A number of other surprises were encountered during 30 years of nuclear weapons development. For example, what was probably man's most extensive modification of the global environment to date occurred in September 1962, when a nuclear device was detonated 250 miles above Johnson Island. The 1.4-megaton burst produced an artificial belt of charged particles trapped in the earth's magnetic field. Though 98 percent of these particles were removed by natural processes after the first year, traces could be detected 6 or 7 years later. A number of satellites in low earth orbit at the time of the burst suffered severe electronic damage resulting in malfunctions and early failure. It became obvious that man now had the power to make long term changes in his near-space environment. Another unexpected effect of high-altitude bursts was the blackout of high-frequency radio communications. Disruption of the ionosphere (which reflects radio signals back to the earth) by nuclear bursts over the Pacific has wiped out long-distance radio communications for hours at distances of up to 600 miles from the burst point. Yet another surprise was the discovery that electromagnetic pulses can play havoc with electrical equipment itself, including some in command systems that control the nuclear arms themselves. Much of our knowledge was thus gained by chance--a fact which should imbue us with humility as we contemplate the remaining uncertainties (as well as the certainties) about nuclear warfare. What we have learned enables us, nonetheless, to see more clearly. We know, for instance, that some of the earlier speculations about the after-effects of a global nuclear war were as far-fetched as they were horrifying--such as the idea that the worldwide accumulation of radioactive fallout would eliminate all life on the planet, or that it might produce a train of monstrous genetic mutations in all living things, making future life unrecognizable. And this accumulation of knowledge which enables us to rule out the more fanciful possibilities also allows us to reexamine, with some scientific rigor, other phenomena which could seriously affect the global environment and the populations of participant and nonparticipant countries alike. This paper is an attempt to set in perspective some of the longer term effects of nuclear war on the global environment, with emphasis on areas and peoples distant from the actual targets of the weapons. THE MECHANICS OF NUCLEAR EXPLOSIONS In nuclear explosions, about 90 percent of the energy is released in less than one millionth of a second. Most of this is in the form of the heat and shock waves which produce the damage. It is this immediate and direct explosive power which could devastate the urban centers in a major nuclear war.