The global warming due to increasing greenhouse gases in atmosphere during recent decades has had significant effects on atmospheric phenomena and biological activities of living organisms on the planet's surface, and this has been the subject of interest by various scientists. The change in the behavior and characteristics of climate variable has shown the effects of global warming on these variables (Stocker et al. 2013). There is observational evidence of long-term changes in weather and climate extremes across the globe with significance dependent on variables, seasons and regions (IPCC 2013).
An extreme event is a rare phenomenon that is statistically located in the upper and lower regions of the statistical distribution, and hence the probability of its occurrence is very low. The extreme weather and climate events include cold and hot waves, floods and droughts occurring under conditions of global warming caused by increase of greenhouse gases, changes in mean climate parameters, and the frequency of extreme meteorological events (Rosenzweig et al. 2001).
The World Meteorological Organization's Climate Commission has introduced 27 precipitation and temperature extreme indices (including 16 temperature extreme indices and 11 precipitation extreme indices) (Peterson 2005; Zhang et al. 2011). Extensive research has been done globally to investigate the effects of global warming on climate extreme indices. The climate models (GCMSs) predict that the hydrological cycle is likely to be intensified and results in occurrence of floods and droughts. On the one hand, winter precipitation is mostly rain and snow zones and spring runoff are reduced and spring and summer droughts are intensified. Also, higher latitudes and heights will have higher temperatures than the global mean temperature, especially in winter and night (the minimum temperature) are expected to increase unevenly (Rosenzweig et al. 2001).
Frich et al. (2002) using 10 temperature and precipitation indices investigated changes in these indices in the second half of the 20th century. The results showed changes in temperature indices, especially increase in summer night heat, reduction in the number of frost days and reduction in maximum annual temperature. In another study, (Yue and Hashino 2003) studied the monthly, seasonal, and annual temperature trends in Japan for the past hundred years. According to the study results, the annual temperature of the 46 stations evaluated by MK test between 1900 and 1996 increased from 0.51 to 2.77 degrees Celsius. Studies conducted in the United States (DeGaetano 1996), Australia, New Zealand (Plummer et al. 1999), China (Zhai et al. 1999), Canada (Bonsal et al. 2001) also showed a reduction in the number of frost days in the upper and middle latitudes of the northern hemisphere and an increase in the length of the growth period compared to the 20th century.
Studies on the effects of climate change on growth period duration include those on the temporal and spatial variability of phonological seasons in Germany from 1951 to 1996 (Menzel 2003), the beginning of spring in China (Schwartz and Chen 2002), changes in the growing season in the last century (Linderholm 2006). In Iran, several studies have been conducted on the effects of global warming on climate parameters, which can be mentioned in the following. Some studies have also examined the accuracy of climate models for projecting the future of climatic elements and extreme indices, including (Khan et al. 2006), (Chen et al. 2013) and (Roshan et al. 2013). Many studies have been conducted on the trend and prediction of climatic elements and extreme indices in Iran. The study results of Nasiri Mahali et al. (2006) show that due to delay in date of occurrence of first autumn frost and early last spring frost date in Iran, growth season duration in all studied stations increased 5–23 days and 16–42 days for 2025 and 2050, respectively. In a similar study, Esmaili et al. (2011) evaluated the changes on growth period duration and spring and autumn frost data caused by climate changes in Razavi Khorasan province in Iran. Others conducted on the trend and projection of climatic variables and extreme indices in Iran ((Jahanbakhsh Asl and Torabi 2004), trend of extreme temperature and precipitation indices in Tehran (Mohammadi and Tagavi 2007), trend Analysis of extreme precipitation Indices in Iran (Asgari et al., 2007), trend of Climate extreme Indices over Iran during 1951–2003 (Rahimizadeh et al., 2009), study of the climate change impacts on agricultural products and agro-climatic variables in Razavi Khorasan (Babaeian and Kouhi 2012), the study of impact of climate change on the probabilistic characterizations of drought events in western stations of Iran (Fattahi et al. 2015), assessing the impact of climate change on water resource using the output of Canadian Global Coupled Model (CGCM 3.1) under A1B, B1 and A2 scenarios (Abbaspour et al. 2009), study the future of extreme precipitation and temperature in Iran (Vaghefi et al. 2019), Comparison of LARS-WG and RegCM4 models’ performance for simulation and post processing of Khorasan temperature and precipitation data (Ahmadi et al. 2016).
In the present study, in addition to investigating the trend of temperature extreme events (SU35, TR20, and DTR) in Razavi Khorasan province, Iran, the future prospects of these extreme events are also provided using downscaled outputs of three general atmospheric circulation models (HadCM3, CNCM3, and NCCCSM) under A1B and A2 scenarios using LARS-WG model for the two forthcoming periods 2011–2030 and 2046–2065.