Ozone and ultraviolet effects

As well as worries about the addition of CO2 to the atmosphere, scientists are also concerned about the depletion of another gas, ozone. Ozone in the stratosphere absorbs much of the ultraviolet (UV) radiation from the sun. This is important because ultraviolet-B (UV-B) is harmful to most life forms. It is the UV-B rays from the sun that cause sunburn and skin cancer in humans and UV light is used to kill bacteria in sewage treatment plants.

In recent decades, the amount of ozone in the stratosphere has been declining. This was first noticed in the Antarctic in the 1970s and since then large 'holes' in the ozone layer have regularly been recorded there. Losses of about 8 per cent per decade have been estimated at polar latitudes. UV levels naturally rise in spring-time but in recent years these higher levels have been arriving sooner and lasting longer than normal. The destruction of the ozone layer is thought to be mainly due to the release of chlorine into the atmosphere. Chlorine destroys ozone through a series of chain reactions. During the winter, chlorine is concentrated over the Antarctic due to complex and unique climatic conditions. This is why ozone depletion is most marked over the Antarctic (Gribbin, 1988a).

Some chlorine enters the atmosphere from natural sources but the majority results from human activity. A major source is from chemicals called chlorofluorocarbons (CFCs) used in air-conditioning plants, refrigerators, aerosol propellants and various manufacturing processes. Alternatives to CFCs are being increasingly used and sought but the ozone layer at present continues to decrease.

UV-B light can burn and damage plants on land. It can also kill and damage photosynthetic algae in surface waters. In fact it is now thought that UV-B may penetrate as deep as 20 m into the oceans although the amounts fall off rapidly below the first metre or so. Experiments have shown that the UV-B in sunlight inhibits phytoplankton growth. Thus an increase in the amount of UV-B entering the oceans could reduce productivity. This in turn would reduce the oceans' ability to absorb CO2 (see Section 10.2.1) and so could increase global warming.

Other experiments have indicated that UV light may have a controlling influence on many plants and animals growing in shallow water. The resistance or sensitivity to UV light of different species and different life stages of reef organisms may partially dictate their abundance and distribution. For example, experiments by Wood (1987) suggest that the kelp Ecklonia radiata may be prevented from colonizing shallow water by the higher levels of UV radiation occurring there.

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