پیش‌نگری رخداد فرین‌های ترکیبی گرم-خشک (CDHE) در ایران

Climate change has amplified the frequency of weather and climate extreme events, such as heatwaves, heavy precipitation, and droughts. These extreme events are increasingly likely to co-occur, resulting in what are known as Compound Extreme Events. Given its geographical location within the global arid and semi-arid belt and the inherent vulnerability of its ecosystems, Iran is recognized as a critical hotspot for the impacts of climate change. A growing body of scientific evidence indicates that the nation is increasingly affected by such compound events. Compound extremes, particularly the concurrence of heat and drought, pose a significant and escalating threat to societies and ecosystems in vulnerable regions. This study aims to assess and project changes in the frequency and duration of Compound Dry-Hot Extremes (CDHE) across Iran. The data utilized in this research are twofold: observational data from synoptic weather stations and outputs from CMIP6 models. The observational dataset comprises daily maximum temperature and precipitation time series from 98 synoptic stations across the country for the 1990–2014 period. For future projections, this study employed six models from the CMIP6 ensemble. To mitigate the uncertainties inherent in individual models and enhance predictive performance, a Weighted Multi-Model Ensemble (WMME) approach was adopted, based on the Interannual Variability Skill (IVS) score method. The analysis involves comparing a historical period (1990–2014) with near-future (2026–2050), mid-future (2051–2075), and far-future (2076–2100) periods under two shared socio-economic pathways (SSP): SSP2-4.5 and SSP5-8.5. The specific phenomenon investigated is the compound hot-dry event. A CDHE is identified by the simultaneous occurrence of two individual extremes, each defined by specific percentile thresholds. A hot event is defined as a day when the maximum temperature exceeds the 95th percentile of daily values for a given period. Concurrently, a dry event is characterized by daily precipitation falling below the 5th percentile for the same period. Analysis of the historical period reveals a statistically significant increasing trend in both the frequency (0.53 events/decade) and duration (0.86 days/decade) of CDHEs. Future projections indicate a positive anomaly, signifying an increase in CDHE occurrences. Under the SSP2-4.5 scenario, the annual frequency of these events is projected to rise from a historical average of 2.8 events to 8.2 events annually by the end of the century. The increase is substantially more severe under the SSP5-8.5 scenario, with the frequency escalating to 10.4 events per year. Similarly, the duration of these events is expected to lengthen considerably, increasing from a spatially averaged 4.4 days in the historical period to 20.6 days under SSP2-4.5 and a notable 36 days under SSP5-8.5. These results unequivocally highlight a significant intensification of risks associated with compound hot-dry extremes in Iran throughout the 21st century. These findings underscore the urgent necessity for implementing proactive adaptation strategies to address the escalating climate challenges.