The first signs of spring are appearing over Antarctica. As the sun returns after months of darkness an alarming annual phenomenon is reappearing — the ozone in the stratosphere is thinning out to form a giant hole. This year the ozone hole appeared quite late but it has grown rapidly in recent weeks, and is already larger than last year’s. It will reach its maximum size between late September and early October before it fills again in mid-December.

The ozone layer shields the Earth’s surface from the worst of the Sun’s ultraviolet rays. Without its protection, dangerous levels of ultraviolet radiation cause sunburn, cataracts and skin cancer in human beings and huge damage to wildlife and vegetation.

During the winter, pollutants such as chlorofluorocarbons (CFCs) are swept up in the stratosphere to form vast whirlpools over the Antarctic. At extremely cold temperatures these chemicals destroy the ozone. Although CFCs are being phased out under the 1987 Montreal Protocol, they linger in the atmosphere for many years, and experts caution that the ozone layer may not fully recover for another 70 years.

Because the ozone layer and the Antarctic are so remote it is difficult to comprehend the damage being done to the atmosphere. Scientists working on Antarctica can see the telltale signs of the ozone hole in strange clouds in the stratosphere, with beautiful colours similar to mother-of-pearl. The temperatures in the clouds are around minus 86C (-123F), and so cold that the tiny ice crystals in them provide the surfaces for the CFCs to attack the ozone layer.