Projected changes in thermal seasons and the growing season in Finland

Abstract The durations of the thermal seasons and the growing season till the end of this century are inferred from projected monthly mean temperatures, separately for the SRES A2 and B1 scenarios. For the baseline period 1971‐2000, we use a high‐resolution observational data set covering Finland, and an average of the temperature responses simulated by 19 global climate models (GCMs) is added to the observed temperatures to obtain projections for the future. Daily climatological temperatures, needed for the determination of the onset and end dates of the seasons and the effective temperature sum, are derived from the monthly means employing a Fourier algorithm that can reproduce monthly mean temperatures perfectly. Under baseline conditions, there are four thermal seasons everywhere in Finland apart from the elevated area in north‐western Lapland. Under the A2 scenario, thermal winter will disappear in the South‐western part of the country by the period 2070‐2099. Elsewhere winter shortens by 2‐4 months. Summer lengthens by slightly over 1 month. Intermediate seasons become longer everywhere except in northernmost Lapland. The thermal growing season lengthens in inland areas by 40‐50 days, on the south‐western coast even more. The effective temperature sum doubles in the north and increases 1.5‐fold in the south. Conditions in Lapland would thus resemble those currently prevailing in southern Finland. Under the B1 scenario the change is smaller, especially in the second half of the century. The robustness of the findings was assessed by considering the differences between the temperature change projections of the various models. The uncertainty in the onset and termination dates was typically of the order of ±2 weeks. Regional downscaling based on regional climate model (RCM) data did not alter the main conclusions. Copyright © 2010 Royal Meteorological Society