Ozone: the basics
The gases in the Earth’s atmosphere act as a shield for solar radiation by scattering or absorbing the radiation. Of these, ozone is the most effective at absorbing UV radiation.
Ozone is generated in the stratosphere by the interaction of solar ultraviolet (UV) radiation with molecular oxygen. Most ozone lies in the lower stratosphere between 15 and 30 km in altitude. Here it absorbs harmful (UVB) radiation from the sun. So small is the amount of ozone above us that if we brought all the ozone down to sea level temperature and pressure, it would be approximately 3 mm thick.
It is natural for ozone to be created and destroyed in the Earth’s atmosphere, but the last century has seen an unnatural amount of thinning and damage. An increase in the amount of UV light can damage important food chains in the Southern Ocean.
Human impact on the ozone layer
The unnatural springtime thinning of the stratospheric ozone layer or “ozone hole” was first detected in the early 1980s. Scientists determined this is a direct result of humans releasing chlorofluorocarbons (CFCs) into the atmosphere.
CFCs were once thought to be safe and unreactive chemicals. They were used in refrigerators, air conditioners, aerosol sprays, solvents and some types of packaging production. These stable compounds cannot be broken down in the lower atmosphere. They can take a decade to migrate up to the stratosphere, where the harsh UV light breaks them down. They become reactive atoms, combining with and destroying ozone molecules before eventually stabilising. A single chlorine atom can destroy thousands of ozone molecules before stabilizing.
So why does this occur in spring, and why is it isolated to Antarctica?
The cycle of the Antarctic ozone hole
During the polar winter night, the polar air cools, contracts and descends. Air flowing towards the pole from the lower latitudes is deflected away due to the rotation of the Earth. A vortex or ‘whirlpool of air’ forms over the Antarctic continent. This polar vortex, which centres on the South Pole, is quite strong and stable. Without sunlight, temperatures in the vortex drop to as low as −85° Celsius in the lower stratosphere. Ice clouds form at these very low temperatures and act as incubators for the ozone-destroying compounds that come from CFCs.
Once the sunlight returns in spring, the destruction of ozone within the vortex begins and rapidly accelerates. It peaks in early October and slowly declines so that things have returned to normal by the end of December.
The Arctic experiences a similar problem, but not on the same scale as the Antarctic. The Arctic stratosphere is more disturbed by the mountain ranges beneath it; the vortex that forms is smaller and does not last as long; and there is less cloud formation, so the net ozone destruction is less.
Impact on the Antarctic – and on us
About 2% of the light the sun emits is in the form of high-energy, ultraviolet (UV) radiation. Some of this UV radiation (UV-B) causes damage to living things, including sunburn, skin cancer and eye damage.
UVB radiation can penetrate into the ocean, stunting the growth of marine life such as phytoplankton. These microscopic creatures are the base of the food web on which all other marine organisms depend. Damage at this fundamental level would affect the ecosystem of the Southern Ocean and beyond.
Solutions & the future
In 1987, after the link between CFCs and ozone depletion became clear, the Montreal Protocol was agreed upon. This historic international treaty came into force in 1989 and was ratified by all 198 United Nations members. It set deadlines for phasing out the production and use of CFCs, as well as promoting research and development of ozone-safe alternatives. The treaty is hailed as an example of exceptional international co-operation.
Because of the Protocol, use of ozone-depleting chemicals has plummeted and the ozone hole is stabilising. Worldwide compliance continues to reduce the yearly emissions of these compounds. At this rate, the ozone layer will continue to improve over the next several decades. This process cannot be sped up, as CFCs are long-lasting and take time to disappear from the atmosphere.
It is vital that we watch the atmosphere to learn if there are other changes to the balance of gases. If there are, scientists will work to understand whether these are natural changes or caused by human activity. To help monitor the atmosphere, scientists regularly release instruments called ozonesondes. Watch the video above to see a meteorologist launch an ozonesonde from Australia's Davis research station.