The ozone layer Ozone depletion The effects of ozone depletion What can be done?

	The Earth is surrounded by a thin layer of ozone O3(g), in the upper atmosphere. It is not very dense and the ozone molecules only make up a small fraction of the gas in the layer. Even so, this thin layer of ozone screens us from 99% of the ultraviolet light from the Sun.
	Ozone is continually being formed and destroyed by the incoming ultraviolet light, cycling between ozone O3(g), normal oxygen gas O2(g), and oxygen atoms O(g). This cycle keeps the amount of ozone in a delicate balance, naturally changing with the seasons.

Ozone depletion
Ozone depletion occurs predominantly in the polar regions and mainly over the South Pole, not over the highly populated regions where the chlorofluorocarbons CFCs, came from! Depletion occurs in spring and early summer. This depletion over the polar ice caps in spring and summer is the result of a combination of factors:

By the time the CFCs have reached the upper atmosphere they have been fairly well spread throughout the whole atmosphere by winds and weather systems.
During the winter, the atmosphere above Antarctica becomes very cold and still during the long nights. It becomes so cold that small ice particles of nitric acid and water form thin ice clouds in the upper atmosphere in the same region as the ozone layer. (This happens to a far lesser extent over the North Pole, because the North Pole is over water and does not get as cold.)
CFCs are absorbed onto these ice crystals. Over the whole of the winter, the ice cloud acts as a "great attractor" for CFCs, absorbing and removing all the CFCs that come in contact with the ice clouds.
When spring arrives, sunlight melts the ice and the CFCs are released back into the atmosphere in huge quantities. They are soon broken down by the ultraviolet light, releasing enough chlorine atoms to destroy all of the ozone in the ozone layer for thousands of square kilometres. Each chlorine atom on average destroys 100000 ozone molecules during a season, before it recombines with other molecules or is swept back out of the upper atmosphere.
This ozone depleted air then starts to circulate out from the polar regions depleting the ozone above Southern Australia and other regions near the poles.
During the summer this air naturally regenerates its ozone, but not quite reaching the previous year's levels.

In this way, the ozone is depleted and reformed each year over the poles. The overall quantity of upper atmosphere ozone is slowly declining at about 1% per year. There are signs that we are presently at the worst level of depletion and that steps to stop the use of CFC's are starting to take effect.

The effects of ozone depletion
If ozone depletion were to go on uncontrolled, scientists believe the whole ozone layer could be at risk. For each 1% decrease in the ozone layer, ultraviolet light intensity on the surface of the Earth increases by 2%. Thus, much more ultraviolet light would reach the Earth's surface creating increased cancer, blindness and mutations in plants and animals. Already, sheep in the Andes mountains of South America have increased incidence of blindness.

Unchecked, life on Earth could be fatally affected, with disease and food shortages dramatically lowering the quality of life and eventually threatening life itself.

What can be done?
Fortunately, a number of protocols have been put in place and the use of CFCs has been greatly reduced. Currently, scientists hope that the ozone hole may be already starting to repair itself and ozone depletion may be a thing of the past by the end of the 21^st century.

Unfortunately, no way has been found of removing CFCs from the atmosphere or of generating ozone in the upper atmosphere. Too many CFC chemicals have already been released into the atmosphere to have any chance of removing them now. We will probably just have to ride out the damage to the ozone layer until all the CFCs are naturally removed from the atmosphere and the ozone depletion ceases.

The only thing that can be done is to make sure CFCs are no longer used and released into the atmosphere.

Copyright owned by the State of Victoria (Department of Education and Early Childhood Development). Used with Permission.