Nuclear Winter occurs when Nuclear units have been exploded, and nuclear fallout has not been cleaned up.
Terrain begins changing into desert and tundra. Settlers, workers, and engineers must be given the clean fallout command to dispose of nuclear waste, which costs three settler-turns per square.
Is it possible to reverse this?
- No. However you can perform the converse operation with Global Warming. However, just like in real life, this tends to mess things up more than help (areas all become extremes).