Nature-Based Solutions to Enhance Urban Resilience in the Climate Change and Post-Pandemic Era: A Taxonomy for the Built Environment

Abstract

Global environmental and health issues such as climate change and the COVID-19 pandemic have highlighted the weaknesses of current urban systems, including the poor availability and accessibility of green and public spaces in cities. Nature-based Solutions are configured as promising solutions to increase the resilience and health of the built environment by addressing climate and pandemic issues, promoting the psycho-physical well-being of users and proposing solutions for the protection of the environment and ecosystems. Following a systematic review of the scientific literature using the PRISMA methodology, this study aims to provide a taxonomic framework for Nature-based Solutions for the built environment that is applicable to the urban and building scales, highlighting key benefits in addressing pandemic and climate challenges and achieving urban resilience. This framework proposes a holistic and multifunctional approach that will prove to be a useful tool for researchers and policy makers to incorporate greening strategies into urban regeneration and redevelopment processes. The application of Nature-based Solutions still seems to be limited. It is therefore necessary to raise awareness of this issue among citizens and policy makers and to promote close co-operation between the different actors in territorial decision-making processes.

1. Introduction

Global environmental and health dynamics such as climate change, environmental degradation and the COVID-19 pandemic have weakened the fabric of cities [1] and highlighted the weaknesses of current urban systems, including the poor availability and accessibility of green and public spaces in cities.

Climate change, driven by natural processes and human activities, is having a serious impact on our planet, especially on the built environment, which is becoming increasingly impermeable due to [2,3] urbanisation [4,5]. This urbanisation leads to increased surface runoff and reduced soil evaporation, which affects urban health by reducing natural areas. High temperatures lead to urban heat islands (UHI), which pose a health risk, especially during heat waves [6]. The growth of cities emphasises the need for permeable green spaces that improve physical and mental health, air quality, temperature regulation and drainage [7,8,9,10].

Climate change influences the spread of diseases, including zoonoses, through changing air quality, rising temperatures, changing precipitation patterns and changing habitats that affect the spread of infection vectors [11]. The COVID-19 pandemic has highlighted the critical role of cities in preventing, managing and responding to health emergencies [12,13]. Access to natural spaces strengthens social resilience in times of crisis and promotes the health of cities and people by providing spaces for recreation and regeneration [14]. Recreational activities and open spaces are essential for improving wellbeing and reducing social anxiety—an important consequence of the pandemic. Private open spaces with good lighting and contact with nature are also essential as they help to lower blood pressure, reduce stress hormone levels and strengthen the immune system [15,16,17]. Courtyards, balconies, terraces and accessible rooftops can provide users with areas for physical activity and leisure while maintaining social distancing and isolation. During the COVID-19 pandemic, green spaces were used more by city dwellers than in pre-pandemic times [18,19]. Unfortunately, however, not all cities, especially those with a compact urban structure and high population density, have sufficient green spaces. The traditional approach to urban development, characterised by extensive infrastructure and resource-intensive practises, has often exacerbated the vulnerability of urban systems to socio-environmental stressors.

The need for greening strategies appears to be an essential necessity to promote urban health, both from a pandemic and climate perspective, to mitigate temperatures and enable water management. Therefore, the discussion of Nature-based Solutions (NbSs) is extremely topical, as it responds to the urgent need to define measures to adapt the built environment to the dual challenge of climate change and pandemics. NbSs are sustainable practises that utilise ecosystems and biodiversity to mitigate and adapt to climate change, increase the resilience of cities and improve well-being, even during pandemics [20,21,22,23,24,25,26,27].

The Horizon 2020 expert group report on “Nature-based Solutions and re-naturing cities” [28] outlines seven key research and innovation actions to address societal challenges in the areas of sustainable urbanisation, ecosystem restoration, climate change adaptation and risk management. These priorities include urban regeneration, coastal resilience, sustainable resource use and increasing carbon sequestration through NbSs with the aim of integrating NbSs into urban development to improve sustainability and resilience [29].

Several recent studies emphasise different aspects of NbSs, but do not address the potential benefits for health emergencies, nor do they clearly refer to the whole system of the built environment in its dual building and urban dimensions. Whilst the information is detailed, it is limited to specific topics. For example, in a recent literature review proposed by Ji et al. [30], the benefits and limitations of the impact of Urban Green Infrastructure (UGI) were analysed only with regard to the aspects related to thermally resilient communities (TRCs). Tsatsou et al. [31] investigate NbSs for water reuse in built environments. Kandel and Frantzeskaki [32] emphasise the role of buildings in integrating NbSs for urban climate resilience. Oukawa et al. propose a tool to implement these strategies to reduce the intensity of the urban heat island [33]. Camacho-Caballero et al. [34] present a decision-making approach to assess NbSs in urban areas. Fang et al. [35] review NbSs for urban sustainability. Diana et al. [36] have proposed a methodology based on greening strategies in cities with high population density, suggesting the integration of greening interventions in buildings and public spaces that are easily accessible and usable by citizens, thus promoting urban resilience and urban health.

