It’s the old Jane Fonda “Nuclear Winter - We’ll All Die” BS.
Nuclear winter was greatly exaggerated in the 1980's by Turco, Toon, Ackerman, Pollack, and Sagan. They predicted that temperatures would fall by 35C which is complete garbage. They wanted unilateral disarmament on our side so that the Soviets could hold us hostage.
Sagan even predicted in 1991 that a nuclear winter would be caused by the fires of Iraqi oil wells. He later admitted he was wrong.
Most of official liberal "scientists" now predict that global temperatures would fall by 7C to 8C in a limited nuclear war. However, they might just be manipulating the data to push their alarmist agenda.
But still, there are examples of volcanic eruptions which casued small scale nuclear winters.
The "Year Without a Summer" of 1816 was due to the eruption of Mount Tambora in Indonesia the year before.
The eruption of Krakatau (Krakatoa) in 1883 caused global cooling.
The eruption of Mount Pinatubo in 1991 cooled global temperatures for a couple of years.