In Carl Sagan and Richard Turco's book "A Path Where No Man Thought," the two nuclear winter theorists propose six classes of nuclear winter, which provide a framework for understanding the possible atmospheric consequences of modern warfare.
- Minimal nuclear winter: In the best-case scenario for nuclear war, a small enough attack would cause minimal cloud cover and little or no environmental impact. While the damage sustained by targeted areas might prove substantial, the rest of the world wouldn't suffer atmospheric consequences.
- Marginal nuclear winter: Sagan and Turco predict a grim scenario for even a "marginal" nuclear winter. They calculate that a few nuclear detonations above urban centers in a contained nuclear war could lower temperatures in the Northern Hemisphere by a few degrees. Agricultural production would suffer, resulting in famine -- especially if accompanied by severe drought. While a great deal of the ash would return to Earth in black rains, much would remain in the upper atmosphere. Sagan and Turco predict that the deaths from such a nuclear winter would equal those killed in the nuclear war. Everything below the equator would remain mostly unaffected, given the hemispheric separation of air currents and the fact that most nuclear targets exist in the Northern Hemisphere.
- Nominal nuclear winter: The authors deem this class of nuclear winter a low-end possibility for a full-scale nuclear war involving the detonation of between 6,000 and 12,000 nuclear weapons. Survivors would endure dark skies, widespread drought, fallout and global temperature drops of 18.3 degrees F (10 degrees C) in the Northern Hemisphere. Noon sunlight would be only one-third what it was before the war. In the following months, these clouds would dissipate, and the sun would seem to burn hotter than before. Because nuclear blasts would have destroyed much of the ozone layer, greater quantities of solar radiation would reach the Earth's surface. The Southern Hemisphere wouldn't experience major climatic change.
- Substantial nuclear winter: This scenario, following full-scale nuclear war, involves catastrophic consequences for the Northern Hemisphere: freezing temperatures, widespread fallout, pollution, ozone depletion and disrupted precipitation. Imagine a deeply overcast day -- now imagine those conditions persisting for years. Green plants would barely receive enough sunlight for photosynthesis. Crops would fail, billions of humans would die, species would go extinct and while humanity would likely survive, civilization as we know it might not. Damage to the Southern Hemisphere would depend on the number of detonations below the equator.
- Severe nuclear winter: In this scenario, less than 1 percent of the sun's light makes it to the Earth's surface for a period of months, resulting in temperature drops around the globe and insufficient light for photosynthesis. In addition to widespread famine and pollution, Sagan and Turco predict that agricultural production would be reduced to levels not seen since the Dark Ages.
- Extreme nuclear winter: In this worst-case scenario, based on the conditions in 1990, nearly all the world's nuclear weapons are deployed. The result would be utter darkness at noon. Much of the planet's life would perish within the chilly confines of this black, atmospheric tomb.
However, nuclear winter is very much a theory -- and a controversial one at that. Next, we'll look at how the theory has evolved and where it stands today.