Abstract
This systematic review aims to appraise the quality of evidence on greenness co-benefits of climate change and health. Although there is evidence of the co-benefits of greenness on climate change and health, the research is of poor quality when it comes to addressing the connections and identifying mediators of greenness and climate change mitigation associations. The evaluation sought to identify areas where there was little or no evidence to guide future research. Current published studies mainly cover six health outcome categories (birth outcomes, physical activity, mental wellbeing, obesity, mortality, and cognitive function). For adaptation, greenness and climate change, associated studies included (a) cooling down effects and urban heat island impacts, (b) air quality improvement, and (c) flood mitigation. For each outcome, we performed a systematic search of publications on Web of Science, PubMed, Google Scholar, and Science Direct databases from 2000 to July 2021. After retrieving records in which full papers were assessed and non-English articles were excluded, a total of 173 articles, including research articles and reviews, were chosen. To ascertain the strength of the evidence, all interventions were assessed using the GRADE approach. The quality of evidence ranged from moderate to high for most categories of health outcomes (birth outcomes, mental wellbeing, mortality). GRADE assessment provided low-quality evidence for studies on air quality, flood mitigation, physical activity, and obesity due to poor study design (observational or limited data) and high heterogeneity (some data provided variability), and the review concluded that there is insufficient evidence on firm recommendations for public health interventions. Due to a huge amount of low-quality evidence and several areas of overlapping study, this evaluation recognized the co-benefits of greenness on climate change and health as an understudied field and hence as a research gap. The evidentiary foundation for greenness-climate change mitigation links was generally weak. Future research on climate change greenness co-benefit interventions should pay special attention to flood prevention, air quality interactions, and health effects including physical activity and obesity.
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1. Introduction
Climate change and the health costs it imposes are the realities of our century, and the negative effects of climate change are already becoming a legacy we will pass on to future generations [1–3]. Recent IPCC (AR6) report uses Shared Socio-Economic Pathways that look at a far greater range of scenarios. There is a greater focus on lower degrees of warming because of these scenarios [4]. Although the report highlights different climate futures and emphasizes the warming impact (1.5 °C temperature rise), a greater focus should be placed on boosting greenness and fighting climate change via multiple approaches [5]. To investigate further, this review was conducted to show evidence on the co-benefits of greenness on climate change and health.
Findings show that taking a health-oriented approach to climate issues will enhance concern and awareness of both global climate change and population health on a worldwide scale [6–8]. Research suggests that increased awareness may provide more green spaces and promote better adaptive strategies [9, 10]. Greenness can help achieve better climate mitigation [11]. It is evident that green areas can curb emissions by absorbing pollutants and providing shade to help reduce energy demand for cooling of buildings [12]. Greenness reduces the likelihood of flooding, and provides a number of environmental health benefits by offering additional space for physical activity, which reduces morbidity and mortality associated with obesity and other chronic diseases [13]. In addition that green space provides shade to help reduce the effects of heat-induced illnesses (heat stroke, exhaustion) [14].
Green space is also important for mental health [15–17]. Many studies have shown that individuals who live near green spaces and have more opportunities to connect with nature generally experience less anxiety, stress, and depression, and can maintain an improved quality of life [18]. Adverse mental health outcomes, cardiovascular disease, and mortality risks can be reduced with greenness, research suggests [19]. Urban green space or green infrastructure, such as green roofs, can be used to build community gardens for community gatherings for younger generations and older adults [20]. Greening strategies can also be altered and improved in light of the research done in climate change and health studies [12, 21].
In light of evidence from previous research, it is clear that green spaces can be a solution to both problems (climate change and health) since they are directly and indirectly connected through many pathways (cooling effect, pollution and heat mitigation, mental and physical health improvements) [22]. However, studies looking into greenness's co-benefits on both health and climate change are limited, and not enough research exists regarding how greenness can help mitigate climate change. This review will look into the literature on the aspects of (a) greenness benefits on climate change for climate adaptation and mitigation, and (b) greenness impacts on health and evaluate the quality of evidence using the GRADE approach [23]. The goal of this literature review is to assess studies conducted over the last two decades that look at the co-benefits of greenness on climate change and health. Previous studies will be carefully analyzed, and the current literature will be critically engaged in the following parts to gather evidence.
2. Methods
2.1. Design
We conducted a systematic review of existing studies on greenness co-benefits of climate change and health. Studies included different methods of analysis, including cohort studies and experimental evidence, in different countries and regions and with different population groups.
2.2. Search methods
Peer-reviewed publications that reported the impact (benefit) of greenness on climate change and health were reviewed between the years 2000 to July 2021. The databases Web of Science, PubMed, Google Scholar, and Science Direct were used for the literature review. The search terms were 'greenness and health and climate change', 'climate change and health', 'greenness benefits and health', and 'greenness and climate change'. If relevant literature was cited based on the search terms, it was included and discussed when providing additional information. This search returned many off-topic articles, as evidenced by their titles and abstracts. In the end, the most important, relevant, and novel publications were considered to form the main body of the review (n = 173). A PRISMA flow chart was constructed for screening, eligibility, and identification of the resources [24] (figure 1).
Figure 1. Prisma flow chart for systematic review.
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The review identified and critically appraised relevant research under two main categories: (a) adaptation and mitigation (with sub-categories: cooling impact, air quality improvement, flood mitigation risk) and (b) health benefits (with sub-categories: birth outcomes, obesity, physical activity, reduced mortality and cardiovascular associations, mental wellbeing and cognitive function). Each sub-category of intervention was rated according to the GRADE approach [23, 25–27]. Study design, study quality, consistency across studies, directness of population, intervention, or outcome, and accuracy in impact and/or effect size estimates all influenced decision-making when it comes to the quality of evidence. Using the GRADE method, the evidence was divided into two categories: high (if studies were randomized) and poor (if they were observational studies). GRADE approach employs five elements to determine whether evidence should be downgraded: (a) individual studies can be biased if they have problems in their design or conduct, which are referred to as 'study limitations' or 'bias risk', (b) inconsistency of findings across studies: impact estimates vary, (c) indirect evidence: the systematic review was conducted with adults in mind, although the participants were youngsters, (d) imprecision: when the studies were meta-analyzed, the results were not statistically significant, but the effect was substantial, and (e) studies that do not suggest a statistically significant effect are less likely to be published, resulting in a biased outcome [28]. Our findings indicated studies with (a) strong evidence (high quality-critical), (b) moderate evidence, and (c) a list of outcomes that are low in evidence and/or controversial.
