Global warming is one of the most controversial topics among climatologists, politicians and even ordinary people. There are different views regrading climate change and global warming. A group of climatologists completely reject the phenomenon of global warming. Some, however, have fully accepted the phenomenon of global warming. In this study, the trend of temperature changes in Iranian metropolises has been studied over a period of 35 years without any bias from a specific perspective. Note that in the central and northern part of Iran, four seasons are quite evident. But in its southern part, four seasons are seen only in the national calendar, but in the climatic calendar, no more than two or three seasons are seen. But in this research, the analyzes have been done in four seasons.
3.1 The trend of temperature changes in Iranian metropolises in spring
Figures 3 to 4 shows 35-year temperature changes in the three months of spring.
In April, while the southern part of Iran is almost warm, in the west and northwestern regions, the weather is still cold and plants are beginning to grow and trees are to sprout. As can be seen, the temperature has increased in almost all metropolises of Iran in this month. The highest increase in maximum temperature with 2.2 ° C occurred in the city of Urmia in the northwest and Birjand in the east of Iran. In contrast, the slightest increase of 0.3 °C in the maximum temperature took place in the city of Bushehr on the Persian Gulf coast. The minimum temperature also rose in all cities of Iran (except Gorgan in northern Iran and the cities of Ilam, Shahrekord and Shiraz in the southwest of the country). The highest elevation of 2.5 ˚C in the minimum temperature occurred in the two cities of Yazd and Zahedan, which are located on the edge of the desert. In this month, except in a few cases, in most metropolises of Iran, the temperature rose. In total, in ten metropolises, the sharpest rise in temperature was in the minimum temperature and in 17 metropolises, the highest increase was observed in the maximum temperature (Figure 3).
Figure 4shows temperature changes in Iranian metropolises in May. During this month, the southern cities of Iran enter the warm season. But, in the northernmost cities, it is the most suitable month in terms of human life and ample vegetation. In other words, it is the most suitable month for tourism in most cities of Iran except for the southern regions. In this month, the highest amount of temperature increase of 3.9 °C in the maximum temperature occurred in the city of Mashhad. Conversely, in the city of Bushehr on the south coast of Iran, the maximum temperature has decreased by about 1.3 ˚C. In other cities, the maximum temperature experience an ascending trend. The highest increase in minimum temperature of at least 3.7 °C happened in Ardabil and Mashhad, which are classified as cold-region cities of Iran. In the three cities of Ilam (tropical region), Shiraz (in the temperate region) and Shahrekord (the highest city in Iran), the minimum temperature has decreased slightly. In other cities of Iran, the minimum temperature has had a rising trend.
Figure 5shows the temperature changes in Iran's metropolises in June. This month, the southern cities of Iran experience high temperatures. However, in the western half and the northwest as well as the northeast, the temperature is still favourable and provides good conditions for human life. Also, the highest rise in temperature occurred in the maximum parameter. This increase of 3.4 ˚C occurred in Sanandaj and Ilam in the west of the country. Also in this month, the maximum temperature has decreased by 0.7˚C in the city of Bushehr in the south of Iran, which is located on the north coast of the Persian Gulf. In another metropolitan, the maximum temperatures have been increasing. In 5 cities, the increase has been higher than 3 ˚C and in 12 cities, this amount was more than 2 ˚C. The highest rate of minimum temperature increase occurred in Khorramabad. In contrast, the minimum temperature in Ilam and Shahrekord decreased by about 1.5 ˚C. In the rest of Iran's metropolises, the minimum temperature also increases. In 4 cities, this increase was more than 3 ˚C and in 8 cities it was more than 2 ˚C.
Table 1 summarizes the situation of temperature changes in each of the metropolises of Iran in the three months of spring. In April, the maximum temperature increased in 27 metropolises and showed a slight decrease in only two cities. This phenomenon has been repeated in the other two months of spring. But, for the minimum temperature parameter, there was an increasing trend in at least 22 metropolises, with five other metropolises experiencing a slight decrease. It has been unchanged in the metropolises of Tabriz and Semnan, though. For the two months of May and June, the upward trend occurred in 26 and 27 cities, respectively. Thus, in several metropolises in other metropolises of Iran, both parameters of maximum and minimum temperature have been increasing.
