TODAY, although you may not know it, is International Day for the Protection of the Ozone Layer. Its objective is to focus popular and media attention on the trials and tribulations of the tenuous veil of ozone that protects our planet from the worst excesses of the ultraviolet radiation streaming from the sun.
September 16th is chosen because it was on that date in 1987 that the Montreal Protocol was opened for signing, introducing internationally agreed measures to limit the production of the CFC gases that destroy the stratospheric ozone.
Perceptions of the ozone problem nowadays might be summed up by that little rhyme on the diversity of human temperament:
Twixt the optimist and pes simist,
The difference is droll
The optimist sees the doughnut.
But the pessimist sees the hole.
The hole in this context is the "ozone hole", an evocative name applied to the dramatic decrease in ozone concentrations that now takes place every southern spring over Antarctica, and whose intensity every year is seen as a barometer of ill health of the ozone layer.
The decrease becomes apparent in mid August, Just as the sun peeps over the horizon after the long polar night, and when temperatures of the South Pole are at the very lowest. It is brought about by the interaction of sunlight with the by products of CFCs at very low temperatures, and is exacerbated by the circumpolar vortex - a band of strong winds in a circle around the pole, acting as a barrier to isolate the depleted atmosphere in that region from "normal" air at higher latitudes.
This year the ozone decline started as usual in August. So far, the severity of the depletion, and the size of the affected area, are similar to those of recent years. In the first two weeks of this month the hole covered most of the Antarctic continent and adjacent ocean areas, and for a day or two it reached the southern tip of South America.
Ozone levels in parts of the affected region were less than 50 per cent of normal values.
The doughnut watchers, however, point to the Montreal Protocol which has now been ratified by an impressive 157 countries. Its provisions have already resulted in a slowing down in the rate of growth of atmospheric concentrations of CFCs and related substances.
Although it will take about a decade for the effects to be seen in terms of higher ozone readings, it is seen as a major success story, and a prototype for effective international action in dealing with global threats to the environment.