2.3. Greenness benefits: climate adaptation and mitigation
Climate change is defined as any change in climate over a long period of time, resulting from natural variability or human-induced activity [29]. Climate change mitigation means finding ways to avoid and reduce the emissions of greenhouse gases into the atmosphere to prevent the world from reaching extreme temperatures [30]. Climate change mitigation strategies focus on reducing greenhouse gas emissions and exacerbating factors [31]. Adaptation to climate change is a response to existing or anticipated climatic influences in natural or human systems, such as metropolitan areas [32]. Moderating the harmful impacts of climate change is part of climate change adaptation strategies.
Climate change adaptation is closely linked to urban greenness [33] as green infrastructure can often serve as an adaptation measure, e.g. floodplain restoration, urban green infrastructure to lessen urban heat island (UHI) effect. Climate adaptation-related greenness measures are an important instrument for sustainable adaptation while creating multiple benefits [32]. Greenness helps in the restoration of floodplain forests; flood prevention; CO2 storage (climate change mitigation) and filtration of pollutants are some of the adaptation benefits. Thus, greenness can help provide more resilience and, additionally, deliver further benefits for human health (table 1) [34].
Table 1. Greenness benefits on mitigation and adaptation with '↓' indicating 'reduction function', and '↑' indicating 'increase function'.
| Measure | Benefits | |
|---|---|---|
| Mitigation | Adaptation | |
| More green infrastructure (green roofs, green walls, landscaping) | ↓Air pollution, noise and heat | ↑Green space and transport physical activity |
| ↑Mental health | ||
| ↓Mortality levels | ||
| ↓Risk of obesity in adults | ||
| ↑Birth outcomes | ||
| ↑Cooling | ↑Flood resilience | |
| ↓air conditioning, ↓energy use | ↑Energy efficiency | |
| ↓water treatment↓GHGs | ↑Air quality needs | |
2.4. Adaptation
All around the world, experts are reporting on the negative impacts of climate change on human health [2, 35]. Climate change is affecting human health in numerous ways, both directly and indirectly [36–38]. The negative effects of heat waves, flooding, and drought on human health are evident (injuries, heat stroke, respiratory diseases, and cardiovascular burdens) [39–41]. Several studies were undertaken on the beneficial impacts of greenness on climate change [22, 42]. Experts are suggesting greenness as a solution that can help curb climate change [43] by providing the co-benefits of improving resiliency and recovery from the impacts of climate change [44, 45]. While some pioneering studies focused on specific climate change impacts (extreme heat, heat island impact [46–48] and air quality interactions [49]), others focused on health interactions in specific regions (Asia, Europe) [50–52]. We review research on the challenges exacerbated by climate change and show how greenness can help accomplish climate change adaptation and mitigation goals while also enhancing human health.
2.5. Cooling down effect-UHI effect
Climate change presents an opportunity to renovate or construct outdoor green areas. The urgent need to control high temperatures and mitigate the consequences of climate change supports the preservation of green spaces since urban greenery plays an important role in regulating metropolitan temperature and safeguarding biodiversity [12, 53]. Greenness and green infrastructure planning enhance our ability to deal with climate change on various scales [33, 54, 55]. The exploration of the global climatic feedbacks of greenness changes may be helpful to identify changes in climate for mitigating global warming [56].
In urban areas, green spaces can provide a cooling impact. Green spaces in urban settings can help to mitigate the effects of the UHI [57]. Green areas have become even more important in the context of climate change, with predicted increases in temperature, dryness, and the intensity of heat waves, as they can generate a cooling impact that spreads to the surrounding areas [58]. Green spaces cool by regulating surface heat exchange, mostly through urban vegetation's transpiration and shading effects, as well as the selective absorption and reflection of solar radiation [59]. Large green spaces, in general, can provide a somewhat constant microclimate, and their cooling effects can be more noticeable and beneficial [7]. The vegetation configuration of green space has a considerable impact on cooling [60]. In an empirical study of London, the cooling effect of green spaces with trees was demonstrated to be larger than that of grass, and the cooling distance was strongly connected to the height of the tree canopy [61].
Studies working on quantifying the cooling impact of greenness (urban parks) are various [62–64]. Climate change in large cities has drawn much attention so that urban planners have been focusing more on green space planning and the green environment protection of cities [65, 66]. This review resulted in mostly observational studies with a moderate level of grading as most studies included expert opinion on the subject. A study on the urban environment suggested that planting more trees or installing green or cool roofs can lower surface and air temperatures while reducing the amount of energy needed to cool down the buildings [67], especially in extreme weather events (heatwaves) [68]. While most studies fail to report quantitative results, research undertaken in the Netherlands has found that on a sunny day a tree can have the power of more than ten units of air conditioning [69]. Although greenness was positively associated with urban cooling in a study in Bengaluru, India, the evidence was not strong enough and with limitations [7]. Firstly, the findings might be tested in a variety of seasons and at various times of day (especially diurnal variations). Second, future research could look at different types of land uses than the ones studied (agriculture, built-up, green space, bare land, and waterbody), as previous research has shown that the amount of built-up area, building heights, and the material used to construct structures, among other things, can affect temperature [70]. Other research has shown that green spaces provide multiple benefits for mitigating excessive heat [71, 72]. Many studies have focused on the role of greenness in regulating urban temperature and protecting biodiversity and the role of nature-based solutions protecting the urban environment and regulating heat [12, 33, 47, 53–56, 69, 71–80]. Yet, most provided moderate evidence as these studies were also observational, based on expert opinions and not quantitative (table 2).
