The amount of ozone overhead at any one place varies considerably, and includes a regular seasonal and annual pattern; these variations generally follow a normal statistical pattern. In this respect the Antarctic ozone layer behaved normally for the first 20 years of BAS measurements, thereafter clear deviations were observed. In every successive spring the ozone layer was weaker than before, until by 1984 it was clear that the Antarctic stratosphere was undergoing a progressive change.
This phenomenon is the result of emissions, mainly in the northern hemisphere, of chlorofluorocarbons (CFCs) and halons. These gases remain in widespread use in refrigeration, air conditioning, insulation, industrial solvents, and fire control. If the provisions of the Montreal Protocol on Substances that Deplete the Ozone Layer of 1987 are revised and strengthened, and followed, there is a reasonable prospect that the Antarctic ozone hole will eventually be repaired, but not before the next appearance of Halley's comet. In the meantime it promises to return every year.
It is important to recall that ozone depletion in the stratosphere was completely unpredicted, and demonstrates amply the power of measurement over theory. The detailed explanation is still the subject of considerable study, but the basic mechanism is clear. Ozone is destroyed in the Antarctic spring by chlorine formed during the sunless winter. The chlorine is generated by an unusual reaction between stable molecules, on the surface of small stratospheric cloud particles which can only form in the intense cold of the polar winter. The stable molecules obtain their chlorine from CFCs which have previously been broken up in sunlit regions. The same conditions apply in the Arctic, but the less intense atmospheric circulation has so far prevented the effect from becoming as severe.
Two tasks have emerged: to discover what processes might give rise to a catastrophic ozone depletion over the middle latitudes, where changes have been relatively slow; and to determine the amount and the biological effects of the increased ultraviolet flux which we may expect to find in the future under a thinner ozone layer. It may then be possible to identify the risks which merit precautionary measures.