3.2 The trend of temperature changes in Iranian metropolises in the summer season
Figures 6 to 8 show the maximum and minimum temperature changes in Iranian metropolises in summer over a period of 35 years. In Iran, the majority of stations in July and August are the hottest months of the year. In the southern stations of the country, these two months are too hot for living and working.
In July, as the first month of summer, the highest increase in maximum temperature, at 3.6 °C, occurred in Gorgan. However, in Bushehr, the maximum temperature decreased by about 0.2 °C. In other cities, the maximum temperature experienced a rise. In 4 cities, this increase was more than 3 ˚C and in 13 cities it was higher than 2 ˚C. The highest increase in the minimum temperature parameter of at least 3.1 °C occurred in Khorramabad. In contrast, in Ilam, Shahrekord and Shiraz, the minimum temperature decreased by at least 2.7, 1.3 and 0.6 °C. But in other cities, the minimum temperature increased. In 8 cities, the minimum temperature this a increased by more than 2 ˚C (Figure 6).
In August, an increase in the highest maximum temperature occurred in the city of Ardabil (the coldest city in Iran). In this month, the maximum temperature increased by about 4.4 ˚C in the city of Ardabil. In this month, in 5 cities, the maximum temperature increased by more than 3 ˚C and in 5 cities, this increase has been higher than 2 ˚C. In contrast, in Bushehr, the maximum temperature decreased by 0.2 °C. The highest increase in the minimum temperature occurred in the city of Khorramabad at 3.7 °C. In contrast, in Shahrekord, Ilam and Shiraz, the minimum temperature decreased by 2.7, 1.9 and 1.3 °C, respectively. In 8 cities, this increase was more than 2 ˚C. The highest elevation in temperature increase in this month occurred in the northwest and northeastern regions of Iran. On the contrary, in other parts of the country, the rise has been negligible (Figure7).
In September, the highest maximum elevation in temperature of 2.8 °C occurred in Gorgan. But still in Bushehr, although slightly but, the maximum (noon) temperature decreased as much as 0.1 °C, rather slightly. In other cities except for Bushehr, the maximum temperature shows a rising trend. In this month, unlike in previous months, the increase in minimum temperature has been much greater. In this regard, the highest temperature increase of at least 3.7 ˚C occurred in Yazd. In 6 cities, this increase was more than 3 and in 6 cities it was more than 2 ˚C. However, in Ilam, the minimum temperature decreased by at least 2.6 ˚C.
Table 2shows the temperature changes of 3 months of summer. The maximum temperature increased in all the three summer months in all cities except in one. This means that the noon temperature is increasing in all metropolises of Iran. Regarding the minimum temperature parameter, in summer, the conditions are slightly different. In July, the temperature dropped in 3 of the 29 metropolitans. The metropolis of Urmia has also remained unchanged. But in other metropolises, the trend has continued to increase. In August, the number of metropolises with a minimum temperature (morning temperature) decreased to 7 cities. But still in 22 metropolises, the minimum temperature is rising. In September, in 4 metropolises, the minimum temperature decreased and in other metropolises, it increased.
3.3 The trend of temperature changes in Iranian metropolises in the autumn season
However, with the onset of autumn, temperatures in the northwest and northeastern parts of Iran drop. In the southern half of the country, the weather is still hot where human life and activity are limited. This phenomenon is quite evident, especially in October. The highest increase in maximum temperature has been seen in Zanjan. In 9 cities, the maximum temperature increase was more than 2 ˚C. Meanwhile, in Bushehr, the maximum temperature dropped by 0.3 ° C. But in other cities, the maximum temperature rose. The increase in minimum temperature was much greater than the maximum temperature. Thus, in the city of Yazd, which is a desert and arid city, the temperature increased by at least 4 ˚C. Also, in 3 cities, the minimum temperature rose by higher than 3 and in 7 cities, the temperature was elevated by more than 2 ˚C. Meanwhile, in the city of Ilam, the minimum temperature has dropped by at least 2.4 ˚C. In the cities of Shiraz, Urmia and Shahrekord, the temperature trend, although small, constantly decreased. In this month, in general, in the arid cities of the deserts of Iran, the temperature increased more than it did in other cities (Figure 9).
In November, metropolitan temperature changes show significant differences. The first noticeable difference is in the amount of temperature change. The highest increase was 2.2 ˚C and the highest decrease was 2.3 °C. Second, in many cities, temperatures fell. In this regard, the maximum temperature in 22 metropolises plummeted. In terms of minimum temperature, in 13 metropolises, there has been a downward trend. Third, in many metropolises, both the minimum and maximum temperatures went down, and in 11 metropolises, both parameters had a falling trend, which is a strange phenomenon in Iranian metropolises (Figure 10).
