Impacts of urban shrinkage on the built environment and its environmental sustainability: an analytical review

The proliferation of case studies of shrinking cities in recent years has stimulated intense debate on the impacts of urban shrinkage. However, assessing the impacts of urban shrinkage from a comprehensive perspective could be more present. Also, there is a lake of analytical review of historical studies about the impacts of urban shrinkage. The built environment has different characteristics under different urban development patterns involving infrastructure, services, and social, economic, and structural factors, which provides a best practice for exploring the impacts of urban shrinkage. This study synthesizes the literature surrounding urban shrinkage and built environment changes, identifying that urban shrinkage notably affects the different components of the built environment and gives rise to four related environmental and sustainability impacts involving urban landscapes and structures, ecological sustainability, socioeconomic vitality, and residents’ perceptions. Furthermore, there are interactions between the environmental and sustainability impacts, involving trade-offs and synergies between residents’ perceptions, ecological sustainability, and socioeconomic vitality. The study also summarized the mainstream methods for assessing the impacts of urban shrinkage and explored the effects of urban shrinkage management strategies on improving the built environment. Finally, a framework for future direction is presented for the final to integrate the theories of urban shrinkage, people and land relationship, and sustainable urban development to guide further exploration in the field. In summary, this study implies that restoring and upgrading the built environment can pave the way for a common goal for long-term sustainable development. The value of this study is to provide relevant researchers with the knowledge to understand the developing frontiers of urban shrinkage impacts on built environments.


Introduction
Human activity has transformed the global natural landscape through the scale effects of population agglomeration, technological progress, and economic growth, commonly known as urbanization.Globally, 75% of the countries have experienced growth in urban population and spatial extent of the built environment (United Nations 2021).The urban population increased by nearly 2.4 billion between 1975 and 2015, meaning that the urban population rose from 38% to 54% of the global population (United Nations 2021).Moreover, it is reported that almost 70% of the world's population will be concentrated in cities by 2050 (The World Bank 2010).However, in the context of overall growth, many countries are already struggling with population decline (United Nations 2022).Regional development imbalance is increasingly prominent worldwide, resulting from the spatial movement of population and capital elements triggered by economic, institutional, and environmental changes (Li et al 2017).Some regions marginalized in the international division of labor and information exchange have lost numerous population and capital elements, described as shrinking cities (Chen et al 2021a).Growth and shrinkage represent the overall patterns of current urban development, and Herrmann et al (2016) argued that both urban growth and shrinkage face challenges in environmental and sustainability dimensions.
The built environment, a human-made space that integrates living, working, and recreating, includes the physical structures, supporting infrastructure, and designed and managed green and blue spaces for people to carry out these activities (Coleman 2017, Nielsen andFarrelly 2019).Some studies argued that from a system perspective, the built environment should also include various flows and scales that are interconnected or independent, including energy, water, waste, people, transportation, and policies, which inform and drive the material manifestation of the built environment (Ingmire and Joo Kim 2022).The built environment has density, diversity, and design attributes that are closely related to accessibility, walkability, thermal comfort, and residents' health (e.g.Yang et al 2017, Bivina et al 2020, Engineer et al 2021), providing the best practice for a comprehensive view of urban development dynamics and their sustainability.The intensity and scale of human activity promote the distribution and accumulation of materials in the built environment, changing it in terms of density, diversity, and design, resulting in a range of environmental and sustainability impacts (Bao et al 2023).Regarding density, a low-density built environment provides more spacious public spaces.However, the inefficient use of urban land is detrimental to enhancing public services and reduces the liveability of cities.A high-density built environment can effectively shorten commuting distances, reduce residents' dependence on cars through intensive mixed land use, and then prioritize the development of public transportation to save energy in cities (Zhao 2018).However, the ecological environment of cities will be destroyed when the density exceeds a certain threshold, which will be manifested as an increase in urban waste and pollution, energy consumption, emissions, the deterioration in air and water quality (Chokhachian et al 2020, Bliznina 2021).There is a non-linear linkage between the density of built environments and eco-environmental quality.Regarding diversity, mixed land-use types are conducive to improving the land-use efficiency of cities (Wang et al 2021).Moreover, vegetation conditions, impervious area scale, and the number of landscape patches in the built environment can have a significant impact on building energy consumption (Wang et al 2023a).Regarding design, the spatial condition presented by various objective physical elements in cities is expressed, such as the distribution and accessibility of facilities (Wang et al 2023b).Outdated infrastructure in the built environment, accompanied by difficulties in upgrading and maintenance, will affect the liveability of cities (Wang et al 2019).Designing green, resilient, equitable, and inclusive infrastructure can help to address environmental and sustainability issues in cities (e.g.Castaño-Rosa et al 2022).It should be noted that differential density, diversity, and design attributes of built environments are closely linked to the environmental and sustainability impacts of cities.In this light, restoring and upgrading built environments is a feasible path to address urban environmental and sustainability issues.
Urban shrinkage has become a universal phenomenon globally, with 37% of cities projected to experience shrinkage by 2050 (Zhai et al 2022).Urban shrinkage is a complex, dynamic non-linear feedback process involving the urban population (size, age, and employment), capital, resources, and public services (Wolff and Wiechmann 2017a, Lym 2021, Wichowska 2021).Many studies have begun to investigate the effects of the shrinking process compared to analyzing the phenomena and mechanisms of shrinkage (e.g.Guo and Li 2019).With population loss as the most visible manifestation, urban shrinkage is accompanied by a reduction in urban economic activity and tax revenues (Slach et al 2019, Kocur-Bera andSzuniewicz 2021), resulting in increased vacant buildings and open space, oversupply of infrastructure, and inefficient public services (Schilling and Logan 2008, Wang and Immergluck 2019, Usui and Perez 2022), which significantly change the urban built environment.Changes in the built environment will raise ecological and sustainability concerns, but this process is often overlooked in shrinking cities.Generally, less population means less land use, energy consumption and emissions, and more open space, potentially optimizing ecological conditions with less human disturbance (Haase et al 2014).Peng et al (2022) argued that weakened industrial production intensity and reduced road traffic flows could mitigate the heat island effect in shrinking cities.However, smaller and dispersed cities are associated with lower energy efficiency and higher pollution, offsetting any local gains (Liu and Sweeney 2012).There are two main reasons why it is challenging to clarify the impacts of urban shrinkage on the built environment and the environmental and sustainability issues associated with it.Firstly, the impacts of urban shrinkage are lagging (Li and Long 2019); secondly, changes in built environments are seen as one of the drivers of urban shrinkage in many studies (e.g.Khavarian-Garmsir et al 2019), which has led to the impacts of urban shrinkage on built environments being missed.Although opposed to growth, it cannot be denied that the built environment is mixed with the critical situation of the inhabitants in shrinking cities, i.e. their well-being, good health, and quality of life are directly related to the built environment.
Integrating socioeconomic and environmental issues is increasingly becoming a global consensus for promoting sustainable urban development.However, how does urban shrinkage affect the built environment?What environmental and sustainability issues are associated with this process?These questions have not been clarified in the current studies.A full understanding of the impacts of urban shrinkage on the built environment will contribute to addressing environmental and sustainability issues from the perspective of restoring and upgrading the built environment.In other words, high-quality built environment construction means reducing unnecessary waste, increasing energy and land use efficiency, and reducing greenhouse gas emissions, contributing to counteracting the unsustainability of shrinking cities, such as underutilized infrastructure, pollution legacies, social fragmentation, built legacy and abandonment (Herrmann et al 2016).Furthermore, a high-quality built environment can encourage residents to stay and even attract new settlers, reversing population decline and revitalizing local economies (He et al 2022).Besides, in today's world, unexpected surges in political, economic, ecological, and other events play a key role in intensifying the lasting impacts of urban shrinkage.For these reasons, shedding light on the impacts of urban shrinkage on built environments is more important now than ever.There are a few reviews on this topic, which hinder researchers and city managers from understanding the complex relationship between shrinkage and built environment changes and pose a barrier to the sustainable development of shrinking cities. Chen et al (2023) provided a review of the ecoenvironmental impacts of urban shrinkage, emphasizing the two sides of the eco-environmental impacts of urban shrinkage, as well as management strategies for shrinking cities in China.In contrast, our study emphasized the impact of urban shrinkage on the built environment and analyzed the related environmental and sustainability impacts.Our study argued that constructing a liveable built environment is the foundation of urban shrinkage management, which is as important as stimulating new economic activities in shrinking cities. Accordingly, a review of the impacts of urban shrinkage on the built environment and its associated environmental and sustainability effects is needed.
The review aims to clarify the impacts of urban shrinkage on the built environment and the environmental and sustainability issues associated with it, providing a reference for managing shrinking cities.In the following, we begin with a summary of research trends on shrinking cities. Next, the methodology of a critical literature review is described.Then, the literature review results are analyzed with a focus on 'urban shrinkage-built environment changes-environmental and sustainability impacts' .Finally, future research directions based on the built environment upgrading are proposed to promote the sustainability of shrinking cities.

