Extreme air temperatures which are of significance in plant growth are influenced by climate change. The aim of this study was to assess the climate change effects on temperature extreme indices (TEIs) in northeastern Iran based on the CMIP6 projections. For this purpose, five extreme indices including maximum of maximum temperature (TXx), minimum of minimum temperature (TNn), cold nights (TN10p), warm nights (TN90p), and summer days (SU25) which are effective on plant growth were evaluated. The projections of the three Earth system models including ACCESS-CM2, MIROC6, and MRI-ESM2-0 were assessed under the three scenarios of SSP1-2.6, SSP2-4.5, and SSP5-8.5 for the period 2026–2050 compared to the historical period (1989–2014). The models’ projections were evaluated by statistical tests on the changes in the average and trend of data. Results showed that the MRI-ESM2-0 model revealed the best efficiency compared to the other models. The projections of all models and scenarios indicated a significant increase in the average of TXx and SU25 indices over the study area at the confidence level of 95% by 1.6 to 2.4 °C, as well as 15 to 18 days under the three scenarios. Also, the results exhibited a significant increasing trend in TN90p and a decreasing one in TN10p overall province at confidence level of 95%. These changes will lead to an increase in evapotranspiration of the plants and an increase in agricultural water demand, and as a result, a decrease in the yield of some main products like wheat and saffron which are the main products in the study area.

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