The Roles of Plants on Mitigating the Urban Heat

International Journal of Agriculture and Economic Development, 2(2), 18-32, December 2014 18

The Roles of Plants on Mitigating the Urban Heat Islands’ Negative Effects Serpil ÖNDER University of Selcuk, Faculty of Agriculture, Department of Landscape Architecture Konya, Turkey [email protected] Ahmet AKAY University of Selcuk, Faculty of Agriculture, Department of Landscape Architecture Konya, Turkey [email protected] Abstract A growing majority of the global population now resides in urban areas. In 2014 urban populations made up approximately 54 % of the world’s population and by 2050 this urban proportion was expected to rise to over 66 %. Rapid urbanization leads to changes in the absorption and reflection of solar radiation, and thus the surface energy balance. Urban heat island (UHI) describes the phenomenon that temperatures are higher in urban areas compared to surrounding rural areas. UHI effect means that air temperatures in urban areas are on average 2–5◦C higher, and in some cases more than 10 ◦C higher, than those in surrounding non-urban areas. Increasing temperatures in the urban area may lead to significant ecological and social consequences. Trees and green spaces contribute considerably to the improvement of the urban climate and to the UHI mitigation. These spaces have been proposed as one of the most important approach to mitigate negative consequences of increased temperatures resulting from UHI. In this study we review the benefits of plants in urban spaces affect thermal comfort such as solar radiation, temperature of exterior surfaces, air temperature, air humidity and wind speed for to destroy the urban heat island.

Keywords: Urban heat island, Urban green spaces, Benefits of plants


Introduction The world’s population in urban areas reached 54 % in 2014 and estimated to increase to 66 % in 2050 (United Nations, 2014). This development underlines the need to understand urban meteorology since this governs the environmental quality of urban districts. Urbanization has brought about several undesirable environmental changes. In the process of urbanization, land cover change sand natural surfaces are replaced by the urban fabric which is characterized by higher temperatures than the surrounding rural environment, a pattern described as urban warming. Large body of urban climate studies have shown that thermal, optical and geometric properties of urban surfaces affect heat absorptive and radioactive properties and lead to the so called Urban Heat Island (UHI) effect (Feyisa, Dons, & Meilby, 2014). Two major phenomena were observed in large cities as compared to its surroundings: -a higher temperature or heat content called Urban Heat Island (UHI), -an occasional lower temperature called Urban Cool Island (UCI) or Urban Cool Valley (UCV), (Rizwan, Dennis, & Chunho, 2008). Increasing temperatures and the risk of heat wave events in urban areas have also represent a serious public health concern. The field of UHI has become highly interesting for scientists and engineers due to its adverse environmental and economic impacts on the society and promising benefits associated with mitigating high heat intensity (Bowler, Buyung-Ali, Knight, & Pullin, 2010). The number of studies related to climate change and urban heat island in the world have started to increase since 1980s. In Turkey today, observations related to urban heating and cooling degrees in cities were made by the Ministry of Water Affairs and Forestry General Directorate of Meteorology (“Isıtma ve Soğutma Gün Dereceleri,” n.d.) Ministry of Environment and


Urbanization prepared the national climate change action plan for cities 2011-23 report. ‘Water Resources Management’, ‘Agriculture and Food Security’, ‘Natural Disaster Risk Management’, ‘Ecosystem Services, Biodiversity and Forestry’, ‘Public Health’ actions were defined for the future in this report and precautions were described (“National climate change action plan,” 2011) Several scientific studies were made to determine the essential precautions and current situation relevant to urban heat islands (Eljadid, 1994; Demirel, 1994; Karaca, Tayanç, & Toros, 1995; Tayanç, 1995; Akay, 1996; Tayanç, & Toros, 1997; Şen, 2002; Çiçek, & Doğan, 2005; Yüksel, 2005; Şimşek, & Şengezer, 2012; Sağlık A., Sağlık E., & Kelkit A., 2014; Gülten, & Aksoy, 2011). This paper aims to contribute the topic of "the roles of vegetation on minimizing the negative effects of urban heat islands" in the light of searching the formation and impacts of Urban Heat Islands by reviewing the relevant available literature. Importance and Formation of Urban Heat Islands The annual mean air temperature of a city with one million or more people can be 1.8 to 5.4°F (1 to 3°C) warmer than its surroundings, and on a clear, calm night, this temperature difference can be as much as 22°F (12°C), (EPA 2012). This difference in temperature between urban and rural areas has been called the ‘urban heat island effect’. Linkages between urban growth and the formation of urban heat islands (UHIs) have been proposed by diverse authors and for various climatic regions (Welch, 1980; Roth, Oke, & Emery, 1989; Kim, 1992; Lee, 1993; Gallo & Tarpley, 1996; Lo, Quattrochi, & Luvall, 1997; Quattrochi & Luvall, 1997; Owen, Carlson, & Gillies, 1998; Kawashima, Ishida, Minomura, & Miwa, 2000; Stone, 2001; Lozada, 2002; Streutker, 2003; Dixon & Mote, 2003). As a consequence of such urban growth, local climate conditions can be highly affected, generating an impact on overall


energy consumption in air-conditioned buildings, or on indoor thermal stress, in the case of free running buildings, due to unintended Urban Heat Island (UHI) effects. An additional problem is posed by present trends as regard climate change. Global mean temperature has increased by approximately 0.8 ◦C since the beginning of the 20th century, most likely according to a rising trend (Diffenbaugh & Field, 2013). Many factors contribute to the development of UHI. Some are related to natural factors such as weather and location. Sato, Murakami, Ooka, & Yoshida (2008), for example, showed a reduction in atmospheric UHI due to the availability of a sea breeze. Other factors are related to wind speed (Radhi et al., 2013). Stone (2012) identified four principle characteristics that make cities hotter than surrounding areas. These characteristics were the reduction in evaporative cooling, low surface reflectivity, vertical surfaces, and waste heat. As a summary, Mobaraki (2012) expressed the following: as the causes of UHI roots, • The complex exchange of heat between buildings in urban streets which act as canyons retaining heat. • The thermal properties of buildings materials that kept the heat within the fabric of urban areas. • Heat released from combustion of fuels, and animal metabolism. • The urban greenhouse effect helps increase the heating effect of the incoming radiation on the polluted and heater urban atmosphere. • Reduced evaporative cooling due to impervious surfaces. • Turbulent transfer of heat, by air movement, from within streets is reduced. • Reduced vegetation due to ever increasing demand for land.


Negative Impacts of Urban Heat Islands Land cover changes such as urbanization may enhance the effect and intensity of the UHI (Antrop, 2004; Kalnay & Cai, 2003; Susca, Gaffin, & Dell'osso, 2012). The UHI induces heat stress, tropospheric ozone formation, and resulting health problems. Higher temperatures lead to increased electricity demand for air conditioning, which, in turn, raises power plant pollution and greenhouse gas emissions. In addition, the UHI may increase water temperatures, resulting in water ecosystems impairment (Chun & Guldmann 2014). Urban warming may also enhance air pollution, for example by increasing surface ozone concentration with several negative impacts on human health (Jacob & Winner, 2009; Weaver et al.,2009). According to Chun and Guldmannb (2014); UHI impacts on the habitability of cities and, for example, increases risk of mortality in the population who are sensitive to high temperature, especially the elderly (>65 years) (Basu & Samet, 2002; Chang, Li, & Chang, 2007; Klenk, Clemens, & Kilian, 2010) and the very young (