There was a slightly different situation for the month of December compared to November This phenomenon is especially evident at maximum temperaturesThus, the maximum temperature continued to rise in 28 metropolises, yet rather slightly. The highest temperature increase with 3.8 ˚C occurred in Ilam station. In terms of the minimum temperature, the situation is almost the same as the previous month. Thus, in 13 metropolises, there was a downward trend in the temperature. The largest decrease occurred in Zahedan at 2.6 ˚C. However, in 16 metropolises, the minimum temperature continued to increase. The highest increase in minimum temperature, with 4.9 ˚C, occurred in Ardabil.
In Table 3, the status of temperature changes in autumn can be observed. In this season, during the first month, the conditions are the same as the previous months, with a rising trend in the maximum temperature in 28 of the 29 metropolises. In terms of the minimum temperature, the value reaches 25 cities. However, the temperature changes in the next two months are remarkably different especially in November. In this month, the maximum temperature in 22 metropolises was reduced and in three metropolises of Ahvaz, Rasht and Bandar Abbas it remained unchanged. This month, the minimum temperature increased in 16 cities and decreased in 13 others. In December, however, the maximum temperature returned to previous conditions, with 28 cities experiencing an upward trend. Despite that, in terms of the minimum temperature, in 13 metropolises the trend was still falling and in 16 metropolises it was rising.
3.4 The trend of temperature changes in Iranian metropolises in winter season
In this season, the northern half of Iran, especially in the first two months of the season, experience cold weather, yet on the south coast, there is a mild weather then. A preliminary examination of temperature changes shows that two important phenomena have occurred in temperature changes in this season. First, in almost all cities, both the maximum and minimum temperature parameters were incremental. Second, there was a significant temperature increase in most cities. Figures 12 to 14 show the temperature changes of Iranian metropolises in winter. In January, the highest temperature increase in the maximum temperature, in Zanjan, reached 5.5 degrees Celsius, which is a high figure. In 13 cities, the maximum temperature increase was more than 3 degrees Celsius and in 10 metropolises, this increase was higher than 2 degrees Celsius, which is considerable. The highest increase in temperature of at least 9.3 °C, which is noteworthy, can be seen in Ardabil. In 6 cities, the increase was larger than 3 degrees Celsius. Meanwhile, in the cities of Ilam, Shiraz and Gorgan, there was a slight reduction in the minimum temperature.
Figure 13 shows the temperature changes of February. In this month, the city of Zanjan with 5.2 degrees Celsius had the highest temperature increase in the statistical period of 35 years among the metropolises of Iran. At the same time, in 7 metropolises, the temperature increased by more than 4 ˚C, and in 4 cities, the temperature rose by more than 3 ˚C. during this month, the maximum temperature did not experience any rise in any city. As can be seen on the map, the west and northwest regions had the highest temperature rises. In total, the temperature in this region of the country increased by more than 3 ˚C at the maximum temperature. The lowest temperature rise in this month belonged to the southern and southeastern half of Iran. The highest elevation in the minimum temperature (morning temperature), still with 5.2 °C, belongs to the city of Zanjan. The cities of Ardabil, Arak and Yazd are next with 4.8, 4.2 and 4.2 ˚C, respectively. In 3 cities, an increase of greater than 3 ˚C is seen. In terms of minimum temperature parameter, the highest rise in temperature occurred in the northwestern and western parts of Iran. The southern and southeastern parts still show the least increase.
It can be observed in the March map that the highest cumulative temperature rise occurred this month. As can be seen, the highest maximum temperature increase of 6.8 °C still occurred at Zanjan station. It should be noted that in Urmia and Arak stations, the temperature increased by 6.4 and 6.3 ˚C. In 11 stations, the temperature of higher than 5 ˚C and in 9 stations, an increase of larger than 4 ˚C occurred. As can be seen, the temperature rose significantly in all cities except the southern coastal cities. The increase in the minimum temperature was the same as the maximum temperature this month when in 13 cities, the temperature increased by more than 3 ˚C. This significant increase in temperature mainly occurred in the west and northwest region and then on the shores of the Caspian Sea. This region is one of the rainiest and at the same time the most pleasant and airy regions of Iran.