Table 2. GRADE evidence profile: Greenness impact on cooling down effect-UHI.
| Reasons to 'downgrade' | Reasons to 'upgrade' | Quality | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Results | Limitations | Inconsistency | Indirectness | Imprecision | Publication bias | Relative risk (95% CI) | Study size | Reduce demonstrated effect | GRADE | |
| Cooling down Impact-UHI impact n = 26 | Role in regulating the urban temperature and protecting biodiversity and role of nature-based solutions protecting urban environment and regulating heat [12, 33, 47, 53–56, 69, 71–80] | Expert opinion not quantitative [12, 69] | No serious inconsistency, the findings were consistent with each other | No serious indirectness | No serious imprecision or sparse data | Non identified | — | — | — | Moderate ⊕⊕⊕ |
| Heating load has decreased by about 1 kWh m−2 per decade, while the cooling load increased by about 5 kWh m−2 per decade [67] | No serious limitations | No serious inconsistency, estimate effect across studies are consistent | No serious indirectness, the outcome did not differ of that in other studies | No serious imprecision or sparse data | Non identified | — | — | Yes | High ⊕⊕⊕⊕ | |
| Increased heat stress caused by global temperature rise can be mitigated by increasing the vegetation cover [7, 67] of surface area and increasing shading [82] effects by changing urban design [68] | The results could be tested in a number of seasons and at different times of day [7, 67, 68] | No serious inconsistency, the findings were consistent with each other | No serious indirectness, the outcome did not differ of that in other studies | No serious imprecision, the CI around the estimate is sufficiently narrow | Non identified | RR 347, (CI: 318 m–376 m) RR 2.23, (CI: 2.13 °C–2.33 °C) [7] | Large scale land | Yes | Moderate ⊕⊕⊕ | |
| A negative linear relationship was found between the large scale land and size of urban forest [62] | No serious limitations | Negative relationship presented | No serious indirectness, the outcome did not differ of that in other studies | No serious imprecision, the CI around the estimate is sufficiently narrow | Non identified | (r = −0.36 to −0.58), size of vegetation cover (r = −0.39 to −0.61), and NDVI (r = −0.42 to −0.93) | Moderate ⊕⊕⊕ | |||
| Observational studies, expert opinions or reviews [65, 66] | Limited to one database search | No serious inconsistency | No serious indirectness | No serious imprecision | Non identified | — | — | No | Moderate ⊕⊕⊕ | |
| An area of 50 m2 covered by vegetation can cool the air by 1 °C [58, 81] | No serious limitations | No serious inconsistency, the findings were consistent with each other | No serious indirectness | No serious imprecision | Non identified | — | — | Yes | High ⊕⊕⊕⊕ | |
Abbreviations: GRADE: grading of recommendations assessment, development, and evaluation; CI: confidence interval; RR: risk ratio; n: number.
Although the literature reports extensively on the available strategies to reduce the UHI effect [73, 74], a moderate level of quantitative research was observed, and some studies were with limitations, such as some results could be tested in a number of seasons and at different times of day and could inform different results (RR 2.23, (CI: 2.13 °C–2.33 °C)) [7]. In general, the methods provided in the literature follow vegetation cover, use of waterbodies, and green infrastructure in different areas and scales [75, 76].
In Japan, one study found that understanding wind corridors and cooling by greening informed Fukuoka city plans [47]. One study in Lisbon also reported that the increase and optimization of urban greenness has been considered as an effective mitigation measure for UHI, with positive effects on human thermal comfort [58, 81]. Reis and Lopes provided high-quality evidence with a quantification method of the cooling effect provided by greenness in order to improve urban climate conditions and human wellbeing [81].
Bowler et al in a review study concluded that green infrastructure (trees, parks, forests, and green roofs) provides a higher level of thermal comfort than other urban spaces [77]. Another study directly associated the cooling effect of urban green spaces with the vegetation cover and tree shade area [82]. A study suggested that it is possible to strategically build greenness and have large effects on city temperatures [78] and a moderate level of evidence proved that increased heat stress caused by global temperature rise can be mitigated by increasing the vegetation cover [7, 67] of surface area and increasing shading effects [82] by changing the urban structure [68]. While greenness provides benefits for altering temperatures [79, 80], it also helps with other climate adverse impacts, such as air quality improvement and pollution mitigation [83].
2.6. Air quality improvement
Residential greenness may provide environmental benefits such as reduced air pollution and improved air quality (PM2.5 positively associated with mortality risk-with lower hazard ratios in greener areas, (RR (relative risk) 1.12, (CI: 1.12–1.13)) [84]. Greenness was stressed as a potential mitigation technique in a crossover study in Germany because it may provide microclimate management and air filtration, however, this study presented nonsignificant and relationships (RR 1.66, (CI: 1.01–2.74)) [85]. One study concluded on urban green spaces' effectiveness on the environment and found moderate evidence that the presence of vegetation can help increase the quality of the air [86]. Another research emphasized the influence of different types of gardens on temperature, humidity, wind, precipitation and the distribution of air pollution [87]. The results of the study showed that greenness (parks, gardens) significantly contributes to improving the microclimate and reducing the rate of pollution [87, 88]. Research undertaken in France has revealed that urban trees are a significant element in reducing air pollution [89]. Greenness and green space benefits were also highlighted in a study in Guangzhou (China) and drove attention to the efficiency of atmospheric cleansing by greenness in Chinese cities, which could be improved by tree plantations and providing effective green space management and provided moderate evidence that the presence of vegetation can help increase the quality of the air [90]. Although numerous studies are looking at the health effects of air pollution and greenness, more evidence is needed to support the link between greenness and improved air quality (table 3).
