Ozone layer depletion: The threat mounts
The Earth's atmosphere is divided into several layers. The lowest region, the troposphere, extends from the Earth's surface up to about 10 kilometers (km) in altitude. Virtually all human activities occur in the troposphere. Mt. Everest, the tallest mountain on the planet, is only about 9 km high. The next layer, the stratosphere, continues from 10 km to about 50 km. Most commercial airline traffic occurs in the lower part of the stratosphere.
Most atmospheric ozone is concentrated in a layer in the stratosphere, about 15-30 kilometers above the Earth's surface. Ozone is a molecule containing three oxygen atoms. It is blue in colour and has a strong odour. Normal oxygen, which we breathe, has two oxygen atoms and is colourless and odourless. Ozone is much less common than normal oxygen. Out of each 10 million air molecules, about 2 million are normal oxygen, but only 3 are ozone.
Laboratory and epidemiological studies demonstrate that Ultra Violet-B (UV-B) causes non melanoma skin cancer and plays a major role in malignant melanoma development. In addition, UV-B has been linked to cataracts. All sunlight contains some UV-B, even with normal ozone levels. It is always important to limit exposure to the sun. However, ozone depletion will increase the amount of UV-B, which will then increase the risk of health effects. Furthermore, UV-B harms some crops, plastics and other materials, and certain types of marine life. Higher air temperatures also increase the concentration of ozone at ground level. The natural layer of ozone in the upper atmosphere blocks harmful ultraviolet radiation from reaching the earth's surface; but in the lower atmosphere ozone is a harmful pollutant. It damages lung tissue and causes particular problems for people with asthma and other lung diseases.
Shockingly, the incidence of skin cancer in the United States has reached epidemic proportions. Health experts predict that one in five Americans will develop skin cancer in their lifetime. Excess UV radiation may also affect the body's general ability to fight off disease. Says immunologist Margaret Kripke M.D. of the Anderson Cancer Centre in Houston, “We already know that ultraviolet light can impair immunity to infectious diseases in animals. We know that there are immunological effects in humans, though we don't yet know their significance”. One impending tragedy is the possible blinding of almost all non-nocturnal animals and insects outside the darkest jungle. While humans can certainly protect their eyes, most other forms of life can't.
However, even the small amount of ozone plays a key role in the atmosphere. The ozone layer absorbs a portion of the radiation from the sun, preventing it from reaching the planet's surface. Most importantly, it absorbs the portion of ultraviolet light called UV-B.
At any given time, ozone molecules are constantly formed and destroyed in the stratosphere. The total amount, however, remains relatively stable. While ozone concentrations vary naturally with sunspots, the seasons, and latitude, these processes are well understood and predictable. Scientists have established records spanning several decades that detail normal ozone levels during these natural cycles. Each natural reduction in ozone levels has been followed by a recovery. Recently, however, convincing scientific evidence has shown that the ozone shield is being depleted well beyond changes due to natural processes.
For over 50 years, chlorofluorocarbons -- CFCs (compounds containing Chlorine, Fluorine and Carbon) were thought of as miracle substances. They are stable, nonflammable, low in toxicity, and inexpensive to produce. Over time, CFCs found uses as refrigerants, solvents, foam blowing agents, and in other smaller applications. Other chlorine-containing compounds include methyl chloroform, a solvent and carbon tetrachloride, an industrial chemical. or bromine. When they break down, they damage the protective ozone layer.
In the early 1970s, researchers began to investigate the effects of various chemicals on the ozone layer, particularly CFCs, which contain chlorine. They also examined the potential impacts of other chlorine sources. Chlorine from swimming pools, industrial plants, sea salt, and volcanoes does not reach the stratosphere. Chlorine compounds from these sources readily combine with water and repeated measurements show that they rain out of the troposphere very quickly. In contrast, CFCs are very stable and do not dissolve in rain. Thus, there are no natural processes that remove the CFCs from the lower atmosphere. Over time, winds drive the CFCs into the stratosphere.
The CFCs are so stable that only exposure to strong UV radiation breaks them down. When that happens, the CFC molecule releases atomic chlorine which is highly active. One chlorine atom can destroy over 100,000 ozone molecules. The net effect is to destroy ozone faster than it is naturally created.
Large fires and certain types of marine life produce one stable form of chlorine that does reach the stratosphere. However, numerous experiments have shown that CFCs and other widely-used chemicals produce roughly 85 percent of the chlorine in the stratosphere, while natural sources contribute only 15 percent. Large volcanic eruptions can have an indirect effect on ozone levels.
One example of ozone depletion is the annual ozone "hole" over Antarctica that has occurred during the Antarctic Spring since the early 1980s. Rather than being a literal hole through the layer, the ozone hole is a large area of the stratosphere with extremely low amounts of ozone. Ozone levels fall by over 60 percent during the worst years.
In addition, research has shown that ozone depletion occurs over the latitudes that include North America, Europe, Asia, and much of Africa, Australia, and South America. Over the U.S., ozone levels have fallen 5-10 percent, depending on the season. Thus, ozone depletion is a global issue and not just a problem at the South Pole.
Reductions in ozone levels will lead to higher levels of UV-B reaching the Earth's surface. The sun's output of UV-B does not change; rather, less ozone means less protection, and hence more UV-B. Studies have shown that in the Antarctic, the amount of UV-B measured at the surface can double during the annual ozone hole.
Just as worrisome is the threat to world's food supply. High doses of UV radiation can reduce the yield of basic crops such as soybeans. UV-B, penetrates scores of metres below the surface of oceans. There the radiation can kill phytoplankton (one celled plants) and Krill (tiny shrimp like animals) which are at the bottom of the ocean food chain. Since these organisms found in greatest concentrations in Antarctic waters and nourish larger fish, the ultimate consumers -- humans -- may face a maritime food shortage.
Let us think for a moment about the world's one billion refrigerators, hundreds of millions of air conditioners, mountains of foam insulation, seat cushions, furniture stuffing and carpet padding, streams of cleaning fluids, rivers of industrial solvents, wafting clouds of aerosol spray, all of them setting free the CFCs that destroy the ozone layer.
Ridding the world of the millions of tons of ozone depleting chemicals is a huge, costly and complex task requiring unprecedented international cooperation. Since du Pont began marketing the miracle refrigerant called Freon, CFCs have worked their way deep into the machinery of what much of the world thinks as modern life. Extricating the planet from the chemical burden of that high-tech lifestyle will require not only technical ingenuity but extra-ordinary leadership qualities.
The technical challenge is to find substances and processes that can replace CFC-based systems without doing further harm to the stratosphere. Happily, that endeavour has worked. In fact, it has turned out to be much easier than anyone expected. Except for medical aerosols, some fire-fighting equipment and certain metal cleaning applications there are now effective substitutes for virtually every ozone-depleting chemical. Assuringly, in a surprising number of cases, the new processes are actually cheaper and better than the old.
But the harm or danger is lurking in somewhere else and so replacing CFCs in newly built equipment, however, is only half the job. Virtually every existing refrigerator and air conditioner is a CFC reservoir. The chemicals are not a problem as long as they continue to circulate within an appliance. But if the machine is carelessly drained, junked or damaged, the CFCs can escape to attack the ozone. The real task for those countries that invested heavily in CFCs in the past is to develop systems for recovering and recycling the chemicals they have already used. Undoubtedly true, the US, Europe and other industrialised countries bear direct responsibility for most of the damage that has been done and they can best afford the costs attached to switching technologies. But what about the countries of the Second and Third World? Many of them are just beginning to enjoy the comforts of CFC technology, and they cannot easily go for a changeover.
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