Observed changes in the climate zones of Iran from mid 20th century onwards (1961-2022)

ABSTRACT
This research evaluates the influence of climate change on precipitation, temperature, and evapotranspiration in Iran, a region with diverse climatic conditions, using UNEP and Demartonne climate classification systems to explore changes in Iran's climatic zones from the reference period (1961-1990) to the most recent normal period (1993-2022). The results show a decrease in the number of humid climate stations and an increase in semi-arid, dry, and desert stations under both classification methods. Mann-Kendall and Sen's slope analyses reveal significant decreases in precipitation, and increases in temperature and evapotranspiration in over 75% of the stations, with the highest annual precipitation decrease observed in Gorgan, and the highest temperature and evapotranspiration increases in Mashhad and Ahvaz, respectively. The findings suggest that Iran's climate is rapidly shifting towards drier conditions due to global warming, with approximately 13 stations having shifted to drier conditions and 29 stations experiencing increased aridity when comparing the two normal periods. These results align with previous studies, highlighting the urgent need to address the impacts of climate change in Iran and providing valuable insights for policymakers and stakeholders to develop effective strategies for climate change adaptation and mitigation in the region.
Extended abstract
Introduction
Climate change is one of the most significant challenges that affect various aspects of human life and the environment. It directly or indirectly impacts human activities and alters the composition of the atmosphere. Consequently, natural and recurring phenomena such as drought occur within specific time periods. Arid and semi-arid regions cover approximately 40% of the world, primarily situated in developing countries. Iran's climate exhibits significant variability in both time and location, with its annual rainfall coefficient experiencing more than a 70% change. Despite previous research on Iran's climate classification, few studies have comprehensively identified the shifts in Iran's climate zones. This research evaluates the influence of climate change on precipitation, temperature, and evapotranspiration. It also examines Iran's climatic classification based on the UNEP and Demartin climate classification systems. Furthermore, it investigates the changes in Iran's climatic zones during the recent normal period (1993-2022) compared to the standard reference period detemined by the World Meteorological Organization (1961-1990).
 
Methodology
The research focuses on Iran, a region with diverse climatic conditions ranging from very dry to very humid. Temperature and precipitation data from 45 stations with various climates were collected on a monthly basis starting around 1961. The main aim of study is estimation of climatic zones for the periods of 1961-1990 (considered as the normal reference standard period by the World Meteorological Organization for Climate Change Studies) and the recent normal period of 1993-2022 using UNEP and Demartonne climate classification indices. Mann-Kendall's and Sen's slope tests were employed to assess the trend of changes and their slopes. Additionally, potential evapotranspiration was calculated based on the Torrent-White method.
 
 
Results and discussion
This research investigates the changes in precipitation, temperature, evapotranspiration, and shifts in the climatic classification of meteorological stations across the country since the mid-20th century using two classification methods of UNEP and Demartonne. The study covers 45 synoptic stations and examines different normal periods, including the reference normal period of 1961-1990 recognized by the World Meteorological Organization for climate change studies and the recent normal period of 1993-2022. Results from the UNEP climate classification for 1961-1990 estimate 9 stations with humid climate, 4 semi-humid stations, 12 semi-arid stations, 13 dry stations, and 4 desert stations. In contrast, for 1993-2022, there are 5 humid climate class stations, 1 semi-humid station, 19 semi-arid stations, 14 dry stations, and 6 desert stations. Comparing the two periods reveals approximately 13 stations have shifted to drier conditions and around 29 stations experiencing increased aridity. Demartonne's climate classification results for 1961-1990 show 7 hyper-arid stations, 12 dry desert stations, 13 semi-arid stations, and 6 Mediterranean climate stations. Additionally, one station is classified as humid and three as very humid. For the period of 1993-2022, there are 12 hyper-arid stations, 8 dry desert stations, 20 semi-arid stations, 2 humid stations, and 3 dry climate stations. Analysis of these periods indicates about 14 stations with tendency of its climate towards drier climates and 28 transitioning to increased aridity. Significance tests on changes and Sen's slope analysis for precipitation, temperature, and evapotranspiration reveal temperature and evapotranspiration increases in over 75% of the stations under study. Approximately one third of the stations has experienced a significant decrease in precipitation. The highest annual precipitation decrease is estimated for Gorgan station and Mashhad station shows the highest temperature increase, while Ahvaz station exhibits the highest evapotranspiration increase. The research findings suggest that Iran's climate is rapidly shifting towards drier conditions due to global warming.
 
Conclusion
The research findings indicate significant changes in climatic classification of Iran based on the UNEP and Demartonne indices. According to the UNEP index, the number of stations with a humid climate decreased from 9 to 5 from the reference normal period of 1961-1990 to the most recent normal period of 1993-2022. Conversely, the number of semi-arid and dry stations increased from 25 to 33, and desert stations increased from 4 to 6, while the number of semi-humid stations decreased from 4 to 1. Regarding the Demartonne index, the number of stations with a semi-arid to hyper-arid climate increased from 32 to 40 during the reference normal period (1961-1990) up to the most recent normal period (1993-2022). Additionally, the number of stations with semi and very humid climates decreased from 10 to 5, and stations with a Mediterranean climate reduced. The Mann-Kendall statistics and Sens slope analysis revealed that precipitation decreased in 14 stations (31%), while temperature and evapotranspiration increased at a significant level of 0.05 in 35 stations (77.78%) and 34 stations (75.56%), respectively. These results align with previous studies by 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), indicating a rapid shift towards drier climates in Iran due to global warming.
 
Funding
There is no funding support.
 
Authors’ Contribution
All of the authors approved the content of the manuscript and agreed on all aspects of the work.
 
Conflict of Interest
Authors declared no conflict of interest.
 
Acknowledgments
We are grateful to all the scientific consultants of this paper.