Table 3. GRADE evidence profile: greenness impact on air quality improvement.
| Reasons to 'downgrade' | Reasons to 'upgrade' | Quality | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Results | Limitations | Inconsistency | Indirectness | Imprecision | Publication bias | Relative risk (95% CI) | Study size (n) | Reduce demonstrated effect | GRADE | |
| Air quality improvement n = 7 | PM2.5 was positively associated with mortality risk-with lower hazard ratios in greener areas [84] | No serious limitations | No serious inconsistency | No serious indirectness | No serious imprecision | Non identified | RR 1.12, (CI: 1.12–1.13) | — | Yes | High ⊕⊕⊕⊕ |
| No significant and consistent associations [85] | No serious limitations | Inconsistency no associations | No serious indirectness | No serious imprecision | Non identified | RR 1.66, (CI: 1.01–2.74) | 3063 | No | Low ⊕⊕ | |
| Moderate evidence that the presence of vegetation can help increase the quality of the air [86–90] | No serious limitations | Consistent with the data from two epidemiological studies; one experiment found no effect and one was inconclusive about the positive effect [86] | No serious indirectness | No serious imprecision | Non identified | — | — | Yes | Moderate ⊕⊕⊕ | |
Abbreviations: GRADE: grading of recommendations assessment, development, and evaluation; CI: confidence interval; RR: risk ratio; n: number.
2.7. Flooding risk management
The impacts of climate change are being felt around the world, including the increased occurrence of extreme events [91, 92]. The interaction between urbanization and climate change has caused a dramatic increase in the global frequency and intensity of flooding disasters [93].
Globally, a number of studies have focused on flood mitigation and greenness association [94, 95]. A study undertaken in Ghent, Belgium, focused on greenness to mitigate urban surface water flooding risk by developing a green infrastructure-based multi-criteria evaluation method [96] and found little research for planners and designers to determine an appropriate strategy for greenness planning. Green infrastructure has considerable reduction impacts and can maximise the advantages for environmental protection, according to a study conducted in Beijing to analyze the effectiveness of greenness on urban flooding reduction [97]. The Hangzhou study reports the results of a survey with green-space users and their tendency toward tree planting in public and communal green spaces as a climate change adaptive response [98]. The results showed that green space users believed that individual actions could reduce climate change impacts (flooding, heatwaves) [99–102]. Zhang et al indicated that larger amounts of green space mitigate flooding risk while aggregating green space into larger, separate areas exacerbate risk in Luohe, China [93].
Green spaces have the potential to reduce flood risk by capturing rainwater, thus greenness should be increased and be protected [103]. Research in Indonesia finds a pattern that the level of greenness in the city center might decrease the potential for flooding but this is not definitive due to poor quality studies as reflected by very low quality of evidence [104]. Green spaces can be strategically implemented to mitigate extreme precipitation, and potentially resulting floods and this could be a nature-based solution to regulating urban impacts of climate change [105]. Improving greenness assists in adapting to climate and urban changes while helping to cope with upcoming increases in precipitation and urbanization [106]. Although there are correlations between flood hazards and greenness interactions, more high-quality evidence is needed in this area to provide useful information to urban planners and help define a course for government climate change mitigation policies (table 4).
Table 4. GRADE evidence profile: greenness impact on flood mitigation.
| Reasons to 'downgrade' | Reasons to 'upgrade' | Quality | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Results | Limitations | Inconsistency | Indirectness | Imprecision | Publication bias | Relative risk (95% CI) | Study size (n) | Reduce demonstrated effect | GRADE | |
| Flood mitigation impact n = 12 | Observational studies focusing on greenness and flooding association [91–95, 97] | Expert opinion, reviews, not quantitative | No serious inconsistency | No serious indirectness | No serious imprecision | Non identified | — | — | — | Low ⊕⊕ |
| Increase in flooding risk and tree plantation, observational and policy recommendations [98–100, 103–106] | Most studies do not give quantitative data and based on recommendations | No serious inconsistency | No serious indirectness | No serious imprecision | Non identified | — | — | — | Low ⊕⊕ | |
Abbreviations: GRADE: grading of recommendations assessment, development, and evaluation; CI: confidence interval; RR: risk ratio; n: number.
2.8. Health benefits
Green space in cities would not only assist to minimize the consequences of climate change and improve air quality but it would also improve human health [107]. Researchers are now exploring how greenness can affect/improve human health by looking into various health outcomes [108]. Exposure to green spaces has been associated with improved-perceived general health, better pregnancy outcomes (e.g. birth weight [109]), enhanced cognitive development in children [110], better brain function in adults; mentally improved health; lower risk of a number of chronic diseases (e.g. diabetes and cardiovascular conditions [111]), and reduced mortality [112, 113]. Now we will look at the evidence for greenness's health benefits in terms of birth outcomes, physical activity, mental well-being, obesity, lower mortality risk, and cognitive performance in various countries and populations.
