Frost days in the beginning of the growing season for Norway

Due to increasing temperatures, the growing season is expanding in Norway, except on glaciers and in high mountain areas. It now starts earlier and ends later in the year, when night–time temperatures may still be low. Therefore, despite a decreasing number of frost days in a generally warmer climate, a longer growing season may lead to an increased risk of frost early or late in the growing season. Frost early in the growing season can pose a risk for several plant species. In Norway, there have been incidents where fruit crops were destroyed after an unusually warm period followed by a cold spell. In this study, we show that frost in the beginning of the growing season is already a risk factor in lowland areas of South Norway, with the highest risk in the southeast. Changes between the periods 1961–1990 and 1991–2020 show the largest increase in the number of days along the west coast of South Norway, which is also where the growing season has increased the most. Future scenarios indicate an even higher risk of frost days in the beginning of the growing season along the coast of Norway, except in the northernmost areas. The increase in risk is highest in the same areas where the growing season is projected to increase the most. In contrast, the risk decreases in many inland areas of South Norway.