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Even in the cool climate of Finland, severe heatwaves occur sporadically, having multiple implications on public health, forestry, fishery, agriculture, and reindeer husbandry, for instance. This study assesses the occurrence and severity of ≥ 3 -day heatwaves in Finland at the 0.5 °C, 1.0 °C, 1.5 °C, and 2.0 °C global warming levels above pre-industrial conditions, utilising bias-corrected daily-mean temperature data from 60 runs performed with 25 global climate models. The severity of a heatwave is measured by the heatwave extremity index, consisting of the sum of exceedances above a fixed threshold of daily mean temperature. Three alternative threshold temperatures, 20 °C, 24 °C and 28 °C, are considered. A shift from the 0.5 °C to 2.0 °C global warming level is projected to result in an increase in the mean annual number of heatwave days above 20 °C from 1 to 5 in central Lapland and from 5 to 20 in south-eastern Finland. Concurrently, the annual sum of the extremity index becomes 4 to 10 -fold. The higher the threshold temperature, the larger is the growth in relative terms. At the 2.0 °C global warming level, heatwaves above 20 °C are experienced in southern Finland nearly every year and in the majority of northern Lapland approximately every second year. Apart from Lapland, heatwaves occurring once in 10 (100) years at the 0.5 °C warming level will then have annual probabilities of 50% (> 10%). Even between the 1.5 °C and 2.0 °C global warming levels, projected changes in heatwave characteristics are substantial, especially for the most severe heatwaves. For example, in southern and central Finland, a heatwave with an annual probability of 12% to 13% at the 1.5 °C warming level is projected to substantially increase in likelihood under the 2.0 °C warming level, up to 19% to 21%. The paper includes a literature review on potential impacts of the intensifying heatwaves. [ABSTRACT FROM AUTHOR]

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°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°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°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. [ABSTRACT FROM AUTHOR]

ABSTRACT 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 °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 °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. [ABSTRACT FROM AUTHOR]