Green and blue infrastructures (GBIs) are also of central importance for urban planning. They form networks of natural and semi-natural areas that provide ecosystem services such as rainwater retention and improve environmental quality and public health [37,38,39,40]. The Biodiversity Strategy 2030, as part of the Green Deal, emphasises the role of GBIs in providing ecosystem services [41]. However, the creation of such infrastructures is often constrained by limited space in cities [42].

The analysis of the recent scientific literature shows the lack of a holistic approach and a specific taxonomic framework for NbSs adapted to the challenges of the built environment in pandemics and climate change. To fill this gap and develop a comprehensive and multifunctional NbS approach, this study aims to develop a taxonomy that is applicable at both building and city level. It emphasises the social and environmental benefits that are critical to improving urban resilience and citizen well-being during health crises and amid climate challenges. Furthermore, this framework serves as a valuable resource for researchers and policy makers, as it facilitates the selection of appropriate solutions in the context of territorial governance and urban regeneration processes. To achieve these goals, this study aims to answer the following research questions:

• What are the main types of NbSs that are suitable for the built environment?
• What are the benefits of NbSs in the post-pandemic and climate change era?
• How can NbSs improve the resilience of built environment?

After the introduction (Section 1), Section 2 provides a theoretical background on urban resilience and the role of NbSs in the challenges of climate change and pandemic health emergencies. Section 3 discusses the methodological approach used, followed by a bibliometric analysis in Section 4, which provides a picture of the current trends in NbSs in the international scientific literature scenario. Section 5 summarises the main findings of the systematic literature review and classifies the potential NbS deployments according to application areas at urban and building level. Section 6 proposes a taxonomy of NbS interventions for the built environment, which is an essential tool for the early stages of greening processes. Section 7 and Section 8 outline the discussions and implications for future research.

2. Urban Resilience: Theoretical Notes

“Resilience is the capacity of individuals, communities, institutions, businesses, and systems within a city to survive, adapt, and grow no matter what kinds of chronic stresses and acute shocks they experience” [43]. The etymological meaning of the term resilience, derived from the Latin resilio, is, strictly speaking, restoration [44].

Understanding the concept of resilience is important to enable cities to prepare for disasters and unexpected events [45], but at the same time implies the ability to adapt and adjust to change [46,47,48]. Analysis of the scientific literature makes it clear that the concept of resilience is not unambiguous, but has a different meaning depending on the discipline, with different methodological and practical implementations [49,50]. Originally, the concept of resilience was associated with the problems of climate change and disaster risk management. However, it has since been extended to socio-ecological aspects in order to improve the resilience of cities from a social and ecological perspective [50]. In relation to cities, Desouza and Flanery [51] define the concept of resilience as “the ability to absorb, adapt and respond to changes in an urban system”. Several studies have investigated the role of socio-ecological resilience due to the complex interdependencies between humans and natural ecosystems [52]. Merrow et al. [53] proposed a definition of urban resilience as “the ability of an urban system-and all its constituent socio-ecological and socio-technical networks across temporal and spatial scales-to maintain or rapidly return to desired functions in the face of a disturbance, to adapt to change, and to quickly transform systems that limit current or future adaptive capacity”.

Urban resilience encompasses a holistic approach to designing and planning cities to withstand and recover from various stresses, including those related to climate change and health emergencies such as pandemics. The resilience of cities requires proactive planning through the implementation of adaptation strategies, such as the development of infrastructure and policies that address the impacts and consequences of an extreme event to ensure the long-term well-being of the urban population. This includes, in particular, the construction of infrastructure that can withstand flooding, sustainable water management systems and the creation or implementation of publicly accessible green spaces. Climate change mitigation efforts aimed at tackling the root causes of the phenomenon, such as reducing greenhouse gas emissions and switching to renewable energy sources, are also important components of urban resilience as they help to prevent future climate disasters.

3. Methods

This section illustrates the methods used in the following study. After defining the aims, the scope and the research questions, the guidelines of the PRISMA method (Preferred Reporting Items for Systematic Reviews and Meta-Analysis) [54,55] were used to conduct a systematic literature review (SLR) [56], following the approach used by Sommese et al. [57]. The PRISMA method provides a holistic picture of the topic through the systematic review of the scientific literature and a quantitative picture of the results of the SLR through the meta-analysis [57]. Figure 1 shows the methodological framework of the following manuscript.

3.1. Data Identification

Once the research questions have been formulated, the next step is to search the documents in the official online repository. For this study, only the Scopus Elsevier database [58] was used, with the search criteria for “title-abstract-keywords”. The search terms used were (“nature-based solutions”) and (“COVID-19” or “pandemic” or “climate change” or “urban resilience”). A review of the Scopus database (retrieved on 10 January 2024) identified 1398 documents from 509 different sources that were suitable for the SLR and bibliometric analysis. The information came mainly from journals, conference proceedings and books. The documents cover the period 2012–2024.

