Answer:
In the 1970s studies were being done on supercomputers of the effects of giant meteor impacts on the earth. It was notices that large firestorm forest fires would be ignited, as well as large amounts of dirt and powdered rock would be thrown out. The particulates (soot from forest fires and particulates from the impact) would end up in the stratosphere, where they could stay for decades. They would nearly block sunlight, causing a several decade worldwide winter.
Somewhat later it was noted that the available nuclear weapons had a similar total yield to the meteor impact. Therefor in a large nuclear war a decades long winter was also possible. This go the name nuclear winter.