Trends of growing degree-days in Turkey

Variations in the surface air temperatures show an association with the atmospheric CO₂ under the hypothesis that warmer temperatures promote increases in plant growth in mid-northern latitudes. Historical daily temperature records at seventy-four weather stations (around 60-yr) in Turkey are used in order to investigate this hypothesis. Separate calculations and analyses are undertaken for monthly and seasonal growing degree-days (GDD). The sequential version of Mann-Kendall rank statistic is applied to demonstrate existence of any possible non-linear trends in accumulated GDDs above 5 °C over the months and seasons. Spatially coherent and statistically significant trends of GDD are shown in some parts of Turkey. The statistically significant trends are negative mostly in summer and autumn seasons over the coastal areas of Turkey. These results strongly agree with the previous studies where weak cooling trends are observed in maximum temperatures over Turkey. They suggest the significant decreases in GDD may lead to long-term decreases in vegetation growth mostly over the coastal regions of Turkey.