3.2. Data Screening

The 1398 records identified in the previous phase were analysed according to the following exclusion criteria. Records written in a language other than English, those which were not open access and those which did not contain information on the authors were excluded from the selection. In addition, the following types of documents were not considered at this stage: reviews, notes, editorials, short surveys, letters and fact sheets. Overall, 644 records were excluded from this screening phase.

3.3. Data Included

In this phase, 754 records were analysed to identify documents that could be considered for systematic review. Overall, 652 papers were excluded, in particular those relating to the proposal of a methodological framework, case studies and those that were off topic. However, 102 papers were identified as eligible papers and included in the review, as they were useful for achieving the aims of the manuscript. Papers on NbSs for urban resilience, climate change adaptation, urban health, COVID-19, pandemics, buildings and the built environment were considered.

3.4. Meta-Analysis

In order to recognise the predominant direction of NbSs in scientific works, a quantitative study was conducted. In particular, a co-occurrence analysis of keywords was performed to identify primary clusters and related compounds. The software VOSviewer, version 1.6.18 [59], was used for the creation and visualisation of bibliometric networks, including co-authorship, citation networks and co-occurrence networks. Each visualisation consists of nodes (or bubbles) connected by lines. The size of the nodes (or bubbles) indicates the frequency of occurrence or citations, while the lines represent the relationship between co-occurring keywords, creating a network map [60,61,62]. The proximity between the two nodes indicates the relatedness; nodes closer to each other indicate a stronger connection.

3.5. Outlines

After a detailed analysis of the scientific literature through the implementation of the PRISMA method, the results were catalogued according to the extent of application of the built environment. In particular, the main types of NbSs identified for buildings and urban areas were categorised. A multifunctional and holistic taxonomic framework was defined that demonstrated the benefits of implementing NbSs for the challenges of health emergencies and climate change to enhance the resilience of cities.

4. Contemporary Tendencies of NbSs in the Scientific Literature: A Bibliometric Analysis

In order to obtain a quantitative picture of the current trends in NbSs in the global panorama of scientific literature, a bibliometric analysis was carried out according to the methodology described in Section 2. In particular, network diagrams of the co-occurrence of the keywords were created, together with those related to the geographical location of the authors of the manuscripts.

4.1. Co-Occurrence of Keywords

Figure 2 shows the network diagram of the co-occurrence of the keywords, grouped into four clusters, each of which was given a name based on the predominant topic: Environment (cluster 1), Urban areas (cluster 2), Water (cluster 3) and Human (cluster 4). Cluster number one, indicated by the red colour, shows a close correlation between the NbSs and the environment. Words such as “ecosystems”, “biodiversity”, “conservation of natural resources”, “forest”, “land use” and “climate change mitigation” appear to emphasise the correlation between the conservation of ecosystems and environmental sustainability. The keywords of the second cluster (green) emphasise the relationship between NbSs and the management of urban areas to promote “urban resilience”, “urban development”, “adaptation to climate change” and the assessment of the “vulnerability” of the urban environment in terms of the “reduction and assessment” of risks, including the “heat island”.

The main keywords of cluster 3 (yellow) concern the relationship between NbSs and the management of flood risks due to the increase in impermeable surfaces and the reduction in open ground. Indeed, there are correlations between the keywords “rain”, “runoff”, “flooding” and “stormwater”. Finally, cluster 4 (blue) illustrates the correlation between NbSs and human health benefits. The core of the keyword “COVID-19” is also present to support the benefits of NbSs, such as green spaces for recreational activities, for the health and physical–mental well-being of citizens.

4.2. Geographical Domains

Figure 3 illustrates the results of the bibliometric analysis based on co-authorship using countries as the unit of analysis [57]. The categorisation into the three main clusters (red, green and blue) does not help to identify a clear correlation between the different countries. Some of them are geographically close to each other, others have the same climate (temperate or Mediterranean climate), but at the same time others have very different landscapes, such as coastal areas, mountains or forests, i.e., climates that require different adaptation measures. The data derived from VOSviewer show that some countries have had a very intensive production of documents on the subject of this manuscript and have produced more than 100 manuscripts, specifically the United Kingdom (n = 279), United States (n = 235), Italy (192), Germany (n = 140), Netherlands (n = 134), Australia (n = 122), China (n = 126) and Spain (n = 111). Such a high number of documents can be explained by the fact that these are countries with important research institutions which therefore receive funding for research on environmental issues. Furthermore, these are contexts with different ecosystems, so each of them must face climatic challenges in a different way; studies and research in these scenarios seem fundamental.

