Planet Earth has its own natural sunscreen that shields us from the sun's damaging ultraviolet radiation. It's called the ozone layer: a fragile band of gases beginning 15 kilometres above our planet, and reaching up to the 40-kilometre level. Human activities have caused a substantial thinning of this protective covering — not only over the North and South Poles, but right over our heads.
Stopping ozone layer depletion is one of the major challenges facing the world today. The stakes are incredibly high. For the ozone layer is truly a "conserver of life," essential to the survival of all living things.
The ozone layer lies in the stratosphere, in the upper level of our atmosphere. The ozone in it is spread very sparsely. In fact, if you could squish the ozone layer to the same air pressure we have at sea level, it would be only about as thick as the sole of your shoe.
Stratospheric ozone filters out most of the sun's potentially harmful shortwave ultraviolet (UV) radiation. This ozone has become depleted, due to the release of such ozone-depleting substances as chlorofluorocarbons (CFCs). When stratospheric ozone is depleted, more UV rays reach the earth. Exposure to higher amounts of UV radiation could have serious impacts on human beings, animals and plants (see The Impacts of Ozone Depletion).
The stratospheric ozone layer sometimes gets confused with the ozone lying near the earth's surface, known as "ground-level ozone." Although some ground-level ozone occurs naturally, most is produced by the reaction of sunlight with chemicals found mainly in automobile exhaust and gasoline vapours. This human-caused ozone is a key, unhealthy ingredient of smog. Ironically, we have too much ozone at ground level and not enough in the stratosphere.
The highest latitudes — the north and south poles — experience the greatest amount of ozone loss, during their spring. Ozone depletion is most pronounced in the Antarctic. But ozone depletion, to a lesser degree, now occurs in the mid-latitudes. For example, the amount of stratospheric ozone over the northern hemisphere has been dropping by 4% per decade.
What does this mean for life on earth? Even the smallest reduction in stratospheric ozone can have a noticeable impact by increasing the amount of UV radiation that reaches the planet. Studies show, for example, that a decrease in stratospheric ozone could cause additional deaths from skin cancer. Even a 1% global reduction in ozone is expected to cause a significant drop in crop yields, in a world that is already struggling to feed itself.
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