excessive carbon emissions also promote pollutant generation and dispersion by altering meteorological factors (e.g., temperature, humidity, precipitation, wind speed), exacerbating urban air pollution. Laumbach et al. suggest further investigating how reducing pollutant emissions can lower exposure risks, particularly in densely populated urban environments with low air circulation [152].

Therefore, future research will focus on the spatiotemporal distribution characteristics of pollutants and the variation in exposure levels and health impacts across different population groups. Through dynamic monitoring and model simulations, research should accurately reveal the exposure levels of carbon emissions and related pollutants, coordinating these data with health risk analyses for urban residents.

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Studies on urban carbon emission mitigation and governance aligned with low-carbon and sustainable development goals.

Research on urban carbon emission governance and mitigation primarily focuses on reducing urban carbon pollutants through policy and technological interventions to achieve sustainable development goals. The emergence of the keyword “mitigation” between 2019 and 2020, with an intensity of 3.7, and “heavy metal” between 2018 and 2019, with an intensity of 3.64, indicates an increased demand for pollution control. Governance of soil and water pollution is also highly prioritized, with “ultrafine particles” showing an emergence intensity of 6.49 from 2005 to 2014, revealing their potential threat to air quality and health. Researchers continue to deepen low-carbon concepts, advance clean energy technologies, and promote energy structure transformation.

Urban carbon emission governance has become vital for sustainable development [153]. Urban carbon governance primarily includes reducing greenhouse gas emissions, mitigating pollutants’ cumulative effects on air, water, and soil, and improving overall urban environmental quality. For instance, Lin et al. argue that future research will focus on developing low-carbon and clean energy technologies to reduce carbon emissions from the source by promoting low-carbon transitions in energy structures [154]. Du et al. suggest that investment in research and development is a critical driving force for technological innovation [155]. Concurrently, innovation in energy technology is essential in reducing carbon emissions [156]. Popp emphasizes that technological change is crucial for promoting low-carbon growth by lowering emission reduction costs [157]. Similarly, Wang et al. highlight the significant impact of energy technology innovation on reducing carbon emissions, suggesting that R&D investments positively influence the scale of clean energy usage [156]. Zhao et al. underscore the guiding role of national policies in urban low-carbon development, noting that applying low-carbon and clean energy innovation technologies, energy structure transformation, and optimized governance strategies can reduce urban carbon emissions [158].

In recent years, advancements in carbon emission control technologies and the application of clean energy have positively impacted emission reduction. However, with accelerating urbanization, challenges in carbon emission governance are increasing, especially within high-density urban environments [159]. Therefore, the first future research direction in urban carbon governance focuses on promoting energy structure transitions through innovative low-carbon technologies, clean energy, and green infrastructure. Exploring efficient pollutant removal technologies, such as advanced filtration materials, pollutant degradation techniques, and carbon capture and storage, will provide a comprehensive approach to reducing pollution loads from source to end-point [160]. Another development direction for urban carbon governance is enhancing urban ecological carbon sinks through strategies like expanding green spaces, implementing vertical greening, and restoring water bodies to strengthen natural carbon sequestration, thus establishing a sustainable pollution control model. Policy-level research on coordinated governance also holds promise by fostering multi-departmental collaboration to integrate policy and technology applications, laying a foundation for low-carbon and sustainable urban futures.

These two research areas will further enhance comprehensive urban environmental governance, providing theoretical and technological support for achieving low-carbon and sustainable urban development goals.

4. Discussion

RUCR research is crucial in understanding and managing potential carbon emission risks in urban environments, reducing the negative climate impact of greenhouse gases, and promoting sustainable urban development. Based on the above literature analysis of the Web of Science (WOS) database, it is evident that researchers from various disciplines have contributed to RUCR-related studies, with a steady increase in publications in recent years indicating growing academic attention. However, RUCR is still in its early stages, as shown by the relatively limited research volume and a primary focus on specific carbon emission sources and control measures, with a lack of systematic and comprehensive urban carbon risk assessment methods.

Regarding content, existing literature primarily explores carbon emission source analysis and control strategies, air pollution and health risk assessment, and carbon emission management. Frequently occurring keywords such as “carbon emissions”, “air pollution”, and “health risk” demonstrate a significant focus on the sources of urban carbon emissions and their public health impacts [59]. Some studies have linked carbon emissions with air pollutant exposure, revealing the adverse health effects of urban carbon emissions through exposure risk analysis [92].

