The ozone layer

Nacreous Clouds at Davis
Nacreous Clouds at Davis (Photo: Nick R)

Life’s protective blanket

The elements that make up the Earth's atmosphere are of vital importance to us all. The balance of gases in the atmosphere has changed recently as a result of the impact of human activity. In order to protect and preserve life as we know it, scientists need to understand the complex factors which control the balance of gases in the atmosphere.

The atmosphere is composed of 78% nitrogen, 21% oxygen and minor gases, 1% argon and minute trace gases, carbon dioxide and ozone, etc. So small is the amount of ozone above us that if we brought all of the ozone down to sea level temperature and pressure, it would be equivalent to a layer of pure ozone approximately 3mm thick.

Ozone occurs throughout the atmosphere, but is more concentrated in the lower stratosphere, between 15 and 40 km above the earth's surface. This 'ozone layer' serves to protect the earth from harmful radiation from the sun. Ultraviolet (UV) light from the sun is harmful to all life on earth. Increases in the amount of UV light can damage important food chains in the Southern Ocean. There is also a strong link between UV increases and an increased incidence of human skin cancer and cataracts of the eye. The atmosphere acts as a shield for solar radiation by scattering or absorbing the radiation through its molecules. Of these, ozone is the most effective at absorbing UV radiation. It is natural for ozone to be constantly created and destroyed in the Earth's atmosphere. It is also natural for a thinning of the stratospheric ozone layer to appear over Antarctica in springtime. This thinning is confined to the area south of latitude 60ºS.

The ozone layer however, is now unnaturally thin. Over the past decade ozone over Antarctica has thinned unnaturally in springtime. Scientists have determined that this depletion is a direct result of humans releasing chlorofluorocarbons (CFCs) into the atmosphere. CFCs were once widely used as propellants in aerosol sprays and as coolants in refrigerators. Once released into the air these stable compounds cannot be broken down in the lower atmosphere and take a decade to migrate up to the stratosphere. CFCs are usually stable molecules but UV light in the upper atmosphere breaks them down into reactive constituents. The reactive atoms then combine with ozone to produce new products. Unfortunately these new molecules are not stable, but go on destroying more and more ozone. A single chlorine atom can destroy thousands of ozone molecules before becoming tied to a stable compound. When a stable compound finally occurs it causes no further ozone loss.

In 1987 after scientists demonstrated the link between CFCs and ozone depletion an international treaty The Montreal Protocol opened for signature. This treaty was designed to protect the ozone layer by phasing out the production of substances believed to be responsible for ozone depletion. CFCs were consequently banned world wide in what has been hailed as an example of exceptional international co-operation.

Why has the ozone thinning occurred over the Antarctic in spring and summer?

During the polar winter night the stratospheric air mass over Antarctica is effectively isolated from the rest of the atmosphere by a strong circulation which is centred on the South Pole. The polar air cools, contracts and descends. Air flowing towards the pole from the lower latitudes is deflected by a force due to the rotation of the Earth and a strong, nearly circular, vortex or 'whirlpool of air' is produced over a region roughly corresponding to the Antarctic continent. This vortex is quite stable and lasts until spring, when the returning sunlight warms the air and disrupts the circulation pattern. During winter the stability of the air in the vortex and its extremely low temperature provide conditions suitable for the formation of high altitude clouds.

These clouds provide the site for ozone destruction when the sunlight returns in spring.

In the Arctic there is a similar problem but not on the same scale as the Antarctic. The Arctic is warmer and 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.

Ozone is a poisonous gas at ground level, but its presence in the stratosphere is vitally important to the health of the Earth. Ozone destroying chemicals can be released anywhere in the world, and given time, they can find their way into the stratosphere. There are definite signs that the ozone hole is stabilising. However, there is unlikely to be any significant reduction in size for many years until the current levels of ozone depleting substances has decreased.

The ban on CFC use, imposed in 1989 in Australia, will not cause any improvement in the stratosphere until well into this century. It is now vital that we closely monitor the atmosphere to ascertain whether there have been other changes to the balance of atmospheric gases and to understand whether these are natural changes or caused by human interference.