The ozone layer — an oxygen compound (O3) found naturally in the Earth's stratosphere — is important because it absorbs UV radiation from the sun and prevents it from causing damage to plants and animals. Ozone is constantly produced and destroyed in a natural cycle; however, the overall amount of ozone is essentially stable. Think of an observer standing next to a stream: Although individual water molecules are moving past the observer, the total depth remains constant. Similarly, while ozone production and destruction are balanced, ozone levels remain stable. This was the situation until the past several decades.
How much of the ozone layer is left?
Large increases in chlorine and bromine in the stratosphere have upset that balance by removing ozone faster than natural ozone creation can keep up. Therefore, ozone levels fall. The discovery that the Earth was losing 4-5% of ozone led to the creation of the Montreal Protocol in 1987. Nations that support this program have pledged to phase out the use of ozone-eroding chemicals, such as chlorofluorocarbon (CFC), which is often found in aerosol cans.
On August 2, 2003, scientists announced that the destruction of the ozone layer has slowed down significantly during the past decade. The study was organized by the American Geophysical Union. Some breakdown can be expected to continue due to CFCs used by nations which have not banned them, and due to gases that are already in the stratosphere. CFCs can last for 50 to 100 years, so the final recovery of the ozone layer will likely take several decades.
To view a report on the latest ozone readings, visit NASA's Total Ozone Mapping Spectrometer.
For a glimpse into the future of ozone recovery, check out this short online video from the EPA and The Weather Channel: Ozone Depletion — Science and Response.