The ozone layer

What is ozone?

Ozone is a naturally occurring molecule made up of three oxygen atoms. It has the chemical formula O3. The word ‘ozone’ is derived from the Greek word óζειν which means “to smell”. Its strong smell allows scientists to detect it in low amounts.

Ozone is found in different levels of the earth’s atmosphere. About 90% of ozone in the atmosphere is concentrated between 15 and 30 kilometres above the earth’s surface (stratospheric ozone). At this level it provides a protective shield from the sun, we think of this as good ozone. It is also found at ground level in lower concentrations (tropospheric ozone). Here ozone is a pollutant that is a key part of smog over cities and we think of it as bad ozone.

A Tale of Two Ozones describes the different effects of ozone depending on where in the atmosphere it is found.

What is the ozone layer?

The ozone layer is the common term for the high concentration of ozone that is found in the stratosphere between 15 and 30km above the earth’s surface. It covers the entire planet and protects life on earth by absorbing harmful ultraviolet-B (UV-B) radiation from the sun.

Prolonged exposure to UV-B radiation is linked to skin cancer, cataracts, genetic damage and immune system suppression in living organisms, and reduced productivity in agricultural crops and the food chain.

What is damaging the ozone layer?

Atmospheric data demonstrates that ozone depleting substances are destroying ozone in the stratosphere and thinning the earth’s ozone layer. Ozone depleting substances are chemicals that include chlorofluorocarbons (CFCs), halons, carbon tetrachloride (CCl4), methyl chloroform (CH3CCl3), hydrobromofluorocarbons (HBFCs), hydrochlorofluorocarbons (HCFCs), methyl bromide (CH3Br) and bromochloromethane (CH2BrCl). They deplete the ozone layer by releasing chlorine and bromine atoms into the stratosphere, which destroy ozone molecules. These and other ozone depleting substances also contribute, to varying extents, to global warming.

When was the depletion of the ozone layer discovered?

In 1974, chemists Mario Molina and Frank Sherwood Rowland discovered a link between CFCs and the breakdown of ozone in the stratosphere. In 1985, geophysicist Joe Farman, along with meteorologists Brian G Gardiner and Jon Shanklin published findings of abnormally low ozone concentrations above the Antarctic, which galvanized world-wide action.

In 1995, Mario Molina, Frank Sherwood Rowland and Paul Crutzen, also an atmospheric chemist, were jointly awarded the Nobel Prize in Chemistry “for their work in atmospheric chemistry, particularly concerning the formation and decomposition of ozone”.

More about ozone layer depletion

The ozone layer is depleted in two ways. Firstly, the ozone layer in the mid-latitude (e.g. over Australia) is thinned, leading to more UV radiation reaching the earth. Data collected in the upper atmosphere have shown that there has been a general thinning of the ozone layer over most of the globe. This includes a five to nine per cent depletion over Australia since the 1960s, which has increased the risk that Australians already face from over-exposure to UV radiation resulting from our outdoor lifestyle. Secondly, the ozone layer over the Antarctic, and to a lesser extent the Arctic, is dramatically thinned in spring, leading to an ‘ozone hole’.

Will the ozone layer recover

The global community has taken action to restore the ozone layer. The Montreal Protocol on Substances that Deplete the Ozone Layer (the Montreal Protocol) came into effect in 1987. It commits countries to phasing out production and import of all the major ozone depleting substances. Australia manages its obligations to this international agreement through the Ozone Protection and Synthetic Greenhouse Gas Management Act 1989.

Every four years, the World Meteorological Organisation and the United Nations Environment Programme review the state of the ozone layer. These reviews show that the abundance of ozone depleting chemicals in the atmosphere is now declining and the ozone layer is expected to recover to pre-1980 levels over the mid-latitudes by 2050 and over the Antarctic by 2065.