From a methodological perspective, current RUCR research is predominantly qualitative and case-based, with a limited focus on quantitative and spatialized risk assessment models. Researchers often assess carbon emission risks by analyzing emission sources and pollutant dispersion pathways [116,117], with minimal integration of multidimensional data and spatial information for carbon risk identification and prediction. The research scale is typically urban or regional, lacking finer-grained, micro-spatial urban assessments.

Theoretically, a unified conceptual framework and systematic assessment method for urban carbon risk have not yet been established. The theoretical framework for urban carbon risk remains fragmented regarding research perspectives, spatial scale, and variable selection, with diverse definitions and measurement standards for carbon risk across studies complicating the synthesis and comparison of research findings.

Therefore, this paper proposes two areas for advancing future RUCR research. First, enhancing spatial identification of urban carbon risk by employing machine learning and other methods to refine the analysis of carbon emission sources and absorption capacities across different areas, enabling more precise spatial and targeted risk identification. Second, scenario-based modeling should be promoted by constructing urban carbon emission scenarios to analyze emission trends and sequestration potential under varying conditions, predict carbon risk trajectories, and assess the effectiveness of response measures. This process should systematically incorporate influencing factors such as climate change, population growth, and energy structure shifts, using advanced simulation tools to model future scenarios and comprehensively assess the climate change risks and socio-economic impacts associated with urban carbon emissions.

4.1. Spatial Identification of Urban Carbon Risk

In RUCR research, space is considered the primary carrier of urban carbon risk. This perspective aids decision-makers and researchers in quantifying and understanding how urban carbon risk manifests across social and economic dimensions and how these dimensions exhibit heterogeneity across different areas. Spatial identification of urban carbon risk allows for systematically categorizing urban areas based on carbon risk levels, providing a scientific basis for formulating tailored and effective adaptation strategies for different regions. “Spatial identification” involves analyzing and delineating specific characteristics or conditions within a defined area. In the context of RUCR research, spatial identification in urban areas aims to pinpoint zones significantly influencing carbon emissions and sequestration, including high-risk carbon emission areas and regions with high carbon sink potential. The primary goal of spatial identification for urban carbon risk is determining which areas should be prioritized in carbon risk management strategies. This process supports more precise assessment, planning, and intervention for urban carbon risk.

While no specific methods and processes for spatial identification of urban carbon risk have been explicitly outlined, existing studies have explored the influence of urban spatial characteristics on carbon emissions from various perspectives. Liang found that population clustering enhances production efficiency and establishes an economic, spatial structure, which can help reduce CO₂ emissions [161]. Furthermore, research indicates that increasing carbon efficiency, enhancing vegetation’s carbon sequestration capacity, and promoting economic spatial clustering can effectively alleviate urban clusters’ Carbon Footprint Pressure [162]. Niemelä et al. share a similar view [86,163], noting that larger green spaces help store more carbon [164]. Krivtsov and colleagues observed that urban water bodies contribute to urban carbon capture and play a vital role in flood risk regulation [165]. Additionally, some scholars have developed refined urban carbon emission models to analyze the dynamic changes in urban carbon emissions [166].

4.2. Scenario Simulation of Urban Carbon Risk

Urban carbon risk scenario simulation aims to define, analyze, and predict urban spaces’ carbon emission and sequestration capacities under various scenarios. This process involves multiple stages, including data analysis, model simulation, and policy analysis, to assess urban vulnerability to carbon emission pressures and evaluate its adaptive capacity. Scenario assessment not only includes analyzing current conditions but also involves predicting possible future pathways. As a multidimensional analysis, scenario assessment must consider potential future events and changes and their impacts on urban spaces. These changes may include the combined effects of climate change, population growth, urbanization, energy use patterns, and technological innovation for urban carbon risk.

The core components of urban carbon risk scenario assessment include model simulation, risk evaluation, and strategy formulation. Using tools such as carbon cycle and urban development models, it is possible to simulate carbon emissions and sequestration under different Shared Socio-economic Pathways scenarios. These models help assess the impact of various policies and measures on urban carbon balance. Based on the simulation results, the assessment identifies carbon risks under different scenarios, particularly the climate change impacts and socio-economic consequences arising from excessive carbon emissions and insufficient carbon sequestration. Ultimately, strategies to mitigate carbon risks and enhance urban climate resilience are formulated, which may include energy structure adjustments, energy efficiency improvements, renewable energy development, urban greening to enhance carbon sinks, carbon pricing, and carbon trading.

