The assessment of the Intergovernmental Panel on Climate Change (IPCC) indicated that human activities are the primary cause of increasing concentrations of greenhouse gases (GHGs) in the atmosphere, which in turn leads to climate change . Thus, emissions of carbon dioxide (CO2), the most abundant of the GHGs, increasingly became an issue of concern for the different governments of nations around the globe. In the past few years, major carbon-emitting countries have developed plans and roadmaps to reduce or offset their CO2 emissions in the coming decades in order to achieve a carbon neutral future [2,3,4]. The building sector is responsible for nearly 40% of total global direct and indirect CO2 emissions . Thus, the decrease in equivalent-CO2 emissions from this sector could have a significant impact on climate change.
The urban green infrastructure, such as urban trees, vegetation, green roofs, and green facades, should be considered as nature-based solutions (NBS) for mitigating direct and indirect CO2 emissions from buildings and communities, given that these NBS can, in accordance with the findings in many existing studies, potentially mitigate the urban heat island effect, lower the temperature of the surrounding environment and inside buildings, and sequester CO2. However, for governments to confidently deploy these strategies at a scale necessary to achieve climate benefits, knowledge of quantifiable and demonstrated outcomes is needed. Thus, in this paper, research articles with respect to how and, to what extent, NBS can contribute to the reduction in carbon emissions of the urban environment and of buildings have first been reviewed. In addition, design guidelines for different NBS and policies to provide incentives and minimum requirements for the implementation of NBS are essential, given that the design and construction of NBS are novel to many building practitioners. As well, the implementation of NBS usually requires additional resources from municipalities and building owners; for example, this may include valuable urban land and additional operational funds to maintain NBS-related systems. The policies and supporting design guidelines should align with the benefits obtained from implementing NBS. As such, a review and discussion of their contents alongside the research findings was also conducted to permit evaluation of the adequacy of existing policies and design guidelines and to identify the information required to facilitate the widespread adoption of NBS by building, municipal, and community practitioners.
2. Literature Search
The research articles considered were initially extracted from the SCOPUS database over the last 15 years (2008–2023) based on keywords or a combination of keywords, as shown in Table 1. Additionally, another database, WoS, was also considered, but similar results to those from the SCOPUS database were generated. These selection criteria were applied due to the fact that most of the pertinent research articles were published post-2008. However, papers from other years that were mentioned in the articles reviewed were also considered during the review process. The terms, ‘carbon reduction’, ‘carbon sequestration’, ‘carbon neutral’, and ‘temperature reduction’, ‘urban heat island (UHI) effect’ are, respectively, directly and indirectly associated with CO2 emissions. Other keywords used include those describing different types of NBS and the different scales of application of such solutions. In total, 1188 papers were found based on the previously mentioned keywords, as shown in Figure 1. According to statistical data, green roofs have received more attention than other types of NBS (Figure 1a). While most studies have focused on investigating the benefits of NBS in mitigating the UHI effect, the number of papers related to carbon sequestration has increased considerably in recent years.
Similar research methods were employed in most research articles that have a similar research objective. Only the values of outputs and discussions were different depending on the particular conditions that were applied or taken into consideration for the research. Moreover, it is essential for reviewed articles to include quantitative data, such that the results can be utilized as evidence and reference for establishing future multi-disciplinary research initiatives. After taking into account all these factors, a review of 128 research articles was conducted, from which 65 research articles were selected based on interest in acquiring information on the ability and efficacy of NBS to reduce CO2 emissions. Another 17 documents on guidelines and policies for different municipalities in Canada were also reviewed.
Similar review articles have also been published in which the functions and benefits of various types of NBS have been analyzed and synthesized with respect to the mitigation of carbon emissions. Typically, these articles focus on specific types of NBS and have a narrower range of impacts. For instance, Zaid et al.  reviewed the potential of green walls for carbon sequestration and the mitigation of the urban heat island effect (UHI). Koch et al.  focused on reviewing green walls for their potential to mitigate the UHI. Weissert et al.  surveyed methods that were used to quantify the carbon storage and carbon sequestration of urban forests. Huamei and Gunwoo  reviewed different types of NBS for mitigating the UHI. Moreover, Elmira et al.  summarized the efficacy of green roofs in mitigating thermal stress for different climatic conditions. There are studies that were also considered to facilitate the review conducted for this study, which discussed the potential of multiple types of NBS in reducing carbon emissions, as well as reviewing relevant design guidelines and policies.
The subsequent sections of the review are organized as follows: Section 3 provides scientific evidence as described in technical research papers, demonstrating the direct and indirect contributions of NBS to reducing carbon emissions. These findings inform the discussions provided in Section 4 and Section 5, in which a review of NBS-related design guidelines and NBS policies are, respectively, given. The design guidelines and policies were discussed separately, given that they offer distinct types of information and serve different purposes.
