تغییرات نواحی اقلیمی ایران از نیمه قرن بیستم تاکنون (2022-1961)

Introduction

The Climate change is one of the most important challenges that affects various aspects of human life and the environment. Climate change directly or indirectly affects human activities and changes the composition of the atmosphere. As a result, the natural and repeatable phenomenon such as drought occurs in a specific time period or periods of drought. Arid and semi-arid regions cover approximately 40% of the world and are mostly located in developing countries. The climate of Iran is very variable in terms of time and place, and the coefficient of change of its annual rainfall is more than 70%. However, the researches that have been carried out on Iran's climate classification so far, but few have been able to fully identify the changes that have occurred in Iran's climate zones. Based on this, the current research has evaluated the impact of climate change on precipitation, temperature, and evapotranspiration, and investigated and classified Iran's climatic regions based on the climate classification of UNEP and Demartin. Then, the changes that occurred in the climatic regions of Iran in the recent normal period (1993-2022) compared to the standard reference period determined by the World Meteorological Organization (1961-1990) have been investigated.



Materials and methods

The studied region in this research is Iran, which has diverse climatic conditions from very dry to very humid. In this research, the temperature and precipitation data of 45 stations from around 1961 with different climates were collected on a monthly basis.Then the climatic zones in the periods of 1961-1990 (as normal period determined by World Meteorological Organization for Climate Change Studies) and recent normal period of 1993-2022 were estimated, using UNEP and Dumartin climate classification indices. Mann-Kendall's and Sen's slope test were used to check the trend of changes and their slopes. In this research, potential evapotranspiration was calculated based on the Torrent-White method.



Results and discussion

In this research, the changes of precipitation, temperature, evapotranspiration and changes in the climatic classes of meteorological stations of the country since the middle of the 20th century based on two classifications methods of UNEP and Dumartin have been investigated for 45 synoptic stations. This study was conducted in a different normal period, including the reference normal period set by the World Meteorological Organization for climate change studies 1990-1961 and the recent normal period 1993-2022. The results of the UNEP climate classification for the period 1961-1990 showed that 9 stations are in humid conditions, 4 semi-humid stations, 12 semi-arid stations, 13 dry stations and 4 stations are in desert conditions. While, in the period of 1993-2022, 5 stations are in wet climate class, 1 semi-humid, 19 semi-arid, 14 dry and 6 are in desert conditions. Examining the changes of two periods shows that about 13 stations are in the new drier climate conditions and about 29 stations are in the new drier climate conditions. The results of Dumarten's climate classification in the period of 1961-1990 showed that 7 stations were in ultra-arid climate conditions, 12 stations were in dry desert climate conditions, 13 stations were in semi-arid climate conditions, and 6 stations were in Mediterranean climate conditions. One station is in humid conditions and three stations are in very humid conditions. While, in the period of 1993-2022, 12 stations are in hyper-arid conditions, 8 stations are in dry desert conditions, 20 stations are in semi-arid conditions, 2 stations are in humid conditions, and 3 stations are in dry conditions. The examination of the changes of two periods shows that about 14 stations are in new and drier climatic conditions and 28 stations are changing toward drier climatic conditions. The results of the significance test of changes and the Sens slope of each of the studied stations for precipitation, temperature, and evapotranspiration showed that there was an increase in temperature, evaporation, and transpiration in more than 75% of the stations, and a significant decrease in precipitation in about one third of the stations Also, the results of the Sense slope showed that the highest slope of annual precipitation decrease was -3.7 mm in Gorgan station. The highest slope of temperature increase was in Mashhad station with 0.729 degrees Celsius per year and the highest increase in evapotranspiration was in Ahvaz station with 25.883 mm. The results of this research show that Iran's climate is rapidly changing to drier climate under global warming, and in a significant number of them, drought can no longer be considered as a temporary disturbance, but as a normal and permanent state of the climate.



Conclusion

As the research findings show, the results of the UNEP index showed that the number of stations with a humid climate has decreased from 9 stations in the reference normal period (1961-1990) to 5 stations in the recent normal period (1993-2022), and on the other hand, the number of semi-arid and dry stations from 25 stations to 33 stations and desert stations from 4 to 6 stations, and the number of stations with semi-humid climate has decreased from 4 to 1.

The results of the Dumarten index showed that the number of stations with a semi-arid to hyper-arid climate has increased from 32 stations in the reference normal period of 1990-1961 to 40 stations in the recent normal period of 1993-2022 and on the other hand, the number of stations with Semi and very humid weather has increased. It has been reduced from 10 stations to 5 stations in the stations with Mediterranean climate. Also, using Mann-Kendall statistics and Sens slope, the results showed that precipitation decreased in 14 stations (31 percent), temperature increased in 35 stations (77.78 percent), and evapotranspiration increased in 34 stations (75.56 percent) on the 0.05 significant level. It is noteworthy that it shows that Iran's climate is rapidly becoming drier under the global warming. The results of this research are in line with the findings of Rahimi et al., (2013), Ashraf et al., (2014), Tawusi et al., (2019), Tawusi et al., (2021), Fathi Tepe Rasht et al., (2022) and Ranjber and Tabatabai(2022).

Key words: Climate classification, Demarton index, Evapotranspiration, Iran, UNEP index.