An effort has been made to present this paper in language that does not require a scientific background on the part of the reader. Nevertheless it must deal in schematized processes, abstractions, and statistical generalizations. Hence one supremely important perspective must be largely supplied by the reader: the human perspective--the meaning of these physical effects for individual human beings and for the fabric of civilized life. Fred C. Ikle Director U.S. Arms Control and Disarmament Agency INTRODUCTION It has now been two decades since the introduction of thermonuclear fusion weapons into the military inventories of the great powers, and more than a decade since the United States, Great Britain, and the Soviet Union ceased to test nuclear weapons in the atmosphere. Today our understanding of the technology of thermonuclear weapons seems highly advanced, but our knowledge of the physical and biological consequences of nuclear war is continuously evolving. Only recently, new light was shed on the subject in a study which the Arms Control and Disarmament Agency had asked the National Academy of Sciences to undertake. Previous studies had tended to focus very largely on radioactive fallout from a nuclear war; an important aspect of this new study was its inquiry into all possible consequences, including the effects of large-scale nuclear detonations on the ozone layer which helps protect life on earth from the sun's ultraviolet radiations. Assuming a total detonation of 10,000 megatons--a large-scale but less than total nuclear "exchange," as one would say in the dehumanizing jargon of the strategists--it was concluded that as much as 30-70 percent of the ozone might be eliminated from the northern hemisphere (where a nuclear war would presumably take place) and as much as 20-40 percent from the southern hemisphere. Recovery would probably take about 3-10 years, but the Academy's study notes that long term global changes cannot be completely ruled out. The reduced ozone concentrations would have a number of consequences outside the areas in which the detonations occurred. The Academy study notes, for example, that the resultant increase in ultraviolet would cause "prompt incapacitating cases of sunburn in the temperate zones and snow blindness in northern countries . . ."