The stratosphere is the layer of the atmosphere that begins at an altitude of around 10 kilometres. The stratosphere contains about 90 per cent of the ozone in the atmosphere, which helps to make Earth inhabitable by absorbing a large proportion of incoming solar ultraviolet (UV) radiation before it reaches Earth’s surface. UV radiation is harmful to a range of biological systems, including human health. It damages cells, and causes sunburn and premature skin ageing in low doses. At higher levels, it can cause skin cancer and suppress the immune system. Note that although stratospheric ozone protects human health, ozone near the ground, where it can be breathed in, is a pollutant and harmful to health. (See Ozone for further discussion of ozone as a pollutant.)
The stratospheric ozone layer was threatened by human-produced ozone depleting substances (ODSs), which were widely used in refrigerators, air-conditioners, fire extinguishers and electronic equipment; as solvents for cleaning (including dry-cleaning); and as agricultural fumigants. These substances contain chlorine and bromine atoms, which are released over time when ODSs break down and then react with ozone molecules to break them up. This has led to a reduction in stratospheric ozone in the mid-latitudes and a particularly severe reduction over Antarctica, known as the ozone hole (described in the Antarctic environment report). Since peaking in the 1990s, the atmospheric abundance of nearly all ODSs that are controlled under the Montreal Protocol has declined. However, these substances are long-lived in the atmosphere and many are also powerful greenhouse gases.
Figure ATM5 shows total column ozone levels above Melbourne, Darwin, Brisbane and Hobart for January each year from 1979 to 2015, where data are available. The dark line shows a 5-year running mean. Although considerable year-to-year variability is evident, such as the influence of the 11-year solar cycle (which peaked around 1980, 1991, 2002 and 2013), the decreasing trend seen in the 1980s and the first half of the 1990s indicates recovery of stratospheric ozone levels, as actions under the Montreal Protocol have become effective.