False spring events in the eastern part of the Baltic Sea region

Abstract Spring frosts can cause substantial economic and ecological damage, especially when occurring after the growing season has begun and leading to compound climate events, known as false spring (FS) events. This study examines the recurrence, spatial distribution and intensity of FS events in the eastern part of the Baltic Sea region from 1950 to 2022. These events were identified by determining the dates of the last spring frost (LSF) and the start of the growing season (SGS) for each grid cell annually. FS events were distinguished when the LSF date occurred later than the date of the SGS. The sum of growing degree days (tbase = 5°C), accumulated until the event, and the minimum air temperature (tmin) during the day of the FS event were used to assess the intensity of FS events. Throughout the study period, LSF and SGS dates shifted earlier by an average of 6.3 and 10.6 days, respectively. In 85.9% of the grid cells, the SGS date changed more rapidly than the LSF date. Although the number of FS events increased over the 73 years in most of the study area, the changes were statistically insignificant according to the Mann–Kendal test. However, comparing 1950–1969 and 2003–2022, 85.9% of grid cells saw more FS events, with the largest increase in the eastern part of the study area and southwestern Lithuania. Significant changes were determined in 30% of cells according to the t-test.