The theory of nuclear winter is essentially one of environmental collateral damage. While a nuclear attack might target a nation's military infrastructure or population centers, the assault could inflict massive harm to Earth's atmosphere.
It's easy to take the air we breathe for granted, but the atmosphere is a vital component of all life on this planet. In fact, scientists believe it co-evolved to its present state along with Earth's unicellular organisms. It protects us from dangerous levels of solar radiation, but also allows the sun to heat our world. Sunlight shines through the atmosphere and warms the planet's surface, which then emits terrestrial radiation that heats the air. If sufficient ash from burning cities and forests ascended into the sky, it could effectively work as an umbrella, shielding large portions of the Earth from the sun. If you diminish the amount of sunlight that makes its way to the surface, then you diminish the resulting atmospheric temperature -- as well as potentially interfere with photosynthesis.
Examples of this scenario have occurred on a smaller scale in recent history. For instance, the 1883 eruption of the Indonesian volcano Krakatoa blasted enough volcanic ash into the atmosphere to lower global temperatures by 2.2 degrees F (1.2 degrees C) for an entire year [source: Maynard]. Decades earlier in 1815, the eruption of Mount Tambora in Indonesia blocked enough sunlight around the globe to cause what came to be known as "the year without summer" [source: Discovery Channel]
That following year, residents in the United States experienced summer snows and temperatures between 5 and 10 degrees F (3 and 6 degrees C) less than average. This drop in temperatures devastated crops and caused hundreds of thousands of deaths -- not counting those who perished in Indonesia. Some archeologists theorize that an even greater cataclysm occurred 65 million years ago when an asteroid collided with Earth. Called the K-T boundary extinction event, some experts believe this collision may have ejected enough ash and debris into the atmosphere to cause an impact winter. The premise is the same as nuclear winter, only with a different method of generating the atmospheric debris. Some paleontologists suspect such an impact winter brought about the extinction of the dinosaurs.
Natural disasters aren't the only proven temperature changers. At the close of the 1991 Persian Gulf War, Iraqi President Saddam Hussein torched 736 Kuwaiti oil wells. The fires raged for nine months, during which average local air temperatures fell by 18.3 degrees F (10.2 degrees C) [source: McLaren].
As severe as these examples seem, nuclear winter theorists provided a far bleaker forecast should nuclear war erupt between the nuclear superpowers of United States and the then-Soviet Union. In the 1980s theorists predicted decade-long temperate decreases of as much as 72 degrees F (40 degrees C) [source: Perkins]. Such a winter could finish the destruction that nuclear war started, sending the survivors down a chilling path of famine and starvation.