Cooling Effect of Urban Green Space: A Nature-Based Solution for Mitigation of Urban Heat

Abstract

Today, urban green space (UGS) is recognized as a nature-based solution to alleviate heat in urban environments by intensifying hot surfaces in urban areas. This chapter aims to provide a comprehensive understanding of the cooling effect of UGS, its characteristics, mechanisms, and their implementation in urban planning and design to combat urban warming concerns. This chapter aims to outline relevant contents in three sections: (1) Cooling effects of UGS: mechanisms and dimensions, which will be allocated to explore the diverse mechanisms that contribute to the cooling performance of UGS in urban areas regarding their physical and spatial features; (2) Quantification of the cooling effect of UGS: methods and implementations. This section will focus on recent quantitative methods and implementations at an urban scale to apply in urban planning and design processes; and (3) Planning and design of cooling cities: urban greening challenges and strategies will provide an overview of urban planning and design approaches, current challenges and recommending effective integrated solutions to improve cooling efficiency.

1. Introduction

Over the past few decades, urban livability has been increasingly affected by global warming (climate change) and rapid unplanned urbanization [1, 2]. According to the latest UN report, global population growth is expected to reach roughly 8.5 billion in 2030, 9.7 billion in 2050, and 10.4 billion in 2100. Currently, 4.7 billion people −56% of the world’s population—reside in urban areas. The trend is projected to continue until 2050, when the urban population is expected to grow to more than double its current size, indicating that nearly 7 out of 10 people will live in cities by then [3, 4]. The uneven patterns of population growth and urban sprawl are causing significant changes that negatively impact socioeconomic prosperity, environmental quality, and sustainable development on local, regional, and global scales. Environmental pollution, land use change, desertification, ecosystem degradation, landscape fragmentation, and biodiversity loss are some severe effects.

Besides these, our cities have experienced a significant shift towards impervious surfaces, dramatically impacting surface radiation, energy balance, and thermal behavior in urban environments. These alterations, along with the heat emissions caused by human activities and climate change, contribute to the phenomenon known as the urban heat island (UHI) effect, exacerbating the impact of heatwaves in urban areas [5, 6]. UHI and land surface temperature (LST) are the main drivers of further urban heat issues alongside global warming and climate change. These factors can amplify heatwaves, which can lead to adverse effects such as impairing air quality, increasing energy and water consumption, and declining health and well-being for people living in cities. Therefore, cities must adapt to the changes to ensure their residents remain safe and sustainable.

Numerous cities have made initiatives to enhance resilience and address the rising climate-related dangers posed to populated regions to achieve sustainable development to combat these threats. Some of the most prominent global and regional initiatives have successfully implemented various approaches and strategies to reduce the UHI effect. These include using cooling materials and colors [7], adopting densification in urban planning [8], establishing urban blue-green infrastructure [9], and incorporating green roofs and walls [10] to make cooler cities and promote greater climate change resilience. Although these well-known approaches and strategies have proven useful in providing initial direction, they have encountered several challenges that impede their practical application. For example, broad recommendations for developing urban greenery may be challenging, where most cases are located in arid and hot climates and face water-supplying limitations. In addition, adopting the strategy of increasing densification and high-rise buildings may result in the loss of open and green spaces, ultimately impacting the overall quality of urban life. Furthermore, the efficiency of cooling systems can only be stated as an average value rather than being tailored to a particular region or area. Even when studies identify a specific locality, models that may be easily duplicated in other locations are frequently lacking [11].

This chapter focuses on an emerging area of research that seeks to tackle the impact of extreme heat on urban environments through nature-based solutions (NBS). In this context, the primary objective of this chapter is to provide a comprehensive understanding of the cooling effect of UGS, mechanisms, and implementation in various intervention strategies based on the NBS approach. It aims to cope with the challenge of UHI by recommending an efficient strategy that can optimize the potential of UGS in city planning, design, and optimization, ultimately leading to cooler cities. This approach may include green infrastructure like green roofs, urban forests, or vegetated vertical surfaces. In addition, it involves optimizing existing UGS by utilizing NBSs to improve the built environment while preparing to plan for future projects. Overall, NBS stands for solutions inspired and supported by nature to create, manage, and restore adaptable and resilient natural and modified ecosystems with cost-effective results while simultaneously having co-benefits for local communities and nature [11].

Although this approach is still relatively new and primarily conceptual, it is gaining popularity among policymakers and practitioners for urban planning applications [12]. NBS has been successfully applied in several empirical cases to address various integrated methods for tackling significant environmental challenges and multiple societal issues [13, 14]. However, this chapter can be a valid source to synthesize insights from a growing body of literature and provide a holistic understanding of the UGS’s cooling effects on UHI mitigation. Moreover, this chapter advises urban planners and decision-makers on incorporating NBS into their policies and plans to promote urban resilience and mitigate the effects of urban heat emission more effectively through integrating UGS implementation strategies.

