Exploring Research Fields in Green Buildings and Urban Green Spaces for Carbon-Neutral City Development

Abstract

The international community is striving to build carbon-neutral societies in response to urban environmental challenges. Green Buildings (GBs) and Urban Green Spaces (UGSs) are recognized as key elements in future city development, as they contribute to both the reduction and absorption of carbon emissions. This study analyzed research fields related to GBs and UGSs by collecting and examining keywords from academic articles indexed in the Scopus database: 2880 articles from 1971 to 2025. After refining the dataset to 1685 articles, centrality, betweenness, and cluster analyses were conducted, and the results were visualized through a keyword network map. The findings are summarized as follows: (1) Research on GBs predominantly focuses on experimental and technological aspects, especially in the areas of heat and energy. (2) UGS-related studies are largely policy-driven and comprehensive, centering on green infrastructure and ecosystem services. (3) The international research landscape highlights key topics such as the greening of existing building stock, green roofs, and rooftop agriculture integrating advanced technologies, focusing on how these GB and UGS strategies address barriers to urban carbon cycling. This study offers valuable insights for researchers in architecture, landscape architecture, and urban planning who are working toward the realization of carbon-neutral cities.

1. Introduction

Due to ongoing urbanization, climate change has become a critical issue recognized by the international community [1]. Climate change is documented as a global challenge with adverse impacts on ecosystems, societies, and economies worldwide [2,3]. The primary cause of climate change is identified as greenhouse gases resulting from human and anthropogenic activities, and it is scientifically agreed that addressing this issue presents significant challenges in the short term [4,5]. As such, carbon neutrality, adopted by the international community, is an essential strategy for mitigating climate change. Urbanization accelerates the negative impacts of climate change, with the excessive heat emitted from urban areas being considered a major contributor to the urban heat island (UHI) phenomenon [6,7,8]. Due to urbanization, approximately 70% of the global population is expected to reside in cities by 2050 [9], and around 77% of the world’s 520 major cities are projected to experience changes in their climatic systems [10]. Therefore, achieving carbon-neutral cities requires challenging experiments focused on carbon emission reduction, carbon absorption and storage, and carbon recycling.

The increase in population density and infrastructure due to urbanization is a major cause of urban climate vulnerability, creating an important space for discussions on changes in urban climate environments [11]. Cities serve as experimental and ideal hubs for implementing the technologies, tools, and policies required for the construction of carbon-neutral cities [12,13]. Achieving a carbon-neutral society necessitates the phased elimination of fossil energy in key carbon-emitting processes, such as building heating and cooling, and transportation [14,15]. The building sector accounts for one-third of global greenhouse gas emissions (12.3 GTCO2 in 2022) [16], with 9.9 GTCO2 emitted from building operations alone [17]. To promote decarbonized buildings, it is essential to replace fossil fuels with electrification suitable for the climatic conditions of each country, improve energy efficiency, and construct low-carbon buildings [16]. With the adoption of the Paris Agreement reports, the international community has initiated efforts to reduce and limit carbon emissions. The World Green Building Council (WGBC) emphasized the goal of halving carbon emissions from the building and construction sector by 2030 and achieving complete decarbonization by 2050 [18,19]. All buildings are required to contribute to reducing carbon emissions into the atmosphere through improvements in building energy efficiency and management. Additionally, predicting future building energy consumption in an unstable and rapidly changing climate environment is crucial.

Nature-based Solutions (NbSs) play a crucial role in spatially addressing both the causes and consequences of climate change [20,21]. Spatially based NbSs have been shown to reduce anthropogenic carbon emissions by an average of 17.4% [13]. Carbon emission mitigation through NbSs includes approaches such as ecosystem services and green infrastructure, which isolate carbon emissions from human activities [13,21]. In a broader sense, green infrastructure, urban forests, urban parks, gardens, green walls, and green roofs are various forms of urban green spaces, which provide a range of ecosystem services, including carbon sequestration, sustainable drainage, flood mitigation, and microclimate regulation [22,23]. Accelerated urbanization increases surface temperatures, exacerbating the UHI effect [24,25]. Given this, the application of NbSs can help delay additional heat generation and support biodiversity, securing ecosystem services that contribute to mitigating the UHI effect [26,27,28,29,30]. Urban green space is one of the ecologically friendly means to alleviate the UHI effect [31,32].

