During the past 40 years or so, the relevant authorities in most countries have established a small number of what are called phenological gardens, dedicated sites at which the growth cycle of plants, trees and shrubs is closely monitored by regular observation.
Careful note is taken of the day on which the first sprouts appear, and of the date in spring on which each plant comes into leaf. The first flowering is observed, the date on which the fruit appears and the first signs of autumn leaves dropping to the ground. Meanwhile, a climatological station nearby allows these events to be related to relevant weather variables like rainfall, temperature and sunshine.
This activity is co-ordinated on a European scale by the International Directorate of Phenological Gardens, based in Germany, where the data from about 60 gardens are collected. Among other things, it is essential that the observed plants of each species, at all gardens in the network, have a common source of origin, so that the possibility of individual hereditary influences can be eliminated.
All specimens of the network, which stretches from the north of Norway down to Macedonia, and from Valentia Observatory's phenological garden in Co Kerry eastwards to Finland, are therefore clones of a single sample of each species, bred in Germany when the first gardens were established.
All the data gathered over the past 40 years has recently been analysed by two German scientists, Annette Menzel and Peter Fabian from the University of Munich. Apparently spring events, like leaf unfolding, have advanced by about six days during the period, whereas autumn events like leaf colouring now occur, on average, 4.8 days later. The conclusion is that the average growing season in Europe has lengthened by 10.8 days since the early 1960s.
These changes can be attributed almost entirely to changes in the average temperature, particularly the winter temperature. Other variables like soil composition, water supply, biological factors and the general surroundings, have as far as possible been kept unchanged.
The results agree with a similar study carried out in the United States some years ago. That study concluded that over three decades the average first leaf date had moved progressively earlier each year, reverting from around May 4th in 1965 to April 23rd by the early 1990s.
Both studies confirm what we already know - that average winter temperatures in both regions have been rising slightly, but significantly, throughout the period concerned. But more importantly, the results establish phenology as an important tool for the monitoring of global warming.