2. The cooling effect of UGS: mechanisms and dimensions

“Cooling effect” in contemporary urban research endeavors in energy, urban planning, and urban climate refers to many heat mitigation tools, procedures, and strategies that have been put forward to relieve the various kinds of urban heat effects in varying climatic settings. To date, these have included the use of albedo materials as well as the development of green space and management of blue bodies [15, 16]. However, UGS implementation has been widely acknowledged as a viable NBS approach for alleviating urban heatwaves, effectively lowering temperatures, and enhancing the urban microclimate in surrounding areas [17, 18]. Although there are several definitions and classifications of UGS in urban landscape research, the relevant literature includes extensive surveys of urban greenery in different types and scales [19]. UGSs are generally known as all kinds of urban vegetation that cover surfaces, provide solar protection, influence air circulation and heat exchange, and constantly absorb solar radiation to contribute to the cooling effect. Specifically, UGSs are also included as the following due to the necessity to define in this chapter: remaining farmland, urban parks, forests, gardens, green roofs and walls, street vegetation, and green courtyards. In an urban context, NBS specifically considers an interconnected and multifunctional network of such UGS typologies, commonly known as urban green infrastructure. This approach is acknowledged widely as one of the most effective strategies to reduce the adverse effects of urban warming, particularly UHI, by restricting the flow of hot air currents, evapotranspiration, and shading [20].

Overall, shadowing effects depend on additional factors like the solar position, urban 3D geometry, the orientation of the street, the height-to-width ratio of street “canyons”, and the amount of “sky view” that must be considered. Regarding urban vegetation, it is essential to note that the diverse angles of a tree experience differential exposure to light, resulting in some sides remaining relatively shaded. Due to the limited penetration of sunlight and restricted energy storage in the soil of these shaded regions, the temperature is noticeably lower. Consequently, shadowing can potentially contribute to a reduction in surface temperature within urban areas [21]. In recent decades, the concept of cooling induced by UGS in urban areas has been investigated extensively through many physical, bioclimatic, and physiological assessments [22]. Irrespective of their adopted methodology, these investigations have consistently demonstrated that generating a cooler environment through UGS is primarily associated with two interrelated aspects, namely, (a) plant shading and (b) the evapotranspiration process.

In addition to evapotranspiration facilitated by vegetation leaves, the shading mechanism generated by tree canopies essentially involves the creation of a barrier against radiative energy within the urban canyon layer, thereby reducing temperature [23]. Due to having a higher albedo and lower heat capacity in green leaves and canopies, shading is among plants’ most significant inherent characteristics, which are closely influenced by their type, species, shape, and size [24, 25]. In other words, any distinction in the characteristics of greenery in urban areas has an inevitable consequence on the surrounding temperature. Accordingly, it has been revealed that various planned and designed UGS, like urban parks and forests, exhibit better cooling performance than lawns, grasses, or shrubs.

Furthermore, the shad engendered by UGS is influenced by the structure, height, and density of the vegetation canopies (i.e., trees) and their relative proportion in built-up areas, which can lead to a cooler environment [26, 27]. In particular, a row of fully grown trees possessing appropriate canopy generates interconnected shaded areas, termed Shadeways, that operate as a green corridor, providing relatively higher amounts of natural shading [28]. Although not all scenarios specifically involving trees may lead to a considerable decrease in energy loads, vitality and leaf temperature are crucial variables that have a bearing on the ability of trees to elicit cooling effects. It has been empirically validated that there are notable variations in the thermal performance of diverse species of urban vegetation surfaces regarding their ability to assimilate radiative energy into latent heat generation and its subsequent conversion into convective heat [29]. Also, the cooling potential of trees is subject to fluctuation based on their distinguishing features, such as the leaf area index, tree height, canopy shape, density, and width [25, 30, 31, 32].

Evapotranspiration, typically expressed in terms of depth, represents an additional mechanism plants employ to foster a cool atmosphere. This process denotes the cumulative dissipation of water into the atmosphere from a terrestrial expanse. It encompasses the aqueous vapor emanating from the soil surface and the aqueous phase on the vegetative surface, in conjunction with the water transpiring from the interior of the plant surfaces. Evapotranspiration can be an effective cooling technique to combat UHI via UGS and all penetrable surfaces. Nevertheless, as an NBS approach, planting and optimizing existing UGS are widely acknowledged as a sustainable measure supported by vegetation. This approach prevents heat retention by generating shadow and increasing relative humidity through evapotranspiration [33]. The role of UGS in the balance of energy and turbulent fluxes is essential, as it substantially increases latent heat compared to sensible heat, resulting in a significant evapotranspirative cooling effect [34]. For instance, despite having a higher albedo than conventional dark roofs, green roofs primarily rely on the vegetative role of capturing sunlight and conducting evapotranspiration to achieve cooling effects. This process involves releasing stored water into the atmosphere, resulting in lower surrounding urban temperatures [35]. The cooling of the surface due to evapotranspiration is typically inferior to that attained through shading; however, this mechanism proves to be efficient in diminishing UHI on a more extensive scale [36, 37]. Generally, it has been demonstrated that the relationship between temperature and canopy cover exhibited a nonlinear trend, whereby the most significant cooling effect was observed when canopy cover surpassed 40%.

