Introduction:

Climate change is the long-term changes in the average conditions of the atmosphere, oceans and land surface, such as temperature, precipitation, wind, etc. Climate change can be caused by natural factors, such as changes in the sun's activity, volcanic eruptions, or orbital changes, or human factors, such as greenhouse gas emissions, land use changes, or air pollution. Therefore, the main problem of this research is to examine the combination of temperature and precipitation limit profiles at the level of Razavi Khorasan province.

Materials and methods:

The study area of this paper is Razavi Khorasan province located in the northeast of Iran. Razavi Khorasan province is located between 56°19' to 61°16' east longitude and 33°52' to 37°42' north latitude, from the north to the country of Turkmenistan, from the east to Afghanistan, from the west and north The west is limited to the provinces of North Khorasan, Semnan and Yazd, and the south and southwest are limited to the provinces of South Khorasan and Yazd.

The data used are from three synoptic stations of Qochan, Sabzevar and Kashmer for the period 1990 to 2014 for temperature and precipitation variables.

Coupled models of the Sixth Phase Intercomparison Project (CMIP6):

The output of the sixth phase or CMIP6 models under common socio-economic scenarios (SSPs) along with the representative scenarios of greenhouse gas concentrations on the analysis of feedbacks between climate changes and socio-economic factors such as global population growth, economic development and technological advances in are available In this research, MPI-ESM-HR model scaled with CMhyd model is used. Two intermediate (SSP2-4.5) and pessimistic (SSP5-8.5) scenarios have been used to examine  extreme climate indices of temperature and precipitation for three periods: the near future (2026-2050), the middle future (2025-2075), and the far future (2076-2100).

Validation of temperature and precipitation of CMIP6 models:

In this research, three statistics include root mean square error (RMSE), mean bias error (MBE) and percentage bias (PBIAS)  were used to validate the model results against station data. Positive values indicate overestimation and negative values indicate underestimation of the model. The RMSE method has been used to check the error. A value close to zero in this method indicates the optimality of the model.

Results and discussion:

The results showed that the MPI-ESM-HR model of the CMIP6 model series estimates the temperature more accurately than the precipitation in Razavi Khorasan province. The minimum deviation for temperature in Kashmer station was obtained with a value of 0.33 degrees Celsius and the maximum deviation was obtained with a value of 0.78 degrees Celsius in Sabzevar station. The value of the average temperature bias in Selane scale for Qochan station is also calculated as 0.51 degrees Celsius. The investigation of the bias method shows that the model has an overestimation in temperature estimation in selected stations of Razavi Khorasan province. The value of RMSE also varies between 0.87 in Kashmehr station and 1.20 in Sabzevar station among the investigated stations.

Investigating the trend of climate extreme indices of temperature and precipitation:

The results have shown that the highest number of consecutive dry days (CDD) is seen in Kashmer station with 158 days and the lowest in Qochan station with 67 days per year. In the same way, the highest consecutive wet days (CWD) in Qochan with 7.5 days per year and the lowest consecutive wet days with 5.7 days have been obtained in Kashmer station. The Prcptot index, which shows the amount of precipitation on days with more than 1 mm of precipitation, also has a decreasing trend from north to south in the province. The highest value of this index is seen in Qochan station with 238.9 mm and the lowest value with 138.9 mm in Kashmer station.

The average temperature (TMm) in the three investigated stations was calculated as 12.19, 15.72 and 18.29 degrees Celsius in Qochan, Sabzevar and Kashmer, respectively. For the maximum temperature values of 18.88, 22.18 and 24.74 degrees Celsius have been calculated for these station respectively. Among the selected climate extreme indices, five extreme indices of temperature include TMm, TNm, TNx, TXm and TXx in all three studied stations had a significant increasing trend at the level of 0.05. The result shows that frosty days are affected by the global warming phenomenon in The level of Khorasan Razavi province has recorded a decreasing trend during the last three decades.

projecting the extreme  temperature and precipitation indices under the  SSP2-4.5 scenario, the result showed that under the conditions of future climate change, the share of torrential rains and floods in the stations of the province will increase. The average temperature (TMm) in the province under the SSP2-4.5 scenario will increase by at least 1.39 degrees Celsius in the near future (2026-2050) at Qochan station and 3.04 degrees Celsius in the far future (2076-2100) at Kashmer station. The results related to the extreme climate indices clearly showed that global warming has caused a significant increase in  extreme temperature indices in Razavi Khorasan province. Also, the climate change has caused the rains to become torrential in the future and the intensity of the rains will increase.

Conclusion:

Examining the extreme temperature and precipitation indices under future climate change conditions has shown that temperature extreme indices will increase under both SSP2-4.5 and SSP5-8.5 scenarios. The intensity of the increase under the pessimistic scenario of SSP5-8.5 is higher than the scenario of SSP2-4.5. Also, with the increase of the period, from the near future to the distant future, the abnormality of the selected extreme temperature indices increases. The analysis of extreme precipitation indices has also shown that the intensity of daily precipitation (SDII) and the maximum daily precipitation (RX1day) in all periods and scenarios in the studied stations have increasing anomalies. The increase of  precipitation has also been reported in previous researches, and frost days (FD) will also decrease in the future in all stations and scenarios. In general, the increase in frequency and intesity of extreme temperature and precipitation indices in the future can be a serious threat in the management of water resources and agriculture at the level of Razavi Khorasan Province, which is necessary to adopt the necessary plans in this field.