Climatological effects

The climatological importance of the ozone layer lies in its contribution to the earth's energy budget (see Figure 6.8). It has a direct influence on the temperature of the stratosphere through its ability to absorb incoming radiation. Indirectly, this also has an impact on the troposphere. The absorption of short-wave radiation in the stratosphere reduces the amount reaching the lower atmosphere, but the effect of this is limited to some extent by the emission of part of the absorbed short-wave energy into the troposphere as infrared radiation.

Natural variations in ozone levels alter 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 total impact would depend upon a number of variables, including the amount by which the ozone concentration is reduced, and the altitude at which the greatest depletion occurred (Schneider and Mesirow 1976).

A net decrease in the amount of stratospheric ozone would reduce the amount of ultraviolet absorbed in the upper atmosphere, producing cooling in the stratosphere. The radiation no longer absorbed would continue on to the earth's surface, causing the temperature there to rise. This simple response to declining ozone concentration is complicated by the effects of stratospheric cooling on the system. 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. Since the cooling effect of the reduction in infrared energy would be greater than the warming caused by the extra long-wave radiation, the net result would be a cooling at the earth's surface. The magnitude of the cooling is difficult to assess, but it is likely to be small. Enhalt (1980) has suggested that a 20 per cent reduction in ozone concentration would lead to a global decrease in surface temperature of only about 0.25°C.

An increase in total ozone in the stratosphere would be likely to cause a rise in surface temperatures as a result of greater ultraviolet absorption, and the consequent increase in infrared energy radiated to the surface. Since current concern is with ozone depletion, the question of rising ozone levels has received little attention. However, the possibility that natural, ozoneenhancing processes might at times be sufficiently strong to reverse the declining trend cannot be ruled out completely.

Stratospheric ozone is not evenly distributed through the upper atmosphere. Its maximum concentration is 25 km above the surface (Crutzen 1972). 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 effect, and lead to cooling (Schneider and Mesirow 1976). CFCs begin to be most effective as ozone destroyers at about 25 km above the surface (Enhalt 1980). They therefore tend to push the level of maximum concentration down, and promote warming.

Thus any estimate of the impact of ozone depletion on climate must consider not only changes in total stratospheric ozone, but 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 be enhanced by an increase in the altitude of maximum concentration or retarded by a decrease in altitude.

Just as there are variations in the vertical distribution of ozone, there are also variations in its horizontal distribution. The latter are seasonal and associated with changing wind and pressure systems in the lower stratosphere (Crutzen 1974). Most ozone is manufactured above the tropics, and is transported polewards from there. Increased levels of ozone have been identified regularly at middle and high latitudes in late winter and spring in the northern hemisphere (Crutzen 1972), and the redistribution of ozone by upper atmospheric winds has been implicated by a number of authors in the development of the Antarctic ozone hole (Shine 1988). Such changes have local and short-term effects which might reinforce or weaken the global impact of ozone depletion.

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. Ramanathan (1975) 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. Similarly, oxides of nitrogen absorb solar radiation so effectively that they are able to reduce the cooling caused by their destruction of ozone by about half (Ramanathan et al. 1976).

Changes in the earth's energy budget initiated by declining stratospheric ozone levels are integrated with changes produced by other elements such as atmospheric turbidity and the greenhouse effect. The specific effects of ozone depletion are therefore difficult to identify, and the contribution of ozone depletion to climatic change difficult to assess.

The Basic Survival Guide

The Basic Survival Guide

Disasters: Why No ones Really 100 Safe. This is common knowledgethat disaster is everywhere. Its in the streets, its inside your campuses, and it can even be found inside your home. The question is not whether we are safe because no one is really THAT secure anymore but whether we can do something to lessen the odds of ever becoming a victim.

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