Trends in the length of the thermal growing season in the north-eastern Poland

In the context of climate change and global warming, a growing number of studies reported lengthening of the growing season for most of the Northern Hemisphere, which is associated with increase of air temperature. Evidences of the increase of the growing season length were noted through satellite data and phonological and meteorological observations. This article presents the studies on the temporal and spatial tendencies in the thermal growing season (TGS) length in north−eastern Poland in 1951−2015. The analyses were conducted using long−term and homogeneity daily data sets from 13 meteorological stations (fig. 1). The dates of beginning and end of the TGS were determined by Huculak−Makowiec method. Trends in the TGS length as well as start and end dates were analysed for the whole study period as well as for successive 30−years periods: 1951−1980, 1961−1990, 1971−2000 and 1981−2010. The results show a general increase of the length of the TGS in the north−eastern Poland in 1951−2015 period, however, there are spatial and temporal differences in TGS evolution. In the western and central part of the region, the growing season in the past 65 years became ca. 20 days longer, while in the eastern part the lengthening of TGS was weaker and amounted to 7−10 days. Most of this change is due to earlier onset of the TGS in spring (average for the western part =2.1 days/10 years, while for the eastern one – 1.2 days/10 years). The observed trends and tendencies show spatial and temporal variability in TGS evolution. Trends in parameters of TGS, calculated for 30−years period, varied directionally and spatially with no significant trends before 1980. In the period 1981−2010 the growing season was longer than in the previous years, the largest increases were found in the central part of the region (1.2 days/year), while the smallest in the south−eastern part (0.5 day/year). Since the 1980s the frequency of long growing seasons increased. A particularly large and steady increase of TGS occurred over the first 15 years of the 21st century. Length as well as start and end dates of the TGS displayed large inter−annual and decadal variability with tendencies for increased range of variability