During the 1970s, scientists began to observe abnormally low levels of ozone in the upper atmosphere (stratosphere) over Antarctica in the spring each year. This phenomenon was termed the ‘Ozone Hole’. Further investigation revealed that concentrations of the gas were decreasing on an annual basis and these findings alarmed the international community. Areas of depleted stratospheric ozone have been observed over both Polar Regions. At times, their limits have spread out from the poles extending over the mid-latitudes in both hemispheres. Overall, there has been a depletion of stratospheric ozone around the globe.

Ozone ( O₃) is a gas that has the ability to absorb ultra-violet solar radiation. Ultra Violet (UV) radiation causes sun burns, cancer and can cause genetic damage. Ozone in the upper atmosphere acts as a ‘sunscreen’ for the earth and effectively reduces UV solar radiation to safe levels when the gas is in sufficient concentrations. It also affects the temperature of the upper atmosphere by absorbing solar radiation.

Ozone is a toxic gas and normally does not occur in the lower atmosphere in significant concentrations. However, ozone is a product of pollution in urban areas and can cause severe health problems such as respiratory diseases if it exists in high concentrations.

CFCs (chlorofluorocarbons) are chemical compounds called Halogens containing chlorine that were once used extensively around the world in aerosols, refrigeration (Freon, for example), Styrofoam, and other industrial practices. Once dispersed into the atmosphere, these chemicals end up in the upper atmosphere where they are broken down to produce chlorine (Cl). Due to circulation in the atmosphere, concentrations develop over the poles. In Antarctica, this occurs predominantly during the winter months. When the sun returns in spring, the chlorine is able to react with ozone in the presence of sunlight in a very complex catalytic reaction. However, the end result is demonstrated below:

Cl + 0₃-->ClO + O₂
ClO + O-->Cl + O₂
----------------------
Net Cl + 0₃ + O - > Cl + 2O₂

One molecule of ozone (O₃) is effectively broken into two molecules of oxygen (O₂). On a large scale the overall result is the ozone "hole". Once the upper atmosphere warms up in late spring and early summer the reaction ceases. New ozone is brought into the area by atmospheric circulation and the hole disappears.

In 1987, The Montreal Protocol for Substances that Deplete the Ozone Layer was agreed upon and signed by many countries and since then, 183 nations have ratified the treaty. The treaty set forth targets for phasing out the use of CFCs and other chemicals (such as carbon tetrachloride) that are responsible for the destruction of ozone in the upper atmosphere. Amendments to the treaty were signed in 1990, 1992, 1997 and 1999. Governments and industries around the world have launched many programs and shared initiatives to develop, implement and utilize CFC alternatives in industrial practices. To date, scientific monitoring of ozone depletion has observed that CFC concentrations appear to have peaked and have recently begun to decline. Although this is promising news, the lifetime of these chemicals in the upper atmosphere is lengthy, and the corresponding decrease in the rate of ozone depletion is expected to be slow. Nonetheless, the Montreal Protocol appears to have been effective and is a good example of international cooperation on issues affecting the global community.

For more information on the Montreal Protocol, including specific policies and regulations, please visit http://www.unep.org/ozone/montreal.shtml.

For the most recent scientific assessment of Ozone Depletion, a UNEP download is available at http://www.unep.org/ozone/pdf/execsumm-sap2002.pdf .