Worldwide Effects of Nuclear War: Some Perspectives

by United States. Arms Control and Disarmament Agency

B. Ozone

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.