Urban shrinkage: definitions and driving forces
Since German scholars, Häußermann and Siebel (1988) proposed the shrinking city concept in 1988 for the urban problems of the Ruhr region in East Germany, research on shrinking cities has been gradually enriched and become a mainstream topic in urban studies recently.The topic has also received attention from the fields of economics, ecological economics, and social sciences (e.g.Delken 2008, Schilling and Logan 2008, He et al 2017).However, due to the diversity of spatial and temporal scales, the concept of the shrinking city is still not unified.The Shrinking Cities International Research Network (SCIRN) ( 2004) considers a shrinking city to be an area of more than 10 000 inhabitants that has experienced population loss for more than two consecutive years due to a structural economic crisis (Wiechmann 2008, Lee et al 2018).In contrast, Oswalt and Rienets (2006) consider shrinking cities to have lost at least 10% of their inhabitants or to have an average annual population loss rate of more than 1%.In addition to these population-focused concepts, Martinez-Fernandez et al (2012) defined urban shrinkage from a multidimensional perspective, and they argued that a city, part of a city, an entire metropolitan area, or a small town that experiences a structural crisis characterized by social problems such as population loss, economic recession, and declining employment could be referred to as shrinking cities.These definitions of urban shrinkage involve different spatiotemporal scales, criteria, and essence of shrinkage.
The global distribution of shrinking cities is represented based on the study area extracted from the relevant literature on the topic of urban shrinkage (figure 1).Shrinking cities are concentrated in four regions globally, namely the northeastern United States, Europe, the Middle East, and East Asia dominated by China and Japan (figure 1).The spatiotemporal heterogeneity in geographic contexts and social development has led to complex driving mechanisms for shrinking cities.The driving factors of urban shrinkage can be classified into three categories, demographic shifts, economic shifts, and spatial shifts.In terms of demographic shifts, aging and population migration are the leading causes of population decline.The increase in the older population and population migration triggers urban shrinkage by reducing the labor force of cities.This urban shrinkage caused by aging and population migration occurred mainly in East Asia, dominated by Japan and some less developed regions of China (e.g.Wang and Fukuda 2019, Tong et al 2020, 2021, Iwasaki 2021).Economic shifts drive urban shrinkage in several ways, the most influential of which is globalization.The identifying results of shrinking cities are vulnerable to scale variability due to the diversity of definitions.Spatial units such as 'urban areas' , 'densely populated areas' , 'parts of cities' , 'entire metropolitan areas' or 'a small town' are used as basic spatial units to identify urban shrinkage at both administrative and physical city scales (Li et al 2017).Moreover, the time interval at which urban shrinkage is measured needs to be clarified.Population changes over a given period may not be consistent across different spatial scales, which leads to a blurring of the identification between shrinkage and growth at different spatial scales, making it difficult to classify and empirically test.This is already the case for demographic data, so data on labor markets, economic conditions, fiscal systems, and social structures are even more difficult to compare (Bernt 2016, Li et al 2017).Research on the drivers of shrinking cities, where phenomena and causes are intertwined in arbitrary combinations centered around population loss, has exacerbated the uncertainty about the concept of shrinkage and hindered progress in theoretical studies of urban shrinkage (Bernt 2016).Much research is beginning to focus on the processes and relationships of cities as opposed to the quantities and things of cities (e.g.Deng et al 2019, Wu and Yao 2021).Based on this, the focus of urban shrinkage research will shift from 'What are the causes and consequences of shrinkage' to 'How does shrinkage change our lives' (Bernt 2016), which will facilitate our understanding of the nature of urban shrinkage.