The form of a city is highly correlated with changes in the urban climate environment and directly influences the UHI effect [33,34,35,36,37,38,39]. The forms of buildings and urban green spaces, which occupy the majority of urban areas, are inversely related to UHI dynamics. Areas with high building density tend to exacerbate the UHI effect, whereas increasing green spaces mitigate this effect [40]. However, it is well established that buildings often exist within green spaces, and green areas can be found in narrow spaces such as building walls, rooftops, and the interiors and exteriors of buildings. Green-covered walls have been shown to reduce indoor temperatures by up to the maximum compared to conventional buildings [41], and expanding green spaces on walls enhances insulation performance [42]. Strategically greening building surfaces can reduce surface temperatures by up to 20 °C and decrease air conditioning usage by 25–50%, with reductions of up to 80% [43,44]. Rooftop greening in non-insulated buildings provides significantly better insulation compared to insulated buildings [45]. Rooftop greening can reduce energy consumption in buildings and be used as a strategy for mitigating the UHI effect [46,47]. Even small-scale green spaces like rooftop gardens, combining buildings and greenery, provide numerous environmental benefits, including energy savings, UHI mitigation, improved air quality, and reduced stormwater runoff [48,49,50,51]. In the absence of adequate urban green spaces, the UHI effect worsens due to greenhouse gas emissions [52]. An increase in building area within urban spaces exacerbates both UHI and air pollution, while green spaces indirectly mitigate air pollution. Urban green spaces are considered a cost-effective means of climate adaptation [53]. A significant amount of carbon is sequestered and stored by urban greenery and vegetation [54,55], and urban green ecosystems play a crucial role in carbon sequestration and absorption [56]. Although urban ecosystems are a part of the broader ecological system, urban green spaces play a vital role in maintaining the carbon balance of both urban and global ecosystems [57]. However, urban spaces are under pressure for development due to high land prices [38]. Therefore, it is necessary to closely examine the interactions between buildings and green spaces within cities and the research fields related to these interactions.

To establish a sustainable carbon-neutral city, a harmonious integration of Green Buildings (GBs) and Urban Green Space (UGS) is essential. In response to the evolving urban environment and the associated environmental challenges, it is necessary to expand the areas of both GBs, which reduce carbon emissions, and UGSs, which absorb carbon. NbSs, such as the integration of buildings with green spaces, can optimize infrastructure, revitalize urban ecosystems, and ensure resilient pathways toward carbon-neutral futures. Achieving carbon neutrality requires not only quantitative expansion of GB and UGS areas but also the development of technologies, materials, and related research aimed at maximizing functional performance, such as energy efficiency and carbon sequestration capacity. However, current research in this field is disproportionately concentrated in specific regions or countries, such as those in tropical climates, making it difficult to obtain a comprehensive understanding of the global research landscape. Therefore, this study aims to provide a systematic overview of the research fields related to GBs and UGSs, with the goal of supporting the realization of future carbon-neutral cities.

2. Literature Review

2.1. Green Buidlings

Green buildings are a critical component in driving a low-carbon economy and serve as a key tool for achieving sustainable development goals within the building sector [58,59]. As one of the proposed strategies to mitigate the significant environmental, social, and economic impacts of conventional buildings, green buildings have been shown to reduce energy consumption by up to 25%, water usage by 41%, and carbon dioxide emissions by 34% compared to traditional buildings [60,61,62]. These reductions contribute significantly to minimizing carbon emissions, energy and water consumption, and waste generation associated with the built environment, making green building promotion a vital aspect of carbon-neutral urban development.

