Urban greening to cool towns and cities: A systematic review of the empirical evidence

Abstract

‘Urban greening’ has been proposed as one approach to mitigate the human health consequences of increased temperatures resulting from climate change. We used systematic review methodology to evaluate available evidence on whether greening interventions, such as tree planting or the creation of parks or green roofs, affect the air temperature of an urban area. Most studies investigated the air temperature within parks and beneath trees and are broadly supportive that green sites can be cooler than non-green sites. Meta-analysis was used to synthesize data on the cooling effect of parks and results show that, on average, a park was 0.94 °C cooler in the day. Studies on multiple parks suggest that larger parks and those with trees could be cooler during the day. However, evidence for the cooling effect of green space is mostly based on observational studies of small numbers of green sites. The impact of specific greening interventions on the wider urban area, and whether the effects are due to greening alone, has yet to be demonstrated. The current evidence base does not allow specific recommendations to be made on how best to incorporate greening into an urban area. Further empirical research is necessary in order to efficiently guide the design and planning of urban green space, and specifically to investigate the importance of the abundance, distribution and type of greening. Any urban greening programme implemented would need to be appropriately designed and monitored to continue to evaluate benefit to human health through reducing temperature.

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.