K. Jylhä – Geophysica

Vol. 58, No. 1, 2023

Abstract

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 centralFinland, a heatwave with an annual probability of 12 % to 13 % at the 1.5°C warming level is projected tosubstantially 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

Climate scenarios for Finland were updated to correspond the Shared Socioeconomic Pathway (SSP) greenhouse gas (GHG) scenarios, considering nearly 30 global climate models (GCMs) that participated in Phase 6 of the Coupled Model Intercomparison Project (CMIP6). The SSPs and CMIP6 GCMs had also been used by the Intergovernmental Panel on Climate Change (IPCC) in composing the Sixth Assessment Report, published in 2021. Projections are provided for three future 30-year periods and all four SSP scenarios from which enough GCM data were available. The signs and geographical patterns of the projected changes are mostly similar to those derived from the previous model generation, the amplitude of the changes depending on the GHG scenario. For example, by the period 2040–2069 under SSP2-4.5, mean temperatures are projected to increase in Finland by 2.4 (1.0–3.8) ◦C in summer and 3.3 (1.2–5.4) ◦C in winter (the multi-model mean change relative to 1981–2010 and, in parentheses, the 90 % uncertainty interval reflecting inter-model differences). Compared to the projections calculated from the previous model generations using comparable GHG scenarios (RCP4.5 or SRES B1), warming is fairly similar in winter but stronger in summer. The diurnal range of temperature is projected to be reduced in the cold season. Precipitation increases by 5 (−6 to 17) % in summer and 12 (0 to 24) % in winter, in good concordance with the previous projections. In summer, the sign of change is fairly uncertain, particularly in southern Finland. Estimates for seasonal incident solar radiation change have become brighter by 1–4 percentage points. For air pressure and wind speed, the multi-model mean changes are close to zero butinter-model spread is sizeable.