A green building is defined as an environmentally responsible structure that minimizes its impact on the environment throughout its life cycle, including site selection, design, construction, operation, maintenance, and demolition. The term is often used interchangeably with high-performance or sustainable buildings [60]. Since the 1990s, the Life Cycle Assessment (LCA) methodology has been adopted to evaluate the environmental impacts of buildings across their full life cycles [63,64]. The green building concept emerged with a particular focus on enhancing energy efficiency, particularly in heating and cooling systems, to reduce environmental burdens. As a result, well-insulated buildings have become new benchmarks for green performance [63], leading to the development of certification systems that assess energy efficiency and environmental sustainability. Globally, approximately 600 Green Building Rating Systems (GBRSs) have been established, including LEED (USA), BREEAM (UK), CASBEE (Japan), DGNB (Germany), Green Star (Australia), and ASGB (China) [59,64,65,66,67,68]. These rating systems inform occupants about the environmental quality and healthiness of buildings and emphasize the integration of green features, thereby reinforcing principles of sustainability [68].

Despite the global community’s ongoing efforts to expand the adoption of green buildings, significant challenges remain in achieving carbon neutrality within the building sector. As of 2021, buildings accounted for 34% of global energy consumption and 37% of carbon emissions [69]. Moreover, global building-related carbon emissions have increased by 43% since 2007 and are projected to reach 42.4 billion tons by 2035 [60]. These emissions are largely attributable to the existing building stock [70,71,72,73], with new buildings accounting for only 1–1.5% of total building inventories in most developed countries [74]. If the number of existing buildings doubles by 2050 as projected, energy demand and carbon emissions could increase by more than 50% [75]. Therefore, improving the energy efficiency and managing carbon emissions of existing buildings is crucial in preparing for the future [76]. Optimizing existing structures and strategically planning new constructions should be the core objectives of green building practices to effectively reduce carbon emissions from the built environment on a global scale [73,77,78,79].

2.2. Urban Green Space

Urban green space refers to both natural and artificial green areas and water bodies within urban environments [80,81,82], serving as significant reservoirs for carbon sequestration and storage [56,83,84]. UGS encompasses a wide range of green infrastructure within cities, including urban forests, parks, and green infrastructure (GI), and provides various ecosystem services depending on their size, structure, vegetation cover, environmental quality, and amenities [85,86]. The carbon sequestration capacity and microclimate regulation of UGS have been recognized as critical in the context of climate change mitigation [80]. Notably, UGS tends to reduce more carbon during the summer than the amount emitted during the winter, thereby contributing positively to local climate regulation and mitigating UHI effects [84,87,88,89,90]. Through processes such as transpiration [91] and the provision of shade [92], UGS effectively lowers ambient urban temperatures and functions as a continuous source of carbon absorption. Consequently, strategic planning and management of UGS are essential to enhance urban thermal comfort, mitigate UHI effects, and reduce climate-related environmental damages [93].

UGS is considered a core element of NbSs [27,92,94]. Numerous studies have shown that increasing the density of tree-covered UGS significantly enhances urban cooling effects [95]. For example, an increase of just 16% in UGS coverage can lead to a reduction in urban temperatures by approximately 1 °C [92,95]. Through this cooling function, UGS can reduce carbon emissions by at least 25% [95], and has contributed to an average temperature decrease of 1.1 °C across European cities [92,96]. Additionally, UGS plays a role in improving air quality by removing carbon from the atmosphere [97,98], and can offset up to 3.6% of carbon emissions generated from fuel combustion [99]. Therefore, the temperature regulation services provided by UGS possess significant potential to continually reduce urban carbon emissions [100,101].

As such, UGS serves as the most ecologically natural environment within urban areas and plays a pivotal role in achieving urban carbon neutrality [102]. It is crucial in offsetting atmospheric carbon concentrations through natural sequestration processes [103,104]. Since the COVID-19 pandemic, the positive effects of UGS on physical and mental health, overall well-being, and stress reduction have been increasingly recognized. As a result, UGS is now considered a fundamental infrastructure for maintaining urban resilience [105]. The development of UGS and public open spaces can enhance cities’ ability to withstand urban environmental and public health crises, such as pandemics, while simultaneously promoting resilience and sustainable development capacity [106].

