More worrisome is the possible effect of nuclear explosions on ozone in
the stratosphere. Not until the 20th century was the unique and
paradoxical role of ozone fully recognized. On the other hand, in
concentrations greater than I part per million in the air we breathe,
ozone is toxic; one major American city, Los Angeles, has established a
procedure for ozone alerts and warnings. On the other hand, ozone is a
critically important feature of the stratosphere from the standpoint of
maintaining life on the earth.
The reason is that while oxygen and nitrogen in the upper reaches of
the atmosphere can block out solar ultraviolet photons with wavelengths
shorter than 2,420 angstroms (A), ozone is the only effective shield in
the atmosphere against solar ultraviolet radiation between 2,500 and
3,000 A in wavelength. (See note 5.) Although ozone is extremely
efficient at filtering out solar ultraviolet in 2,500-3,000 A region of
the spectrum, some does get through at the higher end of the spectrum.
Ultraviolet rays in the range of 2,800 to 3,200 A which cause sunburn,
prematurely age human skin and produce skin cancers. As early as 1840,
arctic snow blindness was attributed to solar ultraviolet; and we have
since found that intense ultraviolet radiation can inhibit
photosynthesis in plants, stunt plant growth, damage bacteria, fungi,
higher plants, insects and annuals, and produce genetic alterations.
Despite the important role ozone plays in assuring a liveable
environment at the earth's surface, the total quantity of ozone in the
atmosphere is quite small, only about 3 parts per million.
Furthermore, ozone is not a durable or static constituent of the
atmosphere. It is constantly created, destroyed, and recreated by
natural processes, so that the amount of ozone present at any given
time is a function of the equilibrium reached between the creative and
destructive chemical reactions and the solar radiation reaching the
upper stratosphere.
The mechanism for the production of ozone is the absorption by oxygen
molecules (O2) of relatively short-wavelength ultraviolet light. The
oxygen molecule separates into two atoms of free oxygen, which
immediately unite with other oxygen molecules on the surfaces of
particles in the upper atmosphere. It is this union which forms ozone,
or O3. The heat released by the ozone-forming process is the reason
for the curious increase with altitude of the temperature of the
stratosphere (the base of which is about 36,000 feet above the earth's
surface).
While the natural chemical reaction produces about 4,500 tons of ozone
per second in the stratosphere, this is offset by other natural
chemical reactions which break down the ozone. By far the most
significant involves nitric oxide (NO) which breaks ozone (O3) into
molecules. This effect was discovered only in the last few years in
studies of the environmental problems which might be encountered if
large fleets of supersonic transport aircraft operate routinely in the
lower stratosphere. According to a report by Dr. Harold S. Johnston,
University of California at Berkeley--prepared for the Department of
Transportation's Climatic Impact Assessment Program--it now appears
that the NO reaction is normally responsible for 50 to 70 percent of
the destruction of ozone.
In the natural environment, there is a variety of means for the
production of NO and its transport into the stratosphere. Soil
bacteria produce nitrous oxide (N2O) which enters the lower atmosphere
and slowly diffuses into the stratosphere, where it reacts with free
oxygen (O) to form two NO molecules. Another mechanism for NO
production in the lower atmosphere may be lightning discharges, and
while NO is quickly washed out of the lower atmosphere by rain, some of
it may reach the stratosphere. Additional amounts of NO are produced
directly in the stratosphere by cosmic rays from the sun and
interstellar sources.
It is because of this catalytic role which nitric oxide plays in the
destruction of ozone that it is important to consider the effects of
high-yield nuclear explosions on the ozone layer. The nuclear fireball
and the air entrained within it are subjected to great heat, followed
by relatively rapid cooling. These conditions are ideal for the
production of tremendous amounts of NO from the air. It has been
estimated that as much as 5,000 tons of nitric oxide is produced for
each megaton of nuclear explosive power.
What would be the effects of nitric oxides driven into the stratosphere
by an all-out nuclear war, involving the detonation of 10,000 megatons
of explosive force in the northern hemisphere? According to the recent
National Academy of Sciences study, the nitric oxide produced by the
weapons could reduce the ozone levels in the northern hemisphere by as
much as 30 to 70 percent.
To begin with, a depleted ozone layer would reflect back to the earth's
surface less heat than would normally be the case, thus causing a drop
in temperature--perhaps enough to produce serious effects on
agriculture. Other changes, such as increased amounts of dust or
different vegetation, might subsequently reverse this drop in
temperature--but on the other hand, it might increase it.
Probably more important, life on earth has largely evolved within the
protective ozone shield and is presently adapted rather precisely to
the amount of solar ultraviolet which does get through. To defend
themselves against this low level of ultraviolet, evolved external
shielding (feathers, fur, cuticular waxes on fruit), internal shielding
(melanin pigment in human skin, flavenoids in plant tissue), avoidance
strategies (plankton migration to greater depths in the daytime,
shade-seeking by desert iguanas) and, in almost all organisms but
placental mammals, elaborate mechanisms to repair photochemical damage.
It is possible, however, that a major increase in solar ultraviolet
might overwhelm the defenses of some and perhaps many terrestrial life
forms. Both direct and indirect damage would then occur among the
bacteria, insects, plants, and other links in the ecosystems on which
human well-being depends. This disruption, particularly if it occurred
in the aftermath of a major war involving many other dislocations,
could pose a serious additional threat to the recovery of postwar
society. The National Academy of Sciences report concludes that in 20
years the ecological systems would have essentially recovered from the
increase in ultraviolet radiation--though not necessarily from
radioactivity or other damage in areas close to the war zone. However,
a delayed effect of the increase in ultraviolet radiation would be an
estimated 3 to 30 percent increase in skin cancer for 40 years in the
Northern Hemisphere's mid-latitudes.