Scholars have already calculated urban carbon emissions and sequestration across various scenarios. For instance, Zhong simulated land use changes for Shandong Province, China, under natural and sustainable development scenarios by 2030, calculating the differences in land carbon emission risks across scenarios [95]. Chen defined three scenarios—baseline development, farmland protection, and ecological protection—and calculated the impacts and drivers of urban expansion on carbon storage [96]. Nakamichi and colleagues examined Japan’s carbon emission scenarios by considering climate mitigation and adaptation, along with the introduction of photovoltaic systems and electric vehicles. They set up four urban land use scenarios and five photovoltaic and electric vehicle adoption scenarios to calculate the resulting mitigation capacity [81].

5. Conclusions

This study conducted a knowledge mapping analysis of 2012 RUCR publications from 1991 to June 2023 in the WOS database, resulting in five key research findings:

(1)

Characteristics of Literature Distribution

Publication Sources: Before 2005, annual RUCR publication numbers were low but steadily increased afterward, reaching a peak in 2022. The primary disciplines of RUCR research are Environmental Sciences, Public Environmental Occupational Health, and Engineering Environmental. Among 1635 RUCR publications, the top 10 journals mainly focus on environmental studies, ecology, and sustainability.

Co-citation Analysis: Analysis of the top 20 co-cited RUCR papers reveals three main research areas: carbon sequestration assessment and technology updates, urban carbon risk factors and their consequences, and strategies for managing carbon risk.

Research Collaboration: Chinese research institutions lead in collaborative publications (accounting for 55%), followed by the United States and the United Kingdom. The top five countries in collaborative research are China, the United States, the United Kingdom, India, and Germany, contributing 74% of global publications. The United States was the first to conduct RUCR research (1991), and it, along with France, exhibits the highest centrality in the field.

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Stages of Research Development

Based on keyword time zoning analysis, high-frequency keyword trends, and relevant policy influences, this study divides RUCR’s development into four stages: Single Exploration (pre-2001): Focused on fundamental issues of air pollution and health risk, particularly the effects of individual exposure on health, with keywords like “air pollution” and “particulate matter” as research highlights. Evaluation and Construction (2002–2009): Prompted by international agreements, research expanded to assess carbon risk impacts and emission reduction measures, focusing on sectors like construction and transportation. Innovation and Breakthrough (2010–2017): Under frameworks like the Paris Agreement, research deepened into low-carbon technology, emission reduction mechanisms, and ecosystem services, emphasizing carbon sequestration and urban emission reduction policies. Technological Synergy (2018–present): With increasing decarbonization demands, research has become more interdisciplinary, employing new technologies like remote sensing and machine learning and exploring multi-perspective approaches, including land use change and carbon financing.

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Key Research Issues

Five primary RUCR research clusters were identified: “0. air pollution”, “1. climate change”, “2. heavy metals”, “3. source apportionment”, and “4. pregnancy”, with “0. air pollution” as the largest cluster. Key research issues within RUCR are summarized across three areas: objectives (carbon reduction and sequestration), methods (qualitative and quantitative analyses), and strategies (promoting interdisciplinary and international collaboration and technological innovation).

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Research Development Trends

Analysis of the 47 burst keywords from 1991 to mid-2023 reveals four research hotspots: “urban pollution and health risk research”, “ecological environment and land use change research”, “carbon emissions and energy utilization research”, and “pollutant dispersion and monitoring technology research”. Based on keyword clustering, key issues, and research hotspots, this paper predicts three future research trends: “urban carbon emission source analysis and control”, “air pollution and health risk research”, and “urban carbon pollution management and mitigation research”.

Based on an analysis of RUCR’s keyword clustering, key issues, and research hotspots, this paper forecasts three significant future research trends in this field: (1) “Studies on the source apportionment and monitoring of urban carbon emissions within the context of urbanization”, (2) “Research on the impacts of urban carbon emissions related to air pollution and health risks”, and (3) “Studies on urban carbon emission mitigation and governance aligned with low-carbon and sustainable development goals”.

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Recommendations for Future Research

Current RUCR research is limited in volume and primarily focuses on specific carbon emission sources and control measures, lacking systematic and comprehensive urban carbon risk assessment methods. From an urban planning perspective, this paper suggests that future research should prioritize urban carbon risk spatial identification and scenario simulation.

These conclusions help researchers from various disciplines quickly understand RUCR’s current status and development trends while offering clear recommendations for future research. However, this study has limitations in two respects. First, due to the authors’ professional backgrounds, the recommendations primarily reflect urban planning and architecture perspectives, suggesting a specific disciplinary scope. Second, some conclusions are based on qualitative analysis from data interpretation, specifically regarding research trends and future recommendations. Researchers should adapt these insights based on their specific expertise and research objectives.