Climate change stand as the most important challenge in the future. Horticulture is one of the most sensitive and vulnerable sectors to the climate change. Climate change and global warming will endanger the production of agricultural products and food security. Because of required longer time to fruit production, fruit trees are heavily susceptible to damage from climate change. The purpose of this study was to investigate the impacts of climate change on thermal accumulation pattern in Apple tree cultivation regions of Iran based on the outputs of new CMIP5 models and radiative forcing (RCP) scenarios.

The present study was carried out using a statistical-analytical method. In this study, two types of data was used; baseline data for past period and model output simulation data for the future period. Observation data for baseline period for 53 weather station was extracted from the Iran meteorological organization (IMO). Afterwards, the data for the upcoming period up to the 2090 horizon were processed using the HadGEM2-ES model from the series of CMIP5 models of the MarksimGCM database based on the radiative forcing scenarios RCP8.5 and RCP4.5. The future period will be refined in the mid-term (2020-2055) and the far future (2056-2090). Afterwards, based on the thermal thresholds, thermal accumulation in Apple tree cultivation areas in Iran processed.

The results showed that based on statistical indices on the output of CMIP5 models, the output of the HadGEM2.ES general circulation model is accompanied by fewer simulation errors in illustrating the climate change of the future period than the observation or baseline period. In fact, based on the evaluation criteria or error measures, this model shows a higher compliance with observational data. In general, the model has a lower accuracy than precipitation in the simulation of rainfall, which is due to the complexity of the precipitation process as well as the structure of the climatic models. One of the fundamental issues that have emerged in recent decades is the change in the potential status or heat accumulation of different regions due to the increase in air temperature. The results showed that due to temperature increase, in the mid and far future heat accumulation will increase compared to the baseline period in Apple tree cultivation areas. Increasing of heat accumulation will reduce the length of the Apple tree growth period, and in fact the Apple tree will complete its vegetative and reproductive cycles sooner. This condition will have negative effects on the quality, taste and color of the Apple varieties. For example, according to the RCP8.5 scenario in the physiological threshold of the apple tree 4.5 C^° , in the mid term (2020-2055) and far future (2056-2090) will be 1132 and 2171 active degree days respectively compared baseline period. These conditions equivalent to the 51% and 42% respectively. Based on the RCP4.5 scenario, these conditions will be 390 and 680 active degree day, equivalent to 9.3% and 15.1%, respectively, compared to the baseline period.

The results showed that the heat accumulation in Apple tree cultivation areas in the future period will increase compared to the baseline period. One of the most important effects of climate change on the Apple tree cultivation will be due to increased heat accumulation in the upcoming period. Increasing the heat accumulation will reduce the length of fruit tree growth period, and in fact the fruit tree will complete its vegetative and reproductive cycles earlier. According to these conditions, the areas of Apple tree cultivation in the future will be extended to higher regions. These conditions are important for cold regios fruit tree such as Apple tree, in facr increase in heat accumulation will reduce the length of the growing season and, as a result, reduce the quality and yield of the fruit. Based on the spatial distribution, the least heat accumulation in the highlands, especially Northwest and central Alborz, will occured. In natural landscapes of low elevations, valleys and plains in the Northeast, central Southern part of the Zagros and around Lake Urmia, higher heat accumulation will occured in the future. Therefore, one of the effects of climate change on fruit trees will be due to increased heat accumulation in the upcoming period. Increasing the potential or heat accumulation will reduce the growth period of the fruit trees, in fact, the fruit trees will complete their vegetative and reproductive cycles sooner.