Evaluation of temporal trends of growing degree days index for major crops in Türkiye

Climate change, previously considered an environmental problem, has begun to be defined as a climate crisis since its effects intensify faster than previously thought, cause severe economic losses, and threaten food security. Climate indexes have been defined as the primary descriptive indicator to acquire a consistent perspective on observed and projected changes resulting from weather and climatic extremes. In this study, the growing degree days (GDD) index, one of the essential indicators in planning agricultural activities, was examined for wheat ( Triticum aestivum ), maize ( Zea mays ), and sunflowers ( Helianthus anuus ) across Türkiye. The GDD index was examined between 1971 and 2020 for the observed change analysis and between 2023 and 2098 for future projections. Three global climate models and two representative concentration pathways (RCPs), that is, 4.5 and 8.5, were selected to assess climate change impact. The analysis of the entire observed period (1971–2020) indicated a considerable increasing trend in the annual total GDD index for wheat, except in the southeast of Türkiye, and maize and sunflower across the country. Assessment results of the climate projection analysis revealed that there would be increases over 1400 and 2300°C day/year for the RCP4.5 and RCP8.5 scenarios, respectively, for all three crops considered. Additionally, the findings indicated further increases in the southern latitudes of Türkiye. Projection results also confirmed that there will be longer suitable periods for agricultural production, and the crop vegetation periods will be shorter and shift earlier in Türkiyein the future periods.