Earth's sunscreen – the ozone layer
Box 3 | How ozone is lost
Ozone loss is most severe in the world’s coldest regions because of clouds in the stratosphere known as polar stratospheric clouds. They provide a surface on which the chemical reactions that result in the destruction of ozone can take place.
The final link in the destruction of ozone requires light. During the winters in the polar regions, the sun never rises. Polar stratospheric clouds form in the cold conditions, and chemical reactions that are necessary for ozone destruction occur. But the final link in the chain does not start until the sun returns in spring. Then, the ozone is destroyed rapidly. Fortunately, the polar stratospheric clouds also disappear as the stratospheric temperature warms up. Net ozone loss then ceases, and the layer will gradually be replenished, but not quite to its former level. Then, the following spring, further depletion will occur.
Arctic ozone loss does occur, and is worsening, but it is not as severe as that measured over Antarctica. The reason is that strong winds develop around Antarctica in the winter, isolating the atmosphere there from the rest of the world. As a result of being shut off from any warmer air from elsewhere, temperatures in the stratosphere fall so low that the formation of polar stratospheric clouds is greater than over the Arctic. (And there is no ‘fresh’ air coming in to dilute the build-up of reactants in the chain of destruction.) In September, ozone loss occurs at its greatest rate over Antarctica as the sun rises. In November, warm air from the rest of the world starts to break through and ozone-poor air moves away thus reducing the average concentration of ozone in the southern hemisphere.
Recently, scientists have become concerned about ozone loss over other regions of the world. Although these losses are nowhere near as great at that recorded over Antarctica, they are worrying because of their possible effects on humans and our crop plants. Over North America, for example, ozone levels fell by about 0.5 per cent per year from 1978 to 1990. The details of the mechanisms causing these mid-latitude losses remain obscure. Polar stratospheric clouds don't seem to be involved. The only region where stratospheric ozone has not diminished is the tropics – and that’s just as well because levels of ultraviolet radiation are at their highest there.
Over Australia, the average amount of stratospheric ozone has also declined, with the losses being greatest in the more southern latitudes. As a result, ultraviolet radiation at ground level has increased. Hobart, for example, has recorded increases of UV-B of about 4 to 6 per cent since 1980.
Boxes
Box 1. Meet the ultraviolet family
Box 2. Can plants get sunburn?
Box 4. Australia finds a replacement for methyl bromide
Posted February 1997.