Literature analysis methodology
The literature review methodology aims to provide a full understanding of the current status, gaps, and future research directions for the effects of urban shrinkage on built environments and related environmental and sustainability impacts.The methodology of the literature review has four main steps.The first step is topic searching.The topic includes ('urban shrinkage' or 'shrinking city' or 'shrinking cities') and ('built environment' or 'living environment' or 'residential environment' or 'environment').This study identified 113 articles published in English journals during 2000-2022 through the Web of Science online database.The second step is a further screening based on article types.In this step, 106 blind-reviewed articles were selected.In the third step of further screening based on abstracts, 88 articles dealing with the environmental effects of shrinking cities were identified.For the final step, considering the availability of the articles, 83 articles were reviewed in full text (figure 2).
This study used Citespace software to construct knowledge maps of the effects of urban shrinkage on built environments and related environmental and sustainability impacts, which is based on theories and methods from applied mathematics, computer science, and graphics, with the advantages of being scientific, comprehensive, standardized, accurate and simple (Chen 2006).The knowledge maps in Citespace rely on time slices (Li et al 2021).The literature was analyzed in Citespace in terms of themes and annual publication numbers in a 1 year time slice to explore advanced knowledge and trends about the impacts of urban shrinkage on built environments.

Classification of research category
Based on the selected literature, a network in the research category was constructed using the cooccurrence analysis and cluster analysis in Citespace, as displayed in figure 3.In figure 3, the nodes and the links between the nodes of the network are visualized.Specifically, the larger the font of a node, the greater the amount of literature involved in the category, and the thicker the line between nodes indicates a stronger link between the two categories (Li et al 2021).In general, the literature related to the impacts of urban shrinkage on built environments is concentrated in the fields of environmental science and ecology, followed by urban studies, social sciences, environmental studies, region and urban planning, public administration, science, and technology, which indicates that various categories contribute to the research progress in this field.

Classification of publication year
The 83 papers reviewed for this study were published between 2008 and 2022.Only since 2008 that the number of articles related to the urban shrinkage impacts on built environments has gradually increased.The selected articles were published in 2019, 2020, 2021, and 2022, with more than ten articles in each (figure 4).
The number of publications reveals two distinct stages.In 2008-2015, the number of publications was only 13.25%, and up to three papers were published in any year.During 2008-2015, issues such as urban wilderness (Rink 2009), vacant land (Schilling and Logan 2008, Couch and Cocks 2013), infrastructural dereliction (Naumann and Bernt 2009), and the happiness of the residents (Delken 2008) were noted.This phase is not rich in quantity or contents of studies and is an exploratory stage to investigate the effects of urban shrinkage on built environments and related environmental and sustainability impacts.Since 2015, the studies have reached a 'rise' stage, with no less than five papers published yearly.And the

Effects of urban shrinkage on built environments and the related environmental and sustainability impacts on cities
Towards a clear overview of the effects of urban shrinkage on the built environment and the related environmental and sustainability impacts, the built environment is categorized into four systems in this review, including shelter systems (buildings), life-support systems (utilities such as energy and water supply), movement systems (transportation infrastructures), and open-space systems (parks and recreational-use utilities) according to Al-Humaiqani and Al-Ghamdi (2022).By analyzing the impacts of urban shrinkage on these groups, four types of environmental and sustainability issues were identified related to urban landscape and structure, ecological sustainability, socioeconomic vitality, and residents' perceptions (figure 5).The literature review will focus on these four areas.

