Study of Time Trend Changes in Annual Mean Temperature of Iran

A number of researches have indicated a gradual increase in mean temperature throughout the world. Yet, there are some reports on the reduction of annual mean temperature. In this research we investigated a long-term trend of annual mean temperature in 34 synoptic stations in Iran (2 stations in cold and humid climates 14 in humid and moderate climates, 11 in Steppe climate and 7 in desert climate - following Kopen climatic division) with a minimum statistical record of 30 years by applying the minimum square-error and Man-Kendall methods (Wald-Wolfowitz method had a different result). The results confirmed a positive trend in 59% stations, while 41 % of the stations were negative for the whole time-horizon of data. Considering the significance level, 3 zones of positive, negative, and no trends for annual mean temperature were detected in Iran. However, it was hard to define a specific spatial theme for such a division. By taking another approach, we proceeded with a shared statistical time period of 1968-1998 for all stations. In this case, 68% of the stations showed a positive trend, while the remaining 32% was negative. There were some shifts in direction from one trend to another in some of the stations in the study, yet no well-defined spatial structure was reported. In contrast, at 5% level of significance, 44, 15 and 41 percentages of the stations demonstrated positive, negative, and no trend for annual mean temperature, respectively. In general the behavior was different for different climates and no specific pattern was found. So, despite the fact that some stations did not show significant trends, one may hypothesize that more regions in future will experience higher temperature values and their positive trends would be a clue for future warming.