Urban warming and global warming: Combined effect on thermal discomfort in the desert city of Beer Sheva, Israel
Introduction
Urban populations are exposed to both urban-induced local climate modification and to the effect of climate change known as global warming. According to the 2007 Intergovernmental Panel on Climate Change (IPCC) report, in the twentieth century the average global air temperature increased by 0.76 °C, and the linear warming trend over the past 50 years is nearly twice that for the last 100 years. This trend of rising global air temperature is likely to continue (IPCC, 2007). Base on intensive literature review, Alcoforado and Andrade (2008) concluded that the impact of global warming including its affect on human well-being and health may be exacerbated in urban areas. Global and regional warming can aggravate urban warming during summer months (Fujibe, 2009) and may increase the frequency of air temperature extremes in urban areas and prolong the duration of these events, which are more pronounced in arid regions (Golden, 2004).
Global warming is accompanied by several climatic changes, such as an increase of specific humidity (Gaffen and Ross, 1999, Wang and Gaffen, 2001, Willett et al., 2008). Between 1973 and 2003, surface-specific humidity has increased significantly throughout the world, and warmer regions exhibit greater increases in specific humidity for a given air temperature change (Willett et al., 2008). In the United States, Gaffen and Ross (1998) found an increase of several percentage points per decade in specific humidity, and several tenths of a degree increase per decade in dew point. These specific humidity increases are consistent with upward air temperature trends. Similar results were found in China (Wang and Gaffen, 2001).
These global climatic changes can potentially increase human thermal stress. Hence, for example, the mean summertime heat stress in the United States between 1949 and 1995 increased due the combined effect of an increase in air temperature and humidity. These trends may be partly associated with increased urbanization (Gaffen and Ross, 1998).
Climatic changes have also been observed in Israel, and are associated with global warming (Alpert, 2004). Ben-Gai et al. (1999) analyzed daily maximum and minimum temperatures from 40 stations between the years 1964 and 1994 and found minimum summer temperature increases of 0.26 °C per decade and maximum summer temperature increases of 0.21 °C per decade. Ziv and Saaroni (2011) analyzed temperature and humidity data from 16 stations in Israel between the years 1975 and 2008 and found that the average temperature significantly increased by 0.4–1.1 °C per decade. Changes in humidity levels, however, were found to be inconsistent (Shafir and Alpert, 2011).
The urban climate is characterized by amplification of air temperature values, lower humidity values, and moderation of wind velocity (Landsberg, 1981, Oke, 1987). Increased air temperature trends in urban sites have been found in many regions globally, and are mostly due to urban growth (Choi et al., 2003, Karl et al., 1988). Multiple studies have shown that the intensity of urban warming increases with the size of the city (i.e., Oke, 1987, Roth, 2007). DeGaetano and Allen (2002) examined long-term air temperature trends across the United States during the twentieth century and found that urbanization exerts a strong influence on extreme trends found in recent air temperature. For minimum air temperatures, the composite slope at urban stations is nearly three times greater than at rural stations.
Mean air humidity values are lower for urban areas than rural ones; occasionally, the absolute humidity can be higher at night in urban sites (Holmer and Eliasson, 1999, Kuttler et al., 2007, Landsberg, 1981). Therefore, the urban effect is likely to decrease long-term air humidity trend values due to both air temperature increase and modifications in land surface; such modifications can evoke a rapid runoff of precipitation and decreased vegetation. The overall result of such modifications is reduced evapotranspiration (Henry et al., 1985). Jauregui and Tejed (1997) reported that the mean annual values of relative humidity and specific humidity in Mexico City decreased rapidly between 1974 and 1992. Liu et al. (2009) examined seasonal trends of air humidity in Beijing, China and suggested that long-term urban effects can decrease both relative humidity and vapor pressure in the urban area.
In desert cities, long-term humidity trends may differ from those in temperate climate zones, generally because a desert city acts as an oasis in the bare and dry environment of the surrounding desert (Golden, 2004). Therefore, an increase in long-term humidity trends is expected, due to the development of irrigated urban greenery (i.e., parks, lawns, street trees and private gardens). However, an examination of long-term monthly averages of dew point levels in Phoenix, Arizona showed that in between 1896 and 1984, dew points were generally unchanged. Changes in local land use and a developing urban heat island (UHI) are suggested causes of the observed patterns (Brazel and Bailing, 1986). Cicek and Türkoğlu (2009) found a decreasing trend in the long-term annual values of water vapor pressure in Turkey's semi-arid climate, due to urbanization. Shafir and Alpert (2011) found that both temperature and relative humidity have increased over the last four decades in the Israeli city of Eilat, which is located in an extremely hot and arid climate.