3. Reduction in Carbon Emissions
The use of NBS could result in direct and indirect contributions to the reduction in carbon emissions, both for buildings and communities. Such strategies include reducing the temperature of an urban area, improving the energy efficiency of buildings, and sequestering carbon from the atmosphere by way of photosynthesis.
3.1. Temperature Reduction for Urban Areas
The growth of urbanization has been driven by the growth of population and as well, the ever-increasing demand for new dwellings all over the world. According to the Population Reference Bureau , by 2022, the worldwide urban population was, on average, 57% of the entire population, and this percentage increased to 79% for developed countries. The UHI effect is an undesired side effect of urbanization and could considerably aggravate the outdoor air quality by intensifying pollutant concentration , increasing the energy demand for cooling buildings within urban centers , worsening the thermal comfort of pedestrians [14,15,16], as well as increasing the risks of occurrence of overheating in buildings which can be lethal to elderly, vulnerable people [17,18,19]. The increase in energy demand is correlated to CO2 emissions, as the majority of energy is still generated through the burning of fossil fuels. A higher temperature in urban areas as compared to non-urban surroundings is a result of the absorption of shortwave and longwave radiation, heat emissions from buildings and vehicles, reduced air circulation, and less evapotranspiration as occurs from vegetation [20,21,22,23]. The NBS can contribute to mitigating elevated temperatures in urban areas by increasing surface shading and providing evaporative cooling [24,25,26,27,28] to reduce the surface temperature of the ground and buildings. The reduction in excessive temperatures in urban areas can also lead to improved thermal comfort and the mitigation of overheating in buildings.
Alexandri and Jones  investigated the thermal effects of vegetation on the building envelope for nine cities across the world with nine different types of climates and different configurations of the urban canyon and wind conditions using a two-dimensional, prognostic, micro-scale model. Ground-covering grasses and ivies were the types of vegetation covering green roofs and green walls, respectively. Simulations were conducted for a typical day of the hottest month in a given year. The maximum and average reduction in air temperature at the roof level were, respectively, 26 °C and 12.8 °C during the daytime. For the canyon, the corresponding maximum and average temperature reductions were, respectively, 11.3 °C and 9.1 °C. The effectiveness of vegetation was found to be increased for a hotter and drier climate but reduced for a wider canyon. Francoeur et al.  compared the performance of lawns located in Montreal, Canada, to another three types of urban NBS that had a low height, in terms of heat mitigation. It was concluded that the efficiency of NBS in mitigating urban heat could be improved by increasing the number of plant species. For a larger scale application, Bass et al.  investigated the effectiveness of green roofs to mitigate the UHI for the city of Toronto. The study was conducted using a Meso-scale Community Compressible simulation model at a 1 km resolution over a 48 h period. It was found that when the green roof area was 5% of the total area of the city, it could reduce the temperature by up to 0.5 °C at a height of 5 m above the ground. In addition, the cooling effects could be intensified by adding irrigated green roofs in high-density urban areas.
In another study, the effectiveness of cool, irrigated green roofs was compared to green roofs without irrigation in reducing the air temperature for the entire Berlin area ; the green roofs with irrigation were found to be able to reduce the average daytime and nighttime temperature by 0.71 °C and 0.26 °C, respectively. The study was conducted using the Weather Research and Forecasting (WRF) model coupled with the Urban Canopy Model. The evapotranspiration cooling effect was the primary factor considered when parameterizing the vegetation. No other information was provided on the implementation of vegetation in the model.
Imran et al.  evaluated the effectiveness of mixed forest (MF), a combination of mixed forest and grasslands (MFAG), and a combination of mixed shrublands and grasslands (MSAG) in mitigating UHI effects in the city of Melbourne during heatwave events. Numerical simulations were conducted using the Weather Research and Forecasting (WRF) model coupled with the single layer urban canopy model. The three types of NBS analyzed, MF, MFAG, and MSAG reduced the near-surface temperature by 0.6–3.4 °C, 0.4–3.0 °C, and 0.6–3.7 °C during the night, respectively, when the fraction of these NBS varied from 20% to 50%. The cooling observed at night was attributed to a decrease in thermal energy stored in urban surfaces during the day as a result of the replacement of high-density urban areas with vegetated surfaces. Meanwhile, no cooling effect was observed for the daytime as a result of the method used to model vegetation which excludes the shading effect.
Dardir and Berardi  estimated the effect of green roofs, green walls, and trees to mitigate UHI for three urban typologies (Brampton, Malton, Downtown) within the Greater Toronto Area using a validated Urban Weather Generator. For the investigated sites, the best-performing scenario had 60% of the tree canopy with green roofs and green walls, which reduced the 10-day average canyon air temperature by 4.6 °C as compared to a reference performance scenario for which only 20% of the tree canopy without green roofs and green walls was evaluated. In this study, the relationship between the effectiveness and affordability of different NBS strategies was also investigated. It was concluded that the most cost-effective option was increasing the tree canopy.