2.3. Towards Carbon Neutrality: The Role of Green Buildings and Urban Green Spaces

In the face of continuously changing urban environments, forecasting future pathways toward carbon neutrality is essential for ensuring urban sustainability. As previously discussed, the building sector accounts for approximately one-third of global greenhouse gas emissions, while UGSs play a direct role in absorbing and storing these emissions. Therefore, integrating green spaces into urban and building design processes is crucial for enhancing thermal resilience and mitigating the adverse impacts of UHI and global warming [93]. Buildings, where people reside and work daily, and their adjacent UGSs are inherently interdependent. Transitioning to green buildings and strategically positioning suitable UGSs can significantly improve urban thermal comfort and contribute to achieving carbon-neutral cities.

Various types of UGSs—such as green roofs, green walls, and vertical gardens—are already being implemented in combination with building exteriors and surfaces. Methods to enhance thermal insulation and energy efficiency include window replacements, façade retrofitting, application of roof coatings, and the installation of building-integrated photovoltaics (BIPV) and solar panels [43]. Green roofs, a representative NbS and a form of UGS, not only extend the lifespan of roofs and walls by providing protective coverage but also contribute to UHI mitigation by removing heat from the atmosphere [45,46,47,107]. Furthermore, vegetation on buildings enhances ecological functions, such as noise reduction, stormwater retention, and biodiversity support [43], while improving insulation performance [108]. The integration of buildings and green infrastructure has positive physical, ecological, environmental, and economic impacts, thus contributing to the long-term sustainability of urban environments.

In a carbon-neutral society, buildings and green spaces are considered essential components of urban systems. From a carbon-neutral urban perspective, retrofitting existing buildings into green buildings is critical for reducing GHG emissions, while expanding green spaces, especially those integrated with buildings, is increasingly necessary. Theoretically, while buildings emit carbon, green spaces absorb and sequester it. However, quantifying the relationship between carbon emissions and sequestration requires interdisciplinary research. Although numerous studies have explored carbon emissions and sequestration, UHI mitigation, thermal comfort, and energy efficiency standards within their respective fields, few have examined these two domains in an integrated manner. To effectively predict and respond to the impacts of dynamic urban climate changes, a comprehensive analysis of the interrelationship between green buildings and UGS is needed from the perspective of urban planning and management.

3. Materials and Methods

3.1. Data Collection

To identify research fields regarding the interrelationship between GBs and UGS, this study conducted a literature search using the international academic database Scopus. The Scopus database encompasses a wider and more diverse spectrum of subject areas than the Web of Science database [109,110,111]. A total of 2880 articles, published between 1971 and 17 March 2025, were retrieved from the subject areas of Environmental Science and Social Science using the search terms “Green Building” and “Green Space”. These fields were selected due to their extensive exploration of the social, economic, and environmental dimensions of urban systems [112,113] (Figure 1). To ensure focused analysis of research fields specifically related to buildings and green spaces within urban contexts, only articles from these two subject areas were included.

Bibliographic data containing missing keywords, metadata errors, or written in languages other than English were excluded during the data refinement process. Following data cleaning, 1685 articles and 5103 unique keywords were selected for the final analysis. The main search terms, “Green Building” and “Green Space”, were excluded from the keyword analysis due to their direct relation to all retrieved articles.

To avoid duplication caused by variations in spelling, abbreviations, singular/plural forms, capitalization, hyphenation, and quotation marks, keyword standardization was performed during the data refinement phase. For example, terms such as “Nature-based Solutions”, “green-roof”, and “Geographic Information System” were standardized to “NbS”, “Green Roofs”, and “GIS”, respectively. Although authors may prefer their keywords to be interpreted in their original form, keyword standardization was necessary for this study to ensure consistency across diverse journal formats and to facilitate trend analysis in related research domains (Figure 2).

3.2. Methods

Keyword network analysis, a method derived from social network analysis, has been widely used to explore research fields in fields such as physics [114], information science [115,116,117], environmental health [118], and animal ecology [119]. Recently, it has also been applied in machine learning to identify emerging fields [117] and in journalism to explore relationships between news topics. As keyword network analysis relies on author-defined terms, additional processes are often required to extract meaningful insights [113], and the final results should be visually represented through structured networks [120]. By analyzing both keyword co-occurrence and co-authorship networks, it is possible to construct knowledge networks and examine the relationships between collaborative efforts and research outputs [121]. Furthermore, keyword analysis based on metrics such as closeness and betweenness centrality can help determine their influence on citation frequency [122] (Figure 3).