2.9. Birth outcomes
Much research has been conducted to investigate the link between population greenness and birth outcomes. Greenness was linked to a higher birth weight and a lower risk of prematurity in a study conducted in Southern California (RR 0.963, (CI: 0.947–0.978)) [114]. However, another study in New Zealand found no association between birth weight and greenness index [115]. While some studies showed improved pregnancy outcomes [116], other studies showed adverse impacts (RR 0.84, (CI: 0.78–0.90)) or weak or no association between greenness and birth outcomes (RR 25.5, (CI: 15.4–35.5)) [117, 118]. A study in Munich, Germany, on greenness and birth outcomes indicated high-level evidence on the association between greenness and birth weight (500 m buffer was associated with an average birth weight increase of (RR 17.6, (CI: 0.5–34.6)) [119]. Some studies have also reported on the reduction of maternal stress in green spaces [120]. An experimental study assessed effect modification based on the low surrounding greenness and maternal stress and found increased risks for low birth weight (RR 1.84, (CI: 1.07–3.18)) [121]. One study in Beijing identified a positive association of greenness and fetal growth in utero, but observations did not conclude an association with birth weight [109]. The research undertaken in Rome (2001–2013), however, looked into socioeconomic position and greenness impact on the heat and air pollution on preterm births [122]. The study concluded that the effect of a 1 °C increase in temperature on the daily number of preterm births was higher in women with low socioeconomic position (RR 2.49, (90% CI: 1.29–3.71)), with no association found between greenness and particulate matter [122]. A study in Massachusetts, USA, associated higher greenness exposure with higher birth weight with stronger associations in a higher range of greenness [123]. A Spanish cohort study also reported on the link between surrounding greenness and improved conditions on pregnancy outcomes [124, 125]. Overall, the evidence levels in the literature ranged from moderate to high, and studies found significant evidence of a positive relationship between birth outcomes and greenness (table 5).
Table 5. GRADE evidence profile: greenness impact on birth outcomes.
| Reasons to 'downgrade' | Reasons to 'upgrade' | Quality | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Results | Limitations | Inconsistency | Indirectness | Imprecision | Publication bias | Relative risk (95% CI) | Study size (n) | Reduce demonstrated effect | GRADE | |
| Birth outcomes n = 14 | Greenness is associated with birth weight increase and lower prematurity risk [114, 119, 123] | No serious limitations | No serious inconsistency | No serious indirectness | No serious imprecision | Non identified | RR 0.963, (CI: 0.947–0.978) [114] RR 17.6, (CI: 0.5–34.6) [119] RR 0.98, (CI: 0.97–0.99) [123] | 72 632 [114] | Yes | High ⊕⊕⊕⊕ |
| Maternal exposure to green space during pregnancy was not associated with birth weight for the entire cohort [67, 109, 115] | No serious limitations | Inconsistent no association | Serious Indirectness | No serious imprecision | Non identified | — | 5901 [67] 18 665 [109] | No | Moderate ⊕⊕⊕ | |
| Improved pregnancy outcomes [116] | No serious limitations | No serious inconsistency | No serious indirectness | No serious imprecision | Non identified | — | — | Yes | High ⊕⊕⊕⊕ | |
| Adverse impacts [118] or weak association [117] between greenness and birth outcomes [115] | No serious limitations | Inconsistent weak or no association | No serious indirectness | No serious imprecision | Non identified | RR 25.5, (CI: 15.4–35.5) [117] RR 0.84, (CI: 0.78–0.90) [118] | 73 221 [117] 39 132 [118] | Yes | Moderate ⊕⊕⊕ | |
| Reduction of maternal stress in green spaces [120, 121] | No serious limitations | No serious inconsistency | No serious indirectness | No serious imprecision | Non identified | RR 1.84, (CI:1.07–3.18) [121] | 2455 | Yes | High ⊕⊕⊕⊕ | |
| 1 °C increase in temperature on the daily number of preterm births was higher in women with low socioeconomic position [122] | No serious limitations | No serious inconsistency | Indirectness | Imprecise-socioeconomic position impact | Non identified | RR 2.49, (90% CI: 1.29–3.71) | 56 576 | No | Moderate ⊕⊕⊕ | |
| Link between surrounding greenness and improved conditions on pregnancy outcomes [124, 125] | Not quantitative | No serious inconsistency | No serious indirectness | No serious imprecision | Non identified | — | — | No | Moderate ⊕⊕⊕ | |
Abbreviations: GRADE: grading of recommendations assessment, development, and evaluation; CI: confidence interval; RR: risk ratio; n: number.
2.10. Physical activity
Although the mechanism underlying green space's health benefits has yet to be discovered, it is clear that green spaces can improve health by giving chances for physical activity [126, 127]. Greenness was associated with 52% greater odds of being physically active (RR 1.52, (CI: 1.22–1.90)) according to a cohort study (n = 846, 78% men) [128]. Research in Europe showed significant interactions between apparent temperature and surrounding greenness on hourly physical activity [129].
A study undertaken in Finland associated residential greenness with the level of low and moderate physical activity and a variety of positive health outcomes (RR 174, (CI: 140–209)) and limitations of this study included significant associations after adjustments only in men [130]. A review focused on the three most commonly studied outcomes (physical activity, birth and developmental outcomes and cardiovascular disease) and found significant positive correlations between greenness and physical health, focusing on larger buffer sizes, up to 2000 m, better predicted physical health than smaller ones [131].
A Lithuanian study found that children residing in greener environments were found to be more physically active (RR 2.32, (CI: 1.43–3.78)) [110]. One study found a non-significant association between greenness and physical health, including only expert opinion without explicit and critical appraisal [17]. Although research of moderate-to-high quality found beneficial links between physical health and greenness, research of low quality found a complex relationship between environmental variables, adolescents' physical activity, and/or no link with expert opinion. More quantitative data and high-quality research could help to inform greenness and physical health relationships, and the literature could benefit from more quantitative data and high-quality research (table 6).