Wang et al. , using the ENVI-met model, tested the effectiveness of different NBS strategies for Guangzhou, China; to mitigate the surface UHI and improve the urban thermal comfort. The “green cover area” was increased by 10% to 20% through the addition of grove and street trees, and green roofs. The six (6) proposed strategy scenarios, including a base condition, were applied to three (3) randomly selected sites within the city, which were circular areas having a 1 km diameter. The simulations were carried out for 24 h with a 1 h interval using meteorological data for 29 October 2019. The study indicated that grove and street trees were more successful in reducing surface UHI as compared to green roofs, for which green roofs had a negligible effect in reducing surface UHI.
Evola et al.  compared the effectiveness of green walls, green roofs, and a combination of trees and urban vegetation for the mitigation of UHI at a building scale and urban scale. The study was completed using the ENVI-met model for a 200 m2 area located in Catania, Southern Italy. It was discovered that the combination of trees and urban vegetation was more useful in decreasing the outdoor air temperature, with a reduction of between 0.5 °C and 1 °C, whereas green walls and green roofs were highly effective in mitigating high indoor temperatures.
In the process of applying NBS to mitigate the excessive temperature in urban areas, the importance of different factors influencing the performance and efficacy of various NBS was explored in the vast majority of studies. In general, the number of NBS, their different types, and combinations of types with respect to the selection of vegetation were found to be significant in determining the degree of effectiveness of NBS in reducing urban temperature [25,26,35,37,38,39,40,41]. However, there was no consensus regarding the most effective vegetation type or combination of vegetation in reducing the urban temperature, given that the urban environment and local climatic conditions differed from one city to another. The implementation of NBS is evidently designed specifically for a particular city. Other considerations to be taken into account to achieve an optimal NBS design are the surrounding micro-climate and substrate, which acts as the foundation for vegetation to grow on. In general, extensive green roofs tend to have lower cooling and insulation properties compared to intensive systems due to their thinner substrate layer. On the other hand, intensive green roofs, with their thicker substrate layer and diverse plantings, provide enhanced insulation and cooling benefits, as well as the capability for carbon sequestration and storage. For instance, the vegetation is less effective in reducing the temperature in a wider urban canyon , and the effectiveness of a vertical greenery system is affected by its orientation . The cooling effects of a green roof can be improved by situating the green roof within an urban area where there is a greater building density . Moreover, the water content in the substrate is crucial to the performance of NBS as it provides the water resource for the evapotranspiration process, from which the heat from the surroundings is absorbed and thus cools the adjacent air [24,28,38,42]. It was also observed from these studies that the most commonly used numerical tools to undertake numerical simulations at an urban scale are the Weather Research and Forecasting (WRF) model coupled with the Single Layer Urban Canopy Model [32,33,34] and the ENVI-met model [35,36,37].
One major knowledge gap identified from these studies when assessing the performance of NBS in reducing air temperature was that it is still difficult to describe the mechanism of convective heat transfer between the plant and the ambient air. Qualitative and quantitative information were derived from statistical methods using mathematical models and empirical data, which were simultaneously affected by factors such as wind, humidity, microbiological factors, and other relevant factors. It is somewhat challenging to accurately isolate the effect of one particular factor and to extrapolate observations to a broader context . Thus, it is also challenging to develop a reliable model that can accurately measure the effect of vegetation on temperature reduction. In addition, for studies at an urban scale, estimations of temperature reduction are performed primarily using numerical simulation tools, given the difficulty and complexity of obtaining accurate measurements before and after implementing the NBS with high resolution on such a large scale. Nevertheless, validation tests, i.e., validation of models using a smaller area and validation of parameters used in these models from experiments, can be conducted to improve the reliability of numerical models. It was also noted that many studies used scenarios with extreme weather, such as heatwave events, and the investigations were usually only conducted for a period of less than one month. The quantitative information obtained from such short periods may not adequately represent the usefulness of incorporating NBS over the long term, e.g., over a year. Thus, to permit further quantifying the reduction in CO2 emissions due to the temperature reduction in urban areas, long-term simulations, i.e., one or multiple-year simulations, the results from which can potentially minimize the impact of seasonal climate factors and short-term extreme weather on the interpretation of results, are essential, although extensive computational resources may be required.
3.2. Improvement of Building Energy Efficiency
The NBS are effective in reducing the air temperature in urban areas. Less energy may be needed for the heating and cooling of buildings in such areas to maintain a comfortable indoor environment during the summer time and thus reduce anthropogenic CO2 emissions. Meanwhile, the application of NBS on individual buildings may have a more significant impact on their energy efficiency. Specifically, heat flux through the building envelope can be reduced by the coverage of green walls and green roofs; insulation effects of green roofs and the increased thermal mass from green roof layers can moderate influences of outdoor temperature to the fluctuations of indoor temperature, which leads to improvements of building energy efficiency; lower wind speeds on the surface of buildings can be achieved from the buffer effect of NBS in proximity to, or on the surface of buildings, as such buffering reduces air infiltration across the building envelope and thus contributes to the reduction in heat loss or heat gain.
