In 1983, R.P. Turco, O.B. Toon, T.P. Ackerman, J.B. Pollack, and Carl Sagan (referred to as TTAPS) published a paper entitled "Global Atmospheric Consequences of Nuclear War" which is the foundation that the nuclear winter theory is based on.
The theory states that nuclear explosions will set off firestorms in the cities and surrounding forest areas. The small particles of soot are carried high into the atmosphere. The smoke will block the sun's light for weeks or months. The land temperatures would fall below freezing.
This combination of reduced temperatures and reduced light levels would have catastrophic ecological consequences. Average light levels would be below the minimum required for photosynthesis during the first 30-40 days after the explosion and most fresh water would be frozen. The TTAPS study concluded: "...the possibility of the extinction of Homo Sapiens cannot be excluded." This effect is similar to what may have killed the dinosaurs.