Table 6. GRADE evidence profile: greenness impact on physical activity.
| Results | Limitations | Inconsistency | Indirectness | Imprecision | Publication bias | Relative risk (95% CI) | Study size (n) | Reduce demonstrated effect | GRADE | |
|---|---|---|---|---|---|---|---|---|---|---|
| Physical activity n = 9 | Complex relationship between environmental variables, adolescents' physical activity [126, 127] | Cross-sectional, not possible to make an inference about causal directions | Inconsistent with other literature | No serious indirectness | No serious imprecision | Non identified | — | 382 [127] | No | Low ⊕⊕ |
| Association with 52% greater odds of being physically active [128] | No serious limitations | No serious inconsistency | No serious indirectness | No serious imprecision | Non identified | RR 1.52, (CI: 1.22–1.90) | 846 | Yes | High ⊕⊕⊕⊕ | |
| Interaction between apparent temperature and surrounding greenness on hourly physical activity [129] | No serious limitations | No serious inconsistency | No serious indirectness | No serious imprecision | Non identified | — | 352 | Yes | High ⊕⊕⊕⊕ | |
| Residential greenness was independently associated with low physical activity [130, 131] | Significant associations after adjustments only in men | No serious inconsistency | No serious indirectness | No serious imprecision | Non identified | RR 174, (CI: 140–209) [130] | 5433 | Yes | Moderate ⊕⊕⊕ | |
| Lower greenness exposure and shorter park usage were associated with the risk of poor health [110] | No serious limitations | No serious inconsistency | No serious indirectness | No serious imprecision | Non identified | RR 2.32, (CI: 1.43–3.78) | 1489 | Yes | High ⊕⊕⊕⊕ | |
| Non-significant association between greenness and physical health [16, 17] | Expert opinion without explicit, critical appraisal | Inconsistent with previous research | Indirectness study was more strongly associated with mental health instead | Serious imprecision | Non identified | — | — | No | Low ⊕⊕ |
Abbreviations: GRADE: grading of recommendations assessment, development, and evaluation; CI: confidence interval; RR: risk ratio; n: number.
2.11. Mental wellbeing associations-reduced stress levels
Availability and access to green spaces could help improve mental conditions by providing calm and tranquility and help one to concentrate [132]. Greenness may help to protect mental health by boosting physical activity and providing direct psychological benefits [133, 134]. One study in Adelaide, Australia (n = 1895) found a significant positive relationship between greenness and the mental health of the participants, predicting a 1.60 times higher odds of better mental health [17].
Perceived greenness at home and at university was positively associated with mental health in Graz, Austria, with a significant but moderate correlation [135]. On the other hand, lower residential greenness levels and less time spent in a park were associated with poorer general and mental health among 4 to 6 year-old children (RR 1.43, (CI: 1.03–2.18)) [110]. A study in Korea (n = 65 128) found that individuals in regions with the highest greenness levels represented the lowest odds for depressive symptoms (RR 0.813; 95% CI: 0.747–0.884) [136]. While most studies evaluated the association between greenness and mental impacts, one study in the USA found a direct association of greenness with perceived stress among the elderly residents (n = 4118; aged 57–85 years), and an indirect association mediated through physical activity and respiratory disease history [137]. Some other research undertaken in China also found significant associations between greenness and depression and anxiety levels in urban Chinese adults [138]. A Canadian study assessing the impact of school greenness on mental health among students found no association between school-based greenness and mental health [139]. The results of a cohort study in Taiwan showed that the surrounding greenness has the effect of reducing the risk of schizophrenia, regardless of gender or financial status [140]. Most studies observed a beneficial association between green space exposure and mental health in children, adolescents, and young adults with moderate-to-high levels of evidence [141]. One study observed a 13% reduction in the risk of depression among older females (54–91 years) (RR 0.87, (CI: 0.78–0.98)) [142] (table 7). This literature review found moderate-to-high quality of evidence suggesting that there is a positive and significant relationship between greenness and mental health associations.
Table 7. GRADE evidence profile: greenness impact on mental health.
| Reasons to 'downgrade' | Reasons to 'upgrade' | Quality | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Results | Limitations | Inconsistency | Indirectness | Imprecision | Publication bias | Relative risk (95% CI) | Study size (n) | Reduce demonstrated effect | GRADE | |
| Mental wellbeing—stress levels n = 13 | No association between school-based greenness and mental health [139] | No serious limitations | No serious inconsistency | No serious indirectness | No serious imprecision | Non identified | — | 6313 | No | High ⊕⊕⊕⊕ |
| Positive Relationship between greenness and mental health [17, 132–134, 140–142] | Mostly expert opinion without explicit, critical appraisal [17, 132–134] | No serious inconsistency | No serious indirectness | No serious imprecision | Non identified | 1.60 times higher odds of better mental health [17] RR 0.87, (CI: 0.78–0.98) [142] | 1895 [17] 38 947 [142] | Yes | Moderate ⊕⊕⊕ | |
| A significant but moderate correlation [135] | The 'clinical' relevance of the results might be limited | No serious inconsistency | Causal direction of the results is not clear | No serious imprecision | Non identified | — | 601 | Yes | Moderate ⊕⊕⊕ | |
| Lower residential greenness were associated with poorer general and mental health [110] | No serious limitations | No serious inconsistency | No serious indirectness | No serious imprecision | Non identified | RR 1.43, (CI: 1.03–2.18) | 1489 | Yes | Moderate ⊕⊕⊕ | |
| Highest greenness levels represented the lowest odds for depressive symptoms [136] | No serious limitations | No serious inconsistency | No serious indirectness | No serious imprecision | Non identified | RR 0.81, (CI: 0.747–0.884) | 65 128 | Yes | High ⊕⊕⊕⊕ | |
| Direct association of greenness with perceived stress [137] | No serious limitations | No serious inconsistency | No serious indirectness | No serious imprecision | Non identified | — | 4118 | Yes | High ⊕⊕⊕⊕ | |
| Significant associations between greenness and depression and anxiety [138] | No serious limitations | No serious inconsistency | No serious indirectness | No serious imprecision | Non identified | — | 27 366 [138] | Yes | High ⊕⊕⊕⊕ | |
Abbreviations: GRADE: grading of recommendations assessment, development, and evaluation; CI: confidence interval; RR: risk ratio; n: number.