Shading and evapotranspiration are the two major cooling mechanisms that may be provided by NBS coverage . The shading prevents a fraction of solar radiation from reaching and warming the underlying surfaces, and the evapotranspiration process absorbs heat from ambient air. As the number of leaf layers increases, the capacity for shading to reduce solar radiation is enhanced. Experiments from Ip et al.  indicated that 45% to 12% of solar radiation could transmit through 1 to 5 leaf layers of Parthenocissus Quinquefolia (Virginia creeper). In the numerical simulation conducted by Wong et al. , the heat transfer values of the building envelope were reduced by 0.59%, 21.2%, and 40.7% when the shading coefficients of species were 0.986, 0.500, and 0.041, respectively. Feng and Hewage  estimated the energy performance of a five-story high building located at the University of British Columbia (UBC), Okanagan, Canada, and having a LEED Gold standard for three green vegetation coverage scenarios, i.e., green building without a green roof, green building with full green roof, and green building with full green wall. These coverage scenarios were compared to the benchmark scenario using validated simulation models in Design Builder software. The results indicated that the full green roof and the full green wall could save 3.2% and 7.3% of the annual cooling energy, respectively. The function of vegetation was represented by a green roof energy balance model developed by Sailor .
Tan et al.  investigated the efficiency of different rooftop options, i.e., cool roof, green roof, and solar panel roof, on saving cooling energy during heatwave events over the Chicago metropolitan area using the WRF regional climate model and a Building Effect Parameterization model coupled with the Building Energy Model. The parameterized green roof used in the simulation was developed by Munck et al.  and it was able to save 14% of energy usage for the investigated area.
Lin et al.  investigated the performance of green walls applied to a high-rise residential building with a height of 54 m subjected to a hot, humid climate in reducing the cladding surface temperature through in-situ measurements. The measurements were taken over 6 days in August 2017. The average surface temperature of the green wall facade was reduced by 4.7 °C.
Bano and Dervishi  explored the efficiency of green walls applied to a high-rise office building exposed to a Mediterranean climate having several design variables, i.e., window-to-wall ratio, leaf area index, and plant height. For those scenarios having window-to-wall ratios of 50% and 70%, the green wall could, respectively, reduce whole-building energy consumption from 9% up to 11% and 3% up to 6%. The green wall was found to be more efficient on fully glazed buildings, as in this study, it covered a larger surface area. This resulted in energy savings for cooling of 14% to 34% over the summer period. The Design Builder was used for the simulation. Vegetation characteristics were parameterized using the Energy Plus built-in module.
Djedjig et al.  investigated the impact of green walls on a group of buildings and the surrounding area using a reduced-scale mock-up of buildings and streets. The green wall system was able to reduce the inner surface temperature by up to 10 °C, lower the inside air temperature of the mock-up by up to 6 °C, and decrease the air temperature near the façade by up to 1 °C.
In addition to energy savings over the summer time, it was noted that NBS are also beneficial to building energy savings over the winter time. In particular, the compositions of NBS structure can increase the thermal resistance of the building envelope by reducing its thermal conductivity and thus reduce the heat loss during the colder time periods. The analysis of the thermal performance of NBS during colder time periods should be divided into two phases. One phase is for the time period during which vegetation can still thrive. A second phase occurs when the temperature drops below freezing points, and most of the vegetation has withered.
In respect to Phase 1, for colder time periods in winter where vegetation can still thrive, Foustalieraki et al.  took measurements for temperature and humidity for a two-story commercial building located in Athens, Greece, during the winter time. The green roof increased indoor air temperature by up to 0.7 °C during the winter period, according to the simulation using Energy Plus, and energy reduction for heating attained a value of up to 11.4%.
Collins et al.  compared energy consumption for heated boxes installed with green roofs and bare roofs during the winter time in southern Finland. Prior to the freezing period, the energy loss of green roofs was 25% to 38% less than that of bare roofs. Cameron et al.  monitored the energy consumption to maintain 16 °C for a water tank surrounded by brick cuboids covered with different vegetation during winter time. The use of ivy or other foliage could reduce the heating energy consumption of the water tank by 21% to 37% in comparison to the bare cuboids surrounding the water tank. The energy efficiency was found to be further improved during periods of extreme weather.
In the previously discussed study of a LEED Gold standard building located in Kelowna, Canada, and conducted by Feng and Hewage , 20% of heat loss through walls and roofs could be reduced by completely covering them with vegetation during a typical week in the winter (i.e., 13 January 00:00 a.m. to 20 January 00:00 a.m.). However, the status of vegetation affected by the harsh winter environment was not considered in this study.
Serra et al.  found that the thermal conductance and transmittance of the substrate for a vertical greenery modular system (VGMS) located in Turin, Italy, was lower than that of the reference wall. Overall, the VGMS could have a beneficial effect on the reduction in heat loss during the winter period, despite its capacity to reduce the amount of absorbed solar radiation.