Additionally, daytime cooling increased with spatial scale, with the most significant cooling effect observed at the scale of a typical city block (60–90 m) [38]. Systematically, the evapotranspiration process is generally regulated by meteorological parameters, including but not limited to solar radiation, air temperature, and wind speed [39, 40]. In addition, numerous studies have reported that vegetation types, density, occupied area, tree height, and complex structures have displayed remarkable proficiency in tempering temperature through the augmentation of humidity [41, 42]. Besides these, plant species also play a crucial role in determining the cooling effect through shading and evapotranspiration. Specifically, evergreen coniferous clusters have recorded the highest cooling potential, amounting to 1.40°C, whereas deciduous broad-leaf clusters had a relatively lower cooling potential of 1.31°C [43].

While the majority of recent works have given prominence to the combined effects of urban vegetation shading and evapotranspiration, there appears to be evidence suggesting that the arrangement and distribution of green structures within a city can potentially enhance the cooling performance of UGS [44, 45, 46]. From a Landscape Ecology point of view, UGS’s spatial features and qualities are commonly known as spatial patterns, comprising two primary components: composition and configuration. Landscape composition is not spatially explicit since it relates to the diversity and abundance of green patch type dominance, definition, and richness, not their distribution or placement within the landscape unit. In contrast, landscape configuration considers green patches’ spatial arrangement and features by their shape complexity, connectivity, and fragmentation in the unit [47]. Landscape metrics are widely applied to assess the cooling effect of UGS on UHI and LST within the patch-matrix model to measure compositional and configurational characteristics [48].

In recent studies, a comprehensive range of landscape metrics has been employed extensively to develop quantifying models for the UGS’s cooling. These landscape metrics have included several green patches, which explore UGS’s patterns, processes, and functions by evaluating area, edge, shape complexity, density, fragmentation, and connectivity [49, 50, 51, 52]. Concerning the cooling impact of UGS, several distinctions exist between the compositional and configurational features. For the compositional dimension, substantial scholarly research has concurred that increasing UGS coverage could yield more favorable cooling outcomes [48, 52, 53]. On the contrary, configurational investigations have aimed to establish a correlation between UGS configuration and more efficacious cooling, emphasizing the spatial arrangement of UGS relative to other urban elements [46, 54]. While this approach offers a systematic method, it remains uncertain whether a consensus can be reached regarding the association between UGS’s spatial patterns and their cooling performance. Considering the diverse range of conditions, scales, and data resolution observed in case studies, the primary challenge lies in generalizing research findings to establish a practical framework for spatial patterns, cooling mechanisms, and heatwaves in urban environments [45]. However, the landscape ecology paradigm may serve as a reputable framework for optimizing the capabilities of UGS and providing substantiated support to both planning and design practices and future scientific research. The optimization of existing UGS is an effective NBS that enables urban planners and policymakers to enhance cooling efficiency by revising UGS configuration instead of the need for more water resources, change in land use, or vacant land for the expansion of green areas [48, 54, 55, 56].

The cooling capabilities of UGS are not exclusively limited to shading and evapotranspiration processes, as other mechanisms can be regarded as NBS in the planning of urban greenery. A supplementary method employed in the cooling process in vegetation is reflection. Indeed, the presence of vegetation enhances thermal efficiency by utilizing the reflection factor. This attribute operates by the elevated albedo of green areas, which stands at 25% compared to standard surfaces and buildings with a lower albedo of 15% [57]. Contrary to the shading mechanism employed in UGS, the evaluation of reflections in vegetation necessitates intricate soil-vegetation combinations that must be integrated with ‘urban’ models of building, urban geometry, building density, and road interactions with the atmosphere. Furthermore, this factor is highly dependent on soil moisture and often a measure of vegetation ‘intensity’ such as leaf area index (LAI), which makes the scenarios highly complicated for ranking NBS effectiveness [58]. In summary, Figure 1 presents a schematic diagram of the types, mechanisms, and dimensions of the UGS cooling effect.

Figure 1.

The type, mechanism, and dimension of the UGS cooling effect.

The shading and evapotranspiration are critical aspects of the UGS cooling that significantly minimize the UHI effects and reduce energy consumption. The spatial patterns of green patches and the reflection mechanism also play an equivalent role in this regard. However, the suggested NBS techniques should consider the potential and limits of these processes. To date, a few UGS-based NBS approaches can be applied to improve the cooling performance in and across urban areas, depending on the scale of the respective strategy. Fundamentally, the spatial scale of the study and the advocated strategy is a vital aspect to consider, especially when high-resolution UHI and green data are required, both in time and space [59].