2.12. Obesity
Numerous studies have been found in the literature on the relationship between predicted obesity and green areas, however, most were unable to draw firm conclusions. Research undertaken among adults in China has revealed that greenness is beneficially associated with obesity levels [143, 144], with one study reporting (RR 0.67, (CI: 0.57–0.78)) high quality of evidence (n = 5849) [144]. A cross-sectional study of 10 208 young adults (16–24 years), middle-aged adults (25–64 years), and older adults (over 65 years) in Perth, Western Australia (2004–2009) showed that higher levels of neighborhood greenness were associated with lower odds of obesity among adults [145]. Findings from the 33 Chinese community health study also associated greenness with a lower risk of obesity in adults (18% lower odds for obesity) [146]. A study in the US with women (aged 37–67) hypothesized that greater residential greenness would be associated with reduced obesity; however, this study concluded no statistically significant association between greenness and the risk of obesity [147].
A study from Denmark showed that access to parks, gardens, or green areas was associated with a lower likelihood of obesity [148]. Low quality of evidence concluded that the majority (68%) of studies found a positive or weak association between green space and obesity-related health indicators, with inconsistent and mixed findings across studies and between locations and populations [149]. A recent study in Shanghai using three greenness features: green access, green exposure and a view-based green index with 9524 respondents from 40 communities found that green spaces had an adverse impact on body weight and obesity although the effects were observed only in women and air pollution could partially mediate the association [146].
Overall, the GRADE approach suggested that the quality of the evidence on greenness's impact on obesity was moderate due to limitations and imprecision in between studies (table 8).
Table 8. GRADE evidence profile: greenness impact on obesity.
| Reasons to 'downgrade' | Reasons to 'upgrade' | Quality | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Results | Limitations | Inconsistency | Indirectness | Imprecision | Publication bias | Relative risk (95% CI) | Study size (n) | Reduce demonstrated effect | GRADE | |
| Obesity n = 7 | Greenness is beneficially associated with obesity levels [143–146, 148] | No serious limitations | No serious inconsistency | No serious indirectness | No serious imprecision | Non identified | RR 0.67, (CI: 0.57–0.78) [144] | 5849 [144] | Yes | High ⊕⊕⊕⊕ |
| No statistically significant association between greenness and the risk of obesity [147] | Limited evidence and mostly cross-sectional | Inconsistent with literature | No serious indirectness | No serious imprecision | Non identified | — | 119 549 | No | Moderate ⊕⊕⊕ | |
| Inconsistent and mixed findings across studies [149] | Mostly expert opinion without explicit, critical appraisal | No serious inconsistency | No serious indirectness | No serious imprecision | Non identified | — | — | No | Low ⊕⊕ | |
| Adverse impact on body weight and obesity [146] | No serious limitations | No serious inconsistency | Air pollution may partially mediate the association | The effects were observed in women but not in men | Non identified | RR −0.15, (CI: −0.22, −0.09) | 9524 | Yes | Moderate ⊕⊕⊕ | |
Abbreviations: GRADE: grading of recommendations assessment, development, and evaluation; CI: confidence interval; RR: risk ratio; n: number.
2.13. Decreased risk of mortality—cardiovascular associations
Residential greenness and mortality are often associated in the global literature. Research relating urban green space with mortality in Ontario, Canada, found reductions in mortality levels with increased residential green space [150]. Another study undertaken with adults found that residential greenness was associated with a decreased risk of mortality [151]. Studies examined cause-specific mortality, and found that higher levels of greenness were associated with a lower risk of cardiovascular mortality [151–153]. High quality of evidence from Taiwan based on the Taiwan Death Certificate Database reported that one unit increase in greenness was associated with reduced mortality (RR 0.901; 95% CI: 0.862–0.941), cardiovascular diseases (RR 0.892; 95% CI: 0.817–0.975) [154]. A cohort study in China, however, found no apparent association between greenness measurements and mortality and this study was not able to compare the effects of residential greenness on cause-specific mortality with other studies [155]. One study in Australia (n = 11 404) found the odds of hospitalization for heart disease or stroke were 37% (95% CI: 8%–57%) lower among adults in neighborhoods with more surrounding greenness [156]. In Canada, a 10% decrease in cardiovascular mortality in adults was linked to residential greenness [157]. An observational study in Scotland showed that areas least green in the summer were found to have higher mortality rates and this association was thought enough to be of importance for policy making [158]. A moderate-to-high level of evidence throughout the literature indicates that more research could inform on mortality reduction and greenness associations (table 9).
Table 9. GRADE evidence profile: greenness impact on risk of mortality and cardiovascular discomfort.
| Reasons to 'downgrade' | Reasons to 'upgrade' | Quality | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Results | Limitations | Inconsistency | Indirectness | Imprecision | Publication bias | Relative risk (95% CI) | Study size (n) | Reduce demonstrated effect | GRADE | |
| Decreased risk of mortality—cardiovascular associations n = 10 | Reductions in mortality levels with increased residential green space [150–154] | No serious limitations | No serious inconsistency | No serious indirectness | No serious imprecision | Non identified | RR 0.901, (CI: 0.862–0.941) [154] | 1173 773 | Yes | High ⊕⊕⊕⊕ |
| No apparent association between greenness measurements and mortality [155] | No reliable data about cause-specific mortality due to the self-report nature of the study design | Not able to compare the effects of residential greenness on cause-specific mortality with other studies | No serious indirectness | No serious imprecision | Non identified | RR 0.73, (CI: 0.70–0.76) | 23 754 | No | Moderate ⊕⊕⊕ | |
| Odds of hospitalization for heart disease or stroke was 37% [156] | No serious limitations | No serious inconsistency | No serious indirectness | No serious imprecision | Non identified | RR 37%, (CI: 8%–57%) | 11 404 | Yes | High ⊕⊕⊕⊕ | |
| 10% decrease in cardiovascular mortality in adults [102, 157, 158] | No serious limitations | No serious inconsistency | No serious indirectness | No serious imprecision | Non identified | RR 6%, (CI: 4%–7%) [157] | — | Yes | High ⊕⊕⊕⊕ | |
Abbreviations: GRADE: grading of recommendations assessment, development, and evaluation; CI: confidence interval; RR: risk ratio; n: number.
