1. Introduction
The combination of global climate change and increasing urbanization has worsened urban environmental problems and profoundly modified the composition and function of urban ecosystems [
1,
2,
3]. The heightened urban heat island phenomenon has resulted in frequent occurrences of extreme weather conditions [
4,
5,
6], increased intensity of land usage, and has negatively impacted the health and well-being of urban residents [
7,
8,
9].
As near-natural biological spaces inside cities, urban green spaces (UGSs) are strategically important for raising living standards in a society growing more urbanized by the day [
10,
11,
12]. They not only mitigate the urban heat island effect and offer social and psychological advantages but are also crucial for integrating urban growth with the urban ecological environment [
13,
14]. Urban environmental research places major emphasis on the influence and composition of ecosystem services provided by UGSs [
15]. Ecosystem services serve as a conduit via which humans can obtain a wide range of advantages from the ecosystem. The association between ecological supply and societal demand relates to the production and utilization of these services [
16,
17,
18]. At present, the supply and demand for environmental services play a crucial role in determining the progress of sustainable urban environmental development [
19,
20]. Effectively managing energy supply within the framework of sustainable ecosystem development reduces demand and ecological pressure, hence promoting urban ecosystem stability and mitigating the urban heat effect [
21,
22]. Moreover, multi-scale UGS studies can offer insights for future provincial and larger-scale UGS structural layout optimization, whereas study into the factors influencing UGS demand mostly focuses on environmental justice [
23] or the scale of specific cities [
24].
Several studies have examined the relationship between supply and demand for urban green spaces in regulating the thermal environment. These studies have found that changes in land use, impervious surfaces, and socio-ecological systems are strongly interconnected with the management of urban thermo-environmental conditions [
25,
26,
27]. While Land Surface Temperature (LST) is commonly used to measure the risk of urban thermal environments [
28,
29], it is necessary to conduct more comprehensive studies that take into account socio-ecological systems and other aspects in order to effectively determine the actual human needs for mitigating thermal environments. Although research on urban thermal environment control has mostly focused on urban green spaces (UGSs), it has also acknowledged that Green Infrastructure (GI) plays a vital part in providing urban ecosystem services [
30,
31]. For meso–macro-scale analysis of the urban thermal environment, UGS is more significant than GI and is essential in minimizing urban thermal effects [
32,
33].
Presently, the integration of supply and demand has emerged as a popular subject in ecosystem service research [
34]. Due to the intricate nature of urban environmental problems, the integration of ecosystem services’ supply and demand has emerged as a significant advancement in tackling urban thermal environmental challenges [
35]. Shen Sijian et al. conducted a study on the supply and demand of UGS for regulating the urban thermal environment. The researchers found that the assessment system for urban thermal environment management in UGS is constrained by data availability. They also identified geographical variations in the interaction between UGS and supply and demand. Measuring the amount of cooling effect’s supply and demand can help analyze how this effect is distributed in space and is an essential requirement for reaching equilibrium in urban spaces [
36]. Wilhelmi and Hayden’s high-temperature vulnerability theory posits that urban thermal environmental risk is influenced by three primary factors: exposure, sensitivity, and adaptive capacity. These factors are driven by a mix of climate change and socio-economic development processes [
37]. In addition, while allocating resources, it is important to take into account the requirements and preferences of various stakeholders. It should be emphasized that unequal distribution of environmental resources can result in substantial environmental and social injustice [
38]. This paper examines the demand for thermal environment regulation services among urban residents and the capacity of urban green spaces to provide such services. It identifies areas where there is an imbalance between supply and demand and uses the Priority Ranking Index (PRI) to guide urban green space planning. The goal is to regulate the impact of urban green spaces on the thermal environment and improve the provision of urban ecosystem services.
3. Results
3.1. Demand for Green Space Regulation’s Spatial Distribution
3.1.1. Spatial Distribution of Heat Exposure
The Heat Exposure Index (
) (0.00–1.00) for Chengdu is influenced by both population density and surface temperature inversion (see
Figure 3a). The spatial distribution of Chengdu’s Heat Exposure Index (
) closely mirrors its population density pattern. High-value
areas (
> 0.58), indicating greater thermal exposure include the Qingyang District, **jiang District, and the East Station area in Chenghua District, in total account for 3.78% of the metropolitan area. By contrast, low-value
zones (
< 0.06) are primarily located outside the Fifth Ring, representing 57.30% of the area. This indicates that Chengdu’s inner city experiences high population density, elevated surface temperatures, and significant urban heat exposure, with heat exposure decreasing outward from the Third Ring.