In this study, a co-occurrence network approach was used, where the number of times a pair of keywords appears together across multiple articles determines the weight of the link between them [115,123]. The resulting network map consists of nodes representing keywords and weighted links representing the frequency of co-occurrence. Keywords with a minimum co-occurrence frequency of 30 were included in the analysis [113]. The primary objective of this study is to explore the current fields and trajectories in research related to GBs and UGS as critical urban components for achieving a carbon-neutral society. To this end, centrality, betweenness, and cluster analyses were performed. Centrality analysis identifies the most important nodes (keywords) within the network, while betweenness analysis evaluates the extent to which a keyword functions as a bridge between other terms. Centrality measures help uncover structural importance, whereas betweenness measures indicate the potential for influence within the network [113,124]. Cluster analysis groups keywords based on co-occurrence, allowing for the identification of semantic relationships between terms—especially useful when individual associations are difficult to detect [124,125,126]. All network analyses were conducted using NetMiner 4 (Cyram, Seongnam, Korea), a dedicated social network analysis software.

4. Results

4.1. Keyword Analysis

In this analysis, only keywords with a co-occurrence frequency of 30 or more were included, to ensure the semantic significance of the terms. A co-occurrence frequency of 30 (Top 10% of all co-occurrence frequency) indicates that a particular keyword appeared in at least 30 different articles. A total of 1295 co-occurrences across 20 distinct keywords met this criterion and were used in the analysis.

The keywords, ranked in descending order of frequency, are as follows: Urban Greening (178), Existing Building Stock (96), Urban Planning (94), Low Energy Building (83), Sustainability (78), Urban Heat Island (UHI) (74), Environmental Justice (72), Green Roof (64), Thermal Comfort (64), Green Infrastructure (62), Environmental Policy (56), Urban Environment (56), Ecosystem Services (52), Nature-based Solutions (NbSs) (44), Land Use (42), Life Cycle Assessment (LCA) (39), Renewable Energy (39), Indoor Environmental Quality (36), Carbon Sequestration (35), and Urban Morphology (31) (

Table 1. Frequency, degree centrality, and betweenness centrality of keywords used in the analysis.

Keywords	Frequency	Centrality Degree	Betweenness Degree
Urban Greening	178	0.263158	0.005117
Existing Building Stock	96	0.157895	0.004094
Urban Planning	94	0.368421	0.082749
Low Energy Building	83	0.105263	-
Sustainability	78	0.210526	0.200000
UHI	74	0.473684	0.143762
Environmental Justice	72	0.315789	0.039669
Green Roof	64	0.578947	0.333821
Thermal Comfort	64	0.157895	0.010136
Green Infrastructure	62	0.578947	0.140741
Environmental Policy	56	0.210526	0.006823
Urban Environment	56	0.210526	0.001462
Ecosystem Services	52	0.368421	0.117593
NbS	44	0.368421	0.087281
Land Use	42	0.105263	0.002193
LCA	39	0.105263	-
Renewable Energy	39	0.052632	-
Indoor Environmental Quality	36	0.052632	-
Carbon Sequestration	35	0.052632	-
Urban Morphology	31	0.105263

).

Figure 4 illustrates the centrality and betweenness of keywords derived from the analysis. In the network map, the nodes are distinguished by both size and color. A larger node indicates higher degree centrality, while a smaller node represents lower centrality. Regarding color, the closer a node’s color is to green, the higher its betweenness centrality; conversely, colors closer to white indicate lower betweenness.

In the degree centrality analysis, Green Roof and Green Infrastructure showed the highest values (0.578947). This suggests that these keywords serve as major topics or focal points in the analyzed research articles. While the two keywords share a similar centrality score, they exhibit differences in their associated domains. Green Roof is predominantly studied in relation to energy and architecture, and many of these studies involve experimental or empirical methods, indicating its close connection to green building research. On the other hand, Green Infrastructure is more widely examined in contexts involving nature, environment, society, ecology, and urban planning, often through theoretical and quantitative approaches—suggesting a strong association with Urban Green Space (UGS). Moreover, the keyword Urban Heat Island (UHI) also demonstrated high degree centrality, signifying its presence in overarching research topics. This implies that keywords with high degree centrality are widely referenced and have received significant academic attention within the field [118].

