The New Nation - Internet Edition

Delowar Hossain

It had become clear the mid-1970s that human activities had the potential to bring about sufficient degradation of the ozone layer that it might never recover. The threat was seen to come from four main sources, associated with modern technological developments in warfare, aviation, life style and agriculture and involving a variety of complex chemical compounds, both old and new.

When a modern thermonuclear device is exploded in the atmosphere, so much energy is released, so rapidly, that the normally inert atmospheric nitrogen combines with oxygen to produce quantities of oxides of nitrogen .The rapid heating of the air also sets up strong convection currents which carry the gases and other debris into the stratosphere and it is there that most of the NOx is deposited. Since natural NOx is known to destroy ozone, it is only to be expected that the anthropogenically produced variety would have the same effect, and one of the many results of nuclear war might be the large scale destruction of the ozone layer. Most of the studies, which originally investigated the effect of nuclear explosion on the atmosphere, used data generated during the nuclear bomb tests of the 1950s and 1960s. After 1963s when a moratorium on atmospheric tests of nuclear devices was declared information from these sources was NOx longer available and recent investigation has been based on statistical models.

The results of the studies of the effects of nuclear weapon tests on the ozone layer were not conclusive. The analysis of data collected during a period of intense testing in 1961 and 1962 produced no proof that the tests had had any effect on the ozone (Foley and Ruderman 1973 ), although it was estimated that the explosions should have been sufficient to cause a reduction of 3% in stratospheric ozone levels( crutzen 1974). Techniques for measuring ozone levels were not particularly sophisticated in the 1960s, and in addition a reduction of 3% is well within the normal annual fluctuation in levels of atmospheric ozone. Similar result were obtained by crutzen (1974) using a photochemical diffusion model. He calculated that the amount of nitric oxide injected into the stratosphere by a 500-megatonne conflict would be more than ten times the annual volume provide by natural process. This was considered sufficient to reduce ozone levels in the northern hemisphere by 50%. Dramatic as these values may appear they remain approximation - based on data and analyses containing many inadequacies. Interest in the impact of nuclear war on the ozone layer peaked in the mid-1970s and declined thereafter. It emerged again a decade, latter as part of a larger package dealing with nuclear war and climatology which emphasized nuclear winter.

The planning and development of a new generation of transport aircraft was well under way in North America Europe and the USSR by the early 1970s. These were the supersonic transports designed to fly higher and faster than convectional subsonic civil airliners and undoubtedly a major technological achievement. It became cleat however that they could lead to serious environmental problems if ever produced in large numbers. Initial concerns included elevated noise levels at airport s and the effects of the sonic boom produced when the aircraft passed through the sound barrier but many scientists and environmentalists saw the impact of theses high flying jets on the structure of the ozone layer as many times more serious and more universal in its effects. Supersonic transports received a great deal of attention between 1971s and 1974s as a result of congressional hearings in the united states into the funding of the Boeing SST and a subsequence climatic impact assessment program commissioned by the us department of transportation to study the effects of SSTs on the ozone layer. The findings in both cases were extremely controversial and gave rise to a debate which continued for several years at times highly emotional and acrimonious. It was fulled further y a series of legal and legislative battles which ended only in 1977, when the USS Supreme Court granted permission for the Anglo-French Concorde to land at New York. In 1970, crutzen drew attention to the role of NOx in the destruction of ozone through catalytic chain reactions and in the exhaust gases of 500ssts could reduce ozone levels by as much as 22-50%. Latter predictions by suggested a 3-22 % reduction while Hammond and Maugh reported in 1974 that the net effect of the NOx emissions from a fleet of 500ssts would be a 16 % reduction in ozone in the northern hemisphere and an 8% reduction in the southern hemisphere. When all of this was under consideration in the early 1970s it was estimated that the world's fleet of SSTs would grow to several hundred aircraft by the end of the century and perhaps as many as 5000 by the year 2025 (Dotto and Schiff 1978). The NOx emission from such a fleet were considered capable of thinning the ozone layer sufficiently to produce an additional 20,000 to 60,000 cases of skin cancer in the united states alone ( Hammond and Maugh 1974)

If there was some doubt about the impact of SST exhaust emissions on the ozone layer the effects of some other chemicals seemed less uncertain. Among these the CFCs group and related BFCs or halons have been identified as potentially the most dangerous. The cfcs sometimes referred to by their trade name Freon came to prominence as a result of lifestyle changes which have occurred since the 1930s. They are used in refrigeration and air conditioning units but until recently their major use was as propellants in aerosol spray cans containing deodorants, hair spray, paint, insect repellant and a host of their substances. When the energy crisis broke they were much in demand as foaming agents in the production of polyurethane and polystyrene foam used to improve home insulation. Polymer foams are also included in furniture and car seats and with the growth of the convenience food industry they were used increasingly in the manufacture of fast food containers and coffee cups. The gases are released into the atmosphere from leaking refrigeration or air conditioning systems or sprayed directly from aerosol cans. They also escape during the manufacture of the polymer foams and are gradually released from the foams age. Halons are widely used in fire extinguishers and fire protection system for computer centre industrial control rooms and aircraft. Although they are less abundant in the atmosphere than cfcs their ability to cause ozone depletion may be 3-10 times greater (environment Canada 1989)