Chengdu’s population density has a tendency of being higher in the center and lower in the outskirts (see
Figure 3b). High-density areas (Dr > 13,744 persons/km
2), accounting for 4.32% of the city’s area, mainly include Qingyang District and **jiang District within the inner ring, the East Railway Station area in Chenghua District, and the North Railway Station area in **niu District. Conversely, low-density areas (Dr < 1433 persons/km
2) are mostly found beyond the Fifth Ring, accounting for 57.84% of the area.
Surface temperature inversion results reveal that Chengdu’s surface temperatures range from 41.10 °C to 46.58 °C, with an average of 44 °C (see
Figure 3c) The distribution of surface temperatures is characterized by lower temperatures in the north and south and higher temperatures in the central region. Areas with high surface temperatures (
T > 45.15 °C) are predominantly located within the Third Ring and in Qingbaijiang City, while low-temperature areas (
T < 42.48 °C), such as **ndu and the outskirts of **tang, cover 8.65% of the total area.
3.1.2. Spatial Distribution of Thermal Sensitivity
The thermal sensitivity (
) (0.00–1.00) results reveal that the distribution of Chengdu’s elderly population is predominantly concentrated in the city center and dispersed around the outskirts (see
Figure 4). Areas with high thermal sensitivity values (
> 0.36), accounting for about 3.78% of the region, include the Qingyang District within the inner ring, **jiang District within the inner ring, and the East Railway Station area in Chenghua District. By contrast, areas with low thermal sensitivity (
< 0.05), mainly located in the eastern and northwestern parts of Chengdu, comprise approximately 58.38% of the area. This pattern underscores that urban thermal sensitivity is elevated in central Chengdu and diminished in its eastern and northwestern sectors.
3.1.3. Spatial Distribution of Thermal Adaptation
The examination of thermal acclimatization demonstrates a spatial pattern that is defined by higher temperatures in the eastern region and lower temperatures in the central region, as shown in
Figure 5. Approximately 23.78% of the area has high thermal acclimation indices (
Ai > 0.85) (0.00–1.00). These regions include the Western Forest Park in Longquanyi District, Tianfu New District, Qingbaijiang Old Town, and the outskirts of Dujiangyan. By contrast, regions with low indices (
Ai < 0.11), accounting for 17.30% of the total area, include Fenghuang Mountain and the Botanical Gardens in **niu District, ** Temple in Wuhou District, Qingbaijiang District, the airport industrial park and South Lake in Shuangliu District, among others. These areas have a generally equal supply and demand of green spaces, requiring only suitable conservation measures. In contrast, Level III and Level IV areas, which necessitate immediate attention due to a significant disparity between the supply and demand of green spaces, are mainly located in the inner rings of Qingyang and **jiang. These areas also extend to Cuijadian, Caotang, and Donghu.
Figure 9b identifies these specific areas as the focus of regulatory initiatives aimed at addressing mismatch and improving urban livability.
4. Discussion
Assessing the balance between both the demand and supply of green spaces and illustrating their spatiotemporal variations have been the main objectives of this research. By doing so, we have provided guidance and direction for urban green space planning and management.
4.1. Indicators for Thermal Environment Regulation of Green Spaces
Thermal environment evaluation of urban human settlements involves an exhaustive list of factors including climate, social economy, urban land use, population demographics, among others. In most cases [
46,
47], researchers have solely focused on evaluating urban green spaces’ demand by incorporating diversified indicators from social, economic, and ecological disciplines. However, there is a paucity of research on the supply side of green spaces. This study aims to combine the demand and supply aspects of green spaces to construct a thermal environment regulation evaluation system suitable for urban settings, replacing the problems of using a single or limited number of evaluation indicators in classical research. Chengdu’s urban thermal condition is greatly influenced by the distribution of green spaces and population dynamics. In urban areas with high population density, inadequate green spaces hinder the efficient distribution of resources. Accordingly, the units classified as “insufficient supply and high demand” were divided into four priority intervention levels using the natural breakpoint technique, which combines the supply–demand relationship with the priority index. This approach specifically targets the areas where there is a mismatch between the supply and demand of green spaces. In this manner, the relevant departments can prioritize areas based on their deficiency level.