In respect to Phase 2, for which colder periods occur where the temperature drops below freezing and most vegetation has withered, it has been determined that the properties of substrates affected by ice and snow become the dominant factor when assessing the thermal performance of the building envelope. The process of water freezing could improve the energy efficiency of green roofs by reducing heat flux at the inner surface of the roof by up to 30% . The thermal conductivity of the substrate affected by frost before and after freezing was, respectively, 0.41 W/m·k and 0.12 W/m·k . The snow cover can be considered as an extra insulator to the building envelope [53,55,56]. Thus, it reduced the relative contribution of the green roof in saving heating energy.
In all the literature reviewed, no studies were found that investigated the variations in thermal performance of NBS over the long term, i.e., several years, and few studies were conducted to compare the performance of different types of NBS for buildings having different configurations. As well, the influence of NBS on the integrity of the building envelope over the long term is unknown. For instance, water leakage from blue–green roofs (green roofs incorporating a stormwater management system) may damage the roof membrane, which would have a negative impact on the thermal performance of the original roof. Consequently, the overall benefit of using a green roof would thereby become questionable. Moreover, in most studies, the net energy savings by considering the consumption of operational energy to maintain the NBS is not discussed, and neither were there any studies for which reductions in building energy correlated to carbon reductions.
The list of models and tools that were used in the studies reviewed in this paper to investigate the effects of NBS in reducing urban temperature and building energy consumption are shown in Table 1.
A summary of models used to investigate reducing urban temperatures and improving energy efficiency in buildings.
A summary of models used to investigate reducing urban temperatures and improving energy efficiency in buildings.
Model typesApplicationsLocation and ClimateFindings and ConclusionsA two-dimensional, prognostic (dynamic) micro-scale mode developed in C++
Green roofs and green wallsLondon, UK
Riyadh, Saudi Arabia
Hong Kong, China
Brasília, BrazilThe maximum and average temperature reductions at the roof level were the greatest in Mumbai and Riyadh during daytime, reaching 26.1 °C and 12.8 °C, respectively. On the other hand, the smallest decreases were observed in London and Moscow.Mean Information Gain (MIG) computed from photo
Lawns and 3 species of plants with low heightMontreal, CanadaAbility of plants to reduce urban heat could be improved by complexifying the plant structureMesoscale community compressible model, ISBA SVAT scheme and an urban canyon model
Green roofs City of Toronto, CanadaTemperature of urban area could be reduced by 0.5 °C if 5% of city area is covered by green roofsWeather Research and Forecasting (WRF) model coupled with the Single Layer Urban Canopy Model (SLUCM)
[32,33,34]Green roofs, green walls, forest, grassland. Berlin, German
Toronto, CanadaTemperature of the investigated urban area could be reduced by applications of NBS, although, effectiveness of different NBS was affected by many factors, e.g., vegetation species, irrigation, areas. ENVI-met model
[35,36,37]Green roofsGuangzhou, China
Catania, ItalyDesign Builder
[46,51]Green roofs and green wallsOkanagan, Canada
Tirana, AlbaniaFull green roof and full green wall could save 3.2% and 7.3% of the annual cooling energy, respectively.
The green wall could save 14% up to 34% energy for cooling during summer time.WRF regional climate model and Building Effect Parameterization model coupled with the Building Energy model
Green roofsChicago, USAThe green roof could save 14% of the cooling energy.
3.3. Biological Carbon Sequestration and Storage
Urban vegetation has the capacity to store and sequester carbon in the atmosphere via its growth and photosynthesis processes. Nowak et al. quantified the carbon storage and sequestration by trees in the United States . On average, the carbon storage density was 7.96 kg CO2/m2 per year of tree cover, and the carbon sequestration rate was 0.28 kg CO2/m2 per year of tree cover. The carbon storage density was estimated by calculating the ratio of total carbon storage, which was calculated using biomass equations, as given in Equation (1), to tree cover area , and the rate of carbon sequestration was estimated using the urban tree growth rate . For the United States, the total annual urban net carbon sequestration was estimated at 18.9 million tonnes per year.
where M is the oven-dry weight of the tree (kg), D is the diameter at breast height (DBH), a and b are parameters. Chen  quantified carbon storage and carbon sequestration by urban green infrastructures, i.e., urban green space developed for recreation, amenity, and ecological purposes, in 35 major Chinese cities that covered most types of climates in China from a review of existing studies for these locations. The average carbon storage density was 2.13 kg CO2/m2. The average carbon sequestration capability was 0.22 kg CO2/m2 per year, and the corresponding total amount of carbon sequestration was 1.9 million tonnes per year.
