Ozone depletion: 2000 and beyond
Paul Holper
Dec 2000

The 2000 Antarctic ozone hole was the largest (in area) on record. It began earlier than usual and peaked in September, attaining an area of almost 30 million square kilometres (nearly the size of Africa).

Minimum total ozone values usually occur at the end of September or early in October. Ozone values above Antarctica then, based on satellite data, were below 200 Dobson Units (DU) over much of the continent, with large areas below 125 DU and a few places below 100 DU. Pre-ozone hole values are about 300 DU. However, the hole did not reach the very lowest values of some recent years.

Every year since the mid-1980s, springtime has brought massive destruction of stratospheric ozone over Antarctica. (The stratosphere is the part of the atmosphere that stretches from about 15 to 50 kilometres above the Earth’s surface. The ‘ozone layer’ is found in the lower stratosphere.)

It is not just Antarctica that suffers from ozone depletion. Over most of the planet, levels of ozone in the stratosphere are now lower than they were a decade ago.

Stratospheric ozone depletion over mid-latitudes means that ultraviolet levels in major population areas of southern Australia are likely to have risen by 10-15 per cent over the past twenty years. This increase is likely to translate to more than a 15 per cent increase in some skin cancers if we do not protect ourselves. ‘Slip, slop, slap’, is the clear message.

CFCs and halons are doing the damage. In the stratosphere, CFCs and halons release, respectively, reactive chlorine and bromine that destroy ozone. The destruction is catalytic, with each chlorine or bromine radical destroying thousands of ozone molecules.

Destruction is particularly efficient over Antarctica in spring. During the preceding dark, winter months, stratospheric temperatures fall to below –80°C. Under these severe conditions polar stratospheric clouds form, with chlorine- and bromine-containing compounds settling on the cloud particles. When the sun reappears in September its ultraviolet rays initiate chemical reactions involving these compounds. The result is a rapid release of active chlorine and bromine capable of attacking and destroying ozone.

The ozone is replenished at the end of spring, when air above Antarctica warms and ozone-rich air from the rest of the stratosphere moves in and brings ozone levels almost back to normal.

Springtime Arctic ozone holes also occur, although they are less severe than their Antarctic counterparts because the Arctic stratosphere doesn’t get as cold as that above Antarctica.

Ozone layer recovery

The Montreal Protocol, signed in 1987, regulates the production of CFCs and other ozone-depleting substances. Production of the most damaging ozone depleting substances was eliminated, except for a few critical uses, by 1996 in developed countries and will be eliminated by 2010 in developing countries.

Measurements from the Cape Grim Baseline Air Pollution Station in north-western Tasmania and similar stations elsewhere around the globe are showing a decline in atmospheric concentrations of many ozone depleting substances.

However, the problem is that ozone depleting chemicals are very stable — one of their major attractions in the past to the refrigeration, aerosol and plastics industries. Some CFCs last in the atmosphere for more than a hundred years. As a consequence, recovery of stratospheric ozone will not be quick.

‘More than half a century may pass before the Antarctic ozone hole is repaired and the first clear signs of repair may not be apparent for 20 years,’ says CSIRO’s Dr Paul Fraser.

In a twist, which emphasises that environmental problems cannot be dealt with in isolation, the decrease in stratospheric ozone and the global increase in greenhouse gases are contributing to colder conditions in the lower stratosphere. The temperature drop could lead to more stratospheric clouds and expand the area of intense ozone loss over Antarctica in spring. It may also exacerbate ozone depletion elsewhere, particularly over the Arctic.

‘Global warming may delay recovery by 10-20 years,’ says Dr Fraser.

Scientists are unable to tell whether the early arrival and record size of the 2000 ozone hole are within expected variations of the hole or are due to the additional influence of other human activities, such as release of greenhouse gases.

According to Dr David Hofmann, from the US National Oceanic and Atmospheric Administration, ‘year-to-year fluctuations in the geographical size of the ozone hole and the timing of the ozone reduction are believed to be related to meteorological factors such as temperature and winds, rather than further increases in ozone-destroying chemicals in the atmosphere’.

Good news is the fact that the observed rate of ozone loss in Australian latitudes has slowed over the last decade and is approaching zero. The impact of the large Antarctic ozone holes of recent years, well to the south of Australia and New Zealand, has never accounted for more than a small fraction of the ozone loss over Australia.

December 2000

Modified: April 3, 2008