Impacts on urban landscape and structure
Density links urban shrinkage to the built environment, as decreasing densities of urban development factors such as population and economy lead to changes in the built environment due to less demand (Wolff et al 2017b).In shrinking cities, 'dedensification' causes an oversupply of housing and infrastructure, which changes the shelter system of Vacant buildings that lack maintenance and repair over long periods are often demolished, giving rise to large amounts of vacant lots in shrinking cities, which resulted in fragmented urban landscapes and large amounts of marginal land (land that is difficult to cultivate using industrial agricultural techniques) (Schwarz et al 2016).Furthermore, Usui and Perez (2022) found that once vacant land accumulates randomly in urban areas, the process tends to continue, shifting urban vacant land from random to cluster generation, which can lead to severe urban perforation.
In shrinking cities, the rate of structural loss tends to be higher than the rate of replacement

Impacts on the ecological sustainability
Urban shrinkage changes the life support systems of built environments, which is closely related to urban ecological sustainability.Typically, shrinking cities consume less energy but without any improvements in energy efficiency (Zeng et al 2022).Lower density patterns of population and economic activities are linked to higher work-housing imbalances and longer commutes, increasing transport energy consumption (Liu et al 2020a).Furthermore, infrastructure such as pipelines, electricity grids, and central heating are overcapacity and inefficient in shrinking cities (Moss 2008).In East Germany, low-income households tend to minimize housing costs and occupy the least energy-efficient houses (old and unrenovated housing), compared to high-income households who tend to choose energy-efficient housing (Großmann et al 2014).In Northern Europe, vacant housing is likely to continue to use energy.In the case of Finland, for example, multifamily buildings use central heating systems, which means that adequate heating must Pollution is another considerable aspect of ecological sustainability associated with life support systems of built environments.In shrinking cities, lower demand and utilization rates lead to water remaining in the network for longer, increasing the risk of bacterial blooms and microbial contamination (Naumann and Bernt 2009).A study of shrinking cities in the US showed that unused impervious surfaces allowed rainfall to enter the sewer system through surface runoff, leading to sewage overflows and deterioration of surface water quality (Faust et al 2016).Another investigation showed that declining quality of service made residents of shrinking cities more inclined to use bottled water, which further led to underutilization of water facilities and exacerbated water quality problems in the USA (Yang and Faust 2019).
Pollution issues are also related to shelter systems of the built environment under the influence of urban shrinkage.Urban brownfield sites, represented by large amounts of abandoned buildings, factories, or entire areas, experience the highest soil lead burdens because of the large amounts of lead-based materials used during past industrial development (Schwarz et al 2016).Sivakoff et al (2020) found that industrial abandonment in shrinking cities has led to elevated levels of heavy metals in topsoil in the postindustrial era, which may negatively impact honeybee populations.Besides, Qiang et al (2021) suggested that air pollution levels increase significantly with the growth of built-up areas in some shrinking cities in China.Overall, the current study provides a preliminary exploration of the pollution effects of urban shrinkage, showing that the pollution effects are associated with the changes in life support systems and shelter systems in the process of urban shrinkage.However, the mechanisms of urban shrinkage on pollution need to be further analyzed, and focusing on changes in the built environment in the process of urban shrinkage may provide a potential way to investigate.
The absorptive capacity of the ecological environment itself also has an essential effect on ecological sustainability.Green spaces, woodlands, grasslands, wetlands, rivers, and lakes in cities play a role in the retention, adsorption, precipitation, and dilution of pollutants, reflecting the absorptive capacity of ecological environments (Tietenberg and Lewis 2011).Liu et al (2022) showed that the increase in open spaces, such as green park spaces, mitigated haze pollution in shrinking cities.From an environmental absorption perspective, a reduction in the scale of industrialization and economic activity would contribute to recovering the urban open space system.However, this process is not yet fully understood in shrinking cities.

Impacts on the socioeconomic vitality
In the process of urban shrinkage, changes in the built environment significantly affect the socioeconomic vitality of cities. Socioeconomic vitality refers to the intensity of socioeconomic activity and is usually expressed in terms of the abundance, mobility and activity of the population, enterprises, and capital (Xia et al 2020, Wang et al 2021).In shrinking cities, housing vacancies will lead to the devaluation of real estate (collateral), resulting in the paralysis of economic activity and social segregation in cities (Qin et al 2020, Kiviaho and Toivonen 2023).Typically, resource-based shrinking cities trigger the disintegration of agglomeration economies, with the most considerable negative impact being the rapid depreciation of residential properties due to reduced demand (Saraiva et al 2017, Kiviaho and Toivonen 2023), which puts governments under fiscal pressure as tax revenues decrease.Slow upgrading of infrastructures and public services reduces the attractiveness of the residents and is detrimental to population agglomeration and economic development in shrinking cities.Some cities have attempted to mitigate the decline in socioeconomic vitality by upgrading infrastructure and public services to attract development factors such as population and capital, but this is challenging.Population loss increases the cost and risk of urban infrastructure construction spread over individual enterprises, leading enterprises to relocate to areas with more optimistic development prospects, which exacerbates the socioeconomic development decline in shrinking cities (Zhong et al 2023).In another way, the young generation and educated people in shrinking cities are more likely to leave for better living conditions (Elzerman and Bontje 2015), accelerating the ageing process (Lauf et al 2016).The growth of the urban open space system in shrinking cities facilitates green infrastructure construction (Lima et al 2020), alleviating the environmental pressure on the elderly.Based on this advantage, shrinking cities can potentially develop the elderly care industry for socioeconomic vitality (Nefs et al 2013).From a regional perspective, shrinking cities face significant external challenges.The development of transportation infrastructure can exacerbate the socioeconomic vitality of shrinking cities through a siphoning effect that promotes the flow of urban development factors to developed areas (Li and Chen 2023).The depressed built environment also increases the cost of aggregating socioeconomic factors, which makes it difficult for shrinking cities to attract investment.Wu and Yao (2021) argued that shrinkage affects opportunities for cities to access external investment, although this effect is small.In Northeast China, Gao et al (2022) suggested that an increase in GDP per capita, fiscal expenditure, and upgrading of industrial structure in neighboring cities would weaken the productivity of shrinking cities, indicating a low level of external synergies for shrinking cities at regional scales.