Additionally, in the lack of a comprehensive evaluation of geographical and environmental limitations, suggesting identical measures for various case studies to tackle urban warming is not justifiable. Accordingly, as the UGS shading mechanism has been revealed to yield better results than evapotranspiration, this will be implemented in cities located in arid and hot regions at the cost of exerting significant pressure on water resources. It is necessary to perform a thorough quantification of the water footprint of urban green cover, with particular emphasis on their evapotranspiration, to achieve an equilibrium between the implementation of urban greening and the preservation of water resources. Therefore, to analyze the potential effectiveness of these UHI mitigation strategies in urban areas, it will be necessary to conduct simulations of the cooling effects of UGS, especially under varying climatic conditions.

3. Quantifying the cooling effect of UGS: methods and implementations

In terms of studying UGS as a cooling tool, extensive studies have been well documented in the recent decade [37, 45, 47, 49, 55, 60, 61]. At an urban scale, diverse methodologies have been employed in these investigations to measure the efficiency of UGS in regulating the thermal environment. Although these studies have generally confirmed that UGS can help alleviate UHI effects, most of these academic works focus on a specific aspect of UGS performance and fail to consider other potential impacts of green areas. As a result, there is a consensus among all these works that any increase in the amount of green spaces can lead to more cooling of the environment [62]. However, concerning other findings, there is little agreement due to differences in methodologies, the scale of the study, and the type of variables [48, 63, 64].

The present section seeks to evaluate the most current research and systematize the data gathering and analysis techniques frequently employed in the context of UGSs and their cooling impact. Many cooling indices have been developed to assess the cooling effects of UGSs, including but not limited to cooling intensity, cooling extent, cooling efficiency, and cooling lapse [49, 65, 66]. Nevertheless, a wide range of studies examining green spaces’ cooling effect have emphasized two indices: cooling effect intensity [61, 67, 68, 69] and cooling effect distance [70]. These indices have broadly been explored through various techniques and methodologies in the reviewed articles. Despite the variability in outcomes, a comprehensive classification can be proffered from these multiple methods to help us better understand the relevant methods and approaches. The methods currently employed in evaluating cooling effects have similarities to those used in UHI research and can thus be classified into three distinct categories [19]. These categories include a) field measurements, b) spatial analysis, and c) simulation and model analysis.

3.1 Field measurements method

Field measurements encompass all methodologies that require the presence of a scholar, observation, and direct utilization of measurement tools such as portable infrared thermometers or thermal infrared images, which are used to gather extensive data for evaluating the extant thermal state of a designated area. Consequently, many of these investigations are frequently executed on a small scale, encompassing green patches, urban parks, and street canyons, to acquire data and numerical models [71]. Field measurements are also utilized to confirm the authenticity of the findings and calibrate the model by juxtaposing data from meteorological stations and remotely sensed data [34, 72, 73]. Although this quantification method covers the possibility of quick and easy data collection and numerical analysis for small scales, it is essential to note that the unpredictable and varying conditions of the urban microclimate, as well as the precision of measuring instruments and equipment, and potential human fallibility, can have a considerable impact on the overall generalizability of the results. For instance, Bowler et al. stated that larger parks and those with trees could be cooler during the day. However, evidence for the cooling effect of green space is mostly based on observational studies of small numbers of green sites [74].

This particular characteristic indicates a linear correlation between the area of UGS, the extent of tree cover, the normalized difference vegetation index (NDVI), and cooling efficiency, particularly about LST, as discussed in the previous investigations. On the contrary, it was determined by Xu and Zhao that there exists a dependence on scale between the size of green spaces and their cooling rate. Accordingly, it was found that the green patches of medium size, with an area ranging from 0.3 to 0.8 ha, have the most efficient cooling effect [63]. Extensive research has also been carried out on UGS and its efficacy in terms of cooling distance. The findings indicate that UGS can lower temperatures by a range of 0.5 to 2.5°C, with a cooling distance effect extending as far as 60 to 540 meters [75, 76]. Concerning the size of UGS, it was discovered that medium-sized urban green spaces with more complex shapes and higher levels of greenness exhibited greater cooling efficiency. In the findings of the investigation conducted by Xu and Zhao [63], it has been determined that the implementation of UGS can effectively reduce LST by a range of 0.06 ± 0.05°C to 3.81 ± 1.01°C.

