This means it’s normally one of the most pleasant months of the year outdoors.
The weather’s generally good and there’s lots happening. The last of our deciduous trees are breaking bud, the last of the summer migrant birds have arrived, there are butterflies on the wing and the weeds in the garden are mustering for a great offensive. It’s also a busy month for phenologists.
Phenology is the science of recording the dates on which certain annual events in the world of nature occur over as long a time span as possible and using this data to come to conclusions about such things as climate change.
The person who writes to the letters’ page of the paper to say they’ve heard the first cuckoo or seen the first swallow of the year is a phenologist, though usually an amateur one. Migrant birds, deciduous trees and the first appearance of butterflies, and some other winged insects, are favourite subjects for phenology.
There are precise records for the dates of harvest of the pinot noir grape in France that go back, without interruption, for over 500 years. This is far longer than meteorological records, or even thermometers, so the records are valuable. The wine-maker’s records can be checked against the scientific records for later centuries and turn out to be very accurate.
One of the problems with temperature recording over long periods of time is micro-climate. Most weather stations are now in built-up areas, though when they were first established decades or even centuries ago they might have been in suburban or even rural sites. If they show a gradual increase in temperature is this recording climate change or is it recording the build-up of heat leaking from pipes, lights and engines?
Similar problems can affect phenology. In central London trees break bud about three days earlier and lose their leaves about three days later than in the surrounding countryside. It has also been noted that trees will keep their leaves for several days longer in the autumn on a branch that’s close to a street light.
There are some other problems too. Temperature is not the only factor that controls plant activity. Day length and soil moisture content play a part as well. And when temperature is crucial it’s often soil temperature at a certain key depth, and this is seldom the same as air temperature.
Some phenologists specialise in the aquatic or marine branch of the science and for them, of course, water temperature is the important thing, and this also is seldom the same as air temperature.
But solving problems like these is what makes the science interesting.
Another attraction is that it’s a game that amateurs can play on an equal footing with the professionals and the academics.
The data provided by the ‘Greenwave’ scheme in our National Schools is scientifically valuable. In fact the science was invented by amateurs. Although people began recording the timing of events in nature in very ancient times – it’s really the reason for Newgrange – modern phenology starts in 1736. A wealthy English landowner called Robert Marsham with an estate at a place called Stratton Strawless in Norfolk meticulously recorded ‘indications of spring’ in that year and his descendants remained on the estate and continued the records, with one short break, until the death of Mary Marsham in 1958.
The results of this, and other records, will not please climate change deniers. In England the leafing of oak trees appears to have advanced by eight days over the past 250 years, corresponding to a rise in temperature of 1.5 degrees Celsius. There is evidence that the rate of advance has accelerated since 1960.