Climate change detection update over Iran during 1958-2017

This project is an updated estimate of the climate change trend in Iran. The time series of monthly and annually mean, maximum and minimum temperature (°C), relative humidity (%) at 2 m height, wind speed (m.sat 10 m height, sun hours (h), precipitation (mm), radiation (Mj.m-2day-1) and cloudiness data at 27 weather stations over Iran are collected from IRIMO for the period 1958-2017. Quality control, detection and modification of non-climate heterogeneity of data was performed. Due to missing data, the Sun hours trend was calculated for the period 1992-2017.

Methodology and data:

Among all stations of Iran, 27 stations have 60-yearly data in the period of 1958-2017. In this research the changes in temperature, precipitation, humidity, radiation, cloudiness and wind during the period of 2017-1958 (for sunshine the period was 1992-2017) were investigated. At first, the data were checked for quality controlling. Then their non-climatic heterogeneities were fixed. The slope of the trend was determined using the least squares method and the slope estimator and their significance was assessed using nonparametric Man-Kendal test and regression.

Temperature: The results showed that all stations in the country face a significant increase in the annual minimum temperature. The minimum temperature increasing rate per decade calculated to be between 0.2 and 0.4  degree of Celsius in Bandar-Abbas and Tehran, alternatively. In general, minimum temperatures rise were detected at all stations and in all seasons, especially in autumn and winter. The annual maximum temperature trend is also increasing, but the rate of increase in maximum temperature is less than the minimum temperature. The increasing rate per decade calculated to be between 0.08 and 0.3  degree of Celsius in Zanjan and Ahwaz, alternativelyPrecipitation: The 60, 30, and 10-years averaged annual precipitation of Iran calculated to be 230.8, 222.4 and 199.3 mm, respectively. The results showed that during the 60-year period, the average rainfall of the Iran decreases, with a rate of 0.43 mm per year (4.3 mm per decade), although the precipitation decline is not significant at 95% level.However, during the last 30 years (2017-1988), the average precipitation of the country has dropped by 2.2 millimeter per year (22 mm per decade), which is significant in 95% confidence level; meaning that precipitation reduction in the most recent 30 years is about four times higher than that of past 60 years.In Figures 2 and 3, the all-country time series of rainfall and temperature changes are shown in the 60- and 30-yearly basis.The country's declining precipitation for the most recent 30 years is about four times faster than the decline of most recent 60 years. The rapid decline in country precipitation over the past 30 years, which is significant   in 95% level, is consistent with the intensification of global warming in the most recent 30 years period.Other parameters: The average wind speed in many parts of the country has increasing trend, which is significant in many stations located in the west, center, and northern part of the country. Average relative humidity has decreased in many regions of the country. The most decreasing trend was observed in southwest and west of the country. Of course, in a few cases such as Gorgan and Rasht, there was an increase, which was not statistically significant. Sunny hours trend was calculated in the period of 1992-1992, and interestingly, seasonal and annual trends at most stations indicate an increase in the number of sunny hours. The total number of days with sky overcast in the west of the Caspian Sea, western part of the country, and stations such as Kerman, Sabzevar and Shahrood has decreased significantly..