Unexpected ray of hope for the ozone layer
The ozone hole has come and gone again. Its annual wax and wane has been a regular feature of the high Antarctic atmosphere for 20 years or more.
It is facilitated by very low temperatures over the South Pole; the stratospheric presence of chlorofluorocarbons (CFCs); and the development during the southern spring of a circular stream of strong winds that prevents the polar air from mixing with the ozone-rich atmosphere outside its ambit.
The result is a disc-like area above the South Pole with ozone concentrations well below the normal values - the so-called "ozone hole".
This year a high rate of ozone depletion was expected. Temperatures high over Antarctica were significantly lower than usual in July and August, which would facilitate the destructive action of the manmade catalysts.
Indeed, these abnormally low temperatures were probably themselves a consequence of a periodic lack of ozone; solar radiation that would otherwise be absorbed by ozone in the stratosphere, raising the temperature in those regions, now often passes more readily downwards towards the surface of the Earth to contribute to the other great meteorological problem of our age - global warming.
By the end of September 2000, the Antarctic ozone hole had become the "largest" and "deepest" hole on record - meaning that there was less ozone in the upper atmosphere over a larger area of the South Pole than has been the case in any previous year since observations of the phenomenon began.
Over an area of 20 million square kilometres, 2 1/2 times the size of Europe from the Atlantic to the Ural Mountains, stratospheric ozone amounts were less than 50 per cent of normal values.
But then a ray of hope - the ozone hole began to shrink much earlier than usual, and by late October it had become for that time the smallest ozone hole of the last decade. By the second half of November it was a tiny fraction of its former self, and it has now completely disappeared. This is in stark contrast to 1999, when the hole persisted strongly until late November.
The hope is, of course, that this early disappearance of the ozone hole may have something to do with the Montreal Protocol.
Complying with this international agreement, industrialised countries have almost phased out their use of CFCs, and developing nations have begun a freeze on their production.
The aim is to introduce ozone-friendly practice on a global scale sufficient to achieve by 2050 the restoration of the ozone layer to a state of health comparable to that which it enjoyed around 1975.
And perhaps it is already happening.