Managing the Adverse Thermal Effects of Urban Development in a Densely Populated Chinese City
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Journal of Environmental Management 70 (2004) 145–156 www.elsevier.com/locate/jenvman Managing the adverse thermal effects of urban development in a densely populated Chinese city Qihao Wenga,*, Shihong Yangb aDepartment of Geography, Geology, and Anthropology, Indiana State University, Terre Haute, IN 47809, USA bDepartment of Geography, South China Normal University, Guangzhou 510630, Guangdong, China Accepted 20 November 2003 Abstract Guangzhou city in South China has experienced an accelerated urban development since the 1980s. This paper examines the impact of the urban development on urban heat islands through a historical analysis of urban–rural air temperature differences. Remote sensing techniques were applied to derive information on land use/cover and land surface temperatures and to assess the thermal response patterns of land cover types. The results revealed an overriding importance of urban land cover expansion in the changes in heat island intensity and surface temperature patterns. Urban development was also related to a continual air temperature increase in the 1980s and 1990s. The combined use of satellite-derived vegetation and land cover distributions with land surface temperature maps provides a potential useful tool for many planning applications. The city’s greening campaigns and landscaping designs should consider the different cooling effects of forest, shrubs and grassy lawns for temperature control and should plant more tall trees. q 2003 Elsevier Ltd. All rights reserved. Keywords: Urban development; Urban heat island; Land surface temperature; Urban and environmental planning; Guangzhou 1. Introduction climatologists have long been interested in the differences in observed ambient air temperature between cities and their Urban development, as a major type of land cover change surrounding rural regions (Landsberg, 1981). The UHI in human history, has a great impact on the environment. In effect is not restricted to large metropolitan areas; in fact, it the process of urbanization, trees are cut and natural has been detected in cities with population less than 10,000 vegetation cover is largely replaced by paved surfaces. people (Karl et al., 1988). Moreover, higher urban Open spaces are maintained for recreational or ornamental temperatures generally result in higher ozone levels due to purposes rather than for the production of food or timber, so an increased ground-level ozone production (DeWitt and that the ecosystem dynamics of the remaining ‘green’ areas Brennan, 2001). Higher urban temperatures also mean of the city are usually quite different from those of the open increased energy use, mostly due to a greater demand for air countryside (Mather, 1986). Moreover, urban areas gener- conditioning. As power plants burn more fossil fuels, they ally have higher solar radiation absorption and a greater drive up both pollution level and energy costs. thermal capacity and conductivity because of being covered Effective ‘greening’ campaigns may reduce urban heat with buildings, roads and other impervious surfaces. Heat is and alleviate the UHI effect. Properly planted trees seize stored during the day and released during night. Therefore, particulates and reduce nearby air temperatures through urban areas tend to experience a relatively higher tempera- both transpiration and shade in the summer and increase ture compared to the surrounding rural areas. This thermal temperatures through wind reduction in the winter difference, in conjunction with waste heat released from (McPherson et al., 1998). Tree planting not only can modify urban houses, transportation and industry, contribute to the microclimate, but also has other ecological as well as development of an urban heat island (UHI). Urban economic and aesthetic benefits (Gatrell and Jensen, 2002). Because of the importance of urban trees, many * Corresponding author. Tel.: þ1-812-237-2255; fax: þ1-812-237-8029. environmentally conscious planning authorities have taken E-mail address: [email protected] (Q. Weng). on the strategy of greening campaigns worldwide (Nichol, 0301-4797/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvman.2003.11.006 146 Q. Weng, S. Yang / Journal of Environmental Management 70 (2004) 145–156 1994). In this context, detailed information on the UHI and (1) how has urban land cover changed over the past several its relationships with urban growth and with urban forest/ decades and what are the temporal and spatial processes of vegetation would be very valuable to urban and environ- urban expansion? (2) how has such urban growth promoted mental planners. Knowledge on these relationships can be the UHI effect and altered surface temperature patterns? and used by planners to recommend tree planting programs and (3) what types of greening campaigns have been adopted to to evaluate the need for new or revised urban design and mitigate the UHI effect and are they adequate and effective? landscaping policies for mitigating the adverse thermal effects of building geometry, building mass and poor landscape layouts. 