Long‐Term Variability and Spatial Differentiation of the Frost‐Free Period in Iceland

ABSTRACT Climate change in subarctic regions leads to significant transformations in thermal conditions; however, the long‐term variability of the frost‐free season (FFS)—and consequently, the growing period—remains poorly documented in Iceland. This study systematically addresses this gap by analysing data from 1951 to 2024. The main objective was to determine the spatial differentiation and long‐term trends of FFS. The analysis included the start, end and duration of FFS, as well as the number of days and sums of minimum temperatures below and above 0°C during May–September, based on daily ERA5 reanalysis data (ECMWF/C3S). Trends were calculated using linear regression and their significance assessed with the Mann–Kendall test. Based on six thermal indicators, areas of Iceland characterised by different degrees of change in thermal conditions were identified. Opposing trends in the start (earlier) and end (later) of FFS have led to its systematic lengthening—on average by 5.9 days per decade, with maxima reaching 21.8 days per decade. The highest rates of change were observed in periglacial regions, particularly around Vatnajökull, where the FFS lengthened markedly and frost days (Tmin < 0°C) decreased. Over large glaciers, a distinct type of warming was noted—reflected not in higher Tmin > 0°C, but in rising cumulative Tmin < 0°C values. Spatial classification showed the greatest changes in central Iceland, especially around Vatnajökull, Drangajökull and the Hornstrandir Peninsula, and the smallest in western and coastal regions. The lengthening of the FFS may have significant implications for agriculture, biodiversity and phenology.