ONE OF the greatest unknowns about the depletion of the ozone layer is the effect it will have on the amount of harmful ultraviolet radiation - UV-B light - reaching the ground.
There is insufficient information to determine the exact relationship between the destruction of the ozone layer - a known filter of UV-B light from the sun - and the increase in ultraviolet radiation.
Scientists nevertheless estimate that for every 1 per cent loss in ozone, there is a corresponding increase in UV-B radiation of between 1.3 and 2 per cent.
This means that there could be about a 25 per cent increase in harmful ultraviolet radiation reaching Britain this year as a result of the ozone thinning detected by the Nasa satellite.
Scientists emphasise, however, that anyone who spends two weeks on a Mediterranean beach in high summer is exposing themselves to a much higher risk of UV-B damage to the skin, immune system and eyes.
Ultraviolet radiation can vary naturally between the seasons and is greatest when the sun is at its highest point in a cloudless sky. Ozone scientists said that the latest satellite data merely underlined the importance of staying in the shade or using sunscreens and wearing a hat and sunglasses when in the sun.
Scientists at the United Nations estimate that for every 1 per cent decrease in ozone there could be a 3 per cent increase in non-melanoma skin cancer and a smaller increase in melanoma, which is potentially fatal.
UV-B radiation is also associated with damage to the cornea, lens and retina of the eye. Researchers estimate that a 1 per cent decrease in ozone will be accompanied by a slightly lower increase (between 0.6 and 0.8 per cent) in eye cataracts.
Although far less research is done on UV-B damage to the immune system, medical scientists fear that a significant and long- term depletion of ozone over populated areas would make people vulnerable to infections.
Scientists are equally concerned about the effects of UV-B light on crops and other plants. John Pyle, an atmospheric chemist at Cambridge University, said: 'This is a different issue and gives us more grounds for concern because plants cannot avoid the sun.'
Brian Jordan, a plant scientist at Horticulture Research International in Littlehampton, said there were suggestions that increases in ultraviolet light that result from a dwindling ozone layer can lower the productivity of certain crops, such as soya bean, by 20 per cent.
It appears that too much UV- B light can trigger the switching off of crucial genes in a plant responsible for photosynthesis. Dr Jordan said: 'For a plant, UV-B light is an additional stress and crops use their resources to protect themselves.'
Researchers are worried that a thinning ozone layer will seriously affect crop production early next century, when food demand from a growing population will be greater than ever.
One of the most important areas for concern is the effect of ozone depletion on marine plants, or phytoplankton. Yet scientists know next to nothing about this.
Plankton in the oceans account for over half of the total global amount of carbon dioxide absorbed each year from the atmosphere. Any harm to plankton could, therefore, have a knock-on effect for the greenhouse effect; carbon dioxide levels could rise faster, making global warming more likely.
More than 30 per cent of the world's animal protein comes from the sea and in many countries, such as Japan, this percentage is considerably higher. Anything that kills the plankton on which shrimps and fish depend will therefore have a significant effect on the human food supply.
Another effect of increasing levels of ultraviolet light could be greater degradation of materials such as plastics that are sensitive to this type of radiation. Dr Pyle said the Nasa data shows the importance of implementing the international Montreal Protocol, which aims to phase out and end the production of ozone-destroying chemicals.
Leading article, page 21
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