Impacts on the perception of residents
The impacts of urban shrinkage on the built environment can also be reflected in residents' perceptions.Low density and diversity of built environments means less human interaction with the environment, but people are more sensitive to changes in built environments of shrinking cities.Some studies have shown that residents of shrinking cities express concerns about their safety and health.Based on the broken window effect, vacant buildings in shelter systems often serve as harbors for looters and criminals (Hollander 2012, Frazier et al 2013, Ryberg-Webster 2016), reducing the sense of security of urban residents.Some urban open spaces provide a place for illegal dumping and can also support unwanted animals, such as disease vectors, with health consequences for urban residents (Gulachenski et al 2016).In addition, some of the impermeable surfaces left behind after demolition raised surface temperatures and affected the thermal comfort of the shrinking cities. Emmanuel and Krüger (2012) argued that the heat island effect continues in some shrinking cities even if the population decreases.Jang and Kim (2021) evaluated the demolition strategy in Daegu, South Korea, where they concluded that the demolition strategy without greenery raised surface temperatures because the open areas without shade were more extensive, creating heat stress zones around open spaces.Additionally, the cost of adapting to climate change increases as reduced tax revenues discourage urban infrastructure renewal in shrinking cities.In Japan, where some cities are expected to shrink by more than 10%, the lack of green space is detrimental to climate change mitigation as declining tax revenues prevent the development of urban parks (Rupprecht 2017).Moreover, numerous residents in shrinking cities are unwilling to pay higher fees for utility service reliability due to the high percentage of residents who are poor (Faust et al 2018).
At the community scale, the perforated, fragmented urban fabric hinders neighborhood connections, which increases community segregation and class divisions, and also leads to inequalities in public service accessibility in shrinking cities (Lima et al 2020, Chae et al 2021), further promoting population loss and triggering a vicious cycle of shrinkage.Lima et al (2020) found that depopulated communities have a higher awareness and demand for outdoor green spaces to gain a sense of social connection.Schetke and Haase (2008) argued that demolition provides residents of Leipzig with huge open lawns and comfortable backyards.It is, therefore, necessary to consider the perceptions of residents and assess the impacts of urban shrinkage on the built environment, which will facilitate the reorganization of urban green space to create a positive built environment.

How to evaluate the impacts of urban shrinkage on built environments and the related environmental and sustainability impacts of cities? 4.5.1. Qualitative methods
Qualitative methods were first applied to analyze the effects of urban shrinkage on the built environment, with interviews and documentation being the most used.Through interviews, the opinions of city managers, residents, and service providers were fully gathered (Schetke and Haase 2008, Naumann and Bernt 2009, Hollander 2012).The documentation approach focused on shrinking cities with a specific development context, such as cities with a historical heritage (Ryberg-Webster 2016) and cities that are expected to attract retirees (Nefs et al 2013), and it assessed the environmental impacts of shrinking cities through extensive documentation and aimed to explore a liveable built environment for shrinking cities (table 1).
The assessment results on the built environment changes of urban shrinkage based on qualitative methods have vivid and specific characteristics, but they cannot be used to infer environmental effects in general and can only represent the environmental effects in some specific shrinking cities.In addition, the results of qualitative assessments are often influenced by subjectivity.

Experiment
During the past decade, several researchers have found the effect of urban shrinkage on the built environment accompanied by pollution effects through experiments.Schwarz et al (2016) collected soils in Sacramento, tested them for lead, and concluded that abandoned industrial facilities had triggered significant soil lead pollution.Sivakoff et al (2020) examined lead levels in bumblebees.They found that industrial abandonment in shrinking cities has led to soil lead pollution, which is a major challenge for bumblebee population conservation.However, current studies have not involved the detection of pollutants other than lead (table 1).Based on experiments, studies have confirmed that pollutants such as organic matter, toxic components, and particulate matter are produced during building demolition (Chen et al 2021b), but this fact has not yet been focused on in the context of large-scale demolition activities in shrinking cities.  1).The spatial analysis methods performed a large-scale assessment of environmental effects, but they mainly present general information, as the spatial heterogeneity of shrinkage is not considered.

Modeling
Modeling is an advanced method to evaluate the changes in the built environment of shrinking cities. ENVI-met is a computational fluid dynamic modeling software that can simulate the effects of building materials, meteorological conditions, and urban morphology on climate change in a threedimensional environment, which is widely applied in environmental planning (Bruse and Fleer 1998).The basic processes of ENVI-met include three steps: (1) model setup, (2) model verification, and (3) scenario simulation.Jang and Kim (2021) used the software to simulate the effects of different demolition strategies on thermal comfort in shrinking cities.
The integration of a system dynamics model (SD), a cellular automata model (CA), and an agent-based model (ABM) were utilized to simulate landscapes