4.2. Characteristics of Green Space Regulation Areas
The green space regulation demand index Hi indicates that green space regulation demand in Chengdu follows a “regional diffusion and local concentration” spatial pattern. The overall demand index for managing the urban thermal environment follows a similar spatial pattern as heat exposure and heat sensitivity, with high-demand zones clustering in the city’s central area. Regions characterized by abundant supply are predominantly situated in sparsely populated areas, including the western section of Longquanyi District, Tianfu New District, and the outskirts of Dujiangyan. Conversely, districts located within the third ring road and heavily populated areas such as Qingbaijiang District exhibit a low supply capacity for green spaces. It has been reported that in urban central districts, concentrated development leads to the consumption of land resources, dense population, and industrial activities which could deprive the region of its green spaces [
48,
49]. In contrast, the outskirts of cities enjoy more green spaces, which need merely to be conserved. In specific functional areas such as airports and high-speed railway stations, intervention opportunities are limited, which aligns with the findings of this study. The ratio of green spaces to surface temperature is the primary factor influencing the uneven distribution of thermal environment regulation in Chengdu. To ensure the balance between green spaces’ supply and demand, it is necessary to consider reducing the inequality in supply levels in central urban districts. In light of this, different areas could be classified from I to IV based on their priority level. In this manner, the areas with the highest priority level (IV) were found to be located in strategic positions such as Caotang in Qingyang District, Cuijiadian in Chenghua District, and Donghu in **jiang District. These regions displayed significant disparities in supply and demand levels and therefore require immediate optimization and management. This can be achieved by increasing the per capita green space area through measures such as vertical greening and green infrastructure, which could improve thermal environment conditions in Chengdu’s central sections.
4.3. Limitations of the Study
Urban green spaces regulate the thermal climate, but different indicators may yield different outcomes. The pertinent indicators in this study are influenced by multiple factors, including residents’ income, cultural preferences, and occupation, which could potentially impact the demand of urban inhabitants for thermal environment regulation. This study considered only the proportion of urban green space area and did not take into account indicators such as the quality and usage rate of green spaces. This might have led to an incomplete understanding of the mechanisms by which urban green spaces regulate the thermal environment. In order to accurately analyze the supply–demand balance of green spaces and their potential in regulating the thermal environment in urban settings, it is necessary to consider the thermal environment regulation factors of each administrative unit and study how their location, type, function, and potential impact the urban thermal environment [
50,
51]. This study considered a grid of 185 units, which makes it difficult to analyze finer landscape features. Additionally, studying landscape patterns can help explain the thermal environment regulation function. By utilizing landscape geometry and distribution patterns, the optimal layout of urban green spaces can be identified. The evaluation system used in this paper mainly focuses on urban and district-county scales, and hence fails to differentiate supply demand levels under otherwise diverse urban settings. However, it is recommended to refine the scale of assessments to the block level in future investigations to accommodate more accurate multi-source data sets (such as heat-related medical and health data, small-scale social survey data, etc.) to improve the results applicability.
5. Conclusions
Supply and demand balance in urban green spaces serves as a vital metric for evaluating urban ecological development and the well-being of its residents. This study, based on the heat vulnerability analysis and latest scientific findings, established a framework for assessing thermal environment regulation by urban green spaces. For this purpose, a grid of 185 units was established. Along with analyzing demand and supply levels, the Z-score method and the priority index were utilized to evaluate their balance and identify intervention priority levels. Assessing the balance between both the demand and supply of green spaces and illustrating their spatiotemporal variations have enabled us to provide guidance for urban green space planning and management. As far as the demand for thermal environment regulation in Chengdu is concerned, it is driven by heat exposure, heat sensitivity, and heat adaptability. High-demand areas are mainly distributed in Qingyang District, **jiang District, and Chenghua District, where the pressure from thermal environmental stress is significantly high. As for the supply condition, there is a scarcity of high-supply areas. Given that high-supply districts such as Longquanyi District and Tianfu New Area are located on the outskirts of the Chengdu metropolitan area, we reached the conclusion that intense human activities are the main culprit for lower green space regulatory services inside the city. Regarding the supply–demand balance, 32 units (17.30%) were found to have supply shortage. This imbalance, which directly impacts the urban ecosystem, is mainly found in the central parts of Chengdu City. However, we believe that Chengdu enjoys a suitable supply–demand situation given the negligible number of units suffering an imbalance between supply and demand of green spaces’ regulatory services. In total, only two level four priority areas were identified requiring urgent intervention. These priority areas must be given special treatment in green space development, planning, and management when boosting urban well-being.