In terms of betweenness centrality—which indicates the extent to which a keyword acts as a bridge between different topics—Green Roof exhibited the highest score (0.333821). This makes it the most influential keyword in the network. It functions as a connecting term between keywords that are otherwise weakly associated, such as Sustainability and Green Infrastructure. Similarly, Sustainability links to Low Energy Building and Indoor Environmental Quality, thus serving as a thematic connector across diverse research domains. These results indicate that keywords with high betweenness centrality play a pivotal role in connecting interdisciplinary themes. In particular, Green Roof emerges as a critical concept, heavily studied across theoretical, experimental, and methodological research. It serves as a nexus within both Environmental Science and Social Science domains, and must be considered one of the most essential topics in the discussion of urban sustainability and climate resilience.

4.2. Cluster Analysis

To analyze the relationships among keywords and identify research fields, a cluster analysis was conducted. This method is particularly effective in understanding related research areas within a large-scale framework, as clusters are formed based on the cohesiveness of individual keywords. In other words, keywords within the same cluster exhibit a high degree of semantic or topical affinity, thereby reflecting shared research themes and directions [113].

Cluster 1 centers around the keyword Green Roof and comprises keywords associated with green buildings (GBs), thermal and energy efficiency, urban environments, and morphological types. The cluster includes terms related to Urban Heat Island (UHI), energy, thermal comfort, Life Cycle Assessment (LCA), urban morphology, and urban environment. Collectively, these keywords suggest that research in this cluster primarily focuses on the technical planning, application, and evaluation of buildings as a solution to mitigate urban environmental issues such as UHI. It reflects studies in which green roofs contribute to reducing indoor energy consumption, enhancing thermal comfort, and serving as integral urban components that influence city form and microclimate.

Cluster 2, on the other hand, revolves around the keywords Green Infrastructure and Ecosystem Services, and is primarily associated with Urban Green Space (UGS). This cluster includes themes such as urban greening using existing building stock, Nature-based Solutions (NbSs) for carbon sequestration, and environmentally just urban policies and frameworks. These keywords collectively indicate a research focus on UGS as a foundation for developing sustainable, ecologically friendly urban futures. Specifically, the research direction of Cluster 2 emphasizes UGS-centered approaches that align with long-term urban sustainability and environmental justice goals.

It is important to note that while the clusters are formed based on the internal cohesion of keywords, they do not imply that all related research topics are exclusively represented within their respective clusters. Instead, these groupings highlight general research fields and thematic associations among keywords, providing a valuable perspective on the broader structure of interdisciplinary studies in green building and green space domains.

4.3. Research Fields of Both Green Building and Urban Green Space

As presented in Section 4.1 and Section 4.2, this study collected literature that simultaneously used both Green Building (GB) and Urban Green Space (UGS) as keywords, and analyzed the keywords of the retrieved documents. According to Cluster 1, research fields in GBs are largely centered around green roofs, with a strong association to the fields of thermal and energy performance (Figure 5). Globally, leading researchers have been actively studying renewable energy systems [31,47], building energy efficiency [72,75], and thermal comfort [41,42] in relation to green roofs and rooftops as a way to mitigate Urban Heat Island (UHI) effects. Additionally, the Life Cycle Assessment (LCA) methodology, which evaluates the environmental impact of buildings over their life span, has been widely applied in this field [63,64]. Research has also been conducted on how building form and layout affect UHI [24,35].

It is noteworthy that GB research is already collaborating with various disciplines such as energy, heat, and environmental health. Rather than remaining confined to the domain of architecture or construction, high-impact researchers are continuously expanding their scope, pursuing interdisciplinary approaches and the development of applicable, forward-looking technologies.

A key observation in Cluster 2 is the presence of the keyword Existing Building Stock in relation to UGS. Most of the keywords in this cluster are associated with green-oriented environmental policies and ecological benefits, indicating that existing buildings may hold significant potential to be redefined or transformed into types of green infrastructure. In this context, Cluster 2 reflects a field toward policy-driven approaches to solving urban environmental issues through Nature-based Solutions (NbSs).