Managing the urban shrinkage to improve built environments
Built environments without management will feed back into the shrinking process, potentially putting cities in a vicious cycle of shrinkage.The effects of urban shrinkage described above suggest that cities need to be re-planned to accommodate a declining population, i.e. growth-oriented urban planning should gradually shift towards de-growth.However, in some shrinking cities, there is a high overlap of areas with low building quality, a high proportion of elderly households, and high energy consumption, suggesting that reducing overall land use and resource consumption does not create a sustainable future for shrinking cities (Weinsziehr et al 2017).Some managing strategies cantered on built environment improvement are being implemented in shrinking cities, with the primary aim of preventing further degradation of built environments.
According to the improvement in the built environment, managing strategies in shrinking cities can be classified into two categories: adaptation-oriented strategies and transformation-oriented strategies.Adaptation-oriented governance strategies include right-sizing strategies (Schilling and Logan 2008), decline-oriented strategies (Jang and Kim 2021), green-oriented strategies (Asl 2022), strategies of green infrastructure expansion (Lima et al 2020), strategies for smart decline (Frazier and Bagchi-Sen 2015), strategies beyond growth (Segers et al 2020), etc. (table 2).These strategies emphasize the longterm depopulation trend, and the urban space is reconfigured through the demolition and greening of vacant spaces.Specifically, vacant lots and brownfields are designed as urban squares, gardens, and urban eco-agricultural areas, which play an essential role in increasing landscape connectivity in shrinking cities, mitigating exposure to heavy metals in soils, and reducing the extent of flood-prone areas while providing ecosystem services such as food production, recreation, and biodiversity (Schilling and Logan 2008, Németh and Hollander 2016, Schwarz et al 2016).However, the urban redevelopment strategy in Cleveland, USA, showed that large-scale demolitions and green schemes in the area had increased segregation and left residents in a poor environment as the government withdrew from urban forestry and side yard programs (Akers et al 2020).Another category of transformationoriented urban management strategies includes the comprehensive blight strategy plan (Newman et al 2020), Green growth strategies (Ortiz-Moya 2020), and 'Slim-Recover-Strengthen' strategy (Zhang et al 2021) (table 2).These strategies not only focus on the multi-level restructuring of urban space but also explore new opportunities for urban development.Based on Green growth strategies, Kitakyushu, Japan, has been changed into a city at the frontier of fighting global warming and using new clean energy sources, even though it was one of the most polluted shrinking cities in the past (Ortiz-Moya 2020).A series of measures that emphasize prevention, counteraction, and deterrence of urban shrinkage are designed into Youngstown's land use zoning strategy, which can achieve an increase in open space, the reuse of vacant buildings, and an increase in employment opportunities in the city (Newman et al 2020).

Region
Strategies Environmental improvements Sources Daegu city, South Korea Decline-oriented strategies: (1) demolition strategy of abandoned houses: (2) demolition and greening the site strategy: (3) small-scale urban renewal strategy The demolition strategy without greening raises the surface temperature; the greening strategy partially offsets the negative effects of the demolition strategy by providing a thermally comfortable environment; the small-scale urban renewal strategy improves the thermal environment and provides a potentially cooler area for pedestrians and residents.

Lisbon, Portugal
Strategies of green infrastructure expansion: (1) squares and small gardens (2) private and semi-private lots (3) big public park There is no denying the importance of large public parks as a strategy of green infrastructure in shrinking, but urban citizens living in depopulated communities are more aware of the importance of small, enclosed outdoor green spaces to enhance a sense of social connection.(1) prevention: giving higher development potential to permanent sites through a series of multifunctional plans (2) counteraction: integrating vibrant small functional sites into large permanent sites to halt the current trend of decline (3) deterrence: balancing the number of temporary and permanent sites to minimize the impacts of urban decay on the target area.(4) allowance: respond to community needs by adding green infrastructure to improve the landscape in decaying areas An average of around 72% of unused land per site will be regenerated, with permanent and temporary sites accounting for 53% and 47% respectively.The total area of new open space will exceed 12.5 hm 2 .59% of existing derelict buildings will be reused and more than 3600 new jobs will be created.
Newman et al (2020) (Continued.)The 'Slim-Recover-Strengthen' strategy attempts to achieve the redevelopment of resource-depleted shrinking cities in China, and Hegang provides a best-practice platform (Zhang et al 2021).Both adaptation-oriented and transformationoriented urban shrinkage management strategies are based on creating sustainable built environments to advance a sustainable future.However, the transformation-oriented management strategy seems to have long-term effects and greater stability, as it creates new development opportunities for shrinking cities, which may form a virtuous circle, contributing to spatial optimization, climate change adaptation and mitigation, and pollution management in shrinking cities through technological solutions.

Discussion
This study seeks to review the literature surrounding shrinking cities and the built environment, intending to provide an understanding of how urban shrinkage affects the built environment and gives rise to a range of associated environmental and sustainability impacts.This review suggests that urban shrinkage changes the shelter systems, life support systems, transportation systems, and open space systems of the built environment, resulting in a series of environmental and sustainability issues in four aspects: urban landscape and structure, urban ecological sustainability, socioeconomic vitality, and residents' perceptions.This study not only provides policy implications for the management of shrinking cities but also proposes future directions to contribute to the research progress on the impacts of urban shrinkage.