Furthermore, it has been demonstrated that the cooling intensity is nonlinearly correlated with the size of the UGS and is closely associated with the complexity of green patches’ shape and vegetation quality. Moreover, the results of this investigation reveal that the cooling efficiency of small, medium, and large sizes of UGS is −0.004 ± 0.03 (n = 2201), 0.79 ± 0.01 (n = 3570), 0.19 ± 0.03 (n = 151), respectively. These findings suggest that urban greening strategies to combat urban heat should prioritize promoting medium-sized UGS and strategically managing green space layout [63]. Similarly, several works have also confirmed the presence of a nonlinear correlation between the size of UGS and their cooling impact in the context of local cool island intensity [77]. Meanwhile, another prevalent utilization of field measurements pertains to evaluating plant species and UGS types concerning their efficacy in ameliorating UHI impacts and enhancing thermal comfort. During these investigations, empirical assessments of air temperatures at the urban scale were conducted to determine the quantitative variance in temperature or cooling effect accompanying the proportional coverage of various urban vegetation types [26, 32, 78]. The findings revealed no significant temperature decrease associated with grass and shrub vegetation due to the inadequacy of both shading and evapotranspiration. Regardless of planting patterns, different species of urban vegetation vary in the cooling they provide, while trees are the most effective.

3.2 Spatial patterns analysis

The technological innovation of Remote Sensing (RS) has successfully addressed the limitations of traditional data acquisition by meteorological stations regarding achieving comprehensive coverage. RS is a progressive instrument for urban environmental research, given its ability to swiftly acquire extensive spatial data of the Earth’s surface and, mainly, retrieve green cover through processing RS images. Given that advancements in cooling UGS through RS are heavily influenced by research on LST and UHI, obtaining the necessary information and satellite imagery for such investigations must align with the fields of LST and UHI research [48]. Currently, the focus of research on UHI and green cover encompasses various aspects, namely, the selection of thermal infrared images, algorithms, spatiotemporal factors, spatial analysis methods, thermal comfort, and spatial simulations. Depending on the image source, a thermal infrared image can have a high, medium, or low spatial resolution, which is determined by ATLAS (5–10 m), Landsat 8 (30 m), Landsat ETM (60 m), Terra ASTER (90 m), and MODIS (250 m). Furthermore, in certain instances, more detailed analyses necessitate the utilization of either QuickBird (0.65 m), IKONOS (1 m), Spot 5 (2.5 m) or Sentinel-2 (10 m) images [79, 80].

When conducting comprehensive investigations on a large scale, implementing satellite maps and remote sensing methodologies to evaluate the collective cooling influence of a cluster of UGSs is a conventional approach. In recent years, there has been extensive empirical research on the topic of cooling effect, which has been exclusively investigated through remote sensing methods. Furthermore, in addition to remote sensing methods, numerous supplementary techniques have been employed in recent studies, including using portable field observations and temperature sensors to measure cooling intensity and distance accurately [60]. To delve deeper into the cooling effect of UGS, landscape ecology scholars have recently proposed analyzing spatial patterns based on landscape metrics that provide an efficacious foundation for quantifying the cooling intensity and distance. For instance, Wu et al. [61] employed eight metrics to evaluate the cooling efficacy of pocket green spaces (PGS) within densely populated urban regions of Shanghai. This study observed that the local cool island intensity could attain a maximum value of 3.6°C with an average of 1.2°C.

Similarly, the maximum cooling area could be as large as 5.7 ha on average, and both these values exhibited a logarithmic increase with the area of PGS. The maximum cooling distance was found to be 132 m on average, while the maximum cooling efficiency was observed to be 6.8 times the size of PGS. The cooling effect was also found to be influenced by the vegetation type present in the PGS. The cooling effect indicators showed a negative correlation with the patch density in the surrounding regions of PGS. Moreover, the cooling effect was observed to be positively correlated with the landscape shape index and the mean Euclidean nearest neighbor distance, highlighting the crucial role played by fragmentation, connection, and complexity of the surrounding landscape in determining the cooling effect at class and landscape levels, respectively [61].

Moreover, Yan et al. [65] have reported that the percentage of vegetation area (PLAND) predominantly demonstrates a negative correlation with cooling intensity. It suggests that a greater quantity of surrounding vegetation could potentially conceal the cooling effect of UGS within its surrounding environment. Furthermore, a positive correlation between the area-weighted mean Euclidean nearest-neighbor distance (ENN_AM) and cooling intensity demonstrates that the existence of more vegetation within a limited spatial range could diminish the cooling intensity by reducing the temperature disparity between UGSs and the neighboring environment. As a conclusion, it is evident that the internal factors, namely NDVI and Landscape Shape Index (LSI), exert a greater influence on the cooling effect of UGSs as compared to the external factors [65]. Despite continuous endeavors, achieving a precise understanding of the interplay between urban heat issues and UGS cooling efficiency remains elusive. This issue is due to the inherent heterogeneity in the spatial patterns of UGSs [52].