In summary, the scope of Cluster 1 suggests a technical and diverse research direction involving green building-based approaches such as sustainable heating, energy efficiency, and renewable energy integration. In contrast, Cluster 2 focuses on environmental planning and policies rooted in green spaces and nature-based resources. Achieving a carbon-neutral future, as emphasized in international discourse, will require a synergistic collaboration between eco-friendly technology development and environmental policy research.

5. Discussion

5.1. Research Trends of Both Green Buildings and Urban Green Space

The temporal distribution of publications revealed two notable inflection points in the years 2000 and 2015, where significant increases in publication frequency were observed (Figure 1). Prior to 2000, the number of publications related to GBs and UGS remained consistently low. Beginning in 2000, a gradual upward trend emerged. After 2015, the growth rate accelerated markedly, with a steep increase in annual publication counts. The years 2000 and 2015 correspond to major international agenda-setting events. Specifically, 2000 marked the announcement of the Millennium Development Goals (MDGs), and 2015 saw the declaration of several global frameworks, including the Sendai Framework for Disaster Risk Reduction (March), the Addis Ababa Action Agenda on financing for development (July), the adoption of the Sustainable Development Goals (SDGs) (September), and the Paris Agreement on climate change (December) [127,128]. The sharp increase in publications post-2015 suggests an alignment between global environmental policy initiatives and academic research interests. This is further reflected in the increasing relevance of GBs and UGS topics in relation to climate change adaptation, urban sustainability, and ecosystem services (Figure 5).

5.2. Possible Shortcut Towards a Carbon-Neutral Transition: Existing Building Stock

While the importance of GBs is rapidly gaining recognition, improving energy efficiency through the greening of the Existing Building Stock remains a critical challenge [129]. Although new construction accounts for only 1–1.5% of the building stock in major developed countries, the vast majority of the built environment is composed of existing buildings [74,130]. Transitioning approximately 20% of the existing building stock into green buildings by 2030 is considered one of the essential strategies for realizing a carbon-neutral society [131].

Existing Building Stock (EBS) accounts for a substantial portion of total energy consumption and represents the most cost-effective potential for energy savings [132,133,134,135]. Therefore, upgrading the energy performance of building stock to its maximum potential is imperative for carbon neutrality [74,132,136,137,138,139]. However, the diverse characteristics of individual buildings often make it difficult to uniformly apply green building standards. This discourages developers and slows the transition process, making it difficult to achieve theoretical goals in practice [140,141,142].

Although investments to improve energy efficiency in EBS are increasing, their direct impact on energy use and intensity is not yet clearly evident [143]. Furthermore, various barriers hinder EBS from obtaining green building certifications, such as disparities in governmental regulations across countries, lack of market support for the profitability of construction firms, challenges in meeting certification standards, and difficulties in promoting interdepartmental collaboration [142]. International discourse must therefore address the need for flexible regulations and standards that consider building age, materials, and structure, raise awareness in the construction industry, and develop strategies to enhance cooperation among relevant departments.

5.3. Discovering a New Type of Urban Green Space: The Evolving Green Building

Although approximately 75% of the urban infrastructure expected to exist by 2050 has not yet been built [142], both UGS and GBs are critical components in forecasting carbon-neutral pathways for future cities [96,144]. Proper and harmonious spatial integration of UGS and GBs can structure livable urban environments that support citizens’ well-being [143].

From a carbon cycle perspective, UGS contributes to carbon sequestration through NbSs, whereas GBs play a role in carbon emissions reduction through engineered technologies. Thus, both UGS and GBs serve essential, complementary functions in achieving carbon neutrality.

The fact that GBs contribute to greenhouse gas mitigation is now widely accepted. The greening of existing buildings enhances environmental quality within buildings and contributes to external urban spaces by creating greenery, regulating microclimates, and shaping urban wind corridors [145]. In particular, green roofs help moderate urban climates and mitigate UHI effects [146]. From a carbon-neutral perspective, GBs can be reinterpreted as a new type of UGS, suggesting a broader conceptual scope of UGS.