Wind velocity within the urban canopy is usually lower in comparison with rural winds at the same altitude. However, when regional winds are very light or calm, with a cloudless sky at night (ideal conditions for urban heat island development), the horizontal air temperature gradient across the urban/rural boundary is sufficient to induce a breeze from the rural area into the city (Oke, 1987). Bornstein (1975) and Bornstein and Johanson (1977) investigated the effect of New York City on regional wind flow and demonstrated that wind velocity over New York City was below the values observed outside the city in a non-urban environment, due to the higher roughness in the city. Examination of wind velocity in New York City's Central Park from 1929 to 2006 showed a drop in velocity over the course of these years (Gaffin et al., 2008). The same wind pattern was found in São Paulo (Oliveira et al., 2003) and in Shanghai (Shun Djen, 1992). The development of an urban heat island was found to cause an increase in wind velocity during nighttime hours when wind velocities were low, in New York City (Bornstein, 1975) and in Phoenix, Arizona (Balling and Cerveny, 1987).
One of the most important implications of urban warming is the increase of heat stress during the day in the summer, which aggravates humans' thermal discomfort (Baker et al., 2002). A study of Phoenix, Arizona showed that rapid urbanization over a period of fifty years has caused the city to warm up by 3 °C, and has doubled the number of hours with T > 38 °C per day during the months of July and August (Baker et al., 2002). In contrast, in Kuwait City, Kuwait, no significant changes in the urban warming and heat stress were observed between 1962 and 1989, mainly due to the city's proximity to the sea, lower overall building heights and the use of local building materials (Balling and Nasrallah, 1991).
Only a few studies address the relationship between global warming and urban warming and their effect on human comfort (Alcoforado and Andrade, 2008). Thorsson et al. (2011) predicted that according to global warming scenarios, heat stress is expected to intensify in northern cities like Göteborg, Sweden, with the occurrence of hot extremes (strong and extreme heat stress) increasing by 100–300% in summer. Lam et al. (2010) found that summer discomfort in Hong Kong has increased from 1968 to 2008 (while winter discomfort decreased) due to warmer weather in recent years.
The study of long-term urban warming trends and their effect on human discomfort and heat stress is an important issue. Moreover, the implications of heat stress during the hottest hours of the day are especially important in urban areas located in hot and arid zones, since these areas are the frontier for the extension and establishment of new desert settlements.
This research examines the combined effect of global warming and urban warming on human thermal stress, as is demonstrated in the rapidly growing desert city of Beer Sheva. The aims of the present study were to:
- a)
Examine the long-term trend in climatic changes in air temperature, humidity and wind velocity in the city of Beer Sheva and in comparison to its surrounding desert environment.
- b)
Demonstrate an urban effect that is superimposed on an environment of climatic change.
- c)
Study the accumulated effect of climatic changes on heat stress values due to the combined effect of regional warming and urban warming.
Section snippets
Study area
The primary location for this case study was the city of Beer Sheva, Israel, located north of the Negev desert at 31°15E/34°50N. Beer Sheva is ideal for UHI examination due to its location on a large plateau and its rapid growth: the Beer Sheva population grew from 45,000 inhabitants in 1965 to 200,000 inhabitants in 2007 (Statistical Abstract of Israel, 2007). The Beer Sheva climate is hot and dry, BWh according to KÖppen Classification (Potchter and Saaroni, 1998). Average annual
An examination of climatic variables between 1967 and 2007
Climactic variables between 1967 and 2007 were investigated for the months of May, June, July, August and September, which together represent the hot season. Statistical analysis of air temperature, specific humidity and wind velocity trends in Beer Sheva and Wadi Hatzerim revealed the following (Table 2a): all summer months showed an increasing trend of air temperature values; the trend in urban Beer Sheva was more significant and pronounced than its corresponding one in the rural area of Wadi
Discussion
The aim of this study was to examine the combined effect of global warming and urban warming on human thermal stress, as is demonstrated in a rapidly growing desert city. An investigation of climate trends and changes over the last several decades indicates that background environmental warming and urban warming are likely to amplify heat stress values. Since 1958, Beer Sheva's population has tripled, causing significant changes in air temperature, humidity and wind velocity in this
Conclusion
The results obtained in the study suggest that:
- •
The accumulated effect of urban climate modification and on a larger scale the effect of environmental climate change and global warming may induce heat stress values in constructed urban areas.
- •
In the case of a desert city irrigation of urban vegetation can cause in the long-term significantly increased specific humidity, which aggravates heat stress values.
- •
Universal or empirical indices such as PET, rather than rational heat stress indices such as
Acknowledgments
The authors wish to thank to Mr. Ido Mintz and Ms. Ilana Gelernter for their statistical analysis of data presented in this paper. The authors are also grateful to Mr. Yaron Yaakov for his technical assistance and to Professor Haim Tzoar for his fruitful advice and remarks.
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