Gratani et al.  estimated the carbon sequestration capabilities, in smaller-scale studies, of four parks located in Rome, Italy, filled with different tree and shrub species. Carbon sequestration rates of these four parks varied from 0.664 to 0.998 kg CO2/m2 per year, which were found to be significantly correlated with the leaf area index. According to George and Palmyra , the carbon sequestration for a green roof was 1.22 kg CO2/m2 per year. In a study completed by Heusinger and Weber , a green roof contributed to 0.313 kg CO2/m2 per year of carbon sequestration. Luo et al.  compared the capability of different green roofs configurations located in Chengdu, China, for carbon sequestration, i.e., the degree of carbon sequestration provided by two different roof substrates, three different roof substrate depths, and various tree plant species. The mean carbon sequestration rate was estimated at 6.47 kg CO2/m2 per year, and the best-performing green roof had a carbon sequestration rate of 7.03 kg CO2/m2 per year. Kong et al.  estimated the contributions to carbon storage of an urban turfgrass system, located in Hong Kong, by considering grasses, soils, and carbon emissions from maintenance operations. The carbon storage capacity of grasses and soils were, respectively, 0.05 to 0.21 kg CO2/m2 and 1.26 to 4.89 kg CO2/m2, whereas the operational carbon emission was 0.17 to 0.63 kg CO2/m2 per year. Based on these numbers, it was concluded that the benefits of carbon storage for grasses and soils could be offset in 5 to 24 years.
Grossi et al.  considered a carbon sequestration rate of 0.575 kg CO2/m2 of tree coverage in the life-cycle assessment of an all-electric laboratory and a natural gas-powered single-detached house located in Montreal. The gardens of the laboratory, with an area of 410 m2, and the single-detached house, with 505 m2, featured full urban tree coverage; the total life cycle carbon emission for these two buildings was offset approximately 17% and 3%, respectively, from urban tree coverage.
The capability of carbon sequestration as can be achieved from NBS varies depending on numerous different aspects. The carbon sequestration rate of trees is affected by species, tree size, tree health, growth rates, and corresponding site conditions . Regarding the carbon sequestration capability of green roofs, the roof plant species, depth, and moisture content of the substrate have a direct impact on the carbon sequestration and storage capability. Moreover, plants and substrates also affect each other’s performance as the growth and survival of the roof plants rely on the substrate [67,68].
With respect to the carbon sequestration capability of vertical greenery systems, water content, percentage of dry matter, mortality of plants, and changes in the carbon storage rate of plants over the long term are factors that affect the carbon sequestration capability . In addition to the biomass Equation (1), which was specifically applied to trees, the carbon sequestration of the vegetation is typically assessed by measuring the carbon content using an allometric equation and laboratory work in both the above and below-ground biomass (AGB); i.e., stems and leaves found above ground, and below-ground biomass (BGB), as is evident in the plant roots .
The initial step when applying an allometric equation is to assess the total dry weight (TDW) of the plant or biomass, followed by determining the total carbon weight (TCW), and, finally, the total CO2 weight (TCO2W). The relationship between these parameters varies based on the species of vegetation. For instance, according to Othman and Kasim , the TCW is 0.5 times the TDW, and the TCO2W is 3.67 times the TCW for shrubs.
One of the major limitations of current studies when assessing the carbon sequestration capability of plants and vegetation is the uncertainty in the outcomes induced by the uncertainty in the input data. Further, many relevant studies have been undertaken, but only on a limited scale, and thus, their conclusions may not be generalizable to scenarios of a greater scale. Weissert et al.  indicated that there was a considerable amount of uncertainty in the currently available results for the evaluation of the potential of urban forests to reduce CO2 emissions in urban areas. Lin et al. estimated uncertainties in input data, sampling methods, and models when using the i-Tree model to estimate carbon sequestration from the tree canopies of 15 American cities . Bootstrap and Monte Carlo simulations were used for the estimation. Sampling methods were found to be the most significant factor contributing to the uncertainty associated with carbon sequestration, surpassing the accuracy of the models and input data. It was also noteworthy that for most studies, the degree of soil respiration as a means of offsetting the contribution made by NBS in carbon sequestration was not considered.
4. Design Guidelines
It is evident that NBS can deliver a number of benefits to the urban environment, including, but not limited to, mitigating urban heat island effects, saving energy, and sequestering carbon. However, given that numerous influencing factors need to be considered to determine the performance of NBS and considering the potential interactions between NBS and building components (e.g., green roof and building roof layers; vertical greenery system and building façade), design guidelines should be developed to facilitate the planning, design, construction, and maintenance of NBS. This allows practitioners to maximize the benefits of NBS whilst maintaining the integrity and functionality of the building to which the NBS is attached in different situations over a long-term period.
4.1. Green Roof
The Guidelines for the Planning, Construction, and Maintenance of Green Roofing  is perhaps the first comprehensive design guideline exclusively developed for green roofs, and it has been used as a benchmark for the development of guidelines by other institutions. The technical part of this guideline provides information regarding (1) types of green roofs and forms of vegetation; (2) function and effects for urban planning, ecology, and economy; (3) requirements for construction and building materials; (4) structural requirements; (5) requirements for the construction of vegetation areas, drainage layer, filter layer, vegetation stratum; (6) requirements for seeds, plants, and vegetations; (7) planting and seeding; as well as; (8) maintenance of green roofs.
