Projections for the duration and degree days of the thermal growing season in Europe derived from CMIP5 model output

Global warming leads to a prolongation and intensification of the thermal growing season. In this study, we present projections for the growing season length and growing degree day sum (GDD) in Europe by the end of the 21st century using two threshold temperatures, 5 and 10 degrees C. The analysis was based on simulations performed with 22-23 CMIP5 global models under the RCP4.5 and RCP8.5 scenarios. Systematic errors in the temporal mean and variability of modelled temperatures were eliminated, and the data were downscaled spatially by employing a bias-correction method. To determine the onset, termination and GDD of the growing season, two methods have been used. The previously developed Fourier method is suited for exploring long-term means, while the novel temperature deviation integral method is applicable to inter-annual variations. According to the multi-model mean of the RCP8.5 simulations in the late 21st century, for the majority of Europe the growing season is prolonged by 1.5-2 months, the GDD above 5 degrees C increasing by 60-100%. Responses to RCP4.5 are qualitatively similar but smaller. A decomposition of the uncertainty variance reveals that in the near-term future the contribution of internal variability is pronounced, but by the end of the century inter-model differences dominate. In studying growing-season conditions on an annual basis, we found that in coming decades years with a GDD below the recent past (1971-2000) mean become very uncommon. In the majority of years, GDD will exceed the 10-year or even the 20- or 50-year return level derived from recent past data.