5. Nature-Based Solutions for the Built Environment

NbSs, when applied to the built environment, are an important tool for addressing the various challenges of urban life [63]. At the building level, the application of green technological solutions, such as green roofs and walls, not only contributes to energy efficiency by providing natural insulation and various benefits in mitigating and adapting to climate change, but also improves the aesthetic quality of the building and, more generally, of the built environment. Interventions at the building level also contribute to improving the quality of urban space, especially in compact urban structures with high population density and a lack of green and accessible spaces. For example, widespread interventions at the building level, such as green roofs, favour a reduction in runoff and a reduction in runoff heat islands, which has a positive impact on the urban environment. At the urban level, the creation of green spaces such as urban forests, urban gardens, street trees or wetlands improves the urban landscape while playing a fundamental role in stormwater runoff, reducing flood risks and strengthening urban resilience. The European Commission’s report [28] on NbSs provides examples of how nature can be brought back to cities and degraded ecosystems to improve urban health, and citizens’ well-being, thereby promoting climate change adaptation [64]. According to the work proposed by Diana et al. [36] in 2024,

Table 1. Some advantages of NbSs. Repurposed from [36].

Interventions	Environmental Factor Regulation	Advantages
Protect urban green spaces Plant trees along the roads	Air quality regulation	Absorb gaseous pollutants and trap particulate matter
Protect urban green spaces	Climate regulation	Store carbon
Green roofs and green walls	Water flow regulation	Facilitate the interception of rainfall
Ponds and wetlands	Water purification and waste treatment	Collect, store and clean water before storing it in streams
Protect urban green spaces Use permeable surfaces and vegetation	Disease regulation	Improve the quality of the air and the environment Promote biodiversity Reduce sources of stagnant water Regulation of vector insects
Provide bird feeders and promote the establishment of species Encourage the planting of plants from appropriate resources and food plants for caterpillars	Pollination	Encourage nesting
Sustainable urban drainage systems Green roofs and green walls Trees in urban areas Permeable surfaces	Disaster risk reduction	Promote the recharging of the aquifers
Trees and bushes between streets and houses	Soundscape management	Reduce unwelcome sounds in public places Provide shelter for songbirds
Attractive green spaces for access Connect the various utilities (schools, work, homes) through green spaces Increase biodiversity	Health	Improve the quality of life Improve human and urban health

reports the main NBSs at an urban scale and their associated beneficial functions, as reported by EC reports. In line with the main objective of the present study, only NbSs that can be implemented in the built environment during urban regeneration processes to achieve urban resilience and urban health were considered; therefore, strategies related to different aspects, such as territorial infrastructure engineering, were excluded from this analysis. Figure 4 illustrates a general framework for nature-based solutions for the built environment that are applicable at both the building envelope and urban scales.

5.1. Nature-Based Solutions for Buildings

The implementation of NbSs for buildings takes place at the level of the building envelope, which is understood as a separating filter between the internal and external environment and is able to regulate energy flows [65]. The most important solutions can be designed for roofs and façades, with benefits for indoor and outdoor comfort, i.e., for the microclimate and the health of the city and its citizens. The following subsections detail the types of NbSs that are applicable at the building envelope level, divided into roofs and façades, indicating for each of them the technological characteristics and application benefits. A systemic diagram summarising these concepts can be found in Figure 5.

5.1.1. Roofs: Green Roofs, Blue Roofs and Blue–Green Roofs

Green roofs (GRs), roofs with a layer of vegetation, have become very popular in recent years due to their economic–ecological benefits, not only for the external environment but also for the indoor comfort of living spaces [66]. In fact, they are considered as NbSs for climate adaptation as they allow normally unused spaces to be utilised, providing economic, social and environmental as well as aesthetic benefits [26].

Various articles in the literature emphasise the benefits of these solutions, which go beyond aesthetic and architectural interest. These include the mitigation of the urban heat island (UHI) effect thanks to evapotranspiration, thermal insulation and shading, the limitation of runoff surfaces, the absorption of air pollution, the reduction in CO₂ and the development of biodiversity, flora and fauna [67,68,69,70,71,72]. The functional layers of a green roof, which are arranged above the roof covering, are a protective anti-root layer, drainage and accumulation layer, filter layer, soil, humus and vegetation. The thickness of the substrate and the type of vegetation vary depending on the type of green roof. From a technological point of view, green roofs can be categorised into extensive (EGRs), intensive (IGRs) and semi-intensive (SIGRs) roofs.

