It has been estimated that a 10,000-megaton war with half the weapons exploding at ground level would tear up some 25 billion cubic meters of rock and soil, injecting a substantial amount of fine dust and particles into the stratosphere. This is roughly twice the volume of material blasted loose by the Indonesian volcano, Krakatoa, whose explosion in 1883 was the most powerful terrestrial event ever recorded. Sunsets around the world were noticeably reddened for several years after the Krakatoa eruption, indicating that large amounts of volcanic dust had entered the stratosphere. Subsequent studies of large volcanic explosions, such as Mt. Agung on Bali in 1963, have raised the possibility that large-scale injection of dust into the stratosphere would reduce sunlight intensities and temperatures at the surface, while increasing the absorption of heat in the upper atmosphere. The resultant minor changes in temperature and sunlight could affect crop production. However, no catastrophic worldwide changes have resulted from volcanic explosions, so it is doubtful that the gross injection of particulates into the stratosphere by a 10,000-megaton conflict would, by itself, lead to major global climate changes. 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.