2. Guangzhou’s climate and socioeconomic settings The objective of this study is to investigate the relationship between urban development and UHI in Guangzhou (also known as Canton) is located at latitude Guangzhou and to discuss the effectiveness that the city’s 238080N and longitude 1138170E and lies at the confluence of greening campaigns have brought about to lessen urban two navigable rivers of the Zhujiang River (literally the temperatures and to dampen the heat island effect. This ‘Pearl River’) system (Fig. 1). With a population of 3.99 coastal city in South China has undergone a fundamental million and total area of 1444 km2 (Guangdong Statistical change in land use and land cover due to accelerated Bureau, 1999), Guangzhou is the sixth most populous city in economic development under the reform policies since the nation. It has been the most important political, 1978. Because of the lack of appropriate land use planning economic and cultural center in South China. The and the measures for sustainable development, rampant Guangzhou municipality comprises eight administrative urban growth has created severe environmental conse- districts in the city proper (Baiyun, Dongshan, Fangcun, quences. Specific questions addressed in this paper include: Haizhu, Huangpu, Liwan, Tianhe and Yuexiu) and four Fig. 1. Study area map showing major geographic features and the eight districts, with inset maps of location and the urban core area. Q. Weng, S. Yang / Journal of Environmental Management 70 (2004) 145–156 147 rural counties (i.e. Huadu, Zengchen, Panyu and Conghua). This study focuses on the city proper of Guangzhou. Guangzhou has a subtropical climate with an average annual temperature of 22 8C. The lowest temperature is normally found in January, averaging 18.3 8C and the highest temperature in July, averaging 32.6 8C. The annual average precipitation ranges from 1600 to 2600 mm. Because of the impact of the East Asian Monsoonal (and less importantly Indian monsoon) circulation, about 80% of the rainfall comes in the period of April to September with highest concentration in spring. The humidity is high in all seasons and the average relative humidity is 80%. This stems principally from the wetland environment and the basin-like terrain of the city. The most uncomfortable season is in the spring’s plum rain period, when the weather is characterized by continuous rain, with rising tempera- tures, low wind velocity and extremely high relative humidity. Clothes may become moldy and people may feel stifled. Research by Professor Yang at South China Normal University (SCNU), who has been studying the Fig. 2. Historical trend in population and housing, 1949–1998. city’s climate for decades using weather station and in situ 3. Data acquisition and image processing data, confirms the existence of the UHI (Yang et al., 1984). The combination of high temperature and high humidity The temperature data between 1985 and 2000 are creates certain degree of thermal discomfort in the obtained from the Central Weather and Chenchun Stations period from May to September and this discomfort in Gunagzhou, including annual mean temperatures, becomes extreme in July and August even for people at monthly mean temperatures, monthly mean of low tem- rest (Yang, 1986). peratures and monthly mean of high temperatures. The UHI Guangzhou has a history of urban development spanning intensity is computed based on the difference of monthly across 2000 years (Xu, 1990). It originated as a small village mean temperatures at the Central Station and those at about 300 BC. The city wall was first built in the third Chenchun Station. century, after China was united in the Qin Dynasty. During Land surface temperatures were derived from geome- the Sui, Tang and Song Dynasties, Guangzhou developed trically corrected Landsat Thematic Mapper (TM) thermal into the largest seaport in the nation as international sea infrared (band 6) images dated on December 13, 1989 and going trade developed rapidly. With the economic devel- August 29, 1997. The TM thermal band has a spatial opment and the population increase, the city wall was resolution of 120 m and a noise level equivalent to a expanded several times toward the east and the north. temperature difference of 0.5 8C(Gibbons and Wukelic, During the Qing Dynasty, the city was further expanded to 1989). The local time of satellite overpass was in the the south, approaching the