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The ozonosphere, commonly known as the ozone layer, is the Earth's first defense against the harmful ultraviolet rays emanating from the sun. Ozone, the three-atom form of oxygen, is comparable only to the oceans in its ability to shield living organisms from radiation. It has, however, been compromised. The discovery of a hole in the ozone layer above Antarctica brought the public's attention to their part in the process of ozone reduction.

The hole itself is a nine million square mile area the size of North America where levels of stratospheric ozone have been drastically reduced. Fluctuating in size at different times during the year, it reaches its largest size in the spring, during the Southern Hemisphere's growing season. The hole then moves from Antarctica to the skies over Australia and New Zealand. The high amounts of radiation let in through the hole can cause higher instances of skin cancer, injure the eyes, harm the immune system, and upset fragile ecosystems. The high-energy UV light could kill photosynthesizing plankton and cause a global reduction of breathable oxygen.

The introduction of chlorofluorocarbons (CFCs)-substances found in cleaning solvents, aerosol propellants, and coolants in refrigerators and air conditioners-into the atmosphere has caused the hole to continue growing. As the CFCs move higher into the atmosphere, the ultraviolet radiation is strong enough to split the molecule and allow chlorine atoms to break away. The chlorine atom then encounters an ozone molecule and steals an oxygen atom, leaving behind an ordinary two-atom oxygen molecule. The chlorine monoxide molecule that is formed encounters a free oxygen atom, the two oxygens combine, and the chlorine atom is free to steal another oxygen atom from another ozone molecule.

Confronted with this knowledge, scientists urged the public to cut down on the use of ozone depleting chemicals like CFCs. Their efforts led to the 1987 Montreal Protocol where 150 countries agreed to a phased reduction of CFCs, and later, in a 1996 amendment, a global ban on CFC production. Even with the ban in effect, chlorine from CFCs will continue to accumulate in the atmosphere for another decade and it will take until the middle of next century for the level of atmospheric ozone to return to 1970s levels. However, because the harmful effects of CFCs were discovered in time to take action, the potential consequences are much less severe than they otherwise could have been.