Extensive green roofs (EGRs) require a shallower growing substrate and less maintenance, in contrast to intensive green roofs (IGRs), which require a deeper substrate and more complex maintenance [67,73,74]. Semi-intensive green roofs (SIGRs), on the other hand, occupy an intermediate position due to the average thickness of the substrate [68]. Intensive roofs reduce runoff by 85% compared to conventional roofs and thus provide better benefits for stormwater management [75]. Bellini et al. [74] presented a quantitative study on extensive green roofs, showing that the main benefits resulting from their application are mainly related to the mitigation of temperatures and the reduction in stormwater runoff. In 2020, Snep et al. [76], dealing with NbSs for urban resilience, presented a classification of green roofs based on the technological level: no-tech, low-tech or high-tech. No-tech roofs are compared to normal grey roofs, i.e., without drainage systems and without vegetation layers; low-tech green roofs are conventional green roofs, i.e., those that are able to capture, drain and temporarily store water for subsequent passive or autonomous infiltration into the body of the available substrate; and high-tech roofs, on the other hand, are able to capture, store and reuse water through an open geocellular structure that allows water retention [76]. The latter solution is also known as a blue–green roof. This refers to a green roof that is installed over an empty modular system (blue roof) and has the function of retaining rainwater [77,78]. Snep et al. [76] say that “high-tech” green solutions that can store and reuse rainwater outperform traditional natural capacities, favouring water management and improving urban resilience without taking up space.

It is necessary to point out that there is no regulated definition of blue–green roofs, but the various interpretations emphasise the ability of blue–green roofs to store more water than is necessary for vegetation [79]. Blue–green roofs are therefore a cutting-edge technology that combines the benefits of traditional green roofs with the ability to store rainwater [80].

A blue roof (BR) is a special roof system that is capable of retaining rainwater, temporarily damming it through barriers at the drainage inlets and then releasing the water again within a short time [81]. The blue roofs essentially function as temporary rainwater collection basins. The substrate and vegetation that are typical of green roofs are replaced by a layer of gravel/aggregate in blue roofs [82].

5.1.2. Green Walls: Green Facades and Living Walls

Green walls are to be understood as vegetated vertical elements of the building envelope. They offer benefits for the environment as well as for living comfort. They can mitigate high temperatures and thus the urban heat island, promote the natural shading of buildings, support evapotranspiration in summer and provide thermal insulation in winter [83]. Thanks to the reduction in surface temperatures and the mitigation of thermal fluctuations, green walls improve indoor comfort and reduce the energy requirements of air conditioning systems by improving their thermal performance [83]. The vertical plant cover also acts as an acoustic insulation layer. In contrast to green roofs, green walls also offer excellent ecological benefits thanks to their larger surface area, as they cover a much larger area than green roofs [68]. The refreshing effect of green walls in hot seasons is due to the absorption of heat through evapotranspiration, the shading and/or reflection of solar radiation by the vegetation, the absorption of solar energy during photosynthesis and the limitation of the passage of hot air due to the insulating effect created by the vegetation layer itself [84,85].

Due to the benefits of vegetation, green walls are also able to reduce air pollution and improve the quality of urban air by favouring the filtration of fine dust and harmful particles thanks to the absorption capacity of plant species [83]. Green woods promote biodiversity as they serve as a habitat for some plant species.

From a technical point of view, green façades can be distinguished from living walls. The former are characterised by plants that are rooted directly in the ground or in pots or planters and, thanks to their morphological abilities, can climb independently on the surfaces of building facades or with the help of supports attached to the wall, such as wires, nets or metal trellises [83,86].

Living walls, on the other hand, also known as vertical gardens, are systems characterised by modular panels that contain soil and the appropriate substrates for vegetation and require hydroponic cultures to promote the growth and maintenance of plants [86,87]. These modular panels are anchored to the wall or to a self-supporting frame with an in- built irrigation system. The space created between the building wall and the greening system acts as a thermal buffer and improves the thermal insulation of the building [88]. Similar to green roofs, green walls can also be classed as intensive and extensive. Fernandez-Canero et al. [89] present a detailed classification of green walls, and define green façades as extensive systems characterised by simplicity and a limitation of functional layers, while defining living walls as intensive systems, as they are characterised by a more complex number of functional layers and a thicker substrate.

5.2. Nature-Based Solutions for Urban Areas

The implementation of NbSs for buildings takes place at the level of the building envelope, considered the interface between the internal and external environment. Apart from the issues related to the management of green spaces and the associated benefits already defined in the previous sections, NbSs are configured at the urban level as privileged opportunities for the management of rainwater, since flooding is considered one of the main problems related to climate change. In fact, according to the Centre for Research on the Epidemiology of Disaster (CRED), floods are the type of disaster that occurs most frequently: about 170 floods per year worldwide [90,91]. Therefore, urban areas are the places where drainage solutions can be trialled and put into practise to intercept, retain and distribute rainwater by creating reservoirs in which a green area can also be created. The increasingly popular Sustainable Drainage Systems (SuDSs) mimic natural drainage processes to reduce flooding, improve water quality and increase biodiversity: they can include permeable pavements, ponds and infiltration basins. In recent debates on urban drainage for urban areas, we often hear about sponge gardens, which are also designed to mitigate the impact of urbanisation on the water cycle by reducing flooding, restoring aquifers and protecting water quality. The concept of sponge cities is also growing rapidly in response to the damage caused by severe storms. This concept was developed in the early 2000s in China to describe the water storage capacity of natural areas by comparing them to a sponge [92].

