Twentieth-century trends in the thermal growing season in the Greater Baltic Area

Phenological data have shown an increase of ca. 10 days in European growing season length in the latter part of the twentieth century. In general, these changes have been associated with global warming. Here we present a study of thermal growing season (GS) trends in the Greater Baltic Area, northern Europe. Yearly dates for the start, end and length of the GS were computed for 49 stations in the studied area, using daily mean temperature measurements. Trends and tendencies of the GS parameters were analysed within the twentieth century. We also examined GS trends in long records (starting before 1850) from the region. The results show a general increase of the length of the GS of ca one week since 1951 in the area, where the most considerable change has occurred in spring (starting similar to 6 days earlier). The largest increases were found at stations adjacent to the Baltic Sea and North Sea, where some Danish stations showed significant increasing trends in the length of the GS of more than 20 days. The only tendency for a shorter GS was found in Archangelsk, north western Russia. The three longest records displayed large inter-annual and decadal variability, with tendencies for increased frequencies of longer growing seasons since the 1950s.