EVERY year when the temperature begins to fall with the approach of winter, deciduous trees begin to lose their chlorophyll. The colour green that was formerly predominant very quickly disappears, and other pigments, like oranges and reds and browns, are left for a brief but glorious moment to reign supreme until the weakened foliage is swept away by autumn's winds.
A late spring or a summer drought, by slowing growth, may delay the onset of this autumnal wave for several weeks; a moist, mild spring, on the other hand, combined with vigorous growth throughout the summer months, can often promote an early fall of leaves. Either way, once autumn starts, its progress can be monitored by satellite. Daily scans of the northern hemisphere show a "brown wave" of colour moving south, as the foliage at each successive latitude changes from green, through yellow and red, to the familiar brown that marks the dying year. The pastel wave sweeps southwards over the middle latitudes at an average rate approaching 40 miles per day.
There is a corresponding "green wave" in the spring, which moves northwards just around this time of year. On the Atlantic coast of Europe it travels north at about the same speed as its autumnal counterpart moves south, sweeping from Portugal to Norway at about 40 miles per day; from Greece to Finland the northerly progress is much slower, about 20 miles per day, and in all cases there is a three or four day delay for every 300 ft of altitude.
The green wave is more difficult to spot from satellites, but it is, on the other hand, a better guide to what is happening to the local climate - particularly when the satellite images can be calibrated by regular observations on the ground of the average budding times of the different kinds of plants.
The study of the "green wave has produced some interesting findings. It has been discovered, for example, that over the last 30 years or so, the average "first leaf date" in the United States has been advancing progressively earlier each year - reverting, on average, from around May 4th in 1965 to April 23rd by the early 1990s. The change confirms what we already know - that average temperatures in the region have been rising slightly but significantly throughout these years. More importantly, however, the "green wave" as observed by satellites will prove to be a useful tool for monitoring global warming in the future.