Thermal seasons in northern Europe in projected future climate

Global warming acts to prolong thermal summers and shorten winters. In this work, future changes in the lengths and timing of four thermal seasons in northern Europe, with threshold temperatures 0 and 10 degrees C, are derived from bias-adjusted output data from 23 CMIP5 global climate models. Three future periods and two Representative Concentration Pathway (RCP) scenarios are discussed. The focus is on the period 2040-2069 under RCP4.5, which approximately corresponds to a 2 degrees C global warming relative to the preindustrial era. By the period 2040-2069, the average length of the thermal summer increases by nearly 30 days relative to 1971-2000, and the thermal winter shortens by 30-60 days. The timing of the thermal springs advances while autumns delay. Within the model ensemble, there is a high linear correlation between the modelled annual-mean temperature increase and shifts in the thermal seasons. Thermal summers lengthen by about 10 days and winters shorten by 10-24 days per 1 degrees C of local warming. In the mid-21st century, about two-thirds of all summers (winters) are projected to be very long (very short) according to the baseline-period standards, with an anomaly greater than 20 days relative to the late-20th century temporal mean. The proportion of years without a thermal winter increases remarkably in the Baltic countries and southern Scandinavian peninsula. Implications of the changing thermal seasons on nature and human society are discussed in a literature review.