Since the end of the 1980s stong changes in air temperature have been observed in Europe. Mainly the temperatures in winter and in the early spring, which are decisive for the plant development in spring, changed distinctly. Most of the last years were warmer than the long-term averages.

These observed changes in temperature correspond well to changes in the circulation pattern over Europe. The increased frequency of positive phases in the North Atlantic Oscillation (NAO) index since 1989 led to milder temperatures in winter and in the early spring, because of the prevailing westerly winds from the Atlantic Ocean during this time of the year.

This led to clear changes in the timing of live cycle events. For instance the growing season in Europe, here defined as the average time between leafing and leaf fall of selected trees, has extended by 11 days in the last 30 years, mainly because leaf infolding begins earlier by about 2.7 days per decade in spring. Events such as the blossoming of fruit trees in Germany (apple, cherry, etc.) were affected by the higher temperatures in the end of winter and in the early spring. An increase of average air temperature between February and April by 1 degree Celsius lead to an advanced leaf unfolding or blossom of trees in Europe and Germany by about one week.