Further, factors that should be considered by practitioners when making decisions for each of the above aspects were provided.
The supplementary guideline for the Toronto Green Roof Construction Standard  provides information regarding the minimum requirement for the design of green roofs in Toronto, Canada; this is a companion document to the Toronto Green Roof Bylaw. This guideline includes six chapters (1) an introduction to the green roof components; (2) considerations for structural impact, e.g., additional deadload, roof slope stability, and parapet height requirement.; (3) suggested factors to be considered for fire safety and occupancy safety; (4) suggestions for water proofing; (5) preservation of vegetation performance; (6) quality assurance for green roof operations.
For each chapter, a corresponding code and bylaw are cited to enhance the understanding of performance requirements. In addition, detailed recommendations to achieve best practices are also provided in each chapter. In comparison with the FLL guideline , this guideline provides more detailed information regarding the local code requirements and more practical recommendations, although only limited information on the impact of influencing factors on the performance of green roofs is provided.
Other design guidelines for green roofs, such as [74,75], also provided similar information. A major limitation for all the design guidelines reviewed is that no approaches or data were provided to quantify, for a given green roof, the potential benefits, in terms of, e.g., probable temperature reductions, reductions in energy consumption, or degree of carbon sequestration. However, this is perhaps understandable given that the expected performance of different green roof applications, even should these be located in similar climatic and geographic regions, would vary, case by case. A more practical approach would be monitoring the performance of green roofs designed and constructed based on performance requirements given in the design guideline from which empirical data for performance assessment could be obtained. In addition, the design guidelines, as were reviewed, have no information in regard to the green roof structure on the lifespan, long-term performance, and changes in performance over a long period of time; such information is crucial to being able to estimate the overall benefits of green roofs.
4.2. Urban Forestry
Municipalities across Canada take the primary responsibility for the development and management of urban forestry. Guidelines for urban forestry are usually nested within municipal design guidelines or form part of the city’s development strategies or plans, given that the urban forest is a necessary and important part of the municipal system and is implemented by incorporating it within other urban infrastructures. Most of the guidelines of the different cities investigated included similar topics: tree planting, tree protection, tree maintenance, tree inventory, and forest management. These guidelines are used as a companion document for local bylaws, although the details of these guidelines may not be the same owing to the unique requirements of the different municipalities.
The Urban Forest Management Plan , approved by the council of the City of Ottawa, provides 26 recommendations for expanding the urban forest, improving the health of vegetation, and promoting resilience. These recommendations were developed based on the conditions of the existing urban forest, the city’s resources, best practices, and scientific and technical literature.
The Municipal Design Guideline  for Halifax, Canada, contains a section (5) for the management and expansion of growing trees. It provides specific guidelines for tree placement, tree planting, health maintenance, requirements for the quantities of trees, and tree protection subjected to an urban environment.
The Street Tree Guidelines , established by the City of Vancouver, Canada, describes 14 aspects of forestry issues, such as tree placement, tree species selection, soil mix, tree protection, and other tree-related elements that need to be considered when planting and landscaping urban trees. The minimum requirements and recommendations were stated throughout each section of the guideline.
Similar guidelines for the urban forest can also be found in many municipalities across Canada. In these guidelines, no information or method was provided to quantify the carbon sequestration capabilities of an urban forest, whereas, in the United States, a nationwide guideline  is available to assist practitioners in carrying out their roles and includes a standardized process to evaluate CO2 reduction, cost-effectiveness for CO2 reduction, and a comparison between benefits and costs for tree design.
Additionally, it is recognized that there would be a great benefit to incorporate the design guideline with tools, such as the i-Tree model, which can facilitate forest management and help develop strategies by providing quantitative information regarding the forest structure and the assessment of benefits.
Documentation in which Canadian urban forestry policies are described, explained, and for which instructions are given in, similar to guidelines, are issued and managed by municipalities and can be classified into urban forest plans and strategies and bylaws. Urban forest plans and strategies usually comprise an introduction in which the benefits of the implementation of urban forest are highlighted; expectations for the development of the urban forest over the long-term; principles to be followed for decision making, specific targets, as well as a description of the operational framework.
In addition to providing guidelines to facilitate the design of NBS, the Urban Forest Management Plan (UFMP) issued by the City of Ottawa  was also intended to enhance an understanding of urban forest-related policies, practices, and standards. A framework and recommendations are provided in this UFMP that guides practitioners to work towards achieving the vision and targets as specified for the time period of 2018–2037. The 20-year UFM Plan has been designed to be implemented in two phases. The first phase is currently underway and includes six projects that are to fulfill eight of the twenty-six (8/26) recommendations outlined in the UFMP. Specifically, (i) review and update current NBS-related bylaws for the City of Ottawa, i.e., Urban Tree Conservation Bylaw, the Municipal Trees, Natural Areas Protection Bylaw, as well as the Heritage Tree Bylaw; (ii) develop a Forested Areas Maintenance Strategy to enhance the health, resilience, and safety of Ottawa’s City-owned woodlands; (iii) assess the benchmark canopy cover data; (iv) approve new policies to balance better the protection of urban natural heritage features and policies for efficient of land use; (v) increase awareness among city staff and other decision-makers involved in the management of the current and future urban forest; (vi) maintain and improve the health and conditions of existing trees in an urban area.