Policy implication
This review indicated that restoring and upgrading the built environment can pave the way for addressing the environmental and sustainability impacts of shrinking cities.Therefore, regardless of the spatiotemporal heterogeneity and formation mechanisms of shrinking cities, creating a sustainable built environment can be a common goal for long-term development.
In the process of urban shrinkage, decomposing the changes in the built environment into different components and taking corresponding measures are conducive to improving the efficiency of built environment restoration and upgrading.The shelter system of the built environment and the open space system are closely related, i.e. the demolition of the shelter system will increase the scale of the open space system.In depopulated conditions, vacant buildings generated randomly should be maintained in time to be put back on the market.Government-led demolition measures should focus on areas where vacancies occur at scale, which will help to reduce urban perforation and landscape fragmentation.Urban vacant lots can be reorganized into plazas and parks to provide rest and recreation for citizens through green infrastructure development (e.g.Pallagst et al 2017).In addition, vacant plots should be utilized to minimize the irregularity and fragmentation of the plots (Song et al 2020).For the life support and movement systems of the built environment, streamlining redundant infrastructures to improve efficiency in water supply, power supply, and transportation and mobility is conducive to enhancing urban ecological sustainability and public service accessibility.For example, in cities with a high proportion of elderly people, prioritizing public transportation can provide convenience for the elderly (Buhnik 2017).In conclusion, this study argues that constructing an environmentalfriendly and sustainable built environment is the foundation of urban shrinkage management.On the other hand, a high-quality built environment can provide continuous and stable support for new economic activities (reducing the cost of economic factor agglomeration), although this is not the focus of this study.
Urban shrinkage changes the built environment of cities, and there are interactions between the related environmental and sustainability issues.Ecological sustainability is closely linked to socioeconomic vitality in shrinking cities.The loss of urban development factors such as population and capital make it challenging for shrinking cities to supply labor, accumulate social capital, and have low technological innovation capacity (Jiang et al 2022).The economic development of some shrinking cities with low resource efficiency and high carbon emission intensity impacts the quality of ecological sustainability.Some measures to stimulate socioeconomic growth through urban land expansion ignore the efficiency of resource allocation (Li et al 2022), which is not conducive to creating a liveable built environment.Only focusing on strengthening the socioeconomic vitality of shrinking cities may deteriorate ecological sustainability (Sun and Zhou 2023).From an efficiency perspective, higher value outputs with fewer resource inputs and less environmental pollution are essential in shrinking cities (Sun and Zhou 2023), as shrinking cities face more problems of reduced human resources, capital, and natural resource.Besides, increased open space in shrinking cities is beneficial for ecological conservation, yet much more open space may inhibit the social networks of city dwellers, involving an Ecology and Sociology trade-off (Riley et al 2018); the improvement of ecological sustainability is also conducive to the socioeconomic vitality in shrinking cities (Nefs et al 2013).Therefore, developing a liveable built environment in shrinking cities is more challenging.It can provide a possible path for developing liveable built environments in shrinking cities by assessing the effect of shrinkage on the built environment, then clarifying the law of the impacts of shrinkage on the built environment, and finally adopting differentiated measures to promote the harmony of the built environment.

Future research
Under the impacts of urban shrinkage, when does the built environment start to change?In which areas does it start to change?Does the change cause negative or positive environmental and sustainability effects?And how to assess the quality of the built environment in shrinking cities?These questions have not been answered.Although much research has already noted that built environments changes in shrinking cities, the mechanisms by which urban shrinkage affects the built environments are unclear.This study proposes the following three research directions in response to these questions.

Mechanisms of urban shrinkage impacts on built environments
A recent study showed that the externality of housing vacancy was observed after three years, while within one or two years, the externality of vacancy was not detected in Japan (Suzuki et al 2022).A study for Qiqihar, China, suggests that the built environment of cities does not deteriorate in the early stages of urban shrinkage, but the capital investment to improve and maintain spatial quality will decrease as population and capital elements escape further, which allows urban spaces decaying (Li and Long 2019).The impacts of urban shrinkage on the built environment have a lag in terms of urban forms and structures, i.e. changes in the urban landscape and structures are not parallel to the decrease of population and capital elements (e.g.Saraiva et al 2017, Luo and Wang 2021).In temporal terms, the impacts of urban shrinkage on the built environment can be concluded as an implicit accumulation phase and an explicitly visible phase, the duration of both phases is likely related to the shrinking intensity.In the implicit accumulation phase, the impacts of shrinkage on the built environment are attending in a 'point' pattern, with a weak radiative capacity to the surrounding area.As the density of urban development elements decreases further, the changes in the built environment gradually spread to the surroundings and become visible.Most studies have focused on the visible phase of built environment change, with an emphasis on restructuring the built environment to match the declining population density in shrinking cities (Schilling and Logan 2008, Jang and Kim 2021).It is possible to effectively link the non-linear process of urban shrinkage and built environment change by treating the density and the cumulative process of built environmental changes as a function of time.Identifying the time threshold t 0 for changes in the built environment from invisible accumulation to visibility is essential, which is reflected in two aspects: 1.The population density corresponding to t 0 is the lowest limit to prevent the deterioration of the built environment from becoming visible; 2. t 0 is the best time to restructure the built environment (intervention) from a cost-saving point of view.Generally, shrinking areas are often the source of the built environment change.Going beyond the administrative city scale and identifying the location and extent of shrinking areas at a finer granularity will help to quantify the dynamic processes of shrinkage and cumulative built environment changes using time series data, which will contribute to clarifying the relationship between the two processes.Furthermore, it is possible to reveal the impacts of urban shrinkage on built environments by identifying the mediating factors and threshold factors between shrinkage and changes in the built environment using spatial measurement theory, which can pave the way for shedding light on the mechanisms of urban shrinkage on built environments (figure 6).

Built environment quality assessment in shrinking cities
There is a lack of research on assessing the built environment quality in shrinking cities.Although He et al (2022) assessed the liveability of resourcebased shrinking cities in China, mainly considering residents' willingness to move and possibly ignoring some objective environmental changes.The traditional evaluation criteria of built environments do not apply to shrinking cities, e.g.many studies suggest that the lower the population density of cities, the better the urban living environments, but the matching relationship between population density, public services, and infrastructure cannot be ignored in shrinking cities.Therefore, developing a comprehensive evaluation system from three perspectives, including efficiency, ecology, and perception, can systematically assess the quality of the built environment in shrinking cities. Regarding efficiency, the land use mix needs special consideration in shrinking cities, where a reduced land use mix indicates a less efficient use of land in cities associated with vacant housing and a perforated urban structure.In addition, energy and resource efficiency and public service accessibility should also be given attention.Regarding ecological sustainability, thermal environment, air quality, and greenness should be considered in shrinking cities.Finally, the built environment quality assessment of shrinking cities should focus on residents' subjective perceptions regarding health, safety, and comfort (figure 6).