2.14. Cognitive function
Given the many benefits of green space, the health of urban residents, who often have limited access to these spaces, can be improved by increasing the amount of green space [159]. Some other benefits of greenness include stress and anxiety reduction, improved cognitive functioning, lowered risks of depression, and overall greater mental and physical wellbeing [160]. Evidence from a range of disciplines has uncovered numerous social, psychological, and health benefits of human exposure to greenness [16, 161].
Many studies have examined the impact of greenness exposure and cognition among adults, and some results varied from studies among school-aged children [141, 162–164]. A study in China provided evidence on the association between an increase in residential greenness and better cognitive function among elderly citizens (RR 1.25, (CI: 1.18–1.34)) [165]. While some studies reported on green window view positively affected adults' cognitive capacity [159, 166, 167], other studies provided findings on green spaces' benefits for child's brain (cognitive and behavioral) development [168, 169]. One study measuring school performance in Massachusetts supported a relationship between the 'greenness' of the school area and students' academic performance [170]. Two reviews provided results of exposure to natural environments on mental health outcomes and cognitive function, reporting mostly positive associations [171, 172]. Much of the research undertaken in the association of cognitive function and greenness has focussed on older populations [173–176]. One study with limitations provided moderate quality of evidence due to its focus group (being women and ethnic minorities), which might have affected the generalizability of findings, and found that higher residential surrounding greenness was associated with slower cognitive decline (RR 0.02, (CI: 0.003–0.037)) [174] (table 10).
Table 10. GRADE evidence profile: greenness impact on cognitive function.
| Reasons to 'downgrade' | Reasons to 'upgrade' | Quality | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Results | Limitations | Inconsistency | Indirectness | Imprecision | Publication bias | Relative risk (95% CI) | Study size (n) | Reduce demonstrated effect | GRADE | |
| Cognitive function n = 17 | Positive correlations with greenness [141, 159–165] | Several limitations | No serious inconsistency | No serious indirectness | No serious imprecision | Non identified | RR, 1.25 (CI: 1.18–1.34) [165] | 38 327 [165] | Yes | Moderate ⊕⊕⊕ |
| Higher residential surrounding greenness was associated with slower cognitive decline [16, 160, 161, 168–176] | Women and ethnic minorities, which might have affected the generalizability of findings | Inconsistency due to generalizability of results | No serious indirectness | No serious imprecision | Non identified | RR 0.02, (CI: 0.003–0.037) [174] | 6506 [174] | Yes | Moderate ⊕⊕⊕ | |
Abbreviations: GRADE: grading of recommendations assessment, development, and evaluation; CI: confidence interval; RR: risk ratio; n: number.
3. Discussions and conclusions
The influence of climate change on human health is becoming increasingly widely recognized; yet, research on the relationship between greenness and its inhibitory effects on climate change mitigation is limited in comparison to its health advantages. Green space is changing dramatically all over the world, and the number of green places is decreasing [177]. It is evident that green spaces have enormous potential for mitigating future climate change. Green space preservation and expansion have a significant impact on climate change mitigation, and governments and researchers should make it a priority to increase its coherence as an effective mitigation technique.
This review highlighted the broad range of climate and health benefits linked to greenspace management and greenness exposure [178]. Despite a growing literature on the topic, the association between neighborhood greenness and climate change mitigation impact is inconsistent and lacks clarity [179]. Although positive associations between greenness and climate change were found, the evidence level was moderate in most of the studies in areas of adaptation and health impacts (mental health, cognitive function, birth outcomes, cardiovascular associations, cooling impact). In particular, quality of evidence being moderate to high, more quantitative and epidemiological studies are required to better understand the link between greenness-climate change and health and the populations affected. Preparation and provision of urban green spaces are not enough, and the effective usage of green spaces is very important for population health [180]. Thus, research on the association between urban green space and health requires further investment in order to build resilience and an adaptive strategy for adverse climate change impacts [19]. With a moderate-to-high level of evidence, the body of literature examining the impact of greenness on health found that greenness may be beneficial for mental health, birth outcomes, cardiovascular outcomes, and mortality. Studies on air quality, flood mitigation, physical activity, and obesity links were scarce and had inadequate evidence, thus more research is needed (table 11).
Table 11. Areas of weak evidence on greenness climate change and health benefits.
| Adaptation | Air quality improvement | No significant and/or consistent associations | Weak evidence |
| Flood mitigation | Expert opinion, reviews, not quantitative most studies do not provide quantitative data | ||
| Health impacts | Physical activity | Non-significant association between greenness and physical health | Weak evidence |
| Obesity | Inconsistent and mixed findings across studies |
Green areas make substantial contributions to healthy urban living and should be viewed as valuable assets to people's environmental health. There is a lack of attention for green space continuity, and it is critical to underline the co-benefits of greenness in terms of climate change and population health. The IPCC should explore the co-benefits of green spaces further, and the report should specifically emphasize the effects of greenness on climate change mitigation and human health. Making environmental protection a priority in communities and surrounding areas will aid in tackling climate change-related health effects. Furthermore, the relationship between climate change mitigation and green space enhancement should be researched further globally to better understand how greenness might assist alleviate climate change impacts [181].
In conclusion, the body of literature examining the impact of greenness on health discovered that greenness may be helpful for mental health, birth outcomes, cardiovascular outcomes, and mortality, with a moderate-to-high level of evidence. However, further research is needed because studies on air quality, flood mitigation, physical activity, and obesity linkages are sparse and provide insufficient data. In order to establish resilience and an adaptation strategy in the face of negative climate change consequences, further study into the link between urban green space and climate change and health interactions is needed.
Data availability statement
The data that support the findings of this study are available upon reasonable request from the authors.
Author contributions
PK originally conducted the review with JSJPK initially drafted the manuscript. Both authors discussed the results and commented on the manuscript and contributed to the preparation of the final manuscript.
Conflict of interest
The authors declare no competing financial interests.