Environmental Engineering Reference
In-Depth Information
depletion on people is not surprising or
unexpected. Much more research into other
biological effects is required, however. Human
beings are an integral part of the earth/
atmosphere system and, as Dotto and Schiff
(1978) suggest, humankind may experience the
consequences of ozone depletion through its
effects on plants, animals and climate. The impact
would be less direct, but perhaps no less deadly.
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.
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