Toronto’s Strategic Forest Management Plan  described the current status of the urban forest in Toronto and introduced six strategic goals: (i) increase canopy cover; (ii) achieve equitable distribution; (iii) increase biodiversity; (iv) increase awareness; (v) promote stewardship; (vi) improve monitoring of existing green areas. In 2017, the updated Ontario’s Municipal Act and the City of Toronto Act delegated additional authority and capability to municipalities to foster and safeguard green infrastructure. Specifically, municipalities need to have regulations in place for preserving and improving their tree canopy and vegetation; municipalities are now allowed to pass bylaws regarding the implementation of green roofs.
In the City of Montreal’s Climate Plan 2020–2030 , measures have been outlined for five aspects to make it a more adaptive city in the face of climate change; these include: (i) plant, nurture, and safeguard 500,000 trees, with a particular emphasis on locations that are most susceptible to heat island effects; (ii) provide stimulation for the transformation of parking lots into open spaces; (iii) incorporate minimum greening requirements into existing building construction and renovation bylaws; (iv) create new parks, green corridors and wetlands, and restore shorelines; (v) increase the percentage of the protected area to 10% within the city. Similar plans and strategies can also be found in many other municipalities across Canada, such as the municipalities of Halifax , Edmonton , Vancouver , and Richmond (Vancouver) . Based on conclusions derived from articles reviewed, as well as the recommendations outlined in these policy documents, all such information is both beneficial and entirely useful to achieving carbon neutrality in the coming years, as is the interest of several Canadian municipalities to realize by 2050.
Municipal bylaws for urban forests have permitted, for specific scenarios, the alteration or removal of trees located in public areas. Specific minimum requirements for urban tree protection have also been provided in municipal bylaws. As an example, for Halifax, the bylaw Respecting Trees on Public Lands (ByLaw number T-600)  requires the installation of a barrier with a minimum dimension and spacing to protect public trees should any work be undertaken within the drip line or root zone of the tree. At the same time, the barrier is also required not to affect the sight line of persons using adjacent pathways.
In Canada, the only current green roof by-law  has been issued by the City of Toronto. This bylaw requires a minimum 20% coverage ratio for green roofs of buildings having over 2000 m2 gross floor area. The threshold ratio increases for buildings having larger gross floor areas. Failure to comply would result in the rejection of a building permit being issued. Relevant by-laws are also to be found for other Canadian cities, such as those for Ottawa, Calgary, and Vancouver. In addition, a Green Bylaw Toolkit  has been developed and regularly updated for municipalities to facilitate the establishment of green infrastructure-related bylaws.
6. Discussion and Conclusions
This paper reviewed the advantage and limitations of NBS in addressing the challenge of reducing CO2 emissions in urban areas. Design guidelines and policies implemented in Canada to facilitate the implementation of NBS were also investigated. Suggestions and requirements outlined in these documents are examined in the review while taking into account the scientific evidence from research papers. It is clear that the adoption of NBS can contribute to carbon neutral communities due to their effectiveness in both reducing the energy consumption of buildings and sequestering carbon from the atmosphere. Further, supportive guidelines and policies to facilitate the implementation of NBS at the city scale have been developed or are currently being developed. Nonetheless, it is still challenging to quantitatively assess the significance of NBS to help achieve carbon neutrality at the city scale, both currently and in the future, given the uncertainties of the input data when conducting an assessment, uncertainties in the methods for quantification, and discrepancies in the conclusions obtained from studies having different temporal and spatial scales. Despite the lack of quantitative evidence, the findings and conclusions derived from existing research articles were sufficient to qualify the measures and suggestions outlined in the guidelines and policies.
Future research should focus on closing the existing knowledge gaps; these primarily include quantifying the benefits of NBS by focusing on carbon emission reductions through improvements in energy efficiency by reducing cooling and heating loads as may occur from moderating the micro-climate in proximity to buildings as well as quantifying carbon sequestration and storage. More specifically,
Conduct field measurements of temperature and the levels of CO2 in areas with and without NBS implementation by collaborating with municipalities to provide more evidence for policy-making;
Develop numerical or mathematical models from which the benefits of NBS can readily be correlated to carbon emission reductions;
Conduct long-term simulations (minimum 12 months) using the validated model over a broad geographic area to minimize the impact of seasonal climate factors and microclimate variations on the quantitative findings;
Incorporate these quantitative findings into a life-cycle assessment for specific NBS; such measures will also be beneficial in the development of more appropriate design guidelines to aid practitioners, as well as provide quantifiable and demonstrated outcomes for the benefit of municipal and community policymakers.
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