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6.5 Ozone depletion
Molecules of ozone, a form of oxygen, formed a thin layer in the earth's outer atmosphere hundreds of millions of years ago. This layer has protected all forms of life from the potentially fatal effects of ultra-violet radiation from the sun.
The Lidar at Davis station helps us study ozone.
Photo: Didier Monselesan
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Ozone depletion timeline
In the early 1970s scientists began to be concerned about damage to this protective layer caused by atmospheric pollution.
In 1974 two United States chemists predicted that a class of chemicals called chlorofluorocarbons (CFCs), widely used in aerosol spray cans, would seriously damage the ozone layer.
In May 1985 British scientists produced the first direct evidence that this was actually happening. Antarctic atmospheric data going back to 1957 showed an alarming decline in ozone values over Antarctica each spring - effectively a hole in the ozone layer.
In September 1987 an international meeting in Montreal created the world's first environmental convention. The Montreal Protocol set limits on the use of CFCs and related chemicals, including halons (widely used in fire extinguishers). Nations agreed to freeze production at 1986 levels and gradually cut it back, eventually eliminating use of the chemicals.
In September 1987 an international team returned from the Antarctic to reveal that the cause of the ozone hole was human-produced chlorine and bromine molecules escaping to the stratosphere and, under ultraviolet radiation, breaking down into 'free' atoms which can break ozone molecules apart. The team found that in the very cold Antarctic stratosphere, ice crystals provide a surface on which ozone-destroying reactions can proceed apace. They found that in springtime, the combination of continuing cold conditions with sunlight brought the greatest rate of ozone depletion.
In June 1990 an international meeting in London voted for a strengthened Montreal Protocol under which CFCs, halons and other ozone-destroying chlorine compounds would be phased out by 2005 (most would be phased out by 2000). This target has essentially been met, but the longer-term results of the Montreal and London protocols will take many years to assess, as the chemicals already released will remain in the atmosphere for years to come.
Polar Stratospheric Clouds over Mawson Station.
Photo: Ivor Harris
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In spring 2006 the ozone hole over Antarctica was arguably the largest on record, with average ozone levels over Antarctica the thinnest ever observed. Polar stratospheric clouds (PSCs), which form during periods of very low temperatures, chemically alter normally-inert CFCs and related chemicals to destroy ozone in the presence of sunlight, effectively putting a hole in earth's protective ozone blanket.
In about 2018 a decrease in the size of the ozone hole is expected and decreasing levels of ozone-destroying CFCs have already been observed. Temperature trends in the stratosphere are responsible for the delayed effect.
Work in the Ice, Ocean, Atmosphere and Climate (IOAC) programme at the Australian Antarctic Division is aimed at helping to assess trends in temperatures and ozone concentrations in the stratosphere, as well as understanding the chemical processes related to PSCs. Climate modelling and observations indicate that lower atmosphere is warming as a direct consequence of enhanced greenhouse gas emission. While increased CO2 warms the lower atmosphere, it cools the stratosphere. A long-term cooling trend has been observed in the stratosphere (with short-lived warmings due to large volcanic events), and this is likely influencing the size of the polar vortex, which in turn influences the formation of polar stratopheric clouds and the size of the ozone hole.
- What are the consequences of ozone depletion? How does human activity add to the problem? What is being done to prevent it? What do you think should be done? What can you do?
There are some wonderful online materials (for teachers and students) on the science of ozone depletion.
Following are listed a selection of the best:
The frequently asked questions at the Bureau of Meteorology's site, including a terrific experiment for students The Ups and Downs of Ozone that looks at how the ozone layer has changed over the decades and during a year. It includes an interactive animation that models the destruction of ozone by CFCs and provides a teacher's lesson plan and student's worksheet.
See also the United States Environment Protection Agency's ozone depletion website with information on the science of ozone depletion, regulations, and many links and its illustrated article Ozone: Good Up High, Bad Nearby and On the Trail of the Missing Ozone, a cartoon on the causes, effects, and solutions to ozone depletion.
Environment Canada's Stratospheric Ozone information for children.
The University of Cambridge's Centre for Atmospheric Science's multiMedia Ozone Hole Tour.
The US National Science Foundation's ozone layer depletion and global warming - great articles and animations for children.
Keep track of the ozone hole, and near-real time ozone profiles with NASA's ozone mapping spectrometer.
The United Nations Environment Programme's site frequently asked questions about ozone depletion page.
The NASA-supported site Exploring the Environment offers an educational module on ozone depletion and its effects. - Ask students to research the effects that changes in the ozone layer might have on Antarctic wildlife.
See the article Effect of ozone depletion on Antarctic marine microbes.
See the effects of ozone depletion on humans, animals and plants for a description of them.
Here is a detailed and fairly technical account of the effects of ozone depletion on aquatic ecosystems and a summary of the research on Ultraviolet-B Effects on Aquatic Ecosystems.