Table 4 demonstrates the trend of temperature changes in Iranian metropolises during the winter months. As can be seen, in every three months of winter, the maximum temperature increased in all metropolises of Iran. The minimum temperature slightly decreased in only three metropolises in January. But in other cities, the trend was still rising. In the other two months of the winter, the minimum temperature increased in all cities.
Discussion
Global warming is a challenging issue for countries that are located in the hot and dry regions of the world and are struggling with the problem of water shortage and drought. In countries facing the problem of water shortage, drinking water supply for the urban population, especially for Metropolises, is a major problem. Most of these metropolises in hot and dry areas lack large rivers or rich groundwater resources. As a result, the increase in temperature, in addition to affecting the comfort, living and activities of human beings, but also with increasing urban water consumption (drinking, cleaning, etc.) makes the water supply of cities with serious problems. Water shortages in hot seasons may become a crisis too.
Iran has 31 metropolises, most of which are located in provincial capitals. Except for the metropolis of Ahvaz, through which a river flows, other metropolises of Iran do not have strategic rivers. The vast majority of these cities are located in large plains or foothills.
In this study, the trend of maximum and minimum temperature changes in all metropolises of Iran except Sari and Yasuj was studied in a statistical period of 35 years (1985-2019) (These two cities did not have a sufficient statistical period). The findings of the present study are as follows:
1. In the three months of spring, the three metropolises of Ilam, Shahrekord and Shiraz (which are generally located in the southern part of Iran) as well as Gorgan, which is a coastal city in the north of the country, experienced a slight decrease in temperature. In other metropolises, there was an increasing trend every three months of spring. This increase in temperature between one and a maximum of 3.9 ˚C was observed in the city of Mashhad (in the northeast of the country). Early in the season, most changes occurred in the cities of the eastern half of Iran. However, in the following months, the highest temperature increase occurred in the cities of the northern and northwestern parts. Figures 15 and 16 show the trend of temperature changes for Iranian metropolises in the 35-year period in spring. In this chart, cities are adjusted based on latitude and from low to high. As can be seen on the graphs of temperature changes, the increasing trend of temperature has intensified from the south to the north of the country. Basically, the increasing trend in the maximum temperature parameter was more homogeneous than the minimum temperature. In the two metropolises of Ilam and Shahrekord, the minimum temperature has decreased abnormally and in the metropolis of Ardabil there has been an unusual increase.
2. In summer, except for the city of Bushehr in the south, in all metropolises of Iran, the maximum temperature shows an increase. The increase in temperature in a city like Ardabil, which is one of the coldest cities in Iran, reached 4.4 ˚C. As for the minimum temperature parameter, at least in 4 metropolises, the temperature trend was falling, but in many cities, the trend was rising. The highest increase in minimum temperature reached at least 3.7˚C. Figures 16 and 17 show the trend of temperature changes in the summer months. As can be seen in the diagram, there was a sharper trend of increase, especially for the maximum temperature parameter from south to north of the country. In this season, the highest values of increase in both maximum and minimum temperature parameters occurred in the northwestern region of Iran.
3. In the first month of autumn, the temperature is almost the same as in previous seasons. In this connection, except in one city in the maximum temperature parameter and 4 cities in the minimum temperature parameter, in other cities the temperature constantly rose. Therefore, the minimum temperature in the city of Yazd increases to 4 ˚C. An important phenomenon that distinguishes this season from other seasons is that the temperature changes in the other two months of autumn, especially in November, show a significant difference with other months of the year. In many metropolises, the temperature decreased in both maximum and minimum temperature parameters. In November, the maximum temperature in 22 metropolises and the minimum temperature in 13 metropolises decreased. This is an unusual phenomenon in this month of autumn. Figures 18 and 19 demonstrate the trend of temperature changes in autumn.
4. In winter, the phenomenon of heating is serious in the metropolises of Iran. As seen on the chart, the maximum temperature rose constantly in all metropolises without exception. This phenomenon was repeated in the minimum temperature parameter except for the three cities of January. Another notable phenomenon is the amount of temperature rise. In this regard, the temperature rises to about 9.3 ˚C in a cold city like Ardabil. In other cities, increases of 4 to 6 ˚C are seen in the maximum and minimum temperatures. Figures 20 and 21 show the trend of temperature changes in winter.