Furthermore, various extraneous variables influence the findings, resulting in ambiguous conclusions. For instance, due to geographical conditions and different climate zones, several research studies have demonstrated that an optimal green area exists that contributes to the cooling effect and variability among green spaces in various cities. Examples of such cities include Nanning, where the optimal green area is 0.3 ha, as documented by Tan et al. in 2021 [75]. Additionally, Yang et al. found that the optimal green area in Copenhagen was 0.69 ha [76]. Similarly, Yu et al. [81] reported optimal green regions of Rome and Florence to be 0.51 ha and 0.37 ha, respectively [81]. Although this method covers a large scale of areas in most case studies, the findings do not reflect the dynamic characteristics in detail nor detect internal influencing factors of the UGS cooling effect [82]. Therefore, the conclusions drawn from UGSs’ cooling performance depend on several factors. These include the spatial resolution of the data, the scale, the type of UGS, statistical methods, and the climatic conditions of the study area.

3.3 Simulation and model analysis

The final section concerns those investigations that have extensively employed computer simulation and model analysis. Despite being a comparatively new technique in the UGS cooling quantification, computer simulation has significantly expedited the qualitative evaluations that prioritize the impact of vegetation quality, type, and the location of green areas, as opposed to cooling intensity and distance measurements [60]. Overall, these types of simulations employ mathematical models commonly validated by comparing output parameter values with those obtained from field measurements. Ultimately, the proposed scenarios are simulated in the model and subsequently identified through analyzing scenarios that exhibit superior performance in enhancing cooling and outdoor thermal comfort [83]. The remarkable progress in computational resources over the past few decades has made them a viable substitute for traditional research methods in urban microclimate modeling and observational techniques [84]. Establishing numerical micro-scale models depends on the intricate interplay between the urban fabric fundamentally, encompassing buildings and ground surfaces, and local weather parameters. In principle, such models incorporate solar radiation, comprising direct, diffuse, and reflected radiation, airflow patterns, and heat transfer from urban surfaces to the atmosphere [85]. In this particular context, one of the most important and widely used simulation software for evaluating the cooling effect of trees is ENVI-met microclimate simulation and modeling, developed by Michael Bruse at the Ruhr University of Bochum [86]. Its interactive applications afford a scientific analysis of the impacts of diverse planning scenarios conceptualized by architects and urban planners. The resolution offered by this model is typically 0.5 m in space and 1–5 sec in time, making it highly suitable for a range of applications such as Architecture, Landscape Architecture, Building Design, and Environmental Planning. It is important to note that the ENVI-met is a prognostic model based on the fundamental laws of fluid dynamics and thermodynamics. This model offers a comprehensive simulation that includes the modeling of various factors such as:

• Flow around and between buildings.

• Exchange processes at the ground surface and building walls.

• Building physics.

• Impact of vegetation on the local microclimate.

• Bioclimatology.

• Pollutant dispersion.

The ENVI-met system allows for the simulation of urban microclimates as interactive systems comprising numerous dynamic subsystems spanning atmospheric dynamics, soil physics, vegetation response, and building indoor climate. These systems, including soil hydrology and building energy simulation, are calculated within a single extensive model, such as an urban quarter, allowing for interaction and adaptation akin to an actual environmental system. The ENVI-met provides highly detailed data for each component, whether a single building among 500 or a solitary tree within 1500 trees.¹ The ENVI-met model has been utilized in numerous studies to examine current microclimatic conditions and evaluate various mitigation strategies against the UHI effect comparatively [86].

In simulating and evaluating the cooling impact of UGS, this software computes some effective factors, including the shading effect, the prospective temperature of leaves, the photosynthesis rate, soil moisture, and local evaporation. Moreover, this model simulates the daily variations within complex urban layouts, consisting of buildings and vegetation of differing configurations, sizes, and shapes, leaf types and rendering relevant outputs for thermal comfort indicators from a microclimatic point of view [87]. There has been a growing scientific interest in urban climate issues and UGS in recent years. This issue has led to an increased utilization of the ENVI-met simulation model, which has gained popularity among researchers. For instance, Makido et al. [11] utilized ENVI-met microclimate modeling to demonstrate that the reduction of paved surfaces, increased vegetation, and enhanced road and roof albedo can potentially diminish temperatures in the studied regions. As an NBS, they recommend that in areas already abundant with tree coverage, such as high-canopy neighborhoods and vegetation urban areas, planning efforts should be directed towards preserving the status quo instead of augmenting vegetation or albedo [11]. The ENVI-met model has also been utilized to simulate selecting suitable trees and planting locations for alleviating urban heat by Morakinyo et al. [88]. The results of the study have revealed that the heat reduction potential of trees was location-dependent and species-specific.

Moreover, the results showed that urban trees with varying forms and species regulate variable temperatures in different urban morphology. Similarly, the simulation approach has been utilized to indicate effective patterns in tree planting design. Abdi et al. [83] demonstrated that the most optimal condition in enhancing outdoor thermal comfort can be achieved by employing rectangular planting of evergreen trees in the outer rows and deciduous trees in the inner rows, perpendicularly oriented to the prevailing wind [83].

