Differential air temperature cooling performance of urban vegetation types in the tropics
Introduction
Air temperatures are typically higher in cities than in surrounding rural areas, a phenomenon known as the urban heat island effect (Arnfield, 2003; Rizwan et al., 2008). In the tropics and subtropics, the maximum intensity of the urban heat island effect is generally lower than in temperate cities (Roth, 2007), but urban areas can still be warmerby as much as 7 °C than their rural surroundings (Chow and Roth, 2006; Tso, 1996). The impacts of urban heat islands on human well-being may be particularly damaging in the tropics due to high temperatures and humidities throughout the year (Chang and Lau, 1993). These conditions discourage people from outdoor recreation (Hwang et al., 2015; Jendritzky and Tinz, 2009), which may reduce overall physical activity and impair health (Mora et al., 2017). Also, more electricity is used for air conditioning; in a study in the United States, for example, electricity consumption increased by 2–4% for every 1 °C increase in ambient temperature over 20 °C (Akbari et al., 2001).
Reducing urban temperatures has become a priority in tropical cities such as Singapore (Roth and Chow, 2012), partly because of growing concerns about the additional impact of future climate warming upon the urban environment. Vegetation is often proposed as one means of mitigating the urban heat island effect (Norton et al., 2015); air temperatures within urban vegetation are typically around 1 °C cooler than in built-up areas (Bowler et al., 2010a), and there is evidence that the cooling benefit can extend beyond the vegetation itself (Chen and Wong, 2006). The reasons for the “cooling effect” of urban vegetation are complex, resulting from interactions between several processes. One of these is evapotranspiration, through which latent heat is removed from air close to the ground surface (Taha, 1997). A second mechanism is shading by tree canopies, which reduces the input of radiative energy into the urban canopy layer and generally results in a lowertemperature (Wang et al., 2008, 2016). The cooling effect of vegetation is closely related to plant characteristics such as vertical structure, transpiration rate and the capacity of leaves to intercept and evaporate rain (Rahman et al., 2015). The shade provided by vegetation is influenced by the structure, height, and density of the vegetation canopy (Shashua-Bar and Hoffman, 2000). In addition to these direct effects of vegetation, there are also indirect effects: for example, urban areas with a high proportion of vegetation tend to be cooler because less energy is used for buildings and transport (Boehme et al., 2015; Taha, 1997).
Urban environments contain several types of vegetation, including lawns, street trees, ornamental shrubs, parks and patches of secondary forest, most of which have been designed and are maintained through management (Khew et al., 2014). These different types of vegetation perform differently in cooling the urban environment (Fung and Jim, 2019). To design urban landscapes that best ameliorate high air temperatures, a knowledge of the cooling capacities of different types of vegetation is needed (Fung and Jim, 2019; Hwang et al., 2015; Yang et al., 2013; Zhang et al., 2013). In this study, we conducted field measurements of air temperatures across the tropical city of Singapore, to quantify the relative temperature difference or “cooling effect” associated with the proportional cover of five common urban vegetation types.
Section snippets
Study area
The study area was the island of Singapore, which has an area of 720 km2 and is located in tropical Southeast Asia (Fig. 1a). To select sample areas, we first determined the green cover in all 500 × 500 m2 grid squares using a published vegetation map (Richards and Tunçer, 2017). A stratified random sample of 30 grid squares was taken from this dataset, after excluding areas where no sampling access was possible (e.g. military live-firing zones, airports). The intention was to place four
Relationships between vegetation and air temperatures
The study resulted in a total of 755,912 hourly temperature records collected between 20/01/2017 and 29/06/2018. Average daily air temperature over the study period ranged from 22.6–30.1 °C. The average diurnal temperature variation was in the range from 3 to 5 °C, with the coolest period around 07:00, and peak temperatures around 15:00 (Fig. 3).
The vegetation in the 10 × 10 m2 plots containing the sensors significantly affected air temperatures, with the proportional cover of three vegetation
Relative temperature reduction by different vegetation types in Singapore
The cooling effect of vegetation observed in this study was comparable to that seen in previous work. For example, a meta-analysis of the temperature differences between parks and urban areas found that parks were 1.15 °C (95 % CI = 0.86–1.45 °C) cooler during the night, and 0.94 °C (95 % CI = 0.71–1.16 °C) cooler during the day (Bowler et al., 2010a, 2010b). These previously-reported values are smaller than the temperature reduction associated with secondary forest in the present study (Table 1
Conclusions
Different types of urban vegetation vary in the cooling they provide, with secondary forest and integrated trees and shrubs being most effective. These differences must be considered when planning the vegetation of urban areas, particularly in tropical cities. It may be impossible to introduce large areas of secondary forest in dense urban centres, although public education programmes could help build acceptance of the cooling benefits. Alternatively, urban vegetation could be designed to be
CRediT authorship contribution statement
Richards DR: Conceptualization, Methodology, Formal analysis, Validation, Data curation, Writing - original draft, Writing - review & editing, Visualization. Fung TK: Conceptualization, Methodology, Data curation, Writing - original draft, Writing - review & editing. Belcher RN: Validation, Writing - review & editing. Edwards PJ: Writing - review & editing, Visualization, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The research was conducted at the Future Cities Laboratory at the Singapore-ETH Centre, which was established collaboratively between ETH Zurich and Singapore's National Research Foundation FI 370,074,016 under its Campus for Research Excellence and Technological Enterprise programme. The authors would like to thank the National Parks Board, Town Councils, Public Utilities Board, Housing Development Board, Singapore Land Authority and National University of Singapore for permission to conduct
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Appendix a - definition of the humid tropics
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