The effects of climate change on increasing the risk of drought in Tehran using CMIP6 scenarios

Introduction
Global warming threatens human survival in today's scenario; and has become an environmental challenge. The climate data shows the warming trend in many parts of the world, which has led to a wide range of climate effects such as reduced precipitation, drought, and the occurrence of extreme events. The increasing vulnerability of many urban areas, especially in developing countries, has been one of the main concerns of life. Therefore, examining the risk of risks caused by global warming on a national and local scale is a fundamental step to increase the readiness of urban areas to reduce current and future risks caused by climate change. Droughts have effects on vegetation, soil and freshwater quality, and etc., and are a serious ecological problem around the world, its impact on crops and water availability for humans can jeopardize human life. Although drought has always been common, the drought risk has become increasingly prominent because of the climatic warming that has occurred during the past century. Therefore, these effects of are noticeable in all climates, and Iran, having a dry and semi-arid climate, is one of the countries that is always at risk of drought, and this causes great economic damage to the country every year.  in addition to this, the review of the history and climatic texts shows that the importance of the effects of climatic elements (rainfall and temperature) on the drought situation of the region in the coming period is essential. Until now, the study of global warming in terms of increasing the risk of drought using the sixth report and new climate data has not been studied in Tehran province. So this study aims to investigate the effects of climate change on increasing the risk of drought in Tehran province.
 
Materials and Methods
This research, First, the parameters (temperature and precipitation) for the synoptic stations (Abali, Shemiran, Mehrabad) for the period (1988-2020) were received from the Tehran Meteorological Organization. To projection in the future, down-scaling methods (SDSM-DC model) and The Mann-Kendall test were used to investigate the changes in temperature and precipitation. Climatic parameters change in space and time scale for many reasons. that these changes should be checked based on observations and statistical methods. Rainfall is one of those climate parameters that is not normal, and methods such as Menn-Kendall should be used to deal with such conditions; For this purpose, using Macro Excel, the value of age slope and Z statistic was calculated in the period (1988-2020) for the stations (Mehrabad, Shemiran and Abali) to investigate the trend of precipitation and temperature from the past to the present at a confidence level of 95-99% computed. In the following NetCDF data together with CanESM5 predictors from the base period (1979-2014) according to the most recent SSP release scenarios of the IPCC 6th Report were obtained from the Canadian Climate Change website. Among them, only Mehrabad station had a complete basic period compared to other stations, it was chosen as the selected station in Tehran. Drought indices are used as an index to track and quantify drought, this research is SPI index with DIC software has been used to evaluate the drought. in this index Precipitation, which has been one of its main components, whose output and results are more consistent with reality.
 
Results and Discussion
The results using the Mann-Kendall test showed that the examination of temperature changes in the stations (Mehrabad, Shemiran, Abali) was an increasing trend. Rainfall in most months of the year is a stable trend, only jumps are observed which can be justified by the increase in the frequency of rainfall. In the simulation with the CanESM5 model under the SSP5 scenario at Mehrabad station, the highest temperature was assigned to July and the highest rainfall will be in March. in this research, The drought survey showed that Mehrabad station experienced severe drought only during 1989-1993, while the duration of the drought period was longer in Abali and Shemiran stations and both of them faced very severe drought in 2014. in addition, this Investigation of three stations with a common period of 32 years in Tehran shows that Tehran has overcome the drought situation in recent years; But most of the time, the region may be in close to normal climatic conditions. But the risk of dry to very dry conditions will be very close. On the other, the results confirmed that drought depends on both rainfall and temperature.
 
Conclusion
As drought depends on both precipitation and temperature, The more the drought is accompanied by the trend of increasing temperature, decreasing precipitation, and the prediction of climate models, the more likely it is that climate change will occur due to global warming. As a result, the temperature of Tehran will increase. And while the rainfall is decreasing, it will be fluctuating and torrential. so In the future, Tehran's climate will have more fluctuations in rainfall and will be warmer than the current conditions. In addition, rains will occur more randomly but with more intensity. Tehran has had a drought in recent years, but most of the time the region has been in near-normal climatic conditions, but is the risk of dry and very dry conditions. This factor will create environmental challenges in the future. In addition to this, it is necessary to have a plan for climate risk management in the future due to the new climate of Tehran, which is prone to drought.