Spatiotemporal analysis of air temperature indices, aridity conditions, and precipitation in Iran

Global warming has become a major threat to life on the earth, and recognizing its impacts can definitely be useful in controlling and mitigating its adverse effects. In this study, time series variations in air temperature indices (frost days, T_min, T_max, T_mean, T_minmin, T_maxmax, T_soil-min), De Martonne aridity index (I_DM), and total precipitation were investigated using a long-term meteorological data (1960–2019) of 31 synoptic stations throughout Iran. The results indicated that more than 94% of the stations had increasing trend in T_mean, in which about 70% were significant at the 0.05 level. The average increase in T_min was calculated approximately 1.7 times higher than T_max and also the increase in T_minmin was about 2.5 times higher than T_maxmax. Our findings showed that abrupt changes in T_min and T_max mostly observed in the 1990s were upward in 87% of all the stations. Increase in annual T_mean at a rate of 0.3 °C per decade and reduction of 5 mm per decade in total annual precipitation led to decrease in the I_DM aridity index by 0.35 per decade in Iran. The intensity of air temperature increase was higher in tropical regions than in cold regions. Trend analysis in the partial series before and after a change point showed that the trends in T_mean before the change point were negative, but turned to positive afterwards in some stations mostly located in the northwestern cold and mountainous regions of the country. Our results revealed that the climate in Iran, in general, has become warmer and drier in the past 60 years and continuation of the current global warming trend will exacerbate this problem in the future.