Document Type : Research Article
Authors
1
Department of Geography, Faculty of Literature and Humanities, Ferdowsi University of Mashhad, Mashhad, Iran
2
Research Institute for Meteorology and Atmospheric Science, Climate Research Center, Mashhad, Iran
Abstract
The aim of this study is to project future changes in precipitation across the altitudinal zones of Iran under global climate change scenarios. Daily precipitation data from 14 selected synoptic stations located in mountainous and highland regions during the baseline period (1985–2014), along with the outputs of CMIP6 models under three green house‑gas emission scenarios—low (SSP1‑2.6), moderate (SSP2‑4.5), and high (SSP5‑8.5) were employed. The results reveal that precipitation changes over Iran are spatially heterogeneous and largely affected by altitude and geographical position. On an annual scale, among six altitudinal zones, only the northern lowlands (Caspian coast) and the southern lowlands (Persian Gulf and Oman Sea coasts) exhibit statistically significant precipitation trends, whereas the other four zones (mountainous, inner foothill, outer foothill, and central regions) show non‑significant increasing or decreasing trends. Seasonally, spring precipitation is expected to increase under the low‑emission (sustainable development) scenario in both near and distant futures. Summer precipitation is projected to decline in northern lowlands, while an increase in summer and autumn rainfall is anticipated for southern lowlands and outer foothills. Moreover, most altitudinal zones show a positive tendency in autumn precipitation, although not statistically significant in some scenarios. Overall, the findings align with the Sixth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) and with credible regional studies, emphasizing the need to focus water‑resource management on areas with significant precipitation decrease (northern lowlands) and those with marked increase (southern lowlands), as well as on the generally decreasing trends observed in the high‑altitude regions.
Introduction
The aim of this study is to project future precipitation changes across different altitudinal zones of Iran under global climate change conditions. Climate change is widely recognized as one of the greatest challenges facing humanity in the twenty‑first century. According to major international organizations, climate change is defined in two principal ways: the World Meteorological Organization (WMO) describes it as a long‑term increase or decrease in the mean values of climatic variables, whereas the Intergovernmental Panel on Climate Change (IPCC) emphasizes changes in climate anomalies driven primarily by human activities. These anomalies manifest through altered patterns of droughts, floods, heatwaves, cold spells, glacier retreat, sea‑level rise, and other extreme events (Ahmadi et al., 2015).
The specific climate hazards affecting any region depend largely on its inherent climatic characteristics. Climate change influences the hydrological cycle, water availability, and water demand mainly through modifications in temperature and precipitation regimes. In this context, projecting future changes in temperature and precipitation using models from the sixth phase of the Coupled Model Intercomparison Project (CMIP6), which benefit from improved physical parameterizations and higher accuracy, can provide valuable insights for long‑term water‑resource planning and management (Mahabadi et al., 2013). Iran is predominantly an arid and semi‑arid country, receiving less than one‑third of the global average annual precipitation (Alizadeh, 2017). Moreover, this limited precipitation has become increasingly irregular under climate change conditions. For instance, previous studies have projected a declining precipitation trend in northwestern Iran toward the end of the twenty‑first century (Mohammadi et al., 2012), while changes in eastern Iran range from −11% to −15% in the near future and from −8% to +15% in the distant future relative to the observational period (Babaeian et al., 2021).
The natural setting of Iran is unique. The Iranian Plateau lies within the extensive Alpine–Himalayan mountain belt while simultaneously occupying the core of the world’s largest desert belt, stretching from the Sahara Desert in North Africa to the Gobi Desert in Central Asia. Historically, due to pronounced altitudinal variability and limited atmospheric precipitation, human settlements in Iran have mainly developed in foothill regions and along river corridors in plains. Altitude therefore plays a critical role in shaping Iran’s climate, with precipitation generally increasing with elevation in most regions. Given this background, the present study investigates future precipitation changes across Iran’s altitudinal zones at both annual and seasonal scales. Particular emphasis is placed on assessing the role of altitude in modulating precipitation responses under different socioeconomic scenarios, an aspect that has received relatively limited attention in previous studies.
Material and Methods
This study employs both observational data and climate model outputs from the sixth phase of the Coupled Model Intercomparison Project (CMIP6). Daily observational precipitation data were obtained from 14 synoptic stations representing different altitudinal zones of Iran (Alijani, 2002). The baseline period of 1985–2014 corresponds to the historical reference period adopted in the Sixth Assessment Report of the IPCC.
Climate model data for the historical period (1985–2014) and the future period (2021–2100) were extracted from the Earth System Grid Federation (ESGF) database. Future projections were analyzed under three Shared Socioeconomic Pathway scenarios: sustainable development (SSP1‑2.6), intermediate development (SSP2‑4.5), and fossil‑fuel‑based development (SSP5‑8.5). Four Earth System Models CMCC‑ESM2, GFDL‑ESM4, MPI‑ESM1‑2‑HR, and NorESM2‑MM—were selected for analysis.
Future precipitation changes were assessed for two time horizons: the near future (2021–2050) and the far future (2061–2090). Accordingly, precipitation trends under different socioeconomic scenarios were evaluated for mountainous, inner and outer foothill, northern lowland, southern lowland, and central zones. The statistical significance of precipitation trends was assessed using Student’s t‑test.
