Since the beginning of the nuclear arms race four decades ago it has been generally assumed that the most devastating consequence of a major nuclear war between the U.S. and the U.S.S.R. would be a gigantic number of human casualties in the principal target zones of the Northern Hemisphere. Although in the wake of such a war the social and economic structure of the combatant nations would presumably collapse, it has been argued that most of the noncombatant nations—and hence the majority of the human population- would not be endangered, either directly or indirectly. Over the years questions have been raised about the possible global extent of various indirect, long- term effects of nuclear war, such as delayed radioactive fallout, depletion of the protective ozone layer in the upper atmosphere and adverse changes in the climate. Until recently, however, the few authoritative studies available on these added threats have tended to play down their significance, in some cases emphasizing the uncertainty inherent in any attempt to predict the combined effects of multiple nuclear explosions