In recent years, numerous integrated models have been developed in addition to the ENVI-met model. One of the most popular models is an integrated approach comprising a coupled meteorological and urban energy balance model (WRF-SUEWS) and a hedonic pricing simulation model (SULD). This model is utilized to evaluate urban heat fluxes and urban compaction effects, respectively. A recent study conducted by Augusto et al. [89] demonstrated that in the short-term, NBS exhibit a local cooling effect due to an increase in green and blue spaces. However, in the medium to long term, an urban compaction effect is observed due to the attraction of residents from peripheral areas to areas surrounding attractive NBS [89].

In conclusion, it is essential to acknowledge that simulation is a remarkably expedient and expeditious approach to analysis. Nevertheless, the precision of these methodologies necessitates a diligent integration with satellite imagery and field measurements. This approach is appropriate for formulating preliminary hypotheses and assumptions and examining the impact of changes in scenarios and variables on the responses [60].

4. Planning and design of cooling cities: urban greening challenges and strategies

In the previous sections, the focus was predominantly on presenting the mechanisms by which UGSs can diminish the ambient temperature. Among these, shading and evapotranspiration are considered as the most effective mechanisms. However, effective strategies that can effectively apply these mechanisms to alleviate temperature are still the subject of ongoing debate. Therefore, the challenge of selecting appropriate NBSs to attain multiple objectives in the urban setting has posed a significant difficulty for urban planners and policymakers [36]. Planning and implementing efficient strategies while refraining from heuristics presents a formidable challenge. Moreover, acquiring the requisite expertise to create, maintain, and optimize the chosen strategy is complicated because ascertaining how to equilibrate the synergies and trade-offs achieved through NBSs may add more complexities to this process [90].

In general, each urban area will experience the impacts of excessive heatwaves in a distinct manner. This issue is due to various factors, such as geographical location, climate and meteorological conditions, population density, urban form and structure, building types and construction practices, and existing land cover. The varying conditions will consequently prompt cities to implement a unique combination of cooling solutions. For instance, a variety of highly effective solutions in a temperate and humid climate may not yield the same results in a hot and arid desert climate [91]. In contrast to other strategies that deal with technology and non-living materials, plants are living organisms whose function is directly based on the dynamism of their immediate surroundings conditions. Thus, within the framework of urban heating, it is imperative to consider that the utilization of plants not only possesses considerable and varied capabilities but also entails significant constraints that may impede the implementation of any strategy on the way to success. Fundamentally, the successful implementation of strategies based on UGS planning and design faces various challenges, which can be classified into two categories: 1) internal challenges and 2) external challenges.

4.1 Internal challenges

Internal challenges are primarily related to knowledge gaps in plant-related fields, encompassing various aspects ranging from selecting the appropriate plant to maintenance techniques. Moreover, insufficient knowledge concerning planting design, implementation techniques, and the horticultural limitations of UGS has led to a disparity between landscape architectural and urban design visions [92]. Furthermore, issues regarding soil quality, planting substrate selection, placement, the timing and method of planting, and ultimately, water quality and irrigation techniques are crucial and can impede the effectiveness of strategies to mitigate temperatures. Meanwhile, issues relating to water are significant, so it is vital to supply more thorough details regarding the water requirements of plants, particularly during dry seasons. It means that maintaining and expanding green plant covers of trees, shrubs, and grasses in dry and hot zone climates requires more water resources, which eventually results in higher costs [56]. Consequently, this section necessitates the development of specific frameworks that simultaneously consider the esthetic characteristics of plants, such as form, color, texture, and size; and the expected functions they perform, such as creating shade and windbreak and reducing pollution.

4.2 External challenges

UGSs constantly face various external challenges that can impact their cooling efficiency. These challenges include the approaches related to policy and strategy formulation for the city, planning, and design visions, as well as implementation techniques. External challenges encompass a group of exterior factors that affect the quantity and quality of the cooling effect of UGSs. These factors include a wide range of land use types, urban morphology, layout geometry and urban form, density, spatial structure, and networks. In addition, urban planning and design processes, policies, and strategies are also drivers that can influence the effectiveness of UGS in terms of cooling and temperature reduction over a long time. These factors exert a considerable influence not solely on the UGS composition but also on configuration and spatial structure [44]. However, the foremost impact of these challenges on the configuration of green areas is the escalation of spatial heterogeneity, resulting in the decline of the efficiency and functionality of such areas.

