Trends and changes in air temperature and precipitation over different regions of Iran

Observations unequivocally show that climate change is happening in most regions of the globe. Warming which has been observed in most regions of the globe, particularly in recent decades, is the best manifestation of the climate change. In contrast to the warming of the most regions of the world, many places across the globe have experienced different changes in the amount and intensity of precipitation, such that under the global warming both increases and decreases of precipitation have been reported. Using meteorological records of fifteen ground stations across Iran for a 63-yr period from 1951 to 2013, trends of the minimum, maximum and daily mean near-surface air temperatures and annual accumulated precipitation are examined. Results indicated that the annual minimum, maximum and daily mean near-surface air temperatures in most regions of Iran have experienced increasing trends. Thus, Iran, like most regions of the world, has been rapidly warming over the past few decades. The observed increasing trend in air temperature is mostly attributed to the increase of the greenhouse gases due to human activities. In most regions of Iran, the increasing trends of the minimum temperature have been greater than those of the maximum temperature, the feature which has been mostly attributed to the urbanization development. Indeed, through blocking the outgoing longwave radiation, the urbanization development has effectively contributed to the more significant increase of the minimum temperature than the maximum. In addition, the urban air pollution decreases the incoming shortwave radiation reaching to the Earth surface; thereby partly contributes to the less increase of the maximum temperature compared to the minimum. As a result, a decreasing trend in the diurnal temperature range (the difference between the daytime maximum and nighttime minimum temperatures) is identified. Temperatures in most regions of Iran have experienced a changing point either in 1980s or 1990s, such that the mean temperature of the all regions during the period after the changing point was approximately 1.2 ˚C greater than the mean temperature during the period before the changing point. Under the warming, most regions of Iran have experienced decreasing trends in the annual accumulated precipitation, although most of the trends have not been statistically significant. The decrease of precipitation, and the increase of air temperature imply that Iran has become drier and more vulnerable to drought over the past few decades. The observed decreasing trend in precipitation over Iran is in contrast to the trend of global mean precipitation, for which the increase of precipitation under the global warming has been noted. Indeed, previous studies have indicated that 1K rise in temperature is associated with 2 percent increase in the global mean precipitation. However, the results of the present study are consistent with previous studies conducted over the subtropical regions. In a warmer climate, saturation of the atmosphere takes alonger time, which delays the onset of precipitation. Thus, in the arid and semi-arid regions of Iran with the dominant subtropical climate, more water vapour can be transported to higher latitudes by the general circulation of the atmosphere before precipitation can form. In contrast, previous studies have indicated that precipitation increases in both subpolar and tropical regions. We, therefore, argue that depending on the geographical location, the intensity and frequency of precipitation vary in response to the warming of the climate.