Results and Discussion
Annual precipitation in the high‑altitude zone (1,340 m), represented by the Tabriz station, is projected to decrease by approximately 5–10% under all scenarios and in both future periods relative to the baseline period. Overall, precipitation in this zone exhibits a decreasing trend; however, this decline is statistically significant only in the distant future under the medium‑emission scenario (SSP2‑4.5). In the inner foothill zone (1,247 m), precipitation at the Mashhad station shows a statistically significant increase in both the near and distant future under the low‑emission scenario (SSP1‑2.6), whereas a non‑significant decreasing trend is observed under the other scenarios. The most pronounced seasonal increase in precipitation is projected for autumn, with the highest increase occurring at the Isfahan station. Seasonally, under the SSP1‑2.6 scenario, winter precipitation shows a significant increase in the near future at Mashhad. In contrast, a significant decrease in precipitation in the distant future under the medium‑emission scenario is observed only at the Isfahan station.
For the outer foothill zone (1,242 m), an increase in annual precipitation is projected, which is statistically significant in the near future under both SSP1‑2.6 and SSP2‑4.5 scenarios. The magnitude of this increase ranges from approximately 10% to 40% compared to the baseline period (1985–2014). In general, autumn precipitation in the outer foothill zone shows a decreasing tendency. Conversely, winter precipitation increases under most scenarios—except for the high‑emission scenario (SSP5‑8.5)—although these increases are not statistically significant.
In the central region (1,300 m), annual precipitation is projected to increase by about 5–10%. This increase is statistically significant only in the near future under the SSP1‑2.6 scenario, while no significant changes are detected under other scenarios. Autumn precipitation increases under all scenarios, with a statistically significant rise observed at the Zahedan station. Winter precipitation generally shows a non‑significant increasing trend, except under the distant‑future SSP5‑8.5 scenario, where a decrease is projected. In the northern lowland region (12 m), annual precipitation changes are negative under most scenarios, except for the near‑future SSP1‑2.6 scenario, and the decreasing trend is statistically significant under the medium‑emission scenario. Summer precipitation across this region shows a significant decline. Changes in autumn precipitation are not statistically significant in any scenario–period combination and generally remain below normal conditions. At the Sari station, precipitation follows the overall regional decreasing trend, whereas at the Gorgan station, precipitation exhibits a non‑significant increase in most scenario periods, except under the medium‑emission scenario in the distant future. Winter precipitation in this region also generally decreases, except under SSP1‑2.6 and SSP2‑4.5 in the near future. Overall, precipitation trends in the northern lowlands are predominantly negative, although most are not statistically significant. In contrast, annual precipitation in the southern lowland region (12 m) shows a statistically significant increase under several scenarios in both the near and distant future. This region exhibits the largest increase in precipitation among all six altitudinal zones. In general, summer and autumn precipitation are projected to increase, whereas winter precipitation tends to decrease. This decrease is statistically significant under the medium‑emission scenario in the distant future, with the most pronounced decline observed at the Abadan station. Conversely, winter precipitation at the Chabahar station increases and is statistically significant under the near‑future SSP5‑8.5 scenario.
Table 1 summarizes precipitation changes across the different altitudinal zones of Iran and highlights trends that are statistically significant at the 0.05 level. Among the six altitudinal zones, statistically significant precipitation changes are observed only in the two low‑lying regions: the northern lowlands (Caspian Sea coast) and the southern lowlands (Persian Gulf and Oman Sea coast). In contrast, precipitation changes in the remaining four higher‑elevation zones mountainous, inner foothills, outer foothills, and central regions are not statistically significant.
Conclusion
This paper projected future precipitation behavior across different altitudinal zones of Iran using observational data from 14 representative synoptic stations and bias‑corrected outputs from climate models participating in the sixth phase of the Coupled Model Intercomparison Project (CMIP6) under multiple socioeconomic scenarios. The results reveal a heterogeneous pattern of precipitation change across Iran’s altitudinal zones, highlighting the critical role of elevation and geographic location in shaping future precipitation regimes. On an annual scale, among the six investigated altitudinal zones, statistically significant precipitation changes are observed only in two low‑lying regions: the northern lowlands along the Caspian Sea coast and the southern lowlands adjacent to the Persian Gulf and the Oman Sea. In these regions, precipitation is projected to decrease and increase significantly, respectively, under different scenarios. In contrast, the remaining four zones mountainous regions, inner foothills, outer foothills, and central highlands exhibit both increasing and decreasing precipitation trends, none of which are statistically significant at the annual scale. Although a decreasing trend dominates in the mountainous zone, it does not reach statistical significance. Similarly, projected increases in precipitation in the central and outer foothill zones remain positive but statistically insignificant.
At the seasonal scale, a notable increase in spring precipitation is projected under the low‑emission scenario (SSP1‑2.6) in both the near and distant future. Summer precipitation is expected to decline in the northern lowlands, whereas an increase in summer and autumn precipitation is projected for the southern lowlands and, to a lesser extent, the outer foothill zones. Moreover, the direction of autumn precipitation changes in many high‑elevation regions of Iran is generally positive, although these changes are not statistically significant in some scenario–period combinations. In the overall assessment, it is important to note that actual water availability and demand are influenced not only by precipitation but also by other climatic factors, particularly temperature. Based on established hydro‑meteorological relationships, an increase of 1 °C in air temperature leads to an approximate 7% rise in atmospheric moisture demand, which may partially or fully offset the benefits of increased precipitation in some regions.
The findings of this study are consistent with those reported in reputable national and international research, including the Sixth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC). Overall, the results demonstrate that future precipitation changes across Iran’s altitudinal zones are highly heterogeneous, underscoring the need for region‑specific adaptation and water‑resource management strategies, with particular attention to areas experiencing significant precipitation reductions in the northern lowlands and pronounced increases in the southern lowlands.
Keywords
Subjects