One of the most critical effects of increasing spatial heterogeneity within the UGS structure is a significant impact on the reduction of their cooling capacity. Given the complexity and spatial variety of ecological factors, spatial heterogeneity is an essential theoretical issue in the science of ecology [93]. From the perspective of Landscape Ecology, urban landscapes are characterized by heterogeneity. The distinct compositions and configurations of the urban landscape across different areas of a city can increase urban heterogeneity, substantially impacting ecological processes. Hence, urban heterogeneity may lead to cooling effects of UGS in other spatial units [94]. While achieving homogeneity in urban layout and structure is nearly impossible, implementing effective urban policies and planning strategies can significantly develop a robust and long-term approach to urban heat mitigation by increasing the cooling capacity and optimizing the efficiency of UGSs. Therefore, understanding how UGS affects UHI intensity poses a significant challenge for urban planners and decision-makers who must design cost-effective urban heat mitigation strategies [48].

Moreover, there has been a growing trend towards an integrating approach by UGS incorporating urban morphology with a concentration on sustainability in the past few decades. This shift is meant to reduce the negative impacts of urban sprawl, congestion, and urban fragmentation issues [95]. Hence, contemporary urban planning and design have adopted more sustainable urban forms by applying various approaches, such as neo-traditional development, urban containment, the compact city, and the eco-city. The compact or dense city form has been particularly emphasized among these forms. The compact city is recognized for its high-density housing, mixed-use, efficient public transport, and encouragement of cycling and walking.

Nevertheless, a detrimental consequence of specific facets of urban densification is progressively being demonstrated by the lack of green space in dense urban areas and the removal of such spaces during the densification process [96]. The compact city development has garnered significant attention as an effective and sustainable approach to urban planning. This approach can create a dense environment that accommodates high-rise buildings and increases population density. However, this approach also poses a significant threat to UGS as it occupies lands, degrades green courtyards and gardens, disconnects green corridors, and restricts space [96]. As an external challenge, the results show that some of the current approaches have progressed towards reducing UGSs and thus decreasing their cooling effect despite their emphasis on sustainability and the use of green spaces.

As demonstrated in this section, the body of reviewed scientific and observational literature implies that adopting a particular urban cooling solution will not lead to a sustainable reduction in urban heat temperatures. Therefore, this section’s present strategy for addressing urban heat-related concerns involves a nature-based approach centered on UGS, which entails a comprehensive approach that considers the scale, monitoring techniques, and urban planning and design. This particular strategy has been achieved by amalgamating three key strategies capable of delivering an extensive array of interventions to cities at various levels: urban scale, neighborhood scale, and building scale. Figure 2 shows the proposed NBS based on the sustainable UGS cooling effect. The following strategies are delineated in brief:

1. Monitoring strategy: uses satellite imagery, environmental sensors, and field measurement to control the quality and quantity of urban green spaces consistently. Additionally, this strategy entails a comprehensive analysis of urban heat islands, the formation of hot and impermeable surfaces, and the meticulous identification of their locations. The resulting reports are then disseminated to urban managers and decision-makers who may utilize this information to make informed decisions concerning future interventions and planning endeavors. It is worth noting that many ranges of intervention in UHI mitigation fall under “control” strategies, like urban development and infrastructure planning decisions.

2. Management strategy: By leveraging the authority of urban managers, the proposed strategy allows cities to facilitate or influence actions towards sustainable cooling at a neighborhood scale. These actions may include raising awareness among residents regarding global warming, climate change, and the significance of maintaining and developing UGS to support urban cooling. Furthermore, this strategy advocates adopting technological instruments like solar panels and online irrigation systems to manage energy and water resources in urban areas.

   Additionally, various measures and practices can be planned at the neighborhood scale to contribute to sustainable urban cooling. For instance, urban neighborhoods can be designed according to wind direction, enabling natural wind to flow through the urban fabric and decreasing heat and polluted air. Moreover, it is possible to enhance ecological connectivity between green areas to homogenize green patterns, optimize their cooling performance, and maximize cooling effects.

3. Planning and design strategy: On the building scale, a wide array of passive design strategies exists to augment energy efficiency and mitigate environmental heat without recourse to any external apparatus or instruments for creating high-performance buildings or spaces. A building may be transformed by precisely calibrating the heating, lighting, and ventilation mechanisms to achieve optimal indoor and outdoor comfort for its occupants. Passive design strategies, in their entirety, encompass building orientation and form, ventilation, shading, green walls and roofs, water bodies, and surrounding landscaping and vegetation. Moreover, in the residential complex, a cluster of buildings can effectively provide self-shading inside the site and enclosed courtyards that help to create a cooling environment.

Figure 2.

NBS based on sustainable UGS cooling effects.

By thoroughly analyzing the most recent research and practical experiences, the current chapter has made a concerted effort to offer a comprehensive understanding of the cooling effect of UGS and has put forth a specific NBS for mitigating urban heat. However, the challenge of addressing the issue of urban cooling is complex and multidimensional, necessitating a holistic approach that considers all dimensions. Notably, applying NBS based on UGS plays a crucial role. It is insufficient to rely on one-dimensional strategies and increase greenery in urban areas to achieve the desired outcomes. Moreover, given that each city possesses unique conditions providing a universal solution for mitigating heat is not feasible.