When concern for the ozone layer was at its height compounds other than cfcs were identified as potentially harmful. These included nitrous oxide carbon tetrachloride and methyl chloroform. It is used extensively as a fumigant to kill pests in the fruit and vegetable industry and may be responsible for as much as 10% of existing ozone depletion. However, its actual impact is still a matter of dispute. Nitrous oxide as one of the oxides of nitrogen group known for their ability to destroy ozone has received most attention. It is produced naturally in the environment by denitrifying bacteria which cause it to be released from the nitrates and nitrates in the soil. It is an inert gas not easily removed from the troposphere. Over time it gradually diffuses into the stratosphere where the higher energy levels cause it to be oxidized into NO, leading to the destruction of ozone molecules. This process was part of the earth /atmosphere system before human beings came on the scene, and with no outside interference, natural ozone levels adjust to the output of N2O from the soil and the ocean. The nitrogen in the fertilizer used by the plants eventually works its way through the nitrogen cycle and is released into the air as n2o to initiate the sequence which ultimately ends in the destruction of ozone. Thus in they the pursuit of greater agricultural productivity through the increased application of nitrogen fertilizers is a threat to the ozone layer. There is however no proof that increased fertilizer use has or ever will damage the ozone layer through the production of N2O.

Declining concentrations of stratospheric ozone allow more ultraviolet radiation to reach the earth's surface. Even after a decade and a half of research the impact of that increase is still very much matter of speculation but most of the effects which have been identified can be classified as either biological or Climatological.

In moderate amounts ultraviolet radiation has beneficial effects for life on earth. It is powerful germicide for example and triggers the production of vitamin D in the skin; vitamin D allows the body to fix the calcium necessary for proper bone development lack of it may cause rickets particularly in growing children. High intensities of ultraviolet radiation however, are harmful to all forms of life. The most serious concern his over rising levels of UV-B-the radiation recognized as causing most biological damage. Intense UV-B rays alter the foundations of life such as the DNA molecule and various proteins. they also inhibit photosynthesis; growth rates in plants such as tomatoes, lettuce and peas are reduced and experimental exposure of some plant to increased ultraviolet radiation has produced an increased incidence of mutation (Hammond and Maugh 1974).insects which can see in the ultraviolet sector of the spectrum would have their activities disrupted by increased levels of ultraviolet radiation. (Crutzen 1974). Most of the concern for the biological effect of declining ozone levels has been focused on the impact of increased ultraviolet radiation on the human species. The potential effects include the increased incidence of sunburn premature ageing of the skin among white population and greater frequency of allergic reactions caused by the defects of ultraviolet light on chemicals in contact with the skin. These are relatively minor, however, in comparison to the more serious problems of skin cancer and radiation blindness both of which would become more frequent with higher levels of ultraviolet tradition. Level of skin cancer is rising among the white-skinned people of the world.

Stratospheric ozone is not evenly distributed through the upper atmosphere. Its maximum concentration is 25km above the surface (crutzen 1972). Natural variation in ozone levels alters the amounts of energy absorbed and emitted, but these changes are an integral part of the earth/atmosphere system, and do little to alter its overall balance. In contrast, chemically induced ozone depletion could lead to progressive disruption of the energy balance and ultimately cause climatic change. The lower temperature of the stratosphere would cause less infrared radiation to be emitted to the troposphere and the temperature of the lower atmosphere would also fall. Destruction of ozone does not occur uniformly throughout the ozone layer and as a result the altitude of maximum concentration may change; a decrease in that altitude will lead to a warming of the earth's surface whereas an increase will have the opposite defect and lead to cooling cfcs begin go be most effective as ozone destroyers at about 25km above the surface. Any estimate of the impact of ozone depletion on climate must consider not only changes in total stratospheric ozone gut also changes in the altitude of its maximum concentration. The depletion of total stratospheric ozone will always tend to cause cooling but that cooling may he enhanced by an increase in the altitude of maximum concentration or retarded by a decrease in altitude. Further complications are introduced by the ability of several of the chemicals which destroy ozone to interfere directly with the energy flow in the atmosphere. Rahmanathan has shown that ozone destroying cfcs absorb infrared radiation and the resulting temperature increase might be sufficient to negate the cooling caused by ozone depletion.