ReviewUrban greening to cool towns and cities: A systematic review of the empirical evidence
Introduction
Climate change has been predicted to have a range of consequences for human health arising from the direct and indirect impacts of changes in temperature and precipitation (McMichael et al., 2003, Patz et al., 2005). One of the primary public health concerns is an increase in the intensity and frequency of heat waves, which have been linked with heat stroke, hyperthermia and increased mortality rates (Stott et al., 2004, Tan et al., 2007). For instance, an estimated 15,000 excess deaths were attributed to the heat wave event across France in August 2003 (Fouillet et al., 2006).
Increased air temperatures can be expected to be particularly problematic in urban areas, where temperatures already tend to be a few degrees warmer than the surrounding countryside. This difference in temperature between urban and rural areas has been called the ‘urban heat island effect’. Urbanisation leads to changes in the absorption and reflection of solar radiation, and thus the surface energy balance. These changes arise from multiple factors, including the thermal conductivity and specific heat capacities of materials used in urban areas, surface albedo, the geometry of urban canyons and the input of anthropogenic heat (Taha et al., 1988, Oke, 1989, Sham, 1990, Taha, 1997). Increasing temperatures resulting from global climate change may exacerbate the health impacts of the higher temperatures that are already common in urban areas (Luber and McGeehin, 2008). Thus, there is a pressing need to evaluate strategies that may mitigate against further increases in temperatures in urban areas and the associated negative impacts on human health.
An adaptation strategy that has been proposed is to ‘green’ urban areas, essentially by increasing the abundance and cover of vegetation (Givoni, 1991, Gill et al., 2007). Vegetation and urban materials differ in moisture, aerodynamic and thermal properties, and so urban greening could affect temperatures through different processes (Oke, 1989, Givoni, 1991). A key process is evapotranspiration, which describes the loss of water from a plant as a vapour into the atmosphere. Evapotranspiration consumes energy from solar radiation and increases latent rather than sensible heat, cooling the leaf and the temperature of the air surrounding the leaf (Taha et al., 1988, Grimmond and Oke, 1991). This contrasts with the effect of impervious urban materials such as asphalt and concrete, which do not retain water for evaporation and quickly absorb and retain heat when exposed to solar radiation. In addition to evaporative cooling, shading from trees can act to cool the atmosphere by simply intercepting solar radiation and preventing the warming of the land surface and air (Oke, 1989). This shading effect may create local cool areas beneath tree canopies, which would be important in otherwise open spaces within an urban area. Finally, vegetation may affect air movements and heat exchange (Bonan, 1997). This effect, however, can be expected to critically depend on the type of vegetation. Tree cover may retain warm air beneath the canopy; in contrast, an open grass field that provides low resistance to air flow may promote cooling by convection.
Some studies have used remote sensing technology to estimate land surface temperature and vegetation cover for a number of different urban localities. Many studies following this approach found a negative correlation between vegetation indices such as NDVI (normalized difference vegetation index) and temperature (Hung et al., 2006, Tiangco et al., 2008). This is consistent with the hypothesis that green cover may be effective in reducing temperature. Mathematical models and computer simulations have also been employed to investigate and make predictions on the potential effects of vegetation cover on urban climates (Taha et al., 1988, Avissar, 1996). In this review, we focus specifically on empirical data based on temperature measurements collected at ground level, rather than surface temperature measurements from satellite imagery or model predictions. We review studies that measure air temperature within green and non-green sites within an urban area; these studies provide a direct test of the effect of green space on temperature. Green infrastructure may be incorporated in a variety of ways, including the creation of parks, tree planting along streets, and green roofs (Givoni, 1991). To be able to assess the effects of different potential infrastructures rather than green cover per se, our review focuses on studies that measure air temperature in a specific greening type (parks, trees, green roofs, ground vegetation) rather than an undescribed green or vegetation surface cover. We use systematic review methodology to provide a robust and transparent framework to collate relevant studies and synthesise their findings. Meta-analysis is used to statistically synthesise data on the temperature differences between urban parks and non-green urban areas to quantify the average cooling effect of a park across studies. Our review also aims to investigate the strength of evidence on variables that may moderate the air temperature of green space, to investigate the context-dependence of their potential climatic benefits.
Section snippets
Search and selection of studies
The methodology of a systematic review has been described in detail elsewhere for both healthcare (Khan et al., 2003) and environmental (Pullin and Stewart, 2006) fields. In brief, it includes construction of an a priori protocol, comprehensive searching of literature and the application of predefined criteria to identify relevant articles. Relevant articles are then subjected to critical appraisal of methodological quality and their findings are summarised, which can include a quantitative
Synthesis
We aimed to characterise the methodology of these studies and summarise their findings based on the data presented in articles. From each article, we extracted information on research methods, times and location of data collection, type of greening, the number of green and non-green sites, and, importantly, information on their findings with respect to the potential cooling effects of green space. We identified the availability of data from articles that would be amenable for statistical
Overview of studies
Our search identified 74 articles that had measured temperature at ground level in a green and non-green urban area, but only 47 of these could be categorised into one of our greening interventions of interest, based on the information presented in the article. Table 1 presents the number of studies that investigated each greening intervention and aspects of their methodology. The effects of parks and trees have received most attention while the effects of green roofs and ground vegetation have
Discussion on the strength of evidence
This systematic review aimed to assess the evidence on the effectiveness of urban greening as a strategy to reduce urban air temperatures. We focused on the subset of studies investigating specific greening types, which may be used to guide the design of urban greening programmes. Most of these studies investigated the difference in temperature between parks or trees and non-green sites within the same urban area. Their findings broadly support the hypothesis that greening can cool the
Conclusions
Increasing temperatures and the risk of heat wave events in urban areas represents a serious public health concern. We reviewed studies that have investigated the effects of green space on temperatures and these studies present evidence that urban greening, such as parks and trees may act to cool the environment, at least at a local scale. Meta-analysis of data from different studies suggested that, on average, an urban park would be around 1 °C cooler than a non-green site. However, this
Acknowledgements
We thank Dave Stone (Natural England) and the stakeholders/subject experts for their input into the review protocol. Funding for the project came from Natural England.
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