Nuclear winter is an the environmental devastation that certain scientists contend would probably result from the hundreds of nuclear explosions in a nuclear war. The damaging effects of the light, heat, blast, and radiation caused by nuclear explosions had long been known to scientists, but such explosions' indirect effects on the environment remained largely ignored for decades. In the 1970s, however, several studies posited that the layer of ozone in the stratosphere that shields living things from much of the Sun's harmful ultraviolet radiation might be depleted by the large amounts of nitrogen oxides produced by nuclear explosions. Further studies speculated that large amounts of dust kicked up into the atmosphere by nuclear explosions (or an asteroid impact) might block sunlight from reaching the Earth's surface, leading to a temporary cooling of the air. Scientists then began to take into account the smoke produced by vast forests set ablaze by nuclear fireballs, and in 1983 an ambitious study, known as the TTAPS study (from the initials of the last names of its authors, R.P. Turco, O.B. Toon, T.P. Ackerman, J.B. Pollack, and Carl Sagan), took into consideration the crucial factor of smoke and soot arising from the burning petroleum fuels and plastics in nuclear-devastated cities. (Smoke from such materials absorbs sunlight much more effectively than smoke from burning wood.) The TTAPS study coined the term "nuclear winter," and its ominous hypotheses about the environmental effects of a nuclear war came under intensive study by both the American and Soviet scientific communities.

The basic cause of nuclear winter, as hypothesized by researchers, would be the numerous and immense fireballs caused by exploding nuclear warheads. These fireballs would ignite huge uncontrolled fires (firestorms) over any and all cities and forests that were within range of them. Great plumes of smoke, soot, and dust would be sent aloft from these fires, lifted by their own heating to high altitudes where they could drift for weeks before dropping back or being washed out of the atmosphere onto the ground. Several hundred million tons of this smoke and soot would be shepherded by strong west-to-east winds until they would form a uniform belt of particles encircling the Northern Hemisphere from 30 to 60 latitude. These thick black clouds could block out all but a fraction of the Sun's light for a period as long as several weeks. Surface temperatures would plunge for a few weeks as a consequence, perhaps by as much as 11 to 22 C (20 to 40 F). The conditions of semidarkness, killing frosts, and subfreezing temperatures, combined with high doses of radiation from nuclear fallout, would interrupt plant photosynthesis and could thus destroy much of the Earth's vegetation and animal life. The extreme cold, high radiation levels, and the widespread destruction of industrial, medical, and transportation infrastructures along with food supplies and crops would trigger a massive death toll from starvation, exposure, and disease. A nuclear war could thus reduce the Earth's human population to a fraction of its previous numbers.

A number of scientists have disputed the results of the original calculations, and, though such a nuclear war would undoubtedly be devastating, the degree of damage to life on Earth remains controversial.

Excerpt from the Encyclopedia Britannica without permission.