The following subsections summarise the most important NbSs for the built environment that emerged from the literature review. Those specifically related to territorial infrastructure engineering and land management have been excluded in order to focus attention only on those that can be implemented in the regeneration processes of urban areas.

5.2.1. Trees, Urban Gardens and Urban Forests

Urban gardens are a cultivated area within a built environment, with plants, flowers or fruit, whose extent varies according to context and use. Otherwise, the expression urban forest (UF) refers to the stock of trees and other plant species in urban areas, e.g., forests, parks, green spaces or even street trees. Urban forests, considered as an integral part of green infrastructures, are configured as fundamental elements of urban systems that provide various ecosystem services to the built environment as well as social and economic benefits; they also play a fundamental role in improving the resilience of cities [93,94,95]. Urban forests and trees not only provide ecological balance, but also improve air quality by removing certain pollutants such as PM10 [96,97], reduce the effects of the urban heat islands and provide habitats for wildlife by ensuring pollination and biodiversity [98]. Urban gardens and urban forests not only have aesthetic benefits for the built environment, but also offer citizens space for recreation and leisure and thus for physical and mental well-being. At the same time, urban gardens, when planted with fruit and vegetable crops, favour the production of local, fresh and healthy products, thus encouraging the participation of local communities.

In this discussion, it seems fundamental to mention the concept of Urban Tree Canopy (UTC), which refers to the amount of leaves, branches and trunks of trees that cover and shade the ground; i.e., it indicates the superficial extent of the tree canopy within an urban area. The UTC is considered an important benchmark for assessing the extent of urban forests. An urban environment characterised by a high and robust UTC mitigates the negative effects of urbanisation, thus reducing heat stress, urban heat islands, stormwater runoff and air pollution [99]. Green spaces, especially trees, offer maximum protection against heat stress, i.e., heat waves, for urban areas [6].

The UTC is also fundamental for the interception of precipitation. The water that falls on a plant is divided into three phases: interception, run-off from the stem and fall-off from the crown [100]. The amount of water that can be retained by the crown of a tree depends on the type of vegetation and the morphological and structural characteristics of the leaf (e.g., shape, total leaf area) [100]. Some of the water that falls on the tree can be directly intercepted and absorbed by the plant without reaching the ground, especially during light precipitation.

Some of the benefits of trees, urban gardens and urban forests are summarised below:

• Better air quality: Trees act as natural air purifiers that remove pollutants such as carbon dioxide, nitrogen dioxide and particulate matter, improving air quality and reducing respiratory problems.
• Reducing temperature: Urban forests counteract the urban heat island phenomenon by providing shade and evaporative cooling, thus reducing energy consumption for cooling buildings and lowering ambient temperatures.
• Runoff control: Trees soak up rainwater, reduce surface runoff and relieve the burden on stormwater management systems. Their root systems also combat soil erosion and clean pollutants in runoff.
• Ecological diversity: Urban forests provide habitats and food for a variety of wildlife, including birds, insects and small mammals, supporting efforts to preserve biodiversity in cities.
• Benefits to visual and mental wellbeing: Trees enhance the urban landscape, create inviting green spaces for recreation and relaxation, and have been shown to reduce stress and promote good mental health.

5.2.2. Rain Gardens

A rain garden is an area defined by a shallow, basin-like depression designed to collect, absorb and filter stormwater runoff in urban areas [101]. Rain gardens are generally characterised by different types of soil that act like sponges, as well as gravel to aid in drainage. They are planted with native vegetation that can tolerate both wet and dry conditions while providing habitats for wildlife. The mulch layer plays an essential role in retaining moisture in the soil and preventing erosion. Rain gardens philtre and purify the collected water in a completely natural way and allow the inflow of surface water to be slowed down to reduce the risk of flooding downstream. In addition to water cycle problems, rain gardens play an important role in controlling urban pollution by intercepting various pollutants present in the soil [102]; in particular, total suspended solids (TSS), nitrogen (N) and phosphorus (P) contained in urban runoff are removed [103]. Peng et al. [104] have highlighted the benefits of rain gardens in reducing carbon emissions, while a more recent study by Onur [105] illustrates the benefits of rain gardens in promoting ecosystem resilience, biodiversity and water management/sustainability.

They can be installed in residential areas, commercial areas and public spaces to counteract surface runoff and support biodiversity. Rain gardens are based on the concept of sponge cities or sponge gardens [104]; in fact, they can take the form of a small part of sponge gardens, and can also include other solutions such as pervious paving or swales.

5.2.3. Filter Drains, Filter Strips and Swales

The various techniques of sustainable urban drainage, which are always summarised under the generic term of NbSs, include filter drains, filter strips and wetlands.