With the advancement of cutting-edge technologies, rooftops—accounting for approximately one-quarter of all urban surfaces—are increasingly being utilized for smart farming. These practices recycle waste heat and energy from buildings to power rooftop farms, reducing airborne carbon and contributing to UHI mitigation [147,148]. Furthermore, installing smart farms improves the thermal insulation of buildings, thus reducing their energy consumption. Rooftop farming represents an evolved, technologically integrated form of green roofing. This innovative use of rooftops can be viewed as a type of UGS that actively engages in the carbon cycle, rather than passively obstructing it [149].

6. Conclusions and Limitations

Carbon-neutral cities have long been envisioned as comfortable and safe urban spaces that address climate impacts while providing environmental, ecological, and cultural services to citizens. However, current urban environments are increasingly confronted by issues such as the UHI effect, which impedes the quality of urban life. The vicious carbon cycle occurring within cities poses a significant threat to the sustainability of urban societies. In response, various fields such as architecture, landscape architecture, and urban planning are making joint efforts to reduce carbon emissions by utilizing GBs and UGS.

This study utilized Scopus to search for literature using GB and UGS as keywords and analyzed the interrelationship between the two fields. The findings indicate that across both domains, a wide range of research methodologies—including theoretical reviews, simulations, experiments, and bibliometric analyses—are being employed to address carbon reduction, energy efficiency, thermal comfort, and UHI mitigation. Most of the studies analyzed in this research emphasize their contributions toward sustainable urban management and the creation of safe, comfortable environments for city dwellers.

Key research fields can be summarized as follows: studies related to GBs primarily focus on technological and experimental approaches concerning building energy performance to reduce carbon emissions and mitigate UHI. In contrast, UGS-related research is largely grounded in the concept of NbSs, focusing on the types and functions of carbon sinks, and is predominantly associated with urban environmental planning and policy.

A noteworthy finding is the evolving nature of GBs, which are increasingly being considered as part of a new type of UGS. While GBs and UGS traditionally occupy distinct academic domains, their shared goal of addressing the carbon cycle and climate change highlights their complementary roles. Particularly, existing building stock management and green roofs have emerged as critical components in the development of carbon-neutral cities. Technological and theoretical integration of GBs and UGS will be essential for effectively addressing future environmental challenges. Continued collaborative research in these fields is expected to play a vital role in enhancing urban sustainability.

This study has certain limitations. While keyword analysis was effective in identifying overarching research fields, the reliance on keywords poses constraints in fully representing the content of each study. During the keyword refinement process, some keywords intended by the original authors may have been altered, potentially affecting the accuracy of the analysis. Moreover, although this study employed quantitative methods, it did not incorporate comparisons with qualitative review articles. Thus, when the relationships between keywords are unclear or lack intersections, there is a risk of misinterpretation of the results. This study did not include time-series data collection. As a result, it has limitations in tracing the precise chronological evolution of research involving the two keywords, GB and UGS. Nevertheless, given that the primary objective of this study was to identify thematic fields and in the two research domains, the limitations are not deemed to significantly affect the validity of the findings. To gain a more detailed understanding of the research trajectory, future studies should incorporate datasets that reflect temporal dimensions and enable longitudinal analysis. Additionally, extending the keyword network to include mediating variables such as technology, health, and related interdisciplinary themes could further clarify and specify the scope of relevant research fields.

Based on the results, three directions for future research are proposed. Firstly, Spatial Integration Strategies for GBs and UGS: Further studies should investigate optimal spatial configurations and morphological arrangements of GBs and UGS that contribute to the mitigation of UHI effects in carbon-neutral urban development. Secondly, Quantification of Carbon Offsets through UGS: Research is needed to determine the appropriate scale of UGS required to offset carbon emissions from general buildings. This will require in-depth efforts to quantify both emissions and absorption levels, demanding substantial time and methodological precision. Finally, Promotion of Urban Rooftop Agriculture: With advancements in technology, rooftops—beyond serving as green roofs or rooftop gardens—are increasingly being used for smart farms as part of urban agriculture. Studies focused on activating these practices can offer valuable guidance for ensuring the future sustainability and management of cities.