3. Photosynthesis disrupted
Solar radiation, important not only for surface temperatures but also for photosynthesis, would drop by 60% to 70% in the first few years of a nuclear winter.
2. Nuclear winter
The varying scenarios for the theoretical nuclear winter depend on the severity of nuclear weapons exchange. Still, dark skies, widespread drought, fallout, and global temperature drops to those last seen in the previous ice age would occur in the Northern Hemisphere.
1. Mass extinction
In the worst-case scenario of a nuclear winter, a mass extinction event would take place.