Restoration and upgrading of the built environment in shrinking cities
Integrating the influencing mechanism of urban shrinkage on the built environment and the results from the quality assessment of the built environment in shrinking cities can help to differentiate the management of shrinking cities in different regions.Built environment restoration is centered on increasing densities.Two ideas for built environment restoration can be adopted based on the perspective of increasing the density of the population and other development factors.For shrinking cities where the environmental effects accumulate implicitly, measures to attract people and capital can be implemented.For shrinking cities where environmental effects are visible, measures to the compact built environment can be implemented.In addition, simulation studies on the built environments of shrinking cities incorporating multi-sourced data and multiple models can support the restoration, optimization, and upgrading of built environments (figure 6).

Limitations
There are two limitations to this research.Firstly, synonyms of 'urban shrinkage' , such as 'declining population and economy' and 'population loss' , were not considered in this study, which may leave some literature out.Future research will expand the scope of the literature search by adding keywords to comprehensively summarize the environmental effects of shrinking cities in a broad sense.Secondly, this study focuses on the general effects of shrinkage on built environments without identifying the specific impacts of different types of shrinkage on built environments, and future research will focus on the effects of different types of shrinking cities on the built environment and related environmental and sustainability issues.

Conclusion
How urban shrinkage affects built environments is gradually becoming an emerging hot topic.Based on the historical explorations, this article summarized the existing knowledge, research gaps, and future research directions concerning the impacts of urban shrinkage on built environments.Relevant literature on the topic and year of publication is also analyzed by Citespace, which suggested that interest in this field has increased after 2015.Based on a full-text review of 83 articles, this research found that urban shrinkage affects the shelter systems, life support systems, movement systems, and open space systems of the built environment and gives rise to four related environmental and sustainability issues involving urban landscapes and structures, ecological sustainability, socioeconomic vitality, and perceptions of residents.Finally, a framework for future research is proposed to integrate the theories of urban shrinkage, people and land relationship, and sustainable urban development and guide further exploration in the field.
Under the influence of urban shrinkage, one aspect of change in the built environment is generally associated with multiple environmental and sustainability impacts.Urban shrinkage increases the number of vacant buildings and lots, changing the landscape and structure of cities. Demolition of vacant structures provides more open space in cities, which facilitates the development of green infrastructure for environmental sustainability and socioeconomic vitality of cities. Inefficient life support and movement systems in the built environment are detrimental to the socioeconomic vitality of the shrinking city while threatening the environmental sustainability of shrinking cities through increased energy and resource consumption intensity.In addition, changes in the built environment in all four systems affect residents' perceptions that urban shrinkage puts residents in a state of health and safety concern.Restoring and upgrading the built environment can pave the way for addressing the environmental and sustainability impacts of shrinking cities.However, there are also interactions between the different aspects of the environmental and sustainability impacts, involving trade-offs and synergies between residents' perceptions, ecological sustainability, and socioeconomic vitality.Therefore, revealing the mechanisms of urban shrinkage on the built environments and developing a scientific and reasonable assessment system for the quality of built environments can contribute to a common goal for longterm sustainable development in shrinking cities.

Figure 1 .
Figure 1.Presence of shrinking cities as indicated in the existing literature.

Figure 2 .
Figure 2. Literature review methodology and resulting article count.

Figure 3 .
Figure 3.A network of web of science categories generated using citespace.

Figure 4 .
Figure 4. Number of published articles by year.
(Thompson and de Beurs 2018), which allows for the growth of urban open space systems in shrinking cities. Open spaces such as urban gardens and parks can enhance the quality of the urban green fabric (Németh and Hollander 2016, Pallagst et al 2017).From the perspective of urban ecology, open spaces have positive ecological effects, such as increasing the connectivity of the urban green fabric.Nevertheless, green is not the whole story, and some studies argue that the cost of green outweighs its own value, citing water use, especially in arid areas (Wolch et al 2014, Schwarz et al 2018).Moreover, some unmanaged open spaces often undergo natural succession, being covered by wild grassland, wild grass mixed with shrub and tree land, bare land, and so on (Song et al 2020), linked to a neglected state of overgrowth (Rink 2009), creating a fragmented urban wilderness.In the 1980s, the German Urban Habitat Survey classified wilderness as 'a king of damaging habitat' because it adds to urban biodiversity but can also produce pests (Rink 2009).
demolition of poorly located, low quality and expensive built space (2) rebuild open spaces Strategic shrinkage favors growth.Create the possibility of demolishing poorly located and low-quality built spaces or rebuilding open spaces, for example in flood-prone areas, for food production, recreation or biodiversity.Segers et al (2020

Figure 6 .
Figure 6.Framework for future research.

X
He et al be available regardless of the large number of vacant units (Huuhka 2016).Inefficient energy consumption means higher carbon emission intensity.At the urban scale, Xiao et al (2019) believed that the carbon emissions of shrinking cities decrease with the decline of the secondary sector in the northeast of China.However, Zeng et al (2022) demonstrated that carbon emissions efficiency decreases by approximately 2.15% with each 1% increase in urban shrinkage in Northeast China.Liu et al (2020a) concluded that shrinking cities are less energy efficient than growing cities.

Table 1 .
Methods for assessing the effects of urban shrinkage on the built environment and related environmental and sustainability impacts.
ory have been used to assess the landscape connectivity of shrinking cities, such as Fragstats and Conefor (e.g.Zhang et al 2019) (table

Table 2 .
Case studies on strategies for urban shrinkage management.