Filter drains are shallow drainage trenches characterised by one or more perforated pipes surrounded by drainage material such as stone or gravel. They are considered rapid infiltration systems that encourage rainwater to infiltrate quickly into the ground [106]. They are not normally treated as green spaces and are located on roadsides.

Filter strips are slightly sloping, planted strips designed to filter and drain rainwater from neighbouring impermeable surfaces. They are usually located near runoff sources such as roads or buildings. They are often combined with swales and serve as pre-treatment components that promote sedimentation, filtration and infiltration [107]. The low or moderate flow velocity favours a reduction in dirt particles and removes sediments, organic material and existing heavy metals [107].

Swales are shallow channels with a flat and vegetated bottom that are designed and used to manage water runoff, but also provide aesthetic benefits and promote biodiversity by creating habitats for various animal and plant species. The ground is vegetated, usually with grass, to slow down the water and facilitate sedimentation and the process of filtration and infiltration in the soil [92,107]. These solutions can replace conventional pipes and, in combination with filter strips, avoid the use of kerbs or manholes.

5.2.4. Pervious Surfaces: Porous and Permeable Pavements

Pervious surfaces are configured as sustainable solutions for paving in urban areas. They are useful to control and mitigate stormwater runoff in the underlying structural layers [92,108,109]. They also help to replenish underground water reserves and reduce the risk of flooding and erosion. At the same time, the passage of water through the pervious surfaces favours the filtration of the water itself, removing pollutants and contaminants. Allowing water to evaporate from the surface of the paving itself improves the microclimate in the surrounding area, cooling the air and thus helping to reduce the heat island. The SUDS manual [107] distinguishes between two types of pervious pavements: porous and permeable. With the former, water infiltration occurs over the entire surface of the pavement (in the case of gravel or grass surfaces). Permeable paving, on the other hand, is characterised by materials that are inherently waterproof, but drainage occurs through the voids or joints that form between one element and another of the paving itself.

6. Results: A Taxonomy for NbS Implementation in the Built Environment

The detailed analysis of the NbSs from the scientific literature has allowed the proposal of a classification of the main interventions for the built environment. In line with the initial framework (presented in Figure 4), Figure 5 and Figure 6 summarise the classification and benefits of the different solutions identified for the building and urban levels, respectively. In particular, Figure 5 shows the typological, topological and technological classifications for green roofs and green walls. For both components of the building envelope, the application benefits at urban and building level are emphasised. Similarly, Figure 6 reports a typological classification and the relative scale of application for intervention at the urban level (neighbourhood, park, street), together with the environmental and socio-human benefits. The typological classification allows the definition of the type of solution (e.g., a green roof, a blue roof or a living wall), and the topological classification distinguishes the different types of solutions by the thickness of the substrate or, more generally, by the depth of the technological package; finally, the technological classification distinguishes the technological complexity of the functional layers of a given solution.

Figure 5. Framework with classification and benefits of NbSs for buildings.

Figure 5. Framework with classification and benefits of NbSs for buildings.

The classification of solutions, divided into interventions at building and urban level, has been summarised in a taxonomic framework (Figure 7) in order to clearly present the results obtained and provide a useful tool for the management of urban regeneration interventions. The usefulness of a taxonomic framework is that a structured system can be proposed for classifying and characterising different entities through a hierarchical and organised structure.

This decision is based on the fact that, although the benefits of NbSs are widely recognised in the scientific literature, a detailed and clear framework for NbSs in the built environment that can be configured as a tool to clarify and facilitate the selection of NbSs for implementation is lacking. By classifying NbSs based on application context and benefits, the taxonomy can be useful to identify the most appropriate solutions for specific urban challenges and facilitate decision-making. Each of the possible interventions applicable at the building or urban level brings different benefits to the built environment, the achievement of which allows us to address the problems of climate change and the pandemic and thus achieve the goal of urban resilience.

The proposed taxonomic scheme is structured in rows and columns. The columns represent the inputs to read and use, while the rows represent the outputs. The taxonomy can be read from left to right or from right to left, following a cyclical path.

It should be emphasised that some NbSs are more specific to a particular action, but consequently they also have indirect benefits. For example, a rain garden is created to capture and collect rainwater, but at the same time it also brings benefits through the use of green spaces and the creation of sustainable spaces for biodiversity. Urban greenery helps to moderate temperatures but also has drainage implications, as some of the water is absorbed by the foliage of the trees and therefore does not reach the ground. It should therefore be emphasised that their integration into built environment policy favours multifunctionality and the management of various challenges.

7. Discussion

Ecosystem health and human health are interconnected, so an integrated and unifying approach is needed to optimise the health of living things and of the environment [110]. Like natural disasters, pandemics also cause economic, social and organisational problems and disruptions [111].

Although the main goal of NbSs is to preserve the environment and biodiversity, the relationship between human health and the natural environment is fundamental in times of pandemic health emergencies. NbSs can therefore significantly contribute to the management of health emergencies related to the pandemic while promoting urban resilience by supporting physical and mental health through sustainable practises that also reduce the risk of infectious disease occurrence.

Urban planning that considers both climate change and pandemic risks is essential for the design of future resilient urban environments and for the regeneration of existing environments. Consequently, resilient cities are indeed better equipped to respond to public health emergencies such as pandemics and to promote urban health. In addition, as indicated in the introduction, the multifunctional approach of NbSs is also in line with the goal of climate neutrality and some of the various goals of the 2030 Agenda, above all SDG11: “Sustainable Cities and Society”.

Table 2. Relationship between NbSs, benefits and the goals of the 2030 Agenda.

NbS	Advantages	SDGs
agricultural and agroforestry activities	promoting the economy in urban and rural areas	SDG1: NO POVERTY
sustainable food production through agriculture and urban reforestation	preserving biodiversity and improving access to nutrients	SDG2: ZERO HUNGER
access to natural environments and green spaces	improving mental and physical health	SDG3: GOOD HEALTH AND WELL-BEING
creation of wetlands, forests and watersheds	purification and disinfection of water	SDG6: CLEAN WATER AND SANITATION
creation of cooling and naturally shaded areas	reduction in energy consumption	SDG7: AFFORDABLE AND CLEAN ENERGY
creating green spaces and infrastructure for more sustainable and liveable cities	promoting the resilience of cities and the well-being of the community	SDG11: SUSTAINABLE CITIES AND SOCIETY
the efficient use of materials and resources	reducing waste, promoting the circular economy and managing resources sustainably	SDG12: RESPONSIBLE CONSUPTION AND PRODUCTION
reforestation and carbon sequestration	mitigating the effects of climate change and promoting climate resilience	SDG13: CLIMATE ACTION
sustainable management of forests, wetlands and natural habitats	Preventing land degradation and protecting ecosystems	SDG15: LIFE OF LAND
cooperation between governments, civil society, businesses and communities	spreading knowledge and implementing NbSs	SDG17: PARTNERSHIPS FOR THE GOALS

shows in detail the benefits of some NbSs for achieving the Sustainable Development Goals.

As mentioned in the introduction, most publications in the literature focus on the benefits of NbSs for the environment and thus for combating climate change, while there is little evidence of their benefits in health emergencies. Therefore, the four main benefits of NbSs for all health pandemics and emergencies are summarised below:

• Mental health and well-being: In pandemic times, when people experience heightened levels of stress and anxiety, spending time outdoors provides an opportunity to relax, de-stress and improve overall wellbeing.
• Physical health: NbSs support physical health by providing spaces for outdoor exercise. Promoting outdoor physical activity is particularly important during the pandemic as it helps to maintain health.
• Air quality and respiratory health: Trees, plants and green corridors help improve air quality by absorbing pollutants and releasing oxygen. Clean air is important for respiratory health and the preservation of ecosystems. In addition, biodiversity favours the regulation of the movement of disease vectors and thus promotes resilience to epidemics.
• Social distancing in urban environments: Access to natural areas for outdoor recreational activities enables social distancing and promotes integration.

8. Conclusions and Future Directions

NbSs offer a multifunctional, holistic approach to improving the resilience of cities to climate change and post-pandemic challenges. By recognising the multiple benefits of urban greening, biodiversity conservation and runoff management strategies, cities can develop adaptive, resilient and highly sustainable urban ecosystems. In this way, cities are not only prepared for current challenges, but also for future uncertainties.

This study investigated the role of NbSs in increasing the resilience of urban areas, taking into account the multiple benefits for human and urban health. In the first part of the manuscript, a bibliometric analysis was conducted to define the main trends in the scientific literature on NbSs. In the second part, by collecting the information obtained from the systematic literature review, a taxonomic framework was created for the built environment. Systematising NbS interventions for the built environment and the associated benefits for improving urban resilience through a taxonomic framework can help policy makers choose the type of intervention to implement.

The topic of NbSs is certainly current and innovative, so it seems obvious that there are future research directions. In particular, future research should focus on using parameters and indicators to assess the applicability of interventions and subsequently monitor the implementation of NbSs in urban contexts in the short and long term. At the same time, it is important to understand how to intervene in particularly compact and dense urban structures where there is not enough space to implement NbSs. A comparative analysis of countries that have successfully integrated NbSs into their urban and regional planning and urban regeneration policies can provide valuable insights for the development of guidelines and best practise.

Research needs to take into account various constraints, including geographical and climatic conditions and territorial specificities, as solutions that are effective in one setting may not be suitable for another. In addition, raising awareness of NbS issues among citizens and policy-makers is essential. This can be achieved through dissemination activities and training events